LATE PLEISTOCENE SLIP RATE OF THE HöH SERH-TSAGAAN SALAA FAULT ZONE, MONGOLIAN ALTAI AND INTRACONTINENTAL TRANSPRESSIONAL DEFORMATION IN CENTRAL ASIA

The Mongolian Altai is an intracontinental transpressional orogen related to the far-field effects of the Indo-Asian collision. This mountain belt, which is located in the transition zone between N-S directed shortening across the Tien Shan and NW-SE directed extension in the Baikal rift, is thought to accommodate ~15% (~4 mm/yr) of the total Indo-Asia convergence along several NNW-SSE-trending transpressional faults. The Höh Serh-Tsagaan Salaa fault system is one such structure acting to partition strain and accommodate uplift and shortening in western Mongolia. This fault zone trends NNW-SSE and displaces late Pleistocene alluvium along the southwest piedmont of the Höh Serh range. Along the central third of the fault zone, strain is partitioned onto two separate strands. One strand strikes ~332º, has a sub-vertical dip and accommodates nearly pure dextral motion. A second, parallel strand strikes ~332º, dips ~25ºNE and accommodates purely dip-slip thrust motion. We determined rates of deformation along each of the Höh Serh-Tsagaan Salaa fault strands based on differential GPS surveys and cosmogenic nuclide ¹⁰Be geochronology. Two alluvial fan channels and a shutter-ridge are displaced right-laterally by an average of ~17.5 m at the Lake Valley offset. Thrust motion has resulted in ~11.8 m of shortening and ~5.5 m of uplift at the North Gorge fan. Five ¹⁰Be samples from the Lake Valley offset cluster tightly and produce a weighted mean age of 14.2 ± 2.0 ka. Six ¹⁰Be samples from the North Gorge fan also cluster tightly and result in an age of 33.5 ± 5.0 ka. Combining the measured offsets and ¹⁰Be dates yields a right-lateral slip rate at the Lake Valley offset of ~1.2 mm/yr; the shortening rate at the North Gorge fan is ~0.3 mm/yr, with uplift occurring at ~0.2 mm/yr. Resolving the shortening and dextral components of deformation yields a slip vector of ~1.2 mm/yr toward 318º. This deformation vector is consistent with the short-term strain field determined by GPS in the region and suggests that at least 25% of Indo-Asian deformation in the Mongolian Altai occurs along the Höh Serh-Tsagaan Salaa fault zone. Although strain in the Mongolia Altai is distributed over a broad region, our results suggest that it is accommodated predominantly on discrete dextral-oblique structures.