The Active Out-of-sequence Thrusting And Folding Of The Southern Junggar Structural Wedges

Structural wedge（structural wedge）is an important Seismogenic structure in the active compressed structural zone.The compressional structural wedge is composed of two oppositely dip and interconnected forward low angle thrust and back-thrust forming a wedge triangle,the two fault units join at the tip of the wedge（Hagke et al.,2018）.The compressional structural wedge is composed of two oppositely dip and interconnected forward low angle thrust and back-thrust forming a wedge triangle,the two fault units join at the tip of the wedge（Hagke et al.,2018）.The displacement of the two faults transmitted in two opposite directions,and resolved the structural displacement at the tip of the wedge.The 1906 Manas M8 earthauqke（Stockmeyer et al.,2014）,1983 Mw 6.5 Coalinga earthauqke,USA（Guzofski et al.,2007）,2015Pishan Mw6.4 earthauqke,Xinjiang（Li et a.,2016;Sun et al.,2016）and 2016 Hutubi Ms6.2,Xinjiang（Li et al.,2018）are commonly associated with deep active tectonic wedging,and related to the cascascade breakdown of multiple faults.Cascade rupture in multiple faults or fault segments can not only increase the maximum magnitude upper limit,but can also lead to more ways of energy accumulation,increasing the potential hazard of triggering large earthquake.The Southern Junggar thrust-and-fold belt which is located in the piedmont of the northern Tian Shan mountains,northwestern China,is a typical low-angle compression-thrust tectonic system.There are a series of deep structural wedge developed in the southern margin of Junggar.These squeezed structural wedges not only controled the reduction of displacement between different structural elements of the foreland thrust belt,but also controled the distribution and deformation mode of these fold-belts developed on the surface of northern Tianshan foreland basin.With the northward expansion of the structural wedge,some part of displacement of SJT reducing along the southward dip backthrust at the tip of the structural wedge,forming the first row of Changji-Qigu fold belt;while other displacement expansion northward along different deep detachments,caused folding deformation at fault ramp and forming the second row of Tugulu-Manas-Huoerguos fold belt and Anjihai fold belt.Due to the polysolutions and limitations of seismic reflection profiles,different scholars put forward the models of structural wedges are not consistent.This study is based on the abundant achievement of previous researches,and carried out a systematic research of the southern Manas、Tugul and Manas structural wedge which composed Southern Junggar thrust（SJT）,with the field survey of the Tugulu-Manas-Huoerguos Fold zone at the southern margin of Junggar basin,with the detailed investigation and large scale geological mapping,high accuracy d GPS survey of terrace surfaces and the integrated analysis of structural interpretation of subsurface seismic reflection profiles（deep and shallow）,combined with chronological method.To reveal the the late Quaternary activity of SJT,and the spatial distribution characteristics of SJT.From this study,the following insights are attained:1.The southern Junggar Thrust（SJT）is the main boundary fault of the current Tianshan block and the Junggar block.It is a more than 70 km widely dispersion deformation zone,which is composed of structural wedge including forward thrust main fault and backthrusts developed at different depths detachment surface.This structural wedge system consist of southern Manas structural wedge（SMNS）,upper TGL anticline and Tugulu north fault（TGNF Fault）,deep TGL structural wedge,upper Manas anticline and MNS north fault（MNS Fault）and deep MNS structural wedge.These branch structural elements convergenced in different detachment surface at different depth.2.The JNM fault is a thrusting northward bedding passive roof back-thrust structure,developed in the grayish-green gypsum mudstone of Anjihaihe formation(E_2-3a)on the north limb of the SMNS anticline.This structural element formed a series of fault landform on surface,which it shown the JNM fault is an active thrust since the late Quaternary.The southern Manas structural wedge（SMNS wedge）forms by the coeval activity of the Southern Junggar Thrust blind ramp forethrusts and the north-dipping JNM backthrust that steps up from the upper Eocene detachment.The dip-slip rate of JNM fault is about～0.4mm/a during～270-11ka and increase to0.8mm/a in the Holocene.It shown that the SMNS wedge is a Holocene tectonically active,out-of-sequence thrust system,which takes up a non-negligible portion of the total deformation across the northern Tian Shan and has a potential seismic hazard.3.The structural style of the west plunging end of Tugulu anticline is fault-propagation fold rather than fault-bend-fold.The foldscarps occur in the south limb of Tugulu anticline are formed by folding of hinge migration,it is related to the activity of lower detachment thrust（SJT1）.Based on the height of foldscarp,underlying bed dip and the age of Manas river terrace,we estimated the uplift rate of SJT is about 1.3±0.2 mm/a since 19.9±1.5 ka（T6 terrace formed）and 1.1±0.4mm/a since 12.4±0.8 ka（T5 terrace formed）,the incremental shortening rate of deep SJT is 2.6±0.4mm/a since T6 terrace formed and 2.1±0.3mm/a since T5 terrace formed,which is far more than the deformation rate of Tugulu north limb faultscarp.The result reveals that the shortening rate of SJT is constant since～20 ka.4.The characteristics of Tugulu northwing scarp and Manas northwing scarp which formed at both side of Manas river are as following:1)The scarps near the surface have“dualistic sedimentary textures”;2)There are northward scarps developed on different terraces,with the same strike as the TGL and MNS anticline trend.The max slope of these scarps on different terraces shown that the earlier scarp formed,the larger slope the scale,and the smaller the steeper slope of the newer scarp,the formation is shown as fold scarp rather than fault scarp;3)The outcrop of the HQZ terrace base surface shown a wide,slow and steep northward scarp,the sediment of fluvial deposits and the near-surface strata deformation exposed a wedge growth strata.These near-surface deformations of scarps are well explained by a trishear fault-propagation fold model.5.The topographic survey of trishear fault-propagation foldscarps on the Manas T6～T4 terraces shown that the scarp height and maximum slope of these foldscarps is about 10.5-12.4m/～18.4°,3.3-5.3m/～12.8°and 0.5m/～5.1°.According to the deformation characteristics of the trishear foldscarp,we use the simulated annealing algorithm for the trishear inverse modeling to calculate the best-fit or spread of possible values of the trishear fold parameters of TGL north fault（TGNF）and estimated the shortening rate of TGNF,with the previous research of chronological results in this area,which is 0.86（+0.29/-0.23）mm/a、0.58（+0.11/-0.09）mm/a from old to young.6.The upper Manas anticline shows a trishear fault-propagation fold,（the southern wing is broad and gentle while the northern wing is steep dipping）,the strata at the south wing is outputting completely while the pattern of stratas at the north wing is incomplete.There is a east-west direction foldscarp at the north wing of Manas anticline,with the same mechanism to the Tugulu anticline north wing fold scarp.This research used the simulated annealing algorithm for the trishear inverse modeling and the previous research of chronological results in this area to estimate the shortening rate of MNS north fault since T6 and T5 terraces formed,which is 1.26（+0.09/-0.1）mm/a and 1.38（+0.13/-0.07）mm/a.7.This study is based on the field survey,topographic measurement,distinguish the paleoearthquake events on the trench and the dating results of samples.And analysized the sequence of paleoearthquake,the age of events revealed in the trench（MNS1）at the west of Manas River.From the research,we had recognized the heritage of the co-seismic surface rupture caused by the 1906 Manas earthquake（MRE）from the MNS1 trench.From the study above,we present evidence of three earthquakes that occurred on the Manas anticline-thrust zone,which revealed in the trench since 6.0ka.These events occurred respectively at 5.7～4.0ka（Event1）,2.9～2.2ka（Event2）,and the most recent event（MRE）is the 1906 Manas M8earthquake.8.This study had established the structural deformation of the middle segment of southern Junggar structural wedges,and summarized the shortening deformation tendency of SJT since the late Quaternary.Our research shown that each branch structural element of the SJT still has cascading interactions along the direction of strike and slip since late Quaternary,which is characterized by an out-of-sequence,the structural deformation along the strike and dip slide was disintegrated by each branch between the Late Pleistocene and the Holocene,while the shortening rates of SJT since the late Pleistocene are essentially uniform with the GPS shortening rate in present.