四川华蓥山断裂带晚第四纪逆走滑特征及地震意义

New Discovery of Late-Quaternary Right-Lateral and Reverse Displacement in Huayingshan Fault Zone in Gao County,Sichuan Province

华蓥山断裂带是四川盆地东界断裂带。前人认为该断裂带的最新活动时代为中更新世中期至晚更新世早期。作者近期野外调查发现了华蓥山断裂带晚第四纪走滑活动证据,位于华蓥山断裂带中段高县罗场镇兴场垇口村,称为兴场垇口断裂,该断裂长15km,走向NE30°,横切该区山体、造成多条水系扭曲,形成地表壮观的逆坡向右旋断错地貌。该断裂断错距今(14.86±1.26)ka地层,断裂的晚更新世晚期垂直活动速率为0.15mm/a,右旋走滑活动速率0.31mm/a,为晚更新世晚期右旋逆走滑活动断裂。

Huayingshan Fault Zone is the biggest fault along the eastern margin of the Sichuan basin.Previous studies considered that the most recent active period of this fault is from middle Pleistocene to late Pleistocene.However,some new evidence has been found recently with the measuring technology of photogrammetry and chronological analysis method of geomorphology and tectonics.Our field investigation indicates that the newest activity of the relatively small Xingchangaokou Fault in the Huayingshan Fault Zone is late Quaternary.The Xingchangaokou Fault is more than 15 km long,NE30°-trending,dipping to southeast.This fault shears the ridge and the water system,results in obvious and grand-scale geomorphologic features by the thrust and right-lateral displacement of the fault.The latest time of rupture was at about(14.86±1.26)ka BP.,during the late Pleistocene.Estimates of moment magnitudes based on empirical relationships with rupture length indicate the possibility of Mw 7 earthquake in this area.According to the empirical relationship of Deng(1993),the lateral displacement caused by a 7~7.9 earthquake is 5~6m in southwest China.So the right-lateral displacement rate is 0.31mm/a.At the same time,the vertical displacement is 2.7m occurred about(17.62±1.49)ka BP.So the vertical displacement rate is 0.15mm/a.According to these observations,the newest active period of this fault is after late Pleistocene.All these results may provide better constraints on the assessment of the crustal stability for this area.