A Study On Gravitational Field And Tectonic Characteristic Of Tianjin Sheet

Area of Tianjin sheet is 1:1000000 scale which includes the whole Tianjin sheet. The geographical position is in the north latitude of 36o~40o, east longitude 114o ~ 120o and total area is 23.4×104km2. It is from west of Taihang mountain to east of Bohai and Laizhou of Shandong, and from north of Yanshan to south of Tai’an City. Because it experienced several tectonic and sedimentary evolution, its surrounding uplifts and Bohaiwan basin have different characteristics of sedimentary strata. The geophysical and geological work of the whole area is in different levels, which is research degree is higher in Bohaiwan basin and little lower in other zones. So far, it has not system completed using gravity data to study structure in area of Tianjin sheet. So studying tectonic system and development of sedimentary strata of this area is done through gravity data combined with the geological and geophysical research.Through systematic collecting and handling datum to get the Bouguer gravity anomaly data, and then a series of work including Trend analysis, Analytical continuation, Moving average filtering, Level first derivative, Vertical second derivative are made to obtain the basis maps of related parameters in Tianjin sheet. Meanwhile, a comprehensive research about the characteristics of gravity anomaly is made using structural geology, regional geology, seismic survey and well drilling. A analysis about factors of local gravity anomaly shows that the high value is caused by basement uplift and the low value is caused by local increasing thickness of Cenozoic & Mesozoic.In this paper, it has used the characteristic of gravity field and previous researches to infer the fracture system in this study area. There are there main types of fracture, including NE(NNE), NW and near EW. And different fractures turn into the grid in the plane. According to the features of Bouguer gravity anomaly, it has been divided into Thaihang uplift, Yanshan fold belt, Bohaiwan basin, Luxinan uplift and Ludong uplift structure units in area of Tianjin sheet. Among Bohaiwan basin has been divided into five depression zones and two uplift zones tectonic units.At the same time, it has been chosen six profiles which can control main tectonic framework of Bohaiwan basin to make inversion fitting on the basis of seismic survey and drilling data. The thickness of Cenozoic in the study area is received through taking advantage of linear regression and making qualitative and quantitative interpretation. The results show that it is a little thiner in the uplift(salient) which mainly contains Neogene and Quaternary, whose minimum deposit depth is about 500 m and maximum deposit is less than 3000 m. And thickness is greater in the depression(sag) which mainly contains Palaeogene, Neogeneand and Quaternary, whose average deposit depth is more than 3000 m, and in subsidence center is more than 8000 m.