THE LATEST TECTONIC DEFORMATION AT THE GROWING POINT OF A NEW FRACTURE ON THE EASTERN TIP OF THE ALTYN TAGH FAULT
阿尔金断裂东端破裂生长点的最新构造变形*

The convergence of the Altyn Tagh Fault on the northern edge of the Qinghai-Xizang Plateau and the North Marginal Fault of the Qilian Mountains is considered to be the growing point of a new fracture of the eastward propagation of the Altyn Tagh Fault.The two faults together with the newly-generated Hongliuxia Fault define a triple junction structure.The latest tectonic deformation at the triple junction structure displays the following characteristic features: The Zhaobishan block which is bounded by the main fault and the branch faults of the same strike on the eastern tip of the Altyn Tagh Fault was dominated by rotation.The main fault propagated outward to form an arcuate shape,while the newest shear fractures occurred on the trail of the main fault.The branch faults are constrainedly bent.The Northern Marginal Fault of the Qilian Mountains was dominated by compression-contraction-detachment deformation,which resulted in a series of imbricate thrust faults and folds.Moreover,horizontal displacement of variable amplitude along different directions occurred on the different segments of the fault.Among them,the western segment was characterized by right-lateral strike-slip,while left-lateral strike-slip became predominant on the eastern segment.The Hongliuxia Fault and its branch faults have caused the uplift and left-lateral strike-slipping of the Hongliuxia Block,which is confined by the faults.The faults along with the Cenozoic strata seized by the faults converge toward the eastern tip of the Altyn Tagh Main Fault,indicating an intensive compressive and traction deformation.The lateral escaping structure along the fault seems to be the main factor that caused the formation of the arcuate mountains on the northern side of the Altyn Tagh Fault.It is postulated that the propagation of fracture from west to east along the Altyn Tagh Fault is the process of the successive formation and linking up of the triple junction.The Altyn Tagh Fault is dominated mainly by creep slip,and so the Yumen Earthquake is believed to be related to the movement of the northen marginal thrust fault of the Qilian Mountains.