Active Deformation And Strong Earthquake Across The Southern Junggar Thrust Along The Hutubi River,NW China

The growth process along the dip-slip and strike and the fracture behaviors in large earthquakes of low-angle thrust faults have always been a fundamental problem and general interest in structural geology research.The South Junggar Thrust and Fold Belt（SJT）is the boundary fault between the Tianshan Block and the Junggar Block.SJT comprises three or four rows of nearly eastern-western out-of-sequence active thrust and fold belts on the surface along the dip-slip direction.Though lots of research on the late Quaternary folding deformation in this area,it mainly focuses on the second and third-row anticlines.There is great controversy about the Late Quaternary activity of the first row anticlines,and still a lack of systematic research of the Late Quaternary deformation characteristics and strain distribution of SJT across the entire foreland thrust belt from the piedmont to the basin along the dip-slip direction.In response to understanding the growth process of low-angle thrust faults and accurately evaluating such faults’seismic risks.A series of issues need to be addressed,such as the characteristics of the late Quaternary deformation of SJT along the dip-slip direction,the deformation rate change on a scale of 1000-10000 years,the recent straining spatial changes in time and space,the deformation characteristics and displacement distribution of the surface fracture zone of the 1906 Manas M 7.7 earthquake,the paleoseismicity characteristics of SJT.This paper study the SJT along the Hutubi river to analyze the late Quaternary activity,shortening distribution,and seismic characteristics of each row of structures by detailed field observations,mapping,and measuring together with seismic balance profile,OSL dating,and trench research.The main findings and conclusions are as follows.（1）The Qigu anticline is an active tectonic wedge composed of a ramp（SJT）and a back thrust（SJT2）.The back thrust fault,thrusting southward,is a bedding fault developed in the grayish-green gypsum mudstone of Anjihaihe formation(E_2-3a)on the north limb of the Qigu anticline.It broked the terrace on the west bank of the Queergou river,forming several fault scarps.The back thrust is a listric-shaped fault bend along the Hutubi river.The continuous activity of the fault causes the synchronous deformation of the terrace base and top on the hanging wall,forming a syncline fault-bend hinge migration fold scarp.According to the deformation characteristics of the fold scarp developed on the west bank T3c of the Hutubi river,it is estimated that the offset and slip rate of the back thrust fault（SJT2）on the Qigu anticline north limb since～17.4 ka B.P.is～4.1-47.4 m and～0.2-2.7 mm/yr along the Hutubi river,respectively.（2）The shallow Tugulu anticline along the Hutubi river is an active mixed-shear fault-propagation fold.This paper modified the shortening calculation formula of the shear fault-bend fold.The shortening of the Tugulu anticline along the Hutubi river was calculated by the modified shortening calculation formula,excess area method,and trishear inversion method according to the deformation of back limb rotation angle,excess area,and the deformation of the forelimb fault scarp,respectively.The shortening of the Tugulu anticline along the Hutubi river is～20.6-29.9 m since 17.4 ka B.P.,estimated by typical limits of different methods.According to the rotation angle of the back limb growth strata,the shortening rate along the Hutubi river has been constant at～1.7^+0.6/_-0.5 mm/yr since 35.5 ka B.P.Assuming the deposition is constant from the beginning of growth,the shortening rate of the Tugulu anticline along the Hutubi river is～1.5-1.7 mm/yr,and its growth started at～700 ka B.P.（3）The Hutubi anticline is an active detachment fold whose detachment layer（SJT3）is at a depth of～9 km underground.The minimum and the mode shortening of the Hutubi anticline along the Hutubi river according to the deformation of the terrace deformation is～6.3 m and～7.5 m by the excess area method,which indicates the minimum and mode shortening rate of the Hutubi anticline along the Hutubi river is～0.36 mm/yr and～0.43 mm/yr since 17.4 ka B.P.（4）The Southern Junggar fold-and-thrust belts are a Holocene tectonically active,out-of-sequence thrust system in which all branch faults of SJT are spreading the late Quaternary shortening.The shortening of the SJT along the Hutubi river mainly spreads by the back thrust fault（SJT2）on the north limb of the Qigu anticline,the shallow detachment layer（SJT1）,and the deep detachment layer（SJT3）.The total shortening of the SJT along with the Hutubi river is～55.2^+21.0/_-20.9 m since～17.4 ka B.P.,which indicates the total shortening rate is 3.1^+1.2/_-1.3 mm/yr since～17.4 ka B.P.Among them,the northern limb back thrust of the Qigu anticline（SJT2）absorbs 53⁺¹⁰/_-36%,the shallow Tugulu anticline（SJT1）absorbs 35⁺²⁸/_-7%,and the Hutubi anticline（SJT3）absorbs 12⁺⁸/_-3%.The results indicate that the regional shallow detachment layer absorbs～90%of the shortening transmitted from Piedmont to the basin.The deep detachment layer absorbed the other 10%.（5）The trishear fault-propagation fold model can explain the Tugulu fault scarp geometry,its near-surface strata deformation,and the 1906 Manas earthquake coseismic surface deformation.The dip angle of the Tugulu fault near-surface is 10°-25°,the coseismic uplift of the Manas earthquake in 1906 is～1.5-1.7 m,and the coseismic dip-slip is～3.5-9.8 m.A total of four paleoseismic events have been identified together with the paleoseismic trenches,kick-points,and morphologic characteristics of the Tugulu fault/fold scarp on the Hutubi river west bank T3c.The results indicate that cascade activities dominate the SJT1,with at least 4 cascade seismic events with a magnitude similar to the 1906 Manas earthquake since～17.4 ka.