Cenozoic Sedimentary Characteristics And Their Tectonic Implications In The West Of Southern Tianshan

The Tian Shan, a Late Paleozoic orogenic belt, has been rejuvenated by the effect of the collision between India and Eurasia during Cenozoic Era.The continuous compression results to the intracontinental deformation in the Tian Shan orogenic belt and accompanied thrusting in the NW margins of Tarim basin. The accumulation of thrust sheets and sediments forms significant gravitational load, which leads to crustal flexing and foreland basin at front of the South Tian Shan, where deposits thick Cenozoic deposits. But the results of this event showed significant spatial variations. We chosed the Tiegerekesazi Cenozoic succession as a representive section to study the sedimentologic features and sedimentary facies changes in the front of the west of South Tian Shan. With the results of the sedimentary research we can get the information about the tectonic activities of the western South Tian Shan in Cenozoic.Detailed study of the Cenozoic events and the sedimentary records at the front of South Tian Shan will provide insights for understanding the spatial and temporal evolution of the Cenozoic tectonic activities in South Tian Shan, the characteristics of the coupling systems between basin and mountain and the intracontinental deformation process.The following primary observations have been highlighted.1.From the Paleogene to middle Oligocene, the study area was dominated by evaporational platform, shallow marine to littoral facies. From late Oligocene to early Miocene the sedimentary environment changed to semi-deep-marginal lacustrine facies and then to shallow-semi-deep lacustrine sand-mudstone sequence in middle Miocene. Late Miocene-Quaternary (upper Pakabulake, Atushi and Xiyu Formation) the sedimentary environment in Tiegerekesazi region changed from braided channel to alluvial fan facies gradually. Since late Miocene the grain size of the sediments and the proportion of gravel were gradually increased. The characteristic of the sediments indicate three individual sedimentary cycles upwardly.2. The South Tian Shan belt has experienced a period of uplifting in late Oligocene which was recorded by the unconformity between Keziluoyi and Bashibulake Formation in Tiegerekesazi region.Since middle Pliocene (upper Atushi Formation) the sediments in Tiegerekesazi is featured by thick layered gravels deposited in alluvial fan system. The gravel was sub-angular to angular, very poorly sorted and dominated by of coarse gravel, suggesting a quick and near-source deposition. These evidences imply that the South Tian Shan belt must have experienced a period of strong and rapid uplifting since middle Pliocene.3. In the east of Tiegerekesazi region, there is a slight angle unconformity between Keziluoyi and Bashibulake Formation in Heiziwei section and a hiatus at the top of the Bashibulake Formation in Keziluoyi section. In the whole piedmont of western South Tian Shan the sedimentary environment changes from marine to terrestrial facies in late Oligocene-early Miocene. All those features reflect an uplifting event in late Oligocene (-25Ma) of South Tian Shan belt, which has also been documented by previous results from apatite fission track thermochronologic and magnetostratigraphic results.Since the middle Pliocene, the South Tian Shan belt must have experienced a period of strong and rapid uplifting. The deformation intensity in the belt descreases gradually from west to east, which may relate to the northward propagation of the Pamir. The strong and rapid deformation since middle Pliocene is recorded by the widespread conglomerates deposited in alluvial fan system, the appearance of igneous and metamorphic gravels, and the rapid increace of the grain size as well. The ongoing uplift also results to the regional unconformity between the Quaternary Wusu and Xiyu Formation.4.The investigations on sedimentary measurement and description and gravel counting of Late Miocene to Quaternary sediments (upper Pakabulake, Atushi and Xiyu Formation) in piedmont Tiegerekesazi sedimentary succession have also been carried out in this study. The results indicate three individual sedimentary cycles, within each of which the sedimentary environment transfers from low-to high-energy system. Consistent with the sedimentary features, the gravels show angular trend in psephicity and increasing trend in grain size within each single cycle. These evidences imply that the distance between the sedimentary source region to the depositional area become shorter and shorter. The results generally suggest three separate tectonisms in western South Tian Shan, which provide new constraints on the mountain propogation in this region.