On the Bouguer gravity anomaly map of southeast China (Figure 3- 1), according to the Bouguer gravity anomaly and the difference of strength and shape with different tectonic units, the southeast region and local Bouguer gravity field can be divided into different regions. Generally speaking, there is an obvious correspondence between the fluctuation zone and the gravity anomaly field. The negative gravity anomaly of Dabie and Wuyi Uplifts is prominent, and the Bouguer gravity anomaly is (-30 ~-40) × 10-5m/s2. The depression zone is mainly consistent with the gravity cascade zone. There are areas with high relative gravity values in Wuchang and Nanchang, and the abnormal values are 0 ~ 40×10-5m/S2, which is the reflection of Cenozoic sedimentary basins in Jianghan and Poyang Lake. In the study area, from the change of Bouguer gravity anomaly value, Bouguer gravity anomaly increases from northwest to southeast, and Bouguer gravity anomaly of Eurasia plate is -50× 10-5 m/s2, and rapidly increases from southeast to Pacific plate to 330× 10-5 m/s2, showing obvious gravity gradient zone, which is different from before. It is worth noting that under the background that gravity anomalies in this area are generally distributed in the northeast, there is a NW-nearly EW-trending gravity kink zone, which shows the composite structural pattern of the deep basement in the east-west direction and the northeast direction, which is closely related to the transformation of the tectonic environment in this area from Tethys tectonic domain to Pacific Rim tectonic domain.
Fig. 3- 1( 1× 1) Schematic diagram of Bouguer gravity anomaly along the southeast coast (according to Yin Xiuhua et al. )
Because Bouguer gravity anomaly is a reflection of the surface topography and rock density balanced by the change of mantle uplift, the fluctuation state of Moho surface can be inverted by using gravity anomaly. According to the research data of Sun Wenzhong et al. (200 1) on Bouguer gravity anomaly in southeastern China, the Moho surface in southeastern China rises sharply to the west with Changsha-Hengyang-Hexian-Nanning line as the boundary, corresponding to the deep mantle depression in Xuefeng Mountain. In the east of this line, the Moho surface is relatively flat, generally 30 ~ 32 kilometers, and it reaches 36 kilometers in some areas. The general trend of Moho depth is consistent with the change trend of regional gravity field and terrain average elevation, that is, Moho surface gradually deepens from east to west, Bouguer gravity anomaly decreases and terrain increases. According to the depth and fluctuation of Moho surface, the mantle morphology of southeast continent can be divided into the following two areas from east to west.
(1) Mantle Depression-Mantle Slope-Mantle Uplift-Mantle Flat Area in Zhejiang, Fujian and Jiangxi
This area is generally distributed in the northeast, and further divided into: (1) Longquan-Guangchang mantle depression area (depth 32.5 ~ 34 km), corresponding to Wuyi uplift; (2) Jiaojiang-Xiapu mantle slope belt (depth 30-32km), which is consistent with the metamorphic uplift belt along the southeast coast; (3) Gutian-Huichang-Yingde mantle slope belt (depth 30-32km) and Yong 'an-Meizhou-Huizhou mantle gentle slope belt (depth 32-32.5km), Dehua still has primary mantle depression areas corresponding to Yong 'an-Meizhou-Huichang depression belt; (4) Xingguo-Lianshan Man Ting area (32-32.5km deep) is equivalent to the northeast of Nanling Belt.
(2) Zhejiang-Jiangxi-Hubei-Anhui-Sumantan depression-floodplain uplift-floodplain slope-floodplain area.
This area mainly includes the following subregions: (1) equiaxed mantle depression (depth 32.5~33.5km), which coincides with Jiangnan uplift; (2) The NE-trending Nanchang mantle uplift area (depth 30 ~ 3 1 km) corresponds to Poyang Lake Basin; (3) Wuchang mantle uplift area (depth 30 ~ 3 1 km) is NW-trending, which is consistent with Jianghan basin; (4) Huangmei-Nanjing mantle uplift (depth 30 ~ 32.5 km) is distributed in the northeast, corresponding to the Lower Yangtze Depression; (5) The Dabie mantle depression (depth 32 ~ 35 km) is elliptic, which is consistent with the Dabie uplift.
The surface structure of the earth is a comprehensive reflection of the deep structure, especially the structural landscape since Cenozoic is related to the ups and downs of Moho surface. In the above mantle structural zoning, the structural landscape of "three ridges and three depressions" can correspond to mantle depression and mantle uplift one by one, indicating that the Cenozoic region has the characteristics of "basin-ridge" structural zoning. The mantle depression area is equivalent to the "mountain system" composed of Dabie Mountain, Wuyishan Mountain and Jiangnan Uplift, all of which show negative gravity anomaly fields. The former two show prominent negative anomaly fields, while the latter only corresponds to relatively weak negative anomaly fields. In Bouguer gravity anomaly, the depression area is mainly characterized by high relative gravity, gravity gradient zone or transition zone of positive and negative anomalies, but it is consistent with Mesozoic and Cenozoic sedimentary basins. On the one hand, this "basin-ridge" structural zoning is caused by Cenozoic extensional rifting, on the other hand, it inherits the result of Mesozoic "basin-mountain" structural evolution, and is also proved by the characteristics of Cenozoic basins superimposed on Mesozoic basins in the region.