Layered azimuthal anisotropy of Rayleigh wave phase velocities in the European Alpine lithosphere inferred from ambient noise,
Résumé
Passive seismic imaging of the earth is a rapidly developing field of study. Recent advances in noise cross correlation techniques allow imaging of isotropic surface-wave and shear-wave velocities in areas where earthquake numbers and distributions are insufficient to implement traditional earthquake based tomography. Furthermore, advances in the theory underpinning surface wave inversion have led to depth-dependent mapping of seismic anisotropy in the lithosphere and upper asthenosphere. We show that by merging these two rapidly advancing fields we can invert noise-based phase velocity measurements for azimuthally anisotropic phase speed, thereby providing a highly resolved image of layered azimuthal anisotropy in continental crust. We apply this new algorithm in the western Alps, an area of complex lithospheric structure. Alpine crustal thickening results from continental collision, indentation of the Ivrea mantle into the middle crust of the European plate, rollback of the European lithospheric mantle, and crustal slicing. We find that anisotropy beneath the central Alps is stratified in two layers -- one with an orogen-parallel fast direction above ∼ 30 km depth and another with a strong orogen-perpendicular fast direction at greater depth. Although our resolution is reduced outside the central Alps, we map orogen-parallel anisotropy in the crust of the northern Alpine foreland. We interpret the results in the central Alps as first-order evidence for a model of azimuthal anisotropy in which (1) near-vertical emplacement of crustal slices following detachment of the lithospheric mantle from the crust gives rise to orogen-parallel fast directions of wave propagation in the crust, and (2) dominantly horizontal tectonics in the thickened crustal root and uppermost mantle yield orogen-perpendicular fast directions at greater depth.