银川盆地构造发展——深地震反射剖面揭示浅部地质与深部构造的联系

横跨银川盆地北西西向的深地震反射剖面,清晰揭示了银川盆地边界断裂以及整个地壳的结构构造特征,这对研究具活动大陆裂谷性质的银川盆地浅-深构造关系具有重大的意义。贺兰山东麓山前断裂、黄河断裂作为银川盆地的西、东边界断裂,前者为一条缓倾斜、延伸至上、下地壳边界的犁式断裂,而后者则为一条切穿地壳并延伸进入上地幔的深大断裂。根据深地震反射剖面揭示的地壳结构特征,银川盆地浅部结构并非前人认为的"堑中堑"结构,而是表现为由一系列东倾犁式正断层控制的新生代断陷。略微下凹的Moho面几何形态以及厚2~3.2 km的层状强反射带为下地壳最显著的反射特征。Moho面深度与强反射带厚度变化趋势与银川盆地沉积厚度变化趋势几乎一致。本文认为,强反射带的成因可能是由源自地幔的基性岩浆以岩席状的形式底侵进入地壳底部造成的,而这部分形成强反射带的物质可能补偿了因银川盆地断陷而造成的地壳减薄,最终导致银川盆地之下Moho面并未像之前所认为的那样隆起。

Development of the Yinchuan Basin: Deep seismic reflection profile revealed the linkages between shallow geology and deep structures

The NW trending deep seismic reflection profile across the Yinchuan Basin precisely revealed the boundary faults and crust structures of the Yinchuan Basin. It has great significances for understanding development of the Yinchuan Basin. Helanshan eastern piedmont fault as the western margin fault of Yinchuan Basin is a gentle dipping listric fault which extends downward to the boundary of upper crust and low crust. While, Huanghe fault of the eastern boundary fault is a deep fracture which cuts through the whole crust. The shallow structure of Yinchuan Basin indicates a listric faults controlled Cenozoic depression reveled by the deep seismic reflection profile. The dramatic characteristics of the low crust are the nearly flatten Moho which have a great contrast with the traditional cognition of uplift Moho under basin and the 2~3.2 km thickness of layered reflections. According to previous studies of layered reflections, we inferred that the layered reflections consist of mafic magma result from the mantle magma underplating. We proposed that the layered reflections under the low crust counteract the crust thinning effect which ultimately lead to the nearly flatten Moho under sedimentary basin.