Research On Mesozoic-Cenozoic Structural Characteristics And Evolution In Northwestern Margin, Tarim Basin

The western segment of South Tienshan and Pamir foreland fold and thrust belts were formed during Late Cenozoic.They met with each in Kash area,northwest of Tarim basin,causing the overlapping,cutting,interfering and transforming of structures with each other,which resulted in a more complex system.Because of the complexity in structure in Kash area,it become one of the best places for structural analysis of the foreland fold and thrust belts and overlapped structures.However,the restriction of low quality of seismic profiles in the deep part and less knowledge of the subsurface structure makes the exploration of oil and gas of this area still in a lower level.Hence,researches of spatial structures,formation processes and Mz-Kz tectonic history of these two fold and thrust systems play an important role in the researches of Tarim basin and central Asia and can also guide the exploration of oil and gas.Based on analysis of field-survey geological data,geophysical data,natural earthquakes and well data,the writer applied modern theories of structural analysis and geological modeling to the Kash area,and established structural geological model. From the model,it is readily to see the spatial structures and overlapping relationship of the two fold and thrust systems.It is also a basis for research of Mz-Kz tectonic evolution history of the basin.After detailed structural analysis of the shallow part and the natural seismic data,a tectonic model of the West Kunlun(Pamir) and South Tienshan,from deep-level subduction to shallow-level thrusting was established. After this study,some innovative results are listed below:(1) There are two main factors might take responsibility for the lateral variability of the western segment of Southern Tienshan fold and thrust belt.One is the Mesozoic fault system(a part of Feierganna fault) which controls the distribution of fault subsidences and the Jurassic sequences.The other factor that causes the lateral variability is the spatial relationship of South Tienshan and Pamir.That is,in different parts of the western segment of South Tienshan,Pamir would influence the thrust pattern of South Tienshan differently,including the pattern in map view,the cross-section structures and the structural domains.Based on those,the authors divided the western segment of South Tienshan into three different parts from east to west,integrating and synthesizing all kinds of data available,detailed medium-level structural analysis are done to different part of western segment of South Tienshan.(2) Constrained by several conditions,structural analysis is made to build a 2D structural model for the different parts of the northwestern margin of the Tarim basin. This provides a basis for the structural analysis and oil and gas exploration in the study area as detail as possible.The author chose several important cross sections through each tectonic unit for forward modeling and inversion modeling and calculated the structural shortening amounts and shortening rates of the different parts of the western segment of South Tienshan and Wuboer fold and thrust belts.The shortening amount of the east part is 32.64-49.1km,with a 40.5%-50.51%shortening ratio.The S-N shortening rate of the crust in Cenozoic is 9.12-13.72mm/a.The shortening amount of the middle part is 24.23km,with a 34.7%shortening ratio and a S-N 6.77mm/a shortening rate of the crust in Cenozoic.The shortening amount,ratio, rate of of Wuboer area is 32km,40.2%,7.17mm/a,respectively.(3) The Kuzigongsu fault subsidence has several asymmetric half-graben depressions from west to east.These half grabens are characteristic of west-faulting and east-overlapping.According to this phenomenon,the source of the sediments may be mainly from "Wulagen Uplift" to the west when the basin formed.During the Cenozoic,the basin was inversed affected by India-Tibet collision.(4) Three long cross-sections across the two fold and thrust belts that reveal the shallow structural of the thrusting were constructed.It seems that he frontier of Pamir fold and thrust belts is an arcuate structural belts,which is a surficial nappe overlapping the thrust sheets of the South Tienshan that formed the overlapping relation.The South Tienshan fold and thrust belts mostly have deep-level folding and thrusting.The geographic boundary of South Tienshan and Pamir fold and thrust belts is plotted:in the west it is along Kezilesu River north to the arcuate frontier of Pamir fold and thrust belts,then the north fault of Kelatuo-Minraole anticline belts,finally along the Qiakewukedaliya River that separates the Kelatuo- Minraole anticline belts and Kash anticline belts.(5) After interpreting natural earthquakes since 1953,the author also did a exploratory research on the lithospheric tectonic relationship of South Tienshan orogen,and considered in west and east side,West Kunlun(Xindukushi) orogen and Tafim(Tajike) block,and considered that Tarim(Tajike) block subducts beneath the West Kunlun(Pamir) orogen and South Tienshan orogen in two direction;In Pamir area,a syntaxis formed with the West Kunlun(Pamir) orogen obducts upon the South Tienshan orogen,and result in the two great orogens that rejuvenated during Cenozoic and the subduction and jointing relationship of the two orogens and Tarim(Tajike) block,showing a specific basin-and-range coupling system.