鄂尔多斯西缘地壳三维精细密度结构及其构造意义

The western margin of Ordos is located in the intersection of Alxa block and Ordos block, where the regional structure is complex, faults are developed and the seismicity is strong. The region is also one of the key area of earthquake disasters in our country. The deep research results mainly focus on the uplift mechanism of northeast edge of Qinghai-Tibet plateau, there is lack of 3-D precise crustal structure researches. And in the research the scale of data is small or the limitation of the field source separation method, the seismogenic structure of strong earthquake is controversial, and the researches on interaction between blocks and contact relations of large fractures are not enough. In this topic, firstly, collect and integrate big scale gravity data and real measuring gravity data; next, use the finite element method to simulate the gravity Bouguer data, the method is adaptive to the complex geometry; build the crustal three-dimensional structure in the region and analyze the deep tectonic deformation characteristics in horizon and in vertical; mainly analyze the depth of the contact relationship between blocks, the large fault zone and its relationship with strong earthquake; finally, discuss the crustal dynamic mode in the research area with integrating the previous seismic，magnetotellurics and geological data what provide the scientific basis for the deep earthquake environment in the western margin of Ordos.

鄂尔多斯西缘地处青藏高原东北缘鄂尔多斯块体和阿拉善地块的交汇地带，区域构造复杂，断裂发育且地震活动强烈，其深部特征及动力学过程一直是学者们的研究焦点，也是我国地震防震减灾的重点地区之一。该区域深部研究成果主要集中于青藏高原东北缘的隆升机制，缺乏地壳精细的三维结构研究，且以往研究中所使用的数据比例尺小、场源分离方法存在局限性，对强大地震的发震构造存在争议，块体间的相互作用以及断裂构造带深浅部的接触关系研究不够深入。本课题通过收集、整合较大比例尺的布格重力数据以及实测重力剖面数据，采用适应复杂几何形状的有限元法对研究区域重力异常进行数值模拟，构建研究区地壳三维密度结构，分析不同深度的横向及纵向构造变形特征，重点分析块体间构造带的深浅接触关系以及大型断裂带与强大地震的关系，同时结合已有地震地质与大地电磁测深等资料，探讨该地区的地壳动力学模型，为鄂尔多斯西缘深部孕震环境研究提供科学依据。

研究区地处青藏高原东北缘鄂尔多斯块体和阿拉善地块的交汇地带，区域构造复杂，断裂发育且地震活动强烈，其深部特征及动力学过程一直是学者们的研究焦点，也是我国地震防震减灾的重点地区之一。特别是贺兰山-银川地堑及邻区地质结构复杂，对该区域深浅结构特征的研究具有重要意义。本研究采用重力归一化总梯度成像和二维小波多尺度分解方法对研究区内重力异常进行了垂向和横向构造分析。重力归一化总梯度成像结果显示高低转换带的倾角、倾向与地质上的贺兰山东麓断裂、银川断裂和黄河断裂分布吻合较好，贺兰山西麓断裂与贺兰山东麓断裂汇交深度约18km，银川断裂与黄河断裂汇交深度约25km；二维小波多尺度分解成像结果表明正谊关断裂、贺兰山西麓断裂、芦花台断裂和银川断裂为上地壳断裂，贺兰山东麓断裂、青铜峡-固原断裂以及黄河断裂为下地壳断裂，且这三大断裂可能分别是阿拉善地块东南边界和鄂尔多斯地块西南边界；1739年平罗M8.0级古地震震中与银川断裂在重力剖面深度约15km汇交，其垂向高低梯度为强变形带，同时古地震震中位于重力正负异常转换部位的低值区，据此可推断此次古地震的发震构造是银川断裂。这些结论可提高对贺兰山-银川地堑及邻区地质结构的认识，为该区地壳动力学过程及强震的孕震机理研究提供一定的科学依据。