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.
The Himalayas and Tibet Plateau are the type example of mountain building by collision of two tectonic plates bearing continental masses. Understanding how this vast uplifted region formed and evolved requires detailed information about the major structures which underlie the dramatic topography. In 1992, an international group of scientists and organizations began a project known as INDEPTH (International Deep Profiling of Tibet and the Himalayas) to apply state of the art geophysical techniques to probe the deep structure of this region. The goal was a modern geophysical transect from the Himalayas in the south to the northeast edge of the Tibet Plateau. Phase IV of Project INDEPTH represents the conclusion of this multi-decadal initiative with a multidisciplinary survey across the boundary of the Tibet Plateau and the adjacent Qaidam basin. This study was cored by a series of seismic profiles using both explosions and earthquake as sources. It also included magnetotelluric probes of lithospheric conductivity. This final stage of INDEPTH confirmed a dramatic offset in the base (Moho) of the Tibet crust where it abuts the Qaidam basin, evidence for injection of lower Tibetan crust northward beneath the Qaidam basin, a fragmented mantle below, and evidence for thrusting of Asia lithosphere southward beneath the plateau. This study led to expanded collaboration of US and Chinese scientists in the interpretation of new SINOPROBE seismic data within and along the boundaries of Tibet, providing further evidence that the Plateau is buoyed up by relatively week lower crustal material that is juxtaposed along its boundaries against stronger, lower, lithospheric blocks ( the Qaidam and Sichuan Basins).
