川东南侏罗山式构造与泸县地区活动构造研究

STUDY ON JURA-TYPE STRUCTURE IN SOUTHEASTERN SICHUAN AND INVESTIGATION OF TECTONIC ACTIVITY INLUXIAN AREA

2019年9月16日四川省泸州市泸县发生Ms6.0级强震,为该地区有历史记录以来的最大地震。地震发生在华蓥山断裂带南段的宜宾-华蓥山一线,位于螺观山-梯子崖背斜和海潮背斜夹持的喻家寺向斜内部。本次研究以泸县Ms6.0地震震区及邻区活动构造为主要研究对象,在野外地质调查基础上,结合遥感影像解译、地震反射剖面解译、断层泥石英形貌学及电子自旋共振测年方法,对研究区地表断层的活动性和深部构造变形特征加以判断。研究结果表明,震中所处的喻家寺向斜内部不存在明显的地表断层;两侧背斜中的走向断层如黄泥垭断裂、堆金湾断裂具有右行走滑为主兼具压性的运动学特点,未见断层切割第四系的确凿证据;构造地貌、水系位错、沿断层局部出现温泉证明它们具有活动断层的迹象;采自黄泥垭断裂的断层泥石英颗粒微形貌特征以代表晚更新世的次贝壳状微形貌占优势,ESR测年获得85±22 Ka的数据,表明该断裂具有晚更新世活动性。地震反射剖面显示典型的侏罗山隔挡式薄皮构造,地表断层和隐伏断层整体构成“人”字形或“入”字形的构造组合样式,总体呈对冲式逆冲构造,向下主要收敛于中-上寒武统底部软弱膏盐层构成的滑脱层中。断层擦痕的构造解析表明,研究区晚更新世以来的最大水平挤压应力来自230°~316°方向,即与川滇地块和龙门山喜马拉雅构造活动有关。

On September 16,2019,a magnitude 6.0 earthquake struck Lu County,Sichuan Province,marking the most powerful earthquake ever recorded in the region.The earthquake epicenter was located within the northeast-trending Yujiasi syncline in the Yibin area,situated in the southern part of the Huay-ingshan Fault Belt.The syncline is flanked by the northeast-trending Luoguanshan-Tiziya anticline and the Haichao anticline.This study investigates the surface fault activity and deep tectonic deformation charac-teristics of the Luxian Ms6.O earthquake using a combination of field geological survey,remote sensing image interpretation,seismic reflection profile analysis,fault lithography,and electron spin resonance dat-ing methods.These methodologies provide insights into the seismic activity and structural changes associ-ated with the earthquake event.The investigation revealed no discernible surface fault in the Yujiasi syn-cline,the epicenter location of the earthquake.Surrounding faults such as the Huangniya and Duijinwan faults exhibit characteristics of right-lateral strike-slip and compressional kinematics.There is no conclu-sive evidence of fault truncation in Quaternary sediments.However,active signs of faults are evidenced by tectonic geomorphology,drainage anomalies,and local hot springs along these faults.Micromorphological analysis of quartz particles from fault gouge samples of the Huangniya fault indicates dominant subconch-oidal features,suggesting late Pleistocene faulting activity.Electron Spin Resonance(ESR)dating result of 85±22 ka also supports late Pleistocene activity of this fault.Interpretation of seismic reflection sec-tions in the study area reveals a typical Jura-type thin-skinned structure.Surface faults and concealed faults collectively form counterthrust structures with chevron or"λ"shaped patterns in profiles.These ac-tive faults predominantly extend into the weak paste salt layer detachment zone at the base of the Middle and Upper Cambrian series.Structural analysis of fault striations indicates maximum horizontal compres-sive stress in the late Pleistocene oriented between 230°to 316°,likely influenced by tectonic activities within the Sichuan-Yunnan Block and the Longmenshan Mountains.