Regional geophysical field characteristics

1. Regional gravity field characteristics

The southeastern Anhui area is a regional low gravity area, the gravity gradient zone is olive-shaped, and the anomalies generally spread in the northeast direction. The macroscopic manifestation is an elliptical ring with relatively high gravity in the middle and low gravity, and the regional field value changes about -20×10-5 ~ -35×10-5ms-2 (Figure 3-3-1).

Figure 3-3-1 Bouguer gravity anomaly map in the southeastern Anhui region

The gravity anomaly trends in the southeastern Anhui region include nearly east-west direction, northeast direction, north-northeast direction, etc., and the shapes are Different. The Qimen-Tunxi (Huangshan City) gravity high zone, which is offset in the east-west direction, is generally distributed in a wave shape, with field values ??>-40×10-5~-30×10-5ms-2. The Guniujiang-Cheng'an, Yixian-Huangshan, Jingde, and Jixi Fuling gravity low belts, the north-northeast gravity low belt at the junction of Anhui and Zhejiang, and the east-west gravity low belt at the junction of Anhui and Jiangxi are distributed around the central gravity high, forming Inner band. To the east, gravity lows such as Fengshulong, Shitaifantan, Qingyang Jiuhua Mountain, Jingxian Langqiao, Ningguo Jichuan, Guangde Liucun, Ningguo Xianxia and other areas are spread along the boundary gradient zone, forming the outer part of the southeastern Anhui anomaly area. ring belt. The gravity low field value of the inner ring is generally <-50×10-5ms-2, and the field value of the outer ring is generally <-30×10-5~-40×10-5ms-2.

Macroscopically, the regional gravity field has a mirror image relationship with the terrain, that is, the mountains show low gravity and the basins show high gravity. The gravity gradient zone corresponds to the transition interval between basins and mountains.

According to gravity field theory, regional anomalies reflect the nature and occurrence characteristics of large-scale deep field sources. Generally, gravity anomalies are caused by lateral residual density. In the southeastern Anhui region, the crust is thickened and the lower interface is depressed. There are large-scale relatively low-density granite layers in the crust. The geological structure is mainly the northeastern section of the Jiangnan Block. The regional gravity gradient zone reflects the steep change zone in the depth of the deep crust interface— - Deep fracture. According to gravity data analysis, it is believed that the Dongzhi-Qingyang-Ningguo gravity gradient zone reflects the structural traces of the Jiangnan Deep Fault Zone, providing important information for deep structural research. Many low-gravity anomaly circles such as Jingde show extensive development of intermediate-acid buried rock batholiths. The Qimen-Tunxi gravity high area indicates that the relatively high-density basement rock series is thicker; the Tunxi area is the gravity high center, which proves that the low-density Mesozoic and Cenozoic sediments are not thick.

2. Regional magnetic field characteristics

The regional magnetic field in the southeastern Anhui Province can be macroscopically divided into a strong magnetic anomaly area in the northeast direction in the southeast of Anhui Province and a northeast-trending low slow magnetic field in the junction area of ??Anhui and Zhejiang. The abnormal zone is the Ningguo-Tunxi (Huangshan City) negative zone. The aeromagnetic T anomaly shows the characteristics of mainly positive values, alternating between positive and negative, and complex changes. Its field value (after polarization) changes in the range of -300 ~ +600 n T (Figure 3-3-2), which illustrates the The area has frequent magma activities and complex geological structures. At the same time, it also clearly reflects the spatial distribution of intrusive rocks and the location and direction of fault structures.

Figure 3-3-2 Aeromagnetic pole anomaly map in southeast Anhui

The magnetic field area in southeast Anhui is roughly bounded by the Huangshan District-Yixian area and can be divided into east and west. part. The Shitai area in the west is a strong magnetic anomaly area. The background anomaly is elliptical in the northeast-northeast direction, extending about 140km, about 70km wide, and with an amplitude of 100-200 n T. The center of the anomaly is located in the Qidu area of ??Shitai, and its southwest side is connected to the strong magnetic field area in the Xingzi area of ??Jiangxi to the east. The surface is mainly Paleozoic strata, with Tanshan granite bodies and some diabase porphyry rocks exposed. However, the Qingbaikou System-Mesoproterozoic rocks in the basement of southeastern Anhui are non-magnetic. It is speculated that such a large-scale background magnetic anomaly is caused by the Paleoproterozoic-Neoarchean crystalline basement. Superimposed on the above-mentioned primary background field are secondary magnetic anomalies caused by intrusive rocks such as Qingyang, Qimen and Chengan.

The eastern part of southern Anhui is dominated by closely spaced strip magnetic anomalies in the north-northeast direction, including Yunling-Hongtian in Huangshan District of Jingxian County, Maolin-Tongshan of Jingxian County, and Tingxi- Abnormal zones such as Langqiao, Huangshan District-Xiancun, Yixian, Jingde Qiaoting-Baidi, Jixi Xiaotou-Shexian Daguyun, etc. have field values ??of 200 to 300 n T, accompanied by obvious low value zones. The abnormal morphology reflects the hidden occurrence of medium-acid intrusive rocks.

The geological structure of the junction area between Anhui and Zhejiang belongs to the western Zhejiang massif. There are magnetic anomalies in the northeast direction such as Shitakeng, Xiaoxiucun, Jixi Xiaoyao, Ningguo Xianxia and Zhongcun in She County. The field value is generally 50 ~100 n T, most of them are caused by tectonic magma zones.