Constraints on the Lithospheric Kinematics in the Aegean and Western Anatolia Unveiled by SKS Splitting Observations

The present study investigates azimuthal anisotropy and its relation to the geodynamical processes beneath the back-arc of the Hellenic subduction zone in the eastern Aegean and western Anatolia where surface tectonics is dominated by the right-lateral strike-slip North Anatolian Fault Zone (NAFZ) in the north and E-W oriented normal fault systems. We obtained apparent SKS splitting parameters from 1,660 good quality and 137 null measurements extracted from 542 events recorded at 40 permanent broadband seismic stations. Overall, the station-averaged splitting parameters indicate NNE-SSW oriented fast directions (∼N20°E) and splitting delays around ∼1.5 s. The large splitting delays, particularly observed beneath the northern Aegean can be explained by either an enlarged mantle wedge thickness or increased strength of upper mantle anisotropy. We constrain complex anisotropy structures within two layer models from notable backazimuthal variations in individual splitting measurements observed beneath a few stations at the north located in a close proximity to the NAFZ and central-western Anatolia. At the western end of the NAFZ, our estimated upper layer anisotropy direction (at ∼120 km) is rather parallel to the NAFZ reflecting the imprint of a lithospheric petrofabric formed by recent deformation while in central-western Anatolia they correlate well with maximum shear directions and small splitting delays (∼0.6 s) appear to further support relatively thin lithosphere (∼90 km). An overall pattern of extension-parallel fast directions (N10°E) within lower layer can be attributed to the slab rollback-induced mantle flow that is highly oblique with respect to the WSW-ward motion of the Anatolian lithosphere.