Research On The Cenozoic Subsidence Evolution In The Tarim Basin And The Process Of Uplift Of The Tian Shan

The collision between the Indian and Asian continents caused large-scale intracontinental deformations and rejuvenated the Tian Shan,the western Kunlun range and other ancient orogens.The mountain buildings in the Tian Shan caused the flexural subsidence in the northern parts of the Tarim Basin and formed the foreland basins coupled with the range.The uplift of the Tian Shan is recorded notably by the sedimentary sequences and subsidence history of the coupling foreland basin.We restrict the age of the initial uplift of the Tian Shan and the underthrusting rate of the Tarim Block beneath the Tian Shan,by deciphering the architecture and evolution of the foreland unit of the southern Tian Shan.Uplift of the Tian Shan and the western Kunlun range caused flexural subsidence in the southern and northern parts of the Tarim Basin.Furthermore,the basin became a closed continental sedimentary basin in the Cenozoic due to uplift of the ranges.The Cenozoic Tarim Basin is formed by flexural subsidence and ponding.We modelled the process of the flexural subsidence and the sediment ponding using the finite elastic plate model.We separated the sediment drape from the flexural subsidence based on the modelling results.Our modelling also reveals migration of the flexural bulge and the evolution of topographic loading on the south and north margins.We analyzed a 353-km-long seismic profile and logging data of four wells in the southern Tian Shan foreland area to decipher its tectonic,stratigraphic,and subsidence history.The sedimentary sequences comprise the Cambrian-Silurian,Devonian-Permian,Triassic,Jurassic-Cretaceous,and Paleogene tectonostratigraphic units,overlain by the late Oligocene-Quaternary foreland basin unit.The Jidike Formation is the oldest sedimentary sequence of the foreland succession,deposited at?26 Ma based on magnetostratigraphic constraints,indicating that uplift of the southern Tian Shan Mountains was initiated by at least?26 Ma.In addition,the tectonic subsidence rate in the southern Tian Shan foreland area increased significantly since?26 Ma due to lithospheric flexure caused by building of the topography of the southern Tian Shan Mountains.The envelope line of the forelandward onlaping points within the foreland unit records the migration of the southward onlaps in the foreland unit.The envelope indicates that the southern Tian Shan foreland basin unit onlap southwards at 1.6±0.1 mm/yr between?26.3 Ma and?12 Ma and at 14.6±0.1 mm/yr after?12 Ma.The southward onlap rate of the the southern Tian Shan foreland basin is the upper limit of the convergence rate between the Tarim and Junggar blocks.The increase of the onlap rate at?12 Ma suggests an accelerated convergence between the Tarim Basin and the Tian Shan.We modelled the basement subsidence history using a two-dimensional(2-D)finite elastic plate model based on the sedimentary architecture shown in the north-northeast trending seismic profile across the basin.The seismic profile reveals that Cenozoic strata comprise a wedge of sediments in the southern and northern regions generated by flexural subsidence overlain by a more uniform regional sediment drape.We obtain the subsidence profiles across the basin at?26 Ma,?13 Ma,?5 Ma and the present day using backstripping based on the sedimentary architecture shown in the seismic profile.We use a numerical finite elastic plate to model these subsidence profiles.Our modelling indicates that the southern and northern flexural depressions started to interfere to form a single flexural bulge within the basin since late Paleogene times.The flexural bulge migrated?52 km toward the western Kunlun range since?26 Ma,reflecting a decrease in the ratio of loads of the western Kunlun range versus the Tian Shan.We separate the sediment drape from the flexural subsidence according to the preceding modelling results.Our separating demonstrates that the thickness of the drape increased from 230±30 m at?26 Ma to 1910±200 m at present,occupying?30%of Cenozoic strata in the profile.The rise in the base level since the Paleogene is estimated as 356±80 m by correcting for sediment loading of the drape.The underthrusting rate of the Tarim Block beneath the Tian Shan and the western Kunlun range were constrained by the sedimentary architecture revealed by fine analysis of the seismic profiles crossing the foreland areas of the western Kunlun range and the Tian Shan.We estimated the historic widths of the Tarim Basin at?26 Ma,?13 Ma,and?5 Ma based on the analysis.We adopt the decreasing basin width to model the evolution of the flexural subsidence,suggesting significant uplifting of the western Kunlun range and the Tian Shan from?26 Ma to?13 Ma,relative stable elevations in these ranges from?13 Ma to?5 Ma,and their rapid uplifts since?5 Ma.The modelling also suggests that the Te values for the ancient Tarim lithosphere must be underestimated if the reduction in the basin width due to the underthrusting is completely overlooked.The predicted increase in the optimal value of Te since?26 Ma is consistent with the cooling history of the Tarim basin.We probably overestimated the Te value because of neglecting the effect of the sediment drape in the subsidence modelling in the Tarim Basin.