Stratification of Seismic Anisotropy in the Aegean Lithosphere and Relation to Deformation

In the Aegean region, the retreating subduction of the oceanic African plate beneath the continental Aegean plate is accompanied by extension in the Aegean Sea and incipient continental collision in the western part of the arc. Besides, the lateral motion of the Anatolian plate towards the SW along the North Anatolian Fault has influenced regional tectonics since the late Miocene. High-quality phase velocity measurements of teleseismic surface waves at frequencies up to 100 mHz offer a valuable opportunity to investigate lithospheric anisotropy caused by these tectonic processes. In this study, we measure Rayleigh dispersion curves along 98 raypaths within the Aegean region with a two-station method and invert them for azimuthally anisotropic phase velocity maps, accounting for both 2Ψ and 4Ψ anisotropic terms. Isotropic velocity anomalies mainly capture differences between crustal velocities, Moho depths and mantle wedge velocities between the forearc area and the Aegean Sea. 2Ψ anisotropy is well resolved with maximum amplitudes around 3.75% of the isotropic variations. 4Ψ terms are shown to be significantly smaller in the well-covered regions. 2Ψ orientations are consistent between adjacent frequency bands, but show variations between two distinct depth ranges. Measurements between 25 s and 30 s lend information about the uppermost mantle beneath the Aegean Sea. Along the forearc, these periods correspond to a low-velocity belt above the slab. The measured 2Ψ directions are parallel to the current direction of stretching indicated by the extensional strain field (Global Strain Rate Map project). Anisotropy is interpreted as due to lattice preferred orientation of minerals in the lithospheric mantle. Reduced amplitudes of anisotropy beneath the volcanic arc and the Sea of Crete are probably an indication of magmatic overprinting of pre-existing mineral orientations. At shallower depth, i.e. between 15 s and 18 s period, 2Ψ axes are oriented differently. Fast propagation directions parallel the GPS-derived surface velocities. Love-Rayleigh discrepancy observed along several raypaths shows that radial anisotropy is present at the same lower crustal depth levels. Changes in 2Ψ orientations outline the boundary between the south Aegean microplate, which is defined by SW-directed GPS vectors, and the region to the north-west, which is stable with respect to Eurasia. The lower crustal anisotropy may be explained by lattice preferred orientation due to ductile flow accomodated since the Miocene, or by extensive-dilatancy anisotropy caused by crack alignment parallel to the maximum compressive stress. Our observations reveal a stratification of anisotropy in the Aegean lithosphere and can help to constrain the depth of anisotropy observed with SKS measurements. The stratification points to a decoupling between lower crust and upper-most mantle or to different mechanisms causing anisotropy at these two depth levels.