A REVISED MODEL FOR INDIA - ASIA COLLISION BASED ON NEW OBSERVATIONS FROM TIBET

Models for the tectonic evolution of Tibet consider that collision between India and Asia began in the earliest Eocene. Estimates of the timing of initial India -Asia contact are based on several lines of evidence and enough significant geological events were recognised to support suggestions of a major tectonic event in the latest Paleocene/early Eocene. However, major uplift, the typical immediate and obvious manifestation of any active collision occurring on Earth today, apparently did not occur until the late Oligocene - early Miocene. At that time numerous other important tectonic events clearly related to collision also occurred in Tibet and surrounding regions. The reasons for such orogenic inertia in a tectonic collision greater than any observable today are, at best, obscure. Despite little evidence of such an event in the intervening (mid Eocene to lower Oligocene) rocks, collision is still inferred to have begun at least 20 million years earlier. The main lines of evidence given in support of a latest Paleocene/early Eocene collision are: the slowdown in the convergence rate between India and Asia; the initiation of compressional tectonics along and south of the Yarlung Tsangpo suture; a lack of data indicating subduction-related magmatic activity north of the suture between India and Asia and the supposed accumulation of molasse deposits along the suture. Recent field-based geological investigations of rocks within the Yarlung Tsangpo suture zone allow reassessment of the existing model. The first two of these features mentioned above can be ascribed to a different collision event and the others must be reassessed because of the earlier assignment of incorrect age ranges to the units in question. Presently available data indicate that collision between India and Asia must first have occurred in the late (rather than early) Paleogene. Similar results are supported from a more indirect geophysical perspective.