Distinct lateral contrast of the crustal and upper mantle structure beneath northeast Tibetan plateau from receiver function analysis

We investigate the crustal thickness, Poisson’s ratio, and the thickness variation of the mantle transition zone using the P receiver function technique on data recorded by the 68 seismic station of the ASCENT experiment beneath the northeast Tibetan plateau. Our results reveal a distinct crustal and upper mantle structure contrast between the western and eastern part of the northeast Tibetan plateau along two migrated profiles. In the eastern region the Moho is relatively smooth and continuous and varies in depth between ∼80 and 51 km, whereas the depth of the Moho beneath the western region varies between ∼77 and 52 km and is offset ∼20 km beneath the boundary between the Kunlun fault and Qaidam basin. The delay times for the Ps phases from both the 410-km and 660-km discontinuities in the western region are ∼1.2 s later than that in the eastern region, indicating that the average velocities of the upper mantle in the western region are lower than that in the eastern region. Crustal rocks with low- to moderate Poisson’s ratios are interpreted to be dominated by felsic to intermediate compositions, implying that large scale middle-lower crustal flow does not occur easily. The nearly constant thickness of the mantle transition zone, with an average value of ∼255 km (24.4 s) from south to north, implies that no lithospheric fragments correlating to the India–Asia collision have entered into the mantle transition zone.