Study On The Reconstruction Of Mesozoic-Cenozoic Paleoelevation In The Western Tianshan

The Tianshan mountain is located in the middle part of the Central Asian orogenic belt,stretching for nearly 2500km from the east to the west as a whole.The formation of the modern Tianshan is not only directly related to the plate subduction and collision orogenesis in Paleozoic,but also influenced by the far-distance effect of Indian-Asian collision in Cenozoic.The Cenozoic uplift and exhumation history of the Tianshan Mountain can help us to study the kinematic and dynamic processes of orogenic belt inside Asian continent.In order to better understand how and when the topography of modern Tianshan was build,this study on reconstruction of paleoelevation in Sikeshu drainage basin,Tuoyun basin,north and middle part of west Tianshan.In addition,from the study of structure deformation and sedimentation in foreland basin,we discuss the the history of the Mesozoic-Cenozoic paleoelevation in different parts of west Tianshan mountion.This paper presents oxygen and carbon isotopic composition from gastropoda fossils,calcareous carbonate cements,and shell sandstone in the Anjihai and Shawan formations,located in the Sikeshu drainage basin,the north of Tianshan.The Anjihai and Shawan formations were previously dated of 17.5-28Ma using high-resolution magnetostratigraphy method.Correlations between the stable isotopic compositions and lithofacies suggest that the Sikeshu drainage basin was mainly a hydrologically open environment.It confirmes that the? 13Cc and ? 18Oc values from the Anjihai and Shawan formations have not yet been reset by late-stage diagenesis based on oxygen isotope of the gastropoda fossil,petrographic examination,and tectonic deformation of strata.The paleoelevations are reconstructed using the corrected paleosurface water ? 18Opsw values,recovered from gastropoda fossils,calcareous carbonate cements,and shell sandstone.The empirical formula of the North Tibet was selected in this study to calculate the palaeoelevation when the Anjihai and Shawan formations deposited.The paleoelevation of the Sikeshu drainage basin of the north of Tianshan had attained an elevation exceeding 2000m at least 28Ma ago.The vesicles in basaltic lava flows can be used to determine paleoelevation at the time of eruption.Here we present basalt-vesicularity based estimates of the paleoelevation of the Eocene basalts in Tuoyun basin of southwest Tianshan mountain.Choosing two suitable basalt flow,two vesicular basaltic rocks have been collected from bottom to top along the lave flow outcrop in Tuoyun basin.The thickness of lava flow could be measured in the field,and sized of the vesicles for samples could be merely determined by 3D X-ray Tomography in situ.Eruptive age of the basalt flow is 48Ma.The final calculation results have indicated that paleoelevation of the Tuoyun basin basalt flow is about 2000m during Eocene.The elevation of the Tuoyun basin is more than 3000m now,which is the result of the late Cenozoic uplift of the Tianshan orogenic belt.Cretaceous uplifting-exhumation process is widely developed in different tectonic units in Xinjiang.Apatite Fission tract dating and thermal history modeling have been carried out from 17 granite samples collected from Boluokelu mountain,Yilianhabierga mountain and Duku road to investigate the uplifting-exhumation processes of mountains.The research area include north part and middle part of western Tianshan.Fission-track ages for these samples range from(81.6±4.9)to(45.4±3.2)Ma with mean track lengths varying from(12.62±0.17)to(13.53±0.14)?m.Temperature-Time modeling of apatite was completed based on date measurements of the fission track length and age.The time for rapid cooling events on north part of west Tianshan mainly concentrated during 50?70Ma,and the time for middle part of west Tianshan mainly concentrated during 70?90Ma.In combination with other geological evidence,it is suggested that the western Tianshan appear differential uplift and exhumation process in late Cretaceous.Using fission track date to reconstruct antiquated topography,which indicate that the change of paleoelevation in the middle part of western Tianshan is larger than the north part of western Tianshan.Isostatic recoil played an important role in the changes of the western Tianshan since the late Cretaceous.Cenozoic deposits on the piedmont of southwest Tianshan indicate the processes of uplift and erosion of the Tianshan mountain.This study area is located in the foreland basin near the Baleigong pluton.Pebbles of Cenozoic conglomerate in the Cangzongse and Liyan formations have been measured and calculated.The characteristics for structure deformation of the foreland basin are also studied.The results show that the components of the pebbles are mainly sedimentary rocks and contain a small amount of carbonate rocks and metamorphic rocks.It is worth noting that there is no granite pebbles in the Cenozoic conglomerate.This result indicates that the Baleigong pluton has not been stripped out of the surface until late Pleistocene.From the study of structure deformation in mountains,the thrust structure in the southwestern Tianshan mountains are the dynamic source of uplift.Based on the analysis of rock uplift and surface uplift,the southwest Tianshan mountain were shaped since late Pleistocene.Based on the reconstruction of the paleoelevation in the typical area of the western Tianshan,and the research for sedimentology and tectonic deformation,this paper discusses the history of the paleoelevation change in different parts of the western Tianshan.The northern Tianshan was slowly rising from the plain to mountain during Cretaceous to Paleogene,denuded in Oligocene to Paleocene,and the rapid uplift of the late Oligocene.The central Tianshan area is mainly Yili basin,which was in a state of stable denudation during Cretaceous to Paleogene,the Yili basin was downfaulted and the surrounding mountains rapid uplift since the Miocene.The southern Tianshan was a rapid uplift event in the early Cretaceous,and keep the denudation stage until the Oligocene,the rapid uplift of the Miocene.The southwestern Tianshan were high mountains during Cretaceous to Paleogene,denuded in Oligocene to Paleocene,rapid uplift from the Pleistocene.it is suggested that the western Tianshan appear differential uplift and exhumation process since the Mesozoic.This event can be controlled by multi-factor.The distant effect of the India-Asia collision is the dynamical source.The main control factor are related to the thermal-rheological structures and different structural boundaries interaction.