Late Quaternary Active Deformation Of Yultuz Basin In The Central Of The Tianshan,China

As one of the most active intracontinental orogenic belts in the world,the Tianshan range has been continuously uplifted in response to the collision of India-Eurasia plate during the Cenozoic time.The Tianshan displays a basin-and-range topography with late Cenozoic relief caused by distributed reverse faulting and folding.Several intermontane basins experienced strong north-south compression deformation during late Quaternary,such as the Yili basin,the Zhaosu basin,and the Yultuz basin,are preserved within the interior of the Tianshan range.Detailed field investigations and mapping of active structures on both sides of the range and in intermontane basin should help understanding the deformation process and tectonic evolution of intracontinental mountain belts.Most investigations of active tectonics in the Tianshan thus far have focused on the timing and rates of slip on thrust fault systems that bound the northern and southern margins.The partitioning of late Quaternary geologic shortening among faults in the interior and at the margins of the belt has not been quantified.We ask how geologic strain rates are distributed across the Tianshan.Do faults on the margins of the mountain belt accommodate a higher proportion of the total strain rate than faults of the intermontane basins?The eastern Tianshan forms an elongate deforming region between two generally stable crustal elements: the Kazakh platform to the north and the Tarim basin to the south.Relative to other basins in the eastern Tianshan,the E-W Yultuz Basin,a wide endoreic sedimentary basin filled by large river fans,is completely enclosed by mountain,which provides an insight for understanding the distribution of active strain in active orogenic belt and the difference of structural deformation patterns in the inner block.This study focuses on the deformation and recent activity on several faults or folds within an intermountain basin,the Yultuz basin,which so far lacks quantitative data on Quaternary deformation rates.We assessed active tectonic activity in the study area based on satellite images and our detailed field observations.These tectonic units include Narat fault,Bayan anticline and Little-Yultuz fault.We looked for geomorphic evidence of active tectonics（deformed fluvial or fan terraces,perturbation of the drainage network）and used different dating and offset measurement techniques to quantify deformation rates.In this work,the Monte Carlo simulation algorithm for obtaining long-term slip histories and deformation rates of fault is optimized,and the listric thrust model is improved according to the tilted fold backlimbs.Based on these deformation rates,we analyzed the strain distri 6 bution characteristics in the Chinese Tianshan and the role of the Yudus Basin in the regional tectonic transformation.Finally,the deformation mechanism of the Tianshan area during the late Cenozoic is discussed.The main findings of those investigations are summarized below.（1）Distribution of tectonic deformation characteristics and rates for Narat faultNorth of the Yultuz basin,Narat fault scarps and associated E-W trending structural ridges,which are located in boundary area between the Narat range and Yultuz basin,affect the smooth surface morphology of the basin.The Narat thrust is a high-angle reverse fault with left-lateral strike-slip component that can be divided into three segments.Of these,in the western segment,the trench reveals several thrust branches offset Quaternary alluvial gravel beds.By quantitative measuring morphotectonic deformation and dating morphological surfaces,the late Quaternary shortening rate of the western segment is ～0.1 ± 0.01 mm/yr.The middle section is composed of two thrust faults at the basin-range junction and a little thrust-related fold in the pidemont of the mountain.The shortening rate of the thrust faults deformed river terraces is ～0.3 +0.2/-0.1mm/yr.Using trishear simulation method combining with terrace ages,crustal shortening rate of the fault controlling the fold development is ～1.5 +0.5/-0.6 mm/yr.In the eastern segment,three palaeoseismic trenches across fault scarps revealed that palaeoseismic events occurred at least 18.42 ± 1.42 ky and 9.46 ± 0.65 ky B.P..The eastern segment consists of six secondary faults,forming a wide fault zone,which absorbed N-S crustal shortening of ～1.2–1.5 mm/yr by different geomorphic scarps.Therefore,these results show that the shortening rates in the middle and eastern segment of are much higher than that in the western segment,indicating that the northeastern part of the basin is the main absorption area of late Quaternary tectonic strain.Meanwhile,the average left-lateral slip rate of Narat fault is constrained to 2.0 +0.7/-0.5 mm/yr according to a displaced gully located in the eastern segment.（2）Kinematic model and deformation rate of the Bayan anticlineThe Kaidu River,flowing through the central part of the Bayan anticline,has formed three terrace levels at tilted fold backlimbs.Based on the field geological investigation,warped and tilted terraces in the Bayan anticline are characterized by broad,continuous backlimbs and abrupt forelimbs and suggest folding through progressive limb rotation of listric thrust model.Combining with the kinematic model and geomorphic age,the slip rate and crustal shortening rate of the underlying fault in Bayan anticline is ～1.6 ± 1.0 mm/yr and ～1.0 +0.9/-0.6,respectively.（3）Tectonic deformation characteristics and rates of the little-Yultuz fault during late QuaternaryIn the western segment of Littile-Yultuz fault,eight episodes of the alluvial fans developed since the late Quaternary.Three rivers incised these alluvial fans,abandoning three levels of fluvial terraces offset by fault scarps（T₃–T₁）,respectively.Based on several sets of displacement and age data,using Monte Carlo analysis method,we obtained the late Quaternary（since ～90 ky）shortening rate is ～0.2 ± 0.1 mm/yr.In addition,this analysis indicates a pulse of accelerated fault slip: since ～15 ka,the median shortening rate was ～0.4 ± 0.2 mm/yr.（4）Discussion on the role of Yultuz basin in tectonic deformation distribution of the TianshanDeformation rates across active faults and folds within the Yultuz basin show that the deformation within the Tianshan are distributed along major fault zones near range-basin margins and their frontal part of the extensions toward the basins.The summed crustal shortening rates of active folds and fault-related folds within the Yultuz basin along N–S trend are ～2.9 ± 1.4 mm/yr during late Quaternary.The shortening rates are derived from faulted and folded river terraces and alluvial fans.And the middle part of the basin is the main absorption area of tectonic deformation.The late Quaternary rate of shortening across the Tianshan range（between longitude 83°E and 87°E）amounts to ～10 ± 2 mm/yr,approximately matching the modern shortening rate of ～8–9 mm/yr derived from the GPS measurements.Therefore,this shortening of Yultuz basin represents ～30–40% of the total shortening rate across the entire range in the eastern Tianshan.In conclusion,A significant fraction of the total Quaternary deformation is accommodated within the Yultuz basin,central part of the eastern Tianshan,which is consistent with the tectonic strain distribution characteristic for delamination as an important dynamic uplift mechanism in the Tianshan area during the Paleozoic and Cenozoic.