Study Of Tectonic,Climatic,and Surface Processes In The Hami Basin Of The Eastern Tianshan Since The Cretaceous

The Tianshan orogenic belt,which spans the hinterland of the Eurasian continent,underwent Paleozoic continental marginal accretion-collision and Meso-Cenozoic intracontinental orogeny to become one of Central Asia’s most significant intracontinental orogenic belts.During the Meso-Cenozoic,complex tectonic,climatic,and surface processes shaped the topographic pattern of Tianshan’s alternately spread basins and mountains.The Turpan-Hami Basin is the largest intermountain basin in the Tianshan.Current research on Mesozoic and Cenozoic geological processes in the Turpan-Hami Basin focuses primarily on the Mesozoic period in relation to oil and gas,coal,sandstone,and uranium resources in the Turpan Basin.Despite the fact that the Meso-Cenozoic geological processes in the Hami Basin are rarely reported,there are a few pressing scientific issues that must be resolved immediately.In order to reconstruct the MesoCenozoic tectonic,climatic,sedimentary,and geomorphic development of the Hami basin,comprehensive research in sedimentology,geomorphology,chronology,and structural geology was conducted on the Hami basin in this work.The following are the most important research accomplishments and prospects.(1)The newly discovered Early Cretaceous Liushuquan Formation aeolian deposits in the Hami Basin were deposited in a typical intermountain erg environment,according to sedimentary facies analyses,sedimentary architecture,and vertical sedimentary packing.The extreme arid environment that was governed by the "rain shadow effect" of the Tianshan and the subtropical high-pressure zone contributed to the formation of the Liushuquan Formation desert.The Hami Basin’s interior geography determines the size of the desert.The sedimentary characteristics and chemical weathering index of the Lower Cretaceous in the Hami Basin indicate a transition from an arid to a humid climate from the early to middle Cretaceous,which is associated with the contraction of the Hadley cycle during the "super greenhouse" period of the middle Cretaceous.(2)The sedimentary facies variation and parallel unconformity between the Oligocene-Miocene Taoshuyuanzi Formation and the Pliocene Putaogou Formation in the Hami Basin suggest that the Pliocene Putaogou Formation is the sedimentary response to the uplift of the easternmost Tianshan.The Taoshuyuanzi Formation was deposited in a semi-humid climate,while the Putaogou Formation was deposited in an arid climate,as indicated by the sedimentary features,magnetic susceptibility,color,and chemical weathering index.The climate change from Oligocene-Miocene to Pliocene in the Hami Basin is responsible for the blocking impact on the north-branching wind of the westerly belt following the uplift of the Harlik.This thesis suggests,when combined with the spatiotemporal evolution of the uplift and aridity,that the Tianshan region’s climate aridity expanded from west to east and was influenced by the interaction between the Tianshan’s uplift and the westerly wind belt.(3)This thesis presents the results of an integrated geomorphologic and chronologic study on the origin of terraces,unconsolidated gravel surfaces,and yardang fields in the Hami Basin,as well as the landscape evolution model.The Quaternary alluvial terraces in the Hami Basin formed mostly during wetter intervals as a result of increased hydrodynamics during the interglacial period,which led to the deposition of alluvial terraces.Strong wind erosion caused the mobilization of silt-to sand-sized fractions from the surfaces of Quaternary alluvial sediments,leaving unconsolidated gravels to assemble into desert pavement surfaces.Yardangs were formed when the coarse-grained alluvium on top of terraces was removed,exposing the underlying strata.The geometry of yardangs was determined by lithology,wind,and water erosion.(4)Based on UAV-3D models and electron spin resonance(ESR)ages,this thesis calculates an erosion rate of～0.232 +0.0586/-0.0389 mm yr-1 for the past 233 ky.Using cosmogenic 10Be measurements,the rates of bedrock erosion cluster around 0.121 ±0.0293 mm yr-1 since 11-23 ky.In the Hami Basin,glacial erosion rates were larger than those during interglacial intervals,and this variation is mostly attributable to variations in atmospheric conditions.When comparing erosion rates and wind speeds between the Hami Basin and the Qaidam Basin,erosion rates in the Hami Basin are anomalously low when considering near-surface wind speeds.This disparity is influenced by the landscape evolution of arid regions.Combining the landscape evolution with a general understanding of East Asian climate variability and erosion rates,this thesis concludes that wind erosion and dust production in the Hami Basin were largely regulated by the synoptically controlled spatial-temporal distribution of precipitation and subsequent landscape evolution since at least the late Pleistocene.