Receiver-function imaging of the lithosphere at the Kunlun-Qaidam boundary, Northeast Tibet

Abstract

We studied the crustal structure and tectonics in the north Tibetan Plateau from the Songpan-Ganzi terrane to the Qaidam Basin using teleseismic receiver-function imaging, across a major lithospheric boundary, the Kunlun-Qaidam boundary, where previous studies suggest a ~15–20-km change in crustal thickness from thicker crust in the Kunlun Mountains to thinner crust in the Qaidam Basin. We report P receiver functions for 70 stations, largely the International Deep Profiling of Tibet and the Himalaya (INDEPTH), phase IV, experiment. Our most dense station coverage is located along the roughly north-south INDEPTH-IV active-source seismic profile at approximately 95° E longitude. Azimuthal and geographical changes in the receiver functions reveal significant changes in crustal structure and Vp/Vs from across the study area. Receiver functions show strong converters that we interpret as the Moho at ~70 km depth beneath the Qiangtang, Songpan-Ganzi terranes and Kunlun Mountains and at ~50 km depth beneath the central Qaidam Basin. This large change in crustal thickness occurs >50 km north of the North Kunlun strike-slip fault, on which the 2001 M8.1 Kunlun earthquake occurred. Receiver functions for some of the stations north of the thickness change at the Kunlun-Qaidam boundary also show a deeper ~70-km bright converter in addition to the 50-km converter. The two converters appear to overlap by up to ~30 km in some locations along the south Qaidam Basin. We combine previous results with these new results to discuss implications for mechanisms for crustal thickening in the north Tibetan Plateau including crustal flow and crustal injection. At depths imaged here, shallower than ~100 km, we see no evidence of southward subduction of Eurasian lithosphere.