| Abstract: | Abstract This paper introduces 8 major discoveries and new understandings with regard to the deep structure and tectonics of the Himalayas and Tibetan Plateau obtained in Project INDEPTH, They are mainly as follows. (1) The upper crust, lower crust and mantle lithosphere beneath the blocks of the plateau form a “sandwich” structure with a relatively rigid‐brittle upper crust, a visco‐plastic lower crust and a relatively rigid‐ductile mantle lithosphere. This structure is completely different from that of monotonous, cold and more rigid oceanic plates. (2) In the process of north‐directed collision‐compression of the Indian subcontinent, the upper crust was attached to the foreland in the form of a gigantic foreland accretionary wedge. The interior of the accretionary wedge thickened in such tectonic manners as large‐scale thrusting, backthrusting and folding, and magmatic masses and partially molten masses participated in the crustal thickening. Between the upper crust and lower crust lies a large detachment (e.g. the Main Himalayan Thrust in southern Tibet, 5–8 km thick) or a very thick shear‐schistose zone (e.g. the Main Qiangtang Thrust—MQT in northern Tibet, up to 20 km thick), which causes the decoupling of the upper crust and lower crust and separation of tectonic activities. (3) During the collision‐compression, the Indian mantle lithosphere was delaminated into two layers from where the crust thickened most rapidly (beneath the High Himalayas). The upper layer extends to 34.5°N and the lower layer to 33.5°. They have been underthrust to depths of 250–300 km into the asthenosphere. Meanwhile the Asian lithosphere (possibly the Qaidam terrane) has also been subducted southwards. Very thick mantle lithosphere does not exist beneath the plateau. (4) The oceanic lithosphere, in light of its lithology and dynamic behaviour, might be close to those of the continental lithosphere and its front might enter the asthenosphere before the continental lithosphere. (5) A 150–200 km deep low‐velocity body below 35°N and a wide low‐velocity zone below the area between 33.5° and 35°N dip north at very steep angles. Volcanism took place frequently in northern Tibet and anisotropy variations are prominent at depths, which might indicate a zone of large‐scale eastward transfer of deep‐seated materials. |
