Seismic Evidence for Lateral Asthenospheric Flow Beneath the Northeastern Tibetan Plateau Derived From S Receiver Functions

We present detailed lithospheric images of the NE Tibetan Plateau by applying the depth migration technique to S receiver functions derived from 113 broadband stations. Our migrated images indicate that the lithosphere-asthenosphere boundary (LAB) lies at depths of 105-120km beneath the Qilian terrane and reaches depths of 126-140km below the Alxa and Ordos blocks. The most prominent variation in the LAB depth is the presence of LAB steps of no less than 20km in the transition zone between the active NE Tibetan Plateau and the surrounding cratonic Alxa and Ordos blocks, which conflicts with the model of southward subduction of the Alxa and Ordos blocks. Furthermore, the marked LAB steps occur at 13010km away from the southern surficial boundary faults between the NE Tibetan Plateau and the surrounding tectonic provinces, corresponding to the North Qilian fault and the Liupanshan fault, respectively. Therefore, we propose that these scenarios of LAB can be attributed to the delamination of fragmented mantle lithosphere in the transition zone between the NE Tibetan Plateau and the surrounding Alxa and Ordos blocks, triggered by lateral asthenospheric flow. In addition, our observations of a thin lithosphere with thickness of 107-115km beneath the Songpan-Ganzi terrane and the west Qinlin orogen greatly facilitate the process of underlying lateral asthenospheric flow. The isostatic uplift of the plateau caused by the delamination of fragmented mantle lithosphere, together with increased horizontal compressive stress, may have led to the outward growth of the NE Tibetan Plateau.