Deformation style in the Munsiari Thrust Zone: a study in the Madlakia–Munsiari–Dhapa section in north-eastern Kumaun Himalaya

ORIGINAL PAPER

Deformation style in the Munsiari Thrust Zone: a studyin the Madlakia–Munsiari–Dhapa section in north-easternKumaun Himalaya

Abhishek Moharana • Anurag Mishra •

Deepak C. Srivastava

Received: 10 October 2012 / Accepted: 16 March 2013

� Springer-Verlag Berlin Heidelberg 2013

Abstract The Main Central Thrust demarcates the

boundary between the Lesser Himalaya and the Higher

Himalaya in the Himalayan orogen. Several definitions of

the Main Central Thrust have been proposed since it was

originally described as the southern boundary of the crys-

talline rocks (the Main Central Thrust mass) in the Ku-

maun-Garhwal Himalaya. The long-held contention that

the Munsiari Thrust represents the Main Central Thrust has

been negated by recent isotopic studies. One way to define

the Main Central Thrust is that it is a ductile shear zone that

is delimited by the Munsiari Thrust (MCT-I) in south and

the Vaikrita Thrust (MCT-II) in north. The alternative

proposition that the Vaikrita Thrust represents the Main

Central Thrust is fraught with practical limitations in many

parts of the Himalaya, including the study area. In the

metamorphic rocks bounded between the Vaikrita Thrust

and the Munsiari Thrust, the isoclinal folds of the earliest

phase are routinely ascribed to the pre-Himalayan orogeny,

whereas all subsequent folding phases are attributed to the

Himalayan orogeny. This article elucidates the structural

characteristics of the kilometre-thick Munsiari Thrust Zone

and revisits the issue of pre-Himalayan orogenic signatures

in the thrust zone. With the help of high-resolution field

mapping and the analyses of mesoscopic scale structures,

we demonstrate that the Munsiari Thrust is a typical fault

zone that is made up of a fault core and two damage zones.

The fault core traces the boundary between the quartzite

and the biotite-gneiss. The damage zones consist of the

low-grade metasedimentary rocks in the footwall and the

gneiss-migmatite in the hanging wall. The entire fault zone

shares an essentially common history of progressive ductile

shearing. Successively developed mesoscopic folds trace

various stages of progressive ductile shearing in the dam-

age zones. Two recognizable stages of the shearing are

represented by the early isoclinal folds and the late kink

folds. As the strain during progressive deformation

achieved the levels that were too high for accommodation

by ductile flow, it was released by development of a tec-

tonic dislocation along a mechanically weak boundary, the

Munsiari Thrust. The isoclinal folds and the Munsiari

Thrust were developed at different stages of a common

progressive deformation during the Himalayan orogeny.

Contrary to the popular notion of consistency with respect

to orientation, the stretching lineations show large direc-

tional variability due to distortion during the late folding.

Keywords Himalaya � Main Central Thrust � Damage

zone � Ductile shearing � Pre-Himalayan orogeny � Sheath

folds

Introduction

The Himalayan orogen evolved as a consequence of the

continent–continent collision between the Indian plate and

the Eurasian plate ca. 55 Ma ago (Klootwijk et al. 1992;

Rowley 1996; Godin et al. 2006; Yin 2006). The orogen is

an ensemble of four juxtaposed lithotectonic terranes: (1)

the Sub Himalaya, (2) the Lesser Himalaya, (3) the Higher

Himalaya, and (4) the Tethys Himalaya (Fig. 1a—after

Jackson and Bilham 1994). The Himalayan Frontal Fault

(HFF) or the Main Frontal Thrust, the Main Boundary

Thrust (MBT), the Main Central Thrust (MCT), and the

South Tibetan Detachment System (STDS) delimit the

southern boundaries of the four terranes (Fig. 1a). Among

A. Moharana � A. Mishra � D. C. Srivastava (&)

Department of Earth Sciences, Indian Institute of Technology

Roorkee, Roorkee- 247 667, India

e-mail: [email protected]; [email protected]

123

Int J Earth Sci (Geol Rundsch)

DOI 10.1007/s00531-013-0892-6

the four terrane boundary faults, the MCT, developed

during 25–20 Ma, is most extensively studied (Hubbard

1989; Metcalfe 1993; Copeland et al. 1996; Arita et al.

1997). The thrust is characterized by a large displacement

of the order of 140–600 km (Schelling and Arita 1991;

Srivastava and Mitra 1994).

Heim and Gansser (1939) first introduced the term ‘Main

Central Thrust mass’ in their geological description of the

Kali and Alaknanda valleys in Kumaun-Garhwal Himalaya.

They describe the ‘Main Central Thrust mass’ as a meta-

morphic pile that rests directly over the Lesser Himalayan

quartzite along a distinct lithotectonic boundary, the Muns-

iari Thrust. Later, Gansser (1964) noted a striking meta-

morphic contrast across the southern boundary of the Higher

Himalaya and named the boundary as the MCT. In many

subsequent studies, the Main Central Thrust is regarded as

1–10-km thick ductile shear zone delimited by the Munsiari

Thrust (MCT-I/MCTL) in south and the Vaikrita Thrust

(MCT-II/MCTU) in north (Bouchez and Pecher 1981; Arita

1983; Mohan et al. 1989; Searle et al. 1993; Martin et al.

2005; Godin et al. 2006; Kohn 2008; Mukherjee 2012; and

others). Following Heim and Gansser (1939) and Gansser

(1964), the Munsiari Thrust has long been considered as the

boundary between the Lesser Himalaya and the Higher

Himalaya, the MCT (Bhattacharya 1987; Bhattacharya and

Weber 2004; Patel et al. 2011; Singh et al. 2012). This view

has been recently contested on the basis of isotopic contrast

between the Lesser Himalayan and the Higher Himalayan

metamorphic rocks across the Munsiari Thrust (Ahmad et al.

2000; Martin et al. 2005; Richards et al. 2005). These studies

reveal that the eNd(0) values in the Lesser Himalayan meta-

morphic rocks are characteristically smaller, -20 to -26,

than those in the Higher Himalayan gneiss, -12 to -19.

Similarly, the detrital zircon ages from the Lesser Himalayan

metamorphics are typically[1,550 Ma, whereas those from

the Higher Himalayan gneiss cluster ca. 1,050–600 Ma

(Martin et al. 2005). As the metamorphic rocks resting above

the Munsiari Thrust exhibit the isotopic affinities with those

Fig. 1 a Schematic cartoon

showing different lithotectonic

terranes in Himalaya (after

Jackson and Bilham 1994). HFFHimalayan Frontal Fault, MBTMain Boundary Thrust, MCTMain Central Thrust and STDSSouthern Tibetan Detachment

System. b Satellite image

showing the Himalayan orogen

and location of study area


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in the Lesser Himalaya, the Munsiari Thrust is no longer

regarded as the MCT demarcating the Lesser Himalaya–

Higher Himalaya boundary. Such interpretations support the

contention that the Vaikrita Thrust, and not the Munsiari

Thrust, represents the Main Central Thrust (Valdiya 1980a,

2010).

Deformation style in the gneiss-migmatite, exposed to

the north of the Munsiari Thrust, has long been elucidated

Fig. 2 a Impure marble beds of the Tejam Group in the footwall of the Munsiari Thrust. b Gneiss in the hanging wall of the Munsiari Thrust

Fig. 3 Geological map of the

study area. The core of fault

zone, the Munsiari Thrust (MT),

marks the contact between the

quartzite and biotite-gneiss. The

hanging wall damage zone

extends up to approximately

2 km north of Dhapa, and its

northern boundary is

characteristically diffused. The

boundaries of dominantly fine-

and coarse-grained augen gneiss

and the dominantly migmatite

units are gradational. The

footwall damage zone is thin,

and its southern boundary

passes through south of

Madlakia


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by polyphase folding and ductile shearing (Bhattacharya

1982, 1987; Thakur and Choudhury 1983; Valdiya and

Goel 1983; Roy and Valdiya 1988; Dubey and Paul 1993;

Paul 1998; Bhattacharya and Weber 2004; Patel and

Kumar 2009; Mukherjee and Koyi 2010; Patel et al. 2011).

Among the successively developed folds in the gneiss-

migmatite, the early isoclinal folds are routinely regarded

as the signatures of the pre-Himalayan orogeny, whereas

all subsequent folds and associated structures are attributed

to the Himalayan orogeny (Thakur 1980; Pognante et al.

1987; Williams et al. 1988; Treloar et al. 1989; Jain and

Patel 1999; Jain et al. 2002). Several issues such as the

effect of the Munsiari Thrust-related deformation on the

footwall rocks and that of the structural correlation

between the rocks in footwall and hanging wall of the

Munsiari Thrust have not as yet been addressed. In this

study, we demonstrate that the Munsiari Thrust is a kilo-

metre-thick fault zone, question the hypothesis that the

fault zone preserves any signature of the pre-Himalayan

orogeny, and propose that the successive folds were

developed during the course of a progressive ductile

shearing in the fault zone.

Geological setting

The study area exposes a rare and accessible strike section

of the Munsiari Thrust for a distance of 35 km from

Madlakia to Munsiari in north-eastern Kumaun Himalaya

(Fig. 1b). The footwall of the Munsiari Thrust exposes the

quartzite/impure marble of the Tejam Group, whereas the

hanging wall rocks constitute a gneiss-migmatite terrane

(Fig. 2a, b). Besides mapping the strike section, we have

also traced a 6 km-long up-dip section, from Munsiari to

north of Dhapa in the hanging wall of the Munsiari Thrust

(Fig. 3).

Fig. 4 Typical examples of complex relationships between gneiss

and migmatite in the hanging wall damage zone. a Obliteration of

migmatite fabric due to granitization (upper left in the photograph).

Thin white line traces the granitization front. Note the lack of fabric

and the occurrence of relict melanosome patches (marked by arrow)

in granite. G—granite, M—migmatite. b A top-to-the-south-west

ductile shear zone cuts through the well-developed gneissic banding

(Gn) in lower left of the photograph. Syn-kinematic migmatization

has transformed gneiss into migmatite (M) within the shear zone. The

migmatite banding traces a series of complex isoclinal folds. c Top-

to-the-south-west ductile shear zones cut through an older gneiss. A

younger gneiss is developed within the shear zones. In the younger

gneiss, the banding is relatively coarser than the fine banding in the

older gneiss. d Development of biotite-rich gneiss (Gn) in a top-to-

the-south-west ductile shear zone that cuts through migmatite (M)


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The Munsiari Thrust Zone

Our mapping shows that the Munsiari Thrust Zone is a

several-kilometre thick fault zone that is made up of a fault

core and two ductilely sheared damage zones, one on each

side of the fault core. The Munsiari Thrust, mappable as the

sharp contact between the footwall quartzite/impure marble

and the hanging wall biotite-gneiss, represents the fault

core. The two damage zones are as follows: (1) footwall

damage zone containing sheared quartzite/marble to the

south and (2) hanging wall damage zone containing sheared

gneiss and migmatite farther to the north (Fig. 3). The

footwall damage zone is \1 km wide, and its southern

boundary with the mildly deformed sediments is rather

sharp. By contrast, the hanging wall damage zone is[5 km

wide, and its northern boundary is characteristically dif-

fused. Occurring as high-strain-concentration zones, several

bands of the marble-phyllonite and the gneiss-phyllonite cut

through the footwall damage zone and the hanging wall

damage zone, respectively (Fig. 3).

Gneiss and migmatite of several generations can be rec-

ognized in the hanging wall damage zone. Some typical

examples of overprinting relationships between different

generations of gneiss and migmatite are as follows: (1)

obliteration of folds and ductile shear fabric in the migmatite

due to granitization, ‘the migmatite-in-breaking’ (Fig. 4a), (2)

migmatite development in metre-scale-thick ductile shear

zones cutting through the gneiss, ‘the migmatite-in-making’

(Fig. 4b), (3) transformation of an older gneiss into a younger

gneiss in the retrogressed biotite-rich shear zones cutting

through the older gneiss (Fig. 4c), and (4) the transformation

of migmatite into gneiss in centimetre–metre-scale ductile

shear zones cutting through the migmatite (Fig. 4d). Consid-

ering the complex relationships between successively devel-

oped gneiss and migmatite, we classify the hanging wall

damage zone into three mappable units: (1) the dominantly

fine-grained augen gneiss, augen size\2 cm (Fig. 5a), (2) the

dominantly coarse-grained augen gneiss, augen size [2 cm

(Fig. 5b), and (3) the dominantly migmatite with distinct

leucosome and melanosome bands (Fig. 5c). The boundaries

between the three units are invariably gradational and inter-

pretative (Fig. 3). We use the terms ‘augen gneiss’ and ‘mi-

gmatite’ in purely descriptive and non-genetic sense for the

purpose of mapping (Fig. 3).

Mesoscopic folds in the damage zones

The mesoscopic folds in the footwall damage zone and the

hanging wall damage zone are characteristically similar with

respect to style, orientation, and relative order of develop-

ment. On the basis of outcrop-scale overprinting relation-

ships, we classify the mesoscopic folds into two major

groups, the early fold group F1 and the late fold group F2.

Early fold group F1; F1A and F1B fold sets

F1 fold group consists of two isoclinal fold sets, F1A and

F1B, that are identical with respect to geometry and ori-

entation. Distinction between the two fold sets is possible

Fig. 5 Typical outcrops of the three mappable units in the hanging

wall damage zone. a Fine-grained augen gneiss. b Coarse-grained

augen gneiss. c Migmatite containing folded leucosome and melano-

some bands


123

only on those outcrops that exhibit distinct overprinting

relationships. F1A folds trace the sedimentary layering in the

footwall damage zone and the gneissic layering in the

hanging wall damage zone (Fig. 6a, b). These folds com-

monly occur as the rootless hinge zones associated charac-

teristically with an axial plane mylonite foliation (Fig. 6b).

F1A folds and their axial plane foliation trace another set of

isoclinal folds, F1B folds (Fig. 6c). Type-3 interference

patterns (Ramsay 1967), developed due to coaxial refolding

of F1A folds by F1B folds, are common in both the damage

zones (Fig. 6 a, d). F1B axial plane foliation is also a myl-

onite foliation showing common development of S–C fabric

(Fig. 6c). It is evident that at least two generations of myl-

onite foliation were successively developed and transposed

along with F1A and F1B isoclinal folds during the course of

progressive ductile shearing (Fig. 6d, e).

The early lineation group consists of three types of

lineations paralleling each other. These are (1) F1A and F1B

fold hinge lines, (2) intersection lineations, and (3)

stretching lineation defined by preferred orientation of

Fig. 6 a F1A and F1B isoclinal folds in the impure marble in the

footwall damage zone. b F1A isoclinal folds in the fine-grained augen

gneiss in the hanging wall damage zone. A mylonite axial plane foliation

cuts through the F1A hinge zone. The folds are characteristically

transposed along the axial plane mylonite foliation. c Hinge zone of a

F1B isoclinal fold in the fine-grained augen gneiss. F1B axial plane

foliation is also a mylonite foliation showing the S–C structure (bounded

between white half-arrows denoting sinistral shear sense). d Type-3

interference pattern formed due to a coaxial refolding of F1A folds

during F1B folding. F1A folds (marked by arrow) are thoroughly

transposed. e F1A axial plane folding by F1B folds. Arrow points to a

transposed and rootless F1A isoclinal fold that is folded by F1B fold


123

stretched quartz, feldspar, and biotite (Fig. 7a). Very tight

to isoclinal sheath folds are observable in both the hanging

wall and the footwall damage zones (Fig. 7b). The sheath

folds were formed due to rotation of F1A and F1B hinge

lines towards the X direction and that of their respective

axial planes towards the XY principal plane of the strain

ellipsoid during the course of progressive ductile shearing

(Ghosh et al. 1999; Alsop and Carreras 2007; Srivastava

2011). Hinge lines of F1A folds, F1B folds and the sheath

folds parallel NNE-NE directed maximum stretching due

to their rotation and extremely tight to isoclinal geometry.

Late fold group F2; F2A and F2B fold sets

Deformation of early lineations, paralleling F1 fold hinge

lines, by F2 folds is the most common overprinting relation-

ship between the two fold groups (Fig. 8a). The F2 fold group,

developed abundantly in the hanging wall damage zone,

consists of two fold sets, F2A and F2B, that characteristically

plunge at low angles towards N-NNE and west, respectively

(Fig. 8b–e). Both F2A and F2B fold sets contain highly

asymmetric open to close kink folds that lack any penetrative

axial plane foliation (Fig. 8c). Of the two kink fold sets,

N-NNE plunging F2A fold set is more abundant than westerly

plunging F2B fold set. Due to lack of any observable over-

printing relationship between F2A and F2B fold sets, their

relative order of development is uncertain. The kink folds are,

therefore, classified into F2A and F2B fold sets purely on the

basis of their characteristic orientation without any implica-

tion on the order of development (Fig. 8d, e).

Compressional structures such as the thrust duplex and

the imbricate structure, and the semiductile extensional set

of planes with normal sense of shear-offset cut all the fold

sets and the mesoscopic ductile shear zones in the gneiss-

migmatite terrane (Fig. 9a, b). While a detailed study of the

late compressional and extensional structures is in pro-

gress, we report their characteristic occurrence in the

hanging wall damage zone in this study.

Fig. 7 a Stretching lineation parallels F1A/1B hinge line in fine-grained augen gneiss. b F1A/1B sheath fold


123

Discussion

Early isoclinal folds; pre-Himalayan or Himalayan?

Deformation style in the gneiss-migmatite terrane has been

elucidated by polyphase folding and ductile shearing in

several studies (Bhattacharya 1982, 1987; Thakur and

Choudhury 1983; Roy and Valdiya 1988; Dubey and Paul

1993; Patel and Kumar 2009; Patel et al. 2011). Many

workers attribute the earliest isoclinal folding to a pre-

Himalayan orogeny and the subsequent folding to the

Himalayan orogeny (Jain et al. 2002 and references therein;

Patel and Kumar 2009; Patel et al. 2011). The isoclinal

geometry of the earliest folds quoted as a criterion for

assigning the pre-Himalayan age to the folds is, however,

questionable in the Munsiari Thrust Zone.

The sharp contact between the quartzite and biotite-

gneiss is invariably tectonic in nature. It traces the core of

Fig. 8 a Deformed F1 intersection lineation (F1 ln), marked by whiteline on the surfaces of F2 folds. Dashed white line—F2 hinge line. b, cHighly asymmetric late folds (F2) in footwall and hanging wall

damage zones. Dashed white line—F2 hinge line in b. d, e Lower

hemisphere equal area projections of s-poles, hinge lines and axial

planes of F2A and F2B folds, respectively


123

the fault zone, the Munsiari Thrust, and parallels the

mylonite foliation in the damage zones (Fig. 3). Two dis-

tinct types of relationships are mappable along contact

between the quartzite and the biotite-gneiss: (1) the biotite-

gneiss overlies the quartzite and (2) the quartzite overlies

the biotite-gneiss (Fig. 10a, b). Two alternative interpre-

tations can explain the two types of observed contact

relationships. First, the two types of contact relationships

represent the tectonic interlayering of the biotite-gneiss and

the quartzite due to imbrication in the Munsiari Thrust

Zone. Second, the two different types of contact relation-

ships correspond to normal and overturned limbs of the

isoclinally folded Munsiari Thrust. The inaccessible nature

of the terrain is, however, a limitation in tracing the indi-

vidual fold hinge zones along the Munsiari Thrust.

Whereas the map scale isoclinal folding of the Munsiari

Thrust remains yet to be tested, the occurrence of abundant

mesoscopic scale isoclinal folds is undisputed in the

Munsiari Thrust Zone.

Fig. 9 a An outcrop-scale thrust duplex cuts through the sheared gneiss in the hanging wall damage zone. b Extensional crenulation cleavage

cuts through early isoclinal folds (marked by arrow) in the augen gneiss

Fig. 10 Two types of contact relationships between the quartzite and biotite-gneiss along the Munsiari Thrust. a The biotite-gneiss overlies the

quartzite. b The quartzite overlies the biotite-gneiss

Fig. 11 Stretching lineations, directed variably from N to E and

towards SW, show a large orientational variation (lower hemisphereequal area plots)


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The Munsiari Thrust Zone is several-kilometre-thick

ductile shear zone that evolved as a consequence of ductile

shearing of quartzite, marble, different types of gneiss, and

migmatite. Mesoscopic scale isoclinal fold sets and atten-

dant axial plane mylonite foliations, sheath folds, ductile

shear zones and kink folds arrest various stages of the

progressive ductile shearing. Towards the last stages of the

shearing, the strain achieved a level that was too large for

accommodation by ductile mechanism. At this stage, the

fault zone broke as the Munsiari Thrust, the fault core, that

accommodated 140–600 km displacement in the Himala-

yan orogen (Schelling and Arita 1991; Srivastava and

Mitra 1994). The isoclinal folds and the Munsiari Thrust

represent different stages of the same progressive defor-

mation during the Himalayan orogeny. The isoclinal folds,

therefore, cannot be regarded as the signatures of any pre-

Himalayan orogeny.

Stretching lineation orientation as indicator

of the regional transport direction

Stretching lineations are extensively used as the transport

direction indicator in the Himalayan orogen (Brun et al.

1985; Brunel 1986; Pecher and Scaillet 1989; Pecher 1991;

Pecher et al. 1991). The notion of stability in the transport

direction during the large-scale regional movements in the

Himalaya is based on the apparent orientational consis-

tency in NE-NNE-directed stretching lineation.

Our study reveals that the stretching lineations, paral-

leling the F1 fold hinge lines, are distorted by the late folds,

F2 (Fig. 8a). As the late folds are characterized by asym-

metric geometry and high limb length ratio, most obser-

vations record the stretching lineation orientation on the

longer limb of the late folds. There is an inevitable sam-

pling bias as the stretching lineation observations from the

long limb of the late folds heavily outweigh those from the

short limb. As shown on the stereoplot, the stretching lin-

eation exhibits a considerable range of orientation due to its

distortion during the late folding (Fig. 11). A few south-

westerly directed stretching lineations are those that occur

on the short limb of the late folds (Fig. 11). We suggest

that the stretching lineations could be used as the transport

direction indicator provided the effect of late folding is

removed for restoration of the original orientation.

Criteria for identification of Vaikrita Thrust

Valdiya (1980b, 2010) cites the distinct metamorphic break

and the fold orientational contrast as criteria for identifi-

cation of the Vaikrita Thrust. Various existing criteria for

delineation of the Vaikrita Thrust are critically evaluated

by Searle et al. (2008). Of the several criteria, the occur-

rence of ‘high-strain zone’ and the presence of ‘metamor-

phic break’ are applied most extensively.

We have mapped several discrete high-strain zones that

occur as the phyllonite bands cutting through the damage

zones in both the footwall and the hanging wall of the

Munsiari Thrust (Fig. 3). The distribution of the phyllonite

bands is too sparse and inhomogeneous for delineation of

any particular high-strain zone as the Vaikrita Thrust. The

lack of staurolite grade in the progressive Barrovian

metamorphism, ‘the metamorphic break’, is another com-

monly used criterion for identification of the Vaikrita

Thrust. In field, however, the Vaikrita Thrust is routinely

mapped by the first megascopic appearance of kyanite. In

the study area, we note a progressive increase in meta-

morphic grade from biotite-garnet through staurolite to

kyanite grade (Fig. 12a, b). The ‘metamorphic break’ cri-

terion for delineation of Vaikrita Thrust is, therefore, dif-

ficult to apply on a regional scale during geological

Fig. 12 a Idioblasts of garnet (Grt) in the garnet-biotite-gneiss

exposed the vicinity of the Munsiari Thrust. b Staurolite idioblast (St)in the migmatite exposed several kilometres north of the Munsiari

Thrust near Dhapa village. Kyanite and garnet appear in the gneiss-

migmatite exposed towards south and north of Dhapa, respectively


123

mapping. Finally, the criterion of structural discordance

remains untested due to lack of detailed structural analyses

and fold orientation data across the Vaikrita Thrust. Irre-

spective of the criteria, there is no regional scale thrust that

could be mapped as the Vaikritra Thrust in the study area.

The Munsiari Thrust, in light of the recent isotopic

signatures, is no more considered as the boundary between

the Lesser Himalaya and the Higher Himalaya. The

Munsiari Thrust is, however, very distinct lithotectonic

boundary that is easily identifiable and mappable at the

regional scale. The proposition that the Munsiari Thrust,

rather than the potentially obscure Vaikrita Thrust, could

be regarded as the Main Central Thrust deserves attention.

Redesignation of the Munsiari Thrust as the Main Central

Thrust would, however, be consistent with the original

nomenclature of the thrusts in the Himalaya (Heim and

Gansser 1939; Gansser 1964).

Summary and conclusions

Geological mapping and structural analyses reveal that the

Munsiari Thrust Zone is made up of a fault core flanked by

two damage zones of asymmetric widths. The sharp dis-

continuity between the quartzite and biotite-gneiss, the

Munsiari Thrust, is the fault core (Fig. 3). The footwall

damage zone is made up of intensely deformed low-grade

metasedimentary rocks. By contrast, the hanging wall

damage zone exposes a sheared gneiss-migmatite terrane.

The southern boundary of the footwall damage zone is

rather sharp, whereas the northern boundary of the hanging

wall damage zone is characteristically diffused.

This study establishes an essentially common style of

deformation in the footwall damage zone and hanging wall

damage zone (Fig. 13a–c). The structural evolution of the

Munsiari Thrust Zone occurred in a progressive shearing

that witnessed development of successive isoclinal folds

and mylonite foliations, sheath folds, and kink folds. The

kink folds, common particularly in the hanging wall

damage zone, were initiated probably due to a change in

shear direction during late stages of the shearing. The late

folds progressively tighten towards north due to progres-

sive increase in the intensity of shearing (Fig. 13b, c).

Intense ductile shearing in the discrete zones resulted

into development of sparsely distributed phyllonite bands

in the hanging wall and footwall damage zones. At advance

stages of progressive shearing, the level of strain became

too high for accommodation by the ductile mechanism and

the deformation mode switched from ductile to brittle. This

resulted into development of the fault core, the ‘Munsiari

Thrust’, as a tectonic dislocation surface along the

mechanically weak boundary between the quartzite and

biotite-gneiss. A large but presently unknown amount of

shortening was accommodated initially by ductile defor-

mation and later by brittle deformation during the tectonic

evolution of the Munsiari Thrust Zone.

Three main limitations in delineation of the Vaikrita

Thrust in study area are as follows: (1) progressive increase

Fig. 13 a–c Lower hemisphere equal area projections of poles to s-

surfaces tracing the late folds in the footwall damage zone and in the

hanging wall damage zone. The distribution pattern of s-poles implies

that the late folds become progressively tighter from south to north.

The fold hinge lines, determined from the distribution of s-poles, are

consistently directed towards N-NNE throughout the damage zone


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in the Barrovian metamorphism, biotite-garnet-staurolite-

kyanite, from south to north in the hanging wall damage

zone, (2) lack of any mappable dislocation surface within

the gneiss-migmatite terrane, and (3) lack of any structural

discordance. The Munsiari Thrust, in contrast to the

Vaikrita Thrust, is distinct and unambiguously mappable at

the regional scale. As the entire structural evolution in the

Munsiari Thrust Zone occurred during the Himalayan

orogeny, it lacks any record of the pre-Himalayan orogeny.

Acknowledgments Erudite comments and constructive suggestions

from Delores Robinson, University of Alabama, and Christopher J.

Talbot, Uppsala University helped improve the manuscript. Discus-

sions with A. K. Jain (CBRI, Roorkee), Pulok Mukherjee (WIHG),

and Rajesh Pandey (IIT Roorkee) were educative during the revision

of the manuscript. Department of Science and Technology, Govern-

ment of India, funded the project through generous Grants (SR/S4/

ES-543-2010-G). We thank Soumyajit Mukherjee for reaching out to

us for the contribution and also for the patient extension of the

deadlines.

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Documents

Deformation style in the Munsiari Thrust Zone: a study in the Madlakia–Munsiari–Dhapa section in north-eastern Kumaun Himalaya