Extension in the West Kunlun Mountains, NW Tibet: Insights from seismicity and analytical modeling

Abstract

Normal faulting in orogens is usually associated with gravitational potential energy (GPE) due to elevation differences and contrasting material rheology between mountains and foreland areas. The 2008–2020 Yutian normal earthquake sequence in the West Kunlun Mountains has been closely studied over the last decade, but the mechanisms of normal faulting in the region are still an open question as both gravitational force and releasing step-overs have been proposed to explain the extension in West Kunlun. We investigated the normal faulting mechanism through (1) co- and postseismic kinematic models constrained by interferometric synthetic aperture radar (InSAR) and (2) analytical force balance modeling with a thin viscous approximation. We used the InSAR time series technique to map the postseismic deformation within 3 years following the 2008 Yutian earthquake. The InSAR-derived afterslip model revealed a clear slip polarity transition from strike-slip to normal faulting, suggesting a tight connection between the normal faulting and bounding strike-slip faults. The global navigation satellite system (GNSS)-constrained thin viscous sheet model gave an estimate of ~1022 Pa∙s for the effective viscosity of the West Kunlun lithosphere, which is generally in line with previous estimates that used a similar methodology. The stress field predicted by the thin viscous model primarily features north–south compression rather than east–west extension, suggesting that normal faulting is caused not by the GPE contrast across the margin of West Kunlun, but instead by the releasing step-overs bounded by strike-slip faults. The main contribution of this paper is therefore identifying the primary causes of normal faulting in West Kunlun and improving our understanding of the active tectonics of the NW Tibetan Plateau.