Active Tectonics in Southern Xinjiang, China: Analysis of Terrace Riser and Normal Fault Scarp Degradation Along the Hotan-Qira Fault System

The northern piedmont of the western Kunlun mountains (Xinjiang, China) is marked at its easternmost extremity, south of the Hotan-Qira oases, by a set of normal faults trending N50E for nearly 70 km. Conspicuous on Landsat and SPOT images, these faults follow the southeastern border of a deep flexural basin and may be related to the subsidence of the Tarim platform loaded by the western Kunlun northward overthrust. The Hotan-Qira normal fault system vertically offsets the piedmont slope by 70 m. Highest fault scarps reach 20 m and often display evidence for recent reactivations about 2 m high. Successive stream entrenchments in uplifted footwalls have formed inset terraces. We have leveled topographic profiles across fault scarps and transverse abandoned terrace risers. The state of degradation of each terrace edge has been characterized by a degradation coefficient 't, derived by comparison with analytical erosion models. Edges of highest abandoned terraces yield a degradation coefficient of 33±4 m2• Profiles of cumulative fault scarps have been analyzed in a similar way using synthetic profiles generated with a simple incremental fault scarp model. The analysis shows that (I) rate of fault slip remained essentially constant since the aggradation of the piedmont surface and (2) the occurrence of inset terraces was synchronous at all studied sites, suggesting a climate-driven terrace formation. Observation of glacial and periglacial geomorphic features along the northern front of the western Kunlun range indicates that the Qira glaciofluvial fan emplaced after the last glacial maximum, during the retreat of the Kunlun glaciers (12-22 ka). The age of the most developed inset terrace in uplifted valleys is inferred to be I 0±3 ka, coeval with humid climate pulses of the last deglaciation. The mass diffusivity constant (k='t!T, T being time B.P.) in the Hotan region is determined to be 3.3±1.4 m2/l 03 years, consistent with other estimates in similar climatic and geologic environments of western China. These results imply a minimum rate for the Tarim subsidence of 3.5 ± 2 mm/yr. If Western Kunlun overthrusts the Tarim platform on a crustal ramp dipping 40'-45' to the south, it would absorb at least 4.5±3 mm/yr of convergence between western Tibet and Tarim.