磁性地层年代对北京平原顺义断裂第四纪活动性的约束

摘要

Shunyi fault is one of Quaternary active faults in northeastern Beijing plain.Its activity is regarded as one of potential geological disasters to the urban security.In order to reveal the activity history of Shunyi fault,the magnetostratigraphy research was conducted on borehole ZK12-1 at the footwall of fault.The detailed stepwise thermal demagnetization isolated two components, and the characteristic remanent magnetization (ChRM)with moderate downward or upward inclinations was identified after removing a low-stability drilling-induced remanence with steep downward inclinations.Combined with the lithologic association of the core,a magnetostratigraphic frame of borehole ZK12-1 was established.Based on the magnetostratigraphic framework of borehole ZK12-1 at footwall and borehole ZK12-2 at the hanging wall and the distance of the vertical movement, the vertical activity rates between the hanging wall and footwall of Shunyi fault were computed to be 0.023 mm/a in the Early Pleistocene,0.001,7 mm /a in the Middle Pleistocene,0.033 mm /a in the Late Pleistocene and 0.051 mm /a in the Holocene.These results indicate Shunyi fault activated strongly in the Early Pleistocene and activated weakly in the Middle Pleistocene.In the Late Pleistocene,this fault became much more active,and in the Holocene it experienced the strongest activity.%顺义断裂是位于北京平原东北部的一条第四纪活动断裂，其活动性对北京市城市安全具有潜在的地质灾害威胁。为了解顺义断裂第四纪以来的活动性，对该断裂下盘的一个钻孔剖面 ZK12-1的岩心样品开展磁性地层学研究。通过逐步热退磁，大部分样品可成功分离出两个剩磁分量，其中低温分量具有正的、高倾角特征，可解释为由钻杆旋转引起的重磁化分量；高温分量为中等倾角且具有倒转特征，为原生剩磁。据此建立了 ZK12-1钻孔的磁性地层年代格架。结合断裂上盘 ZK12-2钻孔的磁性地层年代格架结果，并根据这两个钻孔在第四纪不同时期的地层落差，计算出顺义断裂上、下两盘在早更新世、中更新世、晚更新世和全新世的相对垂直活动速率分别为0.023 mm／a、0.0017 mm／a、0.033 mm／a 和0.051 mm／a。这一结果表明顺义断裂在早更新世活动较强，中更新世活动最弱，晚更新世断裂开始活化，活动性增强，全新世活动性最强。