Generation of relief by fault propagation folding in the Kochkor Basin of the Tien Shan, Kyrgyzstan

The southern margin of the Kochkor basin in the Kyrgyz Tien Shan is actively shortening. At the range front the South Kochkor Fault, a north verging basement bounding thrust fault, is emplacing Paleozoic granite on orogenic Tertiary sediments from the onset of Tien Shan exhumation. Displacement on the fault in the area of focus is at most a few hundred meters; however, the structural relief in the region amounts to 4-5 kilometers. North of the uplifted basement is a 2km thick unit of east-west striking, steeply dipping to overturned sediments that exhibit signs of flexural sheer such as intraformational folds. This steep section is separated from limbs of gently dipping beds both to the north and south by narrow hinge zones. Farther into the basin, the top of the tertiary formation is thrusting over Quaternary deposits in two younger episodes of faulting. Through detailed (1:25,000) mapping west of the Djuanarik River, we can conclude that most of the structural relief in the area is due to fault propagation folding of the entire Tertiary package. Post folding, the original fault has propagated into the basin in two distinct branches. Age constraints from PMAG data and fossil specimens place the conformable uppermost deformed unit (Sharpyldak formation) in the Pleistocene, indicating the relief due to folding in the basin has accumulated in the last 2-5 million years. Elsewhere in the basin the balance between faulting and folding changes. As the South Kochkor fault trends east, displacement on the fault increases as the degree of folding decreases. Within the area of focus the South Kochkor Fault, Achop Hills Fault and the farthest rupturing fault overlap along a north-south transect. Across the three fault traces, Quaternary terraces are clearly uplifted with discrete hinges, indicating all three faults are active.