Geometries And Kinematics Of Cenozoic Thrust And Fold Belt In The Southern Junggar Basin

The southern Junggar foreland fold-and-thrust belt,located in the Northern Tian Shan ranges,one of the most active orogenic belts in the world,represents the far-field effects of the convergence between the India and Asia blocks.In this study,we mainly focus on the structural styles,kinematics and deformation time of the southern Junggar fold-and-thrust belt,which not only can serve for the oil and gas exploration and seismic hazard prevention,but also deepen the understanding of the dynamic processes of the intracontinental orogenic belts.In this thesis,2D and 3D seismic profiles,field observations and drilling data,integrated with various geometrical and kinematic analytical methods(e.g.,axial surface analytical methods,growth strata analytical methods,area-depth-strain methods and displacement-distance maps)to determine the structural styles,kinematics,amounts of shortening,deformation time,deformation episodes and shortening ratios.The analytic results demonstrate that(1)The first row of anticlinal belt of the Kalazha-Aketun structural belt resulted from the imbricated wedge structures which consumed most of fault displacement of the fore thrusts.The Kalazha and Aketun anticlines,secondary fault-propagation folds,are the result of the breakthrough backthrust.The results of area-depth-strain method applied on the Hutubi and northern Hutubi anticlines show that these anticlines are detachment folds which controlled by the decollement within the Lower Jurassic Badaowan Formation.The development of the folds is mainly controlled by the layer parallel pure shearing.Furthermore,the Hutubi anticline underwent three episodes of crustal shortening,the timing of deformation is Late Jurassic-Early Cretaceous,Late Cretaceous-Early Paleogene and Early Pleistocene to present.The northern Hutubi anticline resulted from two phases of crustal shortening,the timing is Late Cretaceous-Paleogene,Late Oligocene-Late Miocene(2)The first row anticlinal belt of the Qigu structural belt is characteristic by the development of wedge structures and fault-bend folds.The Huo-Ma-Tu thrust fault is one of the most important thrust,which formed the northern limbs of the first row of anticlines,and the upper flat within the Anjihaihe Formation transferred fault slip into the basin forming the Huo-Ma-Tu anticlinal belt.What' s more,another part of fault slip was transferred into the basin along the Lower Jurassic Badaowan detachment,which was consumed by the deep structures of the Huo-Ma-Tu anticline belt and the third row of anticline,all of which are detachment folds.(3)The frontal faults of the Huo-Ma-Tu thrust consist of three branch faults,they are Tugulu,Manasi and Huoerguosi faults from east to west.These branch faults connect with each other by soft-linkage.In the eastern termination of the Tugulu anticline,along-strike variations in displacement affect structural styles of the Tugulu anticline,varying from detachment folding,pure-shear fault-propagation folding,and classic fault-propagation folding to fault-bend folding,which may provide insights into the evolution of fault-related folds.(4)Based on the along-strike variation of the fault slip distribution,the growth strata geometry,the kinematics of the Huo-Ma-Tu thrust fault transfer from classic structural wedging to classic fault-bend folding through intermediate case which firstly forms a wedge structure and then a fault-bend fold.(5)Based on the growth strata and the published magnetostratigraphic data,the initial deformation time of the southern Junggar fold-and-thrust belt is Early Oligocene(?26.5 Ma)with two phases of rapid uplift events during the Late Pliocene-Early Pleistocene and the Late Pleistocene-Early Holocene.The deep structures of the second row of anticlinal belt started to deform since Early Pliocene(?5.5 Ma)which is the same as the deformation time of the Anjihai anticline.The timing of the Huo-Ma-Tu thrust is the Early Pleistocene and lasts to present.Accordingly,the structural sequence of the southern Junggar fold-and-thrust belt is break-forward style,however,the Huo-Ma-Tu thrust is an out-of-sequence thrust fault.(6)Within the Kalazha-Aketun structural belt,the total Cenozoic shortening is?23 km,then the average Cenozoic shortening rate is?0.87 mm/yr,the Cenozoic shortening of the Hutubi anticline is?486 m,the average shortening rate is 0.19 mm/yr,suggesting that most tectonic shortening within this belt is consumed by the range front structures.Within the Qigu structural belt,the total tectonic shortening of the fold-thrust belt is?22 km-25 km,then the Cenozoic shortening rate is 0.83-0.94 mm/yr;the amounts of shortening of the deep Tugulu anticline,the Huoerguosi anticline and the Anjihai anticline are?2.16 km,?3.22 km and?1.53 km,respectively,then the average shortening rates are 0.39 mm/yr,0.59 mm/yr and 0.28 mm/yr.The fault slip of the Huo-Ma-Tu thrust is?8-9 km,thus the average Quaternary shortening rate is?3.1-3.5 mm/yr,which accommodated about 60-75%of the modern shortening rate within our study area,suggesting that Huo-Ma-Tu thrust serves as a principal structure accommodating tectonic shortening in the southern Junggar fold-and-thrust belt(7)Based on the area-depth-strain method applied on the Hutubi anticline,the field observations and the seismic data,integrated with the published low-temperature thermochronology data,provenance analysis data and palaeocurrent analysis data,the Northern Tian Shan and the southern Junggar Basin underwent three phases of crustal shortening since Late Jurassic,the deformation time is the Late Jurassic-Early Cretaceous,Late Cretaceous-Early Paleogene,and Early Oligocene to present.