Partitioning of crustal slip between linked, active faults in the eastern Qilian Shan, and evidence for a major seismic gap, the ‘Tianzhu gap', on the western Haiyuan Fault, Gansu (China)

We have studied the Cenozoic and active tectonics of the north-eastern rim of Tibet west of the Yellow River (Gansu, China) where the western Haiyuan Fault enters the eastern Qilian Shana high mountainous region, which was the site of the 1927 May 23, M= 8-8.3, Gulang earthquake. Fieldwork, combined with analysis of aerial photographs and satellite images, reveals consistent cumulative left-lateral offsets of postglacial geomorphic features along the fault, but no recent rupture. West of the Tianzhu pull-apart basin, the levelling of offset-terrace risers implies Holocene horizontal and vertical slip rates on the steeply south-dipping, N110E-striking fault of 11 ± 4 and 1.3 ± 0.3 mm yr-1respectively. The presence of subordinate, mostly normal, throws due to local changes in fault strike, and kinematic compatibility at the SW corner of the Tianzhu basin, constrains the azimuth of the fault-slip vector to be N110-115E. On the less prominent, N85-100E-striking Gulang Fault, which splays eastwards from the Haiyuan Fault near 102.2°E-less detailed observations suggest that the average Holocene left-slip rate is 4.3 ± 2.1 mm yr-1 with a minor component of ≈N-directed thrusting, with no recent seismic break either. East of ≈103°E, coeval slip on both faults thus appears to account for as much as 15 ± 6 mm yr-1 of left-lateral movement between NE Tibet and the southern edge of the Ala Shan Platformin a N105 ± 6E direction. West of ≈103°E structural and geomorphic evidence implies that ≈NNE-directed shortening of that edge across the rising, north-eastern Qilian mountain ranges occurs at a rate of 4 ± 2 mm yr-1, by movement on right-stepping thrusts that root on a 10–20°S-dipping décollement that probably branches off the Haiyuan Fault at a depth of ≈25 km. The existence of fresh surface breaks with metre-high free faces on a N-dipping hanging-wall normal fault south of the easternmost, Dongqingding thrust segment, and of half-metre-high pressure ridges on that segment, indicates that the 1927 Gulang earthquake ruptured that complex thrust system. The ≈4 mm yr-1 shortening rate is consistent with the inference that the thrusts formed and move as a result of orthogonal slip partitioning in a large restraining bend of the Haiyuan Fault.