Collaborative Proposal: Active Faulting Within the Eastern Tibetan Margin and Western Sichuan Basin, P. R. China

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The eastern margin of Tibet is one of the most enigmatic plateau margins in the world. Elevations rise from less than 500 m within the Sichuan Basin to more than 6.5 km over a distance of about 30 to 50 km. This is steeper than the Himalayan front, which owes its origin to the active thrusting of the Indian subcontinent under Tibet. In stark contrast to the Himalaya, however, the eastern margin of Tibet shows insignificant evidence of active thrusting. Moreover, the lack of a significant sedimentary basin adjacent to the margin and the absence of geodetic shortening further testify to the insignificance of active thrusting.New field investigations indicate that while active thrusting is indeed scarce, there is ample evidence for significant active strike-slip faulting. Initial reconnaissance field investigation reveals three and possibly four significant right-lateral strike-slip faults that trend NE, and two major left-lateral strike-slip faults that trend ~N-S. Research is now focused on determining the rates and detailed geometry of these faults, each of which are critical components in planned attempts at constraining relative crustal motions in the region. These observations are able to constrain the various proposed hypotheses and therefore enable us to understand the evolution of this part of the Tibetan margin.Early investigation has also revealed the likelihood of significant active faulting within the Sichuan Basin. This is of particular concern, because the suspect faults may represent significant strong ground motions and the Sichuan Basin is home to about 60 million people.Rates of fault slip are being determined by a careful sampling strategy whereby the ages of landforms that have been offset by faulting are being estimated by cosmogenic radionuclide analyses. The style and pattern of active faulting is being determined by a combination of imagery analysis, topographic analyses, and detailed field mapping in regions previously identified during reconnaissance field work. The combination of slip rates and fault geometry yields a set of kinematics that discriminate among the competing models.