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

A proposal funded by the NSF Continental Dynamics Program   12/01/2006

PROJECT SUMMARY
The northeastern boundary of the Tibetan Plateau is a new focus of 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. Complicating both views is the overprint of major strike slip faulting, which may – or may not— penetrate the entire lithosphere. Critical to any model of surface tectonics are 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. We propose to test key elements in these hypotheses with an integrated, focused program of geophysical surveys across three key segments of the NE Plateau boundary zone: 1) the Jinsha Suture, which separates the Qiangtang from the Songpan-Ganzi- Hoh Xil terranes within the high Plateau, 2) the Kunlun strike-slip and thrust systems, which form the boundary between the Songpan-Ganzi-Hoh Xil terrane and the Qaidam Basin, and 3) the Altyn Tagh strike slip fault, which forms the northwest boundary of the Qilian Shan-Nan Shan thrust system which characterizes deformation of this foreland zone. The geophysical surveys will consist of a) integrated seismic reflection, refraction and densely spaced passive profiles, b) detailed magnetotelluric surveys, and c) a regional broadband seismic array. Specific features to be addressed by the proposed surveys include: a) the existence and nature of proposed Moho offsets, (b) the deep geometry of major thrust faults at the plateau boundary (decollements vs crustal “stacking”), c) the relationship between major thrusts and strike slip faults (which decapitates which?), d) constraints on lower crustal flow from structural continuity (or lack thereof) of crustal markers, e) the existence and nature of subducting Asian lithosphere, and f) the mode of lithospheric thickening beneath the plateau foreland. The proposed experiment, combined with data from previous and ongoing seismic experiments in surrounding regions, will for the first time link high resolution images of crustal structure to the three-dimensional upper mantle seismic structure of the entire eastern Tibetan plateau.

Intellectual Merit: Continent-continent collision is a cornerstone of plate tectonics, and a central process in continental evolution. The intra-continental response to such collision, especially as it relates to plateau formation, is perhaps its least understand aspect. Our study will delineate the 3D geometry of those structures, which have recorded this process as it relates to the Tibetan Plateau, and directly test existing geodynamic hypotheses for the evolution of the distal portions of collision zones.

Broader Impact: The structural evolution of the intracontinental portions of the southern Asian collision zone have implications for a range of phenomena with direct societal implications, including earthquake hazards, geothermal energy, and climate history. The INDEPTH collaboration between Chinese and western scientists has already stimulated geoscience research within China and will continue to lead to valuable spin-off investigations.

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Last updated 12/06