Upper mantle structure beneath the northern South-Nouth Seismic Zone from teleseismic traveltime data

The teleseismic waveform data used in this paper were recorded by 678 portable broadband seismic stations located on the northern South-North Seismic zone from October 2013 to April 2015. The 130309 P wave traveltime residuals were picked with the waveform correlation method. A three-dimensional upper mantle P wave velocity model beneath the northern South North Seismic Zone (30 degrees N-44 degrees N, 96 degrees E-110 degrees E) with ahorizontal grid space of 0.5 degrees x 0.5 degrees was obtained by teleseismic tomography, which reveals significant inhomogenities and strong block characteristics. Velocity structures of the lithosphere beneath western and eastern part of the area show different characteristics: The northeastern margin of Tibetan Plateau, including Qilian block, western Qinling and Songpan-Ganzi blocks, show obvious low velocity anomaly; the Ordos block and the Sichuan Basin, which are cratons, show high velocity anomalies. This comparison indicates that the eastern stable cratonic blocks (the Ordos block and the Sichuan Basin) may obstruct eastward flow of the Tibetan Plateau material. Western Alashan shows low velocity anomaly, and Eastern Alashan, which is adjacent to the Ordos, shows high velocity anomalies, indicating the deformation of the lithosphere in the area may be mainly affected by the extrusion of the northeastern margin of the Tibetan Plateau. Low velocity anomalies are dominant at the depth of 100 similar to 250 km beneath the Qinling Mountains, between Ordos and Sichuan basin, which may implicate an asthenospheric channel. There is a low velocity anomaly beneath the Hetao basin in northern Ordos at the depth of 100 similar to 500 km, indicating that the deep hot mantle materials may be upwelling from the mantle transition zone. Upper mantle beneath northeastern margin of Tibetan Plateau shows low velocity anomaly and the mantle transition zone appears obvious high velocity anomalies, suggesting that lithosphere delamination might have taken place beneath the northeastern margin of the Tibetan Plateau.