Quaternary Tectonic Deformation Of XihHu Anticline Belt Along Foreland Basin Of North TianShan

Xihu anticline is located at the forefront of the foreland basin,the north of Dushanzi anticline,northern TianShan.Previous studies of Northern Tianshan is mainly foucs on the second and third row of active fold-and-thrust belt,and there was no systematic study on the Xihu anticline belt.In this paper,we collect five seismic reflection profiles cross the anticline belt?from east to west the code number is A?B?C?D?E?.We use the deep structure interpretion results of anticline,surface Geology and geomorphology mapping date,topographic lines of folded river terraces and optically stimulated luminescence?OSL?dating,combining the deep and shallow motheds,to investigate the types,growth mechanism,geometry and kinematics characteristics,deformation and deformation rates of the fold.From this study,the following insights are attained:?1?Spatial spread and geometry characteristics of Xihu anticline beltThe Xihu anticline belt is appearance from the west edge of TogulikMoDun Village to east edge of Kuitun river.The gentle fold length is about 33km,width is about 8.5km,uplift range is less than 100m.through the method of axial-surface mapping,we projecting axial surfaces in seismic reflection profiles to horizontal datum and plotting their locations on maps.The deformation length from west to east is more than 47km,the maximum projection width from north to south is about 12km,the projection width decreased gradually to the East and west sides.In the surface deformation range is small,the structure is simple,symmetrical output.Seismic interpretation results show the types of deep structure of fold belt is relatively complex.in addition to a series of secondary faults in the core of Xihu anticline,we find a smaller anticline?called the North Xihu anticline?.The topographic high point of Xihu anticline is located at about 12km to Kuitun river.From the point to east the deformation amplitude decreases rapidly,slower to west.This is consistent with the sesmic reflection profles interpretation results and the shortening results.?2?The type and growth mechanism of the Xihu anticline beltWe can clearly identify a near-level detachment in the five seismic reflection profiles which control the growth of Xihu anticline belt.The detachment may develop in the lower coal strata of the Jurassic.Using the area-depth-strain analysis method,we calculate the depth of detachement is about 7.2km below the surface of the anticline in the east,the detachement is slightly southward and slope 2°;In the west,the detachement is about 6.3 km below the surface,Slightly southward and slope 1°.Although we find a series of secondary faults in the deep part of the seismic reflection profile,the scale is small and no fault ramp was observed to control the growth of the fold.The thickness of the growth strata developed in the anticlinal wings is wedge-shaped thinning toward the core of anticline.the river terraces cross the fold have occurred in the arch deformation,the south wing of anticline have occurred reverse lift.the older the terrace,the greater the arch deformation,the greater the angle of the lift.These deformation characteristics show that the Xihu anticline belt is a detachemeng fold with the growth type of limb rotation?3?The starting and ending growth time of the Xihu anticline beltIn the east section of the appearance area of the anticline,the initial locations of the growth strata are same in the seismic reflection profiles A?B?C,which are located at depth of 1.95km,1.99km and 2.01km respectively,indicating that the initial growth time of the folds is basically the same.There are all river terraces cross the fold have occurred in the arch deformation,even though the modern river bed in the Xiejiakouzi village,indicating that the anticline in there is continuous uplift.In the west section of the anticline,the initial location of the growth strata in the D?E seismic reflection profiles are located at 3.7 km below the ground.At 3.3 km,we can find the stratas are overlying the Xihu anticline,and the above stratas are not deformed,indicating that the anticline is no longer active afterwards.The deformation of the North Xihu anticline is much smaller than that of the Xihu anticline,the initial locations of the growth strata are same.Therefore,the initial growth time of the two anticlinal is basically the same,and it is still in active.?4?deformation characteristics and shortening and uplift of the Xihu anticline beltWe use the area-depth-strain analysis method to calculate the deformation of Xihu anticline,the average shorting calculate from five seismic reflection profiles A,B,C,D and E are 652±72m,1065±70m,778±52m,197±36m,134±31m.At the topographic high point?near the seismic reflection profile B?,the amount of shortening is the largest.Along strike of the anticline,the amount of shortening decrease gradually,and to the eastward the shortening decrease faster,indicates that topographic high point is also the structural high point.The excess area calculate from seismic reflection profiles are 0.56km²,0.36 km²,-0.34 km²,0.56 km² and 0.52 km²,respectively.We find a series of secondary faults in the deep part of the seismic reflection profiles,believeing that the area of??the folds may not be conserved during the deformation,and the excess area is caused by the enter of external material or leave of internal matterThe ratio of dS/dH reflect the change of shortening rate of the anticline to a certain extent when the regional deposition rate is the same and remain constant.The ratio of shortening rate dS/dt to deposition rate dS/dt caculate from growth stratas of seismic reflection profiles A?B?C are 0.23±0.02,0.44±0.03,0.30±0.04,respectively,which may show that the shortening rate of the anticline is largest at topographic high point,and decreases gradually to east and west.The layer-parallel strain increases gradually from the deep upwards found from different seismic reflection profiles,indicating that the proportion of the curvimetric shortening to the actul shortening increases gradually from the deep upwards.The uplift of the anticline calculated from pregrowth interval increases gradually from the deep upwards,fold at the beginning of the growth strata,and then decreases gradually.The maximum uplift of the anticline is about 942 m.The average shortening of the North Xihu anticline calculated from seismic reflection profiles A and B is 58±5m and 133±35m,respectively,and the excess area generated by the entering of external matrial is 0.50 km² and 0.74 km².?5?Late Quaternary deformation of the river terraces of the Xihu anticline beltThe study area developed three rivers across the Xihu anticline belt.These rivers have developed T3,T2 and T1 riverside terraces?from high to low?,and are basically undeveloped in the syncline areas.Among them,T3 river terrace is the most continuous,T2 and T1 terrace in the part of the gully only sporadic development.We collected Silk lens samples on the continuously distributed terraces for OSL dating to define the terraces age.For terrace T3 of east trench the fine-grained quartz?4-11um?OSL dating release age is 72.2±14.4ka,and the middle trench is 77.5±11.6ka,with consistent error range,the average value is 74.8±13ka as the age of terrace T3.For terrace T2 of middle trench the Coarse-grained quartz?125-180um?OSL dating release age is 0.6±0.1ka,¹⁴C age is 660±30a,the two results are basically the same,taking the average age 0.63±0.07ka as the age of T2.The height of these terraces is highest in the core of the anticline,decress gradually to the wings.The river terraces crossing the Xihu anticline belt have occurred in the arch deformation,the south wing of anticline have occurred reverse lift.the older the terrace,the greater the arch deformation,the greater the angle of the lift.we use DGPS system to measure these continuously distributed deformed terraces?the code number of the DGPS line from east to west is 1-9?,adapting area conservation method to calculating the deformation and deformation rate of the anticline of the different Structural position.The shortening and uplift of terrace T3 of east trench?longitude 84.66°?are 15.0±2.1m,20.2±2.0m,the age of terrace is 74.8±13ka,and the corresponding shortening and uplift rates are 0.20±0.05 mm/yr,0.27±0.05 mm/yr.The maximum shortening and uplift of the Xihu anticline?the number of DGPS line is 3?are47.7±6.8m,83.8±8.4m,the age of the geography suface is replaced by the age of terrace T3,the corresponding shortening rate and uplift rate are 0.64±0.14mm/yr,1.12±0.23 mm/yr.The shortening and uplift at the position of fourth line are43.8±6.2m,82.5±8.3 m,the age of the geography suface is replaced by the age of terrace T3,the corresponding shortening rate and uplift rate are 0.59±0.13 mm/yr,1.10±0.26 mm/yr.The shortening and uplift of terrace T3 of middle trench?longitude84.54°?are 23.5±3.3m,56±5.6m,the terrace age is 74.8±13ka,the corresponding shortening rate and uplift rate are 0.32±0.07mm/yr,0.75±0.15mm/yr.The shortening and uplift of terrace T2 of middle trench are 3.9±0.6m,7.7±0.8m,the terrace age is0.63±0.07a,the corresponding shortening rate and uplift rate are6.2±1.1mm/yr,12.2±1.8mm/yr.The shortening and uplift of terrace T3 of west trench?longitude84.34°?are 5.4±0.8m,12.1±1.2m,the terrace age is 74.8±13ka,the corresponding shortening rate and uplift rate are 0.07 0.02mm/yr,0.16±0.03 mm/yr.?6?The lateral expansion of Xihu anticline beltThe topographic profile of the Xihu anticline shows that the topographic high point of the anticline is located at about 12km west of the Kuitun River,closer to the east part of the anticline.The shortenting of fold calculated from seismic reflection profiles show that at topographic high point the amount of shortening is largest,the shortening decreases to the east and west,but decreases faster to the east.The ratio of shortening rate?dS/dt?to deposition rate?dH/dt?at the west side of topographic high point is significantly larger than that of the east side.assuming that the deposition rate is constant,The deformation rate at the west side is also bigger than that of the east side.The deformation and deformation rate at the diffierent part of fold calculated from DGPS line and the results of dating also show that the deformation rate at the west side is also bigger than that of the east side.In summary,we believe that since the beginning growth of the Xihu anticline,the expansion rate of the westward is bigger than that of east.