青藏高原岩石圈三维结构及高原隆升的液压机模型

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

青藏地区可以昆仑断裂和雅鲁藏布缝合线为界分为3个岩石圈地球物理特征各不相同的区域:青海高原、藏北高原和藏南高原。青海高原位于昆仑山脉以北,是重力高和重力低毗连出现的盆山结构。藏南高原位于雅鲁藏布江以南,是印度板块分布的地区,其上是印度板块的陆缘沉积。它的地壳结构是一个向南运动的逆冲推覆系统。INDEPTH反射剖面在藏南发现的主喜马拉雅逆冲断层(MHT)与宽角反射地震扇形剖面得到的T4震相反射面完全吻合。两种地震测深方法得到的结果之间不存在矛盾。T4震相在高喜马拉雅地区没有显示,MHT向南延伸到高喜马拉雅只是一个推论,因而MHT是否为印度板块的俯冲带仍有待于获取新的证据。在昆仑山脉以南到雅鲁藏布缝合带为藏北高原,是广泛发生局部熔融的强流变岩石圈。局部熔融地区呈漏斗状。在藏北广泛存在的深度为15～20km的上部地壳内的低速层是一个最富于流变性能的局部熔融层,它的埋藏深度平坦稳定,可能含大量水质流体。紧挨着上述上部壳内局部熔融层,在藏北岩石圈大范围出现分布不均匀的网状局部熔融。局部熔融体的底部从雅鲁藏布江地区的80km向北逐步加深到200km。漏斗的漏管处位于羌塘—可可西里。藏北局部熔融体的形成是由于印度板块向北运移,受到亚洲板块的阻挡,沿雅鲁藏布缝合带向青藏高原高角度俯冲,在弧后羌塘—可可西里地区产生高热流上升地幔所致。根据卫星重力异常、航空磁测、地震接收函数研究、地球化学资料以及地表地质均揭示,印度板块沿雅鲁藏布缝合带的俯冲仅发生在亚东—唐古拉一线以西的西藏西部。在亚东—唐古拉一线以东,印度板块与西藏块体间仅仅发生碰撞,但没有发生俯冲。高原的整体隆升是由液压效应所造成。青藏高原的隆升像一台液压机。印度板块对青藏俯冲过程中产生的各种应力,通过局部熔融体,传递到地壳深15～20km处的熔融层,在其下形成一个等压面。在这个等压面的驱使下,在低速层以上未被局部熔融的地壳的底部均匀受力,将它们同步向上抬升。高原隆升期后的跨塌,使上部地壳向四周流动。在青海高原,造成毗连阿尔金断裂的一系列由西南向北东方向推动的叠瓦构造。在雅鲁藏布江以南地区,形成一系列向南凸出的弧形逆冲断层。在昆仑山脉与雅鲁藏布缝合带之间,向东的流动便形成上部地壳的滑脱构造。虽然青藏高原的形成是由于印度板块的俯冲,但它的隆升机制不单纯是一个刚体力学问题,更重要的要考虑到流体的作用,简单的用以刚体假设为前提的板块学说去解释高原的隆升机制是青藏高原研究中的误区。西藏高原的深部是一个大热库,西藏热储的开发利用是一个重大的研究课题。

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.