青藏高原中部第四纪左旋剪切变形的地表地质证据

THE EVIDENCE FOR QUATERNARY LEFT-LATERAL SHEAR DEFORMATION IN CENTRAL QINGHAI-TIBET PLATEAU

在青藏铁路的格尔木—拉萨段进行的活动断裂调查发现,在沱沱河—五道梁之间宽约150km的地段内发育了多条由北西西向次级断层左列分布构成的北西西向和北西向左旋张扭性断裂带,在断裂带之间则发育"S"型的北东向裂陷盆地和雁列分布的菱形裂陷盆地,盆地边界断裂也为左旋张扭性质。上述断裂带和裂陷带主要形成于第四纪,它们构成了宽约150km的不均匀的左旋简单剪切变形域,该变形域的整体活动性较弱,属于弱的不均匀剪切变形域。但其中的二道沟断陷盆地是个例外,该盆地边界断裂的垂直活动速率约为0 5mm/a,左旋活动速率介于0 8～1 0mm/a之间。而在整个左旋剪切变形带累计的左旋走滑速率不会超过6mm/a,它们所调节的昆仑山与唐古拉山之间的地壳南北缩短量也可能仅占总缩短量的15%～30%。上述弱剪切变形域与强烈左旋走滑的昆仑断裂系共同构成了高原中部的左旋剪切变形带,它们在印度板块与欧亚板块强烈碰撞的构造动力学背景下,起着调节青藏高原南北向缩短的重要作用。

Based on evidence of Quaternary sediments and active faulting collected between the Kunlun and Tanggula ranges along the Qinghai-Tibet railway and detailed observation of active faults, authors think that left-lateral strike-slip faulting with large component of normal faulting has been the dominant tectonic regime between the Tuotuo river and Kunlun range during Quaternary.From north to south,the Wudaoliang graben,the Wudaoliang southern fault zone,Fenghuoshan northern fault zone, Erdaogou graben, Wuli northern fault zone,the Caco lake and Zhaliwaco lake grabens and Wuli southern fault zone distribute between the Wudaoliang and the Tuotuo river.The Wudaoliang graben and Erdaogou graben are 'S'-shaped and the Caco lake and Zhaliwaco lake grabens are rhomb-shaped, and all grabens are approximately NE-trending. Other faults are WNW-ESE striking or NW-trending and are made up of several left-lateral strike-slip faults with a regular left-lateral en échelon array. Along both the Erdaogou graben margins,Late Quaternary alluvial fans are offset by active normal faults with prominent component of left-lateral strike-slip.The vertical and horizontal slip rates are about 0.5 mm/a and 0.8 to 1.0 mm/a respectively along the faults both the Erdaogou graben margins. The vertical and horizontal slip rates of other faults are more small than those of the faults both the Erdaogou graben margins.The faults result from heterogeneous left-lateral simple shear and form low shear zone which is about 150 km wide from north to south.At the north of the slow shear zone,the Kunlun fault system,which is the most prominent left-lateral slip fault system,forms the high shear zone.The right-lateral shear zone consisting of the south-north-trending rift zones is located at the south of the slow shear zone.The right-lateral and left-lateral shear zones indicate eastward extruding of the Qinghai-Tibet block and absorbed the approximately 30% of the total India-Eurasia convergence on the basis of the results of GPS measurements.