ACTIVE FOLDING AND ACTIVE FLEXURAL-SLIP FAULT SCARPS ON MINGYAOLE ANTICLINE,WEST MARGIN OF TARIM

In recent times,some moderate-large earthquakes occurred in active folds and thrusts,which seem not directly related with known active faults on the surface and did not form surface ruptures. Although such individual earthquakes might correspond to a known surface active fault,most of them occurred under active folds,formed by displacement of burial thrusts which are located at depth of tens kilometers beneath the folds. Stein named such earthquake asfolding earthquake. It is quite a challenging issue to study and assess the seismic hazards of folding earthquakes occurring in compressive tectonic regions with active folds and burial thrusts. Derived from active folding secondary faults such as flexural-slip faults,bend-moment faults,it is easier to identify that the fold itself. These secondary faults have coseismic slip at the surface and record the active history of seismogenic thrusts which provide an effective way to study the seismic activity of blind thrusts. Many flexural-slip fault scarps are developed on several terrace surfaces at the two limbs of Mingyaole anticline,located along the western margin of the Tarim Basin. These scarps mainly form on the limb of steep beds closest to active axial surfaces(dips of 74° ~ 89°,18° ~ 20° and 45° ~ 60°,separately),within a range of 50 ~ 1 200m from active axial surface,and most are 90 ~ 1 000m wide. Overall,the height of the flexuralslip fault scarps gradually deceases away from the active axial surface. These scarps occur at nearly equidistant or multiple distance spacing on the same terrace surface. The strike of the flexural-slip fault scarp is consistent with the strike of underlying bedrock,which is dominated by interbedded medium-thick layered sandstone or fine-grain sandstone with similar rock mechanical properties. Since the abandonment of terrace T3 at the south limb of the Mingyaole anticline,the shortening rate and uplift rate absorbed by flexural-slip faults are at least(1. 0 ± 0. 2)mm / a,(1. 2 ± 0. 1)mm / a, respectively. Movement of the flexural-slip faults is characterized by repeatability and neo-activity.