THE DEEP SEISMIC REFLECTION PROFILE UNVEILS FINE STRUC-TURE AND TECTONIC CHARACTERISTICS OF THE CANGXIAN UPLIFT,HUANGHUA DEPRESSION,AND ADJACENT AREAS
A comprehensive seismic profiling study was conducted across the Jizhong depression,Cangxian uplift,Huanghua depression,and Chengning uplift in the North China Plain to investigate crustal fine structure and the relationship between deep and shallow faults.Two profiles were completed:a deep seismic reflection profile spanning approximately 200km and a middle-shallow seismic reflection profile covering about 66km.Our results indicate a crust thickness of approximately 30 to 35km along the section,with a thin distribution in the east and thick in the west.Notably,there is a significant uplift on the Moho surface beneath the Jizhong depression,with an uplift amplitude of about 2 to 3km.The deep seismic reflection profile reveals distinct upper and lower structural characteristics of the crust.The upper crust displays typical sedimentary layer reflection characteristics,marked by alternating depressions and uplifts.Numerous large-scale faults are concealed beneath the North China Plain,playing a pivotal role in uplift and sag formation.The lower crust's reflection structure comprises events with significant changes in reflection energy,unstable stratification,and complex occurrences,contrasting with the strong reflection energy and good horizontal continuity of the upper crust reflections.The piedmont fault of the Taihang Mountain,identified by the mid-shallow seismic profile and deep seismic reflection section,is a large shovel-shaped normal fault with a low angle,linked to the large detachment structure at the eastern margin of Taihang Mountain.The detachment structure is developed between the basement and the sedimentary cover layer,and is shown on the profile as a reflection zone consisting of 3 to 4 strong reflection phases,lasting 0.3～0.4 seconds.This detachment structure controls the formation of graben and horst structures.The Jizhong depression may have been an extensional tectonic system formed in the upper crust on the basis of the extensional detachment of the Taihang Mountain front fault.The deep seismic reflection section highlights the North China Basin's structural features,characterized by alternating depressions and uplifts,with boundaries clearly delineated by faults such as the Cangxi,Cangdong,and Chengxi faults.These faults control the formation of graben and horst structures and are considered concealed active faults since the Quaternary period.The Cangxi fault,as the eastern boundary of the Jizhong depression,developed in the weak zone of the front thrust nappe tectonic area of the detachment slip structure.The fault plane resembles a plow shape,steep at the top and gently sloping at the bottom.It utilized or transformed the early thrust section,which is now manifested as a west-dip normal fault,controlling the basement structure and stratigraphic sedimentation on the west side of the Cangxian uplift.The Cangdong fault is the eastern boundary fault of the Cangxian uplift,which appears as an east-dipping shovel shaped normal fault on the profile,cut through the reflection waves of the Carboniferous-Permian strata,the Cambrian-Ordovician strata,the Proterozoic strata,and the crystalline basement.It terminates at the interface of the upper and lower crust at a depth of about 18km.The Chengxi fault is a west-dipping normal fault,which cuts through the Cenozoic sedimentary layer at a depth of about 600～700m in the shallow section.It terminates at the interface between the upper and lower crust in a shovel shaped normal fault downward.The deep seismic reflection section also clearly shows the coexisting structural morphology of uplift and depression.Multiple secondary faults that tilt in the same direction or opposite direction to the main fault are developed inside the depression,causing the depression to be divided into multiple secondary structural units,resulting in the complexity of the entire fault basin structure.In conclusion,the development of boundary faults plays a decisive role in the stratigraphic sedimentary and tectonic deformation of the strata within the depression.The existing deep and shallow structures and tectonic patterns in the Wuji-Yanshan section of the North China Basin are formed by the"graben-horst"structure developed in the upper crust,the complex fault combination style near the surface,the stratified reflection and the upper and lower superimposed reflection structure developed in the lower crust,and the undulating Moho surface.The findings of this study contribute to the seismological understanding of the dynamic processes occurring in the North China Basin,as well as to the analysis of the structural relationship between deep and shallow structures in the region.
Cangxian upliftHuanghua depressionJizhong depressionDeep seismic reflectionCrustal structure and tectonics
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NSTL
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