NORMAL FAULTING ON THE EASTERN MARGIN OF THE TIBETAN PLATEAU

Surface extension in convergent settings is often attributed to late-stage gravitational collapse of thickened crust. However, a different mode of extension has been recognized on the eastern margin of the Tibetan Plateau, where a series of normal faults that parallel the steep plateau margin have been identified in the Longmen Shan region. Extension along these structures does not postdate crustal thickening, but instead may have been synchronous with regional uplift. Normal faults separate the Precambrian basement rocks of the Pengguan and Baoxing Massifs, the Danba antiform, and the Ge Zhong dome from Proterozoic and Paleozoic sedimentary cover rocks. The faults often parallel or reactivate Mesozoic and Cenozoic thrust faults, and may be difficult to distinguish from these shortening structures; however, kinematic analysis at both the outcrop and thin section scale reveals a normal sense of motion along the mapped structures. The faults exhibit a range of styles and both low-angle and high-angle faults are present. Although these faults are not currently active, they appear to represent a relatively young phase of deformation. However, there are currently no firm constraints on their timing. Low temperature thermochronology data from sample transects across the faults may help to constrain the timing of deformation. We propose that this extensional deformation on the eastern plateau margin is coeval with regional uplift and related to the development of dynamic topography due to flow of lower crustal material against the strong lower crust of the Sichuan Basin.