Late Quaternary Activity And Tectonic Deformation Kinematics Of Dextral Strike-slip Faults In The Northeast Margin Of The Tibetan Plateau,China

Since the late Cenozoic,the northeastern margin of the Tibetan Plateau has been intensely active under the distant effect of the collision between the Indian and the Eurasian plates.It is the front of the outward growth and uplift of the plateau.The region has developed rich strike-slip fault systems with different trends,between which the interaction involves regional strain distribution,tectonic transformation,and the mutual influence between seismic faults of different strong earthquakes.The study of active faults with different trends is not only of great significance for seismic hazard assessment,but also a key to understanding the deformation patterns of the Tibetan Plateau.However,previous studies mainly focused on the EW-trending strike-slip boundary faults,and relatively few studies on the dextral strike-slip fault system,which hindered the further deciphering of the plateau deformation model.The study selects the NNW-trending right-lateral strike-slip faults between the Haiyuan fault and the Kunlun fault as the research objects.Based on the fine quantitative research method of active tectonics,the paper accurately determines the geometrics of the Elashan and Riyueshan faults,as well as the spatial pattern of the slip rate since the late Quaternary.At the same time,combined with the evolution history of the faults in the NE margin of the Tibetan Plateau,as well as the structural deformation features,the formation of the NNW-trending dextral strike-slip faults were reinterpreted.The progress and new insights made in the paper are as follows:(1)Based on the interpretation of high-resolution terrain data and field investigation,the geometrics and kinematics mechanism of the Elashan and Riyueshan faults have been determined in detail.The north and south segments of both faults are not directly connected due to the influence of the EW-trending thrust faults.Secondary thrust faults are developed at the tip area of the faults.In the structure transformation area,the faults gradually turn to NW direction,and flows into the Erlangdong-Chakabei fault and the Buheteshan fault.There is a right-lateral left-step structural uplift in the Wulan segment,which is a typical strike-slip duplexes.The north and south segments of Riyueshan fault are also not connected.The north segment extends southward and gradually joins the Lajishan thrust fault.The south segment splits into two branches near Guide.The Guide segment ends at the south of the West Qinling fault.The immature Waligong segment may be connected with the Qinghai Nanshan thrust fault.In terms of structural pattern,the West Qinling fault separates the respective tectonic transformation areas of the north and south segments of the Riyueshan fault.(2)The late Quaternary slip rates of the whole Elashan fault and the southern Riyueshan fault has been systematically studied.The slip rate of the northern and southern tips of the Elashan fault is about ～1-2 mm/yr and the late Quaternary slip rate of the central area is up to ～3.1 mm/yr.The slip rate of the north segment of the Riyueshan fault is ～2.4 mm/yr,the slip rate of the Guide segment yields to 3.5 mm/yr,and the slip rate of the Duohemao segment is 1.8 mm/yr.The spatial pattern of high in the middle and low in the end of slip rates along these two dextral strike-slip faults was revealed,further supporting the continuous deformation model of the Tibetan Plateau,and indicating that these two faults play a critical role in the off-fault deformation between large left-lateral strike-slip faults of the Tibetan Plateau.(3)By using the low-temperature thermochronology to determine the initiation age of the Elashan fault,and combining with previous discussions based on paleomagnetism and provenance analysis,the evolution history of the main faults in the NE margin of the Tibetan Plateau is synthesized.It is believed that the possible strike-slip initial age of the Elashan fault is the Middle Miocene,while the left-lateral strike-slip of the Haiyuan fault may not have started at this period.The earlier formation of the NNW-trending dextral strike-slip faults may indicate that their formation is unrelated to the shear movement of the Haiyuan fault and the non-rigid passive bookshelf model proposed by previous researchers may not be applicable to the structural deformation between the Haiyuan and Kunlun faults.(4)Based on the comprehensive analysis of the structural deformation features of the NE margin of the Tibetan Plateau,a new insight is put forward that the formation of the Elashan and Riyueshan faults is the result of adjusting the EW differential shortening.The generation of dextral shear zone,associated with the elevation terrain gradient zone bounded by the Elashan and Riyueshan faults,the distribution difference of the thrust fault zone,the northward rate observed by the GPS velocity field gradually decreasing from west to east,the Anemaqen mountain formed by the left-lateral right-step structural uplift of the East Kunlun fault,and regional tectonic rotation,jointly accommodate the difference of east-west structural deformation in the NE margin of the Tibetan Plateau that may be caused by the convective removal of the mantle lithosphere or a toroidal flow in the asthenosphere.