基于地球物理资料的宁夏南部地区断裂格架特征分析

An analysis of the fault framework in southern Ningxia based on geophysical data

宁夏南部地区处于典型黄土塬地区,其内展布5条构造单元分区断裂,均呈半隐伏—半裸露状态。以区域大地构造为依据,根据地层出露状况,梳理了研究区地球物理异常场分布特征,理清了断裂隐伏段的展布形态及各条断裂的相互关系,确立了宁夏南部地区断裂格架。在1∶20万区域重力与区域航磁资料的基础上,综合利用小波多尺度分解技术与边界识别方法,提取反映断裂的深部重磁异常弱信号,并与MT剖面刻画的深部断裂进行对比。结果显示:研究区的5条主要断裂体现为重力2阶小波细节场中明显的重力高异常条带的边界。牛首山—罗山—崆峒山断裂是SN向长条状重力异常与NW向片状、带状重力异常的分界,具有典型的右行走滑特征,为阿拉善微陆块与鄂尔多斯地块的分界断裂;海原断裂在深部一分为二,海原Ⅰ号断裂隐伏于海原盆地内,无重力异常显示,航磁异常场分区特征明显,海原Ⅱ号断裂裸露于南华山、西华山东北麓,显示出清晰的线性重力异常特征,航磁异常微弱,2条断裂共同构成了北祁连早古生代造山带与阿拉善微陆块的复合边界;阿拉善微陆块内部的天景山断裂、烟筒山—窑山断裂与罗山东麓断裂表现为弧形重力高值异常带的东北边界。

Southern Ningxia,located in a typical loess tableland area,hosts five semi-concealed-semi-exposed faults as boundaries of tectonic units.Based on the regional geotectonic conditions and outcrops,this study analyzed the distribution of the geophysical anoma-ly field in the study area,ascertained the distribution morphology of the concealed fault sections and the relationship between the faults,and established the fault framework in southern Ningxia.Based on the 1:200000 regional gravity and aeromagnetic data,this study ex-tracted the weak signals of deep gravity and magnetic anomalies reflecting faults using both the multi-scale wavelet decomposition tech-nique and the boundary recognition method and compared these signals with the deep faults depicted based on MT profiles.The results show that the five major faults in the study area are the boundaries of the significant gravity high anomaly zones in the detailed second-order wavelet field of gravity.The Niushoushan-Luoshan-Kongtongshan fault is the boundary between the north-south-trending long strip-shaped gravity anomalies and the north-west-trending flaky and banded gravity anomalies.This fault has typical dextral strike-slip characteristics and is the boundary fault between the Alxa microcontinent and the Ordos block.The Haiyuan fault is divided into Haiyuan faults Nos.1 and 2 at depth.The No.1 Haiyuan fault is concealed in the Haiyuan Basin and does not exhibit gravity anoma-lies.Moreover,the aeromagnetic anomaly field of this fault has significant zoning characteristics.The No.2 Haiyuan fault is exposed at the northeastern feet of the northern and southern Mount Huashan and exhibits distinct characteristics of linear gravity anomalies but weak aeromagnetic anomalies.The two faults jointly constitute the composite boundary between the Early Paleozoic North Qilian Oro-genic Belt and the Alxa microcontinent.Three faults in the Alxa microcontinent,namely the Tianjingshan fault,the Yantongshan-Yaos-han fault,and the fault at the eastern piedmont of Luoshan,are present as the northeastern boundary of the arcuate high-amplitude gravity anomaly zone.