Rapid active thrusting along the northwestern range front of the Tanghe Nan Shan (western Gansu, China)

The western part of the Tanghe Nan Shan range southwest of Subei (western Gansu, China) is presently growing on thrust ramps splaying from the left‐lateral Altyn Tagh Fault. Late Cenozoic thrusting has folded and sliced Oligocene‐Miocence red beds into an imbricate wedge, capped by warped and uplifted Quaternary terraces that form a 2‐ to 5‐km‐wide ledge, north of the steeply faceted range front. Seismic scarps 1.5 to 4.5 m high cut young fans along the outer thrusts. Carbon 14 dating of organic remains collected on strath terraces constrains the chronology of deposition and incision by the streams. Most of the fans and terraces in the southern part of the Subei basin appear to have been emplaced after the last glacial maximum, particularly during the early Holocene optimum (9–5 ka). Measurements of the shapes of the warped terraces constrain minimum and maximum throws on the outer thrusts. The minimum vertical throw of 34 ± 2 m of a surface dated at 8411 ± 530 years B.P. at one site provides a minimum vertical uplift rate of 4.1±0.5 mm/yr. The maximum possible uplift (115±15 m) of the oldest terrace surface, whose probable age is 15 to 18 ka, places an upper bound on the uplift rate of 7±2 mm/yr. The thrust geometry at depth and the cumulative shortening (10–20 km) deduced from balancing sections logged across the imbricate thrust wedge are consistent with a shortening rate of about 5 mm/yr and an onset of thrusting at about 4±2 Ma. Such a shortening rate implies a significant northward decrease in slip rate along the Altyn Tagh Fault. The recent growth of the western Qilian mountain ranges thus appears to be intimately coupled with sinistral motion on the Altyn Tagh Fault and the extrusion of Tibet.