Role of inversion tectonics in structural development of the Himalaya

Journal of Asian Earth Sciences 39 (2010) 627–634

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Role of inversion tectonics in structural development of the Himalaya Ashok Kumar Dubey * Wadia Institute of Himalayan Geology, Dehradun 248 001, India

a r t i c l e

i n f o

Article history: Received 14 May 2009 Received in revised form 28 April 2010 Accepted 29 April 2010

Keywords: Himalaya Fault reactivation Listric faults Orogeny Thrusting

a b s t r a c t Analysis of surface and subsurface structures, variation of shortening amounts obtained by restoration of deformed cross-sections, and occurrence of younger hangingwall rocks over the older footwall rocks across the Vaikrita Thrust in the Higher Himalaya suggests reactivation of early normal faults as thrusts. Based on this, an inversion tectonics model is proposed for structural development of the Himalaya. The model explains the geometrical shape of the Himalaya as primary arcuation and helps in resolving superimposed deformation in the region. Ó 2010 Elsevier Ltd. All rights reserved.

1. Introduction Many of the orogenic belts have several aspects in common, e.g. arcuation in their trend, tectonic subdivisions from foreland foothill belt to inner core consisting of high grade metamorphic rocks, pre- and syn-orogenic metamorphism, fold- and thrust belts, early and superposed deformation, reactivation of faults, plane of basal decollement at depth, etc. However, there are certain features that are characteristic of an orogenic belt and these features help in formulating a model for its structural evolution. Some of the characteristic but enigmatic problems of the Himalaya are as follows. 1. The earlier Deep Seismic Section (DSS) profile across the Himalaya reveal a number of vertical faults without any plane of basal decollement (Kaila et al., 1978). However, acceptance of thin-skinned tectonics in some of the orogenies, led to emergence of another seismic profile which displays a prominent plane of basal decollement and various thrusts emanating from the plane (Allegré et al., 1984). It is also to be noted that nearly all the available seismic data reveal a large number of activities below the projected plane of decollement (e.g. De and Kayal, 2003). 2. There appears to be some inconsistency between surface and subsurface fold geometries. The exposed large-scale fold structures are polyharmonic and normally show a rounded profile (e.g. Dubey and Bhat, 1991; Thakur, 1992; Devrani and Dubey, 2008) but the subsurface folds are sometimes shown with typical kink band geometry without any second order folds (e.g. * Tel.: +91 135 2525132; fax: +91 135 2625212. E-mail address: [email protected] 1367-9120/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.jseaes.2010.04.027

Powers et al., 1998) so that the structures can be used for palinspastic reconstruction. Rheological properties of rocks that control the geometric evolution of folds can change with increase in depth but an abrupt variation of fold geometries across the ground surface remains unexplained. The footwall deformation and geometric problems that are associated with the ramp-flat thrust model (Ramsay, 1992) are also ignored. 3. Most of the finite strain data have come from two-dimensional cross-sections and arrive at an inference that the strain values are higher near a thrust (e.g., Bouchez and Pecher, 1981; Bhattacharya, 1987). Correlation of strain with the early and superposed deformation is lacking except a few cases (e.g. Jayangondaperumal and Dubey, 2001; Dubey et al., 2004). 2. Litho-tectonic subdivisions of the Himalaya The Himalaya is divided in the following four main litho-tectonic subdivisions from south to north (Fig. 1) (Gansser, 1964). The division is based on major rock types and prominent structural features. 1. 2. 3. 4.

Foreland Basin or Foothill Belt or Sub-Himalaya. Lower Himalaya or Lesser Himalaya. Higher Himalaya or Greater Himalaya. Tethys Himalaya.

2.1. The Foreland Basin The Foreland Basin lies north of the Indo-Gangetic Alluvial Plain (IGAP). The Alluvial Plain is not a litho-tectonic subdivision of the Himalaya but this is an integral physiographic part because the

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Fig. 1. A simplified map showing main tectonic subdivisions of the Himalaya (Gansser, 1964). ITSZ, Indus Tsangpo Suture Zone; MCT, Main Central Thrust; MBT, Main Boundary Thrust, HFT, Himalayan Frontal Thrust; IGAP, Indo Gangetic Alluvial Plain.

Ganga and Yamuna rivers and their tributaries originating from the Himalaya deposit their sediments in this region. The Alluvial Plain is separated from the basin by the Himalayan Frontal Thrust (HFT) or Main Frontal Thrust (MFT). The thrust continues as a fault zone all along the length of the Himalaya although the contact is not visible everywhere. The Oil and Natural Gas Corporation (ONGC), India has extensively explored the surface and subsurface geology of the area in the last five decades (Karunakaran and Ranga Rao, 1979; Raiverman, 2002). The geological map of the foothills (Raiverman et al., 1990) and subsurface structures (Raiverman et al., 1995) demonstrate a large variation in thickness of individual formations (Table 1) and presence of superimposed folds resulting in weak dome and basin patterns. The early folds are characterized by fault propagation folds near thrusts and thus the anticlines show asymmetric tight fold geometry (small wavelength, large amplitude) with a pinched appearance (Gansser, 1964) whereas the synclines show broad open fold geometry (large wavelength, small amplitude). Thus the folds have steep scarp towards south and gentle slope towards north (Wadia, 1970). Large wavelength buckle folds have developed away from the thrusts. Some of the anticlinal hinges are also cut by faults along the fold hinge lines (Raiverman et al., 1990). Presence of these faults, development of syntectonic veins and flexural-slip mechanism of folding suggest that the deformation has taken place at comparatively upper levels of the Earth’s crust. Conjugate set of strike-slip faults have formed at late stages of early fold formation (cf. Tapponnier and Molnar, 1976). These faults are oblique to the fold hinge lines (cf. Dubey, 1980a) and were regarded as the youngest structures (Nakata, 1989) but now normal faults have been identified as the youngest structure (Srivastava and John, 1999; Kandpal et al., 2006). 2.1.1. Results of cross-section balancing Using the line length balancing technique (Ramsay and Huber, 1987), some of the geological cross-sections have been used for palinspastic reconstruction (Powers et al., 1998). The various crustal

shortenings estimated in the western Himalaya are shown in Fig. 2 (Dubey et al., 2001). The variation in shortening amounts from 22% to 71.3% (or 10.6–60 km) over a lateral distance of 85 km is deceiving and none of the tectonic models can explain such a rapid variation. A large number of collisional plates moving with different rates and separated by strike-slip faults can possibly suggest a solution but in the Himalayan belt, the entire compression is attributed to a single plate (the northward moving Indian plate). Even a conceptual model involving rotation of the plate cannot account for the enormous variation in shortening over a small distance. Some of the possible reasons that can account for minor variations are summarized as follows. 1. The sections are not parallel to tectonic transport direction. 2. Some of the sections are situated in vicinity of oblique fault ramps. For example, transects 1, 2, and 4 are close to the Kangra recess (re-entrant) and transect 5 is crossing the Mohand and Dehradun recesses. However, no consideration is made for oblique-slip displacement along the thrusts. 3. Palaeomagnetic data are not available for determination of rotation of thrust sheets about the vertical axis (cf. McCaig and McClelland, 1992). 4. Some of the seismic lines are disconnected and the sections are completed by extrapolating the surface geological information. 5. The initial compaction and layer parallel shortening at the outset of deformation are not taken into account (Ramsay, 1997). Since there is no lubricating horizon along thrust surfaces for easier slip in the Sub-Himalaya, the layer parallel shortening is likely to be of significant amount. Further, no consideration is made for variation of ratio of buckle shortening to layer parallel strain across the multilayer profile (Dubey and Bhakuni, 1998). 6. The superposed folding and presence of strike-slip faults have been mostly ignored. These structures suggest shortening or extension normal to the plane of the cross-section thereby resulting in deviation from the plane-strain deformation.

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sional phase of the Himalayan orogeny led to reactivation of the early normal faults as thrusts and brought the fault blocks closer to the null point (i.e. the point of change from net extension to net contraction; Williams et al., 1989). Since the method of restoration considers the beds as initially horizontal, a large amount of initial shortening prior to their restoration at the null point has not been taken into account.

Table 1 Generalized litho-tectonic units of the central sector of the Lesser and Outer Himalaya (Srikantia and Sharma, 1976; Srikantia and Bhargava, 1998; Valdiya, 1998; Dubey, 2004a). Indo-Gangetic alluvium

Recent

Himalayan Frontal Thrust Siwalik Group Upper Siwalik (Conglomerate, sandstone) Middle Siwalik (Mainly sandstone) Lower Siwalik (Sandstone, shale) Dharamsala Group Upper Dharamsala (Sandstone, subordinate shale) Lower Dharamsala (Red Shale, sandstone) Subathu Group (Green Nummulitic shale and limestone) Kakara Series (Green, gray and purple shale with siltstone intercalations, lenticular bands of limestone) Unconformity/Krol Thrust/Main Boundary Thrust Tal Formation Quartz arenites and calc-arenite with pebbly quartzite; shale; black phosphoritic and cherty layers Krol Formation Dolomitic limestone and shale alteration Blaini Formation Slate and muddy quartzite, conglomerate, limestone Nagthat Quartz arenites (locally Formation pebbliferous) and subordinate shale Chandpur Olive green and grey phyllite with Formation subordinate slate Mandhali Arenaceous limestone, gritty and Formation slaty quartzite; phyllite

Oligocene to Early Miocene

Paleocene to Eocene Paleocene

Precambrian

Slate, sandstone, and quartzite Cherty limestone and stromatolitic limestone Low to medium grade metamorphic rocks Low grade metasedimentary rocks Medium to high grade metamorphic rocks

R.

SA TL

. AS R. RBE

DEOBAN

BE

SIMLA

HFT

H F T

BH AG IR

I R .

MUSSOORIE

NAHAN NAHAN

I HOSHIAPUR

22 % OR

71.33 %

(Powers et al. 1998)

II

III

62%

H F T

ALMORA

NAINITAL

DEHRADUN

SUBATHU

23.4 Km

SRINAGAR ATH

TANAKPUR

AS

SUNDERNAGAR

YAMUNA R.

KANGRA KANGRA

UTTARKASHI

R.

MANDI

DHARMSALA

RAMPUR

LI

T M B

KA

UJ R.

7. The initial extension during normal faulting of the rift phase (Precambrian to Late Cretaceous; Bhat, 1987) has not been taken into account. It is to be noted that the Tertiary compres-

NA ND A

Jutogh Thrust Jutogh Formation

AK

Chail Thrust Chail Formation (=Haimanta)

AL

Shali Thrust Shali Formation

2.1.2. Himalayan Foreland Basin All the above limitations are valid but in order to understand the estimated large variations over short distances, one has to look deeper into the process of development of a Foreland Basin. The Himalayan Foreland Basin has evolved by a combination of flexural bending of the Indian plate margin and thrust faulting (Beaumont, 1981; Lyon-Caen and Molnar, 1983; Najman et al., 1993; Burbank et al., 1994; Singh, 1999; Meigs, 1997; Storti et al., 1997). The stages in the evolution of the basin are shown in Fig. 3. Initiation of the basin started with displacement along the MBT (the basin margin fault) and development of a flexure in front of the rising mountain (Fig. 3a). Increase in displacement along the basin margin fault leads to gradual subsidence of the basin, synformal curvature of the older beds and deposition of younger beds (Fig. 3b). With progressive deformation, thrust faults initiate and propagate into the basin (TT, Fig. 3c). Upward propagation and displacement along the thrusts produce fault propagation folds (Fig. 3d) with south vergence. Some of the thrusts in the northern flank of the basin may be overlapped by the advancing orogenic load along the basin margin thrust. Since older beds show a greater synformal curvature, unfolding of these beds during restoration reveal a greater shortening as compared to the younger beds. A part of the basin subsidence and curvature of layers is also controlled by the compressive stresses but a simple restoration of deformed sections by bringing the inclined layers to a horizontal position along thrusts will provide incorrect estimation of shortening. Hence these estimates form an unsound basis for seismic predictions in the region (Powers et al., 1998). The Sub-Himalaya has at least one advantage that the subsurface structures are available as a result of detailed investigations by the ONGC. Similar data are almost nonexistent from interior parts of the Himalaya. Therefore previous attempts of estimation of crustal shortening (e.g. Srivastava and Mitra, 1994) are open to questions. The problems associated with restoration of deformed section were already known to the Himalayan workers but the warning note was largely ignored. Searle (1986) has pointed out that ‘‘the

M B T KOTDWAR

IV

26 % OR

OR

OR

10.6 Km

60 Km

34 Km

(Powers et al. 1998)

GANGA R.

Giri Thrust Simla Group

Middle Miocene to Lower Pleistocene

HALDWANI

H F T

V

49 % OR

11.3 Km SIWALIK ROCKS

N

TERTIARY ROCKS LESSER HIMALAYAN ROCKS

SCALE

0

50

100 Km

CRYSTALLINE ROCKS

Fig. 2. A geological map of the western Himalaya showing location of different cross-sections along with the estimated shortenings. MBT, Main Boundary Thrust; HFT, Himalayan Frontal Thrust; R., river.

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(a)

(b)

(c)

(d) Fig. 3. A schematic diagram showing development of the Himalayan Foreland Basin. (a) Displacement along basin margin thrust and initiation of Foreland Basin. N (north), S (south), MBT (Main Boundary Thrust). (b) Advancing orogenic load and gradual deepening of the basin causing synclinal curvature in the older beds. (c) Increase in displacement along the margin thrust and initiation of thrust faults (TT) in the layers. (d) Further deepening of the basin and amplification of fault propagation folds. The diagram reveals that straightening of the older beds during restoration will provide a higher amount of shortening as compared to the younger beds.

amount of inference (fiddling) that has to be made is at present unacceptably great”. This comment was made with reference to the Ladakh and Zanskar regions but it is equally applicable to other parts of the Himalaya. Coward (1996) has also acknowledged that the ‘‘Previous calculations for the shortening across the frontal Himalayas (e.g. Coward and Butler, 1985) considered only one component, the thin-skinned shortening due to plate collision, and hence the results are probably incorrect”. The northern boundary of the Foreland Basin is marked by the Main Boundary Thrust (MBT) that at some places (e.g. Kumaun Himalaya) occurs as a series of nearly parallel faults. 2.2. The Lower or Lesser Himalaya The region consists of sedimentary and metasedimentary rocks ranging in age from Precambrian to Eocene but mostly older rocks dominate (Table 1). Early fold hinge lines trend NW–SE to E–W nearly parallel to trends of the major thrusts as their development is also simultaneous to the thrusting. Superposed fold hinge lines trend N–S to NE–SW and they have formed at a late stage after locking of the thrusts (Ray and Naha, 1971; LeFort, 1975; Dubey, 2004a). The general orientation may vary locally depending upon the interference between simultaneously developing folds, and faults and folds (Dubey and Cobbold, 1977; Dubey and Paul, 1993; Dubey, 1997). Small-scale early folds are more prominent at a distance from thrusts whereas superposed folds are fairly uniformly distributed throughout the area. The uniform distribution of superposed folds provides evidence that they have formed as buckle folds due to compression rather than as a result of interference between two propagating thrust sheets (cf. Coward and Potts, 1983). 2.2.1. Klippe and window structures A large number of thrusts occur in the area and the most characteristic feature is occurrence of klippe structures (Bhargava, 1980). The klippe are of two types. The first one is described as allochthonous klippe which shows a large horizontal translation

from a root zone along a basal detachment thrust. For example, Simla klippe in the Himachal Himalaya provides a minimum displacement of 40 km using the klippe to fenster method (Dubey and Bhat, 1991). The large displacement was possible because of presence of a thick graphitic schist horizon at the base of the Chail-Jutogh Thrust that provided excellent lubrication along the thrust surface. The second type is known as parautochthonous klippe (e.g. Satengal, Banali and Garhwal klippen in the Garhwal Himalaya; Jayangondaperumal and Dubey, 2001; Devrani and Dubey, 2008) which show smaller horizontal and larger sub vertical fault displacement. This type of klippe lies over its root and form as pop-up structure. Tectonic windows occur in the area but none of these exhibits Siwalik rocks indicating a small horizontal translation along the MBT and/or rotation of the MBT to steeper dips and shortening of the basin during progressive deformation. At lower topographic levels, the rocks are sedimentary or metasedimentary with slate and phyllite grades. The grade of metamorphism increases with increase in elevation and the overlying low to medium grade rocks become garnet bearing. The differing grades of metamorphism are separated by a thrust. This increase in metamorphism with increase in topographic level (reverse or inverted metamorphism) can be observed throughout the Himalaya. 2.2.2. Faults–thrusts-ramps A large number of oblique fault ramps have been identified and their simultaneous development with noncylindrical folds has been studied (Dubey, 1997). A combination of frontal and oblique ramp structures form recess (re-entrant) and salient structures along the MBT. The Dehradun recess was earlier attributed to the Delhi–Aravalli Ridge of the peninsular shield but seismic sections do not show presence of the basement ridge below the HFT (Raiverman, 2002). Hence their formation can be attributed to linking of frontal and oblique thrust ramps. These structures are conspicuously absent along the HFT confirming that they are pre-Himalayan structures (cf. Marshak, 1988). The study of these fault ramps is important because some of these ramps are associated with recent earthquakes in the Himalaya. For example, the Uttarkashi earthquake (20 October, 1991; M 7.0) (Gupta and Gupta, 1995), and Chamoli earthquake (29 March 1999; M 6.6) (Mandal et al., 2002) have their epicenters in the vicinity of the ramps. One of the important earthquakes of the century, the Kangra earthquake (4 April, 1905; M 7.0) (Middlemiss, 1910; Molnar, 1987) also occurred along the Kangra oblique ramp. Thrust faults are the dominant structures but normal and strike-slip faults also occur in the region. Normal faults of the following four generations are clearly distinguishable. 1. Pre-orogenic growth faults formed during sedimentation on a rifted basement (Bhat, 1987). 2. Orogenic normal faults attributed to large shear strain along thrusts (Dubey and Bhat, 1991). 3. Normal faults associated with reactivation of oblique thrust ramps during superposed deformation (Dubey, 1997). 4. Youngest normal faults formed due to gravity gliding (Herren, 1987; Royden and Burchfiel, 1987). These can be identified by the fact that they displace all the foliations and are unaffected by superposed folding (Dubey and Bhakuni, 2008). The northern boundary of the Lower Himalaya is marked by the Main Central Thrust (Munsiari Thrust). 2.3. The Higher Himalaya This subdivision constitutes the core of the Himalaya. The exposed rocks, termed as Central Crystalline Zone, represent the

A.K. Dubey / Journal of Asian Earth Sciences 39 (2010) 627–634

main metamorphic belt and root-zone of the allochthonous klippen of the Lower Himalaya. Small-scale superimposed folds and their interference patterns are well represented in the schistose rocks. These folds, in association with micro-structures, help in determination of sense of shear which reveals a general top-tothe-south sense of displacement along the major thrusts. The Central Crystalline rocks are divided into two units, Munsiari Formation and Vaikrita Group separated by the Vaikrita Thrust (Valdiya, 1998). The constituents of the Munsiari rocks are fine to medium grade (size <1 mm) with rare presence of kyanite in small pockets whereas the constituents of the Vaikrita Group are coarse grained (size 1–2 mm and more) and kyanite generally occurs in relatively higher horizons of the Central Crystalline Zone. The Vaikrita Group represents the higher grade of metamorphism and it was regarded as older than the Munsiari Formation. However, recent geochronological data reveal a younger age of the thrust hangingwall rocks (Munsiari, 2600–1800 Ma; Vaikrita, 800–1000 Ma) (Ahmad et al., 2000). Thus Central Crystalline rocks of the Higher Himalaya are age equivalent of the metasedimentary rocks of the Lower Himalaya. The younger age and higher metamorphism of the thrust hangingwall rocks are explained by a model in which the hangingwall rocks have not reached the null point (Williams et al., 1989) during reactivation of the early normal fault as thrust (Fig. 4) (Dubey and Bhakuni, 2007). The northern boundary of the Higher Himalaya is marked by the Tethyan fault. 2.4. The Tethys Himalaya The Tethys Himalaya exposes a continuous rock sequence from the Precambrian to Eocene. The rocks have a thickness of more than 8 km and consist of sedimentary, metasedimentary, and metamorphic rocks (Thakur, 1992; Srikantia and Bhargava, 1998).

(a)

(b)

(c)

(d) Fig. 4. A simplified diagram illustrating occurrence of older rocks in the hangingwall of a thrust. (a) Initial disposition of a layer in a normal stratigraphic sequence and initiation of a listric fault. (b) Normal faulting during pre-Himalayan rift phase in the region. (c) Reactivation of the fault as thrust during the Himalayan orogeny and initiation of a new thrust in the footwall. (d) Locking of thrust 1 prior to reaching the null point and displacement along thrust 2. The younger rock (Y) now occurs in the hangingwall of thrust 1 and older rock (O) in the footwall.

631

The nature of the Tethyan fault has been described differently by different workers. The contact was earlier described as thrust but conformable contact was also reported at some places (Gansser, 1964). Recent literature describes it as a normal fault (Herren, 1987). However, presence of sheath folds at the hangingwall of the Tethyan fault in the Himachal Himalaya suggests earlier thrusting followed by normal faulting which is revealed by displacement of a number of later secondary veins. Reclined and recumbent folds are characteristic structures of the subdivision. The reclined folds have formed as a result of superposed folding of early recumbent folds. Two generations of normal faults parallel and orthogonal to the trend of the Tethyan fault have been described (Dubey and Bhakuni, 2004). Some of the normal and strike-slip faults have formed simultaneously at different structural levels; normal faults at higher elevations due to gravity spreading and strike-slip faults at lower elevations where the rock sequences were confined (Dubey and Bhakuni, 2008). The northern boundary of the Tethys Himalaya is marked by the Indus Tsangpo Suture Zone (ITSZ) (Fig. 1) which is regarded as the zone of collision between the Indian and Tibetan plates. The ITSZ is followed by the Trans Himalaya or Karakoram Mountains which is marked by normal faults and eastward extrusion along strike-slip faults. 2.5. Significant structural features of the Himalaya Some of the structural features that deserve special consideration are summarized below. 1. The Himalayan fold belt presents an arcuate geometrical shape with the convex side facing toward south. A uniform structural pattern throughout the Himalaya suggests a primary arcuation (Ries and Shackleton, 1976). However, little efforts have been made to confirm whether the arcuation is primary or secondary. 2. One of the important features of regional geology is the occurrence of metabasic rocks along the prominent Himalayan thrusts. The tectonic set-up derived from geochemistry of these rocks suggests emplacement during repeated rifting episodes from Precambrian to just before the Tertiary compressional phase of the Himalayan orogeny (Bhat, 1987). 3. There are two distinct phases of metamorphism, i.e. pre-Himalayan and syn-Himalayan. The pre-Himalayan metamorphism is very well recognized (Arita, 1983) but because it is overshadowed by the later phase, a greater emphasis has been provided to the Himalayan metamorphism while providing explanation for inverted metamorphism in the Lower Himalaya (e.g. LeFort, 1975; Hodges and Silverberg, 1988). 4. Superimposed deformation is an established fact of Himalayan geology (Ray and Naha, 1971). The N–S maximum compression required for the formation of early folds (trend of hinge lines, NW–SE to E–W) is attributed to northward movement of the Indian plate. However, formation of the superposed folds (trend of hinge lines, NE–SW to N–S) creates some problem because these folds require maximum compression in E–W sub-horizontal direction. 5. Erratic crustal shortening obtained from restoration of deformed cross-sections. 6. Seismicity of the region indicates that all the three main types of faults, i.e. thrust, strike-slip, and normal, are active simultaneously. The thrust faults develop at depth, strike-slip faults at middle levels closer to the surface and normal faults at higher topographic levels. This needs an explanation since each of these fault types requires a typical stress pattern to develop. 7. Most of the available seismic data (e.g. De and Kayal, 2003) reveal large number of activities below the projected plane of decollement indicating departure from the thin-skinned thrust

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tectonics model. Moreover, presence of the up-warp structure (sub-thrust anticline) at the contact of thrust ramps with the basal decollement is likely to inhibit a large translation along the basal thrust (Dubey, 2004b). 8. The occurrence of younger rocks at hangingwall and older rocks at footwall of the Vaikrita Thrust (Ahmad et al., 2000) suggests inversion tectonics in the region.

3. A model for structural development of the Himalaya In view of the above facts, a model is proposed for structural evolution of the Himalaya (Fig. 5). The model is based on reactivation of pre-Himalayan normal listric faults as thrusts during the Tertiary compressional phase of the Himalayan orogeny (Dubey and Bhat, 1986). The first stage (Fig. 5a) depicts formation of normal listric faults during pre-Himalayan rift phase (Precambrian to Upper Cretaceous) (cf. Bott, 1971; McKenzie, 1978; Bhat et al., 1981). After reaching a deeper level the faults have acted as conduits for upward movement of the magma (Bhat, 1982). This can explain the occurrence of metabasic rocks along the MBT, MCT, and the Indus Tsangpo Suture Zone (ITSZ). The heterogeneous nature of the lithosphere yielded in a number of irregular and anastomosing patterns of fractures that resulted in the formation of oblique ramps. Increase in the tensional stresses led to increase in fault displacement which was accompanied by tilting of the hangingwall blocks toward the fault surface, i.e. toward south (Fig. 5b). The tilt amount depended on the dip and displacement along the fault (Dubey and Bhakuni, 1998). The northernmost hangingwall block reached the deepest topographic level as a result of cumulative displacements

(a)

(b)

(c)

(d)

(e)

(f)

Fig. 5. A simplified diagram illustrating structural development of the Himalaya. N, north; S, south (for explanation, see text).

along the faults and encountered relatively higher temperature– pressure gradients. Initial trend of the basement faults was later modified to an arcuate shape, most probably by shear effects of the Ninetyeast Ridge (Fig. 5c) as well as along the Murray Ridge and Queta Line. The arcuation was prior to the Himalayan orogeny (Carey, 1976). At the outset of the Tertiary compressional phase, the normal faults were possibly reactivated as thrust faults. The initial response of the reactivation was to acquire the stage of null point (Fig. 5d) (Williams et al., 1989). However, the null point could not be reached simultaneously at all the faults. The variation in fault displacement is also likely along the strike of a fault and the null point may not occur all along the strike of the fault. Some of the faults may not have reached the null point because of early locking and initiation of a thrust in the footwall. It is also evident from the diagram (Fig. 5d) that a considerable amount of shortening has taken place prior to reaching the null point. However, this shortening has not been taken into account while restoring a deformed cross-section as the method considers initial disposition of layers as horizontal. This shortcoming of the method adds to the facts that there is no consideration for possible viscosity contrast between different layers of the succession, initial layer parallel (homogeneous) shortening, amount of displacement along basin margin fault (MBT in the Himalaya), departure from the plane-strain deformation as a result of displacement out of the tectonic transport plane, etc. Folding of the cover rocks was simultaneous with thrusting (Fig. 5e). The fault propagation folds initiated over a thrust and buckle folds at a distance from the thrust. The fold hinge lines followed the trend of adjacent thrust except in areas where oblique ramps were present and fold interference took place. Displacement along the arcuate basement faults led to arcuation of the cover folds. Thus the arcuate fold belt is a primary structure. The lower thrust sheets have carried the upper thrust sheets during thrusting and fault tips which were initially at lower altitudes during the normal faulting now occur at higher elevations. Hence, the highest region represents the highest metamorphic grade. The thrusting has also tilted the hangingwall blocks toward the north thereby arranging the successively younger rocks with gradual decrease in metamorphism with increasing distance from the thrust (Metcalfe, 1993). At late stages of fold formation, the folds acquired rotation hardening at low interlimb angles and further decrease in interlimb angles was not possible. Initially, the developing folds showed maximum extension parallel to the axial surface and normal to hinge line but after the rotation hardening the maximum extension direction changed its orientation and became parallel to the fold hinge lines (cf. Dubey, 1980b). The fold amplification was concomitant with rotation of the faults (Dubey and Bhakuni, 1998). At steep dips the thrusts were also locked and the maximum extension took place parallel to its strike resulting in formation of strike-slip faults oblique to the early folds and thrusts (cf. Khattri and Tyagi, 1983). Restriction in the E–W extension because of boundary conditions imposed by transverse faults resulted in formation of superposed folds and associated structures (Fig. 5f). Since the superposed folds are formed at a late stage, their intensity and development are not controlled by thrusting and they are thus distributed throughout the region. The precise time of their formation is not known and it is possible that thrusting along low dip faults and formation of early folds in one area is simultaneous with formation of superposed folds along steep faults in another region. Similar variations may have taken place in depth profiles because of listric fault geometry. At lower levels the fault dips are gentle or moderate hence the faults are likely to act as thrust. However, because of steep dips near the surface, the thrust faults lock and strike-slip faults develop in conjugate sets oblique

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to the trend of the thrusts and early folds. At the same stage of deformation, normal faults may develop at higher elevations because of gravity gliding (Dubey and Bhakuni, 2004, 2008; Devrani and Dubey, 2008). Hence different phases of folding or different types of faults do not represent different phases of deformation. The model can also explain the occurrence of allochthonous klippe (Simla klippe; Dubey and Bhat, 1991) and pop-up klippe (Satengal-Banali klippen; Jayangondaperumal and Dubey, 2001; Garhwal klippe; Devrani and Dubey, 2008) in the Lower Himalaya. A gentle thrust dip and a lubricating horizon along the thrust surface (e.g. graphite schist along the klippe detachment thrust in the Jutogh Formation of the Simla klippe) promotes a large displacement thereby resulting in formation of an allochthonous klippe whereas displacement along a listric fault followed by initiation of back thrust(s) results in formation of a pop-up klippe. Typical listric geometry, poor lubrication along fault surface and deformation at comparatively upper levels of the Earth’s crust favour the development of a pop-up klippe. 4. Conclusions Structural evolution of the Himalaya can be explained with the help of an inversion tectonics based model involving reactivation of early normal listric faults as thrusts. The normal faults with listric geometry were formed during the pre-Himalayan tensional phase in the region that was prevalent from the Precambrian to Upper Cretaceous. These normal faults acted as conduits for basic magma, which exist along the prominent Himalayan thrusts (MBT, MCT, ITSZ). During the Tertiary compressional phase of the Himalayan orogeny, these normal faults reactivated as thrust faults. The compressional phase can be divided into two as follows. 1. Development of early folds and thrusts, and 2. Development of superposed folds, strike-slip and normal faults. These structures have developed all along the length of the Himalaya. The inversion tectonics based model can also explain the occurrence of younger hangingwall Vaikrita rocks over the older footwall Munsiari rocks along the Vaikrita Thrust. The arcuate structure of the Himalaya is suggested to be a primary arc. Acknowledgements I am grateful to my colleagues especially M.I. Bhat, S.S. Bhakuni, R. Jayangondaperumal, S.J. Sangode, N.S. Gururajan, Upasana Devrani, and A.J. Selokar for several discussions, help and cooperation. References Ahmad, T., Harris, N., Bickle, M., Chapman, H., Bunbury, J., Prince, C., 2000. Isotopic constraints on the structural relationships between the Lesser Himalayan Series and the High Himalayan Crystalline Series, Garhwal Himalaya. Geological Society of America Bulletin 112, 467–477. Allegré, C.J. et al., 1984. Structure and evolution of the Himalaya-Tibet orogenic belt. Nature 307, 17–22. Arita, K., 1983. Origin of inverted metamorphism of the Lower Himalayas, Central Nepal. Tectonophysics 95, 43–60. Beaumont, C., 1981. Foreland basins. Geophysical Journal, Royal Astronomical Society 65, 291–329. Bhargava, O.N., 1980. The tectonic windows of the Lesser Himalaya. Himalayan Geology 10, 135–155. Bhat, M.I., 1982. Thermal and tectonic evolution of the Kashmir basin vis-à-vis petroleum prospects. Tectonophysics 88, 117–132. Bhat, M.I., 1987. Spasmodic rift reactivation and its role in the pre-orogenic evolution of the Himalayas. Tectonophysics 134, 103–127. Bhat, M.I., Zainuddin, S.M., Rais, A., 1981. Panjal trap chemistry and the birth of Tethys. Geological Magazine 118, 365–375. Bhattacharya, A.R., 1987. A ‘‘ductile thrust” in the Himalaya. Tectonophysics 135, 37–45. Bott, M.H.P., 1971. Evolution of young continental margins and formation of shelf basins. Tectonophysics 11, 319–327.

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