Collaborative Research: Deep Structure of the Northeast Tibetan Collision Zone- INDEPTH IV

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This project (INDEPTH IV) is a continuation of previously funded projects (INDEPTH I, II, III). The overarching goal of the INDEPTH studies is to understand the basic processes that take place in a continent-continent collision. INDEPTH I, II, and III were joint Sino-U.S. studies to understand the formation of the Tibetan Plateau and Himalayan mountains, which together comprise Earth's type example of an active continent-continent collision zone. Results from these studies have profoundly influenced prevalent theories of mountain building in such tectonic settings. The focus of INDEPTH IV is the northeastern boundary of the Tibetan Plateau. This boundary plays a central role in a contemporary debate concerning continental plateau formation as an intracontinental response to collisional orogeny. Recent geological studies and limited geophysical measurements in this region have been cited to argue that a) uplift is due to internal imbricate "stacking" of Asian crust while b) Asian continental lithosphere is being detached and "subducted" into the Tibetan mantle. Such models contrast with a competing paradigm, derived primarily from observations in the southern and eastern portions of the Plateau, that attribute plateau formation to ductile flow in the deep crust. Critical to any model of surface tectonics is the mantle dynamics beneath the plateau and adjacent regions. Upper mantle geophysical properties beneath the northern plateau have been variously attributed to delamination triggered by instability of a thickened lithosphere, mantle lid detachment and/or asthenospheric counterflow associated with subduction of Indian continental lithosphere. INDEPTH IV will test key elements in these hypotheses with an integrated, focused program of geophysical surveys across three key segments of the NE Plateau boundary zone. Geophysical surveys will consist of integrated seismic reflection, refraction and densely spaced passive seismic profiles, a detailed magnetotelluric survey, and a regional broadband seismic array. Specific features to be investigated by the surveys include: a) the existence and nature of proposed Moho offsets; b) the deep geometry of major thrust faults at the plateau boundary; c) the relationship between major thrusts and strike slip faults; d) constraints on lower crustal flow from structural continuity of crustal markers; e) the existence and nature of subducting Asian lithosphere; and f) the mode of lithospheric thickening beneath the plateau foreland.