贺兰山—银川地堑及邻区重力异常特征及构造意义

Characteristics and tectonic significance of gravity anomalies in the Helanshan-Yinchuan Graben and adjacent areas

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.本文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18 km,银川断裂与黄河断裂汇交深度约25 km;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M 8.0古地震震中与银川断裂在重力剖面深度约15 km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.

The complex geological structure of the Helan-Yinchuan Graben and adjacent areas is of great significance to study on shallow and deep structures of this area.In this paper,the gravity normalized total gradient method and 2-D wavelet multi-scale decomposition method are used to analyze the vertical and transverse structures in this region.The results show that the tendency and dip angle of the value of the high-low gradients match with the fault distributions,such as the eastern Helan mountain piedmont fault,Yinchuan fault and Yellow River fault.The intersection depth of the western Helan mountain piedmont fault and Helanshan-Donglu fault is about 18 km,and the intersection depth of Yinchuan fault and the Yellow River fault is about 25 km.The results of 2-D wavelet multi-scale decomposition show that the Zhengyiguan fault,western Helan mountain piedmont fault,Luhuatai fault and Yinchuan fault are confined to upper crust,while the eastern Helan mountain piedmont fault,Qingtongxia-Guyuan fault and the Yellow River fault extend down to lower crust.These three faults may be the southeastern boundaries of the Alxa block and the southwestern boundaries Ordos block,respectively.The epicenter of the Pingluo M 8.0 paleoearthquake intersected with Yinchuan fault at the depth of the gravity profile about 15 km,where the high-low gradient is a strong deformation zone.At the same time,the epicenter of the 1739 Pingluo M 8.0 earthquake is also located at the place of low gravity anomalies on the gradient belt.Therefore,it can be inferred that the seismogenic structure of this event is the Yinchuan fault.These conclusions can improve the understanding of the geological structure of the Helan-Yinchuan Graben and its adjacent areas,providing a scientific basis for the study of crustal dynamics and seismogenic mechanism of najor earthquakes.