Spatiotemporal Patterns And Geodynamic Mechanism Of Late Quaternary Deformation In The Northern Chinese Tian Shan Foreland

The Tian Shan experienced rejuvenation and uplift during the late Cenozoic as a result of the compressional collision between the India and Eurasia tectonic plates.The Tian Shan,located in the interior of Asia,is recognized as one of the most dynamic orogenic belts.Its unique geological characteristics make it an ideal natural setting for studying the mechanisms of deformation in active orogenic belts within continental interiors.Consequently,it has emerged as a prominent field of interest in the tectonic geomorphology research.A series of thrust-related anticlines developed in the piedmonts as a result of the orogen’s uplift and expansion.Both the tectonic deformation and the geomorphic development of the surrounding area were heavily influenced by these anticlines.Therefore,it is crucial to ascertain the deformation features of these piedmont structures in order to gain insight into the deformation process and strain distribution pattern of the Tian Shan during the late Cenozoic.This information will also aid in unraveling the dynamics mechanism underlying the deformation of active orogenic belts within the interiors of continents.The Urumqi Depression,located in the northern piedmont of the Chinese Tian Shan,features four rows of fold-and-thrust belts（Belts I,Ⅱ,Ⅲ,and IV,denoted basinwards）.These anticlinal formations have a general east-west distribution,laid out in an en echelon pattern with an east-west range of～250 kilometers and a north-south range of～50 kilometers.Tectonics deformation inside fold-and-thrust belts is the focus of this research,which examines its spatiotemporal features.The primary object of structural and geomorphologic studies are the Huoerguos fault and Tugulu fault in the second row of tectonic belts,the Dushanzi anticline in the third row,and the Xihu anticline in the fourth row.Following careful fieldwork to characterize fluvial geomorphic sequences,we use optically stimulated luminescence（OSL）and cosmogenic radionuclide(¹⁰Be)depth profile dating to ascertain the epochs during which the geomorphic surface was formed.The amount of recorded deformation（fault slip,crustal shortening）on the geomorphic surface is calculated using high-precision topographic data acquired via unmanned aerial vehicle（UAV）topographic mapping and differential GPS measurement（d GPS）,in conjunction with the thrusting fault-related fold model.Monte Carlo simulation method is employed to estimate the deformation rates and then determine the spatial deformation characteristics along the strike of single structures.By integrating the findings of previous studies on the southern and northern piedmonts and intermountain basins,as well as those from GPS velocity field analysis,this work provides a thorough comprehending of the spatiotemporal characteristics of strain distribution and tectonic deformation rate in the northern Tian Shan and the Tian Shan orogenic belt during the late Quaternary.Finally,the dynamic mechanism of tectonic deformation in the Tian Shan during late Cenozoic is discussed.The main results of this paper are as follows:1.Deformation characteristics of the individual structure in the northern piedmont of the Chinese Tian Shan（1）Huoerguos thrust fault of Blet ⅡA series of terraces at the Jingou River,which runs through the fault’s middle,and the Sangequan River,which crosses through the fault’s east segment,were displaced by the fault in the late Quaternary.Based on geomorphic dating and deformation measurements,we know that the fault slip rate at the Jingou River is 1.0+0.22/-0.15mm/yr and that at the Sangequan River it ranges from 0.69+0.18/-0.11 mm/yr to1.02+0.15/-0.13 mm/yr since～13 ka.The results show that the slip rates are uniform distributed along the strike of the fault.（2）Tugulu thrust fault of Belt ⅡThe Manas River,Taxi River,and Hutubi River sequentially cross the western,central,and eastern regions of this fault.The terrace surfaces formed by these rivers experience deformation as a result of fault activities.The dependable amount of deformation of the geomorphic surface is derived by integrating high-precision topography data with the fault-bend fold model.Slip rates of the fault of 0.73 mm/yr（Manas River）,1.41 mm/yr（Taxi River）,and 0.82 mm/yr（Hutubi River）have been determined since～15 ka.These findings point to a"bow"shaped distribution of deformation rate along the fault’s strike in the late Quaternary.（3）Dushanzi anticline of Belt ⅢFour alluvial fans and seven terraces are preserved along the path of the Kuitun River as it cuts through the anticline’s middle area.The listric fault-bend fold model and the area conservation model are used together to quantify the magnitude of deformation of the geomorphic surface.Based on the ages of the terraces,we estimate the fault slip rates at the three different locations of the anticline to be 1.1 mm/yr（at the eastern part of the anticline）,2.72 mm/yr（in the middle of the anticline）,and 1.28mm/yr（at the western part of the anticline）since～112ka.Crustal shortening rates of0.83 mm/yr,2.23 mm/yr,and 0.9 mm/yr are corresponding to these slip rates.It is thought that the"bow"shape has been present in the deformation rates along the strike of the anticline since the late Pleistocene.Since～13ka,the central part of the anticline has experienced a fault slip rate of about 2.88 mm/yr and a crustal shortening rate of about 2.15 mm/yr.The structure deformation of the anticline displays a possible acceleration trend during the late Quaternary,when compared to the crustal shortening rate of～0.44 mm/yr for the past 4.8 Ma.（4）Xihu anticline of Belt IVThere are three rivers that runs in south-north direction through the anticline area,from west to east.Each of these rivers features three filling terraces.Using the terrace surface as a reference,the crustal shortening rate at the east-central location of the anticline has been ascertained to be 0.81+0.09/-0.1 mm/yr using the detachment-fold model and the age of the geomorphic surface(～173 ka from ¹⁰Be depth profile dating).At the middle of the anticline,the geomorphic surface has recorded crustal shortening rates of 0.64+0.05/-0.07 mm/yr（since～60 ka）.The rate of shortening calculated at the anticline’s easternmost point was 0.52+0.04/-0.04 mm/yr（since～22 ka）.At the anticline’s westernmost point,the rate was also determined to be 0.81+0.09/-0.1 mm/yr（since～11 ka）.According to the results,the Xihu anticline’s deformation rates show a"bow"-like distribution along its strike.Along the strike of the faults/anticlines,a"bow"-shaped distribution pattern of the deformation rates can be seen.In particular,faster deformation rates are seen in the middle and slower ones at the ends.The Huoerguos fault is the only outlier,which may be because no deformation data are available at its endpoint.In this study,we draw a conclusion on the deformation pattern of in an en echelon structures inside the thrust extrusion tectonic setting.It is hypothesized that the overlapping position of the echelon distributed structures acts as a collectively strain absorber.As a result,the deformation rate of a single structure is greatest at its midsection and diminishes toward its ends along its strike.In addition,the rate of deformation in the individual structure’s central region is taken to be more indicative of the structure’s degree of deformation.2.Spatiotemporal patterns of tectonic deformation in the northern piedmont of the Chinese Tian ShanCrustal shortening rates for individual structures have been determined to be primarily concentrated within the range of 1-2 mm/yr in the second and third rows of the anticline belts,as evidenced by deformed geomorphic surfaces.Nevertheless,the rate of the fourth row of anticline belts has slowed to～0.5 mm/yr since the end of the late Pleistocene.In the late Cenozoic,the rate of crustal shortening in the single structure of the fold-and-thrust belt was less than 1 mm/yr,as revealed by previous investigations employing growth stratigraphy and magnetic stratigraphy,that indicates a very homogeneous distribution of deformation rates over this region.Based on these findings,it appears that compressive strain in the late Quaternary was first concentrated in the second and third tectonic zones,and has since spread to the fourth row of anticlines.Geomorphic evidence suggests an acceleration tendency in the rates of tectonic deformation in the piedmont region during the end of the late Pleistocene.3.Spatiotemporal patterns and possible dynamic mechanisms of tectonic deformation in the Tian ShanThe current active tectonic configuration of the Tian Shan orogenic belt consists of a high-angle strike-slip thrust fault system within the interior mountain and a low-angle thrust nappe located at the piedmont of the mountain,according to previous research on the tectonic deformation occurring in the southern and northern piedmont and the intermontane basins.The fault strike-slip rate and crustal shortening rate,as determined through the use of GPS measurements and geomorphic evidence,exhibit a fundamental level of agreement in the different time scale.Based on geomorphic evidence,we know that the crust is shortening at a pace of 8-11 mm/yr（83^o-87^oE）in the north-south direction.Since this rate is consistent with the 8-10 mm/yr obtained from the GPS velocity field,it is possible that the tectonic deformation of the Tian Shan since the late Quaternary to now has been influenced by a common source of earth dynamics and that the distribution of strain has remained constant over this time period.Analyses of GPS data and geomorphological evidence have shown a similar trend,with a larger tectonic deformation rate in the western part of the Tian Shan compared to the eastern part.However,the deformation rates appear to be fairly evenly distributed throughout the north-south axis.About a third of the north-south direction crustal shortening rate is absorbed by the intermontane basins.Piedmont thrust nappes had long been thought to be the primary sites of crustal shortening,but this research casts doubt on that assumption.This study proposes,on the basis of geophysical evidence and spatial-temporal patterns of tectonic deformation,that the late Cenozoic tectonic uplift in the Tian Shan orogenic belt can be attributed in large part to lithosphere delamination and the bidirectional subduction of the Tarim Block and the Kazak Block（or Junggar Block）.