The Deformation Controlling Factors Of Fold-and-thrust Belt In SW Tarim Basin:Physical Analogue Modeling Study

The collision of Indian and Eurasian plates, resulting the Western Kunlun orogenic belt reactived and the Pamir thrust northwards during the Cenozoic, formed a complex spatial heterogeneous fold-and-thrust belt in SW Tarim Basin. This fold-and-thrust belt caught lots of attention of scholars domestic and abroad beacuse of its abundant geodynamic implications and good oil and gas exploration prospect, however the research in this area was still in a low level, especially in the deformation mechanism as the region was covered by thick Pliocene-Quaternary conglomerate, which significantly decreased the quality of sesmic images. In this thesis, combined with the study of structural deformation characteristics by formers and based on the structural analysis of the fold-and-thrust belt, I used physical analogue modelling method to stduy the deformation mechanism of the fold-and-thrust belt and tried to explore the main deformation controlling factors of the thrust belt and answer the question why the structural styles differed so much among the different parts. The main conclusions are summrized as follows:1. The physical analogue modelling study indicated that the syn-thrusting sedimentation played a key role in the deformation of fold-and-thrust belt. The syn-thrusting sedimentation significantly increases the distance and rate of foreland propagation, and the sedimentation pinching-out point is the preferential place where the front thrust developed. The volume of syn-thrusting sediments would exert an important effect on a underlain frictional detachment (such as mudstone). In the cases of thin or no syn-thrusting sediments, the frictional detachment would not act as an effective detachment but was cut through by all the thrust faults, which means that the structural levels up and down the detachment deformed coordinately. In the case of thick syn-thrusting sediments, the frictional detachment acts as an effective detachment, which causes the deformation uncoordinated. Meanwhile, the volume of syn-thrusting sediments would also determine the structural styles of the fold-and-thrust belt. The syn-thrusting sedimentation would change the dynamic balance of the thrust wedge, render the thrust wedge "sub-critical"(the taper angle lower than the critical taper angle of the wedge), under this condition the deformation would develop in the rear part of the wedge to enlarge the taper angle and remake the wedge critical. In the case of thick syn-thrusting sediments, the syn-thrusting sediments would inhibit the upward propagation of deep basal thrust faults, and instead the faults propagated along the detachment, resulting a duplex in deep to enlarge the taper angle. In the case of thin syn-thrusting sediments, the brittle detachment couldn’t act as an effective detachment and some major back thruts would develop in the foreland, resulting pop-up structure to enlarge the taper angle. The striking similarity between the model with thick syn-thrusting sediments and the real geologic section in Kedong-Kekeya area, SW Tarim Basin implies that the overlying thick Pliocene-Quaternary sediments play a major role in shaping the structure pattern in this area. The thick syn-thrusting sediments inhibited the upward propagation of deep basal thrust faults, resulting duplex structure in deep, at the same time the thick syn-thrusting sediments induce the Early Paleocene gypsum mudstone to be en effecive detachment and helped propagating the deformation front far towards the Tarim Basin, formed the Jiede anticline.2. The physical analogue modeling confirmed that the palaeo-uplift in a fold-and-thrust belt would paly an important role in the later period compressional deformation. The palaeo-uplift under a ductile detachment (such as salt or gypsum) is the preferential place that the front thrust fault develops. At the same time, the palaeo-uplift distribution range in the basin would also greatly influence the structural deformation. During the compressional process, the front thrust fault would firstly develop in the areas with palaeo-uplift distribution, and then tear and propagate towards the areas without palaeo-uplift distribution. During the process of tear and propagation, an arc-shaped front thrust fault was formed on the surface. The front thrust fault of Wupoer fold-and-thrust belt is the Wupoer fault, this fault exerts en arc-shaped on the surface, and the west fragment of the fault thruts up to ground surface while the east part is concealed underground and covered by the Quaternary sediments. Our physical analogue modeling indicated that the Wulagen palaeo-uplift which located in the northwest of the Wupoer fault played a key role in controlling the structural deformation in this area. Because the existence of Wulagen palaeo-uplift, the western part of Wupoer fault firtsly developed in the Wulagen palaeo-uplift area and teared and propagated towards east, resulting the Wupoer fault exerted an arc-shaped on the surface. Because the western and eastern parts of Wupoer fault didn’t develop synchronously, resulting the west part thrust up to ground surface while the east part concealed underground.3. In the SW Tarim Basin, a basal Cambrian salt detachment was widely distributed in the basin but pinched-out towards the orogenic belt. At the same time the spatial distribution of the salt was not homogeneous. In the Sugaite area, the Cambrian salt deposited closely to the West Kunlun Mountain, while in the Kedong area, the salt deposited away from the piedmont, rarely deposited unless Guman area to the north. The physical analogue modeling indicated that the spatial distribution of deep detachment also plays a very important role in controlling the deformation styles and deformation front of a fold-and-thrust belt. In the Sugaite area where the Cambrian salt deposited closely to the West Kunlun Mountain, the deformation style emerged in a thin-skin structure style, and the deformation front propagated faster than the estern Kedong area. And in the Kedong area, where the Cambrian salt deposited far away from the West Kunlun Mountain, it mostly formed thick-skin style imbricate thrusts in the pediment area, in the Guman area where existed Cambrian salt the deformation emerged in a thin-skin like way.