Fault Geometry And Deformation Partition Of WNW-trending Faults In The West Qinling Mountain

The West Qinling mountain lies in the interaction zone among the Tibetan Plateau,the Ordos Block,and the South China Block,which is bounded by the East Kunlun fault in the east,the West Qinling fault in the north,and the Longmenshan fault in the southeast.Since the Late Cenozoic,with northeastward expansion of the Tibetan Plateau,the West Qinling mountain has been experiencing intense tectonic activity,forming a remarkable landform,developing two sets of active faults(trending NWW and NE)on the basis of pre-existing structures.Among them,the NWW-trending fault system(the Bailongjiang fault,Guanggaishan-Dieshan fault,and Lintan-Dangchang fault)is crucial to understand internal deformation and strain partition in the West Qinling mountain,and is of great significance to probe into the tectonic transformation and eastern termination of the East Kunlun fault.This work focuses on three NWWtrending faults in the West Qinling mountain,through quantitative studies on macrogeomorphology and new activity,the long-term deformation history,active fault geometry,slip senses and rates,paleoseismic events,etc.have been analyzed.Combined with the deep extension of NWW-trending faults derived from microseismic profile,geological profile,and deep structures,a new structural model was constructed.Combined results of the structural model and fault kinematics present new indications into the tectonic transformation and eastern termination of the East Kunlun fault.Main results are as follows.1.Regional TopographyRegional topography shows that the west Qinling mountain is divided into the south-and north part by the Dieshan mountain range.South part is characterized by stronger tectonic uplift and downward river incision than the north part,the Bailongjiang watershed was tectonically tilted from south to north.2.Cross-fault topographyThe Bailongjiang fault shows minor long-term vertical offset,which exhibits linear valley predominantly.The Guanggaishan-Dieshan fault shows significant vertical component,which marks the boundaries between the Dieshan mountain range and the main planation surface,the intermontane basin,and the piedmont erosional surface.The main planation surface has been offset several hundred meters and has been tectonically tilted from 1 to 3 degrees.The Lintan-Dangchang fault shows significant vertical motion effects as well,the main planation has been offset ?500 m and been tectonically tilted.All three faults show more significant long-term left-lateral component,and high displacement area is concentrated in the middle segment of NWW-trending faults.3.Fault geometry and kinematicsThrough multi-source remote sensing images interpretation,field investigation,active fault mapping,paleoseismic trenching,differential GPS and UAV photogrammetry,14 C and OSL dating,etc.,this work quantitatively studies the fault geometry and kinematics of the three NWW-trending faults.Results are as follows.The Bailongjiang fault is divided into three segments: the west,middle,and east segment,a 15°clockwise rotation is formed between the west and middle segment,a left stepover is formed between the middle and east segment,within the stepover a pull apart basin(Wuping basin)is formed.The west segment shows a broom-like shape,including at least three branches.The middle segment is straight with good linearity.The east segment includes the south branch and the north branch,and the north branch comprises two secondary branches.This work enriches the geological and geomorphological evidence of late Quaternary activity of the Bailongjiang fault.From Pingya and Getiaoping profiles,the left-lateral dominated slip associated with the north branch of east segment was confirmed.The average vertical-slip rate of north branch fault since late Pliocene is 0.04-0.11 mm/a.The Guanggaishan-Dieshan fault is divided into the west,middle,and east segments.The west and middle segments are separated by the Lazikou left stepover,the middle and east segments are transitioned by the Huama left-turn.The three segments can be subdivided into the south and north branches,and each segment contains 2-3 secondary branches.The west segment runs through bare bedrock mountains and forests,and fault scarps and troughs can be seen on late Quaternary slope fan.The south branch faults of the middle and east segments have been mapped at a 1: 50000 scale,which show a left-lateral dominant slip with minor dip slip.Summarizing left-lateral slip rates at Heiyusi village(<2.6-4.4 mm/a),Laozhuang village(<0.72±0.34 mm/a),and Butuo village(<0.67±0.19 mm/a),vertical slip rate at Butuo village(<0.13±0.03 mm/a),combined with previous studies,it is believed that the overall left-lateral slip rate and vertical slip rate of the Guanggaishan-Dieshan fault is ?1 mm/a and <0.5 mm/a.Four paleoseismic events were determined at three sites(Bagai village,Xiawan Village,and Zhongpai village): 1385-2100 a BP,2765-3320 a BP,12775-13005 a BP and 18495-32950 a BP.The Lintan-Dangchang fault is divided into west,middle,and east segments,the west segment appears to spreading westward,the middle segment is straight,the east segment forms a broom-like shape.This work presents a detailed fault geometry,and reports a new discovered active fault-Xiahe fault,which is closely related to Xiahe Ms5.7 earthquake in 2019.This work reported Holocene activity firstly,we found Holocene fault scarp in the west of Gongqia village,constrained one paleoseismic event at 2090-7745 a BP.Two outcrops(Yugu and Hayekou village)on the Muzhailing fault of the east segment show late Pleistocene-Holocene strata offset as well.The overall slip sense is left-lateral,only some segments are dominated by reverse slip due to tectonic transformation.The left-lateral slip rate of one branch in the middle segment since early Holocene is <0.86-1.10 mm/a,and 0.86-1.65 mm/a on the main fault of the east segment,the vertical slip rate is 0.05-0.10 mm/a.The left-lateral slip rate of Hetuo fault of the east segment is 0.47±0.15 mm/a since middle late-Pleistocene.The overall left-lateral slip rate of the Lintan-Dangchang fault is estimated as 1-2 mm/a.4.Regional structural model and slip partitioningIn the East Kunlun-West Qinling transition zone,the NWW fault system transitions to the East Kunlun fault by a left stepover and intersects the NE fault system,forming multiple closed and open secondary blocks in the West Qinling.Internal strain in the West Qinling is transferred by these blocks.East boundary of the closed secondary blocks(Bailongjiang block,Minxian block)accumulated strain and was released by large earthquakes(1654 South Tianshui M8 earthquake and 1879 South Wudu M8 earthquake),while the open block boundary inherits and continues to transfer strain eastward to the main Qinling orogenic belt.Geological sections,micro-seismicity and deep structures show that the three NWW-trending faults all dip to the north.The Tazang fault,Bailongjiang fault and Guanggaishan-Dieshan fault constitute the Bailongjiang tectonic belt,which is a lowangle subduction system of the Ruoergai Basin beneath the West Qinling Mountain.The Lintan-Dangchang fault extends downward and merges with the steep,southdipping West Qinling fault,they form a high-angle subduction zone from the Longzhong Basin to the West Qinling mountain.Based on regional topography,active faults and tectonic transformation,deep structures,etc.,we believe that the West Qinling orogenic belt is generally controlled by two tectonic belts: the Bailongjiang tectonic belt in the south and the West Qinling tectonic belt in the north,both the Ruoergai Basin and Longzhong Basin subduct beneath the West Qinling,which is the tectonic background of the West Qinling orogenic belt.Ten-thousand-year scale(research on active tectonics)and ten-year scale(GPS,regional stress field,micro-seismicity pattern)exhibit that the NWW-trending faults are all dominated by left-lateral strike-slip,and the sinistral behavior of the Bailongjiang tectonic belt originates from end effects of the East Kunlun fault,the strike-slip of the Lintan-Dangchang fault may partition from the West Qinling fault.The Late Quaternary left-lateral slip rate of the three NWW faults are 1-2 mm/a,?1 mm/a,and 1-2 mm/a,respectively.The decreased slip rate along the East Kunlun fault near the Ruoergai Basin is 1-3 mm/a,which corresponds well with that of the Bailongjiang fault and Guanggaishan-Dieshan fault.What's more,the East Kunlun fault is possibly linked to the Bailongjiang tectonic belt in deep earth,because they are transitioned by a left stepover,within which a series of active faults have been discovered.We hold that the Bailongjiang tectonic belt takes part in the slip partition of the eastern end of the East Kunlun fault,and there is minor component could transfer northward to the LintanDangchang fault.Deformation at the eastern end of the East Kunlun fault is transmitted both to the east and north.As it transfers eastward,deformation is absorbed by the Tazang fault and its lateral structures(Huya fault and Minjiang fault),forming the strongly uplifted Minshan and frequent large earthquakes.As it transfers northward,deformation is partitioned onto the Bailongjiang fault and the Guanggaishan-Dieshan fault,and mainly absorbed and released by Hanan fault and giant earthquakes,a minor component may be transferred to the Liangdang-Jiangluo fault and the Qinling fault.The eastern end of the East Kunlun fault is a broom-like fault system that terminates within the West Qinling mountain.