The thrust structure and slip partitioning of Haiyuan-Qilian systems at NE margin of Tibetan plateau, constrained from geomorphic evidence

Faulting and folding of basement rocks together accommodate convergence within continental orogens. Here we use the river terraces sequences along the Dongda river as the geomorphic tracer to examine deformation patterns at the northeastern Qilian Shan. Five river terraces, T1 (youngest), T2, T3, T4a and T4b (oldest), were identified and dated as 4.2 ± 0.3 ka, 6.1 ± 0.5 ka, 12.4 ± 2.5 ka, 16.4 ± 0.2 ka and 27.4 ± 2.5 ka, respectively. Three major reverse faults, Minle-Damaying fault, Huangcheng-Ta’erzhuang fault, and Fengle fault, contribute to deformation of the region. Based on displaced terrace treads, we estimated the vertical slip rate along the Minle-Damaying fault as 0.7–1.2 mm/a, along Fengle fault as 0.5–0.7 mm/a. Apart from surface displacement across faults, folding of the Dahuang Shan anticline at the hangingwall of Fengle fault adds to an additional uplift rate of ~ 0.2 mm/a at its crest. Inhomogeneous uplift of the intermontane basins between Minle-Damaying fault and the Dahuang Shan anticline indicates a 0.9 ± 0.2 mm/a uplift rate along the Huangcheng-Ta’erzhuang fault. Kinematic modeling shows that the deformation propagated from North Qilian to the foreland along a south-dipping 10° décollement which rooted Haiyuan fault at the depth of 20–25 km, accommodating 2.7–3.8 mm/a total crustal shortening rate. We suggest that the thrusts formed and high strain rate at eastern Qilian as a results of strain partitioning within the Haiyuan-Qilian systems, might coincided with the restraining bend of Haiyuan fault system, and strain rate within this complex structure may bear high regional seismic hazard.