Late Quaternary Faulting In The Southern Margin Of The Chaiwopu Basin,Xinjiang,Northwestern China

The nappe structures are the most intense regions of tectonic deformation in the Tian Shan,and these nappe structural systems have all been responsible for the strong earthquakes which occurred along these structures.Thus,the tectonic activity characteristics study in the background of thrust has always been the focus of active tectonics.The tectonic activity rate as an important index which is used to characterize the activity of crustal structure,that has attached much attention.The Quaternary period is one of the most recent geological periods which can preserve the records of the structure deformation.Thus,a detailed study of tectonic activity in this period are helpful to understand the deformation mechanism of Tian Shan.In response to north-south convergence driven by the India-Asia collision during the Cenozoic times,the EW-trending Tian Shan has experienced a significant crustal shortening.These deformation are mainly recorded by active faulting and fold-and-thrust belts within the range.In the foreland basins,several fold-and thrust belts were formed in response to the Cenozoic propagation of the Tian Shan towards its northern and southern foreland basins,followed by a few reverse faults developed at the internal of the Tian Shan.In the Urumqi depression,three successive,sub-parallel belts of fault-related folds known as belts I to III from south to north and characterized the present topography in the northern piedmont of Chinese Tian Shan.Much researches have conducted about the fold and thrust belts in the northern foreland basin of Tian Shan,such as the geometric characteristics of the faults,the amplitude of the faults activity as well as the time.In recent years,Cities like Urumqi in the northern foreland basin of Tian Shan have been developed rapidly.However,there are potential risks of earthquake caused by the reverse fault activities along the range,especially in the northern and southern of the Urumqi city where the seismic activity is relatively frequent.The deep seismic reflection profile in Urumqi reveals that a low-angle south dipping detachment with 10¹8 km depth converged into the root fault of the range-front Urumqi thin-skin nappe structure?i.e.,the southern Chaiwopu Fault?,which a complete reverse listric fault dominated the tectonic activity of the southern of Chaiwopu basin.Nonetheless,it's inadequate to fulfill a need to unravel expressly the activity of the Banfanggou fault?BFGF?in the southern margin of Chaiwopu basin which a piggyback basin of the northern Chinese Tian Shan foreland.The well-exposed late Neogene-Quaternary strata and well-developed Quaternary alluvial sequences in the southern margin piggyback basin?i.e.,Chaiwopu basin?of northern Tian Shan foreland provided us with a good mark to analyze the tectonic activity of Banfanggou fault.Based on the analysis of sedimentology and geomorphology and combined with the interpretation of remote sensing image,here we provide our geomorphological and sedimentological data,including the division of geomorphic units,the deformation measurements of geomorphic surface?terraces and alluvial fans?and optically stimulated luminescene?OSL?dating.Our results help to reveal the activity of Banfanggou fault in the late Quaternary and further to understand the evolution of Tian Shan.The main conclusions of this paper are as follows:?1?Based on the detailed field investigation with 1:50,000 geological map and combined with the interpretation of remote sensing image of Google earth,according to the topographic continuity,observations in the field and previous studies,three main geomorphic surfaces identified in the southern Chaiwopu basin,they are defined and designated as F₁,F₂ and F₃ in the region.The fan F₂ in the Wushenchenggou can be divided to the high fan F₂?F₂^H?in the east and low fan F₂?F₂^L?in the west based on the fan surface height above the fan F₃.In the Urumqi river,due to the river incision and the resultant abandonment of fan F₁,there developed nine stepped river terraces and defined as T₁ to T₉ increasing systematically in the elevation.?2?The OSL results reveal that the abandoned time of fan F₁ and F₂ was0.5Ma and 20ka-30ka respectively in the northern piedmont of Tian Shan,and the fan F₃ is formed in the Holocene.?3?Based on the analysis in the sedimentology,geomorphology and combined with the optically stimulate luminescene?OSL?dating technique,the characteristics ofthe Bangfanggou fault in the late Quaternary was initially identified in our paper.The Banfanggou fault branched from the Junggar Frontal Thrust Fault bounding the northern Tian Shan and its foreland basin,which crossed over the basin in the direction of NWW from the Wushenchenggou in the edge of the mountain,while in the western of Urumqi river the fault went to the west.In in line with the field data,the Banfanggou fault was a late Quaternary fault which is nearly 45km in the length.?4?Stratigraphic evidence reveals that the Bangfanggou fault has thrust the Neogene mudstone onto the Upper Pleistocene conglomerates with a high-dip angle in the Urumqi river,which the newly tectonic activity is occurred in the Holocene.While,the Banfanggou fault has not reached the surface in the east part.From above,the activity in the eastern of Urumqi river part of Banfanggou fault likely more active than the other parts.?5?The tectonic activities of Banfanggou fault in the late Quaternary are directly reflected in the geomorphic surfaces.There generated a series of apparent geomorphic deformation on the surface.Such as,in the Urumqi river,terraces T₇,T₅,T₄ have been significantly warped up by the Banfnaggou fault,while the accurate deformation can not be defined because of the burial effect.Through the measurement of deformed surface and OSL dating,the terrace T₁^H has been vertically offset about0.6m by the thrust since⁷ka,thus defining the vertical slip rate of 0.09mm/yr since its formation at about 7ka in the east of Urumqi river,In the eastern of Urumqi river,the minimum estimated vertical displacement is 21m since 20 ka of the fan F₂ in the Shuixigou.Further,the vertical deformed range in the fan F₂^H and fan F₂^L in the Wushenchenggou result from the activity of the fault was 32m and 18m respectively,same to the burial effect of the footwall,the accurate deformation can not be obtained.