Late Quaternary Kinematic Characteristics And Paleoearthquakes Along The Western Segment Of The Xiangshan-tianjingshan Fault

The Arc tectonic zone in northeastern Tibet Plateau has a very complex tectonic movementand has many strong earthquakes occurred in history，which located on the junction area amongTibet Block, Ordos Block and Alashan Block. At present, the study about late Cenozoic tectonicformation and evolutionary mechanisms of this area focuses on Haiyuan-Qilian fault zone.However, there are many vital scientific questions to be solved, such as the formation age,expansion mode and deformation mechanisms of the Arc tectonic zone, the process of rupture andfracture propagation direction of each fault, the trigger and response relationship betweenfractures, the earthquake distribution characteristic around the Tianjingshan Sub-bock. Answers tothese questions bear important implication for understanding the outward growth of the TibetPlateau and predicting the locations of large earthquakes in the future.We choose the western segment of Xiangshan-Tianjingshan Fault zone to conduct a detailedstudy, the main aspects including: fault segmetation, offset landforms and the the displacementdistribution, slip rates, paleoearthquakes, fault motion property and tectonic deformationmechanism. And combining the results of previous studies, we have made a preliminary analysisand discussion. The main conclusions are drawn as follows:The Xiangshan-Tianjingshan fault, with a length of200km, shows a central slightlyprotruding arc to the northeast direction. According to geometry, medium structure, activities andrupture history, it can be divided into three significant segments, the western, eastern andsoutheastern segments. The60km-long western segment roughly aligned in E-W directionincludes five sub-segments, such as Jingtai, Guanguanling, Shajing, Zhongwei andQingshan-Gushanzi.To determine the horizontal and vertical offsets distribution characteristic of the westernsegment, we measured240horizontal offsets and62vertical offsets by the means of fieldinvestigation. Through probability density simulation and frequency statistical analysis for thehorizontal offsets, it shows that there is an obvious grouping character and multiple relationships. The six-group horizontal offsets may represent6paleoearthquakes, with a similar event-sequencein the trench excavated on the western segment. The coseismic offsets of the latest earthquake is3m, and the cumulative offsets produced by other older earthquakes is6，9，12，16and20m，respectively, and each earthquake has a similar coseismic offset. Therefore, we suppose that theactivity on the western segment of the Xiangshan-Tianjingshan Fault zone obeys a characteristicslip model since Late Quaternary.Based on the field investigation, we select three typical sites to conduct detailed differentialGPS measurements for the offset landforms, and collect two OSL samples on the geomorphicsurface and terrace for each site repeatedly to define the gully’s yonger and older age. During theslip rates calculation process, we compare the horizontal offsets of multi-level gullies to themultiple successive paleoearthquakes, and correct the vertical offsets of the fault scarp.Meanwhile, we use the ages of geomorphic surface and terrace, the ages of paleoearthquakes toqualify the slip rates, which is closer to the real value. The results show that the horizontal andvertical slip rate of the western segment is0.80±0.04mm/a, and0.05±0.01—0.22±0.06mm/arespectively.It reveals at least six paleoearthquakes during the late Quaternary in the eleven trenchesexcavated on the western segment. The age of the latest earthquake event is1.2±0.1ka BP, andthe ages of the other older events is32.5±1.3ka,28.1±1.6ka,24.3±1.6ka,9.9±0.5ka and6.75±0.35a, respectively. The result suggests that there are two seismically active period, the latePleistocene and Holocene period, whose recurrence interval is5000a.Compared with the easternsegment, Gulang and Haiyuan fault, we find that these faults are in an active state during a relativeshort period, and have remarkable clustering characteristics, which recurrence is about5000a.What’s more, the seismicity of the faults around the Tianjingshan Sub-block is significantlyenhanced again during100-300a BP.The movement property varies from different segments along the Xiangshan-Tianjingshanfault zone: the western segment is normal or reverse-strike-slip fault; the eastern segment isreverse-strike-slip fault; and the southeastern segment is thrust fault. There are two factors thatmay result in the movement property’s change collectively:(1) under the NE-SW or NEE-SWWregional compression stress field, the angle between the principal stress direction and fault strikeincreases with the fault strike’s change, which contribute to the increase of thrust component on the fault plane;(2) it happen compressional deformation in the front due to the block of Ordos andslip-extensional deformation at the end, along with the Tianjingshan Sub-block moves to E or SEEdirection.