Evidence for Trench-normal Flow Beneath the Western Hellenic Slab from Shear-wave Splitting Analysis

The active Hellenic subduction zone is characterized by rapid slab rollback potentially driving widespread extension in the continental upper-plate. Quantifying seismic anisotropy in this region is a method of choice to gain insight into the pattern of mantle flow, and may help elucidate key processes such as the dynamics of slab rollback and the coupling between crustal and mantle deformation. Here we present splitting measurements conducted on teleseismic shear waves (SKS and direct-S) recorded as part of the temporary MEDUSA seismic experiment (2006-2007 and 2008-2009), which consisted of two 400 km-long arrays of closely spaced stations (10 km) running perpendicular to the trench across the Northern and Southern Hellenides. This dense coverage allows us to resolve three distinct domains from the trench to the back-arc, which exhibit the following anisotropic properties: (1) robust trench-normal/oblique fast polarization directions in the first 150 km, followed by (2) a rapid transition to trench-parallel fast directions across eastern Greece and (3) a reversal to trench-normal fast directions over the Aegean Sea. Delay times ranging from 0.5 to 1.5 s suggest a significant mantle contribution to the total anisotropy. We locate this contribution in the sub-slab for domain (1) and in the mantle wedge for domains (2) and (3), based on comparisons with previous anisotropy analyses and ongoing imaging studies. The observation of trench-normal, subslab fast polarization directions is consistent with a simple trench-normal flow of asthenospheric mantle being dragged by the downgoing plate. This pattern is the opposite of that observed in 90% of subduction zones worldwide, where robust trench-parallel fast directions often transitions landward into trench-normal ones. We expect that comparing our results to other settings should provide further constraints on the mechanisms generating seismic anisotropy at subduction zones.