Cenozoic Tectonic Events And Their Sedimentary Records In The West Kunlun

By the effect of the collision and continued convergence between India and Eurasia, the West Kunlun Orogen reactivated and thrusted to the Tarim in the Cenozoic, which located in the northeastern margin of Pamir. Studies of Cenozoic tectonic events and their sedimentary records in the West Kunlun will provide important implications to the evolution of Qinghai-Tibet Plateau, the far-field effects of India-Eurasia collision, and the process of large-scale intercontinental subduction in this area.Based on the studies of Cenozoic sedimentology, structural deformation and evolution in frontal areas of the West Kunlun, combined with thermochronology, the following implications have been achieved.1. The present tectonic pattern of studying area took shape in the late Miocene-early Pliocene. On the aspect of structural style, they display characteristics of S-N zonation, E-W segmentation and vertical-layered structural variation. Four structural segments, for example, the Wupor strike-slipping segment featured by the Quaternary piggy-back basin and the underlying thrusting deformation, the Qimugen strike-slipping segment characterized by the stacking triangular zones and the gentle northern wings of the anticlines, the Qibei-Kekeya thrusting segment featured by the triangular zones and the steep northern wings of the anticlines, and the Pishan-Hetian nappe segment featured by the distanced nappe and the underlain of deep and shallow stratigraphic systems, have also been identified from west to east of the frontal areas of the West Kunlun Orogen.2. Regarding deformation timing, the anticline zones are featured by a forward-breaking propagation pattern, propagating gradually from Orogen to basin. For example, the deformation of the first anticline zones started in the early depositing period of the Atushi Formation of early Pliocene, and the fourth anticline zones began to deform during the late Pleistocene. Meanwhile, the starting deformation time of the anticlines in the same row gets younger gradually from west to east confirmed by the depth of growth strata gradually raise. 3. On the aspect of deformation intensity, the shortening values and rates of Wupor and Qibei-Kekeya segments are greater than those of Qimugen segment, respectively. The early Pliocene intensity is greater than latter stages. Also evident is that the deformation intensity in the same row becomes smaller gradually from west to east.4. Cenozoic sedimentary evolution of the southwestern Tarim Basin can be divided into five stages:the Paleocene-middle Oligocene stable marine sedimentary period, the late Oligocene-early Miocene unstable sedimentary period in west zone, the middle Miocene relative stable sedimentary period, the late Miocene-early Pliocene relative unstable sedimentary period, and post-mid-Pliocene very unstable sedimentary period.Within these sedimentary stages, the unstable sedimentary stage in the late Oligocene-early Miocene has been recorded by unconformity between Keziluoyi Formation and Bashibulake Formation, the fan delta deposits in the west passing to meandering river deposits in the east zone. The relative unstable sedimentary stage in the late Miocene-early Pliocene is featured by the upward-fining sedimentary cycles composed by the interbedded deposition of the coarse and fine clastic, the enrichment of unstable heavy mineral in some segments, the appearance of the granite gravel and the change of the regional paleocurrent directions in the late Miocene-Early Pliocene. The very unstable sedimentary records since middle Pliocene are the two regional unconformities, within which one located between Xiyu Formation and Atushi Formation and the other located between Wusu Formation and Xiyu Formation, gravelly alluvial fan system and the large-scale sediment of coarse clastic.5. Cenozoic subsidence centers do not only transfer from the West Kunlun Orogen to the Tarim basin but also parallel to the strike of the Orogen. The subsidence centers locates in the area between He-1 well and Pishan County in Paleogene, and moved to the triangular-shaped area of the Gu-3 well, Gu-2 well and Yecheng County. This indicates that the West Kunlun Orogen propagated not only from the Orogen towards the Tarim basin in the vertical direction but also from NW to SE in the lateral direction. 6. Based on the analysis of sedimentary records, structural deformation and thermochronology, following uplifting periods have been identified in the West Kunlun Orogen:local uplift in the late Oligocene-early Miocene, pulse uplift in the late Miocene, strong uplift in the late Miocene-early Pliocene, and rapid uplift since the mid-late Pliocene.In the late Oligocene-early Miocene, the uplift of western segment of the West Kunlun Orogen results to the unconformity between Keziluoyi Formation and Bashibulake Formation, and the coarse gravel sediment in Keziluoyi Formation. In the late Miocene, the pulse uplifting activity results to the coarse clastic deposition alternating with fine clastic, and the enrichment of unstable heavy minerals in some segments. In the late Miocene-early Pliocene, the strong uplifting activity leads to the appearance of the granite gravel, the change of the regional paleocurrent directions, and the unconformity between Atushi Formation and Bashibulake Formation in eastern part of Qibei-Kekeya structural segment. The present tectonic pattern in the front of the West Kunlun Orogen took shape after this tectonic period, displaying spatially S-N zonation, E-W segmentation and vertical-layered structural variation, and also the deformation getting younger gradually from Orogen to basin. Since mid-late Pliocene, the rapid uplift of the West Kunlun Orogen results to large-scale alluvial fan system and two regional unconformities, one located between Xiyu Formation and Atushi Formation while the other locate between Wusu Formation and Xiyu Formation.