Eastward extrusion and northward expansion of the Tibetan Plateau—Discussions for the deep processes of the plateau uplift

Abstract

The weak materials beneath the Tibetan Plateau have been escaping and extruding eastward from beneath the thickened and elevated central plateau driven by continuous convergence between India and Eurasia. This paper presents a detailed shear wave velocity cross-section image of crustal and uppermost mantle structure across the Tibet-Qinling transition zone that was generated from a dense linear seismic array. Based on our inverted seismic image together with previous geological and geophysical studies, we discussed about the material extrusion of the Tibetan Plateau, unveiling the depth extent in which the Qinling belt (which could serve as a flow channel) may accommodate the extrusion of the ductile material beneath the Tibetan Plateau, and suggested that how these dynamic processes might accommodate plateau uplift and expansion in the eastern margin of the Tibetan Plateau; furthermore, we compared the mechanisms of crustal deformation/thickening and the behavior patterns of lithospheric mantle between the eastern and northern margins of the Tibetan Plateau. Integrating our seismic image with previous observations, we can obtain the following findings: (1) A vigorous low velocity zone (LVZ) ( V s V s<4.2 km/s) resides beneath the Tibet-Qinling/Sichuan boundary area, contrasting to the high-velocity upper mantle to the east beneath the Qinling block and Sichuan basin. A synthetic analysis based on these findings and previous geological/geophysical observations leads to the corresponding conclusions: (1) Middle to lower crustal flow accompanied by regional fault-related strike-slip/thrusts may be occurring beneath the eastern margin of the Tibetan Plateau, and may be a significant deep seismogenic mechanism of the large earthquakes occurrence in the eastern margin of the plateau, but it has not vastly extruded farther beyond the Tibet-Qinling boundary into the lower crust of the Qinling belt. (2) The hot asthenospheric mantle material beneath the Tibetan Plateau may escape eastward driven by convergence/squeezing between India-Eurasia continents and flow to the Qinling belt, accelerating the delamination or thermal erosion of the base of the lithosphere beneath the Tibet-Qinling/Sichuan boundary area. Combination of the preceding two mechanisms, i.e., the extrusion of ductile middle to lower crustal materials accompanied by regional fault-related strike-slip/thrusts and isostatic buoyancy resulting from lithospheric detachment (triggered by asthenospheric flow), may have jointly engendered the plateau uplift and expansion in the Tibet-Qinling and Tibet-Sichuan transition zones. In contrast, uplift of the Qilian Shan in the northern margin of the Tibetan Plateau significantly depends upon upper crustal shortening processes above a weak midcrustal decollement, which is subject to the North China plate underthrusting beneath the plateau. Obviously, the deep process of material extrusion in the eastern margin of the Tibet Plateau is distinguishable from the deep mechanism responsible for the northward expansion within the Tibet-Alxa transition zone in the northern margin of the plateau. The fundamental geodynamic mechanism leading to the mode discrepancy of plateau uplift between the eastern and northern margins of the Tibetan Plateau may be attributed to the lateral variation of the nature of the lithospheres from central and eastern Tibet (interior part of the plateau) to the northern margin of the plateau. A relatively hot/weak Tibetan lithosphere underlies the Songpan-Ganzi and Qiangtang blocks in the central and eastern Tibet, while a relatively cold/rigid Asian lithosphere underlies the Qaidam and Qilian blocks (as well as the northern bordering Alxa block) in the northern margin of the Tibetan Plateau, with the boundary extending along the Kunlun-West Qinling range on a large-scale view.