title of the thesis - Department of Geology - Queen's University

FROM CESSATION OF SOUTH-DIRECTED MID-CRUST
EXTRUSION TO ONSET OF OROGEN-PARALLEL EXTENSION,
NW NEPAL HIMALAYA
by
Carl Nagy
A thesis submitted to the Department of Geological Sciences and Geological Engineering
In conformity with the requirements for
the degree of Master’s of Science
Queen’s University
Kingston, Ontario, Canada
(September, 2012)
Copyright © Carl Nagy, 2012

View of the northern Karnali valley near the village of Chala. Dr. Borja Antolín is leading the
way, followed by our trusted guide Dawa Tamang. The foreground is Tethyan sedimentary
sequence and the distant peaks are Greater Himalayan sequence. The contact between these
domains is roughly 200 meters towards the peaks from the photograph position.
ii

Abstract
Field mapping and, structural, microstructural, and chronological analyses confirm the
existence of a segment of the Gurla-Mandhata-Humla fault, an orogen-parallel strike-slip
dominated shear zone in the upper Karnali valley of northwestern Nepal. This shear zone forms
the upper contact of, and cuts obliquely across the Greater Himalayan Sequence (GHS). Data
from this study reveal two phases of GHS deformation.
Phase 1 is characterized by U-Th-Pb monazite crystallization ages (~26–12 Ma, peak
~18–15 Ma), consistent with typical Neohimalayan metamorphic ages, and the final stages of
south-directed extrusion of the GHS.
Phase 2 is characterized by south-dipping high-strain foliations and intensely developed
ESE-WNW trending, shallowly plunging mineral elongation lineations, indicating orogen-parallel
extension. Thermochronology of muscovite defining these fabrics implies that the area was
cooling and experiencing orogen-parallel extension by ~15–9 Ma. Mineral deformation
mechanisms and quartz c-axis patterns of these fabrics record a rapid increase in temperature
from ~350°C along the shear zone, to ~650°C at ~2.5 structural km below the shear zone. Such
temperature gradients may be remnants of telescoped and/or flattened isotherms generated during
south-directed extrusion of the GHS.
Overprinting ESE-WNW fabrics record progressive deformation of the GHS at lower
temperatures. Progressive deformation included a significant component of pure shear, as
indicated by symmetric high-temperature quartz c-axis fabrics and a lower-temperature vorticity
estimate (~59% pure shear). A transition in c-axis fabrics from type I to type II cross-girdles at ~
1.2 km below the fault could indicate a transition from plane strain towards constriction.
Together, these data suggest orogen-parallel extension was occurring as a result of transtension.
This study reveals a transition from south-directed extrusion of the GHS to orogenparallel
extension between ~15–13 Ma. Comparing these data with tectonic events across the
iii

Himalaya reveals an orogen-wide middle Miocene transition, coeval with the uplift of eastern
Tibet. This is consistent with interpretations invoking radial spreading of Tibet and east-directed
lower-crustal flow to explain orogen-parallel extension. Our study leads to the suggestion that a
transition affecting mid- to lower-crustal processes may be responsible for the cessation of southdirected
extrusion of the GHS and onset of east-directed lower-crustal flow.
iv

Co-Authorship
This thesis is my own work. The following contributors are co-authors on Chapter 2: My
supervisor, L. Godin, who provided scientific direction, thought-provoking discussion, and
editorial assistance; J. Cottle who provided direction, expertise, and performed the final stage of
U-Th-Pb geochronology; D. Archibald who provided direction, expertise, and guided the
40 Ar/ 39 Ar thermochronology; B. Antolín who provided valuable field assistance, discussion, and
editorial comments.
v

Acknowledgements
It is with utmost pleasure that I am able to acknowledge those that have made the past
years a success, both academically and personally.
Firstly, I must thank Dr. Laurent Godin for the opportunity to work in the Himalaya and
for guidance in the initial stages of my geologic career. His constant enthusiasm and support over
the past years are sincerely appreciated. Borja Antolín is thanked for an exciting 2011 field
season, numerous scientific discussions, and a thorough review of Chapter 2. I must acknowledge
the work of John Cottle at the University of California, Santa Barbara, with whom I collaborated
to perform the U-Th-Pb geochronology, and Doug Archibald, who guided the 40 Ar/ 39 Ar
thermochronology. I would also like to thank the following people for their scientific and
analytical assistance: Brian Joy for guidance with electron microprobe analyses, Dan Gibson
(Simon Fraser University) for his assistance with the presentation of geochronology results, Al
Grant for assistance on the scanning electron microscope, Ashley K. Rudy for her master GIS
assistance, and Martin Wong (Colgate University) for assistance and accommodation during
electron backscatter diffraction analyses. Gratitude is also expressed towards my examining
committee of Drs. Herb Helmstaedt and John Dixon for their constructive reviews of this thesis.
Fieldwork was made exciting and possible by Dawa Tamang, the infamous Birval and too many
other porters to name.
I am beyond grateful for my family, and the friends I have made at Queen's University
and throughout Kingston. Our experiences together have made the last two years much more than
an academic success.
This thesis was funded by a Natural Sciences and Engineering Research Council
(NSERC) discovery grant to L. Godin, a NSERC MSc. Canadian Graduate Scholarship, an
Ontario Graduate Scholarship in Science and Technology, and a Geological Society of America
Student Research Grant awarded to C. Nagy.
vi

Table of Contents
Abstract .......................................................................................................................................... iii
Co-Authorship ................................................................................................................................ v
Acknowledgements ....................................................................................................................... vi
Abbreviations .............................................................................................................................. xiii
Chapter 1 Introduction.................................................................................................................. 1
1.1 Introduction ............................................................................................................................ 1
1.2 Evolution of the Himalayan-Tibetan Orogen ......................................................................... 4
1.3 Transition from south-directed extrusion of the mid-crust to onset of orogen-parallel
extension ...................................................................................................................................... 8
1.4 This study ............................................................................................................................... 9
1.4.1 Geology of northwestern Nepal and the upper Karnali valley ........................................ 9
1.4.2 Purpose of this study ..................................................................................................... 10
Chapter 2 From cessation of south-directed mid-crust extrusion to onset of orogen-parallel
extension, NW Nepal Himalaya .................................................................................................. 13
2.1 Abstract ................................................................................................................................ 13
2.2 Introduction .......................................................................................................................... 14
2.3 Geologic Setting................................................................................................................... 18
2.3.1 Himalayan Overview .................................................................................................... 18
2.3.1.1 South-directed extrusion of the GHS ..................................................................... 18
2.3.1.2 Orogen-parallel extension ...................................................................................... 19
2.4 Geology of the upper Karnali Valley ................................................................................... 20
2.4.1 Background ................................................................................................................... 20
2.4.2 This study ...................................................................................................................... 21
2.4.2.1 Domain I – Folded TSS ......................................................................................... 23
2.4.2.2 Domain II – Transposed TSS ................................................................................. 23
2.4.2.3 Domain III – Ultra-high-strain zone ...................................................................... 25
2.4.2.4 Domain IV – High-strain quartzite and pelitic GHS .............................................. 28
2.4.2.5 Domain V – High-strain gneissic GHS .................................................................. 30
2.4.2.6 Domain VI – Migmatitic GHS ............................................................................... 33
2.5 Microstructural analyses ...................................................................................................... 35
2.5.1 Crystallographic preferred orientation of quartz ........................................................... 35
2.5.1.1 Methodology .......................................................................................................... 36
vii

2.5.1.2 Descriptions of quartz CPO patterns ...................................................................... 38
2.5.1.3 Quartz CPO deformation temperatures .................................................................. 45
2.5.2 Rigid clast vorticity analysis ......................................................................................... 47
2.5.3 Summary of microstructural analyses ........................................................................... 50
2.6 Geochronology and thermochronology ............................................................................... 52
2.6.1 U-Th-Pb monazite geochronology ................................................................................ 53
2.6.1.1 Methodology .......................................................................................................... 54
2.6.1.2 Results .................................................................................................................... 55
2.6.1.3 Summary of U-Th-Pb geochronology .................................................................... 69
2.6.2 40 Ar/ 39 Ar thermochronology .......................................................................................... 71
2.6.2.1 Methodology .......................................................................................................... 72
2.6.2.2 Results .................................................................................................................... 74
2.6.2.3 Summary of 40 Ar/ 39 Ar thermochronology .............................................................. 79
2.7 Discussion ............................................................................................................................ 81
2.7.1 South-directed extrusion of the GHS ............................................................................ 83
2.7.1.1 Initial south-directed extrusion .............................................................................. 83
2.7.1.2 Timing of south-directed extrusion ........................................................................ 84
2.7.2 Orogen-parallel extension ............................................................................................. 85
2.7.2.1 ESE-WNW deformation and exhumation .............................................................. 85
2.7.2.2 Timing of ESE-WNW deformation and cooling ................................................... 87
2.7.3 Transition from south-directed extrusion to orogen-parallel extension ........................ 88
2.7.3.1 Upper Karnali valley .............................................................................................. 88
2.7.3.2 Himalayan transition .............................................................................................. 90
2.7.4 Tectonic model explaining orogen-parallel extension .................................................. 94
2.8 Conclusions .......................................................................................................................... 97
Chapter 3 Discussion ................................................................................................................... 99
3.1 Summary .............................................................................................................................. 99
3.1.1 Middle Miocene orogen-parallel extension and east-directed lower-crustal flow ........ 99
3.2 Future research considerations ........................................................................................... 103
3.2.1 Why did south-directed mid-crustal extrusion cease, and why did east-directed lower
crustal flow initiate ............................................................................................................. 103
3.2.2 Is the Karakoram fault linked to faults of the upper Karnali valley .......................... 104
Chapter 4 Microstructural analyses: proposals for advancing and validating current
techniques ................................................................................................................................... 106
viii

4.1 Introduction ........................................................................................................................ 106
4.2 Modern quartz CPO analyses: future research opportunities ............................................. 106
4.2.1 Evolution of CPO fabrics during progressive strain development .............................. 109
4.2.2 Lateral variations in strain during extrusion of the Greater Himalayan sequence ...... 112
4.3 Evaluation of the rigid grain method for estimating kinematic vorticity numbers; future
research opportunities .............................................................................................................. 113
4.3.1 Analogue modeling of rigid clasts in a 3D flowing matrix ......................................... 117
4.4 Conclusions ........................................................................................................................ 119
Appendix A Station locations and structural data .................................................................. 120
Appendix B Quartz CPO data .................................................................................................. 125
Appendix C Monazite images and elemental maps ................................................................ 149
Appendix D Muscovite 40 Ar/ 39 Ar thermochronology ............................................................. 219
References ................................................................................................................................... 263
ix

List of Figures
Figure 1.1 Color relief map of the Himalaya, Tibet, and surrounding area. .................................... 2
Figure 1.2 Geology of the Himalaya and NW Nepal / SW Tibet. ................................................... 3
Figure 1.3 Extensional and compressional structures in the Himalaya, Tibet, and NW Nepal / SW
Tibet ................................................................................................................................................. 6
Figure 2.1 Geology of the Himalaya and NW Nepal / SW Tibet. ................................................. 15
Figure 2.2 Extensional and compressional structures in the Himalaya, Tibet, and NW Nepal / SW
Tibet ............................................................................................................................................... 17
Figure 2.3 Lithotectonic map of the upper Karnali valley ............................................................. 22
Figure 2.4 Structures and rocks of the Tethyan Sedimentary sequence......................................... 24
Figure 2.5 Structures and rocks of the ultra-high-strain zone ........................................................ 26
Figure 2.6 Structures and rocks of the high-strain pelitic Greater Himalayan sequence ............... 29
Figure 2.7 Structures and rocks of the gneissic and migmatitic Greater Himalayan sequence. .... 31
Figure 2.8 Brittle structures of the upper Karnali valley ............................................................... 34
Figure 2.9 Summary of EBSD data processing ............................................................................. 37
Figure 2.10 Summary of quartz CPO fabrics ................................................................................. 39
Figure 2.11 Samples analyzed for quartz CPOs. ........................................................................... 40
Figure 2.12 Quartz CPO fabrics. .................................................................................................... 41
Figure 2.13 C-axis fabric opening angles versus deformation temperature plot. .......................... 46
Figure 2.14 Vorticity sample and analysis ..................................................................................... 49
Figure 2.15 Summary of microstructural results ........................................................................... 51
Figure 2.16 Monazite images and elemental maps ........................................................................ 56
Figure 2.17 U-Th-Pb concordia and probability diagrams. ........................................................... 60
Figure 2.18 Muscovite 40Ar/39Ar step-heating plateaus. ............................................................. 75
Figure 2.19 Plots of muscovite cooling ages versus distance from shear zone. ............................ 80
Figure 2.20 Summary of CPO, vorticity, thermochronology and geochronology analyses .......... 82
Figure 2.21 Transition from south-directed extrusion to orogen-parallel extension, upper Karnali
valley. ............................................................................................................................................. 89
Figure 2.22 Orogen-wide transition from south-directed extrusion to orogen-parallel extension . 92
Figure 2.23 Numerical model of Himalyan-Tibetan lower crustal flow ........................................ 96
Figure 3.1 Numerical model of Himalayan lower crustal flow ................................................... 100
Figure 4.1 Summary of quartz CPO fabrics ................................................................................. 108
Figure 4.2 Progressive development of quartz CPO fabrics ........................................................ 110
x

Figure 4.3 Microscale CPO variability ........................................................................................ 111
Figure 4.4 Deformation and vorticity analysis planes ................................................................. 115
Figure 4.5 Analogue model of vorticity experiment .................................................................... 118
xi

List of Tables
Table 2-1 U-Th-Pb geochronologic data ....................................................................................... 61
Table 2-2 40 Ar/ 39 Ar thermochronological data ............................................................................... 77
xii

Abbreviations
Mineral
Biotite
Calcite
Garnet
Feldspar
Muscovite
Pyrite
Sillimanite
Tourmaline
Quartz
Abbreviation
Bt
Cal
Grt
Fsp
Ms
Py
Sil
Tur
Qtz
Element
Argon
Cerium
Lanthanum
Lead
Neodymium
Oxygen
Phosphorus
Thorium
Uranium
Yttrium
Abbreviation
Ar
Ce
La
Pb
Nd
O
P
Th
U
Y
Feature
Ama Drime Massif
Chuwa granite
Greater Himalayan sequence
Gurla Mandhata core complex
Gurla Mandhata-Humla fault system
Indus-Yalu suture zone
Karakoram Fault
Kiogar-Jungbwa mantle-type rocks
Leo Pargil gneiss dome
Main Boundary thrust
Main Central Thrust
Main Frontal Thrust
Namche Barwa
Nanga Parbat
South Tibetan Detachment system
Tethyan sedimentary sequence
Thakkhola graben
Abbreviation
AD
CG
GHS
GM
GMH
IYSZ
KF
KJO
LP
MBT
MCT
MFT
NB
NP
STDS
TSS
TG
xiii

Chapter 1
Introduction
1.1 Introduction
The Himalaya-Karakoram mountain belt and adjacent Tibetan plateau are the result of the
ongoing convergence and collision of India and Asia, which initiated approximately 55-50
million years ago (Green et al., 2008; Najman et al., 2010). The Tibetan plateau is the world’s
largest and most elevated plateau, residing at a mean elevation of over 5 kilometers (Fielding et
al., 1994), and spanning from the Kunlun Mountains in central China to the Himalayan front (Fig.
1.1). The topographic front of the Tibetan plateau, also known as the Himalaya, lies between the
Indian shield to the south and the Indus-Yalu Suture to the north (Fig. 1.1). The mountain belt
trends WNW-ESE, spanning 2500 kilometers between two structural syntaxes, Nanga Parbat in
northern Pakistan and Namche Barwa in southeastern Tibet (Fig. 1.1; Hodges, 2000).
The geology of the Himalaya consists of four distinct lithotectonic domains bounded by
crustal-scale fault systems, all of which are laterally continuous across the length of the orogen
(Fig. 1.2). From north to south, these domains are the Tethyan sedimentary, Greater Himalayan,
Lesser Himalayan, and Sub-Himalayan sequences. Their bounding fault systems include the
South Tibetan detachment system, the Main Central thrust, the Main Boundary thrust, and the
Main Frontal thrust (Fig. 1.2). This study focuses on an area of northwestern Nepal, within the
two northernmost tectonostratigraphic domains, the Tethyan sedimentary sequence and Greater
Himalayan sequence, their bounding faults, the South Tibetan Detachment system (STDS), and
the Gurla Mandhata-Humla fault system (Fig. 1.2).
1

Figure 1.1 Color relief map of the Himalaya, Tibet, and surrounding area constructed from the
ETOPO2v2 (2006) database with a Cylindrical Equidistant projection. The upper Karnali valley
(black box) is the study area. Dashed line indicates northern boundary of the Himalaya (IYSZ).
Abbreviations: IYSZ – Indus-Yalu suture zone; KF – Karakoram Fault; NB - Namche Barwa;
NP - Nanga Parbat.
2

Figure 1.2 Simplified geologic and structural map, and cross-section of (a) the
Himalaya, modified after Yin (2006) and Hintersberger et al. (2010), and (b) and
(c), the upper Karnali valley and surrounding regions, modified after Murphy
and Copeland (2005), Robinson et al. (2006) and Yakymchuk and Godin (2012).
Abbreviations: AD - Ama Drime Massif, CG – Chuwa granite, GM - Gurla
Mandhata gneiss dome, GMH – Gurla Mandhata-Humla fault, IYSZ - Indus-
Yalu suture zone, KF - Karakorum fault, KJO - Kiogar-Jungbwa ophiolitic and
mantle-type rocks, LP - Leo Pargil gneiss dome, MBT - Main Boundary thrust,
MCT - Main Central thrust, MFT - Main Frontal thrust, STDS - South Tibetan
detachment system, TG - Thakkhola graben.
3

1.2 Evolution of the Himalayan-Tibetan Orogen
The majority of southern Tibet and the northern Himalaya comprise the Tethyan
sedimentary sequence (TSS), a wedge of Cambrian to Eocene supracrustal sedimentary strata
deposited on the northern margin of the Indian craton (Gansser, 1964; Garzanti, 1999). These
sedimentary rocks accommodated crustal shortening and were metamorphosed at low grades
from the onset of collision through to the late Oligocene/early Miocene (Godin, 2003; Murphy
and Yin, 2003). Significant thickening of the TSS occurred during Eohimalayan deformation
from roughly 45 to 32 Ma, accompanied by metamorphism at high pressure and moderate
temperature conditions (Hodges and Silverberg, 1988; Godin et al., 1999, 2001; Aikman et al.,
2008; Kellett and Godin, 2009; Carosi et al., 2010).
Laterally throughout the orogen, the Greater Himalayan sequence (GHS) typically crops
out south of the TSS. The GHS is the high metamorphic grade and anatectic core of the Himalaya
bounded on its base by the Main Central Thrust (MCT), a south-verging thrust fault, and on its
upper surface by the South Tibetan detachment system (STDS), a low-angle top-to-the-north
normal fault (Fig. 1.2; Burchfiel and Royden, 1985; Burchiel et al., 1992; Hodges, 2000).
Neohimalayan metamorphism is recorded throughout the GHS from early to middle Miocene
(Vannay and Hodges, 1996; Godin et al., 2001; Searle and Szulc, 2005; Cottle et al., 2009b), and
is characterized by extensive anatexis and synchronous motion along the STDS and MCT (see
Godin et al., 2006b for review). This metamorphism is contemporaneous with isothermal
decompression of the GHS and south-directed extrusion during early to middle Miocene (Vannay
and Hodges, 1996; Harris et al., 2004). These relationships prompted the development of a
channel-flow model to explain the process through which the GHS was extruded (Nelson et al.,
4

1996; Hodges et al., 2001; Beaumont et al., 2001, 2004, 2006; Jamieson et al., 2004; Godin et al.,
2006a).
The channel-flow model stipulates that ductile extrusion of the GHS occurred normal to
the trend of the orogen towards the topographic front of the Himalaya via flow in a low-viscosity
channel, potentially coupled to focused surface denudation (Beaumont et al., 2001). However,
this process is controversial and adamantly debated (Grujic, 2006; Harrison, 2006; Harris, 2007;
Kohn, 2008). Alternative models have employed wedge-type extrusion (Burchfiel and Royden,
1985; Grujic et al., 1996; Kohn, 2008) and tectonic wedging (Yin, 2006; Webb et al., 2007, 2011)
to explain the presence of the enigmatic GHS.
By middle Miocene time, motion along the MCT and STDS waned, and shortening was
accommodated south of the GHS, along the Main Boundary thrust and the Lesser Himalayan
imbricate thrust belt (Fig. 1.2; Schelling, 1992; Meigs et al., 1995; Huyghe et al., 2001). From
this time forward, deformation has progressed southward in a typical in-sequence foreland
propagating style.
The onset of orogen-parallel extension throughout the Himalaya and Tibetan plateau
broadly coincides with the cessation of south-directed extrusion of the GHS (Jessup and Cottle,
2010), and on-going orogen-perpendicular shortening in the foreland (Huyghe et al., 2001).
Orogen-parallel extension is indicated by structures that accommodate extension and translation
in an orientation roughly parallel to the Himalayan arc. These structures are typically east-west or
ESE-WNW trending strike-slip faults and north-south or NNE-SSW striking normal faults.
Extensional deformation has been active from middle Miocene up to present day and is
commonly manifested throughout central and southern Tibet as east-west oriented strike-slip
faults and north-south oriented normal faults (Fig. 1.3; Armijo et al., 1986, 1989; Taylor et al.,
5

Figure 1.3 Structures in the Himalaya and Tibet associated with orogen-parallel extension and
north-south contraction. (a) Active structures in the Himalaya, modified after Styron et al.
(2011) and references therein. Red lines represent faults associated, at least in part, with
orogen-parallel extension. The Main Frontal Thrust (black) is associated with ongoing N-S
contraction. Numbers in grey, and italics, refer to initiation ages of structures, and sources,
respectively. (B) and (c) Simplified structural map and cross-section of the upper Karnali and
surrounding regions, constructed from SRTM DEM data sheets 53_06, 53_07, and modified
after Murphy and Copeland (2005), Robinson et al. (2006) and Yakymchuk and Godin (2012).
Structures in black and red are associated with south-directed extrusion of the GHS, and
orogen-parallel extension, respectively. Abbreviations and sources: GMH – Gurla
Mandhata-Humla fault, ITSZ – Indus-Tsangpo suture zone, KC – Kung Co rift,LK – Lopukangri rift, MCT - Main Central thrust, STDS - South Tibetan
detachment system, TG – Thakkhola graben, TY – Tangra-Yumco rift, 1 – Phillips et al. (2004), 2 – Thiede et al. (2006), 3 – Murphy et al. (2002), 4 – Murphy
and Copeland (2005), 5 – Garzione et al. (2003), 6 – Murphy et al. (2010), 7 – Kali et al. (2010), 8 – Lee et al. (2011), 9 - Williams et al. (2001), 10 – Dewane
et al. (2006).
6

2003; Taylor and Yin, 2009). These structures are currently limited to upper crustal levels (see
Fig. 2 from Styron et al., 2011 for comprehensive review).
Within the Himalaya, extensional deformation is expressed as rare strike-slip faults,
normal faults, and migmatite-cored gneiss domes, all exhibiting orogen-parallel slip directions
(Fig. 1.3; Nakata, 1989; Murphy et al., 2002, 2009; Murphy and Copeland, 2005; Thiede et al.,
2006; Jessup et al., 2008; Li and Yin, 2008, Jessup and Cottle, 2010). Gneiss domes related to
orogen-parallel extension (Fig. 1.3) are the Leo Pargil dome in the Western Himalaya (Thiede et
al., 2006), the Gurla Mandhata core complex in the central-west Himalaya (Murphy et al., 2002),
and the Ama Drime Massif in the central-east Himalaya (Jessup and Cottle, 2010 and references
therein). These domes exhume upper amphibolite to granulite facies rocks in settings that are
kinematically and contemporaneously linked to extension throughout the Himalaya and Tibetan
plateau. Note that the terms gneiss dome and core complex, used interchangeably in this study,
refer to migmatite-cored domes, and do not strictly follow the definition of Eskola (1949).
An array of mechanisms have been proposed to explain orogen-parallel deformation,
including lateral extrusion of a rigid Tibet (Tapponnier et al., 1982), oroclinal bending of the
Himalayan arc (Klootwijk et al., 1985; Ratschbacher et al., 1994; Bendick and Bilham, 2001),
gravitational instability of the Tibetan plateau (Molnar and Tapponnier, 1978; Molnar and Chen
1983; Coleman and Hodges, 1995), right-lateral oblique convergence (McCaffrey and Nabelek,
1998; Seeber and Pêcher, 1998), and radial spreading of the Tibetan plateau (England and
Houseman, 1988; Copley and McKenzie, 2007; Copley 2008; Cook and Royden, 2008). To date,
there is little consensus on which model best explains orogen-parallel extension. The kinematics
and spatial distribution of strike-slip and normal faults are commonly used to test predictions of
7

the various models. However, this approach is problematic as a variety of models make similar
predictions (Chapters 2, 3).
1.3 Transition from south-directed extrusion of the mid-crust to onset of orogenparallel
extension
The cessation of south-directed extrusion of the GHS and the onset of orogen-parallel
extension expose a fundamental middle Miocene tectonic transition in Himalayan evolution (e.g.,
Jessup and Cottle, 2010). Characterizing the kinematics, geographic distribution, and timing of
features intrinsic to this transition are key in determining the processes that govern it. Ultimately,
the determination of these processes has a direct bearing on the fashion in which the Himalaya
and other large, hot collisional orogens are perceived and understood.
Thorough constraints have been established for individual components of this transition,
including cessation of south-directed extrusion of the mid-crust and initiation of orogen-parallel
extension; however, limited studies have focused on the transition itself. Such studies require
geologic settings that expose rocks and fabrics associated with both the GHS and orogen-parallel
deformation, such as the Leo Pargil gneiss dome, the Gurla Mandhata core complex, and the Ama
Drime massif.
A compilation of data from the Ama Drime massif and Everest region reveals a cessation
of motion along the STDS between 16-12 Ma and exhumation of melt along the Ama Drime and
other detachments in a setting kinematically linked to extension between 12-8 Ma (Jessup and
Cottle, 2010). Mapping and chronology of the Gurla Mandhata core complex reveals east-west
extension along an evolving low-angle normal-fault system, juxtaposing mid-crustal rocks above
the TSS (Murphy et al., 2002). Orogen-parallel extension along this fault system may have
initiated post 17 Ma, was potentially active by 13 Ma, and was certainly active by 9 Ma. Data
8

from the Leo Pargil dome also reveal the exhumation and juxtaposition of mid-crustal rocks
against the TSS via sustained normal displacement along the west-dipping western flank of the
dome (Thiede et al., 2006). Chronological constraints suggest dome exhumation initiated in a
setting linked to orogen-parallel extension by 16 Ma (Thiede et al., 2006), and potentially post 19
Ma (Leech, 2008).
1.4 This study
The purpose of this study is to characterize the cessation of south-directed mid-crust
extrusion and the onset of orogen-parallel deformation. The area selected to perform this study is
located within the upper Karnali valley of northwestern Nepal (Figs. 1.2b, 2.2b), where a
northwest-striking, strike-slip-dominated shear zone defines the upper contact and cuts obliquely
across the GHS (Murphy and Copeland, 2005).
1.4.1 Geology of northwestern Nepal and the upper Karnali valley
The upper Karnali valley lies to the northwest of the village of Simikot, located in farnorthwestern
Nepal (Fig. 1.2b). The regional geological framework of the area was pioneered by
Fuchs (1977) and Shrestha (1987), and subsequently refined by Murphy and Copeland (2005),
Robinson et al. (2006), and Yakymchuk and Godin (2012).
A compilation of data reveal a high-strain south-dipping contact, which juxtaposes
weakly deformed and metamorphosed TSS over strongly deformed, high-metamorphic grade
GHS (Fig. 1.2b; Murphy and Copeland, 2005). This lithotectonic relationship is consistent with
the STDS, however the kinematics along the high-strain zone are not. Well-developed down-dip
mineral elongation lineations indicating a significant dip-slip component are expected along the
STDS in order to juxtapose units of contrasting metamorphic grades. Contrastingly, shallow ESE-
WNW lineations are observed along the high-strain zone, indicating a dominant strike-slip sense
9

of shear (Murphy and Copeland, 2005). Additionally, this high strain zone is not limited to the
TSS-GHS interface, but also cuts obliquely across the GHS and possibly into the Lesser
Himalayan sequence to the southwest (Fig. 1.2b; Murphy and Copeland, 2005). These
observations, in conjunction with middle Miocene chronologic constraints, prompted the
interpretation that this contact is a component of the Gurla Mandhata-Humla fault system (GMH),
a northwest-striking shear zone associated with orogen-parallel extension, postdating southdirected
extrusion of the GHS (Murphy and Copeland, 2005).
The presence of the GMH strike-slip-dominated fault system atop and along the GHS
provides an ideal geologic context in which to investigate the relationship between south-directed
extrusion of the GHS and orogen-parallel extension.
1.4.2 Purpose of this study
The purpose of this study is to characterize the transition from cessation of south-directed
extrusion of the GHS to onset of orogen-parallel extension. Results from this thesis clarify this
fundamental Himalayan tectonic transition and provide insight on large-scale processes governing
the evolution of large, hot collisional orogens. This problem is approached by addressing the
following questions:
1. What are the relationships between the structures, kinematics, temperatures, and
timing of deformation associated with south-directed extrusion of the GHS and
orogen-parallel extension
2. What do these data imply for tectonic processes governing (1) south-directed
mid-crustal flow, (2) orogen-parallel extension, and (3) orogen-wide evolution of
the Himalayan-Tibetan orogen
10

These questions are addressed through the following approaches: field mapping,
structural, microstructural, and chronological analyses.
Field mapping and sampling took place over two field seasons (Spring 2010, 2011), and
focused on detailed transects across and along high strain zones of the upper Karnali valley (a
short film of the expedition can be seen at: http://vimeo.com/47755261). Structurally-oriented
mapping provided an initial framework upon which additional data were incorporated.
Representative samples were taken throughout the field area. Most samples were oriented, and
optimized for microstructural and chronological analyses.
Microstructural analyses, including petrographic analyses, traditional and modern quartz
crystallographic preferred orientation analyses, and vorticity analysis, provide qualitative to
quantitative assessments of the temperatures and kinematics of deformation. Muscovite 40 Ar/ 39 Ar
thermochronology and monazite U-Th-Pb geochronology provide constraints on the absolute
timing of crystallization and cooling ages across and along the high-strain zone. Results,
interpretations, and consequences of this study are presented in Chapters 2, 3, 4, and Appendices
A through D.
Chapter 2 presents and synthesizes all meaningful data and is written as a stand-alone
manuscript to be submitted for publication in Geological Society of America Bulletin. Chapter 3
incorporates conclusions of this study with previously published models to generate orogen-wide
tectonic conclusions. Conclusions drawn in this chapter involve a degree of interpretation not yet
suitable for publication and thus are not fully incorporated into Chapter 2. Additionally, this
chapter provides suggestions on further work necessary to validate the proposed tectonic models.
11

Chapter 4 outlines proposed future research in the field of microstructural analyses,
including a critique of vorticity analyses and additional applications of quartz crystallographic
preferred orientation studies.
Appendix A presents a comprehensive list of geologic stations, including station
locations and elevation, and structural data. Appendix B presents all quartz crystallographic
preferred orientation data, including samples not suitable for publication. Appendix C presents
thin section maps locating monazite grains, as well as chemical maps, secondary electron and
electron backscatter images of all grains. Appendix D presents data of all muscovite 40 Ar/ 39 Ar
analyses, including additional analyses not suitable for publication.
12

Chapter 2
From cessation of south-directed mid-crust extrusion to onset of
orogen-parallel extension, NW Nepal Himalaya
2.1 Abstract
Field mapping and, structural, microstructural, and chronological analyses confirm the
existence of a segment of the Gurla-Mandhata-Humla fault, an orogen-parallel strike-slip
dominated shear zone in the upper Karnali valley of northwestern Nepal. This shear zone forms
the upper contact of, and cuts obliquely across the Greater Himalayan Sequence (GHS). Data
from this study reveal two phases of GHS deformation.
Phase 1 is characterized by U-Th-Pb monazite crystallization ages (~26–12 Ma, peak
~18–15 Ma), consistent with typical Neohimalayan metamorphic ages, and the final stages of
south-directed extrusion of the GHS.
Phase 2 is characterized by south-dipping high-strain foliations and intensely developed
ESE-WNW trending, shallowly plunging mineral elongation lineations, indicating orogen-parallel
extension. Thermochronology of muscovite defining these fabrics implies that the area was
cooling and experiencing orogen-parallel extension by ~15–9 Ma. Mineral deformation
mechanisms and quartz c-axis patterns of these fabrics record a rapid increase in temperature
from ~350°C along the shear zone, to ~650°C at ~2.5 structural km below the shear zone. Such
temperature gradients may be remnants of telescoped and/or flattened isotherms generated during
south-directed extrusion of the GHS.
Overprinting ESE-WNW fabrics record progressive deformation of the GHS at lower
temperatures. Progressive deformation included a significant component of pure shear, as
13

indicated by symmetric high-temperature quartz c-axis fabrics and a lower-temperature vorticity
estimate (~59% pure shear). A transition in c-axis fabrics from type I to type II cross-girdles at ~
1.2 km below the fault could indicate a transition from plane strain towards constriction.
Together, these data suggest orogen-parallel extension was occurring as a result of transtension.
These data reveal a transition within the upper Karnali valley from south-directed
extrusion of the GHS to onset of orogen-parallel extension between ~15 -13 Ma. Comparing
these data with similar tectonic environments across the Himalaya reveal an orogen-wide middle
Miocene transition. The timing of this transition is consistent with interpretations invoking
models of radial spreading of Tibet and east-directed lower crustal flow to explain orogen-parallel
extension.
2.2 Introduction
Middle Miocene evolution of the Himalaya and Tibetan plateau is characterized by the
progression from south-directed extrusion of the Greater Himalayan Sequence (GHS) to orogenparallel
extension (Jessup and Cottle, 2010). This transition period attests to a fundamental
evolution of tectonic processes governing the architecture of the Himalayan-Tibetan orogen.
The first phase of middle Miocene evolution is characterized by the cessation of southdirected
extrusion of the GHS. The GHS is the high-metamorphic grade and anatectic core of the
Himalaya, typically cropping out at the crest of the Himalaya, along the length of the orogen (Fig.
2.1). The GHS is commonly interpreted to represent a sequence of rocks extruded southward and
normal to the orogen, from mid-crustal depths (Hodges, 2000; Yin and Harrison, 2000; Yin,
2006). South-directed extrusion of the GHS was active up to ~ 16 Ma in a setting linked to northsouth
contraction and shortening (see review of ages in Godin et al., 2006b; Cottle et al., 2009b).
14

Figure 2.1 Simplified geologic and structural map, and cross-section of (a) the
Himalaya, modified after Yin (2006) and Hintersberger et al. (2010), and (b) and
(c), the upper Karnali valley and surrounding regions, modified after Murphy and
Copeland (2005), Robinson et al. (2006) and Yakymchuk and Godin (2012).
Abbreviations: AD - Ama Drime Massif, CG – Chuwa granite, GM - Gurla
Mandhata gneiss dome, GMH – Gurla Mandhata-Humla fault, IYSZ - Indus-Yalu
suture zone, KF - Karakorum fault, KJO - Kiogar-Jungbwa ophiolitic and mantletype
rocks, LP - Leo Pargil gneiss dome, MBT - Main Boundary thrust, MCT -
Main Central thrust, MFT - Main Frontal thrust, STDS - South Tibetan
detachment system, TG - Thakkhola graben.
15

The second phase of middle Miocene evolution is characterized by the onset of orogenparallel
extension throughout the Tibetan plateau and Himalayan arc (Allègre et al., 1984;
Mercier et al., 1987; Styron et al., 2011). With the exception of the enigmatic Karakoram fault
(Zhang et al., 2010), structures related to orogen-parallel extension initiated between ~ 16 -11 Ma
(Fig. 2.2; see Fig. 2 from Styron et al., 2011 for a comprehensive age review). To date, there is
little consensus on the mechanisms that initiated and continue to drive this extension.
In this study we investigate the transition between the cessation of south-directed
extrusion of the GHS and onset of orogen-parallel extension. This is accomplished by integrating
geological, structural, microstructural, and chronological data from the upper Karnali valley of
northwest Nepal (Figs. 2.1, 2.2). Within this valley, a northwest-striking, strike-slip dominated
shear zone, the Gurla Mandhata-Humla fault system (GMH), crops out along a segment of GHS-
TSS interface, and cuts obliquely across the GHS (Figs. 2.1, 2.2; Murphy and Copeland, 2005).
This provides an opportune geologic context to investigate the relationship between southdirected
extrusion of the GHS and ESE-WNW deformation associated with orogen-parallel
extension. To date, this transition has only been constrained in the Everest region of the Himalaya
(Jessup and Cottle, 2010).
Based on our results from the upper Karnali valley, and the integration of our results with
other similar transitions across the Himalaya, we attempt to reconcile the following questions:
(1) What are the relationships between the structures, kinematics, temperatures, and
timing of deformation associated with south-directed extrusion of the GHS and orogenparallel
extension
(2) What do these data imply for the tectonic processes governing the onset of orogenparallel
extension
16

Figure 2.2 Structures in the Himalaya and Tibet associated with orogen-parallel extension
and north-south contraction. (a) Active structures in the Himalaya, modified after Styron et
al. (2011) and references therein. Red lines represent faults associated, at least in part, with
orogen-parallel extension. The Main Frontal Thrust (black) is associated with ongoing N-S
contraction. Numbers in grey, and italics, refer to initiation ages of structures, and sources,
respectively. (B) and (c) Simplified structural map and cross-section of the upper Karnali
and surrounding regions, constructed from SRTM DEM data sheets 53_06, 53_07, and
modified after Murphy and Copeland (2005), Robinson et al. (2006) and Yakymchuk and
Godin (2012). Structures in black and red are associated with south-directed extrusion of the
GHS, and orogen-parallel extension, respectively. Abbreviations and sources: GMH – Gurla
Mandhata-Humla fault, ITSZ – Indus-Tsangpo suture zone, KC – Kung Co rift,LK – Lopukangri rift, MCT - Main Central thrust, STDS - South Tibetan
detachment system, TG – Thakkhola graben, TY – Tangra-Yumco rift, 1 – Phillips et al. (2004), 2 – Thiede et al. (2006), 3 – Murphy et al. (2002), 4 – Murphy
and Copeland (2005), 5 – Garzione et al. (2003), 6 – Murphy et al. (2010), 7 – Kali et al. (2010), 8 – Lee et al. (2011), 9 - Williams et al. (2001), 10 – Dewane et
al. (2006).
17

2.3 Geologic Setting
2.3.1 Himalayan Overview
The geology of the Himalaya consists of four distinct lithotectonic domains bounded by
crustal-scale fault systems, all of which are laterally continuous across the length of the orogen
(Fig. 2.1a). From north to south, these domains are: the Tethyan sedimentary, Greater Himalayan,
Lesser Himalayan, and Sub-Himalayan sequences. Their bounding fault systems include the
South Tibetan detachment system, the Main Central thrust, the Main Boundary thrust, and the
Main Frontal thrust (Fig.2.1a). This study focuses on an area of northwestern Nepal within the
two northernmost lithotectonic domains, the Tethyan sedimentary sequence (TSS) and Greater
Himalayan sequence (GHS), and their bounding faults, the South Tibetan Detachment system
(STDS) and the Gurla Mandhata-Humla (GMH) fault system (Figs.2.1b, 2.2b).
2.3.1.1 South-directed extrusion of the GHS
Laterally throughout the orogen, the GHS typically crops out south of the TSS and north
of the Lesser Himalayan sequence. The GHS is the high-metamorphic grade and anatectic core of
the Himalaya. It is bounded at its base by the Main Central thrust (MCT), a south-verging thrust
fault, and at its upper surface by the STDS, a low-angle, top-to-the-north normal fault (Fig 2.1;
Burchfiel and Royden, 1985; Burchiel et al., 1992; Hodges, 2000). Neohimalayan metamorphism
is recorded throughout the GHS from early to middle Miocene (Vannay and Hodges, 1996; Godin
et al., 1999; 2001; Searle and Szulc, 2005; Cottle et al., 2009a,b), and is characterized by
extensive anatexis and synchronous motion along the STDS and MCT (see Godin et al., 2006b
for review of ages). This metamorphism is contemporaneous with isothermal decompression of
the GHS and south-directed extrusion during Early to middle Miocene (e.g. Vannay and Hodges,
18

1996; Godin et al., 2001; Harris et al., 2004). Motion along the STDS terminated by ~16 Ma in
central and eastern Nepal (Godin et al., 2006b; Cottle et al., 2009a,b), implying that extrusion of
the GHS had also ceased.
2.3.1.2 Orogen-parallel extension
Orogen-parallel extension has been active from middle Miocene up to present day
throughout the Himalaya and Tibetan plateau (see Fig.2.2 from Styron et al., 2011 for
comprehensive review). Extensional deformation is commonly manifested throughout Tibet as
east-west to ESE-WNW oriented strike-slip faults and north-striking rifts (Fig. 2.2a; Armijo et al.,
1986, 1989; Taylor et al., 2003; Taylor and Yin, 2009). The majority of these structures are
located within central to southern Tibet and restricted to the upper crust (Armijo et al., 1986,
1989; Taylor et al., 2003; Taylor and Yin, 2009). Within the Himalaya, extensional deformation
is expressed as orogen-perpendicular normal faults, orogen-parallel strike-slip faults, and
migmatite-cored gneiss domes (Fig 2.2a; Nakata, 1989; Murphy et al., 2002; 2009; Murphy and
Copeland, 2005; Thiede et al., 2006; Jessup et al., 2008; Li and Yin, 2008; Jessup and Cottle,
2010). Three Himalayan gneiss domes (Leo Pargil, Gurla Mandhata, and Ama Drime – Fig.
2.2a,b) exhume rocks of upper amphibolite to granulite metamorphic facies in settings
kinematically linked to orogen-parallel extension (Murphy et al, 2002, 2009; Thiede et al., 2006;
Jessup and Cottle, 2010). Structures related to orogen-parallel extension initiated between ~ 16 -
11 Ma (Fig 2.2; see Fig. 2 from Styron et al., 2011 for a comprehensive age review).
19

2.4 Geology of the upper Karnali Valley
2.4.1 Background
The upper Karnali valley lies to the northwest of the village of Simikot, located in farnorthwestern
Nepal (Figs. 2.1, 2.2). The regional geological framework of the area was pioneered
by Fuchs (1977) and Shrestha (1987), and subsequently refined by Murphy and Copeland (2005),
Robinson et al. (2006), and Yakymchuk and Godin (2012).
First-order mapping reveals low-metamorphic-grade TSS juxtaposed along a southdipping
contact over strongly deformed and metamorphosed rocks of the GHS (Figs. 2.1b, 2.2b;
Murphy and Copeland, 2005). The TSS of the upper Karnali valley is a sequence of lowmetamorphic-grade
rocks interpreted to be Cambrian through Mesozoic based on correlative units
to the northwest (Cheng and Xu, 1987; Murphy and Yin, 2003; Murphy and Copeland, 2005).
The GHS of the upper Karnali valley consists of amphibolite metamorphic facies schist
and gneiss (Murphy and Copeland, 2005; Yakymchuk and Godin, 2012), consistent with high
metamorphic grade and anatectic assemblages observed across the entirety of the GHS (Hodges,
2000; Yin and Harrison, 2000; Yin, 2006). These assemblages imply that the GHS was exhumed
from mid-crustal depths (Yakymchuk and Godin, 2012). However, the processes controlling this
extrusion within northwestern Nepal remain contentious (Robinson et al., 2006; Yakymchuk and
Godin, 2012).
Along the length of the orogen, the STDS is a north-dipping structure, juxtaposing TSS to
the north and GHS to the south (Fig. 2.1a). Within the upper Karnali valley, the same
juxtaposition of units is observed; however, the contact is dips southward, with the TSS to the
south and the GHS to the north (Figs. 2.1b, 2.2b; Murphy and Copeland, 2005). Typical northdipping
STDS structures and relationships are observed south of the upper Karnali valley, along
20

the Seti River (Fig. 2.1b, 2.2b; Robinson et al., 2006). This geometry suggests that the TSS south
of the upper Karnali valley and north of the Seti River forms an approximately 35 km wide
synclinorium plunging to the northwest, and closing to the southeast (Fig. 2.1b). This may be due
to crustal-scale buckling of the GHS following south-directed extrusion, as documented in central
Nepal (Godin et al., 2006b) and northwest Bhutan (Antolín et al., 2012; Kellett and Grujic, 2012).
Alternatively, the current geometry may be associated in part with the formation of the Gurla
Mandhata core complex to the northwest (Fig. 2.1b, 2.2b; Murphy and Copeland, 2005).
Mineral elongation lineations and asymmetric fabric elements of the upper Karnali valley
reveal predominant ESE-WNW strike-slip dominated deformation (Murphy and Copeland, 2005).
This contrasts with the typical top-to-the-north STDS structural fabrics, and thus prompted the
interpretation of this shear zone as a component of the normal, right-slip, WNW-striking Gurla
Mandhata-Humla (GMH) fault system (Murphy and Copeland, 2005). This fault system trends
roughly parallel to the strike of the orogen, from the Gurla Mandhata core complex southeast to
the upper Karnali valley, where it forms the contact between the TSS and the GHS (Figs. 2.1,
2.2). Further eastward, the GMH branches south and southeast, along the GHS-TSS contact and
obliquely across the GHS, respectively (Figs. 2.1, 2.2). The GMH is interpreted to accommodate
arc-parallel extension and translation between 15 and 7 Ma (Murphy and Copeland, 2005).
2.4.2 This study
Detailed mapping of transects across and along the upper Karnali valley reveal six
distinct lithotectonic domains. They are, from south to north, (I) folded TSS, (II) transposed TSS,
(III) ultra-high-strain zone, (IV) high-strain quartzite and pelitic GHS, (V) high-strain gneissic
GHS, and (VI) migmatitic GHS (Fig. 2.3). Deformation associated with the GMH is moderate to
21

Figure 2.3 Lithotectonic map and simplified cross-section of the upper Karnali valley, northwestern
Nepal. D.I, II, III, IV, V and VI refer to lithotectonic domains, see text for descriptions. Inset is identical
scale to main map and shows simplified geology and topography, constructed from SRTM DEM data
sheets 53_06, 53_07. Stereonets were constructed using OSX Stereonet v. 1.3 developed by N. Cardozo
and R. Allmendinger. Abbreviations: GHS – Greater Himalayan sequence, GMH – Gurla Mandhata-
Humla fault system, TSS – Tethyan sedimentary sequence.
_____________________________________________________________________________
pervasive throughout domains II, III, IV, and V. Macro- and micro-scale observations from all
domains are described below (see Appendix A for station locations and additional field data).
2.4.2.1 Domain I – Folded TSS
The folded TSS is the southernmost mapped domain, outcropping between ~2.7 to 1 km
structurally above the GHS-TSS interface (Figs. 2.3, 2.4a,b,c). This domain comprises
interlayered tan to grey marbles and calc-silicates (Cal ± Qtz ± Fsp ± phyllosilicates ± Py). Along
the southern edge of the mapped area, domain I exposes a layer of unknown thickness of dull
green to purple calcareous phyllite (Cal ± Qtz ± phyllosilicates ± Fsp). Calcite is very fine
grained (~30 µm) and no twins are discernable. This may suggest a complete lack of microscale
deformation (Fig. 2.4c; Burkhard, 1993). Both rock types are interpreted to represent the Cambro-
Ordovician stratigraphic level mapped by Murphy and Copeland (2005).
Original bedding is crudely preserved and defines macroscale open to tight folds with
axial planes moderately inclined to the south. Folds verge towards the northeast and fold hinges
plunge shallowly (~0° to 25°) to the ESE and WNW (Figs. 2.3, 2.4a). Rare C/S/C’ fabrics
indicating dextral shear are observed within the purple calcareous phyllite (Fig. 2.4b).
2.4.2.2 Domain II – Transposed TSS
The transposed TSS is located within ~1 km structurally above the GHS-TSS interface
(Figs. 2.3, 2.4d,e,f,g). Throughout this domain, bedding of the folded TSS is transposed planar to
23

Figure 2.4 Representative structures, rock types and deformation mechanisms of domain I and II; the
folded, and transposed Tethyan Sedimentary sequence, respectively. They are from highest to lowest
structural levels: (a) Northeast-verging moderately overturned folds of marble and calc-silicate of domain
I, south of the village of Tumkot. (b) C/S/C’ fabrics indicating dextral shear within a purple calcareous
phyllite of domain I. (c) Fine-grained calcite with no discernable twins within a calc-silicate of domain I.
(d) Recumbant and isoclinal folds of domain II. (e) Calc-silicate of domain II exhibiting thin type I calcite
twins (red arrows). (f) High-strain foliation (S 1 ) within calc-silicate of domain II. (g) Calc-silicate of
domain II exhibiting tabular type II calcite twins (thin red arrows) and curved and tapered type III calcite
twins (thick red arrows). Inset is identical scale from elsewhere in the thin section. Abbreviations: cal -
calcite, qtz - quartz.
24

the main shear zone fabric. The lithology of this domain is identical to the overlying folded TSS,
comprising interlayered tan to grey marbles and calc-silicates (Cal ± Qtz ± Fsp ± phyllosilicates ±
Py).
At upper structural levels within this domain, broad open folds of domain I transition to
recumbent and isoclinal (Fig. 2.4d). Fold hinges plunge shallowly to moderately and trend ESE-
WNW (Fig. 2.3). Axial planes of these folds are roughly parallel to the main shear zone foliation.
Calcite is deformed and shows thin type I twins, and rare type II twins (Fig. 2.4e). This implies
deformation temperatures of less than ~200°C (Burkhard 1993; Ferrill et al., 2004). Quartz grains
within calc-silicate layers are both undeformed and show patchy undulose extinction, suggesting
temperatures of deformation below which quartz dynamically recrystallizes (

Figure 2.5 Representative structures, rock types and deformation mechanisms of domain III, the ultrahigh-strain
zone. They are from highest to lowest structural levels: (a) Angular fragments of marble and
calcite (outlined in white) within a chaotic carbonate matrix. (b) Well-rounded clasts of quartz, feldspar
and phyllosilicate within a matrix of fine-grained calcite. (c) High-strain south-dipping tectonic foliation
(S 1 ) within a pelitic schist. (d) High-strain schist composed of feldspar porphyroclasts exhibiting bent
twins, patchy undulose extinction, and abundant grain-scale fractures, including antithetic micro-faulting
(red arrows), within a matrix dominated by fine-grained dynamically recrystallized quartz. (e) Dextrally
sheared mica fish within a matrix of high-strain dynamically recrystallized quartz (viewed to the south). (f)
Intensely attenuated and dynamically recrystallized quartz grains exhibiting subgrain extinction and
bulging (red arrows). Dotted red line shows length of a single elongate quartz grain. Abbreviations: cal -
calcite, fld - feldspar, ms – muscovite, qtz – quartz.
26

South of the interface, the TSS is composed of mm- to cm-thick, interlayered dark grey,
green, brown, and tan marbles and calc-slicates (Cal ± Qtz ± Fsp ± phyllosilicates). Calcite is
very fine grained and deformation and recrystallization textures are not discernable. Flattened and
elongate layers of carbonate minerals define the pervasive south-dipping foliation. Hinge lines of
microfolds along foliation surfaces define a shallowly plunging ESE-WNW trending crenulation
lineation. South of the village of Puiya (Fig. 2.3), this crenulation lineation overprints an older,
moderately plunging, SE-NW trending mineral lineation defined by aligned mafic minerals.
Within 10 m superjacent to the interface, angular fragments of marble and calcite are
observed within a chaotic carbonate matrix on a centimeter scale (Fig. 2.5a), interpreted to be
related to low temperature cataclastic deformation.
Within 1 m of the interface, microscale observations of high-strain marble show wellrounded
clasts of quartz, feldspar and phyllosilicate within a matrix of fine-grained calcite (Fig.
2.5b). These clasts are internally deformed, showing undulatory extinction and internal brittle
fracturing. Based on these attributes, we interpret the clasts to have been incorporated into the
marble from the underlying GHS and rotated extensively during subsequent deformation.
North of the GHS-TSS interface, the GHS consists of quartzite and pelitic schist (Qtz ±
Fsp ± Ms ± Bt ± Grt). Mylonitic to ultramylonitic fabrics define the pervasive south-dipping
tectonic foliation (Fig. 2.5c,d,e). Pervasive mineral lineations defined by elongation of quartz,
feldspar, and mica plunge shallowly to moderately towards the ESE-WNW. A single leucogranite
dyke (Fsp + Qtz + Ms + Bt ± Tur) crosscuts the ultra-high-strain zone north of the village of
Chala. The dyke is semi-concordant to the main foliation and exhibits a protomylonitic texture,
suggestive of syndeformational intrusion.
27

Quartz grains are dynamically recrystallized and intensely attenuated into elongate
ribbons (Fig. 2.5e,f). Relict grains show subgrain extinction, bulges, and recrystallized subgrains
and bulges (Fig. 2.5f). These textures suggest deformation temperatures approximating the
transition from bulging to subgrain rotation recrystallization at ~400°C (Stipp et al., 2002).
Feldspar grains exhibit bent twins, patchy undulose extinction, and abundant grain-scale
fractures, including antithetic micro-faulting (Fig. 2.5d). Brittle deformation within the feldspar
suggests a temperature of deformation below ~400°C (Tullis and Yund, 1991). Coeval dynamic
recrystallization of quartz and brittle deformation of feldspar constrain deformation temperatures
to 280 -400°C within the ultra-high-strain zone (Stipp et al., 2002; Tullis and Yund, 1991).
Asymmetric fabric elements observed within the ultra-high-strain zone include delta-type
rotated feldspar porphyroclasts and mica fish (Fig. 2.5d,e). Shear sense indicators show a near
even distribution of dextral and sinistral senses of motion. Within the leucogranite dyke,
numerous conjugate shear bands are observed. Additional microscale observations include garnet
and tourmaline grains that are fractured, boudinaged, and pulled apart parallel to the dominant
mineral elongation lineation.
2.4.2.4 Domain IV – High-strain quartzite and pelitic GHS
Highly strained quartzite and pelitic schist of the GHS are found ~1 km structurally
underlying the GHS-TSS interface (Figs. 2.3, 2.6). The mineralogy of the quartzite and pelitic
schist (Qtz ± Fsp ± Ms ± Bt ± Grt) is similar to the overlying ultra-high-strain GHS rocks;
however, this domain has been subjected to lower deformation at higher temperatures. Mylonitic
fabrics and mineral lineations defined by alignment and elongation of quartz, feldspar, and mica
define a pervasive south-dipping foliation and shallowly to moderately plunging ESE-WNW
28

WNW
Figure 2.6 Representative structures, rock types and deformation mechanisms of domain IV, the
high-strain quartzite and pelitic Greater Himalayan sequence. They are from highest to lowest
structural levels: (a) C/S/C’ fabrics and a sigma-type feldspar porphyroclast indicating dextral
shear within high-strain pelitic schist. Note, late stage warping of the outcrop reoriented sample
N-S. Orientation at which fabrics were formed is interpreted to be ESE-WNW. (b) C/S/C’ fabrics
indicating sinistral shear within high-strain pelitic schist (red arrows indicate C’ planes). (c)
Intensely fractured and pulled apart garnets parallel to an east-trending mineral elongation
lineation, and C/S/C’ fabrics indicating sinistral shear within high-strain quartzite (red arrows
indicate C’ planes). (d) Flattened and dynamically recrystallized quartz grains exhibiting
extensive subgrain boundaries (red arrows). Abbreviations: fld - feldspar, grt – garnet, ms –
muscovite, qtz – quartz.
29

trending lineation (Fig. 2.3). Discrete pods and rare crosscutting to semi-concordant leucogranite
dykes constitute < 10 vol% of the domain.
Strongly flattened quartz grains exhibit core and mantle structures, recrystallized
subgrains, and subgrain extinction (Fig. 2.6d), indicating deformation temperatures of ~400-
500°C (Stipp et al., 2002). Garnets are inclusion-rich, intensely fractured and pulled apart parallel
to the mineral elongation lineation (Fig. 2.6c).
Asymmetric fabric elements, including C/S/C’ fabrics and sigma-type feldspar
porphyroclasts, are abundant and well developed within high-strain quartzite and pelitic schist
(Fig. 2.6a,b). Shear sense indicators show a near even distribution of dextral and sinistral senses
of motion.
2.4.2.5 Domain V – High-strain gneissic GHS
High-strain gneiss of the GHS is exposed approximately 1 to 6 km structurally below the
GHS-TSS interface, and spans ESE-WNW across the extent of the field area (Figs. 2.3, 2.7).
Rocks within the structurally uppermost section of this domain are highly strained, and
progressively decrease in strain and increase in volume percent of melt with increasing structural
depth. The high-strain gneiss (Fsp + Qtz ± Bt ± Ms ± Grt ± Tur ± Sil) contains up to 30 vol%
leucosome and is fairly homogeneous, with the exception of rare meter sized quartz/quartzite
layers and a ~100 m thick layer of pelitic schist (Qtz + Bt + Ms + Sil + Grt).
Structurally above this pelitic schist, to east of the village of Yangar, a mesoscale low
strain zone exposes augen gneiss of a similar mineralogy to the dominant high-strain gneiss. This
may suggest that the augen gneiss is the protolith to the high-strain gneissic GHS.
Garnets are inclusion-rich and intensely fractured but not consistently stretched parallel
to the mineral elongation lineation. Sillimanite in the form of fibrolite is abundant within the
30

Figure 2.7 Representative structures, rock types and deformation mechanisms of domain V and VI; the
high-strain gneissic, and migmatitic Greater Himalayan sequence, respectively. They are from highest to
lowest structural levels: (a) Delta-type rotated leucogranite pod indicating dextral shear within domain V.
(b) Feldspar (fld) porphyroclasts bordered by micas and dynamically recrystallized quartz, in a core-andmantle
sigma-type structure indicating dextral shear, within high-strain gneiss of domain V. (c) Sinistral
shear band (red arrows) faulting and drawing into plane sillimanite (sil) and biotite (bt), within pelitic
schist of domain V. (d) Quartz (qtz) grains showing interlobate to amoeboid quartz grain boundaries and
subgrain extinction (red arrows), within gneissic GHS of domain V. (e) Chessboard-style extinction within
quartz of domain VI. (f) Equigranular quartz and feldspar grains of domain VI, exhibiting uniform
extinction, and interlobate grain boundaries.
31

pelitic layer and rare throughout the gneiss. Within the pelitic layer, abundant shear bands both
fault and draw into plane sillimanite and biotite (Fig. 2.7c).
At upper structural levels within the domain, muscovite is common and quartz grains are
mildly elongate and inequigranular. Along strike, feldspar porphyroclasts are bordered by
recrystallized quartz and feldspar in core-and-mantle type structures (Fig. 2.7b), corresponding to
deformation temperatures of ~400-500°C (Tullis and Yund, 1991).
At progressively lower structural levels within the domain, muscovite decreases in
abundance and quartz grain boundaries transition from interlobate to amoeboid, occasionally
exhibiting zones of subgrain extinction (Fig. 2.7d). These textures represent recrystallization
dominantly through grain boundary migration, with a considerable component of recrystallization
by subgrain rotation. This may correspond to the transition between these two recrystallization
mechanisms at ~500°C (Stipp et al., 2002).
These trends are consistent with the weakening of high-strain and asymmetric fabric
elements with increasing structural depth throughout the entirety of the domain. Despite
decreased intensity, tectonic foliations and mineral elongation lineations have a relatively
consistent geometry (Fig. 2.3). Elongate quartz, feldspar, and phyllosilicates define a pervasive
south-dipping foliation and a shallow to moderately plunging ESE-WNW trending lineation (Fig.
2.3). Minor, broad, upright, open folds with fold axes roughly parallel to the mineral elongation
lineation gently warp the domain. Centimeter- to meter-scale isoclinal folds with identical fold
axes are also locally observed throughout the high-strain gneiss.
Asymmetric fabric elements, including C/S/C’ fabrics, sigma-type feldspar
porphyroclasts and delta-type rotated leucogranite pods are best developed within the upper
structural section of the domain (Fig. 2.7a,b,c). Shear sense indicators show a near even
32

distribution of dextral and sinistral senses of motion. Rare sets of conjugate extensional shear
bands are also observed throughout the domain.
Additionally throughout this domain, brittle structures and lower-temperature solid-state
deformation overprint the ductilly deformed high-strain gneiss. Brittle structures include normal
faulting and cataclastic deformation of ductilly deformed gneiss and leucogranite dykes and
injection of pseudotachylite across weakly cataclastic foliation (Fig. 2.8a,b,c). Solid-state
deformation includes the development of centimeter-wide mylonitic to ultramylonitic layers that
cut obliquely across high-strain gneiss (Fig. 2.8d).
2.4.2.6 Domain VI – Migmatitic GHS
The migmatitic GHS is the northernmost and structurally lowest domain mapped (Figs.
2.3, 2.7e,f). The domain consists of migmatitic quartzofeldspathic gneiss (Fsp + Qtz + Bt + Ms ±
Grt ± Tur ± Sil) with up to 70 vol% leucosome. The mineralogy of the migmatitic GHS is similar
to the overlying high-strain gneissic GHS; however, the modal abundance of muscovite and
leucosome, and intensity of strain are markedly different.
Sillimanite, in the form of fibrolite, is rare and anastomoses around clusters of quartz and
feldspar. Garnets are inclusion-rich and intensely fractured, but not in a systematic orientation.
Rare muscovite grains amidst dynamically recrystallized quartz and feldspar show irregular grain
boundaries, suggestive of non-equilibrium conditions. Quartz and feldspar grains are equigranular
and show interlobate grain boundaries (Fig. 2.7e). This indicates recrystallization via grain
boundary migration, which is associated with temperatures of deformation between ~500-700°C
(Stipp et al., 2002). Additionally, quartz displays chessboard extinction (Fig. 2.7f), thought to
represent a combination of slip along the basal and prism lattice planes (Blumenfeld et
33

Figure 2.8 Representative brittle structures and lower temperature deformation within domain V.
(a) Normal faulting of high-strain gneiss and leucogranite. (b) Pseudotachylite (outlined in white)
crosscutting cataclasite. (c) Semi-ductile normal faulting of leucogranite dyke. (d) Ultramylonitic
to mylonitic layers cutting obliquely across high-strain gneiss.
34

al., 1986; Mainprice et al., 1986). This texture corresponds to deformation temperatures in excess
of ~630°C (Stipp et al., 2002).
Along the deepest structural level observed, ~10 km below the GHS-TSS interface, initial
stages of diatexis are seen, in which biotite- and feldspar-rich rafts and leucosome form intermerging
schlieren and nebulitic structures.
Throughout the migmatitic GHS, macroscale anastomosing leucosome and melanosome
define a weakly developed south-dipping foliation. At these structural levels, no mineral
lineations are observed.
2.5 Microstructural analyses
2.5.1 Crystallographic preferred orientation of quartz
Quartz crystallographic preferred orientations (CPOs) are the result of deformation
temperature, strain rate, non-coaxiality of flow, and distortional strain geometry (Sullivan and
Beane, 2010 and references therein). Consequently, the analysis of quartz CPOs has become a
powerful tool in characterizing high-strain zones and exhumed metamorphic terranes, such as the
GHS and its bounding faults (Bouchez & Pêcher, 1976; Brunel and Kienast, 1986; Grujic et al.,
1996; Grasemann et al., 1999; Bhattacharya and Weber, 2004; Law et al., 2004; Larson and
Godin, 2009; Larson et al., 2010a; Yakymchuk and Godin, 2012).
Historically, quartz CPO analysis has been focused on c-axis fabrics due to analytical
limitations. However, development of the Electron Back Scatter Diffraction (EBSD) technique
has provided an alternative methodology whereby complete crystallographic orientations of all
lattice planes and axes can be determined (Prior et al., 1999). This allows for evaluation of both
the quartz c- and a- axes. The analysis of a-axis fabrics provides an independent assessment of
plane strain, flattening, and constriction (Passchier and Trouw, 2005). Furthermore, Sullivan and
35

Beane (2010) demonstrated that for deformation at high temperatures with non-coaxial flow, a-
axis fabrics provide a more accurate representation of strain geometry. We therefore follow the
recommendation of the aforementioned study and record c- and a- axes for all samples.
Additionally, c-axis fabrics of four samples were measured via a universal stage mounted on an
optical microscope for comparative purposes with EBSD-derived data.
2.5.1.1 Methodology
Complete crystallographic orientation data of quartz for all samples were obtained using
the EBSD method. Data were collected through an HKL EBSD detector installed on a JEOL
JSM636OLV Scanning Electron Microscope (SEM) at Colgate University, located in Hamilton,
New York, USA. Oxford Instruments HKL Channel 5.0 software was used for acquisition and
processing of data. The following SEM conditions were used: high-vacuum mode, accelerating
voltage of 20kV and a working distance of 12-20mm (optimal at 15-20mm). Samples were
initially polished to a standard probe polish then subsequently polished on a vibrating table for 4-
6 hours using a slightly basic solution to ensure both a mechanical and chemical final polish.
Following this process, samples were mounted uncoated within the SEM.
EBSD analyses were collected using automated mapping where individual point data for
quartz were measured at established intervals (step sizes) and automatically indexed to determine
crystallographic orientations. Step sizes were set at increments smaller than the smallest grain
size in a given sample to ensure sampling of all grains. Following data acquisition, wild data
spikes were extrapolated and clusters of proximal point data with crystallographic misorientations
less than 10° were grouped and filled in using a nearest-neighbor algorithm (Fig. 2.9). This
eliminated any erratic or misindexed crystallographic orientations. Following this,
crystallographic misorientations greater than 10° were defined as grain boundaries; clusters of
36

Figure 2.9 Summary of CPO post-acquisition data processing. (a) SEM images from localities at which
data was collected. (b) Overlay of software-generated colors on SEM images indicating the identification
of a quartz CPO for individual pixels. A lack of color indicates the software did not identify a CPO
associable with quartz. (c) Rasterized data image where colors represent c-axis orientations; similar colors
represent similar orientation. (d) Rasterized data image after extrapolation of wild data spikes and
application of nearest neighbor algorithm. Using this modified data set, a new data set was created where
each grain (as seen by sets of pixels of like colors) represents an individual point.
37

similar crystallographic orientations were classified as individual grains; and a new data set was
created where each grain was represented as a single point (Fig. 2.9). This eliminated any bias
created by oversampling large grains and allowed for direct comparison with data derived from
the universal-stage (U-Stage) method. A summary of this post-acquisition data processing is
shown in Figure 2.9.
Quartz c-axis orientations were also measured for four samples via an optical microscope
mounted with a universal stage. A single c-axis measurement was taken for each individual grain
and attention was paid to not measure any grain more than once.
2.5.1.2 Descriptions of quartz CPO patterns
Quartz CPO patterns follow the definitions of Lister (1977), in which the plane normal to
the lineation (Y-Z plane) is defined as the symmetry plane. Fabrics that do not mirror themselves
across this plane are described as asymmetric. A summary of the orientation of principal strain
axes, active slip systems, fabric geometries in different strain fields, and evolution of CPO
patterns during non-coaxial strain at various temperatures is portrayed in Figure 2.10.
Photomicrographs of samples analyzed for quartz CPO patterns are seen in Figure 2.11. Quartz
CPO c- and a- axes are presented in Figure 2.12 as lower-hemisphere equal-area projections
oriented normal to the foliation and parallel to the lineation (see Appendix B for additional CPO
data).
Analyzed CPO fabrics were sampled from three domains: the ultra-high-strain zone
(domain III), the high-strain quartzite and pelitic GHS (domain IV), and the high-strain gneissic
GHS (domain V). CPO patterns from within each zone are described below.
38

a
c
a
c
Figure 2.10 Summary of quartz CPO fabrics showing (a) the orientation of principle strain axes,
active slip systems, and fabric geometries in different strain fields, and (b) the evolution of a
CPO fabric with increasing temperature (T.) during non-coaxial deformation (assuming plane
strain). Note that the letters ‘c’ and ‘a’ refer to the quartz crystallographic c- and a-axes. Adapted
and modified after Schmid and Casey (1986); Passchier and Trouw (2005); Toy et al. (2008);
Sullivan and Beane (2010).
Figure 2.11 (Next page) Photomicrographs of samples analyzed for quartz CPOs. From
structurally highest to lowest, they are: Samples (a) HK124B2 and (b) HK125, from millimeter
thick bands of extremely fine-grained and ribbon quartz within domain III. Samples (c)
HK131A, (d) HK139D, (e) HK123, and (f) HK140, from high-strain quartzite and
quartzofeldspathic schist of domain IV. Samples (g) HK145 and (h) HK105A, from quartzite and
quartzofeldspathic schist of domain V. See text for description of quartz textures.
39

Figure 2.12 Quartz CPO c- and a- axes. Columns from left to right are: sample number and distance from
GHS-TSS interface measured perpendicular to the average foliation; EBDS-derived c-axes (),
including shear sense when applicable, and lineation data; EBSD-derived a-axes (), including multiples
of uniform density (mud) for EBSD-derived contours; U-Stage-derived c-axes () and number of
uniform contours. For each analysis, point and contoured data is shown; EBSD data were contoured using
Channel 5 software, and U-stage data were contoured using OSX Stereonet v.1.3 developed by N. Cardozo
and R. Allmendinger. Number of grains measured corresponds to n. All stereonets are lower-hemisphere
equal-area projections oriented normal to the foliation and parallel to the lineation. Note that certain
EBSD-derived CPO fabrics are not centered on the stereonet due to limitations of the EBSD software in
rotating data of poorly-oriented samples.
______________________________________________________________________________
Domain III, ultra-high-strain quartz CPO patterns
Ultra-high-strain quartz CPOs, samples HK124B2 and HK125, were measured from
millimeter-thick bands of extremely fine-grained and ribbon quartz (~100 µm wide) exhibiting
subgrain rotation and bulging recrystallization (Fig. 2.11a,b). Separating these quartz bands are
layers of attenuated micas showing undulose extinction, fine-grained quartz, and brittly deformed
feldspar (Figs. 2.5e,f, 2.11a,b).
CPO patterns analyzed within this domain (Fig. 2.12) are characterized by type I singlegirdle
c-axis fabrics with dominant contributions of rhomb and prism slip and a lesser
component of basal slip. The c-axis pattern of sample HK124B2 is roughly symmetric,
whereas that of sample HK125 is slightly asymmetric indicating a sinistral sense of shear. A-axis
fabrics are concentrated in five to six point maxima around the edge of the stereonet (Fig. 2.12).
Domain IV, high-strain quartzite and pelitic GHS CPO patterns
CPOs of domain IV were measured from high-strain quartzite and quartzofeldspathic
schist across the entire structural extent of the high-strain quartzite and pelitic GHS. With
increasing depth, CPO patterns record a progression from type I single-girdle c-axis fabrics with a
component of basal slip, into a loss of basal and rhomb slip, an intensification of a
Y-maximum, and the formation of pseudo type II cross-girdle c-axis fabrics (Fig. 2.12).
42

Sample HK131A is a high-strain quartzite (Fig. 2.11c). Quartz accounts for ~85 vol%
and is characterized by strongly flattened grains, subgrain extinction and recrystallization through
subgrain rotation. Elongate and flattened mica define non-laterally consistent partings between
the quartz layers (Fig. 2.11c). Contoured EBSD data of sample HK131A indicate a weak
component of basal slip and a more symmetrical distribution of c-axis orientations not
observed in contoured U-Stage data (Fig. 2.12). This is attributed to the quantity of grains
measured; 426 grains were measured using U-Stage, versus 1042 grains using EBSD. Apart from
this discrepancy, the EBSD data are consistent with the U-Stage method. A-axis fabrics are
concentrated in six point maxima around the edge of the stereonet.
Sample HK139D and HK123 are the next structurally lower samples analyzed for CPOs.
Sample HK139D is a high-strain quartzite (Fig. 2.11d). Moderately coarse-grained
porphyroclastic quartz (~300 µm wide) accounts for greater than 95 vol% and is characterized by
flattened grains, serrate grain boundaries, subgrain extinction and recrystallization through
subgrain rotation. Elongated and flattened micas define non-laterally consistent partings between
quartz layers (Fig. 2.11d).
Sample HK123 comes from a centimeter-thick mylonitic layer within a high-strain
quartzofeldspathic schist. Within this sample, CPOs were measured from millimeter-thick
mylonitic bands of quartz showing extensive dynamic recrystallization through subgrain rotation
(Fig. 2.11e).
CPO patterns of both samples reveal type I single-girdle c-axis fabrics approaching a Y-
maxima. Slip systems are predominantly prism , and show weakened contributions of rhomb
relative to overlying samples. A right-lateral asymmetry is observed in sample HK139D. A-
axis fabrics are concentrated in four point maxima around the edge of the stereonet (there would
43

e six point maxima if the fabric was centered on the stereonet – see Fig. 2.12 and caption).
EBSD and U-Stage c-axis fabrics of sample HK139D are consistent (Fig. 2.12).
The next structurally lower sample, HK140, is a mylonitic quartzite (Fig. 2.11f). Coarsegrained
porphyroclastic quartz (~1 mm wide) account for >95 vol% and are characterized by
crudely interlocking interlobate and serrate grain boundaries, and extensive dynamic
recrystallization through subgrain rotation and grain boundary migration. Rare and small (~20 µm
wide) micas are dispersed throughout the sample (Fig. 2.11f). A crudely developed type II crossgirdle
c-axis fabric dominated by prism slip characterizes the CPO pattern of this sample. A-
axis fabrics are concentrated in six point maxima around the edge of the stereonet. EBSD and U-
Stage c-axis fabrics are consistent (Fig. 2.12).
Domain V, high-strain gneissic GHS CPO patterns
CPOs of Domain V were measured from quartzite and quartzofeldspathic schist (Fig.
2.11g). Sample HK145A is a mylonitic quartzofeldspathic schist. CPOs were measured from
quartz clusters comprising ~40 vol%. Well-developed interlobate interlocking quartz grain
boundaries imply recrystallization through grain boundary migration. The remainder of the matrix
comprises perthitic feldspar and rare fractured and inclusion-rich garnets (Fig. 2.11g). The
subsequent structurally lower specimen, HK105A, was sampled from a 10 m thick quartzite layer.
Coarse- to fine-grained quartz represents >95 vol% and is extensively recrystallized (Fig. 2.11h).
Coarse-grained quartz grains show well-developed chessboard style extinction (Fig. 2.7e).
Both samples yield moderately defined type II cross-girdle c-axis fabrics with ambiguous
slip systems, potentially dominated by rhomb with lesser components of prism slip. This
may be consistent with the hypothesis that type II cross-girdle c-axis patterns form during
44

constriction, where rhomb slip dominates over prism slip (Bouchez 1978; Schmid and Casey,
1986).
A-axis fabrics within these samples deviate from the typical six point maxima around the
edge of the stereonet (Fig. 2.12). This deviation may also indicate a departure from plane strain
conditions (Fig. 2.10). EBSD and U-Stage c-axis fabrics from sample HK105A are consistent
(Fig. 2.12).
2.5.1.3 Quartz CPO deformation temperatures
Assuming natural strain rates and plane strain, temperatures of deformation can be
constrained through the following two methods using quartz c-axis fabrics: (1) interpretation of
dominant slip systems active during deformation (Law, 1991; Toy et al, 2008), and (2) evaluation
of quartz c-axis opening angles (Kruhl, 1998; Law et al., 2004).
The first method relies on the transition of c-axis slip systems with varying deformation
temperatures (Sullivan and Beane, 2010 and references therein). However, this methodology can
be problematic as the absolute deformation temperatures of these transitions are poorly
constrained (Toy et al., 2008 and references therein). The second method utilizes an empirical
linear relationship between temperatures of deformation and angles between opposing arms of
quartz c-axis fabric skeletons, referred to as the c-axis opening angle (Fig. 2.13; Kruhl, 1998;
Law et al., 2004). The opening angle method offers a quantitative approach to qualifying
deformation temperatures and has thus been applied in various Himalayan studies (Law et al.,
2004; Larson and Godin, 2009; Langille et al., 2010; Larson et al., 2010a,b; Yakymchuk and
Godin, 2012). This relationship is calibrated on empirical data and subject to deviations from
natural strain rates. Consequently, estimates of deformation temperature are subject to an error of
45

Figure 2.13 Plot of c-axis fabric opening angles from the upper
Karnali valley against deformation temperature. For simplicity,
the prefix “HK” has been removed from sample numbers within
this figure. Modified after Law et al. (2004).
46

± 50°C (Law et al., 2004). Furthermore, the relationship is valid only up to ~650°C, above which
prism slip becomes increasingly active and the linear relationship deteriorates (Law et al.,
2004).
Quartz CPOs from domain III to domain V show an increase in CPO opening angles
from ~45° to 85° (Fig. 2.12, 2.13). This corresponds to an increase in temperatures of
deformation from ~350°C along the GHS-TSS interface within domain III, to ~650°C at depths
of ~2.6km below the interface within domain V (Fig. 2.13). Sample HK105A is an exception to
this trend, exhibiting a lower than expected opening angle relative to its depth. However, this
sample exhibits pervasive chessboard extinction (Fig. 2.11h), resulting from simultaneous slip
along the basal and prism lattice planes (Blumenfeld et al., 1986; Mainprice et al., 1986).
Due to slip along the prism plane, this sample would not be expected to respect the opening
angle linear trend (Law et al., 2004).
2.5.2 Rigid clast vorticity analysis
Vorticity analyses quantify the internal rotational component of flow to estimate the
relative contributions of pure and simple shear throughout deformation (Passchier and Trouw,
2005). Consequently, vorticity analysis has become a powerful tool in characterizing shear zones
and strain regimes throughout the Himalaya (Grasemann et al., 1999; Law et al., 2004; Carosi et
al., 2006; Jessup et al., 2006, 2007; Larson and Godin, 2009; Lee and Wagner, 2009; Langille et
al., 2010; Larson et al., 2010a,b).
For plane strain deformation, the relative proportions of pure and simple shear can be
directly estimated from the kinematic vorticity number (W k ). In a strictly pure shear system W k =
0, whereas in a strictly simple shear system, W k = 1 (Means et al., 1980). The relationship
between pure and simple shear is not linear, with equal contributions of both at W k =0.71 (Law et
47

al., 2004). The W k value refers to instantaneous kinematic vorticity and is only representative of
steady-state deformation, a rare situation in natural systems (Bailey et al., 2004). The mean
kinematic vorticity (W m ) averages the W k value over time and allows for a more accurate
estimation of pure and simple shear throughout non-steady-state deformation (Fossen and Tikoff,
1997, 1998; Jiang, 1998). Vorticity analyses are performed in two dimensions; however the
motion of rigid clasts during deformation is a three dimensional problem. Consequently, error
associated with this method may be significant (Tikoff and Fossen, 1995; Forte and Bailey, 2007;
Iacopini et al., 2008; Li and Jiang, 2011).
W m values were estimated using the rigid grain method (Jessup et al., 2007). Only
samples that abided by the following criteria, as stipulated by Passchier (1987), were chosen for
analysis: (1) presence of abundant rigid pre-deformational porphyroclasts, (2) porphyroclasts are
significantly larger than host matrix grains, (3) no mechanical interaction between porphyroclasts,
(4) significant quantity of porphyroclasts with a wide range of aspect ratios, (5) a homogenously
deforming matrix, (6) having experienced high and protracted strains allowing grains to rotate to
stable-sink positions.
Sample HK146 was the only specimen that respected the above criteria and defined a
clear W m threshold. This sample is located within the lower-temperature mylonitic component of
domain V, the high-strain gneissic GHS (Fig. 2.3). The specimen analyzed was taken from a

Figure 2.14 (a) Photomicrograph of sample analyzed for vorticity showing rigid clasts of
feldspar (fld) and quartz (qtz) in a fine-grained deforming matrix. (b) Results of rigid clast
vorticity analysis displayed on a rigid grain net plot (modified from Jessup et al., 2007),
and the calculation from W m to percent pure shear.
49

temperature. Therefore the calculated vorticity estimate should reflect this stage of deformation.
Analysis of 208 quartz and feldspar porphyroclasts yielded W m values of 0.58-0.61,
corresponding to ~59% pure shear (Fig. 2.14b).
2.5.3 Summary of microstructural analyses
A summary of mineral deformation mechanisms, quartz CPO fabrics, peak metamorphic
assemblages, and temperatures of deformation across the GHS-TSS interface is presented in
Figure 2.15. A vertical profile of the shear zone, from domain I through to domain VI, reveals a
systematic increase in deformation temperatures, and a potential deviation of quartz CPO fabrics
from plane strain towards constriction.
From domain I to the base of domain II, calcite microstructures transition from (1) fine
grained, non-deformed calcite lacking twins (Fig. 2.4c), to (2) moderately deformed calcite
bearing dominantly type I twins (Fig. 2.4e), to (3) recrystallized and deformed calcite bearing
dominantly type II twins, with occasional type I and III twins (Fig. 2.4g).
From the ultra-high-strain zone of domain III, through to domain VI, quartz and feldspar
deformation mechanisms transition from (1) strongly attenuated ribbon quartz exhibiting bulging
and subgrain rotation recrystallization, and brittle feldspar (Fig. 2.5c,d,e,f), to (2) flattened quartz
recrystallized via predominantly subgrain rotation (Fig. 2.6d), to (3) interlocking quartz grains
recrystallized through grain boundary migration (Fig. 2.7d), and (4) interlobate dynamically
recrystallized feldspar and quartz exhibiting chessboard extinction (Fig. 2.7e,f).
The above corresponds to an increase in deformation temperatures from less than 200°C at the
uppermost structural levels of domain I, to ~350-400°C within domain III along the GHS-TSS
interface, and greater than ~630°C within domain IV and V, at depths of ~2.5 -5.5 km below the
GHS-TSS interface (Fig. 2.15).
50

Figure 2.15 Summary of lithotectonic domains (D. I, … VI), mineral deformation mechanisms, quartz
CPO fabrics and slip systems, peak metamorphic assemblages and temperatures of deformation across the
GHS-TSS interface. Numbered boxes refer to mineral deformation mechanisms: (1) predominant type I
calcite twins ( 630 ± 30°C;
Stipp et al., 2002). Distance from GHS-TSS interface measured perpendicular to the average foliation.
Type I, type II, 6-point (pt) maxima, basal , rhomb , and prism refer to c- and a- axis fabrics
and slip systems as described in the text and seen in Figure 2.7. Abbreviations: Bt – biotite, Ms –
muscovite, Sil – sillimanite.
51

Quartz c-axis fabrics show a systematic progression with increasing depth, corresponding
to an increase in temperature and a potential deviation from plane strain toward constriction
(Figs. 2.12, 2.15). From domain III through to domain V, quartz c-axis fabrics transition from (1)
type I cross-girdles with a component of basal slip, to (2) type I cross-girdles lacking basal
slip and approaching a Y-maxima, to (3) moderately-defined type II cross-girdles. C-axis
opening angles increase with depth from ~45° to 85°, corresponding to an increase in
temperatures of deformation from ~350°C within domain III, to ~650°C within domain V (Figs.
2.13, 2.15).
The transition in c-axis fabrics from type I to type II cross-girdles may correspond to an
increase in constriction (Fig. 2.10; Bouchez 1978; Schmid and Casey, 1986). This transition also
corresponds with a minor deviation from the six point maxima a-axis fabrics (Figs. 2.12, 2.15),
which may also indicate a departure from plane strain at the deeper structural levels (Fig. 2.10).
Minor asymmetry, indicating dextral and sinistral motion, is observed in only two quartz
CPO fabrics, both located within ~700 m of the GHS-TSS interface. The remaining CPO fabrics
reveal roughly symmetric c- and a- axis patterns (Fig. 2.12). This attests to an important, and
potentially dominant component of pure shear throughout deformation.
A W m estimate of 0.58-0.61, corresponding to ~59% pure shear, was derived from a lowtemperature
crosscutting mylonitic layer within domain V (Fig. 2.15, 2.14). This attests to the
ongoing importance of pure shear throughout lower-temperature deformation.
2.6 Geochronology and thermochronology
U-Th-Pb geochronology and 40 Ar/ 39 Ar thermochronology were performed on a suite of
samples collected from traverses across and along the GHS portion of the upper Karnali valley
(domains III, IV, V and VI). Integrated with structural analyses and detailed analysis of textural
52

elationships, these data constrain the absolute timing and cooling history of metamorphism and
deformation in the upper Karnali valley.
2.6.1 U-Th-Pb monazite geochronology
Polished thin sections of samples likely to contain monazite grains were examined using
backscatter electron imaging in a scanning electron microprobe (SEM) at Queen’s University.
The presence of heavy elements within monazite grains (Ce, La, Nd, Th, Y) produces an intense
electron backscatter signal allowing for identification of monazite-rich samples. Grains identified
as monazite were subsequently confirmed using high-speed energy dispersive X-ray spectroscopy
analyses. Samples HK109, KH18 and HK117 were deemed to have sufficient numbers of large
monazite grains and were selected for U-Th-Pb in situ monazite geochronology. Detailed textural
relationships between monazites and their host fabrics were investigated by examining highresolution
electron backscatter images of entire thin sections for each selected sample.
Samples KH18a and HK109 are high-strain Bt + Ms + Sil ± Grt pelitic schist. Sample
KH18a is located within the high-strain quartzite and pelitic schist of domain IV, and sample
HK109 is located in a thin pelitic layer within the high-strain gneissic GHS of domain V (Fig.
2.3). In both samples, garnet porphyroclasts are pre-deformational, exhibiting extensive brittle
fracturing and stretching parallel to the mineral elongation lineation. Quartz is recrystallized
dominantly through subgrain rotation with lesser amounts of grain boundary migration. Pervasive
C/S/C’ fabrics are defined by micas ± sillimanite, and within sample HK109, both minerals are
faulted by and drawn into plane along shear bands (Fig. 2.7c). Textures seen in Figure 2.6 are
representative of these samples.
Sample HK117a is a Bt + Ms + Sil ± Grt migmatitic gneiss located within the migmatitic
GHS of domain VI (Fig. 2.3). Garnet porphyroclasts are rare, pre-deformational and fractured in a
53

non-systematic fashion. Quartz is recrystallized through grain boundary migration. Muscovite,
biotite, and sillimanite are present in low quantities and define a moderate anastomosing foliation.
Textures seen in Figure 2.7e,f are representative of this sample.
2.6.1.1 Methodology
Between 8 and 11 monazite grains per sample were compositionally mapped for Th, Y, U
and Nd / La using a JEOL JXA-8230 electron microprobe at Queen’s University. This was
completed to identify chemical zonation within the grains to facilitate targeting individual
compositional domains. The following electron microprobe conditions were used: accelerating
voltage of 15 kV, a beam current of ~350-500 nA, a dwell time of 100 ms and a step size of 0.5-
1.2 μm. High- and low-magnification backscatter-electron and secondary-electron images were
also collected to establish the textural context and physical condition of the monazite grains.
Monazites were analyzed using a laser ablation multi collector inductively coupled
plasma mass spectrometer (LA-MC-ICPMS) system housed at the University of California, Santa
Barbara (UCSB). Instrumentation consists of a Nu Plasma MC-ICPMS (Nu Instruments,
Wrexham, UK) and a 193 nm ArF laser ablation system equipped with a two-volume ‘HelEx’
ablation cell that facilitates rapid transfer and washout of ablated material (Photon Machines, San
Diego, USA). Analytical protocol is similar to that described by Cottle et al. (2009a,b, 2011)
with the modification that the collector arrangement on the Nu Plasma at UCSB allows for
simultaneous determination of 232 Th and 238 U on high-mass side Faraday cups equipped with 10 11
ohm resistors and 208 Pb, 207 Pb, 206 Pb and 204 Pb on four low-mass side ETP discrete dynode
secondary electron multipliers.
U-Th-Pb analyses were conducted for 20 seconds each using a 7 μm spot diameter, a
frequency of 4 Hz and 1.2 J/cm 2 fluence (equating to crater depths of approximately 5- 6 μm). A
54

primary reference material, “44096” (424 Ma Pb/U ID-TIMS age, Aleinikoff et al., 2006) was
employed to monitor and correct for mass bias as well as Pb/U and Pb/Th down-hole
fractionation. To monitor data accuracy, two secondary reference monazites “FC-1” (55.7 Ma
Pb/U ID-TIMS age, Horstwood et al., 2003), and “Managotry” monazite (554 Ma Pb/U ID-TIMS
age, Paquette et al. 1994) and “554” monazite were analyzed concurrently (once every 5
unknowns) and mass bias- and fractionation-corrected based on measured isotopic ratios of the
primary reference material. During the analytical period, repeat analyses of FC-1 gave a weighted
mean 206 Pb/ 238 U age of 56.5 ± 0.4 Ma, MSWD = 0.7, and a weighted mean 208 Pb/ 232 Th age of 54.0
± 0.3 Ma, MSWD = 0.8 (2σ) (n = 15). Repeat analyses of Managotry yield a weighted mean
206 Pb/ 238 U age of 553.5 ± 3.3 Ma, MSWD = 0.5, and a weighted mean 208 Pb/ 232 Th age of 557.8 ±
3.1 Ma, MSWD = 0.7 (2σ) (n = 15). Data reduction, including corrections for baseline,
instrumental drift, mass bias, down-hole fractionation and uncorrected age calculations were
carried out using Iolite version 2.1.2. Full details of the data reduction methodology can be found
in Paton et al. (2010). All uncertainties are quoted at 2σ and include contributions from the
external reproducibility of the primary reference material for the 207 Pb/ 206 Pb, 206 Pb/ 238 U ratios and
208 Pb/ 232 Th ratios. Concordia diagrams were constructed using Isoplot 3.75 (Ludwig, 2012).
2.6.1.2 Results
Monazite grains exhibit a variety of textures, including elongate, rounded and
comminuted into masses of fine-grained fragments (Fig. 2.16, see Appendix C for additional BSE
images). Grains are typically between 20-200 μm in diameter or width/ length. The majority of
monazite grains are strongly deformed and characterized by rounded and ragged edges, brittle
fractures, and occasional pitted interiors. Rounded grains occasionally show a greater degree of
internal deformation, consisting of intense internal fracturing and pitting of the grain. All elongate
55

Figure 2.16 Backscatter electron images (BSE image) of monazite showing textural relationships with
adjacent minerals and the dominant fabric (S), and microprobe generated yttrium and backscatter electron
maps (BSE map) for samples (a) KH18a, (b) HK109, and (c) HK117a. 208 Pb/ 232 Th ages ± 2σ are shown on
yttrium maps for all points analyzed. Diameter of analyzed points (black circles) are 7 μm. “N/A” refers to
analyses not used in the final age interpretation. Note that yttrium color scales are not all identical, and that
warmer and cooler colors indicate higher and lower elemental abundances, respectively.
56

Figure 2.16 – continued.
57

Figure 2.16 – continued.
58

monazite grains are aligned parallel to the main foliation and located adjacent to or within pristine
muscovite and biotite grains defining the dominant foliation (Fig. 2.16).
Characteristic yttrium (Y) and thorium (Th) compositional zoning is present in all
monazite grains. The vast majority of these grains exhibit Y-poor / Th-rich cores and Y-rich / Thpoor
rims (Fig. 2.16, see Appendix C for additional elemental maps). Rare grains show subtle
deviations from the above trend, including the absence of Y / Th zoning, a random distribution of
Y / Th zoning, opposite Y / Th zoning, and an additional Y-rich / Th-poor core. Note, the
chemical formula for the analyzed monazite is (Ce, La, Nd, Th, Y)PO 4 , therefore it is expected
that Y and Th are inversely related, as they may substitute for one another. For all monazite
grains, locations of the spot analyses were based on Y / Th compositional zoning and were
selected to constrain ages associated with different chemical domains (Fig. 2.16). The full U-Th-
Pb data set is presented in Table 2.1, and graphically presented as 208 Pb/ 232 Th vs. 206 Pb/ 238 U
concordia and 208 Pb/ 232 Th age probability distribution diagrams in Figure 2.17.
For sample KH18a, a total of 65 meaningful analyses were obtained on six monazite
grains (Fig. 2.17, Table 2.1). These data yield minor peaks in age distribution at ~26 Ma and ~24
-22 Ma, with the bulk of ages occurring between ~21 -15 Ma. The oldest grouping of ages at ~26
Ma corresponds to a minor group of Y-poor cores. The next youngest age population corresponds
to a minor peak of Y-rich rims between ~24 -21 Ma. The bulk of the ages record a secondary
Figure 2.17 (Next page) U-Th-Pb concordia (left column) and probability (right column)
diagrams for dated monazite. Pink and green color coding in both concordia and probability cures
correspond to analyses of Y-high rims, and Y-low cores, respectively. Blue analyses are
interpreted as mixtures between Y domains. Color-coding within probability diagrams is not
exact and only intended to show general trends. The low frequency area of HK117a, between 12 -
14 Ma, is not color-coded as it does not show a distinct distribution of high/low Y analyses. Total
number of analyses per sample (n) is recorded in the upper right corner of each probability plot.
Concordia diagrams plot 206 Pb/ 238 U against 208 Pb/ 232 Th to avoid complications from 235 U- 207 Pb
disequilibrium. All plots were constructed with Isoplot 3.75 (Ludwig, 2012).
59

Table 2-1 U-Th-Pb geochronologic data
Analysis Position
KH18a
Pb U Th
Th/U
207 Pb/ 206 2σ
Pb
206 Pb/ 238 2σ
U
207 Pb/ 235 U b 2σ
Rho c 208 Pb/ 232 2σ
206 Pb/ 238 U 2σ
208 Pb/ 232 Th 2σ
Th
(ppm) a (ppm) a (ppm) a (%) (%) (%) (%) (Ma) d abs (Ma) e abs
HK18a_2_01 H-Y,R 45 3050 30100 9.8 0.359 6.13 0.00377 5.57 0.192 11.46 0.95 0.001092 4.85 24.2 1.4 22.1 1.1
HK18a_2_02 H-Y,R 59 4860 51700 10.4 0.0929 2.80 0.0027 1.96 0.0345 3.19 0.38 0.000817 2.08 17.4 0.3 16.5 0.3
HK18a_2_03 H-Y,R 66 6390 55700 8.4 0.0864 1.85 0.0028 2.00 0.033 2.61 0.74 0.000856 1.87 18.0 0.4 17.3 0.3
HK18a_2_04 H-Y,R 62 5180 54000 10.0 0.0878 1.94 0.0027 2.11 0.03311 2.84 0.72 0.000831 2.05 17.4 0.4 16.8 0.3
HK18a_2_05 H-Y,R 64 5590 54200 9.4 0.0844 2.13 0.00277 1.77 0.03223 2.79 0.66 0.000854 1.87 17.9 0.3 17.3 0.3
HK18a_2_06 H-Y,R 67 6510 56600 8.4 0.0852 1.88 0.00272 2.06 0.03165 2.91 0.73 0.000854 2.22 17.5 0.4 17.3 0.4
HK18a_2_07 H-Y,R 87 11200 67800 5.9 0.0848 1.13 0.00291 1.99 0.03391 2.12 0.84 0.000921 2.06 18.7 0.4 18.6 0.4
HK18a_2_08 H-Y,R 63 6910 48100 6.9 0.092 1.74 0.00303 1.98 0.0381 2.89 0.73 0.000938 2.13 19.5 0.4 19.0 0.4
HK18a_2_09 H-Y,R 71 7090 58700 8.2 0.0916 1.75 0.00282 2.17 0.03605 2.75 0.70 0.00086 2.21 18.1 0.4 17.4 0.4
HK18a_2_10 H-Y,R 72 7260 54800 7.6 0.1259 1.59 0.00305 1.87 0.0527 2.28 0.73 0.000929 1.94 19.7 0.4 18.8 0.4
HK18a_2_11 L-Y,C 84 13970 64200 4.8 0.0854 1.29 0.00292 1.82 0.03367 2.11 0.77 0.000921 1.95 18.8 0.3 18.6 0.4
HK18a_2_12 L-Y,C 91 13820 67700 5.2 0.1049 1.14 0.00301 2.36 0.0433 2.54 0.87 0.00095 2.32 19.4 0.5 19.2 0.4
HK18a_2_13 L-Y,C 92 10120 70700 7.6 0.0998 1.70 0.00304 2.30 0.041 2.68 0.81 0.000927 2.16 19.6 0.5 18.7 0.4
HK18a_2_14 L-Y,C 85 13290 62400 5.2 0.1167 1.20 0.00302 2.15 0.0487 2.46 0.88 0.000948 2.11 19.5 0.4 19.2 0.4
HK18a_2_15 L-Y,C 82 11810 60200 5.7 0.1185 1.43 0.00308 1.92 0.0498 2.01 0.78 0.00096 1.98 19.8 0.4 19.4 0.4
HK18a_2_16 L-Y,C 66 9500 50100 5.8 0.0937 1.49 0.00299 2.41 0.0383 2.87 0.82 0.000939 2.56 19.2 0.5 19.0 0.5
HK18a_2_17 H-Y,R 103 16470 74500 5.0 0.1459 4.46 0.00311 2.16 0.0642 5.92 0.69 0.000977 2.05 20.0 0.4 19.7 0.4
HK18a_2_18 H-Y,R 59 8230 47100 6.3 0.1182 3.98 0.00287 2.16 0.0471 5.10 0.64 0.000875 2.29 18.5 0.4 17.7 0.4
HK18a_2_19 L-Y,C 58 8490 44300 5.7 0.1082 2.59 0.00296 1.93 0.0443 3.16 0.62 0.00091 2.09 19.0 0.4 18.4 0.4
HK18a_2_20 H-Y,R 71 7670 44100 6.2 0.297 4.38 0.00381 3.41 0.156 7.69 0.91 0.001128 3.28 24.5 0.9 22.8 0.7
HK18a_4_01 L-Y,R 68 5480 39000 7.4 0.3788 1.06 0.00438 1.49 0.2274 2.11 0.85 0.001253 1.44 28.2 0.4 25.3 0.4
HK18a_4_02 L-Y,R 35 2660 24500 8.7 0.3675 1.58 0.0043 2.79 0.217 3.87 0.90 0.001085 4.06 27.7 0.8 21.9 0.9
HK18a_4_03 L-Y,R 146 5870 36740 6.4 0.579 2.59 0.0099 6.46 0.803 8.97 0.99 0.00276 6.16 63.5 4.1 55.8 3.5
HK18a_4_04 L-Y,R 66 4340 41200 9.8 0.4059 1.06 0.00445 3.15 0.2496 3.37 0.95 0.001109 2.71 28.6 0.9 22.4 0.6
HK18a_4_05 L-Y,R 102 4150 32290 7.9 0.6066 0.74 0.00866 2.08 0.728 2.61 0.96 0.002248 2.18 55.6 1.1 45.4 1.0
HK18a_4_06 H-Y,C 120 8240 34240 4.2 0.5096 1.06 0.0062 1.94 0.439 2.73 0.94 0.002513 2.71 39.9 0.8 50.7 1.4
HK18a_4_07 H-Y,C 151 7630 34160 4.5 0.5743 0.68 0.00792 1.77 0.635 2.20 0.96 0.003176 1.64 50.8 0.9 64.1 1.0
HK18a_4_08 H-Y,C 107 9970 32280 3.3 0.4271 0.96 0.00519 1.93 0.3076 2.54 0.94 0.002397 2.29 33.4 0.6 48.4 1.1
HK18a_4_09 L-Y,R 92 10280 37030 3.6 0.3375 2.90 0.00437 2.52 0.205 4.88 0.93 0.001814 2.65 28.1 0.7 36.6 1.0
61

Analysis Position
Pb U Th
Th/U
207 Pb/ 206 2σ
Pb
206 Pb/ 238 2σ
U
207 Pb/ 235 U b 2σ
Rho c 208 Pb/ 232 2σ
206 Pb/ 238 U 2σ
208 Pb/ 232 Th 2σ
Th
(ppm) a (ppm) a (ppm) a (%) (%) (%) (%) (Ma) d abs (Ma) e abs
HK18a_4_10 L-Y,R 140 4840 41700 8.6 0.6163 1.17 0.00919 3.26 0.791 4.17 0.98 0.002423 2.81 58.9 1.9 48.9 1.4
HK18a_4_11 L-Y,R 59 10410 45410 4.3 0.0825 1.21 0.00286 1.72 0.03323 2.29 0.83 0.000943 1.59 18.4 0.3 19.1 0.3
HK18a_4_12 L-Y,R 114 8260 50630 6.1 0.428 3.74 0.00518 4.83 0.309 8.09 0.98 0.001646 3.28 33.3 1.6 33.2 1.1
HK18a_4_13 L-Y,R 148 8330 47900 5.6 0.5347 0.47 0.00695 2.16 0.516 2.33 0.98 0.002262 2.08 44.6 1.0 45.7 0.9
HK18a_4_14 L-Y,R 106 10640 44300 4.1 0.3559 1.85 0.00458 2.05 0.2279 3.33 0.88 0.00172 2.33 29.4 0.6 34.7 0.8
HK18a_4_15 L-Y,R 217 9590 35300 3.6 0.6372 0.42 0.00998 2.00 0.892 2.24 0.99 0.0045 3.11 64.0 1.3 90.8 2.7
HK18a_5_01 Mix 25 2950 12500 4.3 0.415 3.13 0.00401 3.49 0.232 6.03 0.92 0.001559 4.55 25.8 0.9 31.5 1.4
HK18a_5_02 Mix 35 3780 22900 6.4 0.273 1.68 0.00327 2.69 0.1253 3.35 0.83 0.001092 2.75 21.0 0.6 22.1 0.6
HK18a_5_03 Mix 90 9540 71900 7.9 0.1599 2.06 0.00296 1.76 0.0658 2.89 0.68 0.000911 1.87 19.1 0.3 18.4 0.3
HK18a_5_04 Mix 86 9730 61600 6.7 0.1717 3.38 0.0032 2.00 0.0763 4.59 0.77 0.001009 2.08 20.6 0.4 20.4 0.4
HK18a_5_05 Mix 136 12930 85700 7.0 0.3046 1.84 0.00402 2.11 0.1703 3.23 0.86 0.001156 2.08 25.9 0.6 23.4 0.5
HK18a_5_06 Mix 80 9230 56700 6.6 0.2168 2.91 0.00334 2.07 0.1005 4.18 0.82 0.00102 1.96 21.5 0.4 20.6 0.4
HK18a_5_07 Mix 78 10020 62600 6.6 0.0886 2.26 0.0028 1.72 0.0346 2.89 0.65 0.000898 1.67 18.0 0.3 18.1 0.3
HK18a_5_08 Mix 83 8080 56700 7.3 0.3051 2.88 0.00399 3.26 0.17 5.88 0.94 0.001061 2.73 25.7 0.8 21.4 0.6
HK18a_5_09 Mix 84 9670 61780 6.6 0.1323 2.72 0.00309 1.78 0.0564 3.19 0.66 0.000985 1.83 19.9 0.4 19.9 0.4
HK18a_5_10 Mix 79 9610 60100 6.4 0.1926 2.75 0.0031 2.19 0.0817 4.04 0.75 0.000949 2.00 20.0 0.4 19.2 0.4
HK18a_6_01 H-Y,R 69 8750 60700 6.9 0.0877 2.39 0.00265 1.73 0.03195 2.97 0.62 0.000821 1.83 17.1 0.3 16.6 0.3
HK18a_6_02 H-Y,R 69 9730 55900 5.7 0.0782 2.05 0.00276 1.63 0.03022 2.78 0.59 0.000884 1.70 17.8 0.3 17.9 0.3
HK18a_6_03 H-Y,R 90 9960 72000 7.1 0.088 1.82 0.00287 1.74 0.0352 3.13 0.70 0.00089 1.69 18.5 0.3 18.0 0.3
HK18a_6_04 H-Y,R 69 9300 55600 5.9 0.0793 2.14 0.00274 1.94 0.03011 2.99 0.64 0.000877 1.94 17.6 0.3 17.7 0.3
HK18a_6_05 H-Y,R 63 8290 51700 6.1 0.0775 2.32 0.00273 1.69 0.02906 2.99 0.61 0.000867 1.61 17.6 0.3 17.5 0.3
HK18a_6_06 H-Y,R 63 8280 56600 6.8 0.0781 2.43 0.00251 1.71 0.02727 2.97 0.54 0.000794 1.64 16.2 0.3 16.0 0.3
HK18a_6_07 H-Y,R 44 4400 32700 7.3 0.277 5.78 0.00317 5.05 0.123 9.76 0.89 0.000948 4.64 20.4 1.0 19.1 0.9
HK18a_6_08 H-Y,R 79 7160 48100 6.7 0.293 2.66 0.00379 2.01 0.153 4.12 0.88 0.001167 2.06 24.4 0.5 23.6 0.5
HK18a_6_09 H-Y,R 61 7540 49300 6.5 0.0773 1.55 0.00275 1.78 0.02959 2.84 0.79 0.000876 1.83 17.7 0.3 17.7 0.3
HK18a_6_10 H-Y,R 64 7850 50400 6.4 0.0788 1.90 0.0028 1.61 0.03056 2.59 0.63 0.000898 1.78 18.0 0.3 18.2 0.3
HK18a_6_11 L-Y,C 54 7260 45420 6.3 0.0762 2.36 0.0026 2.11 0.02697 3.23 0.69 0.000831 2.17 16.8 0.4 16.8 0.4
HK18a_6_12 L-Y,C 48 6850 40030 5.8 0.1417 4.23 0.00263 1.97 0.0508 4.92 0.55 0.000849 2.12 17.0 0.3 17.2 0.4
HK18a_6_13 L-Y,C 44 4150 32500 7.9 0.224 5.80 0.00309 3.24 0.098 8.78 0.92 0.00094 2.66 19.9 0.7 19.0 0.5
HK18a_6_14 H-Y,R 71 6720 55500 8.3 0.1282 2.57 0.00293 1.95 0.0518 3.09 0.62 0.000895 1.79 18.9 0.4 18.1 0.3
HK18a_6_15 H-Y,R 66 6400 49500 7.8 0.0856 2.34 0.00296 1.72 0.035 3.14 0.62 0.000945 1.59 19.1 0.3 19.1 0.3
HK18a_7_01 H-Y,R 29 1881 14540 7.8 0.4426 1.15 0.00439 1.98 0.2671 2.58 0.89 0.001446 2.21 28.2 0.6 29.2 0.7
62

Analysis Position
Pb U Th
Th/U
207 Pb/ 206 2σ
Pb
206 Pb/ 238 2σ
U
207 Pb/ 235 U b 2σ
Rho c 208 Pb/ 232 2σ
206 Pb/ 238 U 2σ
208 Pb/ 232 Th 2σ
Th
(ppm) a (ppm) a (ppm) a (%) (%) (%) (%) (Ma) d abs (Ma) e abs
HK18a_7_02 H-Y,R 53 5760 47100 8.3 0.0932 1.82 0.00255 1.69 0.03317 2.80 0.71 0.000792 1.64 16.4 0.3 16.0 0.3
HK18a_7_03 H-Y,R 53 5056 43900 8.8 0.1357 2.21 0.00284 1.73 0.0528 3.03 0.63 0.000861 1.63 18.3 0.3 17.4 0.3
HK18a_7_04 H-Y,R 57 5940 47700 8.1 0.1035 1.93 0.00272 1.98 0.0387 2.84 0.69 0.000844 2.01 17.5 0.4 17.1 0.3
HK18a_7_05 L-Y,C 65 6490 49000 7.6 0.1034 1.64 0.00303 1.91 0.04259 2.30 0.69 0.00096 1.88 19.5 0.4 19.4 0.4
HK18a_7_06 L-Y,C 76 7130 55600 7.8 0.141 1.49 0.00313 1.92 0.0601 2.66 0.81 0.000976 1.95 20.1 0.4 19.7 0.4
HK18a_7_07 H-Y,R 70 6390 50000 7.9 0.1674 2.09 0.00326 1.66 0.0751 2.93 0.75 0.001006 1.69 21.0 0.4 20.3 0.4
HK18a_7_08 H-Y,R 71 6280 45590 7.3 0.2647 2.68 0.0036 2.03 0.1299 4.39 0.86 0.001113 1.98 23.2 0.5 22.5 0.4
HK18a_7_09 H-Y,R 71 6140 48000 7.8 0.2285 0.83 0.00349 1.43 0.1101 1.91 0.89 0.001063 1.41 22.5 0.3 21.5 0.3
HK18a_7_10 H-Y,R 97 4780 37100 7.8 0.5796 1.02 0.00662 2.42 0.528 3.22 0.97 0.001896 2.48 42.5 1.0 38.3 1.0
HK18a_7_11 H-Y,R 96 8730 76200 8.6 0.0902 1.22 0.00294 1.87 0.03651 2.14 0.83 0.000896 1.90 18.9 0.4 18.1 0.3
HK18a_7_12 H-Y,R 86 8500 54000 6.3 0.2558 2.78 0.00374 2.08 0.1317 4.40 0.87 0.001152 1.91 24.1 0.5 23.3 0.5
HK18a_7_13 H-Y,R 58 5450 34300 6.2 0.3861 1.74 0.00398 2.36 0.2127 3.67 0.89 0.001195 2.18 25.6 0.6 24.1 0.5
HK18a_7_14 H-Y,R 88 7470 43600 5.8 0.3797 2.45 0.00469 2.56 0.248 4.84 0.94 0.001432 2.65 30.2 0.8 28.9 0.8
HK18a_7_15 H-Y,R 82 8050 49110 6.1 0.3657 1.86 0.00406 1.99 0.2043 3.28 0.91 0.001175 1.96 26.1 0.5 23.7 0.5
HK18a_8_01 L-Y 100 8580 55700 6.6 0.4096 1.05 0.00472 1.78 0.2662 2.40 0.92 0.001267 1.74 30.4 0.5 25.6 0.4
HK18a_8_02 L-Y 78 10520 58600 5.8 0.1334 3.37 0.00303 1.91 0.0556 4.32 0.71 0.000941 1.91 19.5 0.4 19.0 0.4
HK18a_8_03 L-Y 44 4210 29000 7.0 0.313 7.03 0.00382 4.19 0.165 10.91 0.95 0.001076 3.90 24.6 1.1 21.7 0.8
HK18a_8_04 L-Y 108 13340 83300 6.3 0.0993 1.31 0.00295 1.80 0.04022 2.21 0.78 0.000916 1.86 19.0 0.3 18.5 0.3
HK18a_8_05 L-Y 76 6350 41800 6.7 0.3438 1.05 0.00423 1.73 0.1998 2.10 0.89 0.001272 1.81 27.2 0.5 25.7 0.5
HK18a_8_06 L-Y 133 13730 95500 7.0 0.149 1.88 0.00321 1.65 0.0661 2.57 0.73 0.00098 1.63 20.7 0.3 19.8 0.3
HK18a_8_07 L-Y 39 3130 19800 6.3 0.3235 1.89 0.00428 2.57 0.1875 3.25 0.83 0.001466 2.66 27.5 0.7 29.6 0.8
HK18a_8_08 H-Y 69 3272 39670 12.2 0.453 2.65 0.00521 3.84 0.319 6.27 0.96 0.001228 2.52 33.5 1.3 24.8 0.6
HK18a_8_09 H-Y 68 4404 57600 13.0 0.1058 2.84 0.00278 1.94 0.0407 3.69 0.63 0.000836 1.79 17.9 0.4 16.9 0.3
HK18a_8_10 H-Y 127 10540 86100 8.2 0.2044 4.84 0.00347 1.90 0.0987 6.28 0.82 0.00104 1.83 22.4 0.4 21.0 0.4
HK18a_8_11 H-Y 69 6190 56700 9.2 0.0836 2.15 0.00277 1.66 0.03174 2.65 0.61 0.000874 1.72 17.8 0.3 17.7 0.3
HK18a_8_12 H-Y 67 5520 52500 9.5 0.139 1.65 0.00298 2.05 0.0577 2.77 0.75 0.000919 1.96 19.2 0.4 18.6 0.4
HK18a_8_13 H-Y 86 5120 49400 9.6 0.3432 2.16 0.00442 1.99 0.2076 3.61 0.89 0.001261 1.90 28.4 0.6 25.5 0.5
HK18a_8_14 H-Y 83 5635 58500 10.4 0.239 5.86 0.00352 3.13 0.119 8.40 0.91 0.001017 2.46 22.6 0.7 20.6 0.5
HK18a_8_15 H-Y 71 3376 50280 15.0 0.3438 2.12 0.004 2.45 0.1889 4.02 0.92 0.001016 2.26 25.8 0.6 20.5 0.5
HK18a_8_16 H-Y 77 5160 55000 10.7 0.258 6.20 0.0035 3.43 0.122 9.84 0.95 0.001003 2.49 22.5 0.8 20.3 0.5
HK18a_8_17 H-Y 87 1492 22340 15.0 0.72 0.68 0.01593 3.64 1.57 4.01 0.99 0.002808 3.17 101.9 3.7 56.7 1.8
HK18a_8_18 H-Y 73 5810 57500 9.9 0.1332 3.75 0.00297 1.65 0.0545 4.77 0.71 0.000915 1.64 19.1 0.3 18.5 0.3
63

Analysis Position
Pb U Th
Th/U
207 Pb/ 206 2σ
Pb
206 Pb/ 238 2σ
U
207 Pb/ 235 U b 2σ
Rho c 208 Pb/ 232 2σ
206 Pb/ 238 U 2σ
208 Pb/ 232 Th 2σ
Th
(ppm) a (ppm) a (ppm) a (%) (%) (%) (%) (Ma) d abs (Ma) e abs
HK18a_8_19 H-Y 115 10410 89400 8.6 0.0909 1.21 0.00296 1.83 0.03699 2.27 0.80 0.000921 1.63 19.0 0.4 18.6 0.3
HK18a_8_20 H-Y 62 4690 53500 11.4 0.0944 1.69 0.00272 2.02 0.03555 2.62 0.73 0.000838 2.03 17.5 0.4 16.9 0.3
HK109
HK109_2_01 H-Y,R 36 5867 34488 5.88 0.08409 2.97 0.00236 2.16 0.02710 3.46 0.57 0.000708 2.42 15.2 0.3 14.3 0.3
HK109_2_02 H-Y,R 39 6970 36542 5.25 0.08084 2.61 0.00242 2.44 0.02719 3.48 0.73 0.000746 2.43 15.6 0.4 15.1 0.4
HK109_2_03 H-Y,R 42 7859 38601 4.93 0.07828 2.82 0.00246 1.98 0.02649 2.92 0.48 0.000759 2.13 15.8 0.3 15.3 0.3
HK109_2_04 H-Y,R 41 7076 39225 5.61 0.08228 3.05 0.00243 2.29 0.02746 3.64 0.59 0.000740 2.47 15.7 0.4 14.9 0.4
HK109_2_05 H-Y,R 45 7227 41952 5.77 0.07755 2.74 0.00248 2.52 0.02657 3.50 0.60 0.000751 2.73 16.0 0.4 15.2 0.4
HK109_2_06 H-Y,R 49 8150 48583 6.02 0.08200 3.39 0.00234 2.60 0.02683 4.70 0.70 0.000713 2.66 15.1 0.4 14.4 0.4
HK109_2_07 H-Y,R 52 10055 49174 4.93 0.07728 2.81 0.00245 2.54 0.02589 3.51 0.65 0.000760 2.53 15.8 0.4 15.4 0.4
HK109_2_08 H-Y,R 38 6694 40851 6.15 0.09938 3.50 0.00219 2.47 0.02943 3.96 0.55 0.000665 2.13 14.1 0.3 13.4 0.3
HK109_2_09 L-Y,C 77 9813 62883 6.46 0.08099 2.31 0.00280 2.48 0.03128 3.21 0.73 0.000869 2.52 18.0 0.4 17.5 0.4
HK109_2_10 L-Y,C 81 10218 65197 6.37 0.08466 1.92 0.00285 2.58 0.03316 3.34 0.82 0.000884 2.59 18.4 0.5 17.9 0.5
HK109_2_11 L-Y,C 78 9474 64086 6.76 0.08304 2.22 0.00283 2.45 0.03245 3.29 0.74 0.000864 2.50 18.2 0.4 17.4 0.4
HK109_3_01 L-Y 69 11710 61451 5.19 0.07357 2.81 0.00261 2.51 0.02678 4.09 0.80 0.000803 2.78 16.8 0.4 16.2 0.4
HK109_3_02 L-Y 68 11470 60350 5.20 0.07432 2.66 0.00266 3.16 0.02705 3.69 0.72 0.000814 3.46 17.1 0.5 16.4 0.6
HK109_3_03 L-Y 67 11405 59740 5.25 0.07489 2.21 0.00264 2.79 0.02698 3.41 0.75 0.000803 2.67 17.0 0.5 16.2 0.4
HK109_3_04 L-Y 67 11379 59548 5.20 0.07386 2.11 0.00266 3.50 0.02729 4.24 0.88 0.000821 3.46 17.1 0.6 16.6 0.6
HK109_3_05 L-Y 75 11210 62640 5.58 0.07935 2.30 0.00277 3.27 0.03015 4.08 0.80 0.000858 3.39 17.8 0.6 17.3 0.6
HK109_3_06 L-Y 87 12143 72044 5.90 0.08758 2.20 0.00284 3.29 0.03445 3.90 0.82 0.000869 3.12 18.3 0.6 17.6 0.5
HK109_4_01 H-Y,R 39 6869 39827 5.73 0.13569 4.58 0.00234 2.86 0.04434 5.86 0.63 0.000722 3.05 15.1 0.4 14.6 0.4
HK109_4_02 H-Y,R 39 6274 34357 5.40 0.15993 8.06 0.00259 6.49 0.05437 11.50 0.76 0.000834 6.50 16.7 1.1 16.9 1.1
HK109_4_03 H-Y,R 46 9983 50358 4.96 0.09315 4.04 0.00207 2.72 0.02679 4.99 0.62 0.000662 2.75 13.3 0.4 13.4 0.4
HK109_4_04 H-Y,R 54 9944 57213 5.75 0.08418 2.63 0.00225 2.72 0.02632 3.46 0.69 0.000669 2.64 14.5 0.4 13.5 0.4
HK109_4_05 H-Y,R 57 13529 60859 4.48 0.07607 2.36 0.00222 3.16 0.02308 3.84 0.83 0.000673 3.35 14.3 0.5 13.6 0.5
HK109_4_06 L-Y,C 70 9381 60399 6.43 0.07447 2.79 0.00266 3.08 0.02758 4.04 0.73 0.000817 2.93 17.2 0.5 16.5 0.5
HK109_4_07 L-Y,C 68 8584 58399 6.85 0.07471 2.25 0.00266 2.56 0.02737 3.15 0.78 0.000818 2.70 17.1 0.4 16.5 0.4
HK109_4_08 L-Y,C 67 7730 57545 7.41 0.07607 2.36 0.00271 2.75 0.02868 3.42 0.74 0.000839 2.73 17.5 0.5 17.0 0.5
HK109_4_09 L-Y,C 60 7446 52272 7.08 0.07421 2.40 0.00267 3.03 0.02731 3.66 0.75 0.000812 2.80 17.2 0.5 16.4 0.5
HK109_4_10 H-Y,R 63 7041 52058 7.62 0.07419 2.22 0.00279 2.53 0.02860 3.16 0.71 0.000853 2.56 17.9 0.5 17.2 0.4
HK109_5_01 H-Y,R 40 5150 40823 8.15 0.08141 3.37 0.00230 3.21 0.02605 4.68 0.71 0.000695 2.99 14.8 0.5 14.0 0.4
HK109_5_02 H-Y,R 40 5334 40577 7.78 0.08141 3.04 0.00232 3.06 0.02622 4.05 0.68 0.000710 3.16 14.9 0.5 14.3 0.5
64

Analysis Position
Pb U Th
Th/U
207 Pb/ 206 2σ
Pb
206 Pb/ 238 2σ
U
207 Pb/ 235 U b 2σ
Rho c 208 Pb/ 232 2σ
206 Pb/ 238 U 2σ
208 Pb/ 232 Th 2σ
Th
(ppm) a (ppm) a (ppm) a (%) (%) (%) (%) (Ma) d abs (Ma) e abs
HK109_5_03 H-Y,R 35 5934 39858 6.84 0.09178 4.75 0.00203 4.63 0.02549 6.09 0.71 0.000628 4.83 13.1 0.6 12.7 0.6
HK109_5_04 H-Y,R 43 6121 43391 7.18 0.08018 3.26 0.00237 4.04 0.02610 5.10 0.79 0.000719 4.00 15.3 0.6 14.5 0.6
HK109_5_05 H-Y,R 43 6068 43384 7.20 0.07952 3.42 0.00232 3.25 0.02551 4.65 0.69 0.000702 3.39 15.0 0.5 14.2 0.5
HK109_5_06 L-Y 136 7741 53854 6.85 0.06516 1.70 0.00599 3.64 0.05342 3.86 0.91 0.001823 3.52 38.5 1.4 36.8 1.3
HK109_5_07 L-Y 66 7660 51408 6.64 0.07722 2.60 0.00281 3.53 0.02971 4.17 0.82 0.000907 3.39 18.1 0.6 18.3 0.6
HK109_5_08 H-Y,R 58 7943 51836 6.50 0.07634 2.71 0.00252 3.93 0.02652 4.96 0.84 0.000787 3.89 16.2 0.6 15.9 0.6
HK109_5_09 H-Y,R 348 5671 40765 7.14 0.06162 1.30 0.01554 4.58 0.13254 4.67 0.95 0.006171 4.46 99.4 4.5 124.3 5.5
HK109_5_10 M-Y,C 622 3261 23161 6.95 0.05881 0.98 0.06527 3.66 0.52320 3.45 0.96 0.019476 4.11 407.2 ## 389.7 ##
HK109_6_01 H-Y,R 53 7831 50546 6.69 0.07684 2.81 0.00237 3.82 0.02574 4.36 0.80 0.000722 3.52 15.3 0.6 14.6 0.5
HK109_6_02 H-Y,R 55 8356 53705 6.66 0.07975 2.66 0.00239 3.72 0.02638 4.14 0.82 0.000738 3.55 15.4 0.6 14.9 0.5
HK109_6_03 H-Y,R 58 8768 55526 6.64 0.07890 2.45 0.00242 3.70 0.02603 4.04 0.85 0.000737 3.54 15.6 0.6 14.9 0.5
HK109_6_04 H-Y,R 56 8435 54456 6.84 0.08103 3.02 0.00240 4.10 0.02682 4.89 0.79 0.000725 4.06 15.5 0.6 14.7 0.6
HK109_6_05 H-Y,R 64 10273 59909 6.24 0.07919 2.73 0.00250 3.46 0.02779 4.28 0.79 0.000771 3.34 16.1 0.6 15.6 0.5
HK109_6_06 L-Y,C 69 16150 61746 4.09 0.07015 2.16 0.00258 3.04 0.02433 4.01 0.84 0.000813 3.19 16.6 0.5 16.4 0.5
HK109_6_07 L-Y,C 70 15297 61477 4.29 0.06847 2.35 0.00256 3.43 0.02408 4.05 0.83 0.000805 3.31 16.5 0.6 16.3 0.5
HK109_6_08 H-Y,R 58 9043 56834 6.67 0.08013 2.36 0.00240 3.62 0.02593 4.35 0.86 0.000745 3.67 15.4 0.6 15.1 0.6
HK109_6_09 H-Y,R 62 9204 58088 6.70 0.08204 2.43 0.00248 3.29 0.02828 4.39 0.82 0.000763 3.32 16.0 0.5 15.4 0.5
HK109_6_10 H-Y,R 50 7279 49743 7.07 0.08405 2.77 0.00236 3.31 0.02716 3.95 0.73 0.000731 3.55 15.2 0.5 14.8 0.5
HK109_9_01 H-Y,R 56 8315 53777 6.47 0.08691 4.39 0.00244 3.84 0.02973 6.04 0.63 0.000740 3.99 15.7 0.6 15.0 0.6
HK109_9_02 H-Y,R 53 7283 51789 7.07 0.08960 2.59 0.00246 3.94 0.03029 4.45 0.79 0.000738 3.61 15.8 0.6 14.9 0.5
HK109_9_03 H-Y,R 67 11173 57095 4.96 0.07526 2.72 0.00274 5.22 0.02873 5.50 0.86 0.000857 5.04 17.7 0.9 17.3 0.9
HK109_9_04 L-Y,C 69 10626 58723 5.47 0.08263 2.43 0.00276 4.20 0.03147 5.29 0.87 0.000843 4.28 17.8 0.7 17.0 0.7
HK109_9_05 L-Y,C 56 8767 44249 5.00 0.08638 2.33 0.00294 4.16 0.03449 4.49 0.84 0.000926 4.24 18.9 0.8 18.7 0.8
HK109_9_06 L-Y,C 62 9834 51784 5.24 0.08787 2.26 0.00277 3.37 0.03311 3.89 0.79 0.000855 3.60 17.8 0.6 17.3 0.6
HK109_10_01 L-Y 80 14739 71543 4.71 0.07048 2.74 0.00259 3.04 0.02484 4.16 0.77 0.000813 2.87 16.7 0.5 16.4 0.5
HK109_10_02 H-Y 77 8491 63171 7.28 0.08345 2.49 0.00291 3.72 0.03354 4.21 0.82 0.000882 3.67 18.8 0.7 17.8 0.7
HK109_10_03 H-Y 66 8296 54042 6.40 0.07404 3.00 0.00289 3.55 0.02939 4.51 0.76 0.000900 3.68 18.6 0.7 18.2 0.7
HK109_10_04 H-Y 65 7777 54415 6.79 0.07879 2.94 0.00284 4.53 0.03091 5.10 0.82 0.000886 4.36 18.4 0.8 17.9 0.8
HK109_10_05 L-Y 74 10226 65381 6.30 0.07243 2.60 0.00335 6.82 0.03360 6.41 0.91 0.000848 4.59 21.5 1.5 17.1 0.8
HK109_10_06 L-Y 82 10508 73086 6.88 0.07701 2.68 0.00270 3.49 0.02875 3.97 0.81 0.000832 3.45 17.4 0.6 16.8 0.6
HK109_10_07 L-Y 76 8288 65151 7.85 0.07481 2.85 0.00275 4.41 0.02854 4.76 0.83 0.000837 4.23 17.7 0.8 16.9 0.7
65

Analysis Position
HK117A
Pb U Th
Th/U
207 Pb/ 206 2σ
Pb
206 Pb/ 238 2σ
U
207 Pb/ 235 U b 2σ
Rho c 208 Pb/ 232 2σ
206 Pb/ 238 U 2σ
208 Pb/ 232 Th 2σ
Th
(ppm) a (ppm) a (ppm) a (%) (%) (%) (%) (Ma) d abs (Ma) e abs
HK117A_1_1 H-Y 37 6640 30690 4.6 0.2952 3.15 0.00248 4.03 0.1001 5.99 0.85 0.000834 4.20 16.0 0.7 16.9 0.7
HK117A_1_2 H-Y 101 10140 94300 9.4 0.0635 1.89 0.00244 1.80 0.02163 2.40 0.53 0.000747 1.74 15.7 0.3 15.1 0.3
HK117A_1_3 L-Y 51 5520 44300 8.1 0.2521 2.74 0.00302 2.29 0.1052 3.99 0.80 0.0008 2.25 19.4 0.4 16.2 0.4
HK117A_1_4 H-Y 95 10100 88700 8.8 0.0625 1.76 0.00245 1.47 0.021 2.38 0.63 0.000751 1.46 15.7 0.2 15.2 0.2
HK117A_1_5 H-Y 83 10160 73100 7.2 0.2046 2.79 0.00285 2.07 0.0798 4.01 0.76 0.000782 2.17 18.3 0.4 15.8 0.3
HK117A_1_6 L-Y 89 8370 83300 9.9 0.0624 1.92 0.00246 1.75 0.02111 2.79 0.67 0.000745 1.61 15.8 0.3 15.1 0.3
HK117A_1_7 H-Y 79 11120 68300 6.2 0.2093 2.82 0.00281 2.78 0.0825 4.97 0.83 0.000805 2.73 18.1 0.5 16.3 0.5
HK117A_1_8 L-Y 89 8050 82300 10.2 0.0642 2.02 0.00249 1.57 0.02239 2.72 0.52 0.000761 1.71 16.1 0.3 15.4 0.3
HK117A_1_9 H-Y 89 10440 87400 8.3 0.0625 1.92 0.00236 1.66 0.02028 2.51 0.60 0.000717 1.67 15.2 0.3 14.5 0.3
HK117A_1_10 L-Y 86 7850 84100 10.5 0.0637 1.88 0.00247 1.74 0.02169 2.54 0.61 0.000739 1.62 15.9 0.3 14.9 0.3
HK117A_1_11 L-Y 77 6780 71100 10.4 0.0633 1.90 0.00249 1.61 0.02165 2.31 0.56 0.000767 1.56 16.0 0.3 15.5 0.2
HK117A_1_12 L-Y 119 13430 107800 7.9 0.1302 2.84 0.00269 2.60 0.0482 3.94 0.74 0.000804 2.86 17.3 0.5 16.2 0.5
HK117A_1_13 L-Y 79 9850 75300 7.6 0.1815 2.75 0.00269 2.52 0.0677 3.99 0.70 0.000765 2.35 17.3 0.4 15.5 0.4
HK117A_1_14 H-Y 87 7170 83000 11.8 0.066 2.27 0.00253 1.78 0.02284 3.02 0.62 0.000755 1.72 16.3 0.3 15.3 0.3
HK117A_1_15 H-Y 94 6920 88100 12.6 0.0637 1.88 0.0025 1.64 0.02203 2.63 0.67 0.000759 1.58 16.1 0.3 15.3 0.2
HK117A_2_1 H-Y,R 66 10380 68600 6.6 0.0605 1.82 0.00225 1.74 0.01861 2.36 0.60 0.000702 1.85 14.5 0.3 14.2 0.3
HK117A_2_2 H-Y,R 72 7920 69800 8.8 0.0636 2.20 0.00239 1.38 0.02106 2.52 0.41 0.000743 1.62 15.4 0.2 15.0 0.2
HK117A_2_3 H-Y,R 58 4350 48000 10.9 0.3212 1.56 0.00339 2.18 0.1496 3.07 0.89 0.00089 2.47 21.8 0.5 18.0 0.4
HK117A_2_4 H-Y,R 103 7680 91400 12.0 0.1161 1.55 0.00276 1.70 0.0448 2.68 0.63 0.000809 1.61 17.8 0.3 16.3 0.3
HK117A_2_5 H-Y,R 111 8730 105400 12.1 0.0648 1.70 0.00252 1.67 0.02248 2.40 0.63 0.000761 1.71 16.2 0.3 15.4 0.3
HK117A_2_6 H-Y,R 109 7840 102700 13.2 0.0721 1.66 0.00256 1.84 0.02527 2.22 0.56 0.00077 1.82 16.5 0.3 15.6 0.3
HK117A_2_7 H-Y,R 115 8000 104400 13.2 0.1053 2.56 0.00266 1.69 0.0384 3.13 0.58 0.000781 1.79 17.1 0.3 15.8 0.3
HK117A_2_8 H-Y,R 77 5570 67700 12.2 0.1938 2.79 0.00293 1.81 0.0785 3.31 0.71 0.000811 1.85 18.9 0.3 16.4 0.3
HK117A_2_9 L-Y,C 90 6200 81800 13.3 0.0837 2.75 0.00263 1.90 0.0302 3.31 0.51 0.000796 1.88 16.9 0.3 16.1 0.3
HK117A_2_10 L-Y,C 99 6380 83500 13.2 0.1767 1.53 0.00303 1.65 0.0742 2.43 0.72 0.00085 1.53 19.5 0.3 17.2 0.3
HK117A_2_11 L-Y,C 80 6350 73900 11.7 0.0649 2.31 0.0025 1.72 0.02256 2.97 0.49 0.000776 1.80 16.1 0.3 15.7 0.3
HK117A_2_12 L-Y,C 74 8510 71800 8.5 0.0634 2.05 0.00237 1.52 0.02089 2.73 0.66 0.000746 1.47 15.2 0.2 15.1 0.2
HK117A_2_13 H-Y,R 52 5540 44800 8.1 0.1642 3.84 0.00276 2.03 0.0631 4.75 0.65 0.000834 2.04 17.8 0.4 16.8 0.3
HK117A_2_14 L-Y,C 85 6880 80600 11.7 0.0655 2.29 0.00248 1.61 0.0226 3.05 0.56 0.000772 1.55 16.0 0.3 15.6 0.3
HK117A_2_15 L-Y,C 39 3520 35000 9.9 0.2518 1.67 0.00294 2.25 0.1018 2.75 0.82 0.000822 2.07 18.9 0.4 16.6 0.4
HK117A_2_16 L-Y,C 97 6530 78400 11.9 0.2521 1.98 0.00342 2.25 0.117 3.59 0.84 0.000908 2.31 22.0 0.5 18.4 0.4
66

Analysis Position
Pb U Th
Th/U
207 Pb/ 206 2σ
Pb
206 Pb/ 238 2σ
U
207 Pb/ 235 U b 2σ
Rho c 208 Pb/ 232 2σ
206 Pb/ 238 U 2σ
208 Pb/ 232 Th 2σ
Th
(ppm) a (ppm) a (ppm) a (%) (%) (%) (%) (Ma) d abs (Ma) e abs
HK117A_2_17 L-Y,C 100 7930 95700 12.0 0.0677 2.22 0.00242 1.69 0.02301 3.17 0.57 0.000768 1.69 15.6 0.3 15.5 0.3
HK117A_2_18 L-Y,C 95 8330 90700 10.8 0.0668 2.25 0.00243 1.57 0.02229 2.96 0.53 0.000782 1.53 15.6 0.3 15.8 0.2
HK117A_2_19 L-Y,C 82 8390 77300 9.1 0.104 1.73 0.00251 1.87 0.03584 2.51 0.66 0.00079 1.77 16.2 0.3 16.0 0.3
HK117A_2_20 L-Y,C 122 11690 113700 9.7 0.068 2.35 0.00236 1.91 0.02235 3.49 0.60 0.000771 1.95 15.2 0.3 15.6 0.3
HK117A_3_1 H-Y,R 33 3100 26600 8.5 0.226 1.59 0.0027 1.89 0.0836 2.75 0.75 0.00093 1.94 17.4 0.3 18.8 0.4
HK117A_3_2 H-Y,R 34 2999 28320 9.3 0.2621 2.40 0.00302 2.82 0.1086 4.05 0.88 0.000894 2.68 19.4 0.6 18.1 0.5
HK117A_3_3 H-Y,R 40 3466 31890 9.1 0.2864 1.78 0.00319 1.85 0.1245 2.81 0.88 0.000937 1.92 20.5 0.4 18.9 0.4
HK117A_3_4 L-Y,C 110 12030 99790 8.3 0.0649 2.31 0.00226 1.19 0.02005 2.59 0.44 0.000799 1.38 14.6 0.2 16.1 0.2
HK117A_3_5 L-Y,C 118 13120 106600 8.1 0.0677 2.51 0.00226 1.51 0.02089 2.82 0.40 0.000798 1.63 14.5 0.2 16.1 0.3
HK117A_3_6 L-Y,C 116 14620 103700 7.1 0.0661 2.27 0.00224 1.34 0.02039 3.04 0.35 0.000802 1.37 14.5 0.2 16.2 0.2
HK117A_3_7 L-Y,C 114 13220 101500 7.7 0.0665 2.71 0.00225 1.69 0.02031 3.45 0.51 0.0008 1.88 14.5 0.3 16.2 0.3
HK117A_3_8 L-Y,C 104 10550 90800 8.6 0.0678 2.51 0.00227 1.41 0.02089 2.87 0.32 0.000813 1.48 14.6 0.2 16.4 0.2
HK117A_3_9 L-Y,C 130 12600 116900 9.3 0.0757 1.98 0.00222 1.31 0.02282 2.19 0.43 0.000785 1.13 14.3 0.2 15.9 0.2
HK117A_3_10 H-Y,R 125 14760 108900 7.3 0.0767 1.56 0.0022 1.54 0.02286 2.01 0.58 0.000794 1.64 14.2 0.2 16.0 0.3
HK117A_4_1 L-Y,C 154 9430 140300 14.9 0.0613 4.40 0.00239 1.76 0.02001 4.80 0.24 0.000765 1.83 15.4 0.3 15.5 0.3
HK117A_4_2 L-Y,C 146 8900 132500 15.0 0.1352 5.47 0.0026 1.69 0.0476 6.51 0.63 0.000774 1.42 16.7 0.3 15.6 0.2
HK117A_4_3 L-Y,C 109 10680 101900 9.4 0.0568 1.94 0.00222 1.31 0.01716 2.16 0.26 0.000757 1.31 14.3 0.2 15.3 0.2
HK117A_4_4 L-Y,C 154 9600 145100 15.2 0.0527 2.09 0.00231 1.34 0.01679 2.62 0.33 0.00076 1.30 14.9 0.2 15.4 0.2
HK117A_4_5 L-Y,C 147 9350 139500 15.0 0.0527 2.66 0.00231 1.21 0.01667 2.76 0.17 0.000746 1.29 14.9 0.2 15.1 0.2
HK117A_4_6 H-Y,R 90 11740 84500 7.2 0.076 4.34 0.00227 1.54 0.0235 4.68 0.49 0.000759 1.45 14.6 0.2 15.3 0.2
HK117A_4_7 H-Y,R 119 16630 115100 6.9 0.05816 1.44 0.00225 1.69 0.01792 1.62 0.60 0.000749 1.47 14.5 0.2 15.1 0.2
HK117A_4_8 L-Y,C 119 8570 114900 13.4 0.058 2.41 0.00234 1.33 0.01883 2.66 0.31 0.000751 1.33 15.0 0.2 15.2 0.2
HK117A_4_9 L-Y,C 115 8730 110700 12.8 0.063 1.90 0.00235 1.36 0.02031 2.36 0.56 0.000753 1.46 15.2 0.2 15.2 0.2
HK117A_4_10 L-Y,C 116 8680 110900 12.9 0.0743 2.42 0.00239 1.46 0.02476 3.31 0.65 0.000758 1.45 15.4 0.2 15.3 0.2
HK117A_4_11 L-Y,C 63 4356 57900 13.3 0.1852 3.67 0.00284 1.69 0.0724 4.97 0.75 0.000785 1.53 18.3 0.3 15.9 0.2
HK117A_4_12 L-Y,C 113 14490 108300 7.5 0.0849 5.42 0.00245 1.39 0.0289 6.23 0.56 0.00076 1.45 15.8 0.2 15.4 0.2
HK117A_4_13 L-Y,C 139 10750 131700 12.3 0.0544 2.02 0.00244 1.31 0.01849 2.16 0.41 0.000749 1.47 15.7 0.2 15.1 0.2
HK117A_4_14 L-Y,C 130 13100 125400 9.6 0.06393 1.42 0.00241 1.33 0.02144 2.10 0.65 0.000742 1.35 15.5 0.2 15.0 0.2
HK117A_4_15 L-Y,C 133 12510 127200 10.2 0.0612 1.80 0.00242 1.36 0.0205 2.34 0.57 0.000746 1.34 15.6 0.2 15.1 0.2
HK117A_6_1 H-Y,R 69 20490 79800 4.0 0.05789 1.59 0.00194 1.39 0.01563 2.11 0.53 0.000611 1.46 12.5 0.2 12.4 0.2
HK117A_6_2 H-Y,R 76 20060 86500 4.4 0.0599 2.00 0.00199 1.31 0.01649 2.49 0.35 0.000627 1.36 12.8 0.2 12.7 0.2
HK117A_6_3 H-Y,R 82 19660 87500 4.5 0.0613 1.63 0.00209 1.39 0.01769 2.15 0.55 0.000662 1.51 13.5 0.2 13.4 0.2
67

Analysis
Position
Pb U Th
Th/U
207 Pb/ 206 2σ
Pb
206 Pb/ 238 2σ
U
207 Pb/ 235 U b 2σ
Rho c 208 Pb/ 232 2σ
206 Pb/ 238 U 2σ
208 Pb/ 232 Th 2σ
Th
(ppm) a (ppm) a (ppm) a (%) (%) (%) (%) (Ma) d abs (Ma) e abs
HK117A_6_4 H-Y,R 108 14880 101900 7.0 0.0627 2.07 0.00242 1.32 0.02116 2.41 0.41 0.000743 1.48 15.6 0.2 15.0 0.2
HK117A_6_5 H-Y,R 109 15870 103100 6.6 0.0661 2.27 0.00241 1.29 0.02233 2.96 0.35 0.000744 1.32 15.5 0.2 15.0 0.2
HK117A_6_6 L-Y,C 75 19060 82400 4.4 0.1051 2.47 0.00204 1.52 0.02943 2.85 0.48 0.000641 1.56 13.2 0.2 13.0 0.2
HK117A_6_7 H-Y,R 71 16850 75600 4.6 0.1395 2.44 0.00209 1.39 0.0398 2.51 0.59 0.00066 1.41 13.5 0.2 13.3 0.2
HK117A_6_8 H-Y,R 49 10080 44500 4.5 0.2441 1.31 0.0024 1.63 0.0814 2.09 0.82 0.000795 1.76 15.4 0.3 16.1 0.3
HK117A_6_9 L-Y,C 132 21580 129200 6.1 0.0591 1.69 0.0023 1.43 0.01892 2.27 0.44 0.000723 1.52 14.8 0.2 14.6 0.2
HK117A_6_10 L-Y,C 105 13490 98500 7.4 0.0653 1.99 0.00238 1.60 0.02134 2.39 0.52 0.000747 1.47 15.3 0.2 15.1 0.2
HK117A_7_1 H-Y,R 78 6800 22000 3.3 0.0453 7.95 0.0072 22.22 0.045 24.44 0.98 0.00225 ### 46.0 ## 45.0 ##
HK117A_7_2 H-Y,R 34 3150 20600 6.5 0.468 3.21 0.00373 4.29 0.243 4.94 0.77 0.001233 5.68 24.0 1.1 24.9 1.4
HK117A_7_3 H-Y,R 155 12910 154900 11.9 0.0658 1.82 0.00226 1.37 0.02055 2.29 0.58 0.000709 1.41 14.5 0.2 14.3 0.2
HK117A_7_4 H-Y,R 225 15220 220200 14.3 0.0578 1.45 0.00238 1.34 0.01906 1.99 0.43 0.000732 1.22 15.4 0.2 14.8 0.2
HK117A_7_5 H-Y,R 140 9000 140600 15.4 0.069 2.46 0.00231 1.73 0.02197 2.64 0.42 0.000711 1.69 14.9 0.3 14.4 0.3
HK117A_7_6 L-Y,C 105 12870 124100 9.5 0.0813 2.46 0.0019 1.84 0.02151 2.65 0.52 0.000617 1.94 12.3 0.2 12.5 0.3
HK117A_7_7 L-Y,C 114 9370 98800 10.5 0.262 1.49 0.00293 2.12 0.1059 2.64 0.82 0.00083 2.05 18.9 0.4 16.8 0.3
HK117A_7_8 L-Y,C 179 12040 176400 14.6 0.0596 1.85 0.00237 1.69 0.01928 2.54 0.68 0.000725 1.52 15.3 0.3 14.7 0.2
HK117A_7_9 L-Y,C 164 11300 155500 13.7 0.2 3.50 0.00271 1.96 0.0752 4.92 0.86 0.000761 1.58 17.5 0.3 15.4 0.3
HK117A_7_10 L-Y,C 121 9780 126400 12.9 0.0668 2.25 0.00213 1.60 0.01952 2.72 0.39 0.000683 1.61 13.7 0.2 13.8 0.2
HK117A_9_1 H-Y,R 75 2790 16100 5.7 0.6544 0.76 0.01054 2.28 0.951 2.52 0.97 0.00351 3.70 67.6 1.5 70.8 2.5
HK117A_9_2 H-Y,R 37 3610 23800 6.6 0.3195 2.57 0.00324 2.99 0.1426 4.56 0.82 0.001101 3.00 20.9 0.6 22.2 0.7
HK117A_9_3 H-Y,R 44 4590 31500 6.9 0.298 1.58 0.00302 2.81 0.1237 2.75 0.82 0.001015 3.05 19.5 0.5 20.5 0.6
HK117A_9_4 H-Y,R 20 901 4290 4.8 0.5802 1.55 0.00847 4.84 0.677 5.61 0.95 0.0034 6.47 54.4 2.7 68.6 4.4
HK117A_9_5 H-Y,R 112 9440 106900 11.4 0.06275 1.51 0.00235 1.79 0.02053 2.19 0.67 0.000736 1.77 15.1 0.3 14.9 0.3
HK117A_9_6 H-Y,R 122 10470 118400 11.4 0.0608 1.97 0.00237 1.31 0.01989 2.51 0.50 0.000731 1.35 15.3 0.2 14.8 0.2
HK117A_9_7 L-Y,C 122 10360 113100 11.0 0.0604 1.82 0.00243 1.69 0.02015 2.73 0.65 0.000758 1.58 15.7 0.3 15.3 0.2
HK117A_9_8 L-Y,C 114 9950 105400 10.6 0.0631 2.06 0.00244 1.56 0.02123 2.64 0.62 0.000762 1.57 15.7 0.3 15.4 0.2
HK117A_9_9 L-Y,C 116 10470 108800 10.5 0.0629 1.75 0.00242 1.45 0.02102 2.09 0.44 0.000748 1.60 15.6 0.2 15.1 0.2
HK117A_9_10 L-Y,C 117 10530 108200 10.3 0.0624 1.60 0.00246 1.63 0.02154 2.46 0.65 0.000754 1.72 15.8 0.3 15.2 0.3
HK117A_9_11 L-Y,C 104 10470 97400 9.3 0.0633 1.90 0.00242 1.69 0.02089 2.35 0.58 0.000742 1.62 15.6 0.3 15.0 0.3
HK117A_9_12 L-Y,C 117 11150 108900 9.8 0.0651 1.84 0.00251 1.71 0.02243 2.32 0.61 0.000763 1.57 16.2 0.3 15.4 0.2
_______________________________________________________________________________________________________________________________________________
Analysis column sample numbers refer to: SAMPLE_GRAIN_SPOT. H-Y,M-Y,L-Y refer to high, medium, and low yttrium, respectively. C and R indicate core
and rim, respectively. Analyses stroked-through were not included in final age interpretation. (See next page for additional table details).
68

a concentration data are normalized to the primary reference material and are accurate to ~10%
b 207Pb/235U calculated assuming a natural 235U/238U ratio of 137.88
c Rho value is calculated following the method outlined in Paton et al. (2010)
d Age calculations based on the decay constants of Jaffey et al. (1971)
e Age calculations based on the decay constant of Amelin & Zaytsev (2002)
____________________________________________________________________________________
metamorphic event with a maximum age population at ~19 Ma. These ages are roughly
characterized by a ~ 21 -18 Ma peak of Y-poor cores and a ~18 -16 Ma peak of Y-rich rims.
For sample HK109, a total of 54 meaningful analyses were obtained on seven monazite
grains (Fig. 2.17, Table 2.1). These data span between ~19 -13 Ma, and outline two distinct age
populations associated with Y / Th compositional zoning. A peak between ~16 -19 Ma consists of
Y-poor cores, and a second peak of similar intensity between ~13 -16 Ma is characterized by Y-
rich rims. The only exception to this are three outlying ages obtained from monazite grain
HK109_5, at ~37 Ma, 124 Ma and 390 Ma (Table 2.1). These data are not displayed in Figure
2.17.
For sample HK117, a total of 54 meaningful analyses were obtained on seven monazite
grains (Fig. 2.17, Table 2.1). These data yield a main age population at ~15 Ma, and a minor
distribution of ages between ~14 -12 Ma. The main age population consists of a high frequency
of 16 -15 Ma Y-poor cores and a lower frequency of 16 -14 Ma Y-rich rims. Ages between 14 -12
Ma are derived from a mix of Y-rich rims and Y-poor cores.
2.6.1.3 Summary of U-Th-Pb geochronology
Our data reveal a span of 208 Pb/ 232 Th ages from ~26 to 12 Ma, with peaks at 19 Ma and
15 Ma (Fig. 2.17). All samples yield one dominant age range. Sample KH18a shows an additional
minor set of older ages, and sample HK117a reveals a low frequency of younger ages. All ages
69

are derived from intensely deformed, elongate and/or rounded monazite grains that are typically
aligned parallel to the dominant ESE-WNW trending lineation (Fig. 2.16).
For samples KH18a and HK109, distinct older and younger age groups correspond to Y-
poor cores and Y-rich rims, respectively (Figs. 2.16, 2.17). As garnet has a higher partition
coefficient for Y than does monazite, it essentially acts as a Y sink during crystallization (Foster
et al., 2002). Therefore, older age groups associated with Y-poor monazite cores are interpreted to
correspond to a period of simultaneous garnet growth and monazite crystallization. Younger age
groups associated with Y-rich rims are interpreted to reflect monazite crystallization concurrent
with garnet breakdown.
Although the correlation between Y zonation and age domains is generally consistent,
older and younger age groups do not always correspond with analyses of Y-poor cores and Y-rich
rims, respectively (Fig. 2.17, Table 2.1). In certain cases, the analysis of one Y domain may yield
ages expected from the other. This crossover of data is due to mixing of Y zones and associated
age domains during analysis. Laser ablation generates craters with a diameter of ~7 μm and a
depth of ~5-6 μm. Thus, when analyzing elemental zones within in situ grains, it would be
expected that analyses occasionally include underlying zones of different composition and age.
This would lead to a mixing of age domains and a continuous spread of data between age peaks.
Sample HK109 yielded significantly older Eocene, Mesozoic and Paleozoic ages (Table
2.1). The Eocene age of ~37 Ma is interpreted to reflect Eo-Himalayan metamorphism associated
with Eocene-Oligocene crustal thickening (Godin et al., 1999; 2001). The Paleozoic age of ~390
Ma is interpreted to represent a pre-Himalayan metamorphic event, or a mixture between
Paleozoic and younger age domains (e.g. Godin et al., 2001). The Mesozoic age of ~124 Ma is
70

derived from a Y-rich monazite rim, and is interpreted as a mixture between Paleozoic and
younger age domains. These older ages suggest that a portion of this grain was inherited.
2.6.2 40 Ar/ 39 Ar thermochronology
Samples rich in muscovite were collected from traverses across and along the upper
Karnali valley. Mineral separates were obtained through standard crushing and sieving techniques
and muscovites were hand picked under a binocular macroscope. Only grains with good clarity,
greater than or equal to 500 μm in diameter, and void of inclusions and intergrowth were selected
for thermochronology. The 40 Ar/ 39 Ar thermochronology was performed at Queen's University
Argon Geochronology laboratory on muscovite grains from 19 samples located throughout
domain III, IV, V and VI.
Sample HK 125 is taken from an ultra-high-strain pelitic schist of domain III (Fig. 2.3).
Within this sample, intensely stretched and deformed muscovite represent ~10 vol%, and define
the mineral elongation lineation and foliation. Muscovite porphyroclasts show abundant
deformation structures, including mica fish, micro-boudinaging, and undulatory extinction (Fig.
2.5e). No grains within this sample show any indication of recrystallization.
Sample KH13 and samples KH18, KH35, HK131c, HK138 are taken from a quartzite
and from pelitic schist of domain IV, respectively (Fig. 2.3). Within these samples well-formed
muscovite grains represent ~25 vol%, define a pervasive mineral elongation lineation and
foliation, and form distinct C/S/C’ fabrics. Shear bands offset coarse-grained muscovite, while
fine-grained muscovites grow along these same shear planes (Fig. 2.6d). Muscovite located
within these shear bands may be somewhat recrystallized; however by only selecting grains with
diameters ≥ 500 μm we ensured that thermochronology was not performed on any recrystallized
grains.
71

Samples KH10, HK102a, HK105, HK115, HK142 are taken from schist and gneiss of
domain V (Fig. 2.3). Within these samples, well-formed muscovite grains and muscovite
aggregates represent ~15 vol% and define a pervasive to moderate mineral elongation lineation
and foliation. Locally, muscovite grains form C/S/C’ fabrics and mantle rotated sigma-style
quartz and feldspar porphyroclasts (Fig. 2.7b). Shear bands offset coarse-grained muscovite,
while minor amounts of fine-grained and potentially recrystallized muscovite grow along these
same shear planes. Again, only muscovite grains ≥ 500 μm in diameter were selected to avoid
analyzing recrystallized grains.
Samples HK117a, HK118a and HK119a are taken from migmatitic gneiss of domain VI
(Fig. 2.3). Within these samples, muscovite represents ~5 vol% and is characterized by ragged
and serrate edges. Muscovite grains are both randomly oriented and aligned roughly parallel to
the foliation. Aggregates of muscovite are moderately aligned parallel to the mineral lineation
when present. These textural relationships suggest that muscovite grains within these three
samples are not in equilibrium with their host rocks.
2.6.2.1 Methodology
Muscovite separates and standards (used to monitor flux) were packaged in Al-foil,
loaded into an 8.5 cm long and 2.0 cm diameter irradiation capsule and irradiated with fast
neutrons in position 5C for a duration of 5 hours at 3 MWh at the McMaster Nuclear Reactor
(Hamilton, Ontario). Packets of flux monitors were located at ~ 1 cm intervals along the
irradiation container and J-values for individual samples were determined by second-order
polynomial interpolation between replicate analyses of splits for each monitor position in the
capsule. Typically, J-values vary by < 10% over the length of the capsule. No attempt is made to
monitor horizontal flux gradients as these are considered to be minor in the core of the reactor.
72

Analyses were performed in two separate sessions. For each analytical session, a CO 2
laser system was used for total fusion of monitors and step-heating of samples. Between 4 and 15
irradiated muscovite grains per sample were loaded into pits of a copper sample-holder, which
was then mounted beneath a ZnS view-port of a small, stainless-steel chamber connected to an
ultra-high vacuum purification system. For step-heating in the first analytical session, the laser
beam of a 30W New Wave Research MIR 10-30 CO 2 laser was defocused to 2 mm to cover the
entire sample. For step-heating in the second analytical session, the laser beam of an identical
laser was equipped with a faceted lens to allow for uniform heating of the entire sample. Heating
periods were ~3 -4 minutes at increasing percent power settings. The evolved gas, after
purification using an SAES C50 getter (~ 5 minutes), was admitted to an on-line, MAP 216 mass
spectrometer, with a Bäur Signer source and an electron multiplier (set to a gain of 100 over the
Faraday). Blanks, measured routinely, were subtracted from the subsequent sample gas-fractions.
The extraction blanks are typically < 10 x 10 -13 , < 0.5 x 10 -13 , < 0.5 x 10 -13 , and < 0.5 x 10 -13 cm -3
STP for masses 40, 39, 37, and 36, respectively.
Measured argon-isotope peak heights were extrapolated to zero-time, normalized to the
40 Ar/ 36 Ar atmospheric ratio (295.5) using measured values of atmospheric argon, and corrected
for neutron-induced 40 Ar from potassium, 39 Ar and 36 Ar from calcium, and 36 Ar from chlorine
(Roddick, 1983). Dates and errors were calculated using ISOPLOT 3.75 (Ludwig, 2012), and the
constants of Steiger and Jäger (1977). Errors shown in the tables and on the age plateau diagrams
represent the analytical precision at 2σ, assuming that the errors in the ages of the flux monitors
are zero. This is suitable for comparing within-spectrum variation and determining which steps
form a plateau (e.g., McDougall and Harrison, 1988, p, 89). A conservative estimate of this error
in the J-value is 0.5% and can be added for inter-sample comparison. The dates and J-values for
73

the intralaboratory standard (MAC-83 biotite at 24.36 Ma; Sandeman et al., 1999) are referenced
to FCT sanidine at 28.02 Ma (Renne et al., 1998) for the first analytical session, and TCR
sanidine at 28.34 Ma for the second analytical session. Sample KH35 was used as an internal
standard in both analytical sessions, and yielded near identical ages.
Well-defined, and moderately-defined age plateaus are defined as a minimum of three
consecutive steps with ages of overlapping error, releasing 90% or more and 40% or more, of the
total 39 Ar, respectively.
2.6.2.2 Results
Of the 19 samples selected for 40 Ar/ 39 Ar age determination, 10 yielded well-defined age
plateaus, and 4 yielded moderately-defined age plateaus (Fig. 2.18, Table 2.2, see Appendix B for
a complete data set, including inverse isochron diagrams). The remaining samples did not yield
acceptable age plateaus, resulting in a mixture of chaotic plateaus, plateaus with insufficient total
percent argon released, and continuously rising and or descending step-heating profiles
(Appendix D).
Sample HK125 of domain III yielded a well-defined plateau age of 13.01 ± 0.30 Ma
corresponding to 92.8% of released 39 Ar (Fig. 2.18, Table 2.2). Analyzed muscovite grains define
a shallow west plunging lineation and show extensive post-crystallization deformation.
Samples KH13 and KH18 of domain IV yielded moderately-defined plateau ages (Fig. 2.18,
Table 2.2). Analyzed muscovite grains define a shallow ESE-WNW lineation and are offset by
shear bands. Sample KH13 yielded an age of 14.25 ± 0.23 Ma corresponding to 42.3% of
released 39 Ar. Sample KH18 was analyzed in two separate analytical sessions and yielded ages of
13.52 ± 0.16 Ma and 12.79 ± 0.53 Ma, corresponding to 43.0% and 69.5% of released 39 Ar,
74

Figure 2.18 Muscovite 40 Ar/ 39 Ar step-heating plateaus grouped together within their respective
lithotectonic domains. Plateau ages are calculated from grey steps. All errors are reported at 2σ. Plots were
constructed with Isoplot 3.75 (Ludwig, 2012). Additional details can be found in Table 2.2.
75

Figure 2.18 – continued.
76

Table 2-2 40 Ar/ 39 Ar thermochronological data
Sample Domain Rock Type Condition of Muscovite
Plateau Age
(Ma)
Error
(Ma, 1s)
Percent 39 Ar
defining
plateau
MSDW
Probability
HK125 III
Ultra-high-strain pelitic
schist
Deformed: faulted, fractured,
undulatory extinction 13.01 0.30 92.8 0.33 0.96
KH13 IV High-strain quartzite Well-formed, offset by shear bands 14.25 0.23 42.3 1.06 0.38
KH18 (1)* IV High-strain pelitic schist Well-formed, offset by shear bands 13.52 0.16 43.0 0.71 0.62
KH18 (2)* IV High-strain pelitic schist Well-formed, offset by shear bands 12.79 0.53 69.5 0.65 0.62
KH35 (1)* IV High-strain pelitic schist
Well-formed, offset by shear bands,
minor undulatory extinction 13.03 0.17 100.0 0.40 0.96
KH35 (2)* IV High-strain pelitic schist
Well-formed, offset by shear bands,
minor undulatory extinction 13.00 0.45 100.0 0.41 0.90
HK131c IV High-strain pelitic schist Well-formed, offset by shear bands 11.61 0.25 100.0 0.65 0.73
HK138 IV High-strain pelitic schist
Well-formed, offset by shear bands,
minor undulatory extinction 12.29 0.30 100.0 0.27 0.98
HK102a V High-strain gneiss Well-formed 9.27 0.19 54.2 2.10 0.12
HK115 V High-strain gneiss
Well-formed, occasionally offset by
shear bands 15.74 0.28 60.7 0.73 0.48
KH10 V High-strain gneiss
Well-formed, mantles fsp and qtz
porphyroclasts 12.08 0.09 99.4 1.30 0.22
HK105b V High-strain pelitic schist Well-formed 9.90 0.28 100.0 0.20 0.99
HK142 V High-strain gneiss Well-formed 10.16 0.33 100.0 0.19 0.99
HK117a VI Migmatitic gneiss
Ragged and serrate grain boundaries,
possible replacement textures 11.57 0.25 100.0 1.60 0.14
HK118a VI Migmatitic gneiss
HK119a VI Migmatitic gneiss
Small, ragged and serrate grain
boundaries, possible replacement
textures 10.81 0.16 100.0 0.38 0.93
Small, ragged and serrate grain
boundaries, possible replacement
textures 11.39 0.25 100.0 0.46 0.88
*Multiple analyses of individual samples performed in separate analytical sessions have different J-value and are therefore reported as independent analyses (i.e. 1, 2)
Data in italics indicate moderately-defined plateaus corresponding to ~ 40-70% of total 39 Ar released
77

espectively. Note that the plateaus of KH18 cannot be combined into a single plateau age
because they are associated with J-values of their respective analytical session.
Samples KH35, HK131c, and HK138 of domain IV yielded well-defined plateau ages
(Fig. 2.18, Table 2.2). Analyzed muscovite grains define a shallow ESE-WNW lineation and are
offset by shear bands. Sample KH35 was analyzed in both analytical sessions as an internal
laboratory
standard, and yielded ages of 13.03 ± 0.17 Ma and 13.00 ± 0.45 Ma, both corresponding to 100%
of released 39 Ar. Note that plateaus of KH35 cannot be combined into a single plateau age
because they are associated with different J-values of their respective analytical session. Samples
HK131c and HK138 yielded ages of 11.61 ± 0.25 Ma and 12.29 ± 0.30 Ma, respectively. Both
ages correspond to 100% of released 39 Ar.
Samples HK102a and HK115 of domain V yield moderately-defined plateau ages (Fig.
2.18, Table 2.2). Analyzed muscovite grains define a shallow ESE-WNW lineation and are offset
by occasional shear bands. Sample HK102a and HK115 yield ages of 9.27 ± 0.19 Ma and 15.74 ±
0.28 Ma, corresponding to 54.2% and 60.7% of released 39 Ar, respectively.
Samples KH10, HK105, and HK142 of domain V yield well-defined plateau ages (Fig.
2.18, Table 2.2). Analyzed muscovite and muscovite aggregates are present in non-laterally
continuous layers and define a shallow ESE-WNW lineation. From WNW to ESE, samples
KH10, HK142 and HK105 yield ages of 12.08 ± 0.09 Ma, 10.16 ± 0.33 Ma and 9.90 ± 0.28 Ma,
corresponding to 99.4%, 100% and 100% of released 39 Ar, respectively.
Samples HK117a, HK118a and HK119a of domain VI yield well-defined plateau ages
(Fig. 2.18, Table 2.2). Analyzed muscovite grains are not pristine and exhibit ragged and serrate
boundaries, possibly indicative of disequilibrium conditions. From south to north, samples
78

HK117a, HK119a and HK118a yield ages of 11.57 ± 0.25 Ma, 11.39 ± 0.25 Ma and 10.81 ± 0.16
Ma, respectively. All ages correspond to 100% of released 39 Ar.
2.6.2.3 Summary of 40 Ar/ 39 Ar thermochronology
Our thermochronology results provide ages at which muscovite passed through the argon
closure temperature, commonly reported to be 350°C (Hodges, 1991). However, recent studies
have shown argon diffusion in muscovite to cease at much higher temperatures, corresponding to
hotter closure temperatures generally in the range of ~425°C (Harrison et al., 2009), and up to
500-600°C (see review in Villa, 2004). The closure temperature of muscovite in the upper Karnali
valley is estimated to be ~450°C (for pressures of ~5 kbar, muscovite grain diameters of ~500
μm, and a cooling rate of ~60°C/Ma, based of Fig. 7 in Harrison et al., 2009). Based upon the
lack of recrystallization textures observed in the analyzed muscovite of the upper Karnali valley,
we interpret plateau ages obtained in this study to represent muscovite cooling ages.
Based exclusively upon well-defined plateau ages, the GHS of the upper Karnali valley
cooled through ~450°C between ~13 -10 Ma (Fig. 2.18, Table 2.2). If moderately-defined age
plateaus are also considered, then the cooling age of the GHS in this region may span from ~15 -9
Ma (Fig. 2.18, Table 2.2).
Plots of cooling ages along and across the GHS-TSS interface reveal younging trends to
the northeast and ESE (Fig. 2.19). If considering only well-defined age plateaus, cooling ages
young from ~12.5 Ma along the GHS-TSS interface, to ~10.5 Ma at 10 km structurally below the
interface towards the northeast (Fig. 2.19a). A linear regression of this trend yield a R 2 value of
~0.36. This R 2 value may appear low, however, considering the low frequency of data points, and
the distribution of data points along strike where other types of variation would also be expected,
79

Figure 2.19 Plots of muscovite cooling ages against (a) increasing structural distance below the GHS-TSS
interface (measured perpendicular to average foliation), and (b) distance parallel to the GHS-TSS
interface. The line of origin for plot (b) is perpendicular to the GHS-TSS interface and intersects the
village of Tumkot (e.g. plot portrays evolution of muscovite cooling ages towards the southeast). Village
names are provided as reference. See text for description of well-defined and moderately-plateaus.
80

this correlation may be significant. A best-fit line through well-defined and moderately-defined
age plateaus exhibits a similar trend with a lower R 2 value.
Cooling ages also show a younging trend parallel to the GHS-TSS interface (Fig. 2.19b).
Cooling ages of all samples progress from ~14 -13 Ma along the WNW edge of the field area, to
~9 Ma within 25 km to the ESE. Linear regressions of well-defined plateaus, and of all plateaus,
yield R 2 values of ~0.38 and ~0.44, respectively. Although these R 2 values are low, considering
variations that would arise from the low frequency of data points, and the distribution of
data points from various structural levels along strike, this correlation may be significant.
Four of the samples analyzed yield moderately-defined age plateaus. Of these samples,
KH13 yields an upward-stepping profile, KH18 and HK115 yield downward-stepping profiles,
and HK102a yields a subtle U-shaped profile. The upward-stepping profile is interpreted to be the
result of overloading of the sample pit with an excess number of muscovite grains, leading to
uneven heating of all grains during analysis. As a consequence, 39 Ar that should have been
released during initial steps was released in the final steps, generating an upward stepping profile.
Downward-stepping and U-shaped profiles are attributed to the disruption of 39 Ar retention since
initial cooling. This may be the result of continued deformation below the argon closure
temperature in the upper Karnali valley.
2.7 Discussion
Here, data from this study are discussed with regard to south-directed extrusion of the
GHS, orogen-parallel extension and the transition between these two processes. A summary of
results is provided in Figure 2.20. These results are then combined with prior studies to assess the
applicability of the proposed transition on an orogen-wide scale, and to evaluate models
attempting to explain the processes responsible for orogen-parallel extension.
81

Figure 2.20 Summary of results from quartz CPO, vorticity, muscovite thermochronology and monazite geochronology analyses. D. I, II, III, IV, V and VI
refer to lithotectonic domains. Map area is identical to Figure 2.2.
82

2.7.1 South-directed extrusion of the GHS
2.7.1.1 Initial south-directed extrusion
Geologic mapping of the upper Karnali valley reveals low-grade sedimentary rocks of the
TSS juxtaposed along a south-dipping contact over strongly deformed and metamorphosed rocks
of the GHS (Figs. 2.3, 2.20). Mineral elongation lineations throughout the high-strain zones
mantling the GHS-TSS interface plunge shallowly to the ESE and WNW. The kinematics of
these zones show dominant strike-slip and minor dip-slip kinematics (Fig. 2.3). The high-strain
zones are focused along the GHS-TSS interface, but also run obliquely across the GHS
throughout the eastern segment of the field area (Fig. 2.3). These fabrics are inconsistent with the
STDS (Burchfiel and Royden, 1985; Burchfiel et al., 1992; Hodges, 2000), and confirm the
presence of the orogen-parallel, strike-slip-dominated GMH fault system described by Murphy
and Copeland (2005).
However, it is likely that exhumation of the GHS occurred along the STDS prior to
orogen-parallel deformation for two reasons:
(1) The kinematics of the upper Karnali are dominated by strike-slip and minor dip-slip
geometries, which do not provide an effective mechanism for exhuming the mid-crustal
rocks of the GHS. Considering an average lineation plunging ~17° towards 115˚, and an
initial depth of ~26 km for the GHS of the upper Karnali valley (calculated from P-T data
from Yakymchuk and Godin (2012) from samples adjacent to the village of Simikot, with
a geothermal gradient of ~25°C/km), the exhumation of mid-crustal rocks via exclusively
orogen-parallel geometries would have required the exhumation of rocks from ~85
surficial km to the WNW to recreate the GHS-TSS contact currently observed in the
upper Karnali valley.
83

(2) Previously published chronological constraints suggest that the GHS was undergoing
south-directed extrusion prior to the ~15 Ma onset of orogen-parallel extension (see
Godin, 2006b for review of ages, Cottle et al., 2009a,b). This is discussed in the
following section.
With this logic, the final juxtaposition of units in the upper Karnali valley should be a
combined result of south-directed extrusion of the GHS, followed by exhumation during orogenparallel
extension.
2.7.1.2 Timing of south-directed extrusion
Metamorphic monazite ages within the upper Karnali span from ~26 to 12 Ma, with the
highest frequency residing between 19 -15 Ma (Figs. 2.17, 2.20). Neohimalayan metamorphism
throughout the central to central-eastern Himalaya is bracketed between 25 -16 Ma and is
associated with sillimanite-grade metamorphism, extensive anatexis, and south-directed extrusion
of the GHS (Vannay and Hodges, 1996; Godin et al., 1999; 2001; Searle and Szulc, 2005; Cottle
et al., 2009b). Metamorphic ages within the GHS of the upper Karnali valley are consistent with
this time frame. We therefore interpret the monazite crystallization ages from the GHS of the
upper Karnali valley to be associated with south-directed extrusion of the GHS.
The majority of monazite grains show older low-Y cores mantled by younger high-Y
rims (Fig. 2.16, 2.17). As garnet has a high affinity for Y during crystallization, the quantity of Y
in the system (and hence in the monazite) can be considered as an inverse analogue for garnet
growth and prograde-metamorphism. In this regard, we interpret the transition from older low-Y
cores to younger high-Y rims to record the progression from peak to retrograde metamorphism
between ~18 -15 Ma. This time frame is likely concurrent with the final stages of south-directed
GHS extrusion, prior to the onset of orogen-parallel deformation. A study by Kellett et al. (2010)
84

along the STDS in Bhutan documented a similar age progression, in which the growth of 16-15
Ma Y-rich monazite rims are interpreted to record retrograde GHS metamorphism during final
stages of south-directed extrusion.
A minor amount of monazite crystallization occurred between 14 -12 Ma (Fig. 2.17).
These ages are derived from the structurally deepest sample (HK117) located in the migmatitic
gneiss of domain VI (Fig. 2.17, 2.20). High temperatures recorded at these structural levels (Fig.
2.15) may have resulted in a longer cooling history, allowing for protracted crystallization of
monazite, likely post-dating the cessation of south-directed extrusion of the GHS.
2.7.2 Orogen-parallel extension
2.7.2.1 ESE-WNW deformation and exhumation
Structural fabrics observed in the upper Karnali valley define a high-strain ESE-WNW
trending shear zone, associated with orogen-parallel extension (Figs. 2.3, 2.20). Deformation
temperatures record a progression from high-temperature ductile to lower-temperature ductile and
brittle deformation (Fig. 2.15). These data imply that the GHS was deformed during exhumation
at progressively shallower levels. Kinematic data indicate a weak dominance of dextral fabric
elements, a significant contribution of pure shear, and a potential deviation from plane strain
towards constriction (Fig. 2.15). Integrating these data suggests that within the upper Karnali
valley, ESE-WNW oriented extension at progressively lower temperatures was related to ongoing
transtension and orogen-parallel extension.
Overprinting relationships show a progression from high-temperature ductile to lowertemperature
ductile and brittle deformation, indicating progressive exhumation of the GHS at
lower structural levels. This relationship is best observed within the high-strain gneiss of domain
V. Within this domain, the dominant ESE-WNW oriented fabric (deformation T. of ~500-650°C;
85

Figs. 2.7b,d, 2.15) is locally overprinted by centimeter-thick mylonitic layers (deformation T. of
~ 350°C; Figs. 2.8d, 2.14), and brittle structures (Fig. 2.8a,b,c). Cataclastic deformation of
dynamically recrystallized quartz confirms a progression towards lower temperature, while the
injection of crosscutting pseudotachylite (Fig. 2.8b) implies high-strain seismic events occurred
during exhumation.
A temperature progression is also observed with increasing distance normal to the GHS-
TSS interface. Microstructural data show an increase in deformation temperatures of ESE-WNW
oriented fabrics from ~350°C along the GHS-TSS interface to ~650°C at structural depths below
~2.5 km (Figs. 2.15, 2.20). Similar increases in temperature are commonly associated with
telescoping and/or flattening of isotherms as a result of south-directed extrusion of the GHS (Law
et al., 2011 and references therein). Telescoping of isotherms may be possible during normal
faulting and core complex formation; however, it would require either penetrative pure shear
resulting in extensive vertical shortening or heterogeneous simple shear resulting in the
juxtaposition of different particle paths, or some combination of both (Law et al., 2011). These
processes are typically documented in channelized flow type scenarios, as they require high
temperatures and significant transport displacement (Law et al., 2011 and references therein).
We therefore interpret the rapid increase and high deformation temperatures recorded
within ESE-WNW oriented fabrics to be a remnant of telescoped and/or flattened isotherms
generated during south-directed extrusion of the GHS. This thermal inheritance attests to the
intimate temporal link between these two tectonic processes.
Ductile ESE-WNW oriented extension is characterized by significant contributions of
pure and simple shear. This is revealed through, (1) fabric elements that show a near even
distribution of shear senses, with a minor dominance of dextral strike-slip motion (Figs. 2.4, 2.5,
86

2.6, 2.7), and (2) dominantly symmetric quartz CPO fabrics (Figs. 2.12, 2.15). The presence of
asymmetric CPO patterns proximal to the GHS-TSS interface may imply that simple shear was
predominant along this boundary and pure shear, which resulted in WNW-ESE oriented
extension, dominated at lower structural levels. Transitions in CPO fabrics at depths of ~1 km
below the GHS-TSS interface may correspond to a deviation from plane strain towards
constriction (Figs. 2.12, 2.15).
Deformation during exhumation at progressively lower temperatures is also characterized
by pure shear (~59%), as revealed from a vorticity analysis of a crosscutting lower temperature
mylonitic layer (deformation T. of ~ 350°C; Figs. 2.8d, 2.14).
Although sparse, these data are consistent with transtensional deformation, in which noncoaxial
deformation parallel to the fault zone is coupled with coaxial extension oblique to the
fault zone. This simultaneous deformation would generate a bulk constrictional strain (Dewey,
2002), consistent with our observations. This detailed interpretation is also consistent with the
regional network of transtension proposed by Murphy and Copeland (2005).
2.7.2.2 Timing of ESE-WNW deformation and cooling
Intense internal deformation and alignment of monazite grains parallel to the main
lineation (Fig. 2.16) suggest that monazite underwent extensive post-crystallization deformation
and transposition. These grains are typically aligned parallel to, or overgrown by, ESE-WNW
oriented micas. Previously described age relationships suggest that the crystallization of monazite
rims was concurrent with the final stages of south-directed extrusion of the GHS. Therefore, the
deformation and alignment of monazite grains, and growth of ESE-WNW oriented micas should
post-date south-directed extrusion of the GHS and be synchronous with orogen-parallel
extension.
87

Thermochronology of muscovite grains defining the ESE-WNW lineation reveal that by
~15 to 9 Ma, the upper Karnali valley had cooled through ~450°C in a setting linked to orogenparallel
extension (Fig. 2.18, Table 2.2). If only well-defined muscovite age plateaus are
considered, then the cooling age of the upper Karnali valley can be constrained between ~13 to
10 Ma. Undulatory extinction and brittle deformation of these muscovite grains (Fig. 2.5e)
indicate that orogen-parallel extension was ongoing at progressively cooler temperatures.
Cooling ages of the upper Karnali valley show a mild younging trend with increasing
structural depth below the GHS-TSS interface (Fig. 2.19a). This is consistent with slightly
younger monazite crystallization ages at depth (sample HK117, Fig. 2.17), suggesting a
protracted cooling history at deep structural levels due to high temperatures.
Muscovite cooling ages also show a younging trend towards the southeast of the field
area (Fig. 2.19b), potentially due to a southeasterly propagating fault system. This hypothesis was
also proposed by Murphy and Copeland (2005) and is consistent with transtensional deformation
initiating near the Gurla Mandhata core complex and propagating towards the southeast.
2.7.3 Transition from south-directed extrusion to orogen-parallel extension
2.7.3.1 Upper Karnali valley
Our data reveal that the GHS of the upper Karnali valley has been progressively extruded
/exhumed in two distinct tectonic phases (Fig. 2.21):
1. Pre ~15 Ma south-directed extrusion. Monazite ages associated with this stage are
consistent with typical Neohimalayan metamorphism throughout the central to centraleastern
Himalaya (~25 -16 Ma, see prior discussion for references).
2. Orogen-parallel extension resulting in exhumation via oblique strike-slip faulting,
initiating at ~15 -13 Ma. Structures and deformation mechanisms suggest that this phase
88

Figure 2.21 Compilation of chronologic data and deformation temperatures from the upper Karnali valley.
Red (HK117a), and purple (KH18) boxes and arrows show cooling history of individual samples. Grey
vertical box is interpreted as the transition period between south-directed extrusion of the GHS and
orogen-parallel extension.
Box (1) represents previously published metamorphic ages associated with south-directed extrusion of the
GHS (see text for references). Monazite ages from this study are found in the youngest section of this
period.
Boxes (2) through to (6) represent WNW-ESE oriented fabrics and ages associated with orogen-parallel
deformation and extension. Deformation temperatures of these boxes are calculated via microstructures,
and ages are determined relative to one another (i.e. fabrics of box 5 deforms fabrics of box 4 at lower
temperatures). Thus, these boxes are intended to illustrate a trend, and their absolute temperature and age
constraints may vary from what is shown.
Box (2) is constrained between 500 -700°C through quartz recrystallization mechanisms (grain boundary
migration, chessboard extinction) and quart CPO opening angle deformation temperatures (samples
HK140, HK105). These fabrics formed at temperatures lower then the crystallization of monazite, host
deformed monazite, and are hotter then the closure temperature of muscovite. Therefore this box is older
than the cooling of muscovite and younger than the crystallization of monazite. Caption continued on
next page.
89

Figure 2.21 – continued –
Box (3) represents muscovite that define WNW-ESE fabrics. Closure temperatures are estimated at 450 ±
25°C (see text for reference), and ages calculated by 40 Ar/ 39 Ar thermochronology.
Box (4) is constrained between 350 -500°C through quartz recrystallization mechanisms (subgrain
rotation) and quart CPO opening angle deformation temperatures (samples HK131A, HK139D). These
fabrics formed at temperatures similar to the muscovite closure temperature, and therefore should be
roughly coeval with muscovite cooling ages.
Box (5) is constrained between 280 -400°C through quartz recrystallization mechanisms (bulging), quartz
CPO opening angle deformation temperature (sample HK124B), and brittly deformed muscovite.
Deformed muscovites indicate that these fabrics must be younger than muscovite cooling ages.
Box (6) represents brittle deformation at < 280 ± 30°C. Normal faults and cataclasite post-date and
deform all ductile fabrics, therefore they must be the youngest event.
_____________________________________________________________________________
initiated at high temperatures, and progressed to lower temperatures during ongoing
orogen-parallel deformation and exhumation (Fig. 2.21). Monazites grains associated
with south-directed extrusion are typically deformed and transposed parallel to orogenparallel
lineations throughout this phase.
Taken together, these data from the upper Karnali valley reveal a fundamental tectonic
transition from the cessation of south-directed extrusion of the GHS to onset of orogen-parallel
extension (Fig. 2.21). Based upon chronological constraints associated with these two phases, we
suggest this transition occurred between ~15 -13 Ma.
2.7.3.2 Himalayan transition
To characterize the transition between cessation of south-directed extrusion of the midcrust
and onset of orogen-parallel extension, the analysis of field areas containing both styles of
deformation is required. The majority of orogen-parallel deformation is manifested as upper
crustal east-trending strike-slip and N-S striking normal faults throughout Tibet and the northern
Himalaya (Armijo et al., 1986, 1989; Taylor et al., 2003; Taylor and Yin, 2009; Ratschbacher et
al., 2012). There are however, three migmatite-cored domes across the Himalaya that exhume
90

mid-crustal rocks in settings kinematically linked to extension. These domes are: the Leo Pargil
dome (Thiede et al., 2006; Leech, 2008), the Gurla Mandhata core complex (Murphy et al.,
2002), and the Ama Drime Massif (Jessup et al., 2008, Cottle et al., 2009a).
A compilation of chronologic data from across these domes with ages from the upper
Karnali valley suggests a common middle Miocene transition from cessation of south-directed
extrusion of the GHS to onset of orogen-parallel extension (Fig. 2.22). This transition is well
constrained in upper Karnali valley and in the Everest region between ~15 – 13 Ma (this study;
Jessup and Cottle, 2010). In contrast, the transitions in the Leo Pargil dome (data from Thiede et
al., 2006; Leech, 2008) and the Gurla Mandhata core complex (data from Murphy et al., 2002)
are less well constrained, but can still be estimated to be between ~19 – 16 Ma and ~17 – 13 Ma,
respectively.
The age of the transition is oldest in the Leo Pargil dome, located in the western
Himalaya. Although contentious, it can be argued that the collision between India and Asia was
diachronous, having initiated in the west and progressed eastward (Rowley, 1996; 1998). If this is
true, then it may be possible that this transition follows a similar temporal trend. Thus, the
transition within the Leo Pargil dome may be expected to initiate earlier than within gneiss-domes
located in the central and eastern Himalaya.
Based upon the following three criteria, we suggest that the transition from south-directed
extrusion of the GHS to onset of orogen-parallel extension occurred in the middle Miocene (~ 16
Ma) across the length of the orogen.
1. A common middle Miocene transition within migmatite-cored domes across the
Himalaya.
91

92
Figure 2.22 (a) Location of study area
and migmatite cored domes associated
with orogen-parallel extension. Red
lines represent faults associated with
exhumation of these domes, and
orogen-parallel extensional structures.
Modified after Styron et al. (2011). (b)
Compilation from west to east of
chronologic data from across these
domes and the upper Karnali valley.
See text for chronology references.
Abbreviations: ITSZ – Indus-Tsangpo
suture zone, MFT – Main Frontal
Thrust.

2. The cessation of south-directed GHS metamorphism at ~16 Ma in the central to centraleastern
Himalaya (see prior discussion for references).
3. The onset of orogen-parallel and east-west extension at ≤ 16 Ma (see prior discussion for
references). (An exception to this is a set of north-south striking dykes in southern Tibet
dated between 18.3 ± 2.7 Ma to 13.3 ± 0.8 Ma (Williams et al., 2001). If the large error
associated with the older age of onset is considered, then theses age constraints are in
agreement with the proposed transition).
There are, however, some constraints that do not agree with our proposed transition.
Notably, timing constraints from the eastern GHS within Bhutan indicate protracted hightemperature
metamorphism until ~10 Ma (Warren et al., 2012). According to our transition, the
cessation of south-directed extrusion and metamorphism of the GHS should have ceased several
million years prior.
Warren et al. (2012) attribute this young high-temperature metamorphism to the
exhumation of deeper structural levels not exposed elsewhere in the Himalaya. Initial stages of
this exhumation path are constrained by 14 -13 Ma monazite and 15 -14 Ma zircon (Warren et al.,
2012). These initial age constraints are roughly coeval with the onset of the orogen-parallel
extension throughout the Himalaya. Therefore, it may be possible that exhumation from deep
structural levels was in part assisted by the onset of orogen-parallel extension and exhumation. If
this were true, then a middle Miocene transition may have occurred in the eastern GHS.
Alternatively, these young ages may be due to a diachronous transition initiating in the
west and younging towards the east of the orogen (Rowley, 1996; 1998). This would be
consistent with older ages observed in the Leo Pargil dome, and the southeast-directed younging
of orogen-parallel deformation observed in the upper Karnali valley (Fig. 2.19b). However, there
93

is little data to support both aforementioned hypotheses. Therefore, we cannot discount the
possibility that young GHS ages within Bhutan may indicate that our interpretation of a middle
Miocene transition is not applicable on an orogen-wide scale.
2.7.4 Tectonic model explaining orogen-parallel extension
Numerous studies have attempted to explain orogen-parallel extension throughout the
Himalaya and Tibet, invoking models of lateral extrusion of Tibet (Tapponnier et al. 1982),
oroclinal bending of the Himalayan arc (Klootwijk et al., 1985; Rataschbacher et al., 1994;
Bendick and Bilham, 2001), gravitational instability of the Tibetan plateau (Molnar and
Tapponnier, 1978; Molnar and Chen 1983; Coleman and Hodges, 1995), right-lateral oblique
convergence (McCaffrey and Nabelek, 1998; Seeber and Pêcher, 1998) and radial spreading of
the Tibetan plateau (England and Houseman, 1988; Copley and McKenzie, 2007; Copley 2008;
Cook and Royden, 2008). These models are commonly based upon the kinematics and orientation
of faulting with respect to the orogen (e.g. Murphy and Copeland, 2005; Thiede et al., 2006), and
more recently on present-day GPS velocity vectors (e.g. Shen et al., 2000; 2001; Zhang et al.,
2004; Allmendinger et al., 2007; Gan et al., 2007; Styron et al., 2011).
The high-strain zone of the upper Karnali valley defines an approximately orogenparallel
right-lateral fault system in the central-western Himalaya. The geographic position and
sense of motion along this fault are consistent with radial spreading of the Tibetan plateau,
oblique convergence and possibly lateral extrusion of Tibet. Therefore, we cannot rely upon these
data alone to evaluate the mechanism driving orogen-parallel extension.
By integrating chronological data we are able to constrain the onset of orogen-parallel
extension to ~15 -13 Ma in the upper Karnali valley, and potentially to the middle Miocene
94

throughout the Himalaya (Figs. 2.21, 2.22). This high-resolution time frame provides a constraint
that can be used to test the various models seeking to explain orogen-parallel extension.
The only model that predicts a major tectonic change in the middle Miocene leading to
the onset of orogen-parallel extension is a numerical model of modified radial spreading of the
lower crust of Tibet (Fig. 2.23; Royden et al., 1997; Cook and Royden, 2008). This numerical
model simulates three-dimensional deformation during continental convergence during which the
lower crust decreases viscosity, decouples from the overlying upper crust, and flows radially. By
incorporating pre-collisional crustal heterogeneities, including the strong Tarim and Sichuan
Basin, the model predicts that after ~40 million years of convergence the lower crust underlying
the Tibetan plateau would begin to spread laterally to the east and rotate clockwise around the
eastern syntaxis (Fig. 2.23). Orogen-parallel extension throughout central and southern Tibet is
interpreted as a result of the onset of lower-crustal east-directed flow (Cook and Royden, 2008).
Assuming onset of collision between India and Asia at ~55-50 Ma (Najman et al., 2010), the end
of 40 million years of convergence corresponds to ~15-10 Ma. This timing is consistent with the
onset of orogen-parallel extension recorded in the upper Karnali valley (Fig. 2.21) and potentially
orogen-wide (Fig. 2.22).
If this model is correct, then the middle Miocene should also correspond to the uplift of
eastern Tibet due to the buttressing of east-flowing lower crust against the strong Sichuan basin.
This rise in mean elevation of eastern Tibet is confirmed via two independent methods: (1)
initiation of rapid river incision at 13 -9 Ma in southeastern Tibet (Clark et al., 2005), and (2) a
significant change in the oxygen isotopic composition of meteoric water between 13 -12 Ma in
northeastern Tibet, as inferred from the δ 18 O of lacustrine carbonates (Dettman et al., 2003). A
major shift in carbonate δ 18 O between 13-12 Ma indicates a shift to more arid conditions, thought
95

Figure 2.23 Numerical model of modified radial spreading of the lower crust of Tibet,
after Cook and Royden (2008). (a) Modeled elevation and surface velocity of a
Himalayan-Tibetan sized orogen after 40 million years (m.yr.) of convergence. (b)
Present day topography colored to match elevation scale, and select normal and strikeslip
faults. Numbers correspond to the following events synchronous during the middle
Miocene: (1) onset of orogen-parallel extension, (2) east-directed spreading and rotation
of the Tibetan plateau, and (3) cessation of south-directed mid-crustal flow (not
stipulated by the model, but suggested by our results).
96

to represent the time at which the northeastern Tibetan plateau reached the necessary elevation to
block the penetration of moisture from the surrounding oceans (Dettman et al., 2003).
Furthermore, most present-day GPS velocity vector studies suggest that the Tibetan plateau may
be deforming as a continuous medium and extending towards the ESE (e.g. Zhang et al., 2004;
Gan et al., 2007), consistent with a model of radial spreading.
Actual chronologic constraints characterizing the onset of orogen-parallel extension
(Figs. 2.2a, 2.21) are consistent with those predicted in the Cook and Royden (2008) model of
radial spreading of the Tibetan plateau (Fig. 2.23). This does not discount other models
explaining orogen-parallel extension; however, it does offer additional support for models of
radial spreading.
The transition we studied suggests that cessation of south-directed extrusion of the GHS
occurred immediately prior to the onset of orogen-parallel extension. Therefore within the context
of radial-spreading models, our data provide the following additional constraint: initiation of eastdirected
lower-crustal flow and orogen-parallel extension immediately postdates the cessation of
south-directed extrusion of the GHS (Fig. 2.23). It may be possible that there is an intimate link
between cessation of south-directed mid-crustal flow and onset of east-directed lower crustal
flow; however, current state of knowledge has not yet clarified this issue.
2.8 Conclusions
Integrated geological, structural, microstructural, and chronological data reveal a twophase
extrusion /exhumation history of the GHS of the upper Karnali valley. The first phase is
associated with south-directed extrusion of the GHS. Dominant U-Th-Pb monazite ages of 19 -15
Ma are consistent with the final stages of Neohimalayan metamorphism and the cessation of
south-directed extrusion.
97

The second phase is associated with orogen-parallel extension. Structural and kinematic
data suggest that this phase of deformation represents orogen-parallel deformation and
exhumation at progressively lower temperatures, possibly within a larger network of transtension.
Muscovite thermochronology indicate that by ~15 -13 Ma, orogen-parallel extension was active
in the upper Karnali valley.
Assembling these data delineates a transition from cessation of south-directed extrusion
of the GHS to onset of orogen-parallel extension. This transition initiated at high temperatures,
progressed into the brittle realm, and is bracketed between ~15 -13 Ma within the upper Karnali
valley.
A compilation of our chronologic data with three migmatite-cored domes across the
Himalaya that exhume mid-crustal rocks in settings kinematically linked to extension, reveals a
common middle Miocene orogen-wide transition.
A middle Miocene transition supports a model of modified radial spreading (Cook and
Royden, 2008), to explain the onset and continuation of orogen-parallel extension throughout the
Himalaya and Tibet.
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Chapter 3
Discussion
3.1 Summary
Integrating geological, structural, microstructural, and chronological data from the upper
Karnali valley reveals a fundamental tectonic transition between 15 -13 Ma, from cessation of
south-directed extrusion of the mid-crust to onset of orogen-parallel extension. A compilation of
our data with three migmatite-cored domes that exhume mid-crustal rocks in settings
kinematically linked to extension reveals a common middle Miocene orogen-wide transition (Ch.
2).
3.1.1 Middle Miocene orogen-parallel extension and east-directed lower-crustal flow
The studied transition is broadly contemporaneous with the cessation of mid-crustal flow
and onset of orogen-parallel extension across the entire Himalayan-Tibetan orogen during the
middle Miocene (Ch. 2). These processes are also roughly synchronous with initiation of uplift of
the eastern Tibetan plateau (Ch. 2). These observations can be explained by a numerical model of
radial spreading of Tibet, in which the lower crust becomes thermally weak, decouples from the
upper crust and flows radially and southward during the early to middle Miocene, and eastward
thereafter (Royden et al., 1997; Cook and Royden, 2008).
The numerical models of Royden et al. (1997), and Cook and Royden (2008) simulate
three-dimensional deformation of a viscous crust during continental convergence (Fig. 3.1). The
model parameters include an idealized crust with depth-dependent viscosity and a convergence
rate approximating the velocity of the mantle. Modeling results show that after 4 -16 Myr of
convergence, the lower crust becomes so thermally weak that it decoupled from the overlying
99

yr
Figure 3.1 Numerical model of Cook and Royden (2008) simulating the formation of a
plateau after (a) ~16 Myr of convergence, and (b) after ~40 Myr of convergence. (c)
Present day topography and select normal and strike-slip faults (black lines), also
modified after Cook and Royden, 2008. Elevation scale is same for all figures.
Numbers correspond to tectonic and metamorphic events, see text for references.
100

upper crust and begins to flow radially, forming a plateau (Fig. 3.1; Royden et al., 1997; Cook
and Royden, 2008). Assuming onset of collision of India and Eurasia at ~55-50 Ma (Green et al.,
2008; Najman et al., 2010), this timing is consistent with the earliest metamorphic ages obtained
from the GHS at ~39 Ma, corresponding to crustal thickening and prograde metamorphism
(Godin et al., 2001; Prince et al., 2001; Cottle et al., 2009b), and the oldest ages obtained for the
surface of central Tibet at ~35 Ma, corresponding to plateau formation (Rowley and Currie,
2006).
Following ~40 Myr of convergence, modeling results of Cook and Royden (2008) predict
that the Tibetan plateau begins to flow laterally to the east and rotates clockwise around the
eastern syntaxis (Fig. 3.1). This results in orogen-parallel extension across the topographic front
and plateau areas. Furthermore, this model, and a study by Clark and Royden (2000), predict that
buttressing of east-flowing lower crust against the strong Sichuan basin causes thickening of the
lower crust adjacent to the basin, resulting in uplift of eastern Tibet (Fig. 3.1). These predictions
and their timing are consistent with the onset of orogen-parallel extension (~16 -11 Ma), and the
uplift (~13-12 Ma) of and lack of deformation within the upper crust of eastern Tibet (Burchfiel
et al., 1995).
This numerical model of east-directed lower crustal flow is also consistent with middle
Miocene to present day evolution of the Himalaya and Tibet. Within southern and central Tibet,
high plateau elevations have remained relatively stable (Harris, 2006). In contrast, northern Tibet
and eastern Tibet continue to be uplifted (Yang and Liu, 2002; Dettman et al., 2003; Clark et al.,
2005), forming in some regions of eastern Tibet a topographic front somewhat comparable to the
Himalaya (Fig. 3.1; Burchfiel et al., 1995). Despite this gain in elevation, eastern Tibet continues
to show a lack of Cenozoic upper-crustal shortening features (Burchfiel et al., 1995). These
101

phenomena are consistent with ongoing east-directed lower-crustal flow and thickening of the
lower-crust adjacent to the Sichuan Basin.
This time period also coincides with significant regional tectonic changes (Fig. 3.1),
including the cessation of spreading in the South China Sea at ~15 Ma (Li et al., 2006), the
initiation of a major stage of extension within the Andaman Sea at ~15 Ma (Curray, 2005; Hall,
2002), and the termination of east-directed subduction of India below the Burman plates at ~11
Ma (Everett et al., 1990). These tectonic events were proceeded by major environmental changes
including the initiation of the Asian monsoon at 9 -8 Ma (Zhisheng et al., 2001), and 8 -7 Ma
changes in climate observed in the Siwalik foreland basin (Quade et al., 1989; 1995), Arabian Sea
(Kroon et al., 1991; Prell et al., 1992) and Bay of Bengal (Burbank et al., 1993; Derry and
France-Lanord, 1996). Although these tectonic and environmental events are not directly related
to the numerical model of Cook and Royden (2008), it may be significant that they are roughly
coeval with the studied transition and the onset of east-directed flow.
A synthesis of middle Miocene evolution of the Himalaya and Tibet suggest that
cessation of south-directed extrusion of the GHS occurred immediately prior to the onset of
orogen-parallel extension and east-directed lower-crustal flow. If we interpret the GHS as the
result of an extruded low-viscosity channel from mid-crustal depths (Beaumont et al., 2001;
2004; 2006; Jamieson et al., 2004; Godin et al., 2006a), then it may be appropriate to make the
following hypothesis: a middle Miocene transition affecting mid- to lower- crustal processes is
responsible for the cessation of south-directed extrusion of the GHS and onset of east-directed
lower-crustal flow.
102

3.2 Future research considerations
Extensive research opportunities exist throughout the Himalayan-Tibetan orogen. Our
research in particular, has highlighted two important tectonic issues that merit further
investigation.
3.2.1 Why did south-directed mid-crustal extrusion cease, and why did east-directed lower
crustal flow initiate
Results of this study shown an intimate link between cessation of south-directed midcrust
extrusion and the onset of orogen-parallel extension, interpreted to correspond to the
initiation of east-directed lower crustal flow. However, it has not adequately addressed the issue
of why south-directed extrusion of the mid-crust ceased, and why east-directed lower crustal flow
initiated. We briefly hypothesized that a transition affecting mid- to lower-crustal processes may
have been responsible, however we cannot verify this.
The model of Cook and Royden (2008) suggesst that the onset of east-directed lowercrustal
flow may be a result of modified lower crustal flow after ~40 Myr of convergence.
Alternatively, the cessation of spreading in the South China Sea (Li et al., 2006), and the
initiation of a major rifting phase in the Andaman Sea (Hall et al., 2002; Curray, 2005), suggest
that far-field plate reconfigurations may also be partly responsible for middle Miocene transitions
in Himalayan-Tibetan tectonics.
An additional viable hypothesis includes a middle Miocene modification of horizontal
gradients in lithostatic pressure across the Himalaya and Tibet, due to south-directed extrusion of
the GHS. Extrusion of mid-crustal material via a channel-flow model requires a significant
contrast in crustal thickness between the plateau and the foreland (Beaumont et al., 2001; Godin
et al., 2006a). It may be possible that by middle Miocene, south-directed extrusion of the GHS
103

towards the topographic front of the Himalaya had modified horizontal gradients in lithostatic
pressure between the Himalayan foreland and the Tibetan plateau. A diminished contrast in
horizontal pressure gradients between the Himalaya and Tibet could have resulted in the cessation
of south-directed extrusion of the GHS. However, the mid- to lower-crust would have remained
hot and retained low-viscosity due to its depth below the elevated Tibetan plateau (Harris, 2006).
Subsequently, east-directed mid- to lower-crustal flow may have initiated due to lateral pressure
gradients between the elevated Tibetan plateau and the low-lying areas south and north of the
Sichuan basin (Figs. 3.1, 3.2; Clark and Royden, 2000).
Solving this problem is complex and may require detailed investigation and modeling of
tectonic and environmental changes throughout the transition period. Results of this work are
critical to understanding the evolution of lower-crustal processes throughout continent-continent
convergence. Furthermore, considering the chronologic resolution that may be required to resolve
these issues, solutions may be only be possible in modern-day mountain belts such as the
Himalaya-Tibetan orogen.
3.2.2 Is the Karakoram fault linked to faults of the upper Karnali valley
Understanding the link between strike-slip dominated faults of the upper Karnali valley
and larger-scale fault systems is fundamental to determining the role of strike-slip faulting during
continent-continent convergence. The following two contrasting interpretations exist: (1)
deformation is localized along very few large-scale faults that accommodate considerable slip
(e.g. Tapponnier et al., 1982; Armijo et al., 1989; Replumaz and Tapponnier, 2003), and (2)
deformation is distributed amongst many small faults, that each accommodate minor amounts of
slip (e.g. Rothery and Drury, 1984; Taylor et al., 2003).
104

Determining the relationship between faults of the upper Karnali valley and the
Karakoram fault will provide additional support for one of these interpretations. Data from this
thesis, and from Yakymchuk (2010), suggest that faults of the Karnali valley young towards the
southeast. Murphy and Copeland (2005) suggest that the faults of the Karnali are kinematically
and chronologically linked to the crustal-scale Karakoram fault.
Thermochronologic data from a wider area, coupled with micro-seismicity and slip
estimates of active faults, will provide a basis for evaluating and comparing the kinematic,
chronologic and present-day evolution of the Karnali valley and the Karakoram fault.
105

Chapter 4
Microstructural analyses: proposals for advancing and validating
current techniques
4.1 Introduction
Quartz crystallographic preferred orientation (CPO) and rigid clast vorticity analyses are
microstructural techniques used to enhance our understanding of flow kinematics and ductile
deformation (Passchier and Trouw, 2005). Both styles of analyses were performed with variable
degrees of success on samples from the upper Karnali valley (Chapter 2.5).
Quartz CPO analyses from the upper Karnali valley were performed by classic
(universal-stage) and modern methods (electron backscatter diffraction - EBSD). Both
methodologies yielded meaningful results (Chapter 2.5.1), however, the EBSD method offered
additional potential, including the analysis of microscale domains and quartz a-axes. In contrast
to CPO analyses, only a single rigid clast vorticity analysis from the upper Karnali valley yielded
a meaningful result (Chapter 2.5.2).
This chapter outlines two studies made possible with modern CPO acquisition software,
and a third study evaluating the practical application of rigid clast vorticity analyses.
4.2 Modern quartz CPO analyses: future research opportunities
Quartz crystallographic preferred orientations (CPOs) are the result of deformation
temperature, strain rate, non-coaxiality of flow, and distortional strain geometry (Sullivan and
Beane, 2010, and references therein). Thus, the analysis of quartz CPOs has become a powerful
tool in characterizing high-strain zones and exhumed metamorphic terranes, particularly within
the Greater Himalayan Sequence and its bounding faults (e.g. Bouchez and Pêcher, 1976;
106

Bouchez, 1977; Brunel and Kienast, 1986; Grujic et al., 1996; Bhattacharya and Weber, 2004;
Law et al., 2004; Larson and Godin, 2009; Larson et al., 2010a; Yakymchuk and Godin, 2012).
Historically, analysis of quartz CPO fabrics was limited to the c-axis due to analytical
restrictions of traditional methods (e.g. universal stage microscope). However, the development
of the Electron Back Scatter Diffraction (EBSD) technique (Prior et al., 1999), and Fabric
Analyzer (FA) method (Wilson et al., 2003; 2007) have provided alternative and improved
approaches.
Both methods allow for the automatic recognition of quartz c-axes. C-axis fabrics are
commonly used to assess shear sense, active slip systems, deformation temperatures and
deviations from plane strain towards constriction or flattening (Fig. 4.1). The EBSD method
allows for the additional recognition of the complete crystallographic orientations of all lattice
planes and axes. With regard to quartz, this allows for evaluation of both the c- and a- axes. A-
axes provide an additional independent assessment of plane strain, flattening, and constriction
(Fig. 4.1; Passchier and Trouw, 2005). Furthermore, Sullivan and Beane (2010) demonstrate that
at high temperatures and non-coaxial flow, a-axis fabrics are more consistent and betterdeveloped
than c-axis fabrics, and may provide a more accurate representation of strain geometry.
Additional advantages of modern CPO analyses (EBSD and FA) include high-resolution
acquisition of CPO data from rock-forming mineral grains as small as ~1 µm (EBSD) and ~2.8
µm (FA), and fully automated data acquisition and analysis. This enables rapid comparison of
statistically significant data from different microstructural domains. For a full description of
EBSD and FA techniques, and their application to geologic problems, see Prior et al. (1999;
2009) and Wilson et al. (2003; 2007), respectively.
107

a
c
a
c
Figure 4.1 Summary of quartz CPO fabrics showing (a) the orientation of
principle strain axes, active slip systems, and fabric geometries in different
strain fields, and (b) the evolution of a CPO fabric during progressive noncoaxial
deformation (assuming plane strain). Note that the letters ‘c’ and ‘a’
refer to the quartz crystallographic c- and a-axes. Adapted and modified after
Schmid and Casey, 1986; Passchier and Trouw, 2005; Toy et al., 2008;
Sullivan and Beane, 2010.
108

The following section outlines two experiments that use modern quartz CPO analyses to
study (1) the evolution of CPO fabrics throughout progressive strain development, and (2) lateral
variations in strain during extrusion of the Greater Himalayan sequence (GHS).
4.2.1 Evolution of CPO fabrics during progressive strain development
The goal of this study is to document how quartz CPO fabrics evolve at the microscale
during progressive deformation. An understanding of this evolution will provide a context for
evaluating CPO fabrics from extruded and exhumed high strain zones, such as the Greater
Himalayan sequence.
Toy et al. (2008) show that c-axis fabrics of exhumed rocks are strongly influenced by
strain rates at depth (Fig. 4.2). They demonstrate that fabrics formed at depth are inherited by, and
directly influence, lower temperature fabric transitions (Fig. 4.2). CPOs from this study were
measured from entire thin sections.
Peternell et al. (2010) illustrate heterogeneities in CPO fabrics on the scale of a thin
section (Fig. 4.3). They show that bulk CPO fabrics (i.e. those measured from entire thin sections)
are equivalent to the addition of all microscale domains (Fig. 4.3).
The proposed study builds upon the conclusions of Toy et al. (2008) and Peternell et al.
(2010), by examining quartz CPOs of different microstructural domains, and entire thin sections,
at various stages of exhumation. Additionally, quartz CPOs will be obtained from veins formed
during these different stages of exhumation.
Results will provide insight on the evolution of quartz CPO fabrics of different
microstructural domains during exhumation, and their relationship to bulk CPOs. Additionally,
careful observation of microstructural domains could reveal, (1) CPO fabrics that may otherwise
have not been observed due to low abundance or a rapid fabric transition, and (2) controlling
109

Figure 4.2 Results of study by Toy et al. (2008) showing the evolution of bulk c-axis fabrics for
different exhumation paths. Numbers in bubbles correspond to total strain experienced by that
sample; bold black line corresponds to the fault, and bold arrow points arrow from the fault in
the direction of decreasing strain. The different shaded regions correspond to protomylonitic and
ultramylonitic exhumation paths. Note that the strong Y-maxima developed at depth in the
ultramylonite (typically indicative of high deformation temperatures) is preserved during lower
temperature fabric transitions. This is not the case for the protomylonite, as it was unable to
develop a Y-maxima at high temperatures due to lower strains. Modified after Toy et al. (2008).
110

Figure 4.3 Results of study by Peternell et al. (2010) showing (a) different quartz c-axis fabrics
from five micro-domains within a single thin section, and (b) the addition of these five domains
yielding the final bulk c-axis fabric. Note how the asymmetric CPO from domain (ii) is barely
visible on the final addition. Modified after Peternell et al. (2010).
111

factors on the formation of different CPOs and slip systems.
CPOs derived from veins emplaced at various stages of exhumation should record an
unbiased CPO pattern, with no prior inheritance. CPOs of these veins can be compared against
CPOs of the entire thin section and different microstructural domains, to better understand how
preexisting CPOs influence ongoing fabric development.
Lastly, these data will provide an evaluation of the relationship between slip systems, c-
axis opening angles, and temperature of deformation (Chapter 2.5.1.3; Law, 1991; Law et al.,
2004). The aforementioned studies by Toy et al. (1998), and Peternell et al. (2010) both conclude
that CPO patterns did not consistently reflect changes in temperature. This study would attempt to
clarify this by comparing deformation temperatures estimated from quartz c-axis opening angles,
and slip systems, with externally derived temperature estimates.
4.2.2 Lateral variations in strain during extrusion of the Greater Himalayan sequence
The Greater Himalayan Sequence (GHS) is the high-metamorphic-grade and anatectic
core of the Himalaya, commonly interpreted to represent an extruded low-viscosity mid-crustal
channel (Grujic et al. 1996, 2002; Beaumont et al. 2001, 2004; Jamieson et al 2004; Searle and
Szulc 2005; Law et al. 2004; Lee and Whitehouse 2007). A significant kinematic database of the
extruded GHS has been established, largely through analyses of quartz c-axis fabrics and vorticity
(Bouchez and Pêcher, 1976; Brunel and Kienast, 1986; Grujic et al., 1996; Grasemann et al.,
1999; Bhattacharya and Weber, 2004; Law et al., 2004; Carosi et al., 2006; Jessup et al., 2006,
2007; Larson and Godin, 2009; Lee and Wagner, 2009; Langille et al., 2010; Larson et al.,
2010a,b). However, this data set commonly assumes plane strain conditions, and does not
sufficiently address lateral strain variations (i.e. deviations from plane strain towards constriction
or flattening).
112

Understanding deviations from plane strain is critical for two reasons: (1) contributions of
flattening and constriction have a direct effect on extrusion models, and (2) temperature estimates
from quartz c-axis slip system and c-axis opening angles, and vorticity interpretations, rely on the
assumption that deformation occurs in a plane strain environment. Deviation from plane strain
may negate the validity of these methods.
Several quartz CPO studies of the GHS have presented type II cross-girdle c-axis fabrics
(Grujic et al., 1996; Bhattacharya and Weber, 2004; Law et al., 2004; Larson and Godin, 2009;
Larson et al., 2010a,b), indicative of constrictional deformation (Fig. 4.1; Bouchez 1978; Schmid
and Casey, 1986). The goal of the proposed study is to assess deviations from plane strain
towards flattening or constriction throughout the GHS. Quartz c- and a-axes across and along
strike will be analyzed and combined with the current kinematic database. To date, quartz a-axes
are largely undocumented throughout the GHS. Obtaining a-axes may be critical, as they provide
an independent assessment of strain conditions (Passchier and Trouw, 2005; Sullivan and Beane,
2010)
Results from this study will reveal lateral strain variations throughout the GHS. These
results will affect the validity of the plane strain assumption underlying all quartz CPO and
vorticity interpretations, and influence GHS extrusion models.
4.3 Evaluation of the rigid grain method for estimating kinematic vorticity
numbers; future research opportunities
Vorticity analyses quantify the internal rotational component of flow in order to estimate
the relative contributions of pure and simple shear throughout deformation (Passchier and Trouw,
2005). The concept of rigid grain vorticity analysis was initially developed by Jeffery (1922) and
Ghosh and Ramberg (1976), and has since been modified for use in geologic problems (Passchier,
113

1987; Simpson and De Paor, 1993, 1997; Wallis, 1992, 1995; Wallis et al., 1993; Jessup et al.,
2007). It is now commonly used to characterize flow within shear zones, particularly in the
Himalaya (Law et al., 2004; Carosi et al., 2006; Jessup et al., 2006, 2007; Larson and Godin,
2009; Lee and Wagner, 2009; Langille et al., 2010; Larson et al., 2010a,b).
The basis of the rigid grain vorticity method relies on a mathematic relationship between
the aspect ratios and orientations of rigid objects, commonly porphyroclasts, rotating in a flowing
matrix (Jeffery, 1922; Ghosh and Ramberg, 1976). This relationship predicts that during
deformation, porphyroclasts of certain aspect ratios will rotate into a stable-sink orientation.
Assuming the following assumptions of Passchier (1987) are met, this orientation should be a
reflection of the contribution of pure and simple shear active throughout deformation:
(1) Presence of abundant rigid pre-deformational porphyroclasts
(2) Porphyroclasts are significantly larger then host matrix grains
(3) No mechanical interaction between porphyroclasts
(4) Significant quantity of porphyroclasts with a wide range of aspect ratios
(5) Homogenously deforming matrix
(6) High and protracted strains allowing grains to rotate to stable-sink positions
An additional fundamental assumption when performing vorticity analyses, is that all
deformation occurs in two-dimensional flow. This evokes two major limitations. First, flow must
be monoclinic (Fig. 4.4). This implies that the vorticity vector must be parallel to one of the three
instantaneous stretching axes, and both of these vectors must be perpendicular to the plane
containing the best-developed asymmetric structures, as this is the plane that will be analyzed for
vorticity (Fig. 4.4; Passchier, 1987). This assumption may be problematic as not all natural shear
zones have monoclinic symmetry (Lin et al., 1998; Jones and Holdsworth, 1998; Jiang and
Williams, 1998; Forte and Bailey, 2007; Iacopini et al., 2007; Fernandez and Diaz-Azpiroz,
114

Figure 4.4 Illustration of monoclinic (instantaneous stretching axis, ISA, parallel to vorticity
vector), and triclinic (ISA oblique to vorticity vector) shear zone geometries. Blue ellipses
show orientation of finite strain axes, yellow arrows show ISA, green arrows show shear sense
and purple rectangle shows vorticity analysis plane. Modified after Iacopini et al. (2007).
115

2009). Furthermore, there is rising debate on which type of flow, monoclinic or triclinic, is
predominant in the majority of shear zones (Jiang and Williams, 1998; Jiang et al., 2001).
Second, deformation must be plane strain. This implies that all deformation is parallel to
the plane in which vorticity will be analyzed. Deviations from plane strain towards constriction or
flattening are not incorporated into the rigid grain vorticity theory. This assumption may be
problematic when assessing transtensional or transpressional shear zones, which may exhibit
constriction or flattening strain (see review by Dewey, 2002).
Past critiques of the rigid clast vorticity method highlight systematic error in estimating
vorticity due to the two-dimensional nature of the analysis (Tikoff and Fossen, 1995; Forte and
Bailey 2007; Iacopini et al., 2008). Building upon these critiques, a recent study by Li and Jiang
(2011) used numerical modeling to test the reliability of the rigid grain method for evaluating
vorticity. Despite respecting all stipulated assumptions, results of their study yield large
uncertainties that call into question the applicability of this method.
Uncertainties derived from the study of Li and Jiang (2011) were largely attributed to the
inability of randomly oriented clasts to rotate into a stable-sink position under natural amounts of
strain. To perform vorticity analyses, it is essential that rigid clasts rotate into a stable-sink
position. Once in this position, the vorticity vector should be parallel to the intermediate axis of
the clast, and only then will clast aspect ratios and orientations be relatable to vorticity (Fig. 4.4,
Passchier, 1987). Li and Jiang (2011) postulate that this assumption is not valid, as their model
predicts that the alignment of these axes requires unnaturally large amounts of strain. These
authors conclude that the current 2D models of analyzing vorticity do not simulate natural 3D
conditions and therefore do not provide a valid assessment of coaxial versus non-coaxial strain.
116

The following section proposes an alternative study of rigid clast vorticity based on
analogue modeling. Results of this study would provide an additional evaluation of the rigid clast
methods used to estimate vorticity.
4.3.1 Analogue modeling of rigid clasts in a 3D flowing matrix
The goal of this study is to simulate the rotation of rigid clasts in a flowing matrix under
varied natural geologic conditions. Vorticity numbers estimated from the tests will be compared
with known input values and will provide conclusions on the applicability of the rigid grain
vorticity theory to practical geologic problems.
The basic model (Fig. 4.5) would place randomly oriented rigid blocks of known aspect
ratios within a material simulating a geologically-realistic viscous rheology. This material will
then be deformed within a shear box (Fig. 4.5), at a predetermined parameter (listed below). At
intervals throughout the deformation process, the block will be imaged in 2D slices normal to the
vorticity vector, and in 3D (using a tomodensitometer or “CT-scanner” as per Harris et al.,
2012a,b). A similar study was done by Piazolo et al. (2002), however their analogue model was
limited to 2D and was only able to simulate plane strain. Our proposed model will operate in 3D
and will simulate plane strain, constriction and flattening.
This experiment will be repeated with variations in the following parameters: (1) aspect
ratios of rigid clasts, (2) strain rates (low to high), (3) contributions of coaxial and non-coaxial
strain, (4) monoclinic and triclinic flow, (5) plane, constrictional and flattening strain, (6) variable
matrix rheology properties, and (7) alternating layers of variable rheology.
2D images will be used to estimate vorticity at intervals (or “slices”) across the model
using the Rigid Grain Net method (Jessup et al., 2007). These results will show under what
geologic conditions the 2D rigid grain method can accurately estimate the kinematic vorticity
117

Figure 4.5 Simplified set up of analogue model showing (a) randomly
oriented rigid clasts (purple) within a material of geologically realistic
rheology (pink), and (b), the material from (a) within a shear box. Blue
arrows correspond to shear direction. Simultaneous shearing and
widening/narrowing of the box parallel to the black arrows would
simulate constriction/flattening (i.e. transtension/transpression).
118

number. Additionally, comparing 2D slices across the model will show the consistency of repeat
rigid grain analyses within a single sample. Ideally, statistical analyses of results will
establishment of a minimum threshold number of grains required to be measured in order to
accurately estimate vorticity.
2D and 3D images will be used to assess the time required at established strain rates to
bring an axis of all rigid clasts parallel to the vorticity vector (i.e. into stable-sink position). These
values can be compared to geologic time and natural strain rates to verify the assumption that
under natural conditions, rigid clasts will orient themselves in a stable-sink position.
Ultimately, the study will offer an analogue modeling based evaluation of the
applicability of the rigid grain method in estimating vorticity. In combination with prior critiques
of vorticity analyses (Tikoff and Fossen, 1995; Forte and Bailey 2007; Iacopini et al., 2008; Li
and Jiang, 2011), users of the method can then determine if it is still an appropriate tool to
characterize the kinematics of flow.
4.4 Conclusions
Quartz CPO and rigid grain vorticity techniques allow for the analysis and extraction of
kinematic data from ductilly deformed rocks. Notably, quartz CPO analyses have the potential for
innovative studies and for resolving kinematic data on a microstructural scale.
Despite the usefulness of microstructural techniques, the applicability of these
approaches towards geologically realistic scenarios remains poorly understood. In particular,
results derived from numerical modeling of rigid grain vorticity analyses question the validity of
this technique. A robust understanding of the true significance of microstructural results is
required for a proper geologic interpretation.
119

Appendix A
Station locations and structural data
* Latitude and Longitude based on the WGS84 datum
** (2) refers to secondary structures
Station Latitude Longitude
Elevation
(m)
120
Structure
Azimuth
Dip/
plunge
HK100 29 58 21.3 81 48 37.7 3142 foliation (dominant) 270 15
fold axis 260 8
axial plane 260 66
HK101 29 58 21.1 81 47 59.8 3134 foliation (dominant) 281 27
mineral lineation 325 3
HK102 30 01 05.9 81 45 18.8 2432 foliation (dominant) 315 36
crenulation lineation 115 10
HK103 30 01 59.3 81 44 49.0 2431 foliation (dominant) 81 23
mineral lineation 310 7
HK104 30 02 14.5 81 44 34.8 2565 foliation (dominant) 87 50
mineral lineation 135 40
joint 191 65
joint 215 67
joint 210 35
joint 230 50
HK105 30 02 52.0 81 42 22.8 2736 foliation (dominant) 100 34
mineral lineation 270 6
fold axis isoclinal) 100 3
HK106 30 02 56.2 81 41 35.2 2837 foliation (dominant) 120 25
mineral lineation 287 4
fold axis (isoclinal) 290 3
HK107 30 02 52.6 81 04 01.5 3087 foliation (dominant) 272 38
Comments
mineral lineation 295 5 L>S tectonite
minor open
HK108 30 02 32.2 81 38 08.2 2931 foliation (dominant) 120 30 folds
mineral lineation 284 2
HK109 30 03 00.1 81 37 51.9 2941 foliation (dominant) 111 52
mineral lineation 290 5
HK110A 30 03 50.6 81 33 19.9 2950 foliation (dominant) 50 29
HK110B
30 03
48.67
mineral lineation 120 27
81 33
24.11 3001 foliation (dominant) 91 26
mineral lineation 100 1
HK111 30 03 41.7 81 33 47.1 3157 foliation (dominant) 74 30
mineral lineation 128 5
HK112 30 03 45.1 81 33 39.5 3077 foliation (dominant) 89 30
HK113 30 03 39.1 81 34 29.4 2978
mineral lineation 121 15
HK114 30 03 26.1 81 35 20.8 2979 joint 335 84
joint 340 88

Station Latitude Longitude
Elevation
(m)
121
Structure
Azimuth
Dip/
plunge
joint 95 39
joint 98 44
HK115 30 04 39.1 81 36 05.2 3450 foliation (dominant) 355 20
mineral lineation 110 14
HK116 30 05 20.3 81 36 09.3 3575 foliation (dominant) 58 33
mineral lineation 80 5
HK117 30 06 15.5 81 36 19.6 3950 foliation (dominant) 255 88
mineral lineation 70 30
HK118 30 07 42.1 81 37 48.4 4320 foliation (dominant) 90 39
mineral lineation 104 36
mineral lineation (2) 175 57
HK119 30 07 00.2 81 37 09.0 4227 foliation (dominant) 264 49
mineral lineation 40 20
HK120 30 04 24.5 81 36 17.1 3438 foliation (dominant) 74 46
mineral lineation 103 17
HK121 30 03 04.8 81 36 54.9 3145 foliation (dominant) 54 30
mineral lineation 116 28
Comments
HK122 30 03 07.9 81 36 59.9 3074 foliation (dominant) 160 34 cataclasite
fold axis (dominant) 295 41
fold axis (2) 320 64
fold axis (2) 350 25
joint 315 39
striations 295 17
HK123 30 02 35.8 81 36 39.7 3240 foliation (dominant) 145 36
mineral lineation 295 7
HK124 30 02 14.1 81 35 52.5 3600 foliation (dominant) 107 65
foliation (dominant) 115 31
mineral lineation 270 12
mineral lineation 270 45
fold axis (isoclinal) 270 12
axial plane 107 65
HK125 30 02 19.1 81 35 56.7 3589 foliation (dominant) 122 51
mineral lineation 282 13
HK126 30 02 17.3 81 35 45.2 3745 foliation (dominant) 122 28
crenulation lineation 260 23
mineral lineation (2) 250 24
HK127 30 02 17.9 81 35 46.2 3744 foliation (dominant) 105 23
crenulation lineation 250 18
HK128 30 02 17.2 81 35 39.9 3763 foliation (dominant) 113 31
crenulation lineation 138 21
minearl lineation (2) 245 25
HK129 30 02 09.2 81 35 08.9 3975 foliation (dominant) 115 26
cross-cutting
mylonite
cataclastic
carbonate
bedding 140 40 not consistent
mineral lineation 122 11
HK130 30 01 56.9 81 34 55.8 4033 foliation (dominant) 94 32
spaced
cleavage

Station Latitude Longitude
Elevation
(m)
122
Structure
Azimuth
Dip/
plunge
fold axis 265 26
axial plane 94 32
HK131 30 02 22.5 81 36 12.2 3425 foliation (dominant) 117 41
HK132 29 59 26.3
HK133
30 00
05.64
foliation (dominant) 266 14
bedding 146 28
Comments
mineral lineation 287 9
fold axis 282 17
broad open
fold
81 36
49.23 3916 foliation (dominant) 153 28 no GPS lock
fold axis 307 15
81 37
00.99 3547 foliation (dominant) 146 31 no GPS lock
mineral lineation 285 18
fold axis 315 20
HK134 30 00 29.6 81 36 54.6 3768 foliation (dominant) 242 16
HK135 30 00 32.4 81 37 22.3 3713 foliation (dominant) 192 19
foliation (dominant) 101 76
mineral lineation 184 76
HK136 30 00 35.8 81 37 27.4 3685 foliation (dominant) 84 33
HK 137
HK138
HK139
30 00
37.79
30 00
39.53
mineral lineation 120 23
81 37
29.13 3671 foliation (dominant) 142 44
mineral lineation 305 17
fold axis 140 18
81 37
30.87 3655 foliation (dominant) 70 45
mineral lineation 120 20
30 01 03.
0 81 37 42.0 3585 foliation (dominant) 102 35
mineral lineation 130 12
HK140 30 01 19.4 81 37 59.8 3501 foliation (dominant) 140 28
mineral lineation 305 5
HK141 30 01 32.3 81 38 14.1 3924 foliation (dominant) 90 47
HK142 30 01 36.0 81 38 21.3 3421 foliation (dominant) 110 32
TSS - GHS
contact
mineral lineation 204 ~30 no GPS lock
HK143 30 01 52.5 81 38 39.0 3330 foliation (dominant) 89 31
HK144 30 01 56.5 81 38 41.8 3160 foliation (dominant) 116 5
HK145
30 01
59.25
mineral lineation 114 2
crenulation lineation 240 11
fold axis (isoclinal) 300 11
81 38
43.48 3147 foliation (dominant) 110 2
HK146 30 02 18.8 81 44 15.4 2537 foliation (dominant) 8 25
KH001
29 58
21.12
mineral lineation 278 23
foliation (2) 210 26
minearl lineation (2) 260 15
081 48
38.04 foliation (dominant) 350 10
cross-cutting
mylonite

Station Latitude Longitude
Elevation
(m)
123
Structure
Azimuth
Dip/
plunge
KH002 29 59 12.3 81 47 45.9 foliation (dominant) 180 25
KH003
30 00
01.0200
81 47
23.760 foliation (dominant) 335 13
KH004
30 01
03.720
81 45
22.560 foliation (dominant) 326 28
KH005
KH006
KH007
KH008
KH009
30 01
57.30
mineral lineation 120 25
joint 127 37
joint 110 45
Dyke 240 70
81 44
51.06 foliation (dominant) 138 45
mineral lineation 295 20
joint 40 90
30 02
16.62 81 44 23.7 foliation (dominant) 60 35
30 02
20.58
30 02
37.86
30 03
06.66
KH010 30 03 55.5
KH011
KH012
KH013
KH014
KH015
KH016
KH017
KH018
30 03
22.98
30 03
35.34
30 03
49.44
30 05
19.76
30 05
13.80
30 05
07.88
30 05
02.29
30 04
58.35
Comments
Also named
HK004
mineral lineation 108 30
81 38
51.18 foliation (dominant) 255 12
mineral lineation 285 9
81 38
16.25 foliation (dominant) 315 24
mineral lineation 117 18
81 37
German
45.48 foliation (dominant) 110 74 granite
mineral lineation 290 13
81 36
06.000 foliation (dominant) 59 27
mineral lineation 108 22
81 35
13.62 foliation (dominant) 106 54
mineral lineation 125 20
81 34
30.12 foliation (dominant) 65 70
mineral lineation 105 44
81 33
20.04 foliation (dominant) 25 25
mineral lineation 115 20
81 31
59.61 foliation (dominant) 140 55 no GPS lock
mineral lineation 290 32
joint 0 30
81 31
44.27 foliation (dominant) 140 25 no GPS lock
81 31
31.54 foliation (dominant) 100 43 no GPS lock
mineral lineation 110 25
joint 165 90
81 31
30.75 foliation (dominant) 107 55 pseudotachylite
minor open
folds
fold axis 145 40
81 31
30.79 3058 foliation (dominant) 90 52
fold axis 52 63
fold axis 40 45

Station Latitude Longitude
KH019
KH020
KH021
KH022
KH023
KH024
30 04
29.16
30 04
29.57
30 04
32.40
Elevation
(m)
124
Structure
Azimuth
Dip/
plunge
minearl lineation (2) 240 10
81 30
00.36 3338
81 29
59.51 foliation (dominant) 132 37
joint 350 55
81 30
03.96 3330 foliation (dominant) 127 38
bedding 128 15
fold axis 133 1
axial plane 127 38
joint 15 60
30 04 81 30
40.920 11.580 foliation (dominant) 116 33
30 04 81 30
46.82 14.93 3282 foliation (dominant) 110 32
30 04
52.92 81 30 30.3
30 05 81 30
01.86 19.98 foliation (dominant) 115 34
Comments
broad NE
verging fold
N verging
synform
N verging
synform
N verging
synform
transposed
bedding
transposed
bedding
KH025
axial plane 115 34 isoclinal folds
30 04 81 29
KH026 53.69 45.17 foliation (dominant) 130 35
30 04 81 29
KH027 46.980 39.24 4015 foliation (dominant) 120 25
30 04 81 29
KH028 42.84 30.000 4126 foliation (dominant) 280 32
30 04 81 28
KH029 41.28 51.54 4300 fold axis 150 3
axial plane 135 34
30 04 81 28
KH030 29.82 52.56 foliation (dominant) 110 35
30 04 81 28
transposed
KH031 28.980 17.94 4555 foliation (dominant) 110 13 bedding
30 04 81 28
KH032 27.54 13.62 4640 foliation (dominant) 125 26
mineral lineation 280 16
KH033 30 04 28.8 81 28 09.0 4707 foliation (dominant) 150 36
bedding 147 24
mineral lineation 275 25
30 04
KH034 29.04 81 28 06.9 4765 foliation (dominant) 155 40
30 03 81 33
KH035 51.12 09.12 3004 foliation (dominant) 87 45
KH036
KH037
KH039
30 03
56.52
30 03
06.72
30 03
21.03
mineral lineation 115 33
81 33
47.76 3038
81 36
58.98 3077 fold axis 70 30 cataclasite
fold axis 165 24
foliation (dominant) 80 30
81 37
06.75 2928 foliation (dominant) 98 56
mineral lineation 110 20

Appendix B
Quartz CPO data
*For EBSD data, non-processed refers to the raw data, and processed refers to data that has been
reduced whereby each data point represents an individual grain (see Chapter 2.5.1.1).
** Additionally for EBSD data, represents the axis, and , and {11-20}
represent the axis.
HK 140 – U-Stage data
125

HK 140 – Non-processed EBSD data
126

HK 140 – Processed EBSD data (e.g. 1 point per grain)
127

HK 131A – U-Stage data
128

HK 131A (continued)
129

HK 131A – Non-processed EBSD data
130

131

HK 139D – Non-processed EBSD data
132

HK 139D – Non-processed EBSD data
133

HK 105A – U-Stage data
134

HK 105A – Non-processed EBSD data
135

HK 105A – Processed EBSD data (e.g. 1 point per grain)
136

HK 117A – Non-processed EBSD data
137

HK 117A – Processed EBSD data (e.g. 1 point per grain)
138

HK 120A – Non-processed EBSD data
139

HK 120A – Processed EBSD data (e.g. 1 point per grain)
140

HK 125 – Non-processed EBSD data
141

HK 125 – Processed EBSD data (e.g. 1 point per grain)
142

HK 145 – Non-processed EBSD data
143

HK 145 – Processed EBSD data (e.g. 1 point per grain)
144

HK 123 – Non-processed EBSD data
145

HK 123 – Processed EBSD data (e.g. 1 point per grain)
146

HK 123 – Processed EBSD data (e.g. 1 point per grain)
147

HK 124B2 – Processed EBSD data (e.g. 1 point per grain)
148

Appendix C
Monazite images and elemental maps
* Location of monazite grains for each thin section seen on thin section image.
** Numbers on x-ray maps refer to U-Th-Pb spot analyses.
***SL and CP (or COMPO) refer to secondary electron and backscatter electron images,
respectively.
KH18a – Thin section
149

KH18a – Monazite 1
150

151

KH18a – Monazite 2
152

5
6 18
4
19
3
13 16
2
1
12
15
11 14 10
7 8
9
17
20
153

KH18a – Monazite 3
154

155

KH18a – Monazite 4
156

5
10
4
3
9
11 12
2
6 7 8
1
13 15
14
157

KH18a – Monazite 5
158

1 2 5
3 4
6
7
10
9
8
159

KH18a – Monazite 6
160

1
13 14 8 7
12
9 6
11 15 10
5
4
2 3
161

KH18a – Monazite 7
162

10
9
8
1 7
2
3 4 5 6
13
11
12
15
14
163

KH18a – Monazite 8
164

9 8
20 19 7 5 3 1
2
10 6 4
16
15 17
14
12
13 18
11
165

HK109 – Thin section
* For HK109_monazites, scale bars for all x-ray maps are 10 um (seen in top left or
bottom left of image).
166

HK109 – Monazite 2
167

U
Th
4
3
2
1
5
10
9
11
7
8
6
Nd
Y
168

CP
SL
169

HK109 – Monazite 3
170

1
2
3
4
5 6
Y
Th
Nd
171

U
SL
CP
172

HK109 – Monazite 4
173

U
Th
SL
174

Nd
CP
5
4
6
7
8
9
3
2
10
1
Y
175

HK109 – Monazite 5
176

5
3 4
7
8
9
6
10
1 2
Y
Th
SL
177

U
Nd
CP
178

HK109 – Monazite 6
179

U
Nd
Th
180

SL
CP
1
2
3 4
5
9
6 7 8
10
Y
181

HK109 – Monazite 7
182

CP
SL
183

Y
Nd
U
Th
184

HK109 – Monazite 8
185

U
Th
SL
186

P
Nd
Y
CP
187

HK109 – Monazite 9
188

U
Th
SL
189

CP
Nd
2
3 4
6
5
1
Y
P
190

HK109 – Monazite 10
191

U
Th
SL
192

CP
Nd
4
6
5
7
2
3
1
Y
P
193

HK109 – Monazite 11
194

U
Th
SL
195

CP
Nd
Y
P
196

HK109 – Monazite 12
197

U
Th
SL
198

CP
P
Y
Nd
199

HK117a – Thin section
200

HK117a – Monazite 1
201

11
12
10
8
9
13 14 15
7
6
4
2
5
3
1
202

HK117a – Monazite 2
203

4
1 5 9
10
2
11
6 16
3
7 17
14
8
18
19
12
13
15
204

HK117a – Monazite 3
205

9
8
7
10
6
5
4
3
2
1
206

HK117a – Monazite 4
207

7
6
8
9
13
14
15
4
5
1
2
3
10
11 12
208

HK117a – Monazite 5
209

210

HK117a – Monazite 6
211

5
4
9
6
7
8
3
2
1
10
212

HK117a – Monazite 7
213

2
3
4
1
6
7
8 9
10
5
214

HK117a – Monazite 8
215

216

HK117a – Monazite 9
217

6
5
12
11 10
9
8
7
1
2 3 4
218

Appendix D
Muscovite 40 Ar/ 39 Ar thermochronology
219

220

221

222

223

224

225

226

227

228

229

230

231

232

233

234

235

236

237

238

239

240

241

242

243

244

245

246

247

248

249

250

251

252

253

254

255

256

257

258

259

260

261

262

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