![]() ![]() The Himalaya have always been at the forefront of geodetic studies. In Zanskar there is also debate over the timing of the obduction of the Spontang ophiolite onto the Zanskar passive margin sequence, and the relative importance of pre-India–Asia collision folding and thrusting related to the final stages of the obduction, and post- India–Asia collision shortening and thickening. The relatively straightforward tectonic picture along the main Himalayan range is also complicated in the Pakistan sector, west of Nanga Parbat, where the high-grade kyanite and sillimanite metamorphism has recently been dated as Ordovician, not Himalayan in age ( Palin et al. This young metamorphism may be indicative of active metamorphism that is occurring at depth beneath the Himalaya today in rocks that have not yet been exhumed by thrusting, exhumation and erosion. In both these regions, a younger high-temperature metamorphic overprint on the standard Late Eocene–Miocene Himalayan events is apparent with high-grade sillimanite + cordierite crustal melting occurring in the deep basement, as young as Pliocene or even Pleistocene in age. The relatively straightforward structural and metamorphic geometry, and timing constraints along the main Himalayan range are, however, complicated in the two syntaxis regions, the Nanga Parbat–Haramosh syntaxis in the NW (Pakistan), and the Namche Barwa syntaxis (SE Tibet) in the NE. The most recent comprehensive reviews of the structure, metamorphism and tectonic evolution of the Himalaya are given by Kohn (2014), Searle (2015) and Goscombe et al. ![]() Brittle folding and thrusting processes characterize the Lesser Himalaya, structurally below the ductile MCT, and corresponds to the critical taper model. This corresponds to the channel flow (or channel tunnelling) model that is now widely accepted for the GHS ductile structures. Structural mapping and timing constraints suggest the large-scale southward extrusion of a partially melted layer of mid-crustal rocks (sillimanite grade gneisses and leucogranites) bounded by the STD ductile shear zone with right-way-up metamorphic isograds above, and the MCT ductile shear zone with inverted metamorphic isograds below, during the Oligocene–Early Miocene. Decompression melting peaked with widespread partial melting and formation of migmatites and leucogranites along the highest peaks of the Himalaya. The GHS metamorphism is all part of one continuum of crustal thickening and shortening, increasing pressure and temperature following a standard clockwise Pressure-Temperature-Time ( PTt) path. The age of the abundant leucogranite sills and dykes along the top of the GHS, beneath the STD, is concomitant with the sillimanite-grade metamorphic event. Peak kyanite grade metamorphism (Late Eocene–Oligocene) pre-dates the regional higher-temperature, lower-pressure sillimanite ± cordierite-grade event, which was accompanied by widespread migmatization and mid-crustal melting during the Oligocene–Mid-Miocene. In broad terms the timing of major events shows little variation along the entire mountain range, with Late Cretaceous–Paleocene obduction of ophiolites onto the passive margin of India, Late Paleocene ultra-high-pressure (UHP) metamorphism at Kaghan (northern Pakistan) and Tso Morari (India), Early Eocene final marine sedimentation prior to the closure of Neo-Tethys, and Late Eocene to Early Miocene regional Barrovian-type metamorphism along the GHS ( Fig. Chamba klippe in India Lingshi klippe in Bhutan), and far-travelled klippen of GHS rocks occur in places south of the main MCT and GHS rocks (e.g. Klippen of low-grade or unmetamorphosed sedimentary rocks lie above the GHS high-grade rocks in places (e.g. Likewise, the major structures, the Indus–Yarlung Tsangpo suture with north-vergent backthrusts, the South Tibetan Detachment (STD) low-angle normal fault, locally called the Zanskar Shear zone in the west, the Main Central Thrust (MCT) zone and the Main Boundary Thrust are all mapped along the entire length of the mountain belt between the western (Nanga Parbat) and eastern (Namche Barwa) syntaxes. The major structural divisions, the Indus–(Yarlung Tsangpo) suture zone, the Tethyan Himalaya sedimentary units, Greater Himalaya Sequence (GHS) metamorphic rocks, the Lesser Himalaya fold-and-thrust belt and the Sub-Himalaya Siwalik molasse basin are present along the entire 2000 km length of the Himalaya ( Figs 1 & 2). The beauty of the Himalaya is that, on a broad scale they form a relatively simple orogenic belt. They are frequently used as the type example of a continental collision orogenic belt in studies of older Phanerozoic orogenic belts. The Himalaya resulted from collision of the Indian plate with Asia and are well known as the highest, youngest and one of the best studied continental collision orogenic belts. ![]()
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