青藏高原东北缘三关口-牛首山断裂晚第四纪构造活动特征

The active fault zones aroud Tibetan Plateau is a bond which attaching tectonic deformation between plateau and the adjacent blocks，on one hand bounding to plateau uplift and expansion,on the other hand exerting influence on the adjacent block. Sanguankou-Niushoushan fault splits the two blocks of the most active tectonic movement and seismic activity in Chinese mainland, Tibet block and North China block, and there are two very different types of tectonic units on both sides of the fault, arc strike-slip structure and Yinchuan graben. However, the tectonic transition relationship between two tectonic units is not clear.In this study, by taking Sanguankou-Niushoushan fault as the research object,and taking the study of tectonic activity characteristics of the fault since late Quaternary as point of penetration, We will identify the geometry and the movement properties of the fault,and find the deformation characteristics of the fold in Cenozoic strata near the fault. Then, we will obtain quantitative data about the fault after trenching, microrelief measurements and stratigraphic dating, such as fault latest time, periodicity of the paleoearthquakes, slip rate.We also will analyze the relationship between the fault and folds ,and preliminary understand the tectonic evolution of the fault through contrasting the regional strata. Finally, combined with previous data ,we will discuss the constraint effect of the fault on the plateau extension and Yinchuan graben formation, and reveal the relationship between the two main structural system.

三关口-牛首山断裂分割了中国大陆现今构造运动和地震活动最为活跃的两大地块- - 青藏地块和华北地块，断裂两侧分别发育了两类性质截然不同的构造单元- - 弧形走滑构造和银川地堑。本申请以三关口-牛首山断裂为研究对象，以断裂晚第四纪构造活动特征为切入点，通过野外调查和关键部位大比例尺地质填图，查清断裂的几何展布、运动属性、以及断裂附近新生代地层褶皱变形特征；通过槽探、微地貌测量和地层测年，获得断裂最新活动时代、古地震期次、滑动速率等量化数据；分析断裂与褶皱间的关系，并通过区域地层对比，初步认识断裂晚新生代构造演化历史。结合前人资料，探讨断裂对高原扩展和银川地堑形成的约束作用，揭示两大构造体系间的相互关系。

三关口-牛首山断裂位于青藏、阿拉善、鄂尔多斯3大地块的交汇之处，构造位置非常重要。该断裂也是青藏高原东北缘弧形构造带和银川地堑的边界。研究该断裂对认识青藏高原东北缘弧形构造带的扩展方式和银川地堑成因具有重要意义。在野外调查的基础上, 通过大比例地质填图、微地貌测量、槽探及地层年代测试等方法,查清了断裂的几何结构和活动图像，发现了断裂的右旋走滑属性，获得了晚第四纪以来古地震期次和滑动速率，分析了晚新生代以来的构造演化阶段和大致时限，探讨了青藏高原弧形构造带的扩展和银川地堑的成因。. 三关口-牛首山断裂由3条不连续的次级断裂组成，具有显著的右旋走滑特征。在三关口断裂上，古生代、中生代、新生代的地质界线均被断层右旋错动, 位移相近,获得最大走滑位移为961±6m。一系列小冲沟跨断裂发生了同步右旋扭动。通过冲沟位错和断层陡坎测量，获得断裂水平和垂直滑动速率分别为0.35 mm/a、0.086mm/a。三关口断裂3万年以来发生了3次古地震事件，分别发生在距今26.37ka、16.78ka、8.25ka。复发周期约8.8ka，同震水平位移3m，垂直位移0.9m，估算断裂水平和垂直滑动速率分别为0.1mm/a、0.36mm/a。古地震复发属特征地震型，离逝时间较接近复发周期，其地震危险性值得关注。三关口-牛首山断裂附近晚新生代经历了两期构造变形， 早期以地层褶曲变形为主, 后期则转化为断裂的右旋走滑。褶皱变形发生在上新世末（接近2.7Ma），是青藏高原东北向扩展的结果；断裂活动发生在第四纪初，受青藏高原东北向推挤和鄂尔多斯地块逆时针旋转联合作用的控制。. 青藏高原东北缘弧形构造带的扩展是以次级地块分阶段逐次向外推挤而实现的, 高原扩展抵达海原断裂、天景山断裂以及三关口-牛首山断裂的时间分别为10、5.4、2.7 Ma, 次级地块的运动状态控制了边界断裂的运动性质, 次级地块受阻后侧向挤出是导致边界断裂带性质转变的根本原因。青藏高原的东北向推挤不是银川地堑形成的直接原因。