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Journal of Asian Earth Sciences 19 (2001) 17±29
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Microstructures and strain variation across the footwall of the
Main Central Thrust Zone, Garhwal Himalaya, India
Keser Singh*, V.C. Thakur
Wadia Institute of Himalayan Geology, 33, General Mahadeo Singh Road, Dehradun, 248 001, India

Abstract
The microstructural variation with a progressive change in the strain pattern are described in the rocks occurring across the footwall of the
Main Central Thrust (MCT) in an area of the Garhwal Himalaya. In the western Garhwal Himalaya, the MCT has brought upper amphibolite
facies metamorphic rocks southward over the greenschist facies rocks of the Lesser Himalaya. The progressively increasing ¯attening strain
towards the MCT changes either to plane strain or in some cases to constrictional strain. This change in strain is well recorded in the
microstructures. The zone dominated by ¯attening strain is expressed as bedding parallel mylonites. The grain reduction in this zone has
occurred by dynamic recrystallization and quartz porphyroclasts were ¯attened parallel to the mylonite zone. The maximum ®nite strain ratio
observed in this zone is 2.2:1.8:1. The zone, where the ¯attening strain changes either to plane strain or constrictional strain, record an
increase in ®nite strain ratio up to 3.8:1.9:1. This zone represents deformation fabrics like S±C microstructures simultaneously developed
during mylonitization in an intense ductile shear zone. The above zone is either near the MCT or adjacent to crystalline klippen occupying the
core of the synforms in the footwall of the MCT. The microstructural evolution and the ®nite strain suggest that the MCT has evolved as the
result of superposition of southward directed simple shear over the ¯attening strain. The simple shear has played an active role in the rapid
translation which followed the mylonitization at deeper levels. q 2001 Elsevier Science Ltd. All rights reserved.

1. Introduction
The collision of the Indian and Asian plates during the
Palaeogene period produced several thrust planes. The
differential movements along these tectonic planes
produced composite thrust sheets. The crustal shortening,
thus produced, was concentrated on a few major thrust
zones located south of the Indus Tsangpo suture zone.
One of the most important of these is the Main Central
Thrust (MCT). Along this thrust, medium to high grade
metamorphic rocks (Higher Himalayan Crystallines:
HHC) were thrusted over the low grade meta-sedimentary
rocks of the Lesser Himalaya (Heim and Gansser, 1939;
Gansser, 1964; Le Fort, 1975; Thakur, 1987). On the basis
of the presence of klippen of the HHC far south over the
Lesser Himalayan rocks, the MCT zone is thought to have a
cumulative displacement of at least 140±210 km (e.g.
Schelling and Arita, 1991; Schelling, 1992). Studies regarding the crustal shortening and deformation fabrics produced
during this displacement have been limited in number. The
present knowledge about the MCT and other related thrusts

in general, is either restricted to deformation fabrics
(Bouchez and Pecher, 1981; Brunel, 1986) or strain pattern
* Corresponding author. Tel.: 191-135-627387; fax: 191-135-625212.
E-mail address: wihg@giasdl01.vsnl.net.in (K. Singh).

(Roy, 1980; Saklani and Nainwal, 1989; Jain and Anand,
1988; Singh, 1991). The microstructural changes with strain
pattern are required to better understand the internal deformation associated with the MCT. The western part of the
Garhwal Himalaya between Tons and Bhagirathi valleys
were selected for the above studies, since strain markers
are known here across the MCT (Singh, 1991). The footwall
of the MCT has klippen of the thrust sheet rooted within the
crystallines of the HHC. An integrated study of strain
pattern, mylonitization and microstructures across the
MCT zone and also its footwall hold the key to understand
the nature of deformation associated with the MCT in this
part of the Garhwal Himalaya.

2. Geological setting
The geology of Garhwal Himalaya is dominated by a

series of northward dipping and S-verging thrusts. The
Main Central Thrust (MCT), one of the prominent thrusts,
has long been recognized as a major intracontinental ductile
shear zone with an associated inverted metamorphic zone
(Heim and Gansser, 1939; Valdiya, 1980 etc.). The MCT
separates two geologically distinct zones i.e., the Lesser
Himalaya (south of the MCT) and the Higher Himalayan
Crystallines (north of the MCT) (Fig. 1). The rocks of the

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K. Singh, V.C. Thakur / Journal of Asian Earth Sciences 19 (2001) 17±29

Fig. 1. Outline of the tectonic map of the Western Himalaya showing the tectonic framework of the Garhwal Himalaya. The Western Garhwal region is shown
with a rectangle in the map and solid dot in the inset map of India. SHZ-Sub Himalayan Zone, MBT-Main Central Thrust, CH-Chambra Basin, KW-Kishtwar
Window, HHC-Higher Himalayan Crystallines, STD-South Tibetan Detachment.

Lesser Himalaya are largely Precambrian sedimentary units
with a few outcrops of Cambrian and Tertiary strata. The
Higher Himalayan Crystallines (HHC) rocks (1800±2300
Ma old: Bhanot et al., 1980; Singh et al., 1986) occur as a
large, 15±20 km thick thrust sheet and are composed of two
major lithotectonic zones. The underlying inverted metamorphic zone is dominated by feldspar rich augen gneisses,
granodiorite, metasediments, amphibolites and is known as
Munsiari Thrust Sheet. The tectonically overlying prograde
sequence constitutes the high grade metamorphic zone
mainly the kyanite±sillimanite±garnet gneiss, psammitic
gneiss, granulite, magmatite and is known as the Vaikrita
Thrust Sheet. There has been controversy over the exact
location of the MCT. Heim and Gansser (1939) ®rst de®ned
it as the thrust, which places the Crystalline Nappes of the
Higher Himalaya over the sediments of the Lesser Himalaya. Valdiya (1980) maps the Vaikrita Thrust (VT) as the
MCT in the Kumaun Himalaya. This is in agreement with
the tectonic position of the MCT in the Nepal Himalaya (Le
Fort, 1975; Pecher, 1977). Sinha Roy (1982) placed the
MCT at the base of the inverted metamorphic sequence in
the Eastern Himalaya. In Zanskar Himalaya, the MCT is

located at the base of undifferentiated crystalline rocks,
which wrap the Lesser Himalayan rocks in the Kishtwar
Window (Thakur, 1998; Searle and Rex, 1989). The
MCT, in Western Garhwal, is also recognized as thick one
(the MCT zone: up to 12 km) in which the metamorphic
isograds were inverted (Bouchez and Pecher, 1981; Mohan
et al., 1989; Metcalfe, 1993; Searle et al., 1993). This zone,

however, is part of the earlier known Central Crystallines
(Gansser, 1964; Valdiya, 1980). The lower boundary of the
MCT zone (the MCT for present description) corresponds to
the Munsiari Thrust (MT) and the upper boundary to the
Vaikrita Thrust (VT). The HHC rocks are bounded to the
North by the South Tibetan Detachment (STD: Searle,
1986; Valdiya, 1989).
The MCT (MT: Valdiya, 1980) in the studied area has
translated the inverted metamorphic rocks of the HHC
southward over the quartzite of the Berinag Thrust Sheet
(BTS) of the Lesser Himalaya (LS). It dips at about 408 to
558 NE or NNE and the rocks show mylonitic fabric both

along the hangingwall and the footwall. The inverted metamorphic rocks with thickness of several km form the MCT
zone. The lower boundary of the MCT zone (i.e., the MCT)
is observed around Wazri (Fig. 2), Mori (Fig. 3) and Sainj
(Fig. 4) in the Western Garhwal Himalaya. The rocks of the
MCT zone exposed at the higher structural level are within
the amphibolite facies, whereas the base of this structural
pile are in the lower greenschist facies. This observation is
in agreement with the already known metamorphic assemblages in the Bhagirathi valley (Metcalfe, 1993; Searle,
1986) and the other parts of the Himalaya (Hubbard,
1989; Le Fort, 1975; Mohan et al., 1989; Searle and Rex,
1989). A variety of shear zones ranging from mm to outcrop
scale are generally parallel or at acute angle to the schistosity in the MCT zone. These zones are associated mainly
with the change in the lithology in these rocks. The
geometry of the feldspar porphyroclasts in the gneiss

K. Singh, V.C. Thakur / Journal of Asian Earth Sciences 19 (2001) 17±29

19

Fig. 2. Geological and structural map around the MCT in the Yamuna valley.

show abundant evidence of top to south sense of shear
(Metcalfe, 1993) as a result of ductile synmetamorphic
strain. The unit structurally below the MCT to the South,
comprise the quartzite dominated Berinag Thrust Sheet
(Valdiya, 1980) of the Lesser Himalaya. The quartzite is
massive, coarse grained, at places sericitized and generally
white or pale white in colour. This sequence is thrust over
the limestone rich Deoban Formation (Valdiya, 1980) along
the Berinag Thrust (BT). Over the quartzite of the Berinag
Thrust Sheet lies a number of klippen of the Higher Himalayan Crystallines. These klippen are the remnant of the
Chail and or Jutogh Thrust Sheets derived from the HHC.
Two klippen of the Chail Thrust Sheet comprise of chlorite
and muscovite schists, described here as Kanaera Klippe
(Fig. 2) and Chatra Klippe (Fig. 3) along the Yamuna and
Tons valleys respectively. Another klippe of the Jutogh
Thrust Sheet composed of biotite schists, quartz±muscovite
schist and coarse grained gneiss is the Purola Klippe.
The structural analyses at mesoscopic and microscopic
scale in the MCT zone and the part of the Lesser Himalayan

sequence (Berinag Thrust Sheet) forming the footwall indicate three phases of deformation. The earliest one (D1 deformation) has developed penetrative fabric (i.e., widespread

foliation and stretching lineation) observed in both the Berinag Thrust Sheet and the MCT zone. The main planer fabric
(i.e., the foliation in the Lesser Himalaya and the schistosity
in the MCT zone) is dipping NE at moderate to high angles.
It is axial planer to the tight to isoclinal folds, plunging
either NW or SE at moderate angles. The hinges of these
folds (F1) are thickened and appear to be ¯attened. The
foliation plane contains a NE plunging stretching lineation
(L1) expressed by the highly stretched feldspar porphyroclasts in the crystallines of the MCT zones and elongated
quartz clasts in the Berinag Thrust Sheet. The relationship of
stretching lineation (L1) and the foliation (F1) on the equal
area net are represented for the Yamuna (Fig. 2), the Tons
(Fig. 3) and the Bhagirathi (Fig. 4) valleys. The stretching
lineation is also more strongly developed adjacent to the
MCT and below the klippen. The linear fabric represents
the `X'-axis of the strain ellipsoid in the direction of tectonic
transport and is transverse to the trend of the MCT. The
stretching lineation adjacent to the MCT, sometimes,
becomes randomly oriented and almost becomes parallel

to the strike as observed near Sainj along the Bhagirathi
valley. The axes of the F1 folds are nearly normal to the
L1 direction. The quartz clasts in the quartzite progressively

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K. Singh, V.C. Thakur / Journal of Asian Earth Sciences 19 (2001) 17±29

Fig. 3. Geological and structural map around the MCT in the Tons valley.

Fig. 4. Geological and structural map around the MCT in the Bhagirathi valley.

K. Singh, V.C. Thakur / Journal of Asian Earth Sciences 19 (2001) 17±29

21

Fig. 5. Location map of samples for strain analysis from the footwall of the MCT.

become ¯attened when approaching the MCT as is the
mylonitic fabric. The mylonitic banding in the footwall, as

well as the MCT zone, is parallel to the MCT. Additionally
the mylonitic fabric adjacent to the MCT changes to S±C
microstructures (Berthe et al., 1979; Blenkinsop and
Treloar, 1995). This change in the fabric is also observed
adjacent to the crystalline klippen observed over the quartzite, south of the MCT. The inverted metamorphic rocks of
the MCT zone have several discrete zones where the S±C
fabric is observed. Such zones are abundant in the porphyroclastic gneiss and mica schist. The geometry of the feldspar
porphyroclasts and the S±C fabrics indicate abundant
evidence of a top of the south sense of shear. A number of
fold (F2) with large amplitudes were observed in the Berinag
Thrust Sheet on the footwall side of the MCT. The synform
within the quartzite (BTS) accounts for the klippen of both
the Jutogh and Chail Thrust Sheets, and the antiforms as the
windows of the underthrust Deoban Formation. The Kanera
Klippe occurring over the quartzite in the Yamuna valley
(Fig. 2) is occupying the synformal core plunging SE at a
moderate angle. Similarly the Chatra Klippe in the Tons
valley (Fig. 3) is also lying over the quartzite in a synformal
core plunging SE at a moderate angle. The regional schistocity (S1) is folded by these folds (F2). Their limbs have
developed crenulation foliation (S2). These F2 folds are

interpreted as the result of successive and progressive southward translation of the HHC along the MCT. The D3 deformation is non-penetrative and observed as large very open
folds and also as kink folds.

the MCT zone. The two dimensional strain was determined
from each of the three mutually perpendicular thin sections
assuming the foliation plane as the XY plane and stretching
lineation as the `X'-direction for each hand specimen
collected across the MCT (Siddans, 1972; Wood, 1974;
Tullis, 1976). The presence of quartz subgrains along the
margins of the quartz clasts makes it dif®cult to measure
accurately their axial ratios. Therefore the `Center to
Center' technique (Fry, 1979; Ramsay and Huber, 1983)
was used to determine the ®nite strain from the quartz clasts
of the quartzite. About 75±110 grain centres were recorded
from enlarged photomicrographs for the strain determination. The quartz clasts used in this analysis were large detrital grains surrounded by ®ne grains (30±40 mm) of quartz
matrix. The clast sizes are at least an order of magnitude
larger than the matrix grains. Most of the measurements
were made on XZ and YZ sections, although measurements
on some XY sections were also made to check the accuracy
of the computation. By equating Z to unity two-dimensional
strain ratios were integrated into the three dimensional
strain (Ramsay and Huber, 1983). The axial ratios and
orientations of the porphyroclasts were measured accurately, therefore Rf/B (Dunnet, 1969) technique was
applied to determine the strain ratios in the MCT zone.
The stretching lineation served as the arbitrary reference
line for taking the orientation of the porphyroclasts. The
shape of the strain ellipsoid was determined from the plotting of the ®nite strain ratios on Flinn and Hsu graphs
(Hossack, 1968; Flinn, 1978).
3.2. Finite strain variation across the footwall of the MCT

3. Finite strain analysis
3.1. Choice of strain markers and method
The ®nite strain was determined from quartz clasts of the
quartzite of the Berinag Thrust Sheet occurring along the
footwall zone and from the augen of the augen gneiss of

The ®nite strain is determined from `26' oriented samples
of the quartzite from the BTS occurring across the footwall
of the MCT. These samples are distributed along three
valleys (Fig. 5). The quartzite is about 5 km thick in the
Yamuna valley and 6 km thick each in the Tons and Bhagirathi valleys. The ®nite strain ellipsoids along the Tons,

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K. Singh, V.C. Thakur / Journal of Asian Earth Sciences 19 (2001) 17±29

Fig. 6. The distribution of strain ellipsoids along the Tons, Yamuna and Bhagirathi valleys sections across the MCT is superposed over the geological map.
Strain ellipsoid values (X > Y > Z) with specimen location. Lithological symbols are same as in Fig. 1.

Yamuna and Bhagirathi valleys were plotted over the geological map (Fig. 6).
The ®nite strain is determined from `10' quartzite
samples distributed along the Yamuna valley. The ®nite
strain ratios near the base of the quartzite sequence is
1.9:1.7:1. It increases to 3.8:1.9:1 towards the Kanera
Klippe and to 2.3:1.8:1 towards the MCT (Figs. 6 and
7B). The corresponding `K'-value adjacent to the Kanera
Klippe is 1.08 (apparent constriction ®eld) in a progressively increasing trend from 0.21 (near the base) to 0.57
(apparent ¯attening ®eld) toward the MCT along the
Yamuna valley (Fig. 8B). In this section the strain intensity
(Es) values (e.g. Hossack, 1968; Flinn, 1978) corresponding
to quartzite samples adjacent to Kanera Klippe is 2.02 in
progressively increasing values from 0.6 to 0.93 toward the
MCT. The ®nite strain data is also plotted on the Hsu Plot
(Fig. 8b) which corresponds well to the Flinn plot (Fig. 8B).
A near similar trend in the ®nite strain ratios is also
observed in the quartzite sequence along the Tons valley.
The ®nite strain is determined from `9' samples distributed
from the base to the MCT along above valley. The ®nite
strain near the base of the quartzite sequence is 1.9:1.7:1. It
increases to 2.5:1.7:1 towards Chatra Klippe and to
2.3:1.7:1 towards the MCT (Figs. 6 and 7A) along the
Tons valley. The corresponding `K' values adjacent to
Chatra Klippe is 0.55 in a progressively increasing from
0.2 to 0.6 towards the MCT (Fig. 8a). The Es value adjacent
to Chatra Klippe is 1.0 in a progressively decreasing value
from 0.6 to 0.48 toward the MCT. The ®nite strain data were
also plotted on Hsu plot (Fig. 8a) which correspond well to
Flinn plot (Fig. 8A).
The ®nite strain trend observed in the `8' quartzite
samples along the Bhagirathi valley indicates a consistent
and progressive increase from 1.8:1.6:1 to 2.5:2.0:1 towards

the MCT (Figs. 6 and 7C). The corresponding `K' values
progressively increase from 0.25 to 0.48 in the ¯attening
®eld (Fig. 8C) except for sample UM 20 collected adjacent
to the MCT which gives a lower values, i.e. 0.14. The Es
values also represent an increase from 0.64 to 0.9 towards
the MCT. The spatial distribution of variation in the Es
values are represented in Fig. 9.
3.3. Finite strain variation across the MCT zone
The ®nite strain (Rf/B technique) is determined from the
stretched porphyroclasts (more than 30 readings) at six
localities along the Yamuna valley and four in the Tons
valley from the prophyroclastic gneiss of the MCT zone.
It varies from 2.3:1.9:1 to 2.0:1.9:1 adjacent to the MCT
and decreases to 1.9:1.7:1 structurally upsection in the
Yamuna valley. It is 2.2:1.9:1 adjacent to the MCT and
decreases to 2:1 1:7:1 structurally upsection in the Tons
valley. The above ®nite strain is in agreement with the
already known ®nite strain in the Tons Valley. The two
dimensional ®nite strain determined from the augen
decreases from 2.1:1 (X:Z) to 1.6:1 upsection above the
MCT (Jain and Anand, 1988). The above results indicate
an increase in the ®nite strain towards the MCT.
3.4. Microstructural evolution based on strain variation
across the footwall of the MCT
The microstructural variation with progressively increasing Es values toward the MCT were studied along all three
valleys (i.e. the Tons, the Yamuna and the Bhagirathi
valleys). The shear fabric study in relation to the foliation
(S1), the stretching lineation (L1) and the S±C fabrics indicate a progressive deformation history related to MCT as
described below.

K. Singh, V.C. Thakur / Journal of Asian Earth Sciences 19 (2001) 17±29

23

Fig. 7. The geological cross sections along A-A 0 , B-B 0 and CC 0 showing the variation in ®nite strain across the MCT along the Tons, Yamuna and Bhagirathi
valleys respectively. The long and the short diameters represent the X- and Z-axes of the strain ellipse.

The microstructural variation in the quartzite was
described in accordance with Es values along the footwall
of the MCT. The microstructural changes in the quartzite
with different strain intensities (Es) in a progressive deformation suggest that with overall strain intensity (Es < 0.7)
observed in the basal part of the quartzite sequence, the
quartz clasts are relatively less deformed and show undulose
extinction (Fig. 10A). The ¯attened clasts indicate weak
foliation planes. Recrystallized quartz grains are less than
10% of the volume and restricted to the clast margins. Some
of the clasts show diffusive boundaries because of development of quartz subgrains in the marginal zone.
With increasing strain (Es < 0.9) the volume percentage
of the recrystallized quartz subgrains increases and varies
from 30 to 45%. The size of the recrystallized grains appears
to be fairly constant at approximately 20±30 mm. The mylonitic foliation is de®ned by the aligned, and relatively more
¯attened clasts and by the recrystallized quartz grains (Fig.
10B). The quartz subgrains are also developed along the
undulose extinction planes in some of the highly stretched,
deformed quartz clasts. The ®nite strain up to this level is
still in the ¯attening ®eld.
The recrystallization of the clasts into subgrains contri-

buting to the matrix is 50±65% in the highest recorded
strain (Es < 1.1) occurring adjacent to the MCT. The ¯attening strain in this zone change to, or have a tendency
toward plane strain. The quartzite of the Berinag Thrust
Sheet with strain intensity (Es < 1.2) was observed near
the contact zone with the klippen of the Chail Thrust
Sheet. The ®nite strain symmetry here also changes to
constriction. The microstructures in the zone with
Es < 1.1 or Es < 1.2 mark the appearance of S±C fabrics
and are therefore described together. The thickness of this
zone is about 200 m and followed above by a highly
deformed and nearly totally recrystallized zone with well
developed S±C fabrics (cf. Berthe et al., 1979; White et al.,
1980; Lister and Snoke, 1984; Blenkinsop and Treloar,
1995) adjacent to the MCT. The quartz clasts in the zone
are highly elongated with distinct undulose extinction zones
bounded by the microfracture planes (Fig. 10C). The development of subgrains is intense, sometimes leading to nacking within the clasts (Fig. 10D). The majority of the quartz
clasts in the zone are oriented parallel to the `S' planes and
are asymmetrically wrapped by the mylonitic foliation (Fig.
11A). The S±C fabrics, similar to `C' shear band (Passchier
and Trouw, 1996), is developed within some of the quartz

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K. Singh, V.C. Thakur / Journal of Asian Earth Sciences 19 (2001) 17±29

Fig. 8. The variation in the strain ®elds represented on Flinn plots (Figs. A, B and C) and on Hsu plots (Figs. a, b and c) for Tons, Yamuna and Bhagirathi
valleys respectively. Solid dots indicate ¯attening strain in the mylonitized zone and rectangles indicate a tendency for progressive change toward either plane
strain (i.e. Figs. A,a) and of constrictional strain (Figs. B,b) in the S±C fabrics dominated zones.

clasts (Fig. 11C). The excessive recrystallization along the
microfracture planes change the clasts into sigmoidal
shaped subclasts. The shape of such a divided quartz clast
give a top to south sense of shear movement. The quartz

clasts in the zone closer to the MCT no longer preserved
their outer boundaries because of the development of a
higher percentage of quartz subgrains. Still the quartz clasts
that escaped recrystallization present a higher axial ratio

K. Singh, V.C. Thakur / Journal of Asian Earth Sciences 19 (2001) 17±29

25

Fig. 9. Contouring of the ®nite strain intensity (Es) along the MCT superposed over a geological map.

indirectly indicating a further increase in the ®nite strain
ratio towards the MCT. The `S' surfaces in the S±C fabrics
are parallel to the S1 plane outside this zone and `C' surfaces
parallel to the shear plane (Fig. 11B and C) similar to that of
Berthe et al. (1979) model. Generally the intersection
between `S' and `C' is at a moderate to high angle. The
`S' surfaces within the shear zone are identical in appear-

ance and continuous with outside S1 foliation. This indicates
that the slip occur only along the `C' plane and not along the
`S' plane.
Common microstructures of S±C fabric are mica ®sh
observed adjacent to both the MCT as well as the zone
very near the klippen. The mica ®sh are aligned parallel to
the `S' planes while their thin, long and narrow trails are

Fig. 10. Microstructural changes in the quartzite with increasing ®nite strain towards the MCT: (A) Flattening of quartz clasts is minimum. The photomicrograph from XZ section of sample YK-40; (B) Clasts are relatively more ¯attened producing weak foliation, recrystallized quartz subgrains surrounding
the quartz clasts. The proportion of matrix is approximately 30%. The photomicrograph from the XZ section of sample YK-38; (C) Quartz clasts are further
¯attened, well oriented and well foliated. The proportion of matrix is approximately 50%. A photomicrograph from the XZ section of sample YK-132; (D)
Showing extensive development of quartz subgrains leading to necking along a highly stretched quartz clast. The photomicrograph from the XZ section of
sample YK 132. The photomicrograph are obtained from sections cut perpendicular to the foliation but parallel to lineation, scale bar equals 0.04 mm.

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K. Singh, V.C. Thakur / Journal of Asian Earth Sciences 19 (2001) 17±29

Fig. 11. S±C microstructures adjacent to the MCT and the klippen. (A) Quartz clast (q) is parallel to S planes in the quartzite, The photomicrograph is from the
XZ section of sample UM 20. (B) The symmetrical mica ®sh (m) are parallel to the `S' plane (marked as S) with the tails merging into the `C' planes (marked as
C) in the S±C fabrics. The quartz clast (q) is parallel to the `C' planes. The photomicrograph is from the XZ section of sample no. YK-31. (C) The sigmoidal
shaped subclasts (q) resulted from the S±C fabrics within the quartz clast similar to the `C' shear band. The photomicrograph is from the XZ section of a
locality at Wazri adjacent to the MCT. (D) Asymmetrical mica ®sh in the quartzite. The photomicrograph from the XZ section of locality Wazri adjacent to the
MCT. (E) The mica ¯akes in the mica ®sh are straightened out along the extensional plane (b±b 0 ) and a new microfracture plane (c-c 0 ) is developed over one of
its extremity. The photomicrograph is from the XZ section of sample no. YK-31. (F) New growth of mica (m) at an acute angle along the plane of the maximum
extension within the mica ®sh. The photomicrograph is from the XZ section of a locality at Wazri adjacent to the MCT. Scale bar of the Fig. 11 equal to
0.04 mm.

merging into the `C' planes (Fig. 11B). The 001 plane of the
mineral cleavage within both symmetrical (Fig. 11B) and
asymmetrical mica ®sh (Fig. 11D) are curved as a result of
slip along these planes. In some mica ®sh, the 001 planes are
acutely transected by another shear plane (C±C: Fig. 11E)
observed along the outer extremity. Such shear planes
displace the cleavage and die out within the mica ®sh.
The alignment of these shear planes are almost parallel to
the C planes (Lister and Snoke, 1984). The recrystallized
mica is observed within some of the mica ®sh. Such newly
crystallized mica grains are aligned at an acute angle along
the plane of maximum extension (Fig. 11F). The geometry

and orientation of new mica indicate that growth took place
during the late strain-softening period. The sense of shear
deduced from such grains is the same as that recorded by the
asymmetrical mica ®sh geometry. The presence of the
microshear planes and growth of new mica within the mica
®sh indicate a late stage of shearing. Such evolution is
supported by other elements in the mylonitic fabrics, such
as the sense of rotation of quartz clasts and the syn-shearing
folds. The microstructural observations in the mica ®sh of
the S±C fabrics are indicative of typical non-coaxial
deformation.
The correlation of strain intensities with the development

K. Singh, V.C. Thakur / Journal of Asian Earth Sciences 19 (2001) 17±29

of microstructures indicates that in a progressively increasing strain, there is a progressive change in the microstructures toward the MCT. A change in the strain ®eld is well
re¯ected and well recorded in the microstructures. The
correlation further suggests that the mylonitization and
development of S±C microstructures were products of the
same progressive deformation.
3.5. Microstructural evolution across the MCT zone
The mylonitic fabrics are the prominent feature observed
in the mica schist and overlying gneisses. Within the mylonitized zone there are several alternating zones of protomylonite, mylonite and ultramylonite (Twiss and Moore,
1992). The feldspar porphyroclasts with diameter .1 mm
(the ®nite strain is determined from such porphyroclasts) are
generally observed in the protomylonite. The mica schist
adjacent to the MCT of the Yamuna and Tons valleys correspond to high strain zone and consist of thin elongated plates
of quartz within ®ne grained muscovitic aggregates (ªshimmer aggregatesº). The S±C fabrics are also well developed
in the mica schist. The overlying porphyroclastic gneiss
with ultramylonitic fabrics sometimes, exhibit S±C fabrics.
The porphyroclasts of the feldspar consist of tails of pressure shadows at their end mainly in the protomylonite. The
polygonized quartz grains are observed within the shadow
zones. The large sized porphyroclasts show both the fracture
as well as stretching. The recrystallization within the
porphyroclasts is generally con®ned along the fracture
planes. The feldspar porphyroclasts sometimes show undulatory extinction. Therefore these clasts appear to have
deformed by the combined effect of plasticity and fracturing. The aspect ratios of some of the feldspar porphyroclasts
(in cm.: not used in strain calculation) is up to 3:1 and 8:1 in
some of the extremely stretched pull-apart porphyroclasts.
4. Strain controlled microstructural pro®le across the
MCT and the inverted metamorphism
The zonal distribution of the microstructures re¯ects the
amount and type of ®nite strain at a given level in the quartzite occurring along the footwall of the MCT. From the base
of the quartzite sequence, the development of mylonitic
characters are essentially governed by the strain pattern as
indicated by the contouring of Es values (Fig. 9). The zone
between strain intensities (Es) 0.7 and 0.9 indicates that the
strain is of a ¯attening type and there is a gradual increase in
the ®nite strain from 1.9:1.7:1 to 2.2:1.8:1 with a nearly
constant orientation towards the MCT. This zone represents
nearly 40% recrystallization along the quartz clasts and the
recrystallized quartz subgrains along these quartz clasts
increases with the increasing strain. This increase in quartz
subgrains aligns preferentially with higher strain and thus
progressively develops the mylonitic foliation (S1) in the
quartzite. The relationship between the microstructures
and strain pattern in the quartzite indicates a direct bearing

27

on the MCT and therefore can be interpreted as a product of
the MCT. The stretching lineation (L1) on the foliation plane
(`X' direction of the strain ellipsoid) is also a product of
development of the MCT. It tests the assumption that the
stretching lineation represents the direction of tectonic
transport (Roy, 1982; Thakur, 1987). The increase in the
®nite strain towards the MCT when reaching Es equal to
1.1 produces a change in the mylonitic foliation with the
advent of S±C fabrics. This simultaneous change in the
mylonitic foliation with the development of S±C fabrics
coincides with a change in the strain symmetry from the
¯attening strain to either the plane or the constrictional
strain. Such changes in the strain symmetry are observed
in a narrow zone adjacent to the MCT. This change in the
strain symmetry further imparted stretching to the strain
ellipsoids which in turn is re¯ected in a stronger development of stretching adjacent to the MCT. The deformational
changes along the quartzite below the crystalline klippen
along the footwall of the MCT also indicate an increase in
the strain and a change in strain symmetry from ¯attening to
constriction (e.g. Chatra and Kanera Klippen). The S±C
fabrics is also observed in this zone. It can be safely
assumed that similar deformation conditions must have
prevailed during translation of the Chail (also the Jutogh)
thrust sheet from the HHC. Therefore the zone with a higher
strain and a strong component of simple shear has triggered
the translation of crystallines from the MCT zone to now lie
over the footwall in the eroded form of klippen. This also
supports the contention that the stretching lineation represents the tectonic transport direction. The MCT zone has
numerous zones rich in S±C fabrics mainly in the ultramylonitic fabrics. Such zones, in particular and the MCT zone
as a whole can be interpreted as evolved from the superposition of simple shear over the ¯attening strain. Similar
change in the deformation pattern were cited in the N Apennine shear zone (Klig®eld et al., 1981), Caledonian Skerrols
Thrust (Saha, 1989).
The inverted metamorphic sequence in the MCT zone is
recognized all along its length. Several models like: (1)
recumbent folding of the isograds (Frank et al., 1977; Searle
and Rex, 1989); (2) imbricated slices juxtaposed during the
thrusting (Treloar et al., 1989; Mohan et al., 1989); (3)
frictional heating (Bird, 1978; Le Fort, 1975); (4) hot over
cold model (Le Fort, 1975, 1986); and (5) ductile shearing
(Hubbard, 1989; Jain and Manickvasagam, 1993) were
proposed for this inverted metamorphism. None of these
are consistent with all the geological observations. One of
the structural observations that have not been given
adequate consideration is the distribution and changes in
the internal strain and the ductile shear deformation. The
widely accepted model to explain the inverted metamorphic
zonation in the MCT zone is the emplacement of hot rocks
(Vaikrita) over the cooler rocks (MCT zone). Geothermobarometric studies in Western Garhwal (Metcalfe, 1993),
and also in other parts of the Himalaya (Mohan et al.,
1989, etc.), suggest a rapid cooling to preserve inverted

28

K. Singh, V.C. Thakur / Journal of Asian Earth Sciences 19 (2001) 17±29

metamorphic zonation. This is explained by a higher rate of
uplift and rapid erosion. A close relationship between the
strain accumulation and metamorphic inversion is proposed
along the MCT. The study of rocks indicates a mylonitized
zone distributed all along the MCT zone as well as in a part
of the footwall zone. The myylonitized zone with S±C
fabrics indicate an increase in the strain and a stronger
development of stretching lineation adjacent to the MCT,
which is explained here as superposition of simple shear
over the ¯attening strain. This simple shear has played an
active role in the rapid translation of crystallines along the
MCT. It also goes well with a rapid exhumation, which
followed the mylonitization at dipper level.

5. Conclusions
The MCT and associated thrusts evolved in a progressive
non-coaxial type deformation. The correlation of ®nite
strain and microstructures indicate that the ¯attening strain
progressively increased towards the MCT so does the mylonitic signatures. The ¯attening strain adjacent to the MCT
and the crystalline klippen changes to either plane strain or
constrictional strain and this zone is dominated by S±C
microstructures. Such a strain pattern and microstructural
evolution is explained here by superposition of simple
shear over the ¯attening strain. The simple shear strain
adjacent to the boundaries of the MCT zone has resulted
from a rapid translation.

Acknowledgements
KS is grateful to Dr. M.I. Bhat for his constructive
comments and detailed review of the paper. He is also
thankful to the Director, Wadia Institute of Himalayan
Geology, Dehradun for providing various facilities.

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