Mantle flow under the Central Alps: Constraints from shear-wave splitting for non-vertically-incident SKS waves

Abstract

Shear-wave splitting measurements map the orientation of geodynamically induced fabrics in the upper mantle. However, due to the limited depth resolution of shear-wave splitting investigations from core-mantle refracted phases, it has so far been difficult to differentiate between asthenospheric and lithospheric origins of observed seismic anisotropy. To improve this differentiation, we take advantage of backazimuthal variations of the fast orientation ϕ and delay time Δt that is expected for an olivine-mantle block with vertical b-axis as compared to vertical c-axis. This suggests that shear-wave splitting observations can in principle provide constraints that have so far not been recognized. The two end member cases can occur depending on the type of deformation, for example, a subhorizontal foliation in the case of a Simple Asthenospheric Flow and a subvertical foliation when considering transcurrent/transpressional circumstances during Vertical Coherent Deformation in the lithosphere. In this study we investigate the cause of seismic anisotropy in the Central Alps. Fast orientations ϕ show a spatially coherent and relatively simple mountain-chain-parallel pattern without large azimuthal variations per station, likely related to a single-layer case of upper mantle anisotropy. Considering the measurements of the entire study area, our procedure indicates b-up olivine and thus favors an asthenospheric source of the anisotropy, with a horizontal flow plane of deformation. We also test the influence of position relative to the (European) slab under the Central Alps, distinguishing northern and southern subarea based on vertically-integrated travel times calculated from a tomographic model. Differences in the statistical distribution of splitting parameters ϕ and Δt, and in the backazimuthal variation of their variation δϕ and δΔt, become apparent. While the observed seismic anisotropy in the northern subarea shows indications of planar Poiseuille flow around the Alps, the origin in the southern subarea remains uncertain, but it is likely to be affected by the slab itself.