Thermal metamorphism in the lesser Himalaya of Nepal determined ...

Thermal metamorphism in the lesser Himalaya of Nepal
determined from Raman spectroscopy of carbonaceous material
Abstract
Olivier Beyssac*, Laurent Bollinger 1 , Jean-Philippe Avouac 1 , Bruno Goffé
Laboratoire de Géologie, Ecole Normale Supérieure, CNRS-UMR 8538, 24 rue Lhomond, F-75005, Paris Cedex 5, France
Received 18 December 2003; received in revised form 6 May 2004; accepted 20 May 2004
Available online
The determination of metamorphic conditions is critical to the understanding of the formation of mountain belts.
However, all collisional mountain belts contain large volumes of accreted sediments generally lacking metamorphic index
minerals and are therefore not amenable to conventional petrologic investigations. By contrast, these units are often rich in
carbonaceous material, making it possible to determine thermal metamorphism through Raman spectroscopy of carbonaceous
material (RSCM method), a technique that has been recently calibrated [Beyssac et al., J. Metamorph. Geol. 20 (2002) 859–
871]. The Lesser Himalaya (LH) is one of these problematic cases with a very poor mineralogy, but a key structural position
within the Himalayan system that makes LH considered as diagnostic of the overall thermal behaviour of the orogen. This
work demonstrates the performance of the RSCM technique and shows that this technique might thus be used to detect intersample
variations as small as f 10–15 jC, but absolute temperatures can only be determined to F 50 jC due to the
uncertainty on the calibration. This study reveals that the LH has undergone a large-scale thermal metamorphism, with
temperature decreasing progressively from about 540 jC at the top to less than 330 jC within the deepest exhumed structural
levels.
D 2004 Elsevier B.V. All rights reserved.
Keywords: Himalaya; graphitization; Raman spectroscopy; geothermometry; metamorphism
1. Introduction
Earth and Planetary Science Letters 225 (2004) 233–241
The Lesser Himalaya (LH) consists mainly of
Gondwanian sediments (quartzites, marbles and black
schists) of probably Proterozoic ages, that have been
* Corresponding author. Tel.: +33-1-44-32-22-75; fax: +33-1-
44-32-20-00.
E-mail address: Olivier.Beyssac@ens.fr (O. Beyssac).
1 Now at: Geological and Planetary Sciences Division,
California Institute of Technology, MC 100-23, Pasadena, CA
91125, USA.
0012-821X/$ - see front matter D 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.epsl.2004.05.023
www.elsevier.com/locate/epsl
accreted onto the Himalaya over the Cenozoic (e.g.,
[2,3]). They have undergone pervasive thrust shearing
and, supposedly, low grade metamorphism [4,5].
These units must therefore contain key information
about the tectonic and thermal evolution of the Himalayan
orogenic wedge.
At the top of the LH, the Main Central Thrust
fault zone (MCT) separates the LH from the overlying
High-Himalayan Crystalline (HHC) units. A
number of petrological studies have focused on this
zone where a characteristic inverted thermal gradient
has long been identified (e.g., [6]). Due to the

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O. Beyssac et al. / Earth and Planetary Science Letters 225 (2004) 233–241
poverty of the mostly pelitic LH units in index
mineral assemblages, metamorphism in the LH has
been only qualitatively described in terms of garnet
and biotite ‘‘isograds’’ [5] and from illite crystallinity
[7]. Because LH units are rich in Carbonaceaous
Material (CM), metamorphism might alternatively
been quantified from the Raman spectroscopy of
CM (RSCM method). This technique has been
shown to provide a reliable estimate of the peak
metamorphic temperature, as recently calibrated by
[1]. In this paper, we present RSCM results obtained
from 83 samples collected in the LH of central and
far-western Nepal, including data from near the MCT
zone, where a comparison with conventional petrological
approaches is possible.
The aim of this paper is therefore to provide new
constraints on the thermal evolution of the Himalaya
in Nepal using the RSCM method. This emblematic
geological setting allows some comparison of RSCM
with other techniques, offering some opportunity to
demonstrate its applicability and reliability.
2. Geological setting
The dataset consists of 83 samples that were
collected in central Nepal (Fig. 1) and in far-western
Nepal (Fig. 2). A detailed description of the structural
and stratigraphic location of the samples from Central
Nepal can be found elsewhere [8], and is beyond the
scope of this paper. Here, we just give a brief outline.
The Lesser Himalaya in central Nepal forms a broad
anticlinorium [5] at front of the high-range thought to
be associated with some duplex structure at midcrustal
depth [3]. South of this anticlinorium, remnants
of crystalline thrust sheets, such as the Katmandu
and Jajarkot klippes, overly the LH [9]. The
structure in far-western Nepal is similar, although
more complex with several alternates of LH windows
and overlying klippes [2], such as the Dadeldhura
klippe (Fig. 2). Although this is debated [9], the thrust
faults at the base of the klippes are considered to be
equivalent to the MCT. Hereafter, we will distinguish
the data collected from the MCT zone along the front
Fig. 1. Geological map of central Nepal. This map shows the major tectonostratigraphic zones and tectonic contacts. For each sample: label and
mean RSCM temperature are given. The lower-right inset shows location of studied area at the scale of Nepal. (LH: Lesser Himalaya, HHC:
High-Himalayan Crystalline, TG: Tansen group, MCT: main central thrust, MBT: main boundary thrust, MFT: main frontal thrust). Lower-left
inset: zoom on the temperature data from the Damauli area. The Damauli Klippe is considered as an equivalent of the Katmandu klippe, at least
for its thermal evolution.

O. Beyssac et al. / Earth and Planetary Science Letters 225 (2004) 233–241 235
Fig. 2. Geological map of far-western Nepal. This map shows the major tectonostratigraphic zones and tectonic contacts. For each sample: label
and mean RSCM temperature are given. The upper right inset shows location of the studied area at the scale of Nepal. The samples for which a
Raman spectrum is depicted on Fig. 3 are indicated with a yellow label (LH: Lesser Himalaya). Lower inset shows a simplified NE–SW crosssection
AAV with temperature and main thrusts (MCT: main central thrust, MBT: main boundary thrust, MFT: main frontal thrust).
of the high range, from those collected farther south in
the shear zones below the klippes.
Metamorphism near the MCT zone has been widely
studied from conventional petrology (e.g., [10] and
references therein). In these rocks, mainly because of
the high-temperature metamorphism (T>500 jC) there
is a various, even spectacular mineralogy. The mineral
isograds (biotite, garnet, kyanite, and sillimanite,
respectively) are progressively intersected along a
S–N section going upward in the structure through
the MCT zone defining the well-known inverted
metamorphic gradient (e.g., [5]).
There is a large debate about the respective positions
of metamorphic isograds and structural position
of the MCT [11,12]. To avoid any misunderstanding,
we just precise here that all our samples come from
the upper and lower LH and are all located below the
kyanite isograd.

236
3. Estimating temperature from Raman
spectroscopy of carbonaceous material (RSCM)
O. Beyssac et al. / Earth and Planetary Science Letters 225 (2004) 233–241
During diagenesis and metamorphism, CM present
in the initial sedimentary rock progressively
transforms into graphite (graphitization). The
corresponding progressive evolution of degree of
organization of the CM is considered to be a reliable
indicator of metamorphic grade, especially of temperature
[13,14]. Because of the irreversible character
of graphitization (CM is tending toward the
thermodynamic stable phase which is graphite),
CM structure is not sensitive to retrograde metamorphism
and therefore primarily depends on the maximum
temperature reached during metamorphism,
whatever the retrograde history of the sample.
The first-order Raman spectrum of disordered CM
exhibit a graphite G band at 1580 cm 1 ,E2g2 mode
corresponding to in-plane vibration of aromatic carbons,
and several defect bands D1, D2 and D3,
corresponding to ‘‘physico-chemical defects’’ (see
[15] and references therein). The structural organization
of CM can be quantified through the R2
parameter defined as the relative area of the main
defect band D1 (R2 = D1/(G + D1 + D2) peak area
ratio). A linear correlation between this R2 parameter
and metamorphic temperature was calibrated using
samples from different regional metamorphic belts
with well-known P–T conditions (RSCM method,
see [1]). RSCM applies for metasediments of pelitic
lithology in which CM precursor is mainly a kerogen
mixed with minor hydrocarbons trapped during diagenesis
[1,16]. R2 parameter variations are significant
from 0.0 for pristine graphite to f 0.7 for
poorly organized CM limiting the applicability of
the RSCM method to the range 330–650 jC [1].
When R2 is higher than 0.7, the corresponding
spectrum usually exhibit a very large D1 band, a
broad G band at f 1600 cm 1 including both G
and D2 bands and a D3 band. The accuracy in the
determination of the temperature from this calibration
is estimated to be F 50 jC for temperatures in
the range 330–650 jC [1]. This relatively loose
calibration mainly results from the large uncertainties
on the conventional petrologic data themselves. For
maximum reliability, the measurements must be
made (1) on thin-sections cut perpendicular to the
foliation, therefore parallel to the mean c axis of
CM, and (2) below transparent minerals, quartz for
instance, to avoid any effect of polishing on the
structure of the CM [15]. For only one sample
(FW0028) with a very high content of CM but too
fragile to make a polished thin section, the measurements
were directly performed on the rock itself.
Raman microspectroscopy was performed by using
a DILOR XY double subtractive spectrograph with
premonochromator, equipped with confocal optics
before the spectrometer entrance, and a nitrogencooled
SPECTRUM1 CCD detector. A microscope
is used to focus the excitation laser beam (514.5 nm
exciting line of a Spectra Physics Ar+ laser) on the
sample and to collect the Raman signal in the backscattered
direction. The presence of the confocal
pinhole before the spectrometer entrance ensures a
sampling of a 1–3 Am diameter area using the
50 objective, with a final laser power of about 1–
4 mW at the sample surface. Band position, band
intensity (i.e. band height), band area (i.e. integrated
area) and band width (i.e. full width at half maximum,
FWHM) were determined using the computer program
PeakFit 3.0 (Jandel Scientific) following the
fitting procedure described by [15].
4. Results
For each samples, 6–17 spectra were recorded in
order to smooth out the within-sample structural
heterogeneity. All results are reported together with
sample locations in Table 1. In a few samples,
detrital pristine graphite (no defect bands) with a
crystalline morphology was easily identified under
the microscope and separated from the CM diffuse
texture. The structural heterogeneity was found to be
generally small, with a maximum standard deviation
of R2 values of 0.13, contributing to an uncertainty
of about 15 jC in temperature (at the 1 r confidence
level, sample K4, 14 spectra). Here, to get
some estimate of the uncertainty associated with the
fitting procedure we have processed ten times each
spectrum represented on Fig. 3, because they span
the whole range of CM structural organization analyzed
in this study. For samples FW6, FW24, FW29
and FW35, the standard deviation from the mean
value of the ten R2 values for a given spectrum is
systematically lower than 0.01. For sample FW31 in

O. Beyssac et al. / Earth and Planetary Science Letters 225 (2004) 233–241 237
Table 1
Samples with longitude (Long.) and latitude (Lat.) in decimal
degrees (WGS84), number of Raman spectra (Sp.), R2 ratio (mean
and standard deviation) and RSCM temperature (mean and 1
uncertainty)
r
Sample Long. Lat. Sp. R2 T (jC)
Mean SD Mean 1r
Central Nepal-MCT zone
G10 83.7875 28.4092 15 0.225 0.06 540.8 7
G7 83.7128 28.3958 11 0.236 0.03 536.1 4
G9 83.7500 28.3937 9 0.287 0.03 513.4 5
Gum2 85.8914 27.9008 12 0.245 0.08 531.8 10
Gum6 85.8935 27.9063 13 0.169 0.03 565.8 4
K10 85.9569 27.9445 14 0.181 0.11 560.7 12
K4 85.9344 27.9233 14 0.287 0.13 513.5 15
K5 85.9373 27.9261 13 0.237 0.09 535.4 11
K9 85.9470 27.9430 17 0.237 0.12 535.5 13
MB01 84.3646 28.2908 13 0.199 0.04 552.4 5
MB10 84.3326 28.2928 14 0.207 0.07 549.0 8
MB13 84.3296 28.2898 12 0.232 0.07 537.7 8
MB15 84.3272 28.2894 10 0.198 0.07 553.1 10
MB17 84.3192 28.2895 13 0.248 0.03 530.7 4
MB19 84.3135 28.2875 15 0.250 0.06 529.5 6
MB02 84.3779 28.2958 10 0.239 0.08 534.9 11
MB23 84.3099 28.2791 12 0.249 0.05 530.3 7
MB25 84.3128 28.2837 11 0.221 0.05 542.7 6
MB27 84.3191 28.2879 9 0.216 0.04 545.0 7
MB29 84.3337 28.2919 13 0.272 0.07 520.0 8
M9913 83.8185 28.3469 7 0.284 0.04 514.7 7
N0006 83.9583 28.3456 9 0.216 0.06 545.1 9
N0025 83.9639 28.3656 11 0.248 0.06 530.8 8
P1 85.9366 27.9304 13 0.155 0.06 571.9 8
P4 85.9366 27.9304 12 0.163 0.06 568.6 8
MA78 84.3336 28.3047 9 0.202 0.08 550.9 12
MA79 84.3340 28.3162 7 0.191 0.04 555.8 6
MA80 84.3347 28.3271 9 0.220 0.06 543.3 8
MA81 84.3347 28.3298 9 0.163 0.11 568.5 16
Central Nepal-Lesser Himalaya anticlinorium
CW05 83.7669 27.9860 10 0.686 0.03 335.6 4
CW07 83.7068 27.9521 9 0.702 0.02 < 330
CW08 83.6687 27.9374 6 0.681 0.03 338.0 6
CW09 83.6518 27.9178 7 0.710 0.03 < 330
CW10 83.6428 27.8888 10 0.750 0.03 < 330
D04 84.2743 27.9433 6 0.589 0.03 379.0 5
D06 84.2800 27.9204 8 0.429 0.07 450.3 12
D07 84.2744 27.9140 7 0.260 0.05 525.1 9
D11 84.2714 27.8991 10 0.323 0.11 497.5 15
D14 84.2680 27.9013 9 0.465 0.03 433.9 5
D17 84.2569 27.9026 6 0.285 0.06 514.0 11
D24 84.2465 27.9167 7 0.326 0.05 496.0 8
D25 84.2491 27.9225 10 0.339 0.09 490.3 13
D28 84.2531 27.9246 10 0.277 0.03 517.6 4
D32 84.2639 27.9420 12 0.449 0.04 441.2 5
D33 84.2661 27.9477 9 0.374 0.09 474.7 13
D34 84.2656 27.9530 9 0.469 0.06 432.1 9
Table 1 (continued)
Sample Long. Lat. Sp. R2 T (jC)
Mean
Central Nepal-Lesser Himalaya anticlinorium
SD Mean 1r
D40 84.2789 27.9879 9 0.619 0.01 365.5 2
MB36 84.5399 27.8903 13 0.676 0.03 340.0 4
MB37 84.5379 27.8786 6 0.351 0.05 484.6 9
MB38 84.5566 27.8560 11 0.696 0.02 331.3 3
MB41 84.7285 27.7959 11 0.529 0.05 405.8 7
MB42 84.7744 27.8080 11 0.508 0.06 414.9 8
MB50 84.7077 27.8013 14 0.572 0.03 386.3 3
MOI1 84.5565 27.8359 10 0.671 0.02 342.3 2
MOI2 84.6779 27.8144 9 0.651 0.03 351.4 4
MOI3 84.7506 27.8030 8 0.462 0.04 435.3 7
N0001 84.7075 27.8053 14 0.563 0.03 390.5 4
N0035 84.7333 27.7939 9 0.387 0.05 468.9 8
Far-western Nepal
FW0003 80.4886 29.6217 17 0.606 0.03 371.2 3
FW0006 80.3894 29.7528 11 0.324 0.03 496.6 5
FW0008 80.5525 29.8506 9 0.578 0.02 384.0 2
FW0024 80.7155 29.9860 13 0.225 0.06 540.7 8
FW0028 80.6697 29.8744 6 0.367 0.04 477.6 8
FW0029 80.6783 29.8269 8 0.498 0.04 419.3 7
FW0031 80.6581 29.7683 12 0.669 0.03 343.5 4
FW0035 80.6086 29.7019 15 0.467 0.03 433.1 3
FW0038 80.5778 29.4544 3 >0.7 < 330
FW0105 80.6276 29.4101 8 0.562 0.03 391.1 5
FW0115 80.8489 29.5042 9 0.540 0.02 400.8 3
FW0116 80.8708 29.5027 6 0.571 0.04 386.8 7
FW0117 81.1731 29.5695 9 0.670 0.02 342.8 4
FW0120 81.1643 29.5850 6 0.700 0.06 < 330
FW0123 81.1618 29.5987 10 0.543 0.04 399.4 5
FW0124 81.1626 29.5926 14 0.660 0.04 347.2 5
FW0135 81.1201 29.5050 10 0.690 0.02 334.0 3
FW0138 81.0511 29.4868 8 0.638 0.04 357.0 6
FW0142 80.9001 29.4607 10 0.636 0.03 357.9 4
FW0144 80.8727 29.5046 6 0.603 0.06 372.6 11
FW0148 80.8157 29.5337 8 0.500 0.03 418.6 4
FW0150 80.8005 29.5384 10 0.422 0.05 453.2 7
FW0152 80.7723 29.5389 9 0.362 0.05 479.8 7
FW0158 80.6195 29.3975 6 0.212 0.04 546.5 7
FW0162 80.6057 29.1733 8 0.217 0.10 544.5 16
which RSCM is not applicable (0.7 < R2), the standard
deviation is only f 0.02.
The Raman spectra obtained from the LH samples
show a large variety of structural organization, spanning
the whole range from poorly organized CM to
crystalline graphite. There is a clear spatial pattern of
CM structure with respect to sample location. For
illustration, a subset of representative spectra along a
N–S cross-section of the far-western region of Nepal is

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O. Beyssac et al. / Earth and Planetary Science Letters 225 (2004) 233–241
Fig. 3. Examples of Raman spectra obtained from samples collected
in far-western Nepal. Locations are indicated with yellow labels in
Fig. 2. An example of spectral decomposition by the fitting
procedure [15] is given for the less organized sample (FW31).
Position of the graphite G band and D1, D2, D3 defects bands are
indicated. For each spectrum, the value of the mean R2 ratio
(R2 = D1/[G + D1 + D2] peak area ratio) obtained after 10 decompositions
is given (see text).
shown in Fig. 3. As we get closer to the MCT, toward
higher LH structural levels, the peak corresponding to
the graphite G band gets gradually narrower and shifts
towards crystalline graphite ( f 1580 cm 1 ). Simultaneously,
the peaks corresponding to D1 and D2
defect bands vanish gradually. This trend corresponds
to a gradual decrease of the R2 parameter from 0.747
(poorly organized CM) down to 0.198 (microcrystalline
graphite).
The results from the samples collected in the
MCT zone, at a structural distance less than 3 km,
systematically indicate nearly microcrystalline graphite
(0.1 < R2 < 0.3) and yielded remarkably clustered
temperatures around a mean value of 541 jC (Fig.
4). The standard deviation of this data set is only
f 16 jC. One particular sample, MA79, which had
yielded f 520 jC from the garnet–biotite Fe/Mg
exchange thermometer [10] yielded a RSCM temperature
of f 556 F 6 jC. More generally, the
temperatures estimated by the RSCM method in
central Nepal are fairly consistent with the temperatures
estimated from conventional petrology but are
much less dispersed (Fig. 4). The clustering of the
RSCM temperatures from the MCT zone contrasts
with the systematic and gradual decrease of RSCM
temperature as we considered samples farther away
from the MCT zone.
In central Nepal, below the Katmandu klippe,
peak temperature is around 470 jC and also progressively
decreases with distance to the klippe base.
This trend is best illustrated for the monoclinal units
below the Damauli klippe (lower inset in Fig. 1).
The Damauli klippe is constituted by high-temperature
schists (garnet–biotite parageneses) and can be
considered as analogous to the larger Katmandu
klippe [8,17].
5. Discussion
5.1. A reliable geothermometer
Different sources of uncertainty affect the estimation
of temperature by the RSCM method. An
important source is due to the structural heterogeneity
of the CM at the scale of the thin-section. This
term was estimated from repeated measurements on
each thin section. The standard deviation for R2 of
individual measurements within one thin-section is
typically around 0.05. The structural heterogeneity
contributes to an uncertainty on the mean value of
at most 10–15 jC (at the 1 r confidence level),
for typically f 10 measurements on the same thinsection
(Table 1). This heterogeneity can be generated
by the technique (see method), or can be
inherited from the CM precursor itself [16] or
created by a nonhomogeneous graphitization
through CM [18]. The possible mechanical effect
of shear on the CM organization could be an
additional source of uncertainty. This term should
be particularly evident when various samples from a

O. Beyssac et al. / Earth and Planetary Science Letters 225 (2004) 233–241 239
Fig. 4. Summary of peak metamorphic temperatures in the MCT zone. Temperature estimates by RSCM method (this study) and by
conventional petrology [10,11,19,20] are compared in central and far-western Nepal. We distinguish samples collected (1) from the High-
Himalayan crystalline, (2) from the MCT zone (at a structural distance less than 3 km) at front of the High-Himalayan Crystalline, (3)
immediately below the klippes overlaying Lesser Himalaya and (4) in the core of the Lesser Himalaya anticlinorium. The dashed line indicates
the temperature 540 jC.
heterogeneously deformed zone are compared. The
data from the MCT zone are strongly clustered
however, and are found to follow closely a normal
distribution with a standard deviation of only about
16 jC. This shows that the effect of mechanical
shearing, as well as other possible sources of intersamples
heterogeneities is small and might contribute
to an uncertainty of at most 6 jC (at the 1 r
confidence level). Actually, when the data from
particular areas are considered closely, some systematic
spatial patterns generally emerge suggesting
that gradients of peak metamorphic temperatures
within the MCT zone are probably the main reason
for the slight dispersion of the RSCM data in Fig.
4. We therefore contend that the various factors,
other than thermal metamorphism, affecting the
structural organization of the CM, do not contribute
to more than a few jC uncertainty on the estimated
peak metamorphic temperature. The RSCM technique
might thus be used to detect inter-sample
variations as small as 10–15 jC or so, but absolute
temperatures can only be determined to F 50 jC
due to the uncertainty on the calibration.
5.2. Remarkably uniform temperatures in the MCT
zone
The remarkable clustering of the R2 values in the
MCT zone, and hence of the peak metamorphic
temperatures, implies that the temperature attained in
the MCT zone was fairly uniform, around 540 jC,
without any significant variations along E–W strike
in central and far-western Nepal.
When data from the shear zones below the klippes in
both central and far-western Nepal are considered, they
yield temperatures in the range 470–525 jC slightly
lower than those from the MCT zone. This suggests
that the different thrusts separating those klippes from
the upper part of LH should be considered as equivalent
to the MCT, at least for their thermal evolution.
5.3. Evidence for an inverted thermal gradient
throughout the lesser Himalaya
The whole data set reveals a large scale thermal
metamorphism throughout the LH with temperatures
decreasing gradually from about 540 jC at the top of

240
the LH, to less than 330 jC in the core of the LH
anticlinorium. The inverted metamorphic gradient,
first evidenced near the MCT zone, can thus be traced
down throughout the whole Lesser Himalaya.
6. Conclusions
The almost ubiquitous presence of CM within the
pelitic metasediments from the LH made it possible to
map peak metamorphic temperatures throughout the
Lesser Himalaya in Nepal. These data provide quantitative
constraints with a relative uncertainty of less
than 10–15 jC (at the 1 r confidence level) on the
thermal structure and tectonic evolution of the LH.
They are used in a complementary study that
describes the process of accretion over the Cenozoic
based on complementary thermochronological and
structural data in central Nepal [8]. Our case study
shows that RSCM thermometry is a powerful and
reliable technique to explore the thermal structure of
orogenic prisms in general. Indeed, most collision
belts contain large amounts of deep marine sediments
that were metamorphosed and accreted to the orogenic
prism, but exhibit a mineralogy not sufficient for
conventional petrology.
Acknowledgements
Field work was organized with the friendly
assistance of our colleagues at the Department of
Mines and Geology in Katmandu. Raman spectroscopy
measurements were performed at ENS Lyon
(INSU national instrument). Maps were produced
with GMT software. Constructive and sensitive comments
by Michael Tice and Brigitte Wopenka as well
as editorial handling by Ken Farley are gratefully
acknowledged. [KF]
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