重磁三维反演伊犁盆地中部密度和磁性结构

伊犁盆地中部区域地处多个构造单元的交汇区,构造复杂,缺乏高精度地质资料,制约了对盆地构造演化的进一步认识.本文联合使用多种先验信息作为约束条件,利用高精度重磁数据反演获得了研究区地下海拔-10 km以浅的三维密度和磁性结构,增强了反演结果的可靠性.结果揭示以白石墩次凸为中心发育了一个"北断南超"的南西-北东向不对称型凹陷,北部沉积厚度大于南部;区内主要断裂具有高角度特征,生烃中心受断裂控制,阿吾拉勒山前凹陷浅部可能发育逆掩断裂;火成岩以侵入的基性火成岩为主,伴有部分喷出火成岩和中酸性火成岩.

Density and magnetic susceptibility distribution of central Yili Basin by three-dimensional inversion of gravity and magnetic data

Yili basin on a micro-block sandwiched in the Tianshan orogenic belt is a mountainous basin. The central area at Baishidun is located in the intersection of multiple tectonic units. The complicated geological structure and lack of high precision research results restrict the further understanding of Yili basin tectonic evolution. The target area is a nearly rectangular and the two sides are 38 km and 35 km long. Under the constraints of various prior information, the three-dimensional (3-D) density and magnetic susceptibility structure from the ground to the altitude of -10 km were obtained through 3-D inversion based on the 1:50000 high-precision anomaly data that included 5004 gravity anomaly data and 10025 magnetic anomaly data. We used the 3-D inversion method with terrain, designed 1080576 inversion grids and introduced physical properties constraints and structural constraints. We also considered the various weight of different grids which usually has higher weight in region with a prior information. Ultimately, we got higher reliability results after the continuous adjustment of parameters according to each inversion results. The inversion results show that (1) the density of different regions changes slowly and the transition is gentle but the boundary of the high and low density regions is obvious; and (2) the inheritance of different depth density characteristics is significant and the horizontal density nonuniformity decreases gradually with altitude decreasing; and (3) there are two main high magnetic susceptibility bodies in the northeast and east of the study area respectively and extend down deep; and (4) some dispersed magnetic susceptibility bodies are in the shallow area of the central region and their anomalies are less obvious. According to the results of this inversion and other previous research, we speculate that the study area has a southwest-northeast asymmetric depression which has thick sediments. The thickness of the northern sedimentary layer is larger than that of the south and the southern sediments are overriding the Wusunshan uplift. The main faults in the area are high-angle secondary faults and the northern piedmont may develop overthrusts. The igneous rocks in the area are dominated by basic igneous rocks, accompanied by part of intermediate-acidic igneous rocks.