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Tectonics of the syntaxis and the : an introduction

PETER J. TRELOAR 1, MICHAEL P. SEARLE 2, M. ASIF KHAN 3 & M. QASIM JAN 3 l Centre for Earth and Environmental Sciences Research, School of Geological Sciences, Kingston University, Kingston-upon-Thames, Surrey KT1 2EE, UK 2Department of Earth Sciences, Oxford University, Parks Road, Oxford OX2 3PR, UK 3National Centre of Excellence in Geology, University of Peshawar, Peshawar, NWFP,

Often described as a natural laboratory, the Himalaya, access to rocks along the disputed Himalaya are probably the ideal place in which border between India and Pakistan is largely to study ongoing continent-continent collision. forbidden to foreigners. Similarly, easy access This volume focuses on the geology of the northward across the Chinese-Pakistan border northwestern part of the Himalaya which pro- has only been possible since the opening of the vides the most complete and best-exposed Khunjerab Pass in 1986. It is arguable that prior transect across the range. Here, in northern to the Russian invasion of in 1980, Pakistan and in Ladakh in northwest India, the the geologically best known of the Himalayan full profile across the south Asian continental nations was Afghanistan, largely through the margin, and the north Indian margin is superbly detailed mapping of Wittekindt & Weipper exposed in reaching as high as (1973) and Wolfart & Wittekindt (1980). Since (8611 m) and Nanga Parbat (8125 m). The south then, advances in understanding of the geology of Asian geology is exemplified in the Afghanistan have been limited to re-interpreta- and Hindu Kush ranges along the north and tions based on satellite imagery or increases in northwestern frontiers of Pakistan. The unique knowledge of the geology of surrounding states Kohistan-Dras island-arc terrane is sandwiched such as Pakistan. As a result of these problems, within the Tethyan suture zone between India many regions of the Himalayan chain remain and Asia. Rocks of the northern margin of the poorly mapped or completely unmapped. Con- Indian Plate are exposed in both the and taining many new maps, this volume goes part of the Pakistan Himalaya. The northern sedi- the way towards addressing this shortfall. mentary carbonate platform of the Indian Despite the problems of access, the Himalaya Plate, magnificently exposed in the mountains and adjoining ranges continue to be a of Zanskar and Ladakh, is largely missing in magnet to earth scientists. The geology of north- Pakistan where the Kohistan arc has been ern Pakistan was largely unknown until about obducted southward onto the metamorphosed 20 years ago. Until then much of what was known rocks of the internal crystalline zones of the of the region was due to the work of the Indian Plate. The Nanga Parbat syntaxis Geological Survey of India, especially Hayden represents an orogenic bend developed within a (1915) and Wadia (1931, 1932). Some aspects of convergent zone in the thrust belt where the the geology were summarized in volumes that south-vergent thrusts of the central and eastern dealt with the geology of the whole of pre- Himalaya swing around through 300 degrees. partitioning India (e.g. Wadia 1919). Some The history of geological research in the geology was carried out by geologists attached Himalaya extends back to some of the earliest to major expeditions to the Karakoram (e.g. explorers and climbers who visited the region. Desio 1930; Auden 1938). However, really Two significant problems have affected mapping detailed regional scale work truly commenced of the topography and geology of the Himalaya. only in the 1970s during which R. A. K. The first of these is the sheer logistical problem of Tahirkelli from the University of Peshawar attaining access to extremely rugged terrain. The carried out much pioneering work in the northern second is a political problem. In the NW part of the Indian Plate, the Hindu Kush and

From: KHAN, M. A., TRELOAR, P. J., SEARLE, M. P. & JAN, M. Q. (eds) Tectonics of the Nanga Parbat Syntaxis and the Western Himalaya. Geological Society, London, Special Publications, 170, 1-6. 1-86239-061-4/00/$15.00 © The Geological Society of London 2000. Downloaded from http://sp.lyellcollection.org/ by guest on September 29, 2021

2 P.J. TRELOAR ET AL.

Karakoram mountains of the southern part of (1996) and Searle et al. (1999). Collision between the Asian Plate and what is now recognized as the the arc and continental India followed closure of Kohistan arc. This work was summarised in 1982 neo-Tethys. The age of collision is not precisely in a superbly illustrated Geological Bulletin of the determined, although on the basis of the sedi- University of Peshawar (Tahirkelli 1982) as well mentary record is likely to have been at about as in Tahirkelli et al. (1979). A then useful 55 Ma (see discussions in Garzanti et al. 1996, summary of the broader effects of the Himalayan Pivnik & Wells 1996, Rowley 1996 and Treloar orogeny was given in Farah & De Jong (1979). In 1997). A late Cretaceous to Paleocene ophiolite 1980 the (KKH), that emplacement event predated final closure of stretches from Islamabad in Pakistan through to Neotethys (Searle 1986; Beck et al. 1995; Searle Kashgar in China, was opened and this stimu- et al. 1997). Since collision the leading edge of the lated an extraordinary influx of geoscientists into Indian Plate has undergone deformation, burial, the region. The KKH straddles not just the metamorphism and exhumation. In contrast to northern margin of the Indian Plate and the the main Himalayan chain to the east, peak southern margin of the Asian Plate, but also the metamorphism in the Pakistan Himalaya was Cretaceous Kohistan island arc sandwiched Eocene in age (Treloar & Rex 1990). Exhumation between the two. An early synthesis of the during the early Miocene was by a combination geology along the Pakistani section of the KKH of north-vergent extension (Burg et al. 1996; was given in the proceedings of an international Vince & Treloar 1996) and erosion with sedi- expedition that traversed the KKH immediately ments deposited in the foreland basins to the on its completion (Miller 1982). A recent south of the topographic high (Burbank et al. literature audit showed that between 1980 and 1996). Deformation of these sedimentary basins 1997, over 200 papers had been authored or co- continues to the present day. authored on the region traversed by the KKH by The northwest Himalaya differ from the main British, French and German geoscientists alone. Himalayan chain for three main reasons. Here A complete reference list up to 1996 and synthesis the Asian and Indian plates are separated by the of the geology of N. Pakistan is given in Kazmi & Kohistan-Dras island arc, peak metamorphism Jan (1997). This volume builds upon this recent was Eocene rather than Miocene in age, and the explosion of geological knowledge. south-vergent thrust systems are deformed by the The essential geological framework of the West Himalayan syntaxes. First recognized by Pakistan Himalaya is summarized briefly below. Wadia (1931, 1932), the syntaxes are crustal- The Kohistan-Dras island arc was initiated scale, north-trending antiformal structures which offshore of Asia during the late Jurassic or are essentially half windows. The Hazara syn- early Cretaceous. The arc sutured to the southern taxis to the south deforms thrusts of the external margin of the Asian Plate at between 102 Ma, the zones. Within its core, foreland basin sediments age of emplacement of the pre-suturing Matum on the footwall of the Main Boundary Thrust Das pluton (Petterson & Windley 1985) and 85 Ma. The later age is derived from an c. 84 Ma have been tectonically uplifted (Bossart et al. 1988). The Nanga Parbat syntaxis deforms U-Pb zircon age (Zeitler et al. 1981) for unde- formed gabbro-norites of the complex, thrusts of the internal zones. Within its core, which contain xenoliths of Gilgit Formation crystalline rocks of the Indian plate have been gneisses that had been deformed during suturing. tectonically exhumed from beneath their cover of The age is compatible with a Rb-Sr age of volcanic rocks of the Kohistan-Ladakh arc 87+ 19Ma (Mikoshiba et al. 1999) assuming sequence that had been thrust onto the Indian that their sample 92CH60 is included in the Plate early in collision. Present day uplift rates regression. After suturing the arc behaved as an within the Nanga Parbat syntaxis may be as high Andean-style volcanic arc. Thickening of the arc as 6 mm a-1 (Zeitler 1985). Mechanisms of uplift accompanied suturing. Most of the deformation and its deformational effects, timing of uplift, and metamorphism in the arc appears to have and the magmatic effects of rapid uplift and post-dated suturing, but predated collision with exhumation have been the focus of exhaustive India. Deformation and metamorphism within research within the Nanga Parbat syntaxis the southern margin of the Asian plate appears during the last ten years. A number of British to have been diachronous with tectonothermal scientists funded by the Natural Environmental events during the late Cretaceous (after suturing Research Council and the Royal Society have with Kohistan) and the Tertiary. A full review of been working in the syntaxis. At the same time, a the Cretaceous through to Tertiary history of the team from the USA under the leadership of P. K. Asian Plate in Pakistan is given in Searle et al. Zeitler has been working in the area with funding (1999) and of the Kohistan arc in Treloar et al. from the National Science Foundation. Papers Downloaded from http://sp.lyellcollection.org/ by guest on September 29, 2021

INTRODUCTION 3

from both of these groups are included in this that accommodated uplift within the syntaxis. volume. This set of five papers provide a benchmark on In setting the regional scene for the volume, which future structural studies of the syntaxis can Caporali describes the gravity field of the be built. The Nanga Parbat encompasses Karakoram Mountain Range and surrounding greater than 6000m of relief. This relief is a areas. He demonstrates the gravimetric low function of the rapid Neogene uplift and results associated with the Nanga Parbat syntaxis in rapid erosional unroofing of the crystalline which is a function of the rapid differential uplift basement complex. Shroder & Bishop explore within the syntaxis as well as the presence of large aspects of this unroofing and demonstrate that negative anomalies between the Karakoram late Pleistocene processes were sufficiently rigor- Fault and the Main Karakoram Thrust. The ous to produce the present-day pronounced relationship between crustal scale geological relief. Whereas elsewhere in the Himalaya, features (sutures, thrusts and syntaxes) and extensional unroofing has played an important gravitational potential is clear when plotted on part in exhumation, at Nanga Parbat, where a regional scale geology map (Caporali, fig. 3). exhumation is most rapid, it appears not to have The next seven papers deal with the Nanga done so, a point also made by Edwards et ai., Parbat syntaxis. In the first of these, Treloar et ai. Bishop & Shroder argue that accelerated uplift outline the significance of field relationships in and erosion are part of a feed-back loop with defining long term geological histories. Mafic tectonic uplift. They also use remotely sensed sheets within Indian plate gneisses within the imagery to derive a hierarchical order of topo- Nanga Parbat syntaxis clearly document a poly- graphic complexity that is a function of erosion phase deformation and metamorphic history. In dynamics. The use of remote sensing is probably stressing the importance of recognising complex the prime way in which the topography of the histories, the authors recall the seminal work of Himalaya will be mapped in future. John Sutton and Janet Watson in the Lewisian Large crustal-scale structures like the Nanga Complex. Parbat syntaxis should encourage debate. Two The next papers relate to the structural and papers here do just that. In a far-reaching paper topographic evolution of the syntaxis. Butler Burg & Podladchikov model the evolution of the revisits his earlier studies (Butler & Prior Himalayan syntaxes numerically, (the Nanga 1989a, b; Butler et al. 1989) on the western Parbat syntaxis and the Namche Barwhe syntaxis margin of the syntaxis to refine, in the light at the eastern end of the Himalayan chain). Their of recent field-work, an interpretation of the numerical modelling indicates that pure shear structures that accommodated bodily uplift of thickening and symmetric buckling accommo- the syntaxis along west-vergent thrust faults. date shortening until, at a certain strain, an Edwards et al. describe structures exposed along asymmetric thrust-like flow pattern occurs on a the south-west of the syntaxis (i.e. to the south of crustal to lithospheric scale and it is on this that those described by Butler). Uplift here was along the syntaxes grow. A side-effect of the model is steep, east-side up shear zones. Whereas Butler that syntaxial growth is accompanied by the and Edwards et al. describe structures that growth of marginal basins, in the case of the accommodated uplift along the western margin Nanga Parbat syntaxis these are the and of the syntaxis, Argles describes the structural Peshawar basins. That these basins may be the evolution of part of the eastern margin of the result of syntaxial uplift is in contradiction to syntaxis. Butler et al. describe a structural section previous models which infer them to be piggy- that crosses the syntaxis and incorporate data back basins developed above late-stage thrusts presented here and elsewhere to derive a model such as the Main Boundary Thrust. Whittington for the structural evolution of the syntaxis as a et al. use isotopic data to demonstrate that the whole. This model should be viewed in connec- crystalline Indian Plate rocks contained within tion with that published by Schneider et al. the core of the Nanga Parbat syntaxis are typical (1999). As geochronological data were first used of the Lesser Himalaya rather than of the Higher to hint at the speed of syntaxial growth (Zeitler Himalaya. The implications of this are profound 1985), it is appropriate that a more profound as they stress the differences between the Pakistan data set should be used in an attempt to Himalaya and the Indian and Nepalese Hima- constrain uplift mechanisms. To this end, Treloar laya, and indicate that models that suggest that et al. use geochronological data to outline firstly, the syntaxis is developed above the lateral tip of that peak metamorphism within the syntaxis is the Main Central Thrust may be incorrect. Eocene-Oligocene, rather than Miocene, in age, The remaining papers in this volume provide a and also to indicate how geochronological data cross section across the Himalayan collision zone can be used to refine the regional scale features from the Asian plate through the Kohistan arc to Downloaded from http://sp.lyellcollection.org/ by guest on September 29, 2021

4 P.J. TRELOAR ET AL. the Indian plate. It has long been known that, provide a detailed account of the Main Mantle prior to collision, the southern margin of the Thrust (MMT) which separates the Kohistan arc Asian plate was a tectonic collage formed of a from the Indian Plate and which is the true series of exotic blocks sequentially accreted to the western continuation of the Indus-Tsangpo southern margin of continental Asia. The Suture Zone. They highlight the fact that the Kohistan-Dras arc was the last of these blocks MMT is constituted of a number of strands of to be accreted to Asia. Zanchi et al. describe one various ages. Although clearly a major early of the suture zones that encompassed the south- Tertiary tectonic feature, the MMT has been ward growth of Asia during the late Mesozoic transposed many times during the Tertiary. and show how the Tirich Mir Fault Zone can be Three papers detail the geology of the internal described as a suture on the basis of the presence zones of the Indian Plate. Corfield & Searle of ophiolitic peridotites. Also on the Asian Plate, describe the geology of the north Indian con- Hildebrand et al. describe the geological evolution tinental margin in Zanskar. They include an of the Hindu Kush in the NW Frontier of important new map and estimate shortening Pakistan. They document a major late Mesozoic amounts across the north Indian margin. These deformation probably related to suturing of estimates incorporate both late Cretaceous to Kohistan to Asia as well as an Oligocene- Palaeocene ophiolite emplacement events and Miocene metamorphism-deformation event subsequent post-collisional shortening. Lombar- that was likely related to indentation of Kohistan do et al. describe the occurrence of glaucophane- into Asia following collision of Kohistan and and barroisite-bearing eclogites from the Upper India. This paper comes together with an Kaghan Valley. Fontan et al. also describe high- important new geological map of a large part of pressure rocks from Indian plate sequences in the the Hindu Kush. Neelum Valley. Although eclogites have pre- Three papers deal with aspects of the Kohistan viously been described from Indian Plate se- arc sequence. Yamamoto & Nakamura and quences of north Pakistan (Pognante & Spencer Anczkiewiez & Vance date peak metamorphism 1991), it is only recently that the high pressure within the Kamila amphibolite belt at the struc- nature of these rocks has been recognized tural base of the arc at ¢. 95 Ma with amphibolite- (O'Brien et al. 1999). The full significance of facies retrogression at c. 85 Ma (see Treloar et al. these rocks is as yet unclear. 1989). Arbaret et al. describe a variety of Abassi & Friend explore the significance of SW-vergent structures within the Kamila amphi- exotic conglomerates in the Neogene Siwalik bolite belt that range from magmatic through succession and relate them in part to growth of sub-magmatic to amphibolite facies. This defor- the Nanga Parbat syntaxis. They show the mation spans the period documented geochrono- Pliocene-aged Janak conglomerate to be derived logically by Yamamoto & Nakamura and from a topographic high, with erosion shedding Anczkiewicz & Vance. What remains unclear is into the Neogene foreland basin. to what these deformation, magmatic and meta- Finally, Badshah et al. provide a detailed map morphic events relate. Arbaret et al. interpret the of part of the Pakistan-Afghanistan border. This magmatism and deformation as having occurred important map forms a link between those of at the base of the arc during ongoing subduction Jones (1960). Wittekindt & Weippert (1973) and of the Tethyan oceanic lithosphere beneath Bender & Raza (1995) and with these documents Kohistan. An alternative solution is that the an important part of the collision zone between deformation documents a change in subduction the Indian Plate and the Afghanistan Block (see dynamics following on from suturing of Treloar & lzatt 1993). Kohistan to Asia. Three papers deal with the sutures that bound the arc. Weinburg et al. describe the suture zone between Asia and the Ladakh part of the References Kohistan-Ladakh arc sequence and show that closure must have pre-dated 68_+ 1 Ma. AUDEN, J. B. 1938. Geological Results. In: SHIPTON, E. Robertson and DiPietro et al. discuss the (ed.) The Shaksgam Expedition 1938. Geographical evolution of the suture between the Mesozoic Journal, 91, 335-336. BECK, R., BURBANK, D. W., SERCOMBE, W. J., RILEY, arc and the Indian Plate. Robertson finds the G. W., BARNDT, J. K., BERRY, J. R., AFZAL, J., Indus Suture Zone in Ladakh to be a zone KHAN, A. M., JURGEN, H., METJE, J., CHEEMA,A., of complex multi-stage processes involving SHAFIQUE, N. A., LAWRENCE, R. D. • KHAN, M. deformational features which span the time A. 1995. Stratigraphic evidence for an early interval from subduction through emplacement collision between northwest India and Asia. to post-collisional shortening. DiPietro et al. Nature, 373, 55-58. 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INTRODUCTION 5

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