Paleo-mesozoic Fault System And Tectonic Evolution In Gaoqing Area

Gaoqing area is located in the southwest of Dongying sag,the transition area between southeast edge of Huimin depression and Qingcheng salient.Based on the analysis of the static and dynamic characteristics of Paleozoic-Mesozoic fault system,the tectonic evolution process in the Gaoqing area has been defined.Combining with the regional geodynamic background,the mechanism of typical structural styles in Gaoqing area has been discussed.The Paleo-Mesozoic faults in Gaoqing area are mainly composed of nearly EW,NWW and NE faults.The faults in different trends cut and stagger each other.Both extensible and compressional structures occur in Gaoqing area.The main faults contains Linnan fault,Pingnan fault,Gaoqing fault and Huagou fault.The secondary faults are mainly EW trend,and few are NE,NWW and nearly SN trend.The main and secondary faults have typical assemble pattern of parallel,en echelon and oblique on the plane.Extensional,extensional-strike slip,negative inversion structure and extrusion structure style can be observed on the profile sections.The tectonic evolution stages of Gaoqing area can be divided into seven parts:early Paleozoic-Middle Triassic,Late Triassic,Early-Middle Jurassic,Late Jurassic-Early Cretaceous,Late Cretaceous,Paleogene and Neogene-Quaternary.In early Paleozoic-Middle Triassic,Gaoqing area deposited stably.The fault system had not yet developed.In Late Triassic,the Pingnan fault with NWW trend began to develop under the squeezing action from SWW,and Gaoqing area suffered overall uplift and erosion.In Early-Middle Jurassic tectonic stable period,the fault activity rate of Pingnan fault decreased apparently.There was no new fault developing.The deposition process was the erosion of convex and deposition of concave in undulating terrain elevation.In Late Jurassic-Early Cretaceous,with the extension in NS and NE direction,Pingnan fault negative inverted,and secondary normal faults in EW trend had developed.In Late Cretaceous,Gaoqing area suffered squeezing action in nearly EW direction,entirely uplifted and eroded in this period.Fold and reverse fault in nearly NS trend had been developed.In Paleogene,faults in nearly EW and NE trends developed in Gaoqing area under the extension in SN and NW-SE direction.Linnan fault,Gaoqing fault and Huagou fault began to develop.Qingcheng uplift formed in this period.In Quaternary,tectonic activity in Gaoqing area gradually weakened,and the whole area depressed in this period.The study of genesis mechanism of typical tectonic style in Gaoqing area show that Pingnan fault negative inversion is formed by the compressional stress from SWW in Late Triassic and the SW-NE extensional stress in early Cretaceous.The fold near Gao43 well is caused by the regional compressive stress in EW direction formed by two mechanisms in Late Cretaceous.One is the subduction between plate and continent and the formation of new lithospheric mantle in Late Cretaceous.The other is that dextral slip faults can derivate compressive stress in EW direction.The combination of region and local stress field is the reason of fold formation.The grid fault system in the central region is formed by the stress field in the multi period tectonic movement and multi direction,mainly formed in Early Cretaceous and Ek-Es₄ depositional period in Paleogene.