海原断裂干盐池拉分盆地中央断层的形成机制探讨

A POSSIBLE MECHANISM FOR REVERSE CROSS-BASIN FAULT IN GANYANCHI ASYMMETRIC PULL-APART BASIN ALONG THE HAIYUAN FAULT

拉分盆地内部的"对角线式中央断层",不仅在拉分盆地的消亡过程中发挥着重要作用,还对大地震的发生具有重要控制作用,研究其形成演化具有重要的意义。然而,与拉分盆地相比,专门针对中央断层的研究较少,制约了人们对拉分盆地乃至走滑断裂带构造演化过程的理解。文中以海原断裂带中段的干盐池拉分盆地为例,对盆地内的中央断层开展了地质地貌调查、浅层人工地震勘探和钻孔联合探测等工作,着重对该断层的性质和形成机制进行了探讨,获得的主要认识有:1)与前人的认识不同,干盐池盆地中央断层为一条倾向SW的逆走滑断层;2)干盐池盆地为一不对称拉分盆地,其形成演化主要受盆地北缘的南-西华山北麓断层控制,盆地内堆积了厚度> 680m的生长地层且构成了翻转背斜;3)干盐池拉分盆地的实例表明,逆走滑中央断层的形成机制可能是"截弯取直"作用,即初期发育的反向正断层在截弯取直后吸收了边界断层的逆走滑位移而形成,而翻转背斜可能对中央断层的倾向发生旋转有一定影响。

Pull-apart basin and push-up structure are the two most common and important structures formed within a strike-slip fault system.The term of pull-apart basin was firstly introduced when researchers discussing the formation of Central Death Valley,California.A pull-apart basin typically forms and develops between en echelon surficial strands or along a releasing bend of the transform fault.The diagonal cross-basin fault,formed diagonally within a pull-apart basin,connects the two en echelon strands bounding the basin.This type of fault not only plays an important role in the extinction of pullpart basin,but also controls the occurrence of large earthquakes.Therefore,it is of great significance to study the formation and evolution of cross-basin faults.However,compared with extensively studied pull-apart basins,fewer studies have been conducted on cross-basin fault,which greatly hampers our understanding of the structural evolution process of pull-apart basin and strike-slip fault.In this study,we take the Ganyanchi(Salt Lake)basin,the largest pull-apart basin located in the central part of the Haiyuan Fault,northeastern corner of the Tibetan plateau,as an example to investigate the character and general mechanism of the cross-basin fault.Geomorphological investigation,shallow artificial seismic exploration,and composite drilling geological survey are carried out along the cross-basin fault in Ganyanchi Basin.The main conclusions are listed as below:1)The cross-basin fault in Ganyanchi Basin is a reverse strike-slip fault dipping to SW,rather than the previously claimed a strike-slip fault with significant normal component;2)Although with a classic rhombic shape,the Ganyanchi Basin is actually an asymmetric pull-apart basin,which is mainly controlled by the northern boundary fault,i.e.,the Nan-Xi Hua Shan Fault.Under the control of the Nan-Xi Hua Shan Fault,more than 680 m thick growth strata were accumulated in the basin,and a rollover anticline was formed by the growth strata;3)The example of the Ganyanchi asymmetric pull-apart basin suggests that a possible mechanism for the reverse cross-basin fault is probably the"straightening of strike-slip fault",that is,the earlier formed antithetical normal fault adopts reverse component of straightened basin boundary fault,and then undergoes rotation and finally becomes a synthetical reverse fault with great strike-slip component.The rollover anticline,mainly controlled by the master boundary fault of an asymmetric pull-apart basin,may also partly contribute to the rotation of the cross-basin fault.As more natural pull-part basins needed to be investigated,caution should be taken when this suggested model is applied elsewhere.