Slip Partitioning Along the Haiyuan Fault System

Oblique convergence across Tibet leads to slip partitioning with the coexistence of strike-slip, normal and thrust in major fault systems. Both transform and convergent components of deformation are responsible for significant seismic hazard. However, fault interactions and present-day deformation related to thrust faults are poorly explored in current models of interseismic deformation. Here, we use SAR interferometry and GPS data to investigate slip partitioning between the Haiyuan fault system and the Qilian Shan restraining bend at the north- eastern active margin of the Tibetan plateau. The objectives are both methodological (extracting a small deformation signal in mountainous areas by InSAR) and thematic (understanding the present-day kinematics along this complex fault system). For InSAR, we use the most recent developments of our Small Baselines interferometric chain NSBAS (that takes into account, in particular, atmospheric and DEM delays corrections before un- wrapping). We compute and analyze time series of interferograms built from the complete Envisat data archive over track 104 in between 2003 and 2010. We then model the velocity field by oblique slip on a deep crustal decollement connecting the roots of the major Haiyuan and Gulang left-lateral faults and the Qilian Shan thrust. Using this geometry, we determine a horizontal shortening rate of 4+- 2 mm/yr and fault-parallel component of 9+- 2 mm/yr across the whole fault system, with different partitioning west and east of the Tianzhu pull-apart basin. Our simple model gives a quantitative understanding of how crustal deformation is accommodated by the various branches of this thrust/strike-slip fault system and shows that the complexity of the Haiyuan fault system may be accounted for by partitioning, laterally and at depth, of the more simple block motion.