| Summary: | The Greater Himalayan Sequence (GHS) is composed of a sequence of Barrovian facies metamorphic rocks up to kyanite or sillimanite+K-feldspar grade, migmatites, layered stromatic migmatites and leucogranite sheets. These rocks were metamorphosed during the late Eocene to early Miocene, and are bounded below by a large-scale SW-vergent ductile shear zone - thrust fault (Main Central Thrust; MCT), and above by a NE-dipping low-angle normal sense shear zone and fault (Zanskar Shear Zone; ZSZ), part of the South Tibetan Detachment (STD) system. Restoration of the high-grade metamorphic rocks of the GHS reveals that protoliths are Proterozoic shales (Haimanta Group), Cambrian-Ordovician orthogneisses, Permian Panjal volcanics and Palaeozoic-Triassic/Jurassic sedimentary rocks, lateral equivalent rocks of the adjacent unmetamorphosed Tethyan zone. Structural outliers of low-grade or unmetamorphosed rocks (e.g., Chamba syncline) have been mapped overlying high-grade (usually sillimanite grade) rocks of the GHS. The low-grade klippen are underlain by low-angle north-directed ductile shear zones and normal faults, that are interpreted here as earlier equivalents of the ZSZ. These low-angle normal faults formed during continuous NE-SW compression, crustal thickening and southwestward extrusion of the GHS slab along the footwall. STD normal faults propagated structurally upward (northeast) with time, whereas MCT reverse faults along the base of the GHS propagated structurally downward (southwest) with time. Inverted metamorphic isograds along the base of the slab (MCT zone) can be linked on the map with right way-up isograds along the footwall of the ZSZ normal fault at the top of the slab, demonstrating that the recumbently folded isograd model is a viable geometrical representation of the metamorphic structure of the GHS. Post-metamorphic shearing along the ZSZ and MCT has condensed the isograds by a combination of pure shear and simple shear. These observations support the Channel Flow model of ductile extrusion of a mid-crustal layer of Indian plate rocks during the Miocene, from beneath the passive roof stretching fault of the ZSZ.
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