Defining the Himalayan Main Central Thrust in Nepal - Queen's ...
Defining the Himalayan Main Central Thrust in Nepal - Queen's ...
Defining the Himalayan Main Central Thrust in Nepal - Queen's ...
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Journal of <strong>the</strong> Geological Society, London, Vol. 165, 2008, pp. 523–534. Pr<strong>in</strong>ted <strong>in</strong> Great Brita<strong>in</strong>.<br />
<strong>Def<strong>in</strong><strong>in</strong>g</strong> <strong>the</strong> <strong>Himalayan</strong> <strong>Ma<strong>in</strong></strong> <strong>Central</strong> <strong>Thrust</strong> <strong>in</strong> <strong>Nepal</strong><br />
MICHAEL P. SEARLE 1 ,RICHARDD.LAW 2 , LAURENT GODIN 3 , KYLE P. LARSON 3 , MICHAEL J.<br />
STREULE 1 ,JOHNM.COTTLE 1 & MICAH J. JESSUP 2<br />
1 Department of Earth Sciences, Oxford University, Parks Road, Oxford OX1 3PR, UK (e-mail: mikes@earth.ox.ac.uk)<br />
2 Department of Geological Science, Virg<strong>in</strong>ia Tech, Blacksburg, VA 24061, USA<br />
3 Geological Sciences and Geological Eng<strong>in</strong>eer<strong>in</strong>g, Queen’s University, K<strong>in</strong>gston, Ontario K7L 3N6, Canada<br />
Abstract: An <strong>in</strong>verted metamorphic field gradient associated with a crustal-scale south-vergent thrust fault,<br />
<strong>the</strong> <strong>Ma<strong>in</strong></strong> <strong>Central</strong> <strong>Thrust</strong>, has been recognized along <strong>the</strong> Himalaya for over 100 years. A major problem <strong>in</strong><br />
<strong>Himalayan</strong> structural geology is that recent workers have mapped <strong>the</strong> <strong>Ma<strong>in</strong></strong> <strong>Central</strong> <strong>Thrust</strong> with<strong>in</strong> <strong>the</strong> Greater<br />
<strong>Himalayan</strong> Sequence high-grade metamorphic sequence along several different structural levels. Some workers<br />
map <strong>the</strong> <strong>Ma<strong>in</strong></strong> <strong>Central</strong> <strong>Thrust</strong> as co<strong>in</strong>cid<strong>in</strong>g with a lithological contact, o<strong>the</strong>rs as co<strong>in</strong>cident with <strong>the</strong> kyanite<br />
isograd, up to 1–3 km structurally up-section <strong>in</strong>to <strong>the</strong> Tertiary metamorphic sequence, without support<strong>in</strong>g<br />
structural data. Some workers recognize a <strong>Ma<strong>in</strong></strong> <strong>Central</strong> <strong>Thrust</strong> zone of high ductile stra<strong>in</strong> up to 2–3 km thick,<br />
bounded by an upper thrust, MCT-2 (¼ Vaikrita thrust), and a lower thrust, MCT-1 (¼ Munsiari thrust). Some<br />
workers def<strong>in</strong>e an ‘upper Lesser Himalaya’ thrust sheet that shows similar P–T conditions to <strong>the</strong> Greater<br />
<strong>Himalayan</strong> Sequence. O<strong>the</strong>rs def<strong>in</strong>e <strong>the</strong> <strong>Ma<strong>in</strong></strong> <strong>Central</strong> <strong>Thrust</strong> ei<strong>the</strong>r on isotopic (Nd, Sr) differences,<br />
differences <strong>in</strong> detrital zircon ages, or as be<strong>in</strong>g co<strong>in</strong>cident with a zone of young (,5 Ma) Th–Pb monazite<br />
ages. Very few papers <strong>in</strong>corporate any structural data <strong>in</strong> justify<strong>in</strong>g <strong>the</strong> position of <strong>the</strong> <strong>Ma<strong>in</strong></strong> <strong>Central</strong> <strong>Thrust</strong>.<br />
These studies, comb<strong>in</strong>ed with recent quantitative stra<strong>in</strong> analyses from <strong>the</strong> Everest and Annapurna Greater<br />
<strong>Himalayan</strong> Sequence, show that a wide region of high stra<strong>in</strong> characterizes most of <strong>the</strong> Greater <strong>Himalayan</strong><br />
Sequence with a concentration along <strong>the</strong> bound<strong>in</strong>g marg<strong>in</strong>s of <strong>the</strong> South Tibetan Detachment along <strong>the</strong> top,<br />
and <strong>the</strong> <strong>Ma<strong>in</strong></strong> <strong>Central</strong> <strong>Thrust</strong> along <strong>the</strong> base. We suggest that <strong>the</strong> <strong>Ma<strong>in</strong></strong> <strong>Central</strong> <strong>Thrust</strong> has to be def<strong>in</strong>ed and<br />
mapped on stra<strong>in</strong> criteria, not on stratigraphic, lithological, isotopic or geochronological criteria. The most<br />
logical place to map <strong>the</strong> <strong>Ma<strong>in</strong></strong> <strong>Central</strong> <strong>Thrust</strong> is along <strong>the</strong> high-stra<strong>in</strong> zone that commonly occurs along <strong>the</strong><br />
base of <strong>the</strong> ductile shear zone and <strong>in</strong>verted metamorphic sequence. Above that horizon, all rocks show some<br />
degree of Tertiary <strong>Himalayan</strong> metamorphism, and most of <strong>the</strong> Greater <strong>Himalayan</strong> Sequence metamorphic or<br />
migmatitic rocks show some degree of pure shear and simple shear ductile stra<strong>in</strong> that occurs throughout <strong>the</strong><br />
mid-crustal Greater <strong>Himalayan</strong> Sequence channel. The <strong>Ma<strong>in</strong></strong> <strong>Central</strong> <strong>Thrust</strong> evolved both <strong>in</strong> time (early–<br />
middle Miocene) and space from a deep-level ductile shear zone to a shallow brittle thrust fault.<br />
The <strong>Himalayan</strong> <strong>Ma<strong>in</strong></strong> <strong>Central</strong> <strong>Thrust</strong>, with its zone of <strong>in</strong>verted<br />
metamorphic isograds from sillimanite grade down to biotite<br />
grade, is one of <strong>the</strong> largest ductile shear zones known from any<br />
collision-related mounta<strong>in</strong> belt. The <strong>Ma<strong>in</strong></strong> <strong>Central</strong> <strong>Thrust</strong> crops<br />
out along c. 2200 km length of <strong>the</strong> Himalaya from western<br />
Zanskar to Bhutan and Arunachal Pradesh (Fig. 1). It dips north<br />
and places high-grade metamorphic rocks of <strong>the</strong> Greater Himalaya<br />
south over unmetamorphosed rocks of <strong>the</strong> Lesser Himalaya.<br />
S<strong>in</strong>ce <strong>the</strong> discovery of an <strong>in</strong>verted metamorphic field gradient<br />
across <strong>the</strong> Darjeel<strong>in</strong>g–Sikkim Himalaya by Mallet (1874) and<br />
von Loczy (1878), and across <strong>the</strong> Indian Himalaya by Oldham<br />
(1883), it has been recognized that metamorphic grade <strong>in</strong>creases<br />
up-structural section towards <strong>the</strong> north from <strong>the</strong> Lesser Himalaya<br />
to <strong>the</strong> Greater Himalaya. In <strong>Nepal</strong>, pioneer<strong>in</strong>g geological studies<br />
by Hagen (1954), Hashimoto (1959, 1973) and Bordet (1961)<br />
also recognized <strong>the</strong> <strong>in</strong>crease of metamorphic grade up-structural<br />
section. The <strong>Ma<strong>in</strong></strong> <strong>Central</strong> <strong>Thrust</strong> zone was def<strong>in</strong>ed by Heim &<br />
Gansser (1939) and Gansser (1964) as <strong>the</strong> thrust fault that places<br />
high-grade metamorphic rocks of <strong>the</strong> Greater <strong>Himalayan</strong> Sequence<br />
southward over low-grade rocks of <strong>the</strong> Lesser Himalaya.<br />
Unfortunately, this def<strong>in</strong>ition is not useful, because Greater and<br />
Lesser <strong>Himalayan</strong> rocks refer to thrust-bounded structural<br />
packages. Typically thrusts cut up-stratigraphic section <strong>in</strong> <strong>the</strong><br />
footwall, along ramps plac<strong>in</strong>g older rocks over younger rocks.<br />
<strong>Thrust</strong>s may also follow flats plac<strong>in</strong>g similar age or younger<br />
rocks over older rocks. We emphasize here <strong>the</strong> dist<strong>in</strong>ction<br />
between structural term<strong>in</strong>ology (Greater <strong>Himalayan</strong> Sequence/<br />
Lesser Himalaya Sequence) and stratigraphic term<strong>in</strong>ology. S<strong>in</strong>ce<br />
<strong>the</strong> orig<strong>in</strong>al recognition of <strong>the</strong> <strong>Ma<strong>in</strong></strong> <strong>Central</strong> <strong>Thrust</strong> along <strong>the</strong><br />
Himalaya, <strong>the</strong>re has been a great amount of confusion regard<strong>in</strong>g<br />
<strong>the</strong> structural position and tim<strong>in</strong>g of slip along <strong>the</strong> <strong>Ma<strong>in</strong></strong> <strong>Central</strong><br />
<strong>Thrust</strong>. This has come about because many different structures<br />
are called <strong>the</strong> <strong>Ma<strong>in</strong></strong> <strong>Central</strong> <strong>Thrust</strong> by different workers. Clearly<br />
<strong>the</strong>re is an urgent need to f<strong>in</strong>d a common def<strong>in</strong>ition and location<br />
of <strong>the</strong> <strong>Ma<strong>in</strong></strong> <strong>Central</strong> <strong>Thrust</strong>, and <strong>in</strong> this paper we attempt to do<br />
that, based on comb<strong>in</strong>ed stra<strong>in</strong> and metamorphic criteria.<br />
Previous attempts to map <strong>the</strong> <strong>Ma<strong>in</strong></strong> <strong>Central</strong> <strong>Thrust</strong> have used<br />
<strong>in</strong>direct methods such as: (1) a lithological contrast follow<strong>in</strong>g a<br />
dist<strong>in</strong>ctive quartzite unit beneath an orthogneiss unit (e.g.<br />
Gansser 1983; Daniel et al. 2003); (2) follow<strong>in</strong>g <strong>the</strong> kyanite<br />
isograd (e.g. Bordet 1961; LeFort 1975; Colchen et al. 1986); (3)<br />
differences <strong>in</strong> U–Pb detrital zircon ages (e.g. Parrish & Hodges<br />
1996; Ahmad et al. 2000; DeCelles et al. 2000); (4) differences<br />
<strong>in</strong> Nd isotope compositions (e.g. Rob<strong>in</strong>son et al. 2001; Mart<strong>in</strong> et<br />
al. 2005; Richards et al. 2005, 2006); (5) location of young U–<br />
Pb and Th–Pb monazite ages (e.g. Harrison et al. 1997; Catlos<br />
et al. 2001, 2002;). None of <strong>the</strong>se methods <strong>in</strong> <strong>the</strong>mselves can be<br />
used <strong>in</strong>dependently to def<strong>in</strong>e a thrust fault. Lithology, detrital<br />
zircon ages and Nd isotopes give <strong>in</strong>formation on stratigraphy, not<br />
structural relationships. Isograds and monazite ages give <strong>in</strong>formation<br />
on metamorphic reactions, fluids, and tim<strong>in</strong>g of m<strong>in</strong>eral<br />
growth, not structure.<br />
523
524<br />
M. P. SEARLE ET AL.<br />
Fig. 1. Geological map of <strong>the</strong> Himalaya.<br />
Zones of high stra<strong>in</strong> have been documented across <strong>the</strong> <strong>Ma<strong>in</strong></strong><br />
<strong>Central</strong> <strong>Thrust</strong> zone <strong>in</strong> <strong>the</strong> Arun Valley by Brunel (1986) and<br />
Brunel & Kienast (1986) us<strong>in</strong>g k<strong>in</strong>ematic criteria. Abundant<br />
shear criteria (S–C fabrics, rolled garnets, etc.) and stretch<strong>in</strong>g<br />
l<strong>in</strong>eations show southward transport of <strong>the</strong> Greater <strong>Himalayan</strong><br />
Sequence along <strong>the</strong> <strong>Ma<strong>in</strong></strong> <strong>Central</strong> <strong>Thrust</strong> zone. Quantitative<br />
vorticity studies were first reported from <strong>the</strong> Sutlej Valley, India<br />
by Grasemann et al. (1999). Law et al. (2004) and Jessup et al.<br />
(2006) showed that mean k<strong>in</strong>ematic vorticity numbers from <strong>the</strong><br />
<strong>Ma<strong>in</strong></strong> <strong>Central</strong> <strong>Thrust</strong> zone along <strong>the</strong> Everest profile <strong>in</strong> <strong>Nepal</strong><br />
yielded 58–44% pure shear component <strong>in</strong> addition to <strong>the</strong><br />
dom<strong>in</strong>ant top-to-south simple shear. The fact that metamorphic<br />
isograds across <strong>the</strong> <strong>Ma<strong>in</strong></strong> <strong>Central</strong> <strong>Thrust</strong> ductile shear zone have<br />
been compressed or telescoped <strong>in</strong>to a 1–2 km thick section<br />
(Searle & Rex 1989; Hubbard 1996) shows that shear<strong>in</strong>g postdates<br />
peak metamorphism.<br />
Here we discuss <strong>the</strong> various previous methods used to map or<br />
def<strong>in</strong>e <strong>the</strong> <strong>Ma<strong>in</strong></strong> <strong>Central</strong> <strong>Thrust</strong> and <strong>the</strong>n we propose a unify<strong>in</strong>g<br />
def<strong>in</strong>ition and map location, <strong>in</strong> <strong>the</strong> hope that future studies<br />
relat<strong>in</strong>g to <strong>the</strong> <strong>Ma<strong>in</strong></strong> <strong>Central</strong> <strong>Thrust</strong> will refer to one s<strong>in</strong>gle <strong>Ma<strong>in</strong></strong><br />
<strong>Central</strong> <strong>Thrust</strong> ductile shear zone and brittle thrust fault.<br />
Metamorphic isograds and <strong>the</strong> <strong>Ma<strong>in</strong></strong> <strong>Central</strong> <strong>Thrust</strong><br />
The earliest attempt to map <strong>the</strong> <strong>Ma<strong>in</strong></strong> <strong>Central</strong> <strong>Thrust</strong> was carried<br />
out by Bordet (1961), who mapped it along a prom<strong>in</strong>ent kyanitebear<strong>in</strong>g<br />
pelite band with<strong>in</strong> sillimanite gneisses <strong>in</strong> <strong>the</strong> Arun valley,<br />
but did not describe any structural criteria to support this<br />
placement (Figs 2 and 3). The same location for <strong>the</strong> <strong>Ma<strong>in</strong></strong><br />
<strong>Central</strong> <strong>Thrust</strong> was subsequently used by Lombardo et al. (1993)<br />
and Pognante & Benna (1993), who mapped <strong>the</strong> <strong>Ma<strong>in</strong></strong> <strong>Central</strong><br />
<strong>Thrust</strong> fur<strong>the</strong>r north as far as Kharta <strong>in</strong> sou<strong>the</strong>rn Tibet. Above<br />
this kyanite-bear<strong>in</strong>g pelite horizon are some 5–8 km thickness of<br />
sillimanite + garnet + biotite + cordierite orthogneiss (variously<br />
called <strong>the</strong> Barun gneiss, Black gneiss or Jannu–Kangchenjunga<br />
gneiss) with evidence of abundant partial melt<strong>in</strong>g (Brunel &<br />
Kienast 1986; Lombardo et al. 1993; Pognante & Benna 1993;<br />
Searle & Szulc 2005). High-grade calc-silicate gneisses and
HIMALAYAN MAIN CENTRAL THRUST, NEPAL 525<br />
Fig. 2. Map of <strong>the</strong> Everest–Makalu–<br />
Kangchenjunga Himalaya <strong>in</strong> east <strong>Nepal</strong><br />
show<strong>in</strong>g location of <strong>the</strong> <strong>Ma<strong>in</strong></strong> <strong>Central</strong><br />
<strong>Thrust</strong> (MCT) and Greater <strong>Himalayan</strong><br />
Series. Shaded area represents <strong>the</strong> partially<br />
molten channel conta<strong>in</strong><strong>in</strong>g migmatites and<br />
leucogranites. The Bordet (1961) <strong>Ma<strong>in</strong></strong><br />
<strong>Central</strong> <strong>Thrust</strong> follows a prom<strong>in</strong>ent band of<br />
kyanite gneisses at Tashigaon village,<br />
with<strong>in</strong> <strong>the</strong> sillimanite-grade gneisses. Our<br />
proposed location of <strong>the</strong> <strong>Ma<strong>in</strong></strong> <strong>Central</strong><br />
<strong>Thrust</strong> is <strong>the</strong> ductile shear zone<br />
correspond<strong>in</strong>g to <strong>the</strong> zone of <strong>in</strong>verted<br />
metamorphic isograds above Tuml<strong>in</strong>gtar<br />
village along <strong>the</strong> Arun river, and near<br />
Taplejung <strong>in</strong> <strong>the</strong> Tamur river dra<strong>in</strong>age.<br />
LHS, Lesser Himalaya Series; GHS,<br />
Greater Himalaya Series; TSS, Tethyan<br />
sedimentary series.<br />
marbles conta<strong>in</strong><strong>in</strong>g oliv<strong>in</strong>e, cl<strong>in</strong>opyroxene, wollastonite and<br />
scapolite are <strong>in</strong>tercalated with <strong>the</strong> orthogneiss. P–T conditions<br />
reached upper amphibolite facies and even granulite facies at<br />
800–850 8C and 10–12 kbar, with later decompression follow<strong>in</strong>g<br />
a clockwise P–T–t path to 4–6 kbar dur<strong>in</strong>g sillimanite-grade<br />
metamorphism (Goscombe & Hand 2000; Dasgupta et al. 2004).<br />
Below <strong>the</strong> Bordet (1961) <strong>Ma<strong>in</strong></strong> <strong>Central</strong> <strong>Thrust</strong> horizon, <strong>the</strong><br />
Num orthogneiss is a 3–4 km thick unit of sillimanite + K-<br />
feldspar orthogneiss with about 15–20% <strong>in</strong> situ partial melt (Fig.<br />
3). Metamorphic grade is similar above and below this kyanite<br />
gneiss horizon, although protolith rocks are probably from different<br />
stratigraphic levels. Internal stra<strong>in</strong> is extremely high across<br />
this entire package with consistent shear criteria <strong>in</strong>dicat<strong>in</strong>g<br />
south-directed simple shear with a significant component of<br />
coaxial pure shear. Structurally beneath <strong>the</strong> Num orthogneiss,<br />
near Tuml<strong>in</strong>gtar, is a telescoped and highly sheared <strong>in</strong>verted<br />
metamorphic isograd sequence from sillimanite through kyanite<br />
and staurolite to biotite grade, where we map our preferred<br />
location for <strong>the</strong> <strong>Ma<strong>in</strong></strong> <strong>Central</strong> <strong>Thrust</strong>. Above this, all rocks have<br />
a Tertiary metamorphic impr<strong>in</strong>t on protoliths that range from<br />
Proterozoic to Mesozoic. Below our <strong>Ma<strong>in</strong></strong> <strong>Central</strong> <strong>Thrust</strong>, rocks<br />
have little or no Tertiary metamorphic overpr<strong>in</strong>t.<br />
Goscombe et al. (2006) recognized that <strong>the</strong> <strong>Ma<strong>in</strong></strong> <strong>Central</strong><br />
<strong>Thrust</strong> had been <strong>in</strong>correctly mapped as follow<strong>in</strong>g a stratigraphic<br />
boundary (<strong>the</strong>ir ‘<strong>Himalayan</strong> unconformity’) and <strong>the</strong>y mapped <strong>the</strong><br />
<strong>Ma<strong>in</strong></strong> <strong>Central</strong> <strong>Thrust</strong> beneath <strong>the</strong> Ulleri–Phaplu augen gneiss <strong>in</strong>
526<br />
M. P. SEARLE ET AL.<br />
Fig. 3. Simplified schematic section across<br />
<strong>the</strong> Everest–Makalu Himalaya show<strong>in</strong>g key<br />
features of <strong>the</strong> structure, stratigraphy and<br />
m<strong>in</strong>eral isograds, toge<strong>the</strong>r with our<br />
proposed location of <strong>the</strong> <strong>Ma<strong>in</strong></strong> <strong>Central</strong><br />
<strong>Thrust</strong> <strong>in</strong> eastern <strong>Nepal</strong>. bt, biotite; grt,<br />
garnet; st, staurolite; ky, kyanite; sill,<br />
sillimanite; crd, cordierite; ms, muscovite;<br />
kfs, K-feldspar.<br />
eastern <strong>Nepal</strong>. However, <strong>the</strong>y also def<strong>in</strong>ed a new structure, <strong>the</strong><br />
‘High Himal <strong>Thrust</strong>’, close to, or along <strong>the</strong> kyanite pelite band<br />
and <strong>the</strong> Bordet (1961) <strong>Ma<strong>in</strong></strong> <strong>Central</strong> <strong>Thrust</strong>. Sillimanite +<br />
muscovite + K-feldspar grade gneisses and migmatites occur<br />
both above and below this horizon, and we suggest that both are<br />
part of <strong>the</strong> same Greater <strong>Himalayan</strong> Sequence metamorphic<br />
package. Goscombe et al. (2006) def<strong>in</strong>ed a <strong>Ma<strong>in</strong></strong> <strong>Central</strong> <strong>Thrust</strong><br />
zone that extends down-section from this horizon south as far as<br />
<strong>the</strong> garnet isograd beneath <strong>the</strong> Ulleri–Phaplu orthogneiss (Fig.<br />
3).<br />
In <strong>the</strong> Annapurna–Manaslu region of central <strong>Nepal</strong> LeFort<br />
(1975), Colchen et al. (1986) and Pêcher (1989) mapped <strong>the</strong><br />
<strong>Ma<strong>in</strong></strong> <strong>Central</strong> <strong>Thrust</strong> as follow<strong>in</strong>g <strong>the</strong> kyanite isograd (at Dana <strong>in</strong><br />
<strong>the</strong> Kali Gandaki valley and Bahundanda <strong>in</strong> <strong>the</strong> Marsyandi<br />
valley; Figs 4–6). There is no doubt that <strong>the</strong> kyanite gneisses are<br />
highly stra<strong>in</strong>ed, but so too are most of <strong>the</strong> rocks structurally<br />
above and below this horizon. High-stra<strong>in</strong> shear fabrics are<br />
particularly prom<strong>in</strong>ent <strong>in</strong> pelitic schists and K-feldspar augen<br />
gneisses, but less apparent <strong>in</strong> <strong>the</strong> more massive homogeneous<br />
marble horizons. The location of <strong>the</strong> <strong>Ma<strong>in</strong></strong> <strong>Central</strong> <strong>Thrust</strong> as<br />
mapped by LeFort (1975) and Colchen et al. (1986) was followed<br />
by most subsequent workers <strong>in</strong> <strong>the</strong> region (e.g. Harrison et al.<br />
1997; Kohn et al. 2001).<br />
Hodges et al. (1996) recognized several shear zones and<br />
thrusts across a ‘<strong>Ma<strong>in</strong></strong> <strong>Central</strong> <strong>Thrust</strong> zone’ and mapped <strong>the</strong><br />
sou<strong>the</strong>rn limit of <strong>Ma<strong>in</strong></strong> <strong>Central</strong> <strong>Thrust</strong> shear<strong>in</strong>g close to <strong>the</strong><br />
village of Lamdrung <strong>in</strong> <strong>the</strong> Modi khola. Searle & God<strong>in</strong> (2003)<br />
mapped <strong>the</strong> entire <strong>in</strong>verted metamorphic sequence as be<strong>in</strong>g part<br />
of <strong>the</strong> Greater <strong>Himalayan</strong> Sequence and placed <strong>the</strong> <strong>Ma<strong>in</strong></strong> <strong>Central</strong><br />
<strong>Thrust</strong> brittle fault along <strong>the</strong> base of <strong>the</strong> <strong>in</strong>verted metamorphic<br />
sequence (Fig. 5). This locality marks a sharp break between<br />
highly stra<strong>in</strong>ed rocks affected by Tertiary metamorphism <strong>in</strong> <strong>the</strong><br />
hang<strong>in</strong>g wall and rocks beneath this that are not highly<br />
metamorphosed or highly stra<strong>in</strong>ed. Orthogneiss horizons such as<br />
<strong>the</strong> Proterozoic Ulleri augen gneiss <strong>in</strong> <strong>the</strong> Annapurna region, or<br />
<strong>the</strong> Phaplu augen gneiss <strong>in</strong> <strong>the</strong> Everest region, previously<br />
assigned to <strong>the</strong> ‘upper Lesser Himalaya’ thrust sheet, are now<br />
more logically placed with<strong>in</strong> <strong>the</strong> Greater <strong>Himalayan</strong> Sequence<br />
thrust sheet above <strong>the</strong> <strong>Ma<strong>in</strong></strong> <strong>Central</strong> <strong>Thrust</strong>.<br />
Arita (1983) mapped a <strong>Ma<strong>in</strong></strong> <strong>Central</strong> <strong>Thrust</strong> zone of high<br />
ductile stra<strong>in</strong> up to 2–3 km thick, bounded by an upper thrust,<br />
MCT-2 (Vaikrita thrust), and a lower thrust, MCT-1 (Munsiari<br />
thrust). The earlier, upper MCT-2 corresponds to <strong>the</strong> Colchen et<br />
al. (1986) <strong>Ma<strong>in</strong></strong> <strong>Central</strong> <strong>Thrust</strong> whereas <strong>the</strong> lower, later MCT-1<br />
corresponds to our proposed location of <strong>the</strong> <strong>Ma<strong>in</strong></strong> <strong>Central</strong> <strong>Thrust</strong>.<br />
The rocks between <strong>the</strong>se two thrusts show peak metamorphic<br />
temperatures rang<strong>in</strong>g between 550 8C and less than 330 8C, with<br />
an <strong>in</strong>verted <strong>the</strong>rmal gradient as deduced from Raman spectroscopy<br />
of carbonaceous material by Beyssac et al. (2004) and<br />
Boll<strong>in</strong>ger et al. (2004). Those workers accepted <strong>the</strong> location of<br />
<strong>the</strong> Colchen et al. (1986) <strong>Ma<strong>in</strong></strong> <strong>Central</strong> <strong>Thrust</strong>, and so placed<br />
<strong>the</strong>se rocks <strong>in</strong> <strong>the</strong> Lesser Himalaya. However, we <strong>in</strong>clude all<br />
<strong>the</strong>se metamorphic rocks <strong>in</strong> <strong>the</strong> Greater <strong>Himalayan</strong> Sequence<br />
above our proposed <strong>Ma<strong>in</strong></strong> <strong>Central</strong> <strong>Thrust</strong>.<br />
With<strong>in</strong> <strong>the</strong> <strong>Ma<strong>in</strong></strong> <strong>Central</strong> <strong>Thrust</strong> ductile shear zone, Kohn et al.<br />
(2001) showed that garnets from structurally lower locations<br />
grew with <strong>in</strong>creas<strong>in</strong>g P and T (load<strong>in</strong>g), whereas garnets from<br />
structurally higher locations grew with <strong>in</strong>creas<strong>in</strong>g T but decreas<strong>in</strong>g<br />
P (exhumation). This records a snapshot <strong>in</strong> time of <strong>the</strong><br />
cont<strong>in</strong>uously evolv<strong>in</strong>g northward burial (prograde metamorphism)<br />
and southward exhumation (decompression and retrograde<br />
metamorphism) particle paths of Greater <strong>Himalayan</strong> Sequence<br />
metamorphic rocks. Searle et al. (2002) suggested that <strong>the</strong><br />
position of <strong>the</strong> <strong>Ma<strong>in</strong></strong> <strong>Central</strong> <strong>Thrust</strong> along <strong>the</strong> Darondi valley<br />
should be 15 km south of where it was mapped by Colchen et al.<br />
(1986) and Kohn et al. (2001), along <strong>the</strong> base of <strong>the</strong> <strong>in</strong>verted<br />
metamorphic sequence (‘Location of <strong>in</strong>ferred structure’ <strong>in</strong> fig. 1<br />
of Kohn et al. 2001).<br />
Dadeldhura and Ramgarh thrusts<br />
DeCelles et al. (2001) and Rob<strong>in</strong>son et al. (2003, 2006) mapped<br />
two thrust sheets structurally beneath <strong>the</strong> <strong>Ma<strong>in</strong></strong> <strong>Central</strong> <strong>Thrust</strong> <strong>in</strong><br />
western <strong>Nepal</strong>, <strong>the</strong> higher Dadeldhura and lower Ramgarh thrust<br />
sheets. They failed to locate a discrete thrust at <strong>the</strong> position of<br />
<strong>the</strong> <strong>Ma<strong>in</strong></strong> <strong>Central</strong> <strong>Thrust</strong> and <strong>in</strong>ferred its presence from extrapolation<br />
along strike <strong>in</strong> <strong>the</strong> Karnali Valley. The Dadeldhura thrust<br />
sheet consists of garnet–muscovite–biotite schists, mylonitic<br />
augen gneiss and Cambrian–Ordovician granites. The Ramgarh<br />
thrust sheet consists of greenschist-facies metasedimentary rocks
HIMALAYAN MAIN CENTRAL THRUST, NEPAL 527<br />
Fig. 4. Map of <strong>the</strong> Annapurna–Manaslu<br />
Himalaya, show<strong>in</strong>g <strong>the</strong> structure of <strong>the</strong><br />
Greater <strong>Himalayan</strong> Sequence and our<br />
proposed location of <strong>the</strong> <strong>Ma<strong>in</strong></strong> <strong>Central</strong><br />
<strong>Thrust</strong>. The Colchen et al. (1986) <strong>Ma<strong>in</strong></strong><br />
<strong>Central</strong> <strong>Thrust</strong> is co<strong>in</strong>cident with <strong>the</strong><br />
kyanite isograd runn<strong>in</strong>g through Dana and<br />
Bahundanda villages. Our <strong>Ma<strong>in</strong></strong> <strong>Central</strong><br />
<strong>Thrust</strong> is located along a high-stra<strong>in</strong> zone<br />
fur<strong>the</strong>r south, south of Gorhka, and<br />
corresponds to <strong>the</strong> sou<strong>the</strong>rn limit of Tertiary<br />
metamorphism. The mapped locations of<br />
<strong>the</strong> South Tibetan Detachment system<br />
(STDS) normal faults are from Searle &<br />
God<strong>in</strong> (2003).<br />
Fig. 5. Simplified, schematic section across<br />
<strong>the</strong> Annapurna Himalaya show<strong>in</strong>g key<br />
features of <strong>the</strong> structure, stratigraphy and<br />
m<strong>in</strong>eral isograds, and our proposed location<br />
of <strong>the</strong> <strong>Ma<strong>in</strong></strong> <strong>Central</strong> <strong>Thrust</strong> <strong>in</strong> central <strong>Nepal</strong>.<br />
Shaded area represents <strong>the</strong> migmatites and<br />
leucogranites with<strong>in</strong> <strong>the</strong> partially molten<br />
channel.<br />
of <strong>the</strong> Kushma and Ranimata Formations. Both <strong>the</strong> Dadeldhura<br />
and Ramgarh thrust sheets occur <strong>in</strong> a synformal klippe that is a<br />
structural equivalent of <strong>the</strong> Almora klippe to <strong>the</strong> west <strong>in</strong> India<br />
and <strong>the</strong> Kathmandu klippe to <strong>the</strong> east. The Ramgarh thrust forms<br />
<strong>the</strong> roof thrust to a series of imbricated thrust slices of<br />
unmetamorphosed Lesser <strong>Himalayan</strong> rocks of Late Archaean,<br />
Proterozoic and Cambrian age (DeCelles et al. 2001; Rob<strong>in</strong>son<br />
et al. 2006).
528<br />
M. P. SEARLE ET AL.<br />
Fig. 6. Simplified, schematic section across<br />
<strong>the</strong> Manaslu Himalaya, show<strong>in</strong>g key<br />
features and our proposed location of <strong>the</strong><br />
<strong>Ma<strong>in</strong></strong> <strong>Central</strong> <strong>Thrust</strong>. Shaded area<br />
represents <strong>the</strong> zone of partial melt<strong>in</strong>g with<br />
migmatites and leucogranites (crosses). The<br />
Manaslu leuocogranite is wholly with<strong>in</strong> <strong>the</strong><br />
Greater <strong>Himalayan</strong> Sequence, follow<strong>in</strong>g<br />
Searle & God<strong>in</strong> (2003), with <strong>the</strong> South<br />
Tibetan detachment (STD) wrapp<strong>in</strong>g around<br />
<strong>the</strong> upper level of <strong>the</strong> granite.<br />
The Ramgarh thrust marks <strong>the</strong> sou<strong>the</strong>rn limit of Tertiary<br />
<strong>Himalayan</strong> metamorphism <strong>in</strong> western <strong>Nepal</strong> and we prefer to l<strong>in</strong>k<br />
this with <strong>the</strong> <strong>Ma<strong>in</strong></strong> <strong>Central</strong> <strong>Thrust</strong>. The reported location of <strong>the</strong><br />
Ramgarh thrust <strong>in</strong> central and eastern <strong>Nepal</strong> (Mart<strong>in</strong> et al. 2005;<br />
Pearson & DeCelles 2005), however, does not co<strong>in</strong>cide with <strong>the</strong><br />
<strong>Ma<strong>in</strong></strong> <strong>Central</strong> <strong>Thrust</strong>. In central and eastern <strong>Nepal</strong> <strong>the</strong> location of<br />
<strong>the</strong> Ramgarh thrust is almost entirely <strong>in</strong>terpreted from lithological<br />
repetition; a possible fault surface has been observed only<br />
<strong>in</strong> <strong>the</strong> Tribeni area of eastern <strong>Nepal</strong>. A more sou<strong>the</strong>rly location<br />
for <strong>the</strong> Ramgarh thrust is supported by pervasive deformation<br />
documented by quartz c-axis fabrics throughout central <strong>Nepal</strong><br />
(Bouchez & Pêcher 1981). Although <strong>the</strong> recrystallization of<br />
quartz under significantly high stra<strong>in</strong> has been recognized <strong>in</strong> <strong>the</strong><br />
Ramgrah thrust sheet, as mapped by Mart<strong>in</strong> et al. (2005) and<br />
Pearson & DeCelles (2005), it is <strong>in</strong>terpreted to be locally<br />
conf<strong>in</strong>ed to <strong>the</strong> immediate hang<strong>in</strong>g wall of <strong>the</strong> <strong>in</strong>ferred thrust<br />
(Pearson & DeCelles 2005). However, Bouchez & Pêcher (1981)<br />
showed that quartz c-axis fabrics are preserved for more than<br />
6 km far<strong>the</strong>r south than <strong>the</strong> mapped position of <strong>the</strong> Ramgarh<br />
thrust. Both lithological and structural data fit better with a<br />
structurally lower, more sou<strong>the</strong>rly, Ramgarh thrust where it is<br />
co<strong>in</strong>cident with <strong>the</strong> <strong>Ma<strong>in</strong></strong> <strong>Central</strong> <strong>Thrust</strong>.<br />
Restored sections show that <strong>the</strong> Ramgarh, Dadeldhura and<br />
<strong>Ma<strong>in</strong></strong> <strong>Central</strong> thrust sheets of DeCelles et al. (2001) all have<br />
Proterozoic sedimentary rocks, Ulleri augen gneiss and Cambrian–Ordovician<br />
sedimentary rocks and granites as protoliths.<br />
Hang<strong>in</strong>g-wall–footwall cut-offs can be successfully matched <strong>in</strong><br />
restored sections (Fig. 7). We <strong>the</strong>refore propose that <strong>the</strong> <strong>Ma<strong>in</strong></strong><br />
Fig. 7. Generalized restored section across <strong>the</strong> <strong>Nepal</strong> Himalaya show<strong>in</strong>g <strong>the</strong> pre-thrust<strong>in</strong>g trajectories of <strong>the</strong> <strong>Ma<strong>in</strong></strong> <strong>Central</strong> <strong>Thrust</strong> and South Tibetan<br />
Detachment shear zones and faults. The shaded horizon represents <strong>the</strong> Upper Proterozoic sedimentary rocks of <strong>the</strong> Lesser Himalaya, and Greater<br />
Himalaya. With<strong>in</strong> <strong>the</strong> Greater <strong>Himalayan</strong> Sequence <strong>the</strong>se <strong>in</strong>clude <strong>the</strong> metamorphosed rocks of <strong>the</strong> Nawakot Group above <strong>the</strong> Ramgarh thrust (<strong>Ma<strong>in</strong></strong><br />
<strong>Central</strong> <strong>Thrust</strong>) and <strong>the</strong> Bhimpedi Group with<strong>in</strong> <strong>the</strong> Kathmandu nappe, above <strong>the</strong> Mahabharat thrust. Sillimanite-grade pelitic gneisses with<strong>in</strong> <strong>the</strong> Greater<br />
<strong>Himalayan</strong> Sequence are <strong>in</strong>terpreted as metamorphosed Upper Proterozoic sedimentary rocks of <strong>the</strong> Haimanta–Vaikrita Group. The base of <strong>the</strong> Tethyan<br />
Himalaya consists of similar Neoproterozoic sedimentary rocks show<strong>in</strong>g that <strong>the</strong> Lesser, Greater and Tethyan Himalaya were all part of one contiguous<br />
Indian plate.
HIMALAYAN MAIN CENTRAL THRUST, NEPAL 529<br />
<strong>Central</strong> <strong>Thrust</strong> should be mapped along <strong>the</strong> Ramgarh thrust, and<br />
all rocks above that should be <strong>in</strong>corporated <strong>in</strong>to <strong>the</strong> Greater<br />
<strong>Himalayan</strong> Sequence. The major thrust systems propagated<br />
southward with time from <strong>the</strong> Early Miocene motion of <strong>the</strong><br />
Greater <strong>Himalayan</strong> Sequence metamorphic core (Hodges et al.<br />
1996; God<strong>in</strong> et al. 2001) to <strong>the</strong> ,15 Ma motion along <strong>the</strong><br />
Ramgarh thrust (DeCelles et al. 2001; Rob<strong>in</strong>son et al. 2006).<br />
Dur<strong>in</strong>g <strong>the</strong> Late Miocene ductile shear<strong>in</strong>g along <strong>the</strong> <strong>Ma<strong>in</strong></strong> <strong>Central</strong><br />
<strong>Thrust</strong>–Ramgarh thrust ceased, and thrust<strong>in</strong>g propagated downsection<br />
to <strong>the</strong> Lesser <strong>Himalayan</strong> brittle imbricate thrust system.<br />
At least 120 km of southward translation has been estimated<br />
across <strong>the</strong> Ramgarh thrust sheet (Rob<strong>in</strong>son et al. 2006).<br />
Mahabharat thrust<br />
Several klippen or thrust sheets of high-grade metamorphic rocks<br />
and granites (e.g. Almora klippe; Kathmandu complex) overlie<br />
low-grade or unmetamorphosed rocks of <strong>the</strong> Lesser Himalaya to<br />
<strong>the</strong> south of <strong>the</strong> ma<strong>in</strong> <strong>Ma<strong>in</strong></strong> <strong>Central</strong> <strong>Thrust</strong>, as mapped <strong>in</strong><br />
Langtang and <strong>the</strong> Ganesh Himal (Fig. 1). The thrust beneath<br />
<strong>the</strong>se klippen has been variously termed <strong>the</strong> Almora or Munsiari<br />
thrusts <strong>in</strong> India (Heim & Gansser 1939; Valdiya 1980), and <strong>the</strong><br />
Mahabharat and Dadeldhura thrusts <strong>in</strong> <strong>Nepal</strong> (Stöckl<strong>in</strong> &<br />
Bhattarai 1980; Upreti & LeFort 1999; Johnson et al. 2001). The<br />
Mahabharat thrust beneath <strong>the</strong> Kathmandu klippe encircles <strong>the</strong><br />
sou<strong>the</strong>rn Kathmandu valley and l<strong>in</strong>ks up with <strong>the</strong> <strong>Ma<strong>in</strong></strong> <strong>Central</strong><br />
<strong>Thrust</strong> <strong>in</strong> <strong>the</strong> NW and NE (Upreti & LeFort 1999; Johnson et al.<br />
2001). Rocks above <strong>the</strong> Mahabharat thrust <strong>in</strong>clude Proterozoic<br />
Bhimpedi Group and early–middle Palaeozoic Phulchauki Group<br />
sedimentary rocks, which are <strong>in</strong>truded by Ordovician granites<br />
and augen gneisses. Metamorphism reaches kyanite grade at <strong>the</strong><br />
base and isograds are right-way-up from kyanite through garnet<br />
and biotite to chlorite grade (Johnson et al. 2001). Along <strong>the</strong><br />
Mahabharat thrust dynamic metamorphism has locally <strong>in</strong>verted<br />
<strong>the</strong> <strong>the</strong>rmal gradient with formation of garnet–biotite mylonites<br />
and phyllonites. Beneath <strong>the</strong> Mahabharat thrust carbonaceous<br />
pelites of <strong>the</strong> Nawakot Group show an <strong>in</strong>verted <strong>the</strong>rmal gradient<br />
from 468 8C to less than 330 8C, based on Raman spectroscopy<br />
of carbonaceous material (Beyssac et al. 2004; Boll<strong>in</strong>ger et al.<br />
2004).<br />
These crystall<strong>in</strong>e complexes have been termed ‘Lesser <strong>Himalayan</strong><br />
crystall<strong>in</strong>es’ or ‘Outer Lesser <strong>Himalayan</strong> crystall<strong>in</strong>es’, but<br />
<strong>in</strong> reality <strong>the</strong>y are lateral equivalents to <strong>the</strong> Greater <strong>Himalayan</strong><br />
Sequence above <strong>the</strong> <strong>Ma<strong>in</strong></strong> <strong>Central</strong> <strong>Thrust</strong>. They share similar<br />
upper Proterozoic and lower Palaeozoic protoliths, similar Miocene<br />
metamorphism, and similar 21–18 Ma leucogranite pegmatite<br />
dykes as <strong>the</strong> ma<strong>in</strong> Greater <strong>Himalayan</strong> Sequence to <strong>the</strong> north<br />
(Johnson et al. 2001). We concur with <strong>the</strong> conclusions of<br />
Johnson et al. (2001) and Johnson (2005) that <strong>the</strong> Mahabharat<br />
thrust is <strong>the</strong> same structure as <strong>the</strong> <strong>Ma<strong>in</strong></strong> <strong>Central</strong> <strong>Thrust</strong>, but along<br />
a more sou<strong>the</strong>rly, proximal position <strong>in</strong> <strong>the</strong> restoration. The<br />
Mahabharat thrust climbs up-section <strong>in</strong> <strong>the</strong> transport direction,<br />
from be<strong>in</strong>g along <strong>the</strong> base of <strong>the</strong> <strong>in</strong>verted metamorphic sequence<br />
at Langtang <strong>in</strong> <strong>the</strong> north, to along <strong>the</strong> isograd fold h<strong>in</strong>ge at<br />
Kathmandu (Fig. 8). The youngest thrust splays off beneath <strong>the</strong><br />
Kathmandu complex to l<strong>in</strong>k with <strong>the</strong> Ramgarh thrust to <strong>the</strong> south<br />
and west of <strong>the</strong> Kathmandu complex (Fig. 1). All <strong>the</strong>se thrusts<br />
are part of <strong>the</strong> <strong>Ma<strong>in</strong></strong> <strong>Central</strong> <strong>Thrust</strong> but are restored at different<br />
depths, progressively ramp<strong>in</strong>g up-section towards <strong>the</strong> south.<br />
The Darjeel<strong>in</strong>g klippe is ano<strong>the</strong>r structural outlier of Greater<br />
Fig. 8. Geometry of <strong>the</strong> <strong>Ma<strong>in</strong></strong> <strong>Central</strong> <strong>Thrust</strong> zone <strong>in</strong> <strong>the</strong> Langtang–Kathmandu nappe region of central <strong>Nepal</strong> show<strong>in</strong>g <strong>the</strong> relationship of <strong>the</strong> Mahabharat<br />
and Ramgarh thrusts to <strong>the</strong> metamorphic isograds. This geometry comb<strong>in</strong>es <strong>the</strong> folded isograd model of Searle & Rex (1989) with <strong>the</strong> channel flow model<br />
for <strong>the</strong> Greater <strong>Himalayan</strong> Sequence (Law et al. 2006; Searle et al. 2006) and with <strong>the</strong> Johnson (2005) structural model for <strong>the</strong> Mahabharat thrust and<br />
Kathmandu nappe. It also expla<strong>in</strong>s <strong>the</strong> structural location of <strong>the</strong> greenschist- and amphibolite-facies metamorphic rocks of <strong>the</strong> Ramgarh thrust sheet<br />
(Beyssac et al. 2004; Boll<strong>in</strong>ger et al. 2004) structurally beneath <strong>the</strong> Mahabharat thrust. Shaded area shows <strong>the</strong> zone of partial melt<strong>in</strong>g (sillimanite +<br />
K-feldspar gneisses, migmatites), with <strong>the</strong> ma<strong>in</strong> leucogranites (crosses) concentrated along <strong>the</strong> north. Metamorphic isograds have been sheared (by a<br />
comb<strong>in</strong>ation of pure shear and south-directed simple shear), and flattened along <strong>the</strong> South Tibetan Detachment ductile shear zone above (right-way-up<br />
metamorphic isograds) and along <strong>the</strong> <strong>Ma<strong>in</strong></strong> <strong>Central</strong> <strong>Thrust</strong> ductile shear zone below (<strong>in</strong>verted metamorphic isograds).
530<br />
M. P. SEARLE ET AL.<br />
<strong>Himalayan</strong> Sequence rocks thrust above unmetamorphosed Lesser<br />
<strong>Himalayan</strong> sedimentary rocks <strong>in</strong> far eastern <strong>Nepal</strong> and<br />
Sikkim–West Bengal (Fig. 1). The full <strong>in</strong>verted metamorphic<br />
isograd sequence has been mapped here, but unlike <strong>the</strong> Kathmandu<br />
complex, and like <strong>the</strong> Greater <strong>Himalayan</strong> Sequence, <strong>the</strong><br />
isograds are structurally <strong>in</strong>verted from sillimanite down to<br />
biotite–chlorite (Mohan et al. 1989; Dasgupta et al. 2004). The<br />
structures and metamorphic P–T conditions clearly show that <strong>the</strong><br />
Darjeel<strong>in</strong>g klippe is l<strong>in</strong>ked to <strong>the</strong> ma<strong>in</strong> <strong>Ma<strong>in</strong></strong> <strong>Central</strong> <strong>Thrust</strong> to<br />
<strong>the</strong> north (Searle & Szulc 2005; Fig. 3).<br />
Detrital zircon ages and <strong>the</strong> <strong>Ma<strong>in</strong></strong> <strong>Central</strong> <strong>Thrust</strong><br />
Detrital zircon U–Pb ages provide maximum depositional age<br />
constra<strong>in</strong>ts of <strong>the</strong> metamorphic protolith. Parrish & Hodges<br />
(1996) orig<strong>in</strong>ally proposed that Greater and Lesser <strong>Himalayan</strong><br />
rocks, divided by <strong>the</strong> <strong>Ma<strong>in</strong></strong> <strong>Central</strong> <strong>Thrust</strong>, had a significant<br />
difference <strong>in</strong> sedimentary provenance. Zircons from Greater<br />
<strong>Himalayan</strong> Sequence rocks have ma<strong>in</strong>ly late Proterozoic and<br />
early Palaeozoic ages, whereas zircons from <strong>the</strong> Lesser Himalaya<br />
have late Archaean–early Proterozoic ages. DeCelles et al.<br />
(2000) showed that metasedimentary rocks from <strong>the</strong> Greater<br />
<strong>Himalayan</strong> Sequence gave zircon ages of 800–1700 Ma, whereas<br />
quartzites of one unit generally mapped with<strong>in</strong> <strong>the</strong> Lesser<br />
Himalaya Sequence, <strong>the</strong> Nawakot Group, yielded zircons<br />
.1.8 Ga old. This reflects <strong>the</strong> 1866–1833 Ma depositional age<br />
of <strong>the</strong>se units. The upper age limit is given by <strong>the</strong> Ulleri augen<br />
gneiss, which was <strong>in</strong>truded <strong>in</strong>to <strong>the</strong> Nawakot quartzite. However,<br />
a major mylonite zone correspond<strong>in</strong>g to <strong>the</strong> <strong>Ma<strong>in</strong></strong> <strong>Central</strong> <strong>Thrust</strong><br />
was mapped beneath <strong>the</strong> Ulleri augen gneiss <strong>in</strong> <strong>the</strong> Annapurna<br />
region (Searle & God<strong>in</strong> 2003), so <strong>the</strong>se rocks are now <strong>in</strong>cluded<br />
with<strong>in</strong> <strong>the</strong> Greater <strong>Himalayan</strong> Sequence. The Phaplu augen<br />
gneiss <strong>in</strong> <strong>the</strong> Everest profile is a similar age, and at a similar<br />
structural position to <strong>the</strong> Ulleri augen gneiss. The Phaplu gneiss<br />
is highly sheared and overla<strong>in</strong> by staurolite- and sillimanite-grade<br />
rocks typical of <strong>the</strong> Greater <strong>Himalayan</strong> Sequence, so it has also<br />
been <strong>in</strong>cluded here with<strong>in</strong> <strong>the</strong> Greater <strong>Himalayan</strong> Sequence<br />
(Jessup et al. 2006; Searle et al. 2006).<br />
There seems little doubt that many Lesser <strong>Himalayan</strong> protoliths,<br />
particularly <strong>in</strong> <strong>Nepal</strong>, are older than <strong>the</strong> exposed Greater<br />
<strong>Himalayan</strong> Sequence protoliths. The upper structural levels of<br />
<strong>the</strong> Lesser Himalaya <strong>in</strong> India (Cambrian Krol and Tal Formations,<br />
which overlie Proterozoic rocks) are lateral equivalents to<br />
<strong>the</strong> base of <strong>the</strong> restored Greater <strong>Himalayan</strong> Sequence (Steck<br />
2003) It is also widely accepted that <strong>the</strong> Greater <strong>Himalayan</strong><br />
Sequence protoliths were similar <strong>in</strong> age to <strong>the</strong> lower levels of <strong>the</strong><br />
Tethyan Himalaya, which range <strong>in</strong> age from Neoproterozoic to<br />
Eocene. Indeed, <strong>in</strong> <strong>the</strong> Zanskar Himalaya <strong>in</strong> India, Searle (1986)<br />
and Walker et al. (2001) were able to correlate thick garnet<br />
amphibolite units <strong>in</strong> <strong>the</strong> Greater <strong>Himalayan</strong> Sequence with<br />
unmetamorphosed Permian Panjal volcanic rocks <strong>in</strong> Kashmir,<br />
and thick high-grade marbles of <strong>the</strong> Greater <strong>Himalayan</strong> Sequence<br />
with unmetamorphosed Triassic (and possible Jurassic) shelf<br />
carbonate units with<strong>in</strong> <strong>the</strong> Tethyan Himalaya.<br />
Nd isotopes and <strong>the</strong> <strong>Ma<strong>in</strong></strong> <strong>Central</strong> <strong>Thrust</strong><br />
Several workers (e.g. DeCelles et al. 2000; Rob<strong>in</strong>son et al. 2001;<br />
Richards et al. 2005) have described <strong>the</strong> <strong>Ma<strong>in</strong></strong> <strong>Central</strong> <strong>Thrust</strong> as<br />
a ‘discrete ductile shear zone separat<strong>in</strong>g isotopically different<br />
protoliths’. Parrish & Hodges (1996) first proposed that <strong>the</strong>re<br />
was no overlap between <strong>the</strong> ranges of 143 Nd/ 144 Nd ratios between<br />
Greater <strong>Himalayan</strong> Sequence and Lesser Himalaya rocks <strong>in</strong> <strong>the</strong><br />
Langtang region. DeCelles et al. (2000) and Rob<strong>in</strong>son et al.<br />
(2001) showed that ENd(0) average values from Lesser <strong>Himalayan</strong><br />
rocks <strong>in</strong> <strong>Nepal</strong> are 21.5, whereas <strong>the</strong> Greater and Tethyan<br />
Himalaya zones <strong>in</strong> <strong>Nepal</strong> have an average ENd(0) value of 16.<br />
They suggested that <strong>the</strong> Greater <strong>Himalayan</strong> Sequence was not<br />
Indian basement, but ra<strong>the</strong>r a terrane that was accreted onto India<br />
dur<strong>in</strong>g <strong>the</strong> Early Palaeozoic, and that <strong>the</strong> <strong>Ma<strong>in</strong></strong> <strong>Central</strong> <strong>Thrust</strong><br />
had a large amount of pre-Tertiary displacement. However, <strong>the</strong>re<br />
is no evidence of Palaeozoic suture zone rocks (e.g. ophiolites,<br />
deep-sea sediments, etc.) anywhere along <strong>the</strong> <strong>Ma<strong>in</strong></strong> <strong>Central</strong><br />
<strong>Thrust</strong>, and pal<strong>in</strong>spastic reconstructions across <strong>the</strong> Western<br />
Himalaya (Searle 1986; Steck 2003), <strong>the</strong> Everest profile <strong>in</strong> <strong>Nepal</strong><br />
(Searle et al. 2006) and <strong>the</strong> Sikkim–Bhutan Himalaya (Searle &<br />
Szulc 2005) show a cont<strong>in</strong>uous sedimentary succession from<br />
proximal to distal across <strong>the</strong> Lesser, Greater and Tethyan<br />
Himalaya.<br />
With larger Nd isotope datasets, <strong>the</strong> dist<strong>in</strong>ctive differences<br />
between Greater <strong>Himalayan</strong> Sequence and Lesser Himalaya<br />
protolith start to vanish. Ahmad et al. (2000) and Richards et al.<br />
(2005) recognized a separate zone termed <strong>the</strong> ‘Outer Lesser<br />
Himalaya’ that had relatively young source rocks, similar to <strong>the</strong><br />
Greater <strong>Himalayan</strong> Sequence. They concluded that Greater<br />
<strong>Himalayan</strong> Sequence and ‘Outer Lesser Himalaya’ rocks showed<br />
a Meso-Palaeo-Proterozoic source, whereas <strong>the</strong> rest of <strong>the</strong> Lesser<br />
Himalaya showed Late Archaean to Early Proterozoic source<br />
rocks. However, Myrow et al. (2003) showed that samples from<br />
<strong>the</strong> base of <strong>the</strong> Tethyan Himalaya, north of <strong>the</strong> Greater <strong>Himalayan</strong><br />
Sequence, have similar detrital zircon age spectra and Nd<br />
isotopic data to samples from <strong>the</strong> Kathmandu klippe and Lesser<br />
Himalaya south of <strong>the</strong> Greater <strong>Himalayan</strong> Sequence, thus elim<strong>in</strong>at<strong>in</strong>g<br />
<strong>the</strong> need for separate Greater <strong>Himalayan</strong> Sequence–Lesser<br />
Himalaya ‘terranes’. Lesser, Greater and Tethyan Himalaya<br />
represent a proximal to distal section across a cont<strong>in</strong>uous Indian<br />
plate prior to collision with Asia (Searle 1986; Myrow et al.<br />
2003; Steck 2003; Searle et al. 2006). Mart<strong>in</strong> et al. (2005) also<br />
correctly recognized that <strong>the</strong> Lesser–Greater <strong>Himalayan</strong> dist<strong>in</strong>ction<br />
was a protolith designation, fixed at <strong>the</strong> time of deposition.<br />
Because <strong>the</strong> <strong>Ma<strong>in</strong></strong> <strong>Central</strong> <strong>Thrust</strong> is a Tertiary thrust fault that<br />
certa<strong>in</strong>ly cuts across stratigraphy, detrital zircon ages or Nd<br />
isotopes cannot be used to def<strong>in</strong>e <strong>the</strong> location of <strong>the</strong> <strong>Ma<strong>in</strong></strong><br />
<strong>Central</strong> <strong>Thrust</strong>.<br />
Metamorphism, U–Th–Pb monazite ages and <strong>the</strong><br />
<strong>Ma<strong>in</strong></strong> <strong>Central</strong> <strong>Thrust</strong><br />
Inverted metamorphism along <strong>the</strong> <strong>Ma<strong>in</strong></strong> <strong>Central</strong> <strong>Thrust</strong> zone is<br />
almost certa<strong>in</strong>ly related to movement along <strong>the</strong> <strong>Ma<strong>in</strong></strong> <strong>Central</strong><br />
<strong>Thrust</strong>. With<strong>in</strong> <strong>the</strong> <strong>in</strong>verted metamorphic field gradient, <strong>the</strong> rocks<br />
are highly sheared show<strong>in</strong>g ubiquitous C–S–C9 fabrics and<br />
north-plung<strong>in</strong>g l<strong>in</strong>eations that <strong>in</strong>dicate southward transport. Approximately<br />
5–8 km of thickness has been flattened by pure<br />
shear to a section 1–2 km thick along <strong>the</strong> <strong>in</strong>verted metamorphic<br />
isograd zone (Searle & Rex 1989). There are no major metamorphic<br />
discont<strong>in</strong>uities with<strong>in</strong> <strong>the</strong> <strong>in</strong>verted metamorphic sequence,<br />
suggest<strong>in</strong>g post-metamorphic pure shear flatten<strong>in</strong>g. The<br />
geometry of <strong>the</strong> <strong>in</strong>verted isograds along <strong>the</strong> <strong>Ma<strong>in</strong></strong> <strong>Central</strong> <strong>Thrust</strong><br />
zone is similar along <strong>the</strong> entire <strong>Himalayan</strong> cha<strong>in</strong> between <strong>the</strong><br />
Zanskar–Kishtwar area <strong>in</strong> <strong>the</strong> west (Stephenson et al. 2000,<br />
2001) to <strong>Nepal</strong>, Sikkim and Bhutan <strong>in</strong> <strong>the</strong> east (e.g. Boll<strong>in</strong>ger et<br />
al. 2004; Dasgupta et al. 2004; Jessup et al. 2006). Dat<strong>in</strong>g of<br />
peak metamorphism has relied on Sm–Nd dat<strong>in</strong>g of garnet (e.g.<br />
Vance & Harris 1999), or U–Pb dat<strong>in</strong>g of monazites (eg: Walker<br />
et al. 1999; Simpson et al. 2000; Foster et al. 2002), that grew <strong>in</strong><br />
equilibrium with kyanite or sillimanite. These are <strong>the</strong> only<br />
methods that date m<strong>in</strong>erals with high enough closure tempera-
HIMALAYAN MAIN CENTRAL THRUST, NEPAL 531<br />
tures. 40 Ar/ 39 Ar ages of hornblendes or micas record only a po<strong>in</strong>t<br />
on <strong>the</strong> cool<strong>in</strong>g path after peak metamorphism, dur<strong>in</strong>g exhumation.<br />
With<strong>in</strong> <strong>the</strong> Greater <strong>Himalayan</strong> Sequence <strong>in</strong>itiation of garnet<br />
growth and burial metamorphism occurred at 44 Ma, with garnet<br />
rims grow<strong>in</strong>g as late as 29 3 Ma. (Pr<strong>in</strong>ce et al. 2001; Foster et<br />
al. 2002). In <strong>the</strong> Everest region peak kyanite-grade metamorphism<br />
with<strong>in</strong> <strong>the</strong> Greater <strong>Himalayan</strong> Sequence has been dated by<br />
U–Pb monazite at 32.2 0.4 Ma with later HT–LP sillimanitegrade<br />
metamorphism at 22.7 0.2 Ma (Simpson et al. 2000).<br />
Most leucogranite ages, <strong>in</strong>terpreted as dat<strong>in</strong>g peak sillimanitegrade<br />
metamorphism and migmatization, along <strong>the</strong> <strong>Nepal</strong>ese<br />
Himalaya range from c. 24 to 16 Ma (see Searle et al. 1997,<br />
2003, for reviews).<br />
Along <strong>the</strong> base of <strong>the</strong> Greater <strong>Himalayan</strong> Sequence <strong>in</strong> <strong>Nepal</strong>,<br />
Harrison et al. (1997) first reported surpris<strong>in</strong>gly young ages of c.<br />
6 Ma from <strong>in</strong> situ 208 Pb/ 232 Th dat<strong>in</strong>g of monazite <strong>in</strong>clusions <strong>in</strong><br />
garnet. These were <strong>in</strong>terpreted as dat<strong>in</strong>g metamorphic recrystallization.<br />
Catlos et al. (2001) reported ages as young as<br />
3.3 0.1 Ma from <strong>the</strong> Marysandi Valley and Kohn et al. (2001)<br />
reported ages of 9–8 Ma from <strong>the</strong> Lesser Himalaya (re<strong>in</strong>terpreted<br />
here as <strong>the</strong> lower levels of <strong>the</strong> Greater <strong>Himalayan</strong><br />
Sequence above <strong>the</strong> <strong>Ma<strong>in</strong></strong> <strong>Central</strong> <strong>Thrust</strong>). Th–Pb monazite ages<br />
from <strong>the</strong> <strong>Ma<strong>in</strong></strong> <strong>Central</strong> <strong>Thrust</strong> zone <strong>in</strong> Langtang are c. 16Ma<br />
(Kohn et al. 2005), and <strong>the</strong> youngest monazite along <strong>the</strong> Dudh<br />
Kosi transect south of Everest has an age of 10.3 0.8 Ma<br />
(Catlos et al. 2002). In Sikkim, <strong>in</strong> situ Th–Pb monazite ages<br />
from <strong>the</strong> <strong>Ma<strong>in</strong></strong> <strong>Central</strong> <strong>Thrust</strong> zone cluster at c. 22, 15–14 and<br />
12–10 Ma (Catlos et al. 2004). These workers all <strong>in</strong>terpreted <strong>the</strong><br />
Th–Pb monazite ages as dat<strong>in</strong>g metamorphism along <strong>the</strong> <strong>Ma<strong>in</strong></strong><br />
<strong>Central</strong> <strong>Thrust</strong>, and hence tim<strong>in</strong>g of slip. However, Boll<strong>in</strong>ger &<br />
Janots (2006) cautioned that some young <strong>Himalayan</strong> monazites<br />
were a retrograde growth product from <strong>the</strong> breakdown of allanite<br />
at low temperatures (,370 8C). In this scenario, <strong>the</strong> Th–Pb<br />
monazite ages only date a retrograde event at low temperature<br />
and may have noth<strong>in</strong>g to do with slip along <strong>the</strong> <strong>Ma<strong>in</strong></strong> <strong>Central</strong><br />
<strong>Thrust</strong>. It seems quite likely that <strong>the</strong> very young monazite ages<br />
along <strong>the</strong> <strong>Ma<strong>in</strong></strong> <strong>Central</strong> <strong>Thrust</strong> zone may record crystallization<br />
from metasomatic hydro<strong>the</strong>rmal fluids ra<strong>the</strong>r than a ‘metamorphic<br />
event’. The ubiquitous presence of hot spr<strong>in</strong>gs along <strong>the</strong><br />
<strong>Ma<strong>in</strong></strong> <strong>Central</strong> <strong>Thrust</strong> zone today testifies to <strong>the</strong> high level of<br />
hydro<strong>the</strong>rmal fluids channelled up along <strong>the</strong> <strong>Ma<strong>in</strong></strong> <strong>Central</strong> <strong>Thrust</strong><br />
zone, and to <strong>the</strong> <strong>in</strong>significance of shear heat<strong>in</strong>g along <strong>the</strong> thrust.<br />
The <strong>Ma<strong>in</strong></strong> <strong>Central</strong> <strong>Thrust</strong> system evolved through time and<br />
space. Deep crustal levels show a wide ductile shear zone <strong>in</strong><br />
kyanite-grade rocks. Higher-level brittle faults show more discrete<br />
fracture planes (e.g. <strong>the</strong> Ramgarh thrust). Rocks with young<br />
matrix monazite ages would be expected above <strong>the</strong> youngest,<br />
sou<strong>the</strong>rnmost thrust planes of <strong>the</strong> <strong>Ma<strong>in</strong></strong> <strong>Central</strong> <strong>Thrust</strong>.<br />
Restoration of <strong>the</strong> <strong>Ma<strong>in</strong></strong> <strong>Central</strong> <strong>Thrust</strong> system<br />
A generalized restoration of <strong>the</strong> <strong>Ma<strong>in</strong></strong> <strong>Central</strong> <strong>Thrust</strong> system <strong>in</strong><br />
<strong>Nepal</strong> is shown <strong>in</strong> Figure 7. Late Proterozoic rocks extend from<br />
<strong>the</strong> Lesser Himalaya to <strong>the</strong> Greater <strong>Himalayan</strong> Sequence and<br />
across to <strong>the</strong> base of <strong>the</strong> Tethyan Himalaya (Haimanta Group–<br />
Cheka Formation). Unmetamorphosed Nawakot Group sedimentary<br />
rocks <strong>in</strong> <strong>the</strong> Lesser Himalaya pass north <strong>in</strong>to <strong>the</strong> same<br />
protolith age rocks which have been metamorphosed to greenschist–upper<br />
amphibolite facies <strong>in</strong> <strong>the</strong> Ramgarh thrust sheet<br />
(Beyssac et al. 2004), up to kyanite grade <strong>in</strong> <strong>the</strong> Kathmandu<br />
thrust sheet (Johnson et al. 2001), and f<strong>in</strong>ally <strong>in</strong>to high-grade<br />
sillimanite gneisses <strong>in</strong> <strong>the</strong> Dadeldhura and Greater <strong>Himalayan</strong><br />
Sequence thrust sheets <strong>in</strong> <strong>the</strong> high Himalaya. In <strong>the</strong> <strong>in</strong>ternal parts<br />
of <strong>the</strong> Greater <strong>Himalayan</strong> Sequence, <strong>the</strong> sillimanite gneisses<br />
(commonly referred to as Greater <strong>Himalayan</strong> Sequence Formation<br />
1; Colchen et al. 1986) are metamorphosed equivalents of<br />
<strong>the</strong> same late Proterozoic protoliths. As <strong>the</strong>re are no suture zone<br />
rocks along <strong>the</strong> <strong>Ma<strong>in</strong></strong> <strong>Central</strong> <strong>Thrust</strong> and <strong>the</strong> stratigraphy is<br />
cont<strong>in</strong>uous across <strong>the</strong> restored thrusts, <strong>the</strong>re cannot be a Palaeozoic<br />
suture zone along <strong>the</strong> <strong>Ma<strong>in</strong></strong> <strong>Central</strong> <strong>Thrust</strong> (DeCelles et al.<br />
2000; Richards et al. 2005). Instead, <strong>the</strong> restored Lesser–<br />
Greater–Tethyan Himalaya was a contiguous cont<strong>in</strong>ental section<br />
(Searle 1986; Myrow et al. 2003; Steck 2003; Searle et al.<br />
2006).<br />
One major implication of <strong>the</strong> restoration of <strong>the</strong> Himalaya is<br />
that <strong>the</strong> <strong>Ma<strong>in</strong></strong> <strong>Central</strong> <strong>Thrust</strong> follows a flat for a long distance<br />
across strike. This flat follows a rheologically weak horizon<br />
along <strong>the</strong> Neoproterozoic shales. True Indian basement rocks<br />
(Archaean–Lower Proterozoic) are never exposed <strong>in</strong> <strong>the</strong> Himalaya.<br />
Although impossible to determ<strong>in</strong>e accurately because of <strong>the</strong><br />
high degree of ductile stra<strong>in</strong>, m<strong>in</strong>imum crustal shorten<strong>in</strong>g<br />
estimates from <strong>the</strong> Himalaya are between 500 and 900 km<br />
(Searle 1986; DeCelles et al. 2001; Rob<strong>in</strong>son et al. 2006).<br />
Because <strong>the</strong> across-strike width of <strong>the</strong> basement and cover<br />
(Neoproterozoic–Eocene rocks exposed <strong>in</strong> <strong>the</strong> Himalaya) must<br />
balance on <strong>the</strong> restoration, it must be concluded that Indian<br />
basement has underthrust northwards beneath <strong>the</strong> Himalaya north<br />
of <strong>the</strong> <strong>Ma<strong>in</strong></strong> <strong>Central</strong> <strong>Thrust</strong>, and beneath <strong>the</strong> sou<strong>the</strong>rn marg<strong>in</strong> of<br />
Asia (Lhasa block) by a similar distance across strike. Very<br />
large-scale subhorizontal detachments such as <strong>the</strong> <strong>Ma<strong>in</strong></strong> <strong>Central</strong><br />
<strong>Thrust</strong> <strong>the</strong>refore can play a major role <strong>in</strong> <strong>the</strong> mechanical<br />
decoupl<strong>in</strong>g of <strong>the</strong> crust. In <strong>the</strong> Himalaya, <strong>the</strong> <strong>Ma<strong>in</strong></strong> <strong>Central</strong><br />
<strong>Thrust</strong> and <strong>the</strong> South Tibetan Detachment effectively decouple<br />
<strong>the</strong> upper (Tethyan Himalaya), middle (Greater Himalaya) and<br />
lower (Indian basement) crust.<br />
The <strong>Ma<strong>in</strong></strong> <strong>Central</strong> <strong>Thrust</strong>, like all major thrust faults, changes<br />
<strong>in</strong> style <strong>in</strong> both <strong>the</strong> horizontal and vertical planes (Fig. 8), as well<br />
as through time. In deep crustal profiles it is a 1–3 km thick<br />
ductile shear zone <strong>in</strong> kyanite-grade metamorphic rocks. U–Th–<br />
Pb monazite ages of <strong>the</strong>se rocks range between c. 24 and 18 Ma<br />
(for a review, see God<strong>in</strong> et al. 2006). At higher structural levels<br />
ductile shear<strong>in</strong>g passes up <strong>in</strong>to brittle thrust fault<strong>in</strong>g along a<br />
discrete thrust plane. Many of <strong>the</strong>se shallower levels follow flats<br />
plac<strong>in</strong>g similar age or younger rocks over similar age or older<br />
rocks (Fig. 7). In more sou<strong>the</strong>rly, outboard profiles such as at <strong>the</strong><br />
Kathmandu complex, <strong>the</strong> <strong>Ma<strong>in</strong></strong> <strong>Central</strong> <strong>Thrust</strong> (Mahabharat<br />
thrust) ramps up to higher levels where metamorphism <strong>in</strong> <strong>the</strong><br />
hang<strong>in</strong>g wall is right-way-up (Fig. 8). Here, a new, younger<br />
brittle thrust (Ramgarh thrust) developed <strong>in</strong> <strong>the</strong> footwall of <strong>the</strong><br />
Mahabharat thrust, plac<strong>in</strong>g greenschist-grade rocks over unmetamorphosed<br />
Lesser <strong>Himalayan</strong> rocks (Fig. 8). In <strong>the</strong> Darjeel<strong>in</strong>g<br />
area, a late out-of-sequence breakback <strong>Ma<strong>in</strong></strong> <strong>Central</strong> <strong>Thrust</strong><br />
developed beh<strong>in</strong>d <strong>the</strong> Darjeel<strong>in</strong>g klippe (Searle & Szulc 2005).<br />
All <strong>the</strong>se ductile shear zones and brittle thrust faults are part of<br />
<strong>the</strong> <strong>Ma<strong>in</strong></strong> <strong>Central</strong> <strong>Thrust</strong> system, which developed over a period<br />
.14 Ma (from c. 24 to 10 Ma), from depths equivalent to at least<br />
10–12 kbar (35–40 km) to <strong>the</strong> surface. Because <strong>the</strong> <strong>in</strong>verted<br />
metamorphic gradient resulted from post-metamorphic shear<strong>in</strong>g,<br />
<strong>the</strong> observed metamorphic gradient should not be directly compared<br />
with <strong>the</strong> geo<strong>the</strong>rm (Searle & Rex 1989; Boll<strong>in</strong>ger et al.<br />
2004).<br />
Conclusions<br />
None of <strong>the</strong> criteria commonly used for mapp<strong>in</strong>g <strong>the</strong> <strong>Ma<strong>in</strong></strong><br />
<strong>Central</strong> <strong>Thrust</strong> are valid <strong>in</strong> <strong>the</strong>mselves for def<strong>in</strong><strong>in</strong>g <strong>the</strong> <strong>Ma<strong>in</strong></strong><br />
<strong>Central</strong> <strong>Thrust</strong>. Lithology, detrital zircon U–Pb ages, and Nd<br />
isotope signatures reveal <strong>in</strong>formation about provenance and
532<br />
M. P. SEARLE ET AL.<br />
stratigraphy, but not structure. Because thrust trajectories cut up<br />
and across stratigraphic section <strong>in</strong> <strong>the</strong> transport direction, <strong>the</strong>se<br />
methods are clearly not useful <strong>in</strong> def<strong>in</strong><strong>in</strong>g <strong>the</strong> position of thrust<br />
faults such as <strong>the</strong> <strong>Ma<strong>in</strong></strong> <strong>Central</strong> <strong>Thrust</strong>. Isograds are metamorphic<br />
reactions that can be mapped (with difficulty) <strong>in</strong> <strong>the</strong> field by <strong>the</strong><br />
first appearance of key <strong>in</strong>dex m<strong>in</strong>erals (sillimanite, kyanite,<br />
staurolite, garnet). Young monazite ages reveal specific <strong>in</strong>formation<br />
on growth of <strong>the</strong> garnet that armours <strong>the</strong>m, or <strong>the</strong> matrix<br />
that conta<strong>in</strong>s <strong>the</strong>m, and fluid <strong>in</strong>filtration, nei<strong>the</strong>r of which are<br />
def<strong>in</strong>itively associated with motion along a thrust fault. Only<br />
structural mapp<strong>in</strong>g and stra<strong>in</strong> <strong>in</strong>dicators can def<strong>in</strong>e <strong>the</strong> position<br />
of <strong>the</strong> <strong>Ma<strong>in</strong></strong> <strong>Central</strong> <strong>Thrust</strong>. Follow<strong>in</strong>g Hanmer & Passchier<br />
(1991) and Passchier & Trouw (2005), <strong>the</strong> essential criteria to<br />
def<strong>in</strong>e a shear zone are <strong>the</strong> identification of a stra<strong>in</strong> gradient and<br />
<strong>the</strong> clear localization of stra<strong>in</strong>.<br />
As <strong>the</strong> metamorphic isograds are always telescoped along <strong>the</strong><br />
base of <strong>the</strong> Greater <strong>Himalayan</strong> Sequence with up to 50% pure<br />
shear flatten<strong>in</strong>g superimposed on <strong>the</strong> already ‘frozen-<strong>in</strong>’ isograds<br />
(Jessup et al. 2006), <strong>the</strong> position of <strong>the</strong> <strong>in</strong>verted metamorphism<br />
often correlates closely, or precisely, with <strong>the</strong> position of <strong>the</strong><br />
<strong>Ma<strong>in</strong></strong> <strong>Central</strong> <strong>Thrust</strong> ductile shear zone. In <strong>the</strong> western Himalaya,<br />
<strong>the</strong> <strong>Ma<strong>in</strong></strong> <strong>Central</strong> <strong>Thrust</strong> zone <strong>in</strong>verted metamorphic isograd<br />
sequence along <strong>the</strong> base of <strong>the</strong> Greater <strong>Himalayan</strong> Sequence has<br />
been mapped around a NW-plung<strong>in</strong>g recumbent anticl<strong>in</strong>e, and<br />
has been shown to jo<strong>in</strong> up with right-way-up isograds along <strong>the</strong><br />
footwall of <strong>the</strong> South Tibetan Detachment low-angle normal fault<br />
at <strong>the</strong> top of <strong>the</strong> Greater <strong>Himalayan</strong> Sequence (Searle & Rex<br />
1989). The map relationship and tim<strong>in</strong>g constra<strong>in</strong>ts (Hodges et<br />
al. 1996) show that movement along <strong>the</strong> <strong>Ma<strong>in</strong></strong> <strong>Central</strong> <strong>Thrust</strong><br />
and South Tibetan Detachment were synchronous, and that <strong>the</strong><br />
Greater <strong>Himalayan</strong> Sequence moved south, bounded by <strong>the</strong>se<br />
shear zones above and below, dur<strong>in</strong>g southward extrusion of <strong>the</strong><br />
ductile partially molten core of <strong>the</strong> Greater <strong>Himalayan</strong> Sequence<br />
(Fig. 9; channel flow model).<br />
We suggest that a common unify<strong>in</strong>g def<strong>in</strong>ition for <strong>the</strong> <strong>Ma<strong>in</strong></strong><br />
<strong>Central</strong> <strong>Thrust</strong> should be ‘<strong>the</strong> base of <strong>the</strong> large-scale zone of<br />
high stra<strong>in</strong> and ductile deformation, commonly co<strong>in</strong>cid<strong>in</strong>g with<br />
<strong>the</strong> base of <strong>the</strong> zone of <strong>in</strong>verted metamorphic isograds, which<br />
places Tertiary metamorphic rocks of <strong>the</strong> Greater <strong>Himalayan</strong><br />
Sequence over unmetamorphosed or low-grade rocks of <strong>the</strong><br />
Lesser Himalaya’, similar to that suggested for <strong>the</strong> Kishtwar<br />
<strong>Ma<strong>in</strong></strong> <strong>Central</strong> <strong>Thrust</strong> section by Stephenson et al. (2000, 2001).<br />
Whereas <strong>the</strong> Kishtwar section shows an exhumed, deeper, more<br />
<strong>in</strong>ternal section across <strong>the</strong> <strong>Ma<strong>in</strong></strong> <strong>Central</strong> <strong>Thrust</strong> zone, <strong>the</strong><br />
Kathmandu nappe and Ramgarh thrust sheets show a shallower,<br />
more external section across <strong>the</strong> <strong>Ma<strong>in</strong></strong> <strong>Central</strong> <strong>Thrust</strong> (Fig. 8).<br />
The <strong>Ma<strong>in</strong></strong> <strong>Central</strong> <strong>Thrust</strong> ductile shear zone is commonly<br />
bounded along <strong>the</strong> south (base) by a brittle thrust fault, so a<br />
dist<strong>in</strong>ction could be made between <strong>the</strong> <strong>Ma<strong>in</strong></strong> <strong>Central</strong> <strong>Thrust</strong><br />
ductile shear zone (up to 2 km or more thick) and <strong>the</strong> brittle<br />
<strong>Ma<strong>in</strong></strong> <strong>Central</strong> <strong>Thrust</strong> fault (sensu stricto) along its base.<br />
We acknowledge NERC, NSF and NSERC grants respectively to M.P.S.,<br />
R.D.L. and L.G., and UK (M.J.S.), New Zealand (J.M.C.), Canadian<br />
(K.P.L.) and US (M.J.J.) PhD studentships grants. We thank <strong>the</strong> late<br />
Pasang Tamang, and Pradap Tamang and team for excellent trekk<strong>in</strong>g<br />
logistics <strong>in</strong> <strong>the</strong> Annapurnas, Tashi Sherpa and Sonam Wangdu <strong>in</strong> <strong>the</strong><br />
Everest region, and Suka Ghale and team <strong>in</strong> <strong>the</strong> Manaslu region. The<br />
paper benefited greatly from reviews by Paul Myrow and Richard Brown,<br />
and discussions with Mike Johnson, Randy Parrish and Laurent Boll<strong>in</strong>ger.<br />
Fig. 9. Generalized model for <strong>the</strong> <strong>Ma<strong>in</strong></strong><br />
<strong>Central</strong> <strong>Thrust</strong> ductile shear zone and thrust<br />
fault, and Greater <strong>Himalayan</strong> Sequence<br />
channel flow along <strong>the</strong> Himalaya. The<br />
South Tibetan Detachment and <strong>Ma<strong>in</strong></strong><br />
<strong>Central</strong> <strong>Thrust</strong> were active simultaneously<br />
dur<strong>in</strong>g <strong>the</strong> early to middle Miocene, and <strong>the</strong><br />
deeper ductile shear zones pass upward and<br />
outward <strong>in</strong>to brittle faults with time. The<br />
mid-crustal channel of partially molten<br />
crust separates <strong>the</strong> brittle deform<strong>in</strong>g<br />
seismogenic upper crust from <strong>the</strong> rigid,<br />
high-pressure granulite lower crust of <strong>the</strong><br />
subducted Indian Shield.
HIMALAYAN MAIN CENTRAL THRUST, NEPAL 533<br />
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Scientific edit<strong>in</strong>g by Rob Strachan