Lithospheric Structure beneath Northeastern Tibet Plateau and Sichuan Basin revealed by S-Receiver Function Imaging 

Abstract

    Based on the seismic data of 60 portable stations and 90 permanent CEA stations in the northeastern Tibet Plateau and adjacent regions, we utilized the wave equation post-stack migration method of S-receiver function to image the lithospheric structure of northeastern Tibet Plateau and the Sichuan Basin. Fig. 1 Distribution of the seismic stations and imaging profile     The imaging results show that the Moho in the northeastern Tibet Plateau is deeper than 50 km, and it gradually becomes shallow along the profile to the southeast until reaches about 45 km below the Sichuan Basin. The negative anomaly signals corresponding to the Lithosphere and Austhenosphee Boundry (LAB) are obvious in most areas, but under the Sichuan Basin, there are many strong negative anomaly signals in the migration images of different frequencies. In general, the LAB along the profile is undulating and discontinuous: The lithosphere is deeper in the southern Qilian Orogenic Belt, up to ~200 km, with no significant change at the boundary between the Qilian Orogenic Belt and the western Qinling Orogenic Belt. The lithosphere gradually thinned to ~150 km beneath the western Qinling Orogenic Belt, with a step of ~100 km at the tectonic boundary between the Qinling Orogenic Belt and the Songpan-Garze block, and the signal intensity is obviously weakened. LAB was maintained at this depth level until near the Longmenshan Fault, and the lithosphere thickened again to ~190 km after entering the Sichuan Basin. Moreover, there are two discontinuities within the lithosphere of the Sichuan Basin, with depths of ~100 km and ~140 km, respectively, and the latter becomes shallower to ~110 km in the western margin of the Sichuan Basin. Our observations of mid-lithospherci discontinuity (MLD) beneath the Sichuan Basin provide further evidence that the cratonic lithospheric mantle is generally stratified. Fig. 2 The migration results of the profile     It is proposed that the lithospheric thinning along the eastern margin of the Songpan-Garze Block may be related to the eastward flow of hot mantle materials beneath the eastern Tibet Plateau. Blocked by the Ordos block and the Sichuan Basin, which have preserved the ancient and rigid craton roots, the eastward flow of mantle materials from the Tibet Plateau will turn to the west of the two blocks. A small part of the blocked mantle material migrates eastward to the Qinling Orogenic Belt, while most of it migrates southward clockwise along the mantle flow path to the west of the Sichuan Basin. The lithosphere in the eastern margin of the Songpan-Garze block, heated by the mantle flow, will be subjected to thermochemical erosion and destruction in the process of collision with the Yangtze craton, and is more likely to be delaminated, resulting in significant thinning and destruction under long-term action.