柴达木盆地西南缘油砂山断裂的古地震和晚第四纪活动速率

摘要

油砂山断裂位于柴达木盆地西南缘英雄岭背斜南翼,英雄岭背斜是柴达木盆地内新构造表现最为强烈的地区,也是柴达木盆地内部褶皱向S扩展的最前缘位置.对油砂山断裂的古地震和晚第四纪活动速率研究有助于理解该地区的构造变形方式和评价地震地质灾害.通过对油砂山山前洪积扇上沿断裂带发育的挤压鼓包进行探槽开挖、油砂山西侧盆地内褶皱陡坎进行测量及晚第四纪冲、洪积地层进行光释光测年,综合分析认为:1)油砂山断裂是1条全新世活动的逆冲断裂,盆地内的褶皱陡坎表明至少晚更新世中晚期以来该断裂在同一地表迹线上持续活动.油砂山探槽揭露到的断层面产状为N62°W/NE∠17°.探槽揭露到了至少2次构造事件,较新的1次事件Ⅱ发生在距今500a以来,垂直断距约0.25m.目前获得的资料还不能确定它就是1977年茫崖西北的M6.4地震,但也不能排除与该地震有关.较老的事件Ⅰ发生在距今1000~4000a之间,垂直断距约0.55m.事件发生后的侵蚀作用影响了事件Ⅰ年龄的限定和事件Ⅰ确切次数的判定.2)油砂山断裂晚更新世中晚期以来的垂直活动速率约为(0.38±0.06)mm/a.与该区GPS站点的相对速率相比较表明,油砂山断裂是该区非常重要的1条全新世活动的逆冲断裂,在调节区域构造变形上起到了重要作用.%The Youshashan Fault lies in the south flank of Yingxiongling anticline, southwestern margin of Qaidam Basin.The Yingxiongling anticline is one of the most active neotectonics,situated at the front of folds expanding southward in the Qaidam Basin. Research on the paleoseimology and Late Quaternary slip rate of this fault is important for hazard assessment and understanding tectonic deformation in this area. We excavated a 27-m-long trench across the Youshashan fault where a pressure bridge formed on the Holocene alluvial fans, measured a profile of the fold scarp created by the fault west of the Youshashan mountain,and collected several samples of finer sands for luminescence dating.Analysis of these data shows that(1) The Youshashan Fault is a Holocene active feature. The fold scarp in the basin indicates that this fault has been active along a same surface trace since at least mid-late Pleistocene. At least two paleoseismic events are revealed by trenching,both occurred in Holocene.The latest eventⅡin the trench happened after 500a.The current information fails to confidently support that it is the 1977 Mangya M6.4 earthquake, but cannot excludes the possibility of it is related to this earthquake. The other eventⅠoccurred about between 1 000a to 4 000a. Erosion after the eventⅠprevents us to constrain the event age and to identify more events further. (2)The vertical slip rate of the Youshashan fault is about(0.38±0.06)mm/a since mid-late Pleistocene. Comparing with relative speeds of GPS sites across the Yingxiongling anticline suggests that the Youshashan fault is an important structure which is accommodating crustal shortening in this region.