Characteristics of Wanbuge gravity anomaly

1.3.2. 1 regional gravity field characteristics

According to the distribution of 1 ∶ 1 Wanbuge gravity anomaly (Figure 1. 10), the gravity anomaly in Meng Xing-Jihei area is divided into three grades. The first-order anomaly is the background field of the whole region, reflecting the characteristics of Moho surface fluctuation anomaly; The secondary anomaly is a large gravity gradient belt with high gravity, low gravity and large extension, which reflects the abnormal relationship between micro-landmasses. The third-order anomaly is a trap of local anomaly isoline, which reflects the structural characteristics of uplift and depression and their separation in the continental block. According to the distribution characteristics of first-order anomalies, Moho uplift in Meng Xing-Jihei area mainly occurs in Songliao basin and Sanjiang basin east of Mudanjiang fault. Generally speaking, the Moho surface fluctuation reflected by the primary background is controlled by two obvious secondary gravity anomaly terraces, namely, the Daxing 'anling gravity anomaly terrace in the west and the Yilan-Yitong gravity anomaly terrace in the east. These two gravity anomaly terraces divide this area into three gravity anomaly areas, namely Daxinganling anomaly area, Songnen anomaly area and Zhangguangcailing anomaly area. In order to improve the recognition accuracy of tectonic line position, and more accurately reflect the division of tectonic units and the position of boundary zone characterized by abnormal step zones in this area, small-domain filtering and three-dimensional small-domain filtering are adopted to reduce the width of conventional Bouguer gravity abnormal step zones and highlight the distribution characteristics of abnormal zones in all directions. The processing results show that the east-west, northeast, northeast and north-south anomalies are the main abnormal structural lines in Meng Xing-Jihei area (figure1.1,figure 1. 12), which clearly shows the distribution position and relationship of linear anomalies in all directions after contraction. The east-west low gravity anomaly only appears in Mohe area in the north and Hohhot-Chifeng line in the south of this area. The latter is an east-west low gravity anomaly belt with a width of nearly 200km, which is controlled by a compact linear anomaly belt in the south and north respectively. Geologically, the positions of these two linear anomaly zones roughly correspond to the northern margin fault of North China Plate and the Xilamulun River fault. It is worth noting that the east-west low gravity anomaly and bilateral contraction linear anomaly all stop in the west of Shenyang and do not continue to the east. The linear contraction anomalies on both sides of the NNE gravity anomaly belt distributed along Daxing 'anling Mountains are distributed along Huma-Zalantun-Chifeng line and Heihe-Baicheng line respectively. The north-south linear anomalies of contraction only appear in Jilin and eastern Heilongjiang (Figure 1. 12), which correspond to Jiayin-Mudanjiang fault and Tongjiang-Mishan fault respectively. Two narrow NE-trending linear anomalies are distributed in parallel, corresponding to Jiamusi-Yitong fault and Dunhua-Mishan fault respectively, and the north-south anomalies have obvious transformation. The important geological understandings obtained by using the small-domain filtering processing technology are as follows: ① The east-west structure mainly develops between the Meng Xing-Hebei black lithosphere block and the North China lithosphere block, and is transformed by the NNE-oriented structure distributed along Daxing 'anling-Taihang Mountain. The east-west structure stops in the west of Yilan-Lan Shu fault and does not extend to the east of Jilin. (2) The N-S structure is only developed on the east and west sides of Jiamusi micro-landmass, corresponding to Jiayin-Mudanjiang fault and Tongjiang-Mishan fault respectively, in which Jiayin-Mudanjiang fault is obviously transformed by the northeast structure; ③ Abnormal cutting relation shows that the east-west structure in the west of Meng Xing-Hebei-Heilongjiang area is earlier than the northeast structure, and the east-west structure is earlier than the northeast structure. However, the temporal relationship between the western structure and the eastern structure cannot be determined only by the abnormal characteristics.

Figure 1. 10 Northeast China1:1Wanbuge gravity anomaly contour map

Figure 1. 1 1 filtering results of gravity anomalies in northeast China

1.3.2.2 Zoning characteristics of gravity anomalies

According to the results of small-domain filtering and three-dimensional small-domain filtering, combined with the characteristics of regional gravity field and related geological and geophysical data, this area is divided into two first-class gravity anomaly areas, Meng Xing-Jihei and North China, with the east-west low gravity anomaly zone sandwiched by Hohhot-Zhangjiakou-Chaoyang-Siping fault and Xilamulun River-Changchun fault as the boundary. This broad abnormal boundary zone basically corresponds to the location of the accretion zone on the northern edge of the North China plate, and the Hohhot-Zhangjiakou-Chaoyang linear anomaly on the south side of the abnormal zone basically corresponds to the traditional northern boundary of inner mongolian axis (Ji Shao 'an, 199 1), but the eastward extension information is not obvious. In the Meng Xing-Jihei anomaly area, three important anomaly boundary lines and two northeast anomaly zones transformed from anomaly boundary lines were finely determined by the results of three-dimensional small-domain filtering, and the structural units of gravity anomaly response in Meng Xing-Jihei area were divided according to them (Figure 1. 13). Meng Xing-Jihei area is divided into four secondary gravity anomaly areas from west to east, which correspond to the so-called Ergon-Xing 'an block (I area in figure 1. 13), Songnen block (II area) and Songnen block, respectively, with the northeast Heihe-Baicheng anomaly zone, the north-south Jiayin-Mudanjiang anomaly zone and the north-south Tongjiang-Mishan anomaly zone as the boundaries.

Figure 1. 12 east-west small domain filtering results