印度-亚洲碰撞:从挤压到走滑的构造转换

Indo-Asian Collision:Tectonic Transition from Compression to Strike Slip

印度-亚洲板块碰撞导致喜马拉雅山脉的崛起、青藏高原的生长、两倍于正常地壳厚度的巨厚陆壳体，以及大量青藏高原腹地的物质沿着大型走滑断裂朝东、东南、西的方向逃逸。印度-亚洲碰撞如何造成板块汇聚边界由挤压到走滑的构造转换对认识大陆岩石圈的变形机制具有重要意义。本文通过总结喜马拉雅造山带及青藏东南缘～55Ma以来的构造、变质、岩浆记录，发现高喜马拉雅的挤出起始于始新世加厚的喜马拉雅造山带中-下地壳的部分熔融，受控于渐新世以来同期发育的向南逆冲和平行造山带的韧性伸展，并建立了高喜马拉雅“三维挤出”构造模式。晚始新世以来，羌塘地块和拉萨地块的物质通过“岩石圈横弯褶皱和壳内解耦”的运动学机制，围绕东构造结发生顺时针旋转并向青藏高原东南缘逃逸。结合东南亚板块重建的资料，我们认为：印度-亚洲的“陆-陆碰撞”到印度洋板块一亚洲东南大陆的“洋-陆俯冲”的转换是导致从印度-亚洲主碰撞带的挤压到青戚东南缘走滑转换的根本原因。

The Indo-Asian collision caused uplift of the Himalayan orogen, growth of the Tibetan plateau, a 70--km-thick continental crust, and large amounts of materials escaping from the Tibetan plateau towards east, southeast and west along large scale strike-slip faults. How the Indo-Asian collision controlled tectonic transition from compression to strike slip in the convergent boundary is critical for our understanding of deformation mechanisms of the continental lithosphere. On the basis of structural, metamorphic and magmatic records in the Himalayan orogen and SE Tibetan margin since 55 Ma, we establish a 3D kinematic extrusion model of the Greater Himalayan Complex. Extrusion of the Greater Himalayan Complex was triggered by partial melting of the middle and lower crust of the Himalayan orogen in the Eocene, and accommodated by coeval southward thrusting and orogen-parallel ductile extension since the Oligocene. Since the late Eocene, the materials of Lhasa terrane and the Qiangtang terrane rotated clockwise around the eastern structure and escaped toward the southeastern margin of the Qinghai-Tibet plateau, which follows the kinetic theory of lithospheric bending folds and mid-crustal decoupling. In combination with data of plate reconstruction in Southeast Asia, we propose that the transition from the Indo-Asian ＂ continent-continent collision＂ to the Indian Ocean-SE Asian ＂ oceanic- continental subduction＂ was the preliminary cause resulting in the tectonic transition from compression of the Indo-Asian collision zone to strike slip in the SE margin of Qinghai-Tibet.