Activity Of Fold-related Faults In The East Segment Of The Qiulitage Anticline Belt, South Piedmont Of Tian Shan

Active thrust faults and folds are one of major subjects in research of active tectonics. Both the two kinds of tectonic deformation usually occur in a juxtapositional manner. On one hand, changes of the shape and slip of a deep detachment fault or thrust fault can control the growth of the overlying folds, forming different types of thrust-related folds.On the other hand, during the deformation of the folds, secondary faults can be produced to accommodate strain variations related to structural and stratigraphic position. Active tectonic mapping focuses on the surface structure. However, in active folding zones, different types of active faults are likely to expose. Thus, ascertaining the structure attributes of the surface faults is fundamental to thoroughly and scientifically understand seismogenic structures in an active folding zone, and related directly to the quantitative study of active tectonics and assessment of seismic risk.The Kuqa depression located in the middle of the south piedmont of Tian Shan is a foreland basin with very thick Mesozoic and Cenozoic sediments. It began receiving sediments on a large scale since the late Triassic, resulting in a complete sedimentary sequence of the Mesozoic and Cenozoic. The southern Tian Shan began to intensively uplift and thrust to the basin since the Oligocene. The Mesozoic and Cenozoic strata were involved deformation from north to south progressively, forming 4 nearly EW trending thrust-fold belts which are typical thin-skinned structures. Of these, the Qiulitage anticline belt is the third row in the Kuqa depression. The study area of this thesis is the east segment of this anticline belt which lies east of the Yanshuigou River. It is made up of two sub-anticlines:Kuchetawu anticline and east Qiulitage anticline. Based on geological and geomorphologic characteristics of the surface faults acquired by field investigations and subsurface structure from petroleum seismic profiles, this work analyzes the distribution, kinematic property, activity and formation mechanism of the surface faults in the study area. The conclusions of this thesis are stated below.(1) There are 4 reverse faults in the east segment of the Qiulitage anticline belt:F1 in the core of the Kuchetawu anticline, F2 in the north limb of the Kuchetawu anticline, F3 in the south limb near the core of the eastern Qiulitage anticline, and F4 in the south limb of the eastern Qiulitage anticline. F1 is an extension branch of the detachment fault developing in the Paleogene salt layer with a shortening rate of 0.32mm/a. Evidence shows it is a latePleistocene fault. F2 developed in the fold hinge in front of the Kuchetawu anticline in a parallel group and has a discontinuous distribution. It is a fold-accommodation fault controlled by local compressive stress.Trenches around Kurangkan present silp-rate of several segments of F2, and they have a large difference ranging from 0.27mm/a to 0.09mm/a. F3 is a low-angle thrust fault, likely a branch of the upper ramp which controls the development of the eastern Qiulitage anticline. This fault did not offset the T2 river terrace of the Bositan River, thus was an inactive fault since the late Quaternary. F4, is a shear-thrust fault, which developed in the steeply dipping frontal limb of the fault-propagation folds, and also characterized by group occurrence and discontinuous distribution. Geological profile around the outlet of Bositan River shows F2 has a shortening rate of 0.79?1.11mm/a. Trenches around Gekuluoke present silp-rate of several segments of F2, and they have a large difference ranging from 0.09mm/a to 1.23mm/a.(2) The range-frontal faults in the east segment of the Qiulitage anticline belt are merely fold-accommodation ones. However, trenching confirms that these fold-accommodation faults have been active since the late Quaternary and have recorded part of paleoearthquakes in the active folding zone. Fresh fault scarps on either side of the anticline belt (trenches:KCD-1?KC-1?KC-3?DQ-3) have been seen, which may be the products of the Kuqa 71/4 earthquake of 1949. Thus major quakes that took place in the active fold area may have generated surface ruptures on either side of the anticline zone, and the recurrence may be?1.5ka.(3)The surface anticlines of rapid growth and associated accommodation faults are the manifestations of the deep faults that experienced complex folding deformation and propagated upward to the near surface, serving as an indicator of faulting at depth. The fold-accommodation faults are merely local deformation during the folding process, which are indirectly related with the deep faults that control the growth of folds.