Fold Kinematics and Fault Slip Rates From Progressively Deformed Terraces: Implications for Structural Evolution of Basins in the Kyrgyz Tien Shan

Active shortening across the Tien Shan is transforming dispersed basins and ranges into continuous high topography. The Jumgal Basin is an example of a basin in an intermediate phase of closure, with both a basin-bounding reverse fault system and a >25 km long anticline growing in the basin interior. We used geologic mapping and surveying of progressively deformed fluvial terraces preserved across the anticline to measure deformation. A novel method of inverting deformed terrace data allows estimation of fault geometry at depth as well as incremental slip for terraces. The preserved northern portion of the Jumgal anticline is consistent with folding above a fault that ramps up from a low-angle splay following bedding in synorogenic stratigraphy. The eastern part of the fold has grown by limb lengthening above an angular bend in the fault. Areas to the west record progressive rotation consistent with deformation above broadly curved portions of the fault. Slip rates determined with late Quaternary terrace ages from measured and correlated chronologies are consistent with slip rates that vary significantly from similar to 0.6 to 1.2 mm/year along the investigated portion of the anticline, with lower rates in the middle of the anticline. Rate variation may indicate along-strike trade-offs in slip between an emergent splay of the basin-bounding fault and the intrabasin fold. Slip rates are rapid enough to build topography north of the basin since the Pliocene, and observations suggest the Jumgal Basin is progressively closing from west to east, influenced by earlier phases of basin partitioning.