Shortening Rates and Recurrence of Large Earthquakes From Folded and Uplifted Terraces in the Western Danghe Nan Shan Foreland, North Tibet

Understanding the three-dimensional structure, segmentation, and kinematics of complex fault systems is essential to assessing the size of potential earthquakes and related seismic hazards. The Danghe Nan Shan thrust, a major splay of the Altyn Tagh fault (ATF) in north Tibet, is one of these complex fault junctions. Near the town of Subei, the western Danghe Nan Shan thrust composes two left-stepping faults outlined by fault scarps in front of folded and uplifted alluvial fans and terraces. Age constraints and 2D reconstructions of the accumulated slip above a transient base level of four terraces standing 7-60 m above the present stream bed yield shortening and vertical uplift rates of 0.5 +/- 0.1 and 1.1 +/- 0.3 mm/yr, respectively, over the last 130 ka on the southern thrust. Along the northern thrust, vertical terrace offsets of 1.5-3.6 m and horizontal slip of 4.5 m documented in a paleoseismological trench occurred after 12 +/- 4 ka, constraining coeval rates of 0.3 +/- 0.1 mm/yr for uplift and shortening. Overall, 1.4 +/- 0.4 mm/yr terrace uplift and 0.8 +/- 0.2 mm/yr shortening rates are determined, in agreement with late Miocene long-term exhumation rate estimates. Our fault mapping and geomorphic and structural observations imply that the western Danghe Nan Shan thrust accommodates slip transfer from the ATF to the west to thrusting and shortening farther east in the Qilian Shan region. Considering the scarp sizes, their lateral extent, the geometry of the faults at depth, and their slip-rate, we suggest the possible occurrence of Mw 7+ earthquakes near Subei. Plain Language Summary How faults slip is essential to understanding continental deformation and seismic hazards. In the western Qilian Shan region, the Danghe Nan Shan is one of the largest ranges bounded by active thrust faults. West and south of Subei, these thrusts can be followed for several kilometers along meter-high scarps and folded terraces. Quantifying the cumulative deformation of the folded and uplifted terraces and fans enables to constrain shortening rates of about 1 mm/yr over the last 130 ka. Evidence from surface geomorphology and sub-surface trenching across the faults indicates the occurrence of large magnitude potentially up to Mw 7 seismic events in the last 16 ka. Integrating age-controlled cumulative geological observations with field-documented co-seismic deformation increases the knowledge of slip rate and co-seismic displacement for seismic hazards.