RESEARCH ON HOLOCENE PALEOEARTHQUAKES ON THE XIDATAN SEGMENTOF THE EAST KUNLUN FAULT ZONE IN NORTHERN TIBET
东昆仑断裂带西大滩段全新世古地震研究*

The East Kunlun Fault zone in Northern Tibetan Plateau is a typical active fault zone, characterized by sinistral strike-slip faults. The field investigation has reveals a lot of relics along the fault traces of the Xidatan-Dongdatan segment of the Kunlun Fault, including push-up and pull-apart basins, sag pond, and earthquake fault scarps. They imply that the Xidatan-Dongdatan segment of the Kunlun Fault is a seismogenic fault in which surface-rupturing earthquakes often occured in regular or irregular intervals. To determine the chronology of Holocene surface ruptures of this part of the Kunlun Fault, four trenches were excavated across the Xidatan-Dongdatan segment at Xiaonanchuan near Xidatan Town and Jingxian Valley near Kunlun Pass, respectively. All sediment samples collected from the trenches were dated by using thermoluminescence (TL) and optically stimulated luminescence (OSL) methods.Paleoseismic evidence, combined with the optically stimulated luminescence (OSL) and thermoluminescence (TL) dating of sediments and collapse units from the trenches, shows that the Xidatan segment of the Kunlun Fault zone has generated at least 6 earthquakes during the Holocene, whose ages are 10302±651aB.P. , 8650±500aB.P. , 7160±506aB.P. , 2830±170aB.P. , 1985±121aB.P. , and 1540±92aB.P. respectively. The recurrence interval between two adjacent events is 1652±820a,1490±711a,4330±534a,845±209a, and 445±152a respectively. It shows that the recurrence is not uniform with time, that is, the distribution of paleoearthquakes on the Xidatan segment of the Kunlun Fault is characterized by clustering. The Holocene is divided into two subperiods in terms of paleoseismic evidence, the Early Holocene characterized by a recurrence interval of 1571±543a, and the Late Holocene characterized by a higher frequency of tectonic events and a recurrence interval of 645±129a. This result suggests that the Xidatan segment of the fault had displayed a time-dependent seismic behavior. The current research has significant implications for seismic hazard assessment in the Xidatan-Dongdatan segment of the fault, which stretches for 250km. The last large event occurred before 1540±92a, combined with the eastward motion of the Tibetan Plateau along the fault, underscore the possibility that the Xidatan segment of the Kunlun Fault may be a site of major faulting in the near future.