Geological Structure,Formation And Evolution Of Chepaizi High In Western Junggar Basin

With its location at the southernmost end of the Western Junggar Nappe,Chepaizi high is the key combination of the Junggar Basin and the western Junggar Orogenic Belt and records abundant information of intracontinental structural deformation.The study on the geological structure,formation and evolution of Chepaizi high is not only helpful to clarify the structural attributes,but also has important scientific and practical significance on studying the deformation process and mechanism of the continental structure and pushing the progress for the hydrocarbon exploration of the deep Carboniferous.Based on a combination of the superficial geology,the latest drilling and 3-D seismic data and geodetic measurements,this study used the fault related fold theory and the methods of isotopic chronology and structural geology to systematically study the geological structure,formation,evolution and genesis mechanism of the Chepaizi high.The analytic results demonstrate that:(1)The isotopic age results of volcanic rocks revealed that the Carboniferous volcanic eruption events were concentrated around 300 Ma,313Ma and 322 Ma.Combined with the analysis of biostratum and lithostratigraphy,the Carboniferous stratigraphic sequence of Chepaizi high from the bottom to the top are the lower Carboniferous Tailegula Formation,Baogutu Formation,Xibeikulas Formation and the upper Carboniferous Halaalate Formation.According to the characteristics of regional unconformities,Chepaizi high is divided into 5 structural layers: Carboniferous,Permian-Triassic,Jurassic,Cretaceous-Paleogene and NeogeneQuaternary.(2)Based on the analysis of structural deformation characteristics and the low-velocity layer and Devonian detachment,the layered geological structure model of Chepaizi high superimposed with the deep duplex,shallow imbricate fan and superimposed structural wedge is established.It is revealed that this high has the characteristics of E-W zonation(it can be divided into back thrust belt,central thrust belt,Cheqian 5 well thrust belt,frontal thrust belt and anterior monocline belt in the dip direction)and S-N segmentation(Middle Permian structural activity is strong in the north and weak in the south,Late Triassic to Jurassic is weak in the north and strong in the South).“Steep on the top and gentle on the bottom” geometry characteristic of Hong-Che fault and Hong-3 well eastern fault which are the eastern boundary of Chepaizi high is indicated.Chepaizi high is the front of thrust nappe.(3)Time and characteristics of structural deformation revealed by fine analysis of seismic profile indicated Chepaizi high underwent five tectonic evolution stages: Middle Permian intense compressional thrust,Late Permian-Late Triassic inherited thrust superposition,Jurassic extension-compression cycle,Cretaceous-Paleogene intracontinental depression adjustment,and Neogene-Quaternary transpression and tilt.(4)Based on the study of fault activity period,intensity,shortening of structural deformation and structural relationship with peripheral structural units,the dynamic model of rotational progressive deformation of Chepaizi high is established.Our results revealed that the tectonic deformation process of Chepaizi uplift since the Permian is the front response of the West Junggar nappe under the background of the convergence of peripheral plates.