Late Quaternary Activity Characteristics Of The Chongba Yumtso Normal Fault In The Southern Section Of The Yadong-Gullu Rift,Tibet

The Yadong-Gulu rift is the largest active rift within the southern Tibet Plateau.Although tectonic characteristics and geodynamic implications of normal faulting have been widely discussed,precise constraints on the fault slip rates and their spatial distribution are still scarce to date,hindering full exploration of the deformation mechanism of active rifts in the southern Tibet.In this study,we integrated the interpretations of high-resolution satellites and unmanned aerial vehicle(UAV)-acquired images,and topography with the results of field surveys and RTK-GPS,with the OSL and radiocarbon dating to demonstrate the slip distribution along the Chongba Yumtso fault(CYF)bounding the northeastern sector of the Duoqing Co graben.Then,the vertical slip rate and horizontal extension rate of the fault were estimated,following which the spatial variation characteristics of the extension of the major-boundary normal fault of the southern Yadong-Gulu rift were discussed preliminarily,the nearly EW-trending extensional deformation rate of the southern Tibet rift was re-estimated.The coseismic displacement,rupture distribution and characteristics,earthquake magnitude and time of the prehistoric earthquake relic in Chongba Yumtso section of the northern Duoqing Co graben are determined and analyzed,and the potential earthquake risk of the Duoqing Co graben discussed.The results show that:The CYF fault is an active fault that displaced moraine surfaces and river terraces,with mean the vertical slip rate and extension rate across the CYF are about 1.2 ± 0.1and 1.7(+0.7/-0.5)mm/a since late Pleistocene,respectively;combining the vertical slip rate with the maximum rate,maximum basin-mountain height difference and basin sedimentary thickness,and the size of the graben in the plane view in the southern Yadong-Gulu rift,it was proved that the extension of the major-boundary normal fault in the southern Yadong-Gulu rift displayed a generally northward weakening in spatial variation.The average extension rate across the Yadong-Gulu rift zone is about 2.1(+0.5/-0.6)mm/a,assuming that the long-term extension rates of main rifts in the southern Tibet rift zone are similar and active normal faults among them can be neglected,the total extension rate of southern Tibet to be ?16(+5/-4)mm/a.The good agreement between the 10-year or 100-year scale rates and the 10,000-year scale geological rates prove that the overall extension rate of the southern Tibet rift zone is approximately stable on different time scales.It is reasonable to think that at least an Mw7.1?7.2 earthquake occurred after 3260 a BP in Duoqing Co graben,forming the surface rupture between Chongba Yumtso and Duoqing Co,with a total length of up to 62 km,and a maximum coseismic displacement of 4.5 m.Synthesizing the Quaternary slip rate of the fault,ground strain rate,the historical seismic records of the region,and strong earthquakes triggered by the Great Earthquake of Nepal on the southern Tibet rift,it is inferred that the Duoqing Co graben in the southern Yadong-Gulu rift shows high earthquake risk and the present-day Chongba Yumtso fault segment has accumulated a potential for the occurrence of at least Mw 6.6-7.4 earthquakes.