Research Of The Structural Evolution And Its Sedimental Response In Changling, Shiwu Faulted Depression Of The Sourthern Songliao Basin

Guiding by the rules of the global plate tectonics mobilism and the basic theory of spatial and temporal/episodic development of regional geology, using the new concepts and methods of the Modern Tectonic Geology as well as the Basin Analysis, this paper discussed the geometric, kinematical and geodynamical characters of the Changling Area's faulted depressions in the southern Songliao rift by the comprehensive research basing on the geological field investigation, volcanic features, seismic profiles, drilling and logging data Basing on those studies, it described the structural evolution of the fault depressions in Changling Area during the early Cretaceous and discovered the differentiation of the strain pattern during the crustal extension in the southern Songliao Basin.The stratigraphies of Changling Sag Basin from Cretaceous to Quaternary could be divided into three Tectono-stratigraphic units. The lower unit filled into the faulted depression can be also divided into four groups, such as: Huoshiling Group, Shahezi Group, Yingcheng Group as well as Denglouku Group from the bottom to the top, which represent the different stage's sediments of early Cretaceous. The surface between the fault basin and depression basin is defined in the middle of Denglouku Group. The types of sediment facies, such as fan delta sediment, braided delta sediment, alluvial fan sediment, fluvial plain sediment of braided river as well as lacustrine sediment, organized together and reflected the control of the margin of syn-normal fault and the broken-slope to sediment facies.The syn-sedimentary faulting cut into the basement have two mainly directions-the NE and the SW- which controlled the formation of the fault basin. The analysis of the balanced section based on the seismic profiles across the fault basin along NE direction and SW direction indicates that the extension which formed the fault basin turned from NE to nearly E-W along the clock direction during the stages mentioned above. At the beginning, Huoshiling stage, the direction of extension is along NE, and the center of subsidence composed three belts and distributed mostly along the NW at Changling fault depression and Qianan fault depression. Following that, the extension direction turned during the Shahezi stage. Finally, the mainly extension direction is nearly along the E-W during the Shahezi stage and Yingcheng stage. It formed a union subsidence belt along NE direction at the three fault depressions. This process can also be find at Shiwu fault depression-a half-graben formed at the same time southeast to Changling area.Accompanied by the change of extension direction, the structure patterns also changed at different stages during the forming of the fault basin. The analysis of structure pattern basing on the seismic section show there are two kinds of strain pattern forming the fault basin at Changling area. At the first stage, the main structure pattern was graben and horst which usually represented the strain pattern of pure-shear deformation. After that, it changed into imbricate half graben which formed usually under the Sample shear conditions. The second type overlaid on the first and made the structure of Changling area complex.The studies show that there are some relationship between the thickness of crust or lithosphere and the strain pattern during the extensional process. The Pure-shear Model presented by the way of graben or horst-a symmetry extension-was formed on the plateau background with thicker lithosphere and the Simple-shear Model presented by the form of metamorphic core complexes model-a dissymmetry extension- was usually formed on the thinner lithosphere. Both of the two deformation models are not only exist in the different layer of lithosphere, but also exist in different stage during the extension process.