西秦岭勉略带陆内构造变形研究

Study on the intracontinental deformation of the Mian-Lue suture belt, western Qinling

秦岭造山带勉略缝合带是古特提斯洋盆向北俯冲形成的华北与华南最后拼接带。这个主缝合带俯冲-碰撞过程中以由北向南的一系列韧性逆冲推覆构造为特征,形成由前泥盆系、泥盆-石炭系和蛇绿混杂岩等不同构造岩片叠置的复杂构造带,碰撞时代从245Ma一直延续到230Ma左右。最近,作者对勉略缝合带内发育的韧性和脆性左行走滑剪切变形进行了研究,结果表明这些顺造山带的左行韧性走滑剪切变形带的变形时代为223±2Ma,与碰撞后花岗岩所确定的碰撞后构造环境的起始时间(225Ma)一致,显示这些韧性走滑剪切变形带是勉略带陆内变形初期变形产物。亦即华北、扬子大陆碰撞之后很快就转入陆内变形阶段,并且是以顺造山带的侧向走滑位移为主要变形方式。勉略带内顺造山带的脆性左行走滑断层的发育,表明这种顺造山带的侧向位移过程从深部到地壳浅层是一致的。因此,大陆碰撞在直接碰撞之后很快转变为顺造山带的侧向走滑位移为主的陆内变形,这种位移可能表现为两个大陆碰撞后的相对走滑,或是碰撞带中强烈变形部分顺造山带的侧向挤出,从而消减了正向碰撞所造成的地壳缩短和增厚。

The Mian-Lue suture belt in western Qinling orogenic belt,which is formed from the paleo-tethys ocean subducted northdirection, is the latest assemble belt between the north and south China plate.The subducted-collisional process of this main suture belt is charactered by a series of ductile thrust-nappe structures.This process formed a complex tetonic belt imbricated with different tetonic slabs,such as the Predevonian slab,the Devonian-Carboniferous slab and the ophiolitic melange slab,etc.And the coUisional process was continuous from 245Ma to 230Ma.Recently,the author studied the ductile and brittle left-lateral shear deformation in the Mian-Lue suture belt,and indicated that the timing of the sinistral shear deformation along the orogenic belt is 223±2Ma.This age consistents with the initial age(225Ma) of the post-collisional tectonic environment that determined by the post-collisional granite.And we consider that the ductile strike-slip shear deformation is the earliest intracontinental deformation process.It means that the collision between the Yangtze and north China plate switched to the intracontinental deformational stage rapidly.And the mainly deformational mode is lateral slip displacement along the orogenic belt.In addition,the growth of the brittle left-lateral strike-slip faults along the orogenic belt in the Mian-Lue suture belt,indicates the process along the orogenic belt from the deep earth to shallow crust is uniform. Therefore,the continental collision quickly switches to intracontinental deformation with slipped displacement after the direct collision. This displacement represents the slip between coUisional continents relatively or the lateral extrusion of the intensively deformationed part of the coUisional belt along the orogenic belt.This mode reduced the shortening and thickening of the crust within the collision.