Newly discovered backthrust fault and related active fold scarp in the first piedmont anticline belt of the foreland thrust belt of the North Tianshan

The tectonic wedge is an important seismogenic structure in the active compression area.The foreland thrust belts of the North Tianshan developed a complex extruded tectonic wedge and formed 3~4 groups of fold belts on the surface.It was generally believed that the first group of piedmont anticline belt in the southern part had ceased to be active since the Quaternary.In the field investigation, we found that there were two nearly parallel Late Quaternary active back thrusts (Kasha backthrust fault, KBT) arranged southward in the first piedmont anticline belt, the monocline area on the north wing of South Anjihai anticline.It had faulted the Late Quaternary landform on both sides of the Anjihai river and formed a series of southward thrust fault scarps.The gray-green mudstones of the Anjihaihe Formation (E_2-3a) thrust southward along the KBT bedding onto the brick-red sandstones of the Shawan Formation (E₃-N₁s), it causing repeated exposure of the strata.On the west of the Anjihai river, the KBT plane on the south side is shovel-shaped along the bedding to form a fault bend.The continuous activity of the fault caused the simultaneous bending of the upper terrace and the base surface, forming a slanting fault-bending fold scarp.The terraces on the south side of the fold scarp are relatively thin, and they have obvious thickening to the north.They have the characteristics of growing strata, indicating that KBT has continuous activities during the terrace accumulation process.The discovery of the Kasha backthrust fault scarp and active fold scarp indicate that the South Anjihai anticline has been active since the Late Quaternary.These backthrust faults and related fold structures provide a window for us to study the activity of tectonic wedges in the southern margin of Junggar, and provide a reliable and convenient way to further define the deformation mechanism, rate and growth history of the active tectonic wedge.