Vertical coherence of deformation in lithosphere in the NE margin of the Tibetan plateau using GPS and shear-wave splitting data

Abstract

We present 817 new SKS/SKKS shear-wave splitting observations to infer the mantle deformation field in the NE margin of the Tibetan plateau. The obvious pattern of dense splitting measurements shows that the fast wave polarization directions (φ) are oriented along a dominant NW-SE in the northeastern Tibetan plateau, Alxa block, and the western margin of the Ordos block and rotate to near E-W in the Qinling orogen, Weihe graben and Ordos block. Based on a continuous surface deformation field inferred from GPS observations and faults slip data, we model the predicted fast axis orientations (φc) at each location of averaged splitting observation. Exclusion of 32 averaged observations with larger misfits within the interior of the Ordos block, comparing splitting observations (φ) with predictions (φc) produces an average misfit of 11° indicating vertical coherent of lithospheric deformation in the NE margin of the Tibetan plateau. However, in the Qinling orogen, the thin lithosphere (<100km) could not account for the observed delay times (δt, ~1.2s), and the measured fast directions are parallel to the absolute plate motion (APM) direction, we infer that both the eastward asthenospheric flow and lithospheric left-lateral shear deformation contribute to the observed anisotropy. Within the interior of the Ordos block, where has thick lithospheric root (>300km) and weak anisotropy (δt, ~0.6s), the average misfit increases to 63°, and vertical coherence is no longer present, which suggests the rigid and stable Ordos block with little deformation still remains the lithospheric root of North China Craton.