ESTIMATING FAULT SLIP AND GEOMETRY FROM FLUVIAL TERRACES PROGRESSIVELY DEFORMED ABOVE CURVED FAULT BENDS: AN EXAMPLE FROM THE KYRGYZ TIEN SHAN

Surface deformation associated with active reverse faults is often expressed by folded sedimentary strata and geomorphic surfaces. Geometric relationships between folded stratigraphy that predates fold growth and progressively deformed markers like fluvial terraces that formed during fold growth provide constraints on the geometry and kinematics of the structure at depth. Terraces folding above angular fault bends exhibit limb-lengthening behavior, whereas terraces deforming above continuously curved faults show progressive limb rotation. Deformation associated with spatially restricted curved fault bends results in a combination of limb lengthening and rotation. The geometry of pre-growth and syn-growth markers provides constraints on the deeper fault and fold geometry as well as incremental amounts of slip that have occurred since the formation of syn-growth markers.

We have developed a new strategy for modeling deformation of terraces above curved fault bends that allows simultaneous estimation of the geometry of the underlying fault as well as the slip that occurred since the formation of each terrace in a progressively deformed flight. Uncertainties of relevant geometric parameters and slip magnitudes are inverted from surveyed terrace data using bounded nonlinear least squares.

We apply this method to analyzing a well-preserved flight of terraces folded above an active fault system in the Jumgal basin in the Kyrgyz Tien Shan. We find that the deformation of Neogene bedrock and fluvial terraces are consistent with a reverse fault system that ramps upward from a low-angle fault splay connected to the basin-bounding fault. Marked along-strike variation in the form of the fold can be explained by changes in the curvature of the fault ramp underlying the fold, with a trend toward more broadly curved bends to the west. Slip rates estimated from the modeled fault displacement suggest that the late Quaternary structural evolution of the basin is linked to the earlier history of basin partitioning in this portion of the Kyrgyz Tien Shan.