3D lithospheric structure of the Qinghai-Tibet Plateau and hydraulic pressure machine model of the plateau uplift

Abstract: The lithosphere beneath the Qinghai-Tibet Plateau may be divided into three areas with different geophysical characteristics by the Kunlun fault and the Yarlung Zangbo suture. To the north of the Kunlun fault is the Qinghai Plateau, which is a basin and range area with contiguous gravity highs and gravity lows. To the south of the Yarlung Zangbo suture is the southern Tibetan Plateau, which belongs to the Indian plate, covered by continental-margin sediments of the Indian plate. Its crustal structure is marked by a south-vergent thrust nappe system. The Main Himalaya Thrust(MHT) found in southern Tibet by the INDEPTH reflection profile coincides with the T4 reflector obtained by wide-angle seismic reflection fanshooting. The fanshooting profile extends through the Himalaya, but the T4 reflector is not displayed there. The extension of the MHT to the Higher Himalaya is merely a deduction. Therefore, whether the MHT does exist needs new evidence. The region from south of the Kunlun Mountains to the Yarlung Zangbo suture is the northern Tibetan Plateau, where the partially melted, strongly rheological lithosphere occurs. The area of partial melting is funnel-shaped. The prevalent low-velocity layer in the upper crust at 15～20 km depth in northern Tibet is the most rheological partially melted layer. With a persistent burial depth, this layer probably contains abundant water. Immediately below the aforesaid partially melted layer, there appears netlike inhomogeneous partial melting. The depth of the base of the partially melted body increases gradually from 80 km in the Yarlung Zangbo River northward to 200 km. The bottom tube of the funnel is situated at Qiangtang-Hoh Xil. The partially melted body in northern Tibet is formed by the back-arc high heat flow in the Qiangtang-Hoh Xil area produced by high-angle subduction of the Indian plate along the Yarlung Zangbo suture beneath the Qinghai-Tibet Plateau as the northward movement of the Indian plate was hindered by the Asian plate. Satellite gravity anomalies, aeromagnetic anomalies, seismic receiver function study, geochemical data and surface geological observations all show that the subduction of the Indian Plate along the Yarlung Zangbo suture only occurred in western Tibet west of the Yadon-Tanggula line, while in eastern Tibet east of the Yadon-Tanggula line, there only occurred collision between the Indian plate and the Tibet block rather than subduction of the former. The wholesale uplift of the Qinghai-Tibet Plateau resulted from the effect of hydraulic pressure. The Qinghai-Tibet Plateau was like a hydraulic pressure machine and the various stresses produced during the subduction of the Indian plate were transferred through partially melted rocks to the partially melted layer at 15～20 km depth, forming an equi-pressure surface. Driven by this equi-pressure surface, the bottom of the not partially melted crust above the low-velocity layer was uplifted synchronously. At the end of the plateau uplift, the collapse of the plateau caused the upper crust to flow at all sides, thus forming a series of NE-directed imbricate structures on the Qinghai Plateau, a series of arcuate thrusts convex southward south of the Yarlung Zangbo. Between the Kunlun fault and Yarlung Zangbo suture the eastward flow resulted in the formation of detachments in the upper crust. Although the formation of the Qinghai-Tibet Plateau is due to the subduction of the Indian plate, its uplift is not merely a rigid dynamic problem, but more importantly we should consider the fluid processes and cannot explain the plateau uplift mechanism simply using the plate theory based on rigid bodies. The deep part of the Tibetan Plateau is a vast heat reservoir; so the exploitation and utilization of the heat reservoir are an important research subject.