依兰-伊通断裂方正段晚第四纪以来的构造活动及其地貌表现

方正断陷是位于依兰-伊通断裂中北部的一级负向构造单元,依兰-伊通断裂构成盆地边界的控堑断裂,在盆地中部发育走滑断裂(伊汉通断裂),与边界断裂一起构成统一的断裂系统。该中部断裂的最新活动在地貌上具有明显的表现,在卫星影像上显示出清晰的线性。笔者在高分辨率卫星影像解译的基础上,通过野外地质地貌观察测量等手段,分析认为此断裂为一长期活动断裂,普遍错断河流一级阶地,并有连续的断层陡坎展布,最新的活动时代为全新世,应该曾发生过7级以上地表破裂型地震。现今以右旋走滑运动为主,兼具垂向滑动分量,并且垂向上表现为枢纽断层的运动特征。这些结果与前人认为的依兰-伊通断裂晚第四纪以来活动性极弱,东北地区是我国构造最稳定的地区的结论似乎不甚相符。笔者认为还需要更深入的工作,以分析该断裂与盆地边界断裂的关系,并以科学的态度认识依兰-伊通断裂活动特征,充分认识其地震危险性。

Tectonic Activity and Geomorphic Features of the Fangzheng Section of Yilan-Yitong Fault in the Late Quaternary Period

As a negative tectonic unit the Fangzheng depression locates in the north-central part of Yilan-Yitong fault zone, which constitutes the boundary-fault for the Fangzheng basin, while strike-slip fault develops within the basin. The new fault within the basin shows remarkble tectonic deformation and has led to clear geomorphic response, which stands out in the satellite image. Through interpretation of high-resolution satellite images and field work like geological and geomorphological observation and measurement, we find that the new fault has a long-term activity since late Pleistocene, and it offsets the latest terraces resulting in continuous fault scrap. The latest activity should be within the Holocene Epoch and led to a M7 earthquake which created the surface rupture. At present, the tectonic movement of the fault is mainly dextral strike-slip with minor vertical displacement which shows the characteristics of pivotal fault. Our results in this paper differ from previous results that Yilan-Yitong fault has a very weak activity and that the Northeast is the most stable block region in China.