A constant slip rate for the western Qilian Shan frontal thrust during the last 200 ka consistent with GPS-derived and geological shortening rates

Abstract

Active thrust faulting at the front of the Qilian Shan accommodates the northeastward growth of the Tibetan Plateau, however, the lifespan of individual faults and their slip history on different timescales remain largely unknown. Here, we show that the main range-bounding thrust fault of the western Qilian Shan has accrued tectonic slip at an almost constant rate during the last ∼200 ka, and possibly since fault initiation in the mid-Miocene. Our finding is based on 10Be ages from a flight of five deformed fluvial terraces along the Hongshuiba river, which constrain the vertical slip rate of the Qilian Shan frontal thrust to be 1.2±0.1 m/ka during the last 200 ka. With a fault dip of 30±5° constrained by seismic reflection data, we obtain a horizontal shortening rate of 2.0±0.3 m/ka. This value is consistent with both the short-term shortening rate of 1.7±0.3 mm/a derived from GPS data and the long-term shortening rate of 2.1±0.4 m/ka, which is based on a balanced geological cross-section. The latter provides a total shortening estimate of 25±3 km since the thrust fault initiated 12±2 Ma ago. The agreement between the shortening rates on the range of timescales between 100 and 107 years suggests that the western Qilian Shan frontal thrust has slipped at a steady rate since its initiation and implies that this fault is the main structure responsible for the growth of the western Qilian Shan.