1. Gravity anomaly characteristics
The general feature of the gravity field in the south-central segment of the Tan-Lu fault zone is a NNE gravity gradient zone (attached figure 1). Its eastern boundary is a suddenly dense gravity gradient zone, and its western boundary shows a transition zone with different gravity fields. Some local traps are distributed obliquely on the fault zone regularly. The gravity field characteristics of this fault zone are also obviously segmented.
Shandong section and Jiangsu-Anhui section: From Weifang to Juxian, the fault zone is located in a dense gravity gradient zone that transits from Jiaolai positive anomaly area in the northeast to Taiyi negative anomaly area in the northwest. From south to Jiashan, the gravity field of the fault zone is mainly composed of Linyi high gravity, Tancheng low gravity, Suqian-Xinyi high gravity and Suqian south low gravity. The gravity anomaly in Jiashan-Lujiang section is disturbed by the NW-trending structure, and the characteristics of the west boundary are vague and difficult to distinguish, while the characteristics of the east boundary are obvious.
Wei Ying profile: The gravity field in Bohai Sea is obviously a NNE-NE linear anomaly zone. The central Bohai Sea is divided into north-south abnormal sections by gravity height. Laizhou Bay on the east side of the southern section is a gravity gradient zone extending to the sea in Shandong section. The gravity field in this belt is composed of local positive and negative abnormal traps, and its width is increased to 60 kilometers. The west branch of the gravity gradient zone disappeared in the middle of Bohai Sea after being adjusted by the north-south gravity uplift. The northern section turns eastward, turning into a northeast gravity cascade belt, crossing Liaodong Bay and passing through the lower Liaohe Plain to Shenyang.
The Moho contour map of the Tanlu fault zone shows (Attached Figure 6) that the uplift zone of the upper mantle of the fault zone is not obvious, and the Moho surface is slightly shallower than the surrounding areas, and the Moho surface in Bohai Sea is the shallowest, with a depth of 29km, and the burial depth in other areas is between 35 and 36 km.
2. Characteristics of aeromagnetic anomalies
The shallow metamorphic rocks or sedimentary rocks in Guyu, Paleozoic, Mesozoic and Cenozoic in the study area are weakly magnetic or nonmagnetic, and generally do not cause obvious magnetic anomalies. According to the characteristics of magnetic anomalies (amplitude, gradient, shape and scale), the study is divided into three magnetic fields: east, west and Tancheng-Lujiang magnetic anomaly zones, with the middle-south section of the Tancheng-Lujiang fault zone as the boundary.
It can be seen from the aeromagnetic △T contour map (Attached Figure 2) of the south-central section of the Tan-Lu fault zone that the Tan-Lu fault zone generally presents a linear magnetic anomaly zone with a sharp change in NE. The magnetic field in the survey area is complex and changeable, and the amplitude of magnetic anomaly changes greatly. The amplitudes of positive and negative magnetic fields generally range from -200 to +300 nt, and there are magnetic anomalies greater than -300nT and +800nT in some areas.
Positive magnetic anomaly zone of Wei Ying profile: Liaodong uplift profile is a linear negative anomaly zone with ne direction, with an anomaly amplitude of 0~-250nT, and a positive anomaly with an amplitude not greater than 0~30nT is superimposed on this negative magnetic field background. The anomaly in the low uplift area of Dong Bo presents a rising NNE-NE linear positive anomaly zone, with an anomaly amplitude of 0 ~ 200 nt and a local negative anomaly of-50 ~-150 nt. The local abnormal peak value reaches n× 100~ 1000nT, indicating that there are a large number of magmatic rocks.
Positive magnetic anomaly zone in Shandong section: this section runs through the whole territory of Shandong along the direction of NE20, and the magnetic field presents a series of long and narrow beaded anomalies in NNE, which is obviously different from the magnetic fields on both sides.
Jiangsu-Anhui section of positive magnetic anomaly belt: this section is generally a positive magnetic anomaly belt arranged in the northeast-north-south direction, which changes sharply near Tancheng and is a multi-peak anomaly with a peak value of 800n t·mt The intensity of magnetic anomaly weakens from north to south, and moves from north to south. Its west side is a regional negative magnetic field with positive and negative changes, and its strike is mostly near east-west, corresponding to the metamorphic basement of Neoarchean Wuhe Group (Ar2wh) or Proterozoic Fengyang Group (Pt 1yf). The east side is dominated by stable magnetic field, corresponding to metamorphic basement. In the south of Mingguang (Jiashan), the magnetic anomaly zone suddenly narrows, with a width of only about 10km, showing a right-handed diagonal line.