Metasedimentary rocks of the Greater Himalaya are traditionally viewed as Indian shield basement that has been thrust southward onto Lesser Himalayan sedimentary rocks during the Cenozoic collision of India and Eurasia. Ages determined from radioactive decay of uranium to lead in zircon grains from Nepal suggest that Greater Himalayan protoliths were shed from the northern end of the East African orogen during the late Proterozoic pan-African orogenic event. These rocks were accreted onto northern Gondwana and intruded by crustal melts during Cambrian-Ordovician time. Our data suggest that the Main Central thrust may have a large amount of pre-Tertiary displacement, that structural restorations placing Greater Himalayan rocks below Lesser Himalayan rocks at the onset of Cenozoic orogenesis are flawed, and that some metamorphism of Greater Himalayan rocks may have occurred during early Paleozoic time.

Field mapping, geochronological analyses, and cross section construction reveal a protracted deformation history and a minimum of 61 km of Cenozoic NE-SW shortening (in present coordinates) across the Yushu-Nangqian thrust belt in northern Tibet. Cenozoic contraction started prior to 51 Ma and was followed fi rst by northwest-striking right-slip faulting and later by northwest-striking left-slip faulting. Renewed NE-SW contraction is expressed by folding of Neogene strata and thrusting, which again was followed by northweststriking left-slip faults. Late Neogene deformation is expressed by local north-striking normal faults. Shortening across the Yushu-Nangqian belt appears to be accommodated by thinskinned thrusting, which raises the question of how the deformation was accommodated in the lower crustal levels. To resolve this problem, we perform geochemical analysis of igneous rocks dated as 51-49 and 38-37 Ma. The rocks exhibit geochemical signatures characteristic of subduction, which implies that coeval crustal thickening in northeastern Tibet was most likely induced by continental subduction.

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