RATE OF ACTIVE SHORTENING ACROSS THE AKSAI RESTRAINING STEPOVER ALONG THE ALTYN TAGH FAULT, GANSU PROVINCE, NW CHINA

The active, left-slip Altyn Tagh Fault strikes N70E for over 1200 km along the northwestern margin of the Tibetan Plateau, and is thought to accommodate India-Asia convergence. Between 85E and 95E, the fault is punctuated by four, right-stepping restraining double bends. Constraining the rate of shortening across these restraining bends may allow us to determine the slip rate along the Altyn Tagh Fault. The Aksai restraining stepover (93-94E), the eastern-most restraining bend, coincides with a 100-km-long (E-W) and 60-km wide (N-S) mountain range that is centered on the Mt. Altyn uplift. On the basis of field observations and remotely sensed data, we conclude that the only shortening structure with a clear geomorphic expression within this bend is the active Dalong reverse fault. The Dalong Fault bounds the northern edge of the mountains associated with the stepover and it dips 50º, strikes N71W, and has a surface trace of 40 km. In the Liuchenzi valley (39.5N, 93.7E), the Dalong Fault deforms a suite of four unpaired strath terraces (T1-T4), which are elevated 41-126 m above the modern drainage. Through previously published work, we demonstrated that incision of these terraces is related to slip along the Dalong Fault and that the deformation is best modeled using trishear fault-propagation folding. To determine slip rates along the Dalong Fault at the Liuchenzi site, we paired our kinematic model with preliminary ¹⁰Be ages from 3 depth profiles in the deformed T2 and T4 fluvial terraces (85 m and 126 m above the modern drainage, respectively). Samples were collected at 0, 50, and 100 cm below the surface. The nuclide concentrations decrease exponentially with depth and, assuming no erosion, suggest that the surfaces significantly pre-date the last glacial maximum. These ages imply a low (<10 mm/yr) late Quaternary slip rate along the Dalong Fault. Assuming that all of the shortening along this section of the Altyn Tagh Fault is resolved onto the active Dalong Fault within the Aksai stepover, we speculate that the Altyn Tagh Fault slips at a minimum rate of <7 mm/yr.