Faulted Landforms,Slip-rate,and Tectonic Implications Of The Eastern Lenglongling Fault,Northeastern Tibetan Plateau

If there is a continuous Qilian-Haiyuan fault zone in the northeastern Tibetan Plateau,the tectonic movement along the middle part of the Lenglongling fault(LLLF)should directly transition to the Jinqianghe fault,and the eastern section of the LLLF should not be very active.If 1927 Gulang earthquake commonly cross the junction of LLLF and JQHF to rupture both faults within the same earthquake,then the slip-rate along the eastern section of the LLLF(being east of the fault junction)should be distinctly smaller than the slip-rates along the middle part of the LLLF and the JQHF.How strain is transmitted from the LLLF to JQHF over geologic time scales is poorly understood.So the analysis of tectonic geomorphology and the slip rate of the eastern section of the LLLF may break the past understanding of the continuous Qilian-Haiyuan fault zone and the Tianzhu seismic gap.The study provides scientific evidence for further understanding of the present strain distribution model in the northeastern Tibetan Plateau and the division of future seismic risk zones.The Huoshaotai site,at the eastern side of the LLLF-JQHF intersection,was chosen as a research area to address this question.High precision DEM data was obtained by using Sf M(Structure from Motion)technology.We analyzed the evolution process of different displaced gullies,and the gully G1 basically maintained the geomorphic characteristics of vertical crossing faults.LaDiCaoz?2v2 was used to measure the offset of different faulted geomorphologic markers.In order to determine the abandoned age of faulted geomorphologic markers,excavating trenches and collecting and dating radiocarbon were carried out on the geomorphologic unit.We established the correspondence between offset and abandoned age by reconstructing the evolution model of offset landforms.The Monte Carlo analysis is a method that enables slip rate determination while statistically incorporating displacement and uncertainties.The sinistral horizontal geological slip rate of the eastern LLLF was limited to 6.0±0.8 mm/a based on Monte Carlo method.The sinistral horizontal slip rate obtained from the GPS velocity profile of the eastern LLLF is 4.9±0.2 mm/a.Combined with the variation trend analysis of GPS slip rate,it can be considered that the variation trend analysis of geologic and geodetic slip-rates shows that the slip-rates on the eastern section of LLLF are similar with those along middle section,which suggests that this portion of the fault is not commonly bypassed by ruptures crossing from middle LLLF to JQHF or vice versa.This is an indication that multi-fault rupture across LLLF-JQHF intersection is not common.By extrapolation,these observations may tell us about multi-fault rupture probability along Qilian-Haiyuan fault as a whole.A NE trending normal fault developed at the easternmost segment of the LLLF,which terminated the surface rupture zone of the1927 M8.0 Gulang earthquake and cut off the eastern extension of the major part of the Northern Qilian Shan.From Zhuanglang River to Haxi Town,there is probably a near NS trending persistent seismic tectonic boundary zone,in which two seismic tectonic units with relatively independent dynamic processes are formed on the east and west sides,which gave birth to the 1920 M8.5 Haiyuan earthquake and the 1927M8.0 Gulang earthquake,respectively.The result of this study do not support the existence of a Qilian-Haiyuan fault zone characterized by "horizontal transmission" continuous deformation,nor the Tianzhu seismic gap with a length of about 260 km and the risk of large earthquakes(M?7.5)in the future.