THE QUATERNARY TECTONIC EVENTS IN THE YANQING FAULTING-DOWN BASIN, BEIJING

The Yanqing faulting-down basin, with NEE extension, is located at the southern margin of Yan-shan Mountain. The basin has developed since the late Cenozoic, and can be regarded as one of the tension basins in the northeastern tip of Shanxi shear faulting zone. According to detailed investigation of landform evolution and sedimentation variation, a compendium outline of neotectonic deformation of the basin has been suggested as fellows. There were three major tectonic events which controlled the evolution of the basin in 500ka B. P.. It is confirmed that the oldest one of lake sediments which were exposed on the surface in the basin was accumulated in the middle Pleistocene. The first event occurred in the middle period of Middle Pleistoncence and its distinguishing feature is the slipping down of the northern margin fault and the uplifting of the entire basin again. The second event took place between Middle and Late Pleistocene, representing uplifting of the basin and the around mountains, and re-action of normal faulting along the northern margin. The third event occurred in the late period of Late Pleistocene and has the similar features to the second one. Many proper samples for dating of the three events mentioned above were collected. Based on ESR dating, TL dating, IRSL dating and ¹⁴C dating of the samples, age estimates of the three events have been obtained as fellows: the first event in 0.40-0.35 ma BP., the second event in 0.15-0.10 ma BP. and the third event in 18-12 ka BP. respectively.