鄂尔多斯西缘北段大型陆缘逆冲推覆体系

Epicontinental mega thrust and nappe system at north segment of the western rim of the Ordos Block

鄂尔多斯西缘北段是一个自中生代末以来形成的、结晶基底和早古生代大陆边缘沉积盖层同时卷入的巨型陆缘逆冲推覆构造体系。根据区域地层发育、变形岩石属性、冲断层几何学以及它们与联冲断层的关系,大体上可以分为不同形成阶段的3个冲断层构造组合,包括9个次级构造单元（B1-B7,BN,BS）。受冲断层运动自西向东的一致推进,整体呈现一个局部被近东西走向联冲断层切错、向东凸出的弧形：前端为陆缘褶皱冲断带;中部表现为一系列“原地”或“异地”推覆体和冲断席,发育低角度滑脱层和双冲构造;后部又被最晚期的冲断体叠置。侏罗-白垩纪为逆冲推覆构造的主要发展阶段,经历了3期主要的冲断层作用。第Ⅰ期发生在侏罗纪末,沿阿拉善-华北两类不同性质结晶基底之间的主滑脱面发生大规模冲断层作用,形成桌子山-岗德尔山褶皱冲断带。第Ⅱ期冲断层作用的持续位移,形成了具有上、下两个构造层的石嘴山-尖山大型异地推覆体,主滑脱面为石炭纪煤系地层,其中发育典型的双冲构造。新生代（距今65Ma）以来,印度-欧亚板块挤压碰撞和青藏高原早期向北推挤,加剧了鄂尔多斯西缘逆冲推覆构造的进一步发育,第Ⅲ期冲断层作用在东部陆缘褶皱冲断带形成了苏海图反冲构造的同时,在西部将异地推覆体下部的奥陶系再次推至地表。第Ⅰ期和第Ⅱ期冲断层作用累计位移幅度可能达到60~80km,第Ⅲ期冲断层作用的位移幅度为8km。相邻冲断席之间位移矢量的差异,通过近东西走向的联冲断层得到了调整。

A mega thrust-nappe system which has formed since Late Mesozoic are preserved at north segment of the western rim of the Ordos Block, in which both crystalline basement rocks and Early Paleozoic epicontinental sediments were involved. Three tectonic domains, including nine sub-tectonic elements（B1-B7 , BN and Bs） , formed in different stage of the thrust-nappe developing were subdivided according to their stratigraphic records, deformation properties of involved rocks, geometry of the thrusts, and relationship with alternation link thrust. The system showed an overall arc-shaped structure forward eastwards, locally sheared by east-west trending link thrusts, resulting from an eastwards promoting of the consistent thrust movement： a fold-thrust zone in the front, followed by a series of autochthonous or heterochthonous nappes and thrust sheets, in which low-angle detachment zones and duplexes were developed, and fi- nally overlapped at the back of the system by latest thrust sheet. The mega thrust-nappe system was mainly developing during Jurassic-Cretaceous and underwent at least three major thrusting events. The first thrusting event was approximately occurred at the end of Jurassic, during which a principal detachment probably formed along the boundary between two different basement of the Alxa Terrain in the west and the North China Terrain in the east, that resulted in a fold-thrust zone of Zhuozishan-Gangdeershan forming. A large scale heterochthonous nappe of Shizuishan-Jianshan with upper and lower tectonic layers was formed during the second thrusting event. These tectonic layers separated by a low-angle detachment developed in Carboniferous system with typical duplexes. Since Cenozoic（ca. 65 Ma）, the mega thrust-nappe system went a step further along with collision between the Indian and China plates, as a result of a northwards pushing of the Qinghai-Tibet Plateau. A third thrusting event pushed the Ordovician strata, which was previously covered by heterochthonous nappe, up to surface again at western side of the system and, at the same time, the Suhaitu back-thrusts were created in the Zhuozishan-Gangdeershan fold-thrust zone. Distance of the thrust sheets and nappes movement might be up to 60 to 80 km during the first and second thrust events, and up to 8 km during the third event. Difference of thrusting movement vectors at vicinal sheets was adjusted by series nearly east-west trending link thrusts.