Explaining the current geodetic field with geological models: A case study of the Haiyuan fault system

Abstract:

Oblique convergence across Tibet leads to slip partitioning with the co-existence of strike-slip, normal and thrust motion in major fault systems. While such complexity has been shown at the surface, the question is to understand how faults interact and accumulate strain at depth. Here, we process InSAR data across the central Haiyuan restraining bend, at the north-eastern boundary of the Tibetan plateau and show that the surface complexity can be explained by partitioning of a uniform deep-seated convergence rate. We construct a time series of ground deformation, from Envisat radar data spanning from 2001-2011 period, across a challenging area because of the high jump in topography between the desert environment and the plateau. To improve the signal-to-noise ratio, we used the latest Synthetic Aperture Radar interferometry methodology, such as Global Atmospheric Models (ERA Interim) and Digital Elevation Model errors corrections before unwrapping. We then developed a new Bayesian approach, jointly inverting our InSAR time series together with published GPS displacements. We explore fault system geometry at depth and associated slip rates and determine a uniform N86±7E° convergence rate of 8.45±1.4 mm/yr across the whole fault system with a variable partitioning west and east of a major extensional fault-jog. Our 2D model gives a quantitative understanding of how crustal deformation is accumulated by the various branches of this thrust/strike-slip fault system and demonstrate the importance of the geometry of the Haiyuan Fault, controlling the partitioning or the extrusion of the block motion. The approach we have developed would allow constraining the low strain accumulation along deep faults, like for example for the blind thrust faults or possible detachment in the San Andreas “big bend”, which are often associated to a poorly understood seismic hazard.