Many studies of active, strike-slip fault systems have focused on both short-term (0-10 ka) and long-term (>5 Ma) strain, but less attention has focused on how such systems evolve at intermediate (10 ka to 1 Ma) timescales. Sophisticated models of evolving fault geometry at such intermediate timescales are particularly needed. To address this problem, we are investigating how fault-perpendicular shortening is accommodated across the 100 km long (E-W) and 20 km wide (N-S) Aksai stepover (93E-94E), one of four restraining bends along the left-slip, Altyn Tagh fault in NW China. The Aksai bend is defined by northern and southern ranges which flank the restraining bend in its NE and SW quadrants and are ~70 km long (E-W) and ~25 km wide (N-S). Between these ranges, the central Mt. Altyn uplift is cut by a complex network of fault traces which are part of the Altyn Tagh fault system. While these flanking ranges mirror each other in shape and morphology, sharing common features like sharp SE-NW trending range fronts and interior valleys that separate them from the central range, our 1:20,000-scale neotectonic and 1:100,000-scale structural mapping reveals that the northern range has a greater number of active thrust faults and folds that record larger magnitudes of active shortening than the southern range. The northern range is bordered to the north and south by the south-dipping Da Long thrust system and the north-dipping Sai Ma right-reverse fault, respectively. Evidence for active, stepover-perpendicular shortening along these structures includes fresh fault scarps with vertical separations of 2-35 m, offset drainages, >2000m of local topographic relief, and sets of late-Quaternary (?) to Holocene (?) fluvial terraces that are both folded and faulted. In contrast to the distributed deformation along the north side of the stepover, only a single active structure (the Tuo La Yi fault) accommodates stepover-perpendicular shortening within the southern flanking range. This fault defines the NE margin of the range. The southern range has fewer fault scarps than the north range, more muted topography, and no deformed terraces. These results suggest that active shortening is concentrated in the north range. We speculate that the short-term (10 ka to 1 Ma) strain pattern revealed by our mapping may help to explain the long-term (>5 Ma) evolution of the Aksai stepover. In particular, the Mt. Altyn uplift and the complex network of fault traces at the center of the bend may have resulted from earlier bend-perpendicular shortening, which has more recently been transferred to the younger structures within the flanking ranges.

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