Seismic Evidence for North China Plate Subduction beneath Northeastern Tibet and its Implications for Plateau Growth

The mode of lithospheric deformation related to the Tibetan plateau has been clarified as verging subduction of the Indian (northward) and Asian (southward) plates beneath the plateau, but how the lithosphere of the Tibetan plateau interacts with that of the North China craton (NCC) is still unclear due to shortage of data in the northeastern Tibet (NE Tibet). A long passive-source broadband seismic profile with densely spaced stations was conducted across the entire NE Tibet, extending to the southern border of the Alxa block (belonging to NCC). A structural model of lithosphere was derived from P and S receiver function imaging. Some significant features including the north-dipping intracrustal converter (NC) beneath the Kunlun-West Qinling orogen, the Moho offsets with overlaps beneath the fault zones and the low velocity layer (LVL) beneath the Qilian terrane were revealed by our observations. A synthesis of these observations probably indicates the oblique crustal thrusts (accompanied by shear) along the large strike-slip faults, taking the LVL as an intracrustal decollement. We did observe the Lithosphere-Asthenosphere Boundary (LAB), as south-dipping and descending continuously from beneath the interior Alxa to the Qilian block. Combining our profiling results with other seismic evidence, we infer that, the lithosphere of NCC may underthrust beneath the Qilian block and as a minimum reach the southern margin of the Qilian block (bounded by the West Qinling fault zone) to the south. We suggest that, the continental subduction of lithosphere of the NCC accompanied by crustal thrusts is a preferred mode of the lithospheric deformation beneath NE Tibet, which is responsible for the plateau growth in the northeastern margin of Tibetan plateau.