中更新世晚期以来中条山北麓断层滑动速率研究

STUDY ON THE SLIP RATE OF THE NORTH ZHONGTIAO SHAN FAULT SINCE THE LATE MIDDLE PLEISTOCENE

约束断层在不同时间尺度上的滑动速率,有利于认识其活动性和区域构造特征。文中以山西地堑系南部中条山北麓断层为研究对象,利用无人机摄影测量技术获取小李村河断错地貌的高精度DEM,通过光释光和AMS^(14)C年代学约束河流阶地序列的废弃年代和地层的沉积时代,建立中更新世晚期以来小李村河地貌演化与中条山北麓断层活动的关系。结合断层上盘钻孔所揭露的沉积地层和光释光年龄,约束中条山北麓断层在中更新世晚期以来2个不同时段的垂直断距,获得的相应滑动速率为0.3~0.4mm/a,并简要讨论了晚新生代以来该断层滑动速率的变化。

Slip rate is an important parameter for the quantitative study of active fault and can be used to reflect the mode and intensity of fault activity.However,the selection of geomorphic surface,the acquisition of displacements,and the limitation of chronologic methods result in challenges to constrain the slip rate.A series of boreholes and geochronology studies revealed a continuous sedimentary sequence of the Quaternary in the Yuncheng Basin in the southern Shanxi Graben System.Multiple late Quaternary river terraces have developed and been preserved in the northern piedmont of the Zhongtiao Shan.The activities of the north Zhongtiao Shan Fault resulted in the elevation difference between the strata in the Yuncheng Basin and the river terraces.In this study,we chose the geomorphic units of the Xiaolicun River and combined them with the results of boreholes in the Yuncheng Basin to constrain the slip rates of the north Zhongtiao Shan Fault since the Late Pleistocene.Based on field observation and remote sensing image interpretation,we established the distribution and sedimentary characteristics of four terraces and the latest alluvial fan of the Xiaolicun River.Two main faults(F1and F2)and a series of fractures or branch faults have been identified in these sedimentary strata.The high-resolution DEM of the faulted landform of the Xiaolicun River was obtained using UAV photogrammetry technology.Combined with a stratigraphic outcrop survey,the landform and sedimentary section across the fault were constructed.The abandonment ages of the terraces T4,T3,T2,and T1have been determined as(214.3±13.9)ka,(118.5±6.4)ka,(59.6±2.4)ka,and(10.9±0.5)ka by OSL dating,respectively.The chronological results of the AMS14C dating show that the alluvial fan north of F2was deposited at 35~1ka.Based on these results,this study established the relationship between the geomorphic evolution of the Xiaolicun River and the activities of the north Zhongtiao Shan Fault.Since the late Middle Pleistocene,F1had been active,accompanied by the abandonment of the T4.At~120ka,the terrace T3was formed,F1was no longer active,but F2began to be active and raise T3and T4in the footwall.Since then,the Xiaolicun River has undergone rapid incision and formed T2and T1.The continuous activities of F2maintained T4-T1in an uplifted state and formed a series of fractures in the alluvial fan.Based on this evolutionary relationship,T4,T3and their corresponding strata in the boreholes of the Yuncheng Basin were used to constrain the slip rate of the north Zhongtiao Shan Fault in this study.After determining the depth in boreholes corresponding to the abandoned ages of T4and T3,subtracting the influence of the surface slope and the activities of the southern Salty Lake Fault,and considering the depth error caused by climate change,the vertical displacements of the north Zhongtiao Shan Fault since the two periods were obtained with the vertical slip rate of(0.31±0.05)mm/a and(0.34±0.04)mm/a,respectively.Our results indicate that the slip rates of the north Zhongtiao Shan Fault since the late Middle Pleistocene are greater than those since the Late Pliocene and Quaternary.