The Investigation Of Structure And The Mechanism Of Formation Of The Buried Hill In Bohai Area

Buried hill in Bohai Sea underwent the contruction of pre-Mesozoic strata and the destruction of Mesozoic strata,happened severely remould,grew and distributed unclearly,have diversity classification and ambiguity governing factor.All above problems restrict to know the tectonic evolution and dynamic mechanism of the buried hill in Bohai Sea.So this paper depends on drill core,well logging,earthquake,biology fossil,peripheral outcrop section,and so on,and then researches detailly in the stratigraphic characteristics and itsregularties of distribution,genetic types of buried hill,and discusses the main controlling factor of different genetic types of buried hill.The major achievements of this paper show as follow.The material basis of the buried hill in Bohai Sea is the pre-Paleogene strata.By compiling the pre-Paleogene geologic map,the author thinks strata of the buried hill have characteristic in controlling by fault,distribution by annule,full central section,south with older strata and north with younger strata,east and west separately.To be specific,footwall became hill and formed emboss area,and hanging wall formed sunken area.Pre-Cambrian strata and Paleozoic strata mainly distributed in the emboss area,but Mesozoic strata distributed between two emboss areas or in the sunken area,especially the strata in the middle area of the Bohai Sea was most thickness.The emboss area mainly distributed around the sea area and it showed single layer structure with pre-Cambrian or double layer structure with pre-Cambrian and Paleozoic,but because the Mesozoic strata distributed between two emboss areas or in the sunken area,the vertical structure showed double layer structure with pre-Cambrian and Mesozoic or three layer structure with pre-Cambrian,Paleozoic and Mesozoic,respectively.Boundary by Nanpu sunken area-Bozhong sunken area,the south of the Bohai Sea distributed mainly pre-Cambrian and Paleozoic strata,and the north of the Bohai Sea distributed mainly Mesozoic strata.Boundary by Bozhong sunken area-Laizhouwan sunken area,the east of the Bohai Sea distributed mainly Mesozoic and pre-Cambrian strata,and the west of the Bohai Sea distributed mainly pre-Cambrian,Paleozoic and Mesozoic strata.According to the axial direction of fold,the Bohai Sea growed two groups that were nearly EW direction and NE-NNE direction,meanwhile,the fold nearly EW direction suffered remould by the NE-NNE fold and then showed obviously subsection.According to morphological characteristics and mode of motion of fault.the Bohai Sea growed stretch structure,sloping structure and inverted structure.It was an important moulding function to diapir and upwelling for buried hill structure in the granite emplacement mechanism.To sum up above conclusions,the buried hill in Bohai Sea could be divided to erosion model,extrusion model,stretch model,oblique model,transition model and diapir model.In addition,the buried hill in Bohai Sea also could be divided to landform buried hill and structure buried hill by category,emboss buried hill belt,drop-off buried hill belt,hollow buried hill belt,slight grade buried hill belt and the central strike-slip buried hill belt by location,and residual hill,brokeback hill,anticline hill,planing step hill,double break hill,single break hill,negative flower,negative inversion and magmatism by structural configuration.The cause of the buried hill in Bohai Sea is mainly by the nearly EW ancient Asia-tethys tectonic domain formed between the end of Paleozoic and the early of Mesozoic,NE Pacific Ocean tectonic domain formed in the middle and later of Mesozoic,and fault depression formed in the Paleogene.Thereinto,the middle and later of Mesozoic tectonic movement was key mould role of inside story of buried hill,and it showed mainly that the early fault was abviously mould function to the inside story of buried hill.There were NNE or NE fault,nearly EW fault and NW fault in the Cenozoic basin of Bohai Sea.These tree groups fault were all property in inheriting,and inherited or inherited partially and remoulded the pre-Paleogene fracture system.Thereinto,the nearly EW fault appeared at the earliest and formed by squeezing action from north and south in the Indo-china period and the early of Yanshan period.The tectonic style was graben and horst composite construction which was concerning with slippage.The nearly EW fault was remoulded by NNE or NE and NW fault in the middle and later of Yanshan period,and the tectonic style was complex and diversity.So the fault showed short and discrete characteristic in Himalayan movement and the tectonic style grew that horst was overlaid with graben.Because the NNE or NE and NW faults formed in the middle and later of Yanshan period,these faults controlled the taphrogeny in the Cainozoic more highlightly.It reflected in two aspects.One aspect was that the NNE or NE fault controlled the basin in the east of Bohai Sea which was controlled by NE Mesozoic Tan-Lu fault zone and the tectonic styles were graben and horst composite construction,warped tilting fault-block and negative flower.The other aspect was that compared with the east of the Bohai Sea,the inpart of the NW fault was more significant in the west,and the tectonic styles became more diversification.