Active tectonics of the Beichuan and Pengguan faults at the eastern margin of the Tibetan Plateau

The steep, high-relief eastern margin of the Tibetan Plateau has undergone rapid Cenozoic cooling and denudation yet shows little evidence for large- magnitude shortening or accommodation generation in the foreland basin. We address this paradox by using a variety of geomorphic observations to place constraints on the kinematics and slip rates of several large faults that parallel the plateau margin. The Beichuan and Pengguan faults are active, dominantly dextral- slip structures that can be traced continuously for up to 200 km along the plateau margin. Both faults offset fluvial fill terraces that yield inheritance- corrected, cosmogenic 10 Be exposure ages of < 15 kyr, indicating latest Pleistocene activity. The Pengguan fault appears to have been active in the Holocene at two sites along strike. Latest Quaternary apparent throw rates on both faults are variable along strike but are typically < 1 mm yr(-1). Rates of strike- slip displacement are likely to be several times higher, probably similar to 1-10 mm yr(-1) but remain poorly constrained. Late Quaternary folding and dextral strike- slip has also occurred along the western margin of the Sichuan Basin, particularly associated with the present- day mountain front. These observations support models for the formation and maintenance of the eastern plateau margin that do not involve major upper crustal shortening. They also suggest that activity on the margin- parallel faults in eastern Tibet may represent a significant seismic hazard to the densely populated Sichuan Basin.