Active fold deformation and crustal shortening rates of the Qilian Shan Foreland Thrust Belt, NE Tibet, since the Late Pleistocene

Abstract

Quantification of crustal shortening rates across foreland fold-and-thrust belts is essential for evaluating how convergence is accommodated throughout the orogenic belt and analyzing seismic hazard in neighboring cities. In this study, we focused on active fold along the western portion of the Qilian Shan Foreland Thrust Belt in NE Tibet. The shortening rate across the Hujiatai Anticline (HA) along the eastern segment of the Fodongmiao-Hongyazi Fault (FHF) is 0.8 ± 0.2 mm/a based on terrestrial cosmogenic nuclide (TCN) dating of deformed Late Pleistocene fluvial terrace. When this rate is combined with the shortening rates of 1.1–1.4 mm/a and 0.7–1.2 mm/a along the western and central segments obtained by previous studies, it is apparent that crustal shortening rates along the FHF gradually decrease to the east. This eastward decrease in fault activity may be a result of the northward propagation of the FHF or partitioning of strain off to nearby faults or folds. Furthermore, tectonic geomorphology and trenching suggest that several thrust faults around the HA ruptured during the historical 1609 earthquake. Combining the shortening rate of 0.6–0.8 mm/a across the north side of the Jiudong basin with our rate, we suggest that the total crustal shortening rate across the basin is about 1.3–2.2 mm/a assuming little internal deformation, which accounts for roughly 20–40% of the amount of crustal shortening deformation across the Qilian block. These results demonstrate that the Qilian Shan Foreland basin plays an important role in accommodating the crustal deformation of northeastern Tibet.