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.
How do continents deform during collision, and what happens to the collided crust? Collision of India and Asia built the Tibetan Plateau, Earth's largest, but this surface expression of collision lacks the volume to accommodate all the material consumed over 55 million years of ongoing orogeny. There appears to be a striking asymmetry between the "front" of the collision, in the Himalayan mountains that bound southern Tibet; and the "back" of the collision, in the Kunliun mountains that bound northern Tibet. In the south, large volumes of "India" have disppeared below the surtface, in a process akin to oceanic subduction, that gives rise to huge thrust-fault earthquakes, like the devastating 2004 Sumatra earthquake. In the north, equally large earthquakes occur, but they are largely "strike-slip" showing two plates sliding past eash other, like the well-known San Andreas Fault of California. Our project was designed to help map the India-Asia collision in three dimensions, across the northern margin of Tibet. Starting in 2007 our Chinese-US-German collaboration carried out a series of experiments across the Kunlkun mountains (Figure). We detonated small explosive charges from which we could observe the propagation of seismic (sound) waves threough the crust to be recorded at hundreds of geophones (analogous to microphones) we had laid out across the surface. We left dozens of sophisticated seismic recorders deployed in the mountains for two years to record even modest earthquakes on the far side of the world. We used these seismic waves to build up a 3D picture of the boundary of northern Tibet. Althouth the conventional picture of a vertical plate boundary appears to be correct in the upper crust, we discovered that Tibet is being injected to the north beneath the Qaidsam Basin (Asian continent) at the base of the crust (Figure). The overlapping Mohos and weak Tibetan crust extending beneath the Qaidam Moho suggest northward injection of Tibetan lower crust beneath the Qaidam crust. These geometries suggest north-directed lower-crustal channel flow or injection is a better description of geodynamics at this plateau margin than intra-continental transform faulting or southward subduction of Eurasia. These experiments are a telescope turned into the earth to provide a new understanding about the weays continents deform beneath our feet; and also provide an opportunity to train a new generation of young scientists how to study the earth, and carry out sophisticated computational analyses of the scientific data.
