Multistage Evolution Of The Basin-and-Range Structure Of The Eastern Section Of The Tienshan Mountains

The eastern section of the Tienshan Mountains with its typical basin-and-range structural pattern becomes the important window for understanding the tectonic evolution and geodynamic mechanism of the continent abdominal region of Xinjiang and Central Asia.Based on integrated analyses of the sedimentary sequence, tectonic evolution and geodynamic mechanism of the Bogda Mountains and foreland basins on its both sides, namely the eastern Junggar basin and the Turpan - Hami Basin, this thesis acquired the following major understanding:The basin-and-range continent geological structural pattern has already been shaped basically in the eastern section of the Tienshan Mountains and its adjacent areas in the Late Paleozoic era, which was the result of the closure of the Paleo-asian Ocean and its special but complicated Central Asia type orogeny. Afterward this pattern has undergone a multi-phase complex evolution.Using the method of tectosedimentary unit analysis, according to the isopach map and sedimentary facies diagram of 13 structure beds, this thesis recovered the eastern Junggar basin prototype nicely, which could be divided into six stages that have different features in sedimentary formation and structure reform.Reconstruction of the eastern Junggar basin prototype revealed that the shape and the central axis direction of the deposition sub-centers were in continuous changing, particularly in the Jimsar depression. Detailed studies of the sediment and structures of the basement and its cover strata indicate that it is related with the continuous rotation of the basement block, especially in the late Hercynian-Indosinian when the geodynamic environment in the region underwent substantial changes.The chessboard pattern of alternatively arranged uplifts and depressions is the major structural feature of the eastern Junggar basin region. Different structural combination constructed the Kelameili fault zone, Qitai uplift basement deep fault, Santai uplift boundary fault and Zhangbei fault-fold belt, which show different types of tectonic styles and tectonic deformation, and acted as the important geometry and dynamic boundary and strain records of the complicated tectonic evolution of the eastern Junggar basin region.In the Indo-Yanshanian, as the Siberian plate and the North China-Korea plate collided to form the Asian continent assembly, the eastern Junggar basin region was situated at the western end of the compressional front, where the stress was concentrated, and with the compression of transpressional deformation of the Kelameili fault zone and the Bogda piedmont fault zone on both, the nearly triangle eastern Junggar basin region block escaped westward, and to the front of the tectonic escape intense compression constructed a large thrust-back fault-bend type composite fold belt, namely the Zhangbei fault-fold belt. This thesis also argued that the strongest performance and stereotype of tectonic escape in the late Jurassic might be related to the eventual closure of the Okhotsk Ocean and subsequent collision of the Siberia plate with the North China-Korea plate in far east.According to integrated analyses of the stratigraphy sequence of the Paleozoic sediments exposed in the current Bogda orogen, ancient geography situation of the Bogda area since Late Paleozoic was reconstructed, which can be divided into two completely different tectonic evolution stage: the Hercynian paleo-Bogda continental rift and the cyclical revived (paleo-) Bogda intraplate orogen since the Indosinian.The Hercynian sedimentary succession in the Bogda area had similar deposition system constitution and three-dimensional distribution pattern with typical rift in the world elsewhere, which gradually transformed from littoral volcanic eruption to shallow marine clastic-carbonate formation, deep water sea basin flysch, sea-land interfingering and coastal river-lagoon deposition from top to bottom, recording the complete history of the paleo-Bogda rift basin evolved from the initial rift to strong split and from sink to shrink.The paleo-Bogda rift generally matched with the present distribution of the Bogda Mountains, and the morphological character of the rift was similar to that of typical continental rifts, showing half-graben appearance on the cross-section. The south and north slopes differentiated in topography and deposition, but acted as comparable counterparts. And the rift could be divided into the western Urumqi-Qitai segment and the eastern Mulei -Qijiaojing segment, which alternated in polarity and had obviously different sedimentation and structure evolution characters, and the Daheyan-Mulei transfer zone acted as the transition between these two segments.Based the arguments that in late Paleozoic the paleo-Bogda rift had no direct sedimentary comparability with the western Yiliankhabirga residual ocean basin, and terminated by the Kelameili- Maiqinwula- Harlik orogen in the east, it is suggested that the eastern rift segment started rifting earlier than the western segment and had more complex rift evolution features than the later. This thesis concludes that the paleo-Bogda rift was an impactogen, which is a scissor tension crack extending from the orogen into the Junggar-Tuha block when the Junggar-Tuha block and the Siberia plate strongly collided to form the Kelameili-Maiqinwula-Harlik orogen in the Carboniferous.The outcrop sequence analysis revealed that since the Paleo-Bogda rift inverted to orogen in the late Paleozoic, this area had experienced three successively stages of rejuvenation and planation in the Indosinian, Yanshanian and Himalayan. From the perspective of basin-mountain coupling, this thesis has adopted a series sedimentary indicators of different scales to deduce the orogen evolution history and its features, such as the basin-fill stratigraphy framework, unconformity surface, wedge-shaped coarse clastic aggradation, sedimentation patterns, river types, debris constitution, deposition rate, stratigraphy magnetism and magmatic activity, etc., and stressed that structure is the main controlling factor of the basin-range evolution and the sedimentary sequence development.Based on the Shanshan-Gaoquandaban-Mulei highway geological section which crossed the Bogda Mountains completely, and dozen other short cross-sectional observations, this thesis concludes that the overall structural style of the present towering Bogda Mountains is a giant double-thrusting nappe, with a series of faulted block thrusting from the interior orogen to the north and south sides, respectively. This double-thrusting nappe and the corresponding brittle thrust structure in the foreland basin form a complete thrusting system. The double-thrusting nappe is a large thick-skinned inversion structure originated from the half-graben type Paleo-Bogda rift, and this was not formed just simply in the late Cenozoic Himalayan movement, but is full record of the multi-stage revival orogeny since the late Paleozoic, with the main structure formed in the Indo-Yanshanian, and finally stereotyped in the Himalayan.Based on the phenomena that the mountain uplifting was often superposed with synchronous slipping, the sedimentation centers in the foreland basin often migrated directionally, this thesis concludes that there were abundant strain distribution processes in this region, and the (paleo-) Bogda Mountains was a typical transpressional orogen, with the transpression as the dominant mechanism of the cyclical revival orogeny, expressed in geomorphy as a narrow band chain. Further analysis also argues that the right lateral transpression caused the paleo-Bogda rift to invert to orogen in the Indosinian, and the orogen underwent left lateral transpression in the Yanshanian-Himalayan period.The basin-and-range structural pattern of the eastern section of the Tienshan mountain has suffered long-term intraplate tectonic compression since its first emergence in the Late Paleozoic, and experienced a complicated multi-stage evolution. This thesis generalizes the main features of the evolution as follows: 1). The tectonic process directly related to plate separation and assembling is the main factor controlling the evolution of the basin-and-range structural pattern; 2). Basins and mountains have relatively stability as they often remained in the same place over long time; 3). The deep faults (belt) at the edge of the block often acted as the important deformation boundary condition during the basin-and-range tectonic evolution; 4). Strike-slip, transpression, block rotation and tectonic escape are significant forms of the relative movement and adjustment between blocks during the basin-and-range tectonic evolution; 5). The multi-stage evolution of the basin-and-range structural pattern revealed the regional tectonic evolution sensitively. Investigation on the evolution characters of the basin-and-range structural pattern of the eastern section of the Tienshan Mountains since late Paleozoic can help understanding not only the specific process of its evolution, but also the main factors and mechanisms controlling this complex multi-stage intraplate deformation process, which would also provide a better understanding of the regional tectonic evolution of Central Asia and adjacent regions.Placing the basin-and-range structure pattern of the eastern section of the Tienshan mountain under wider regional tectonic evolution background of the north Xinjiang and even the vast Central Asian region could help identifying the main characters and the geodynamic mechanism of the basin-range evolution effectively, and even further provide guidance to the research of regional geology and other similar areas. And this method was tested on the five different basin-and-range evolution stages in the final part of this thesis, obtaining two interesting conclusions worth further investigation:A,In the earlier Indosinian, the continental basin-range pattern similar to present emerged in north Xinjiang. At the same time, large-scale ductile shearing and relevant block rotation occurred in both north Xinjiang and central Asia or even the broader global range. These phenomena may all have been caused by the global scale relative movement between the north and south continents as they were assembling to form the Pangea ,namely the Pangea megashear.B,In later middle Jurassic epoch, the north Xinjiang area underwent intensive tectonic movement and obvious change of the basin-range structure pattern, especially in the eastern Junggar basin area. Most previous studies suggest that the plate colliding at the southern edge of the Asian continent was the driving force, which was related to the closing of the Tethys Ocean. But this thesis would emphasize that the tectonic drive was from the north, namely the closing of the Mongolia-Okhotsk Ocean and the southward indentation of the Siberia plate. Based on that there existed a complex series of regularly arranged structures in north Xinjiang-Mongolia-Xing'an'ling area, such as thrusting, strike slip, tectonic escape, magmatic activity, mountain rise, and basin subsidence, and all their formation were evidently relevant to the closing of the Mongolia-Okhotsk Ocean and the subsequent violent collision and convergence between the Siberia plate and the north China-Amuria plate, this thesis proposes a geodynamic model which figures that there once developed an ancient plateau in the ring zone around the Siberia plate, namely the paleo-Mongolia plateau, whose tectonic evolution was similar to the present Tibetan Plateau.