Downloaded from http://sp.lyellcollection.org/ at Kingston University on January 3, 2020 Introduction to Himalayan tectonics: a modern synthesis MICHAEL P. SEARLE1* & PETER J. TRELOAR2 1Department of Earth Sciences, University of Oxford, Oxford OX1 3AN, UK 2School of the Natural and Built Environment, Kingston University, Penrhyn Road, Kingston-upon-Thames, Surrey KT1 2EE, UK MPS, 0000-0001-6904-6398 *Correspondence: [email protected] The Himalaya resulted from collision of the Indian widespread migmatization and mid-crustal melting plate with Asia and are well known as the highest, during the Oligocene–Mid-Miocene. The age of the youngest and one of the best studied continental col- abundant leucogranite sills and dykes along the top lision orogenic belts. They are frequently used as the of the GHS, beneath the STD, is concomitant with type example of a continental collision orogenic belt the sillimanite-grade metamorphic event. The GHS in studies of older Phanerozoic orogenic belts. The metamorphism is all part of one continuum of crustal beauty of the Himalaya is that, on a broad scale they thickening and shortening, increasing pressure and form a relatively simple orogenic belt. The major temperature following a standard clockwise Pressure- structural divisions, the Indus–(Yarlung Tsangpo) Temperature-Time (PTt) path. Decompression suture zone, the Tethyan Himalaya sedimentary melting peaked with widespread partial melting and units, Greater Himalaya Sequence (GHS) metamor- formation of migmatites and leucogranites along the phic rocks, the Lesser Himalaya fold-and-thrust highest peaks of the Himalaya. Structural mapping belt and the Sub-Himalaya Siwalik molasse basin and timing constraints suggest the large-scale are present along the entire 2000 km length of the southward extrusion of a partially melted layer of Himalaya (Figs 1 & 2). Likewise, the major struc- mid-crustal rocks (sillimanite grade gneisses and leu- tures, the Indus–Yarlung Tsangpo suture with north- cogranites) bounded by the STD ductile shear zone vergent backthrusts, the South Tibetan Detachment with right-way-up metamorphic isograds above, (STD) low-angle normal fault, locally called the and the MCT ductile shear zone with inverted meta- Zanskar Shear zone in the west, the Main Central morphic isograds below, during the Oligocene– Thrust (MCT) zone and the Main Boundary Thrust Early Miocene. This corresponds to the channel are all mapped along the entire length of the moun- flow (or channel tunnelling) model that is now widely tain belt between the western (Nanga Parbat) and accepted for the GHS ductile structures. Brittle fold- eastern (Namche Barwa) syntaxes. Klippen of low- ing and thrusting processes characterize the Lesser grade or unmetamorphosed sedimentary rocks lie Himalaya, structurally below the ductile MCT, and above the GHS high-grade rocks in places (e.g. corresponds to the critical taper model. The most Chamba klippe in India; Lingshi klippe in Bhutan), recent comprehensive reviews of the structure, meta- and far-travelled klippen of GHS rocks occur in morphism and tectonic evolution of the Himalaya are places south of the main MCT and GHS rocks (e.g. given by Kohn (2014), Searle (2015) and Goscombe Darjeeling klippe). et al. (2018). In broad terms the timing of major events shows The relatively straightforward structural and little variation along the entire mountain range, with metamorphic geometry, and timing constraints Late Cretaceous–Paleocene obduction of ophiolites along the main Himalayan range are, however, onto the passive margin of India, Late Paleocene complicated in the two syntaxis regions, the Nanga ultra-high-pressure (UHP) metamorphism at Kaghan Parbat–Haramosh syntaxis in the NW (Pakistan), (northern Pakistan) and Tso Morari (India), Early and the Namche Barwa syntaxis (SE Tibet) in the Eocene final marine sedimentation prior to the clo- NE. In both these regions, a younger high-tempera- sure of Neo-Tethys, and Late Eocene to Early Mio- ture metamorphic overprint on the standard Late cene regional Barrovian-type metamorphism along Eocene–Miocene Himalayan events is apparent with the GHS (Fig. 3). Peak kyanite grade metamorphism high-grade sillimanite + cordierite crustal melting (Late Eocene–Oligocene) pre-dates the regional occurring in the deep basement, as young as Pliocene higher-temperature, lower-pressure sillimanite ± or even Pleistocene in age. This young metamor- cordierite-grade event, which was accompanied by phism may be indicative of active metamorphism From:TRELOAR,P.J.&SEARLE, M. P. (eds) 2019. Himalayan Tectonics: A Modern Synthesis. Geological Society, London, Special Publications, 483,1–17. First published online July 1, 2019, https://doi.org/10.1144/SP483-2019-20 © 2019 The Author(s). This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0/). Published by The Geological Society of London. Publishing disclaimer: www.geolsoc.org.uk/pub_ethics Downloaded from http://sp.lyellcollection.org/ at Kingston University on January 3, 2020 2 M. P. SEARLE & P. J. TRELOAR Fig. 1. Geological map of the Himalaya. that is occurring at depth beneath the Himalaya today and his successor George Everest, mapped out the in rocks that have not yet been exhumed by thrusting, Himalayan ranges for the first time. Amongst the exhumation and erosion. The relatively straightfor- many great achievements of the Survey, these sur- ward tectonic picture along the main Himalayan veyors accurately determined the heights of most range is also complicated in the Pakistan sector, of the highest peaks of the Himalaya and Karakoram, west of Nanga Parbat, where the high-grade kyanite and measured gravity anomalies that led to the devel- and sillimanite metamorphism has recently been opment of the theory of isostasy. Richard Oldham dated as Ordovician, not Himalayan in age (Palin joined the Survey of India in 1879 and made the et al. 2018). In Zanskar there is also debate over first detailed observation of a large Himalayan earth- the timing of the obduction of the Spontang ophiolite quake, the Great Assam earthquake of 1879 (Oldham onto the Zanskar passive margin sequence, and the 1917). Oldham first identified on seismograms the relative importance of pre-India–Asia collision fold- arrivals of primary (P-waves), secondary (S-waves) ing and thrusting related to the final stages of the and tertiary surface waves, previously predicted by obduction, and post- India–Asia collision shortening mathematical theory. The earliest geological and and thickening. geographical explorations of the Himalaya were made in the late 1800s and early 1900s. In 1907 Col- onel S.G. Burrard, Superintendent of the Trigono- History of research metrical Survey, and H.H. Hayden, Superintendent of the Geological Survey of India, published four The Himalaya have always been at the forefront of volumes of their classic work A Sketch of the Geog- geodetic studies. The Great Trigonometrical Survey, raphy and Geology of the Himalaya Mountains and started in 1802 under its founder William Lambton Tibet. During the late 1800s geologists like Downloaded from http://sp.lyellcollection.org/ at Kingston University on January 3, 2020 AN INTRODUCTION TO HIMALAYAN TECTONICS: A MODERN SYNTHESIS 3 Fig. 2. Photograph taken from the Space Shuttle looking west along the Himalaya with the outline of the major structural divisions and major faults of the Himalaya. Photo courtesy of NASA. Medlicott, Middlemiss and Oldham made significant along the central Indian Himalaya, Ardito Desio, discoveries in the western Himalaya, and Mallet and who led the first successful ascent of K2 and mapped von Loczy first discovered the inverted metamorphic a large tract of the Baltoro Karakoram in 1955, and gradient in the Darjeeling klippe. Rashid Khan Tahirkheli, a heroic Pakistani geologist The next breakthrough was the publication of who mapped large parts of remote Kohistan during Arnold Heim and Augusto Gansser’s Central Hima- the 1970s. This work was continued by the studies laya: Observations from the Swiss Expedition of of Qasim Jan and Asif Khan and their students 1936 (Heim & Gansser 1939), and Augusto Ganss- from the University of Peshawar. A regional map er’s classic Geology of the Himalaya, published in of the Central Karakoram Mountains covering the 1964. Heim and Gansser discovered the remnant Hunza, Hispar, Biafo and Baltoro glacier region at ophiolites of SW Tibet in the Kiogar–Amlang-la the scale of 1:250,000 was published by Searle Range, laid the foundations for the stratigraphy of (1991) and a large compilation geological map of the Indian plate and confirmed the inverted nature North Pakistan at scale of 1:650 000 was published of metamorphism along the Main Central Thrust. by Searle & Khan (1996). Other great pioneering geologists were D.N. Some of the most important early geological Wadia, who mapped large tracts of the NW Frontier mapping in the Indian Himalaya was carried out by region, J.B. Auden and K.S. Valdiya, who worked K.S. Valdiya and his colleagues from Kumaon Downloaded from http://sp.lyellcollection.org/ at Kingston University on January 3, 2020 4 M. P. SEARLE & P. J. TRELOAR Fig. 3. Photograph of the central Himalaya in Nepal and plateau of Tibet showing the major high peaks, courtesy of NASA. University, working mainly in the Garhwal– Manaslu region (Colchen et al. 1986). Climbers Kumaon Himalaya, and Vikram Thakur and col- contributed greatly to the early pioneering studies leagues from the Wadia Institute of Himalayan Geol- of Mount Everest. Noel Odell was a geologist– ogy, Dehra Dun, working mainly in Himachal mountaineer on the 1924 British Everest expedition Pradesh, Lahoul–Spiti and Ladakh. More advances and was the last person to see Mallory and Irvine were made by the field studies of A.K. Jain and San- heading up the NNE ridge towards the summit. deep Singh and their students from the Indian Insti- Odell made many original geological observations tute of Technology, Roorkee, and Talat Ahmad on their journey from Darjeeling and Sikkim to the and colleagues from the universities of Kashmir Tibetan side of Everest, and collected many samples.