STAGES OF THE NEOTECTONIC MOVEMENT OF THE MENYUAN BASIN IN THE MIDDLE SEGMENT OF THE QILIAN MOUNTAINS

The Menyuan Basin is an intermontane basin in the central segment of the Qilian Mountains,bounded by boundary faults on the north and south.The basin is filled with the Paleogene Baiyanghe Formation and Quaternary tills and glaciofluvial deposits.The differences in rock deformation,geomorphological evolution and fault movement phases show that the Menyuan Basin underwent four stages of tectonic movements since the Cenozoic.The earliest Cenozoic tectonic movement began in the middle Oligocene(30 Ma B.P),when the I-order planation surface was disintegrated and basin was downfaulted and received sandy gravel deposits of the Baiyanghe Formation.The second stage occurred from the end of the Oligocene to the beginning of the Miocene(25 Ma to 460 ka B.P) and was characterized by the end of the basin subsidence,termination of Tertiary deposition and folding of the Baiyanghe Formation.This process continued to the initial middle Pleistocene.The third stage started in the middle part of the middle Pleistocene(460 ka B.P) and ended at 30 ka B.P.During the stage new boundary faults formed, the basin was again downfaulted and received 400 m thick tills and glaciofluvial deposits,and boundary faults were very active.This process lasted till the late part of the late Pleistocene(30 ka B.P).The latest stage began at 30 ka B.P,which was marked by the wholesale uplift of the basin and mountains at both sides of the basin and transition of the basin surface process from deposition to incision and erosion.The fault movement changed from thrusting with a strike-slip component to strike-slip movement,but the strike-slip rate decreased significantly.