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The tectonic evolution of the Northwestern Himalaya in eastern Ladakh and Lahul, India
A. STECK, L. SPRING, J.-C. VANNAY, H. MASSON, H. BUCHER, E. STUTZ, R. MARCHANT & J.-C. TIECHE Section des Sciences de la Terre, Universitd de Lausanne, BFSH 2, CH-IO15 Lausanne, Switzerland
Abstract: Geological studies along a transect across the Himalaya in eastern Ladakh and Lahul provide new insights into the Tertiary structural evolution of this region. The initiation of the Nyimaling-Tsarap Nappe is related to an early phase of underthrusting of India below Asia. In Lahul, an opposite vergent intra-continental underthrusting develops immediately after continental collision (NE-vergent Tandi Syncline and Shikar Beh Nappe). This NE-vergent nappe stack is responsible for the amphibolite-facies regional metamorphism of the lower Chandra Valley. The subsequent phase corresponds to the main thrusting of the SW-vergent Nyimaling-Tsarap Nappe, developed by ductile shear (87 km Eocene shortening). This nappe pile is responsible for the regional metamorphism of SE Zanskar (kyanite-staurolite near Sarchu). The root zone and the frontal part of the Nyimaling-Tsarap Nappe are subsequently overprinted by two NW-SE-orientated dextral transpressional shear zones. To the south of the investigated area, the Main Central Thrust has been developed as a shear zone in the regional metamorphic ductile crustal rocks below the older nappes to the north. In the Sarchu and Nyimaling regions, the following tectonic phase corresponds to NE-vergent 'backfolding' (Miocene). Normal faults in the Sarchu area record a late extension of approximately 14 km.
Numerous geological observations made during the Upper Precambrian to Lower Cambrian several scientific expeditions in the eastern Lad- meta-sandstones of the Phe Fm. (Figs 3 and 5), akh and Lahul regions of the Himalaya, have in the lower part of the Chandra Valley (Upper unravelled the structural evolution of this moun- Lahul). The basal stratigraphic contact cor- tain chain. A detailed geological map, a cross- responds to an unconformity and the Tandi Unit section and a palinspastic section of this part of represents a normal stratigraphic sequence. the northern Indian plate margin are presented Powell & Conaghan (1973) and Picket et al. in Figs 1, 2 and 3 respectively. (1975) interpret this structure as a SW-vergent The Ladakh Batholith, which marks the antiformal syncline related to the first Tertiary northern limit of the study area, is interpreted as tectonic phase. Frank et al. (1973) and Srikantia the product of partial melting of the subducted & Bhargava (1982) described it as a NE-vergent Mesozoic ocean floor of the Himalayan Tethys synform. Our mapping and structural evidence beneath the Asian Plate (Frank et al. 1977; show that the Tandi Syncline is a NE-vergent Honegger et al. 1982). During continental col- synform, created during the earliest Tertiary lision, two main compressional structures deformation recognizable in the studied area characterized the tectonic evolution of the east- (Dra~). In the lower part of the Chandra Valley, ern Ladakh and Lahul regions, to the south the this structure is associated with a zone of NE-vergent Tandi Syncline and the associated medium pressure and temperature regional Shikar Beh Nappe, and to the north the SW- metamorphism (Frank et al. 1973, 1975). vergent Nyimaling-Tsarap Nappe (Figs 3 and 4). Kyanite-staurolite-garnet mineral assemblages This tectonic evolution is summarized in Table 1 observed near Khoksar imply a lithostatic over- and briefly described in the following sections. A burden of approximately 20 km (Fig. 3). Toward more detailed account of the regional geology of the NE, the regional metamorphism decreases eastern Ladakh and Lahul is presented in Steck to actinolite-pumpellyite facies south of the et al. (1993). Baralacha La. Therefore, the high grade re- gional metamorphism of the lower part of the The Tandi Syncline and the NE-vergent Chandra Valley is not related to the SW-vergent Shikar Beh Nappe Nyimaling-Tsarap Nappe observed to the north, in SE Zanskar. We interpret this metamorphism The Tandi Syncline is an enigmatic structure as a consequence of a NE-vergent nappe composed of Mesozoic limestones in-folded with stacking, associated with the tectonic phase
From TRELOAR,P.J. & SEARLE, M.P. (eds), Himalayan Tectonics 265 Geological Society Special Publication, No. 74, pp. 265-276. Downloaded from http://sp.lyellcollection.org/ by guest on August 14, 2018
266 A. STECK ET AL.
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Fig. 1. Index map of the investigated area in Ladakh and Lahul. responsible for the NE-vergent Tandi Syncline. trasts with classical models of Himalayan tec- We propose the name Shikar Beh Nappe for the tonic evolution, which postulate that the high lower unit of this presently eroded nappe stack. grade metamorphism of the 'High Himalayan This structure is probably the result of a south- Crystalline' is due exclusively to crustal thicken- dipping intracontinental zone of underthrusting, ing related to SW-vergent structures. caused by a mechanical destabilisation in the Indian crust during the progressive underthrust- ing of India below Asia. Deep seismic profiles suggest the presence of such a S dipping under- thrusting zone below the High Himalaya of Fig. 2. Palinspastic reconstruction of the Rohtang Nepal (Him et al. 1984). This conclusion con- La-Leh section. Downloaded from http://sp.lyellcollection.org/ by guest on August 14, 2018
ILl TANDI BARALACHA LA KHARNAG Z O SARCHU NYIMALING U 15 Mesozoiclimestones (TANDI LIMESTONES) UJ COrr 14 UpperJurassicTertiary shales sandstones and limestones [~ ~] 10 .... ~ ~_ 12 (SPITI GIUMAL CHIKKIM) PRESENT 7-~o~c. 13 MiddleJurassic calciturbidites (LALUNG LA) 12 TriassicLiassic shales (DOLTO) 11 UpperTriassic Middle Jurassic limestones (KIOTO) Z ~/X~,~, ~0 NYIMALING-TSARAPNAPPE ~ - ~/+ii!l "~-\ ~ = 3 ' .... 10 Triassicshales limestones and sandstones (LILANG GR) A ° z ~40x, a ~ C overburden maximum D ---.. E o~ GH .,b~" = > W z ...... F 9 Permianshales (KULING) ~n CO__ ~-- ~.. "q ~ - -" -~ " " 1 ,-, 8 Permianshales limestones and sandstones (KULING) 7 PANJALTRAPS Z - ,(./~ . ~ = ~ ~ ~ ~ ra 1 t II/ z,,~." 1 km !)!!i!:i:i.i. 6 LowerPermian polygenic conglomerates (CHUMIK) (/3 I-- DARCHAUNIT ~. KENLUNGSARAI UNIT\, ~ O. ~ KHARNAGUNIT/~P~ "" "" 2" 5 Devoniansandstones (MUTH) and Carboniferous shales limestones and sandstones (LIPAK PO) 4 OrdovicianSilurian sandstones and conglomerates (THAPLE) 3 Cambrianslates sandstones and dolomites (KARSHA + KURGIAKH) 2 PrecambrianCambrian slates and sandstones (PHE) "~O~i?O"l,l,~ ~100 KM 1 Graniticgneisses (protolith of Cambro Ordovician age)
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INDIAN PASSIVE MARGIN TANDI BARALACHALA KHARNAG INDUS MOLASSE SARCHU NYIMALING and LADAKH BATHOLITH 19 20 -Rapey l-- 18 18. Mesozoic limestones (TANDI LIMESTONES) 19 Tertiary olistostrome 16 I,., ~ 17 23 Post Lower Eocene sandstones and conglomerates I 18 i' ii # Upper Jurassic - Cretaceous shales, sandstones and limestones (STOK. SUMDO, HEMIS) , 20 , (SPITI. GIUMAL CHIKKIM) Lower Eocene fiysches (CHOGDO) 17 Middle Jurassic calciturbidites (LALUNG LA) 22 21 Ladakh intrusives (100 -40 Ma) ~/".o#/;/d/Gush~ Gushal 16 Upper Triassic - Middle Jurassic limestones (KIOTO) 15 TANDI SYNC :- Tandi Triassic - Liassic shales (DOLTO) / 14 Triassic shales, limestones and sandstones (LILANG) HIMALAYAN STRUCTURES DTal lkm .//.,.+ 1 13 Permian shales, limestones and sandstones (KULING)
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I I II I I / GEOLOGICAL TRANSECT OF J WESTERN HIMALAYA IN LADAKH AND LAHUL by A. STECK. L. SPRING. J.-C. VANNAY. H. MASSON. H. BUCHER. E R. MARCHANT. E. STUTZ and J.-C. TIECHE J Downloaded from http://sp.lyellcollection.org/ by guest on August 14, 2018
TECTONIC EVOLUTION OF THE NORTHWESTERN HIMALAYA 267
Table 1. Chronology of the Tertiary Himalayan structures in eastern Ladakh and Lahul
Continental collision and beginning of underthrusting of India below Asia (c. 54 Ma) (A) Early initiation of SW-vergent Nyimaling-Tsarap Nappe D1 Early NE-vergent movements (B) NE-vergent folds (FT,1), axial surface schistosity (ST,1) and stretching lineation (bTal): Tandi DTal Syncline, Shikar Beh Nappe Main SW-vergent deformations and subsequent dextral transpression and backfolding (C) SW-vergent folds (FTa2), axial surface schistosity (ST,2) and E-ENE-orientated stretching DTa2 lineation (LTa2): SW-vergent deformation of Tandi Syncline (D) SW-vergent thrusts and folds (F2), axial schistosity ($2) and NE-orientated stretching D2 lineation (L2): main development of the Nyimaling-Tsarap Nappe, Baralacha La Thrust and Chandra Tal Flexure (E) Dextral shear and underthrusting (transpression) parallel to NW-orientated stretching D3 lineation (L3): Nyimaling and Sarchu Shear Zones (F) NE-vergent 'back-folds' F4, doming and dextral transpression: Nyimaling Dome D4 (G) Late dextral shear in Nyimaling-Markha Valley Steep Belt ($5) Late extensional structures of Sarchu region (H) Low-angle normal faults (reactivation of pre-existing D2 thrust planes) D5 (I) Steep Sarchu Normal Fault Late compression (J) NE-SW compression and vertical extension: symmetric box-folds in the Nyimaling Dome D6 W-vergent folds in the Kenlung Sarai Unit near Sarchu
The Tandi Syncline has been overprinted by propagated progressively through the Indian the subsequent SW-vergent deformations. Near upper crust toward the SW. The Nyimaling- Batal, in the Chandra Valley, the main $2 Tsarap Nappe is characterized by a set of high schistosity of the Chandra Tal Flexure and structural level discrete thrust surfaces, which Baralacha La Thrust crosscut the older NE grade with depth into a large ductile shear zone vergent FT,I folds associated with the Tandi with associated isoclinal folds (Figs 2 & 6). The Syncline. The $2 schistosity is related to the geometry of the Nyimaling-Tsarap Nappe be- main SW-vergent thrusting (D2) of the Nyi- fore backfolding and late extension is shown in maling-Tsarap Nappe, farther to the north. This Fig. 7. The Nyimaling-Tsarap Nappe represents local chronological relationship in the front of an imbricate structure ('structure imbriqu6e') as these two opposite-vergent structures does not defined and drawn by De Margerie & Heim exclude the possibility that the Nyimaling- (1888, fig. 105), more than 100 years ago. This Tsarap Nappe initiated at the same time or even imbricate structure differs from the 'imbricate earlier than the Tandi structure (DTal). fan' defined by Boyer & Elliot (1982) as the frontal thrust slices are not related to a unique The Nyimaling-Tsarap Nappe 'sole thrust' but to a large ductile shear zone associated with fold structures and situated in The initiation of the SW-vergent thrusting of the the upper and middle crust. The model of Boyer Nyimaling-Tsarap Nappe, away from the North & Elliott (1982) is only suitable for thrust Indian continental border, probably cor- systems developed at shallow tectonic levels, as responds to the incipient stage of crustal thick- for instance the Jura Thrust system in the Alps ening related to the continental collision of India (Buxtorf 1907; Laubscher 1965). The preser- and Asia. This stage is probably older than the vation of the normal stratigraphic sequence and NE vergent Tandi Syncline deformation (DTal). the continuity between the thrust front and the However, as mentioned above, the main defor- autochthonous root zone of the Nyimaling- mation phase of the Nyimaling-Tsarap Nappe Tsarap Nappe indicate that simple shear is the (D2) is younger than the Tandi structure. The Nyimaling-Tsarap Nappe developed by ductile shear of the upper Indian crust during underthrusting of the Indian continental border Fig. 3. (Plate 1), Geological transect of the below the rigid margin of the Asian Plate. The Northwestern Himalaya in eastern Ladakh and lower and frontal limit of the ductile shear zone Lahul. Downloaded from http://sp.lyellcollection.org/ by guest on August 14, 2018
268 A. STECK ET AL.
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TECTONIC EVOLUTION OF THE NORTHWESTERN HIMALAYA 269
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TECTONIC EVOLUTION OF THE NORTHWESTERN HIMALAYA 271
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272 A. STECK ET AL.
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0m ~ ~ .______~ -- -~_---~:~_ ~ ......
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main mechanism of nappe translation. A simple Tsarap Nappe (D2) is followed by a N to shear model, assuming the boundary conditions NW-orientated dextral transpression (oblique shown in Fig. 8, allows a quantitative estimation underthrusting, D3), associated with a stretch- of the deformation related to this nappe. This ing lineation L3. Two important shear zones are model indicates a crustal shortening of approxi- associated with this tectonic movement: the mately 87 km (Fig. 9). The structural evolution Dextral Sarchu Shear Zone in the frontal part of of the Himalaya along the eastern Ladakh- the Nyimaling-Tsarap Nappe, and the Dextral Upper Lahul studied transect contrasts with the Nyimaling Shear Zone near the root zone of the tectonic style proposed in some models for the nappe (Figs 3 & 4). The transition from SW- Zanskar nappes, about 50 km to the west (Searle vergent thrusting (D2) to N-NNW orientated 1986; McElroy et al. 1990). dextral transpression (D3) can be correlated with the anticlockwise rotation of the under- The Nyimaling and Sarchu dextral shear thrusting direction of India with respect to Asia which took place about 50, 45 and 36 Ma ago zones (Mid-Eocene to Early Oligocene) according to The SW-vergent thrusting of the Nyimaling- plate tectonic models (Molnar & Tapponnier Downloaded from http://sp.lyellcollection.org/ by guest on August 14, 2018
TECTONIC EVOLUTION OF THE NORTHWESTERN HIMALAYA 273
1975; Pierce 1978; Patriat & Achache 1984; between the Sarchu Normal Fault and the Klootwijk et al. 1985; Dewey etal. 1986). Zanskar Normal Fault farther to the west If the proposed correlation between plate (Searle 1986; Herren 1987) is unknown. How- movements and deformation structures is cor- ever, it is quite possible that the discrete ex- rect, the tectonic development of the Nyi- tensional faults of the Sarchu region represent a maling-Tsarap Nappe was completed in the high-structural level tectonic feature of the duc- Eocene or Lower Oligocene (by 35 Ma). The tile extensional shear zone developed in the observed dextral transpressional movements deeper tectonic level that outcrops in Zanskar. agree with the lateral strike-slip escape of the The late extensional structures characterized Tibetan block toward the SE with respect to by normal faulting and backfolding are fre- India (Fig. 1 ; e.g. Tapponnier et al. 1982, 1986). quently interpreted as related to the so-called phenomenon of 'gravitational collapse' (e.g., Burg & Cheng 1984; Burchfiel & Royden 1985; NE-vergent backfolds and late dextral Herren 1987; Searle et al. 1987). In our opinion this model cannot explain the structures in the shear Sarchu region, as the normal faults are younger In the Sarchu and Nyimaling regions, the D4 than the NE-vergent backfolds. We interpret tectonic phase corresponds to a gradual super- these structures as the consequence of a dextral position of NE vergent 'backfolds' on the zones transtensional tectonic phase that followed dex- of dextral transpression. Between Sarchu and tral transpressional movements. In the Hima- Kenlung Sarai, a spectacular isoclinal recum- laya of Nepal, P~cher & Le Fort (1989) and bent syncline composed of Devonian to Carbon- P~cher (1991) proposed a similar transtensional iferous sediments and illustrated in Baud et al. phase to explain comparable structures. (1984, fig. 7), is related to this deformation phase. In the Nyimaling region, these 'back- folds' were subsequently modified by late dex- The age of continental collision and nappe tral shear. Backfolding and associated dextral stacking transpression are probably responsible for the tectonic unroofing of the Nyimaling-Tso Morari The youngest marine sediments incorporated in Crystalline (Stutz & Steck 1986; Stutz 1988). the Himalayan nappes are marls and limestones NE-vergent folds and thrusts are the only Ter- containing reworked Palaeocene nummulites tiary structures observed in the Indus Molasse and were deposited along the North Indian and late dextral shear does not affect this margin. These sediments are probably of Early tectonic unit, situated to the north of the Indus Eocene age. They crop out in the Lingshed and Suture (Fig. 3). Zangla nappes and beneath the Spongtang ophiolite klippe (Garzanti et al. 1987). In the Kharnag region, we observed such reworked Late extension Palaeocene nummulites in a Tertiary olistos- trome within the Nyimaling-Tsarap nappe. Re- During a late period of the Himalayan orogeny, worked Palaeocene nummulites are also present an important extension affected the frontal part in the Indus Molasse sediments of the active of the Nyimaling-Tsarap Nappe. In the Sarchu Asiatic margin (Frank et al. 1977a; Baud et al. region, two generations of extensional struc- 1982; Van Haver et al. 1984; Bucher & Steck tures have been observed (Figs 2, 3 and 4). A 1987). These sediments are also probably of first set of low angle normal faults developed Early Eocene age. The Early Eocene age of the parallel to pre-existing thrust surfaces of the youngest marine sediments of the Indian Tethys Nyimaling-Tsarap Nappe. These low angle and of the oldest continental sediments of the faults are crosscut by the steep Sarchu Normal Indus Molasse, is coeval with a rapid decrease in Fault, which places amphibolite facies meta- the plate motion of India between 54 and 35 Ma morphic rocks to the south in contact with lower (Molnar & Tapponnier 1975; Pierce 1978; greenschist facies rocks to the north (Spring & Patriat & Achache 1984; Scotese et al. 1988; Crespo 1992). The tectonic unroofing of the Dewey et al. 1989). These data indicate that the amphibolite facies rocks south of Sarchu is continental collision of India and Asia occurred probably a consequence of this extension. 4°Ar/ during the Early Eocene and that the nappe 39Ar cooling ages of 33.5 Ma for amphibole thrusting started at this time. As discussed (total gas age) and of 19.3 _+ 0.2 Ma for biotite earlier, formation of the Nyimaling-Tsarap (plateau) are probably related to this unroofing Nappe was completed in the Eocene or Lower (Spring et al. this volume). The relationship Oligocene (by 35 Ma). Downloaded from http://sp.lyellcollection.org/ by guest on August 14, 2018
274 A. STECK ET AL. \ SW % NE Shikar Beh ~. Tandi Baralacha La ~".,, Jakang EXTENSION AND UPLIFT " p ~C \ .-:---. D (UNROOFING BY EXTENSION, UPLIFT AND EROSION 'i "1 \'~'" " -4KU_.[ D5b HIGH ANGLE SARCHU FAULT
B~~~ D D5a LOW ANGLE NORMAL FAULTS (inversion of thrust faults) ] I lO I KM I I i--" COMPRESSION AND UNDERTHRUSTING DTal ~N£AI#YNC~41ANpE4ND~~~~~~....~,~ (REGIONAL METAMQRPHISME) D4 NE VERGENT FOLDS D3 DEXTRALSARCHU SHEAR ZONE --- /;- - - ~,,, ~ ~p~I550:ICT~-F-t,~:ES ,,~--~. ---__o D2 NYIMALING-TSARAP THRUST 87 KM
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J INDIAN CRUST l ..... /- ~ ...... MoHO ...... 50 KM
Fig. 10. Tectonic model for the Rohtang La-Leh section.
Conclusions cance of these early NE-vergent nappe structures in the Himalaya. The tectonic evolution of the Himalayan chain The SW-vergent Nyimaling-Tsarap Nappe is along the investigated transect is summarized in a consequence of the ductile shearing of the the updated geological cross section presented in upper Indian crust during its underthrusting Figs 4 and 10. This profile includes a compilation below Asia. The Nyimaling-Tsarap Nappe is of the results of Frank et al. (1977b, 1987) for the characterized by a change in the tectonic style region situated between the Shikar Beh and the associated with different structural levels. The Siwaliks, to the south of the studied area. The discrete thrust surfaces of the imbricate struc- following deformational events characterize the ture observed in the nappe front grade into a model we propose. The initiation of the SW- large ductile shear zone at deeper crustal levels. vergent Nyimaling-Tsarap Nappe corresponds The structural features of the Nyimaling-Tsarap to the first stage of the continental collision. The Nappe are consistent with the orogenic lid model NE-vergent Tandi Syncline and Shikar Beh ('traineau 6craseur rigide', Fig. 7) described by Nappe developed during an early phase of the Tcrmier (1903). collision, as a consequence of intra continental The superposition of NW-SE-orientated dex- underthrusting developed within the Indian tral transpression zones on the root zone and the Plate. Note that seismic data suggest the pres- frontal part of the SW-vergent Nyimaling- ence of a south-dipping underthrusting zone Tsarap Nappe, indicates an anticlockwise ro- below the High Himalaya of Nepal (Hirn et al. tation of the underthrusting direction of India 1984). This structure might have been generated with respect to Asia. This conclusion agrees with by a 'flake tectonics' mechanism, as described by the plate tectonic models, which show an anti- Oxburgh (1972). These early intracontinental clockwise change in the motion direction of nappe structures were subsequently overprinted India between Mid-Eocene and Early Oligo- by SW-vergent deformation associated with the cene. During a late phase of the Himalayan main thrusting of the Nyimaling-Tsarap Nappe orogeny, an important extensional event affec- to the north and with the Main Central Thrust ted the frontal part of the Nyimaling-Tsarap (MCT) to the south. Our observations contrast Nappe. with the classical models for the Himalayan chain, characterized mainly by SW-vergent We thank M.A. Cosca, E.J. Essene, J.C. Hunziker, nappe structures. However, more regional in- D. Kirschner, D. Marshall, M.P. Searle and G. vestigations are needed to test the overall signifi- Stampfli for critical reviews and helpful discussions. Downloaded from http://sp.lyellcollection.org/ by guest on August 14, 2018
TECTONIC EVOLUTION OF THE NORTHWESTERN HIMALAYA 275
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