The Influence of the North Anatolian Fault and a Fragmenting Slab Architecture on Upper Mantle Seismic Anisotropy in the Eastern Mediterranean

Abstract

AbstractThe eastern Mediterranean hosts, within the span of a few hundred kilometers, extensional, strike‐slip, and collision tectonics above a set of fragmenting subducting slabs. Slab roll‐back, toroidal flow, and lithospheric dripping/delamination processes are also believed to be operating. Associated asthenospheric flow and lithospheric deformation are expected to manifest as seismic anisotropy, measurable via study of SKS shear wave splitting. Surprisingly, previous SKS splitting investigations have resolved only long wavelength patterns of anisotropy in the region, interpreting them as large‐scale asthenospheric flow; moreover, no anisotropic signature has been associated with the North Anatolian Fault (NAF), unlike other major strike‐slip plate boundaries worldwide. We present a 29‐year record of SKS splitting observations, revealing hitherto‐unrecognized short‐length‐scale variations in anisotropy, and backazimuthal variations of splitting parameters that attest to multi‐layered anisotropy. Lithospheric anisotropy beneath the NAF exhibits fast directions either fault‐parallel or intermediate between the principle extensional strain rate axis and fault strike, diagnostic of a relatively low‐strained transcurrent mantle shear zone. Elsewhere, anisotropy is consistent with asthenospheric flow through tomographically imaged slab gaps, and driven by Hellenic trench retreat. Evidence for westward flow of asthenosphere driving Anatolian plate motion is lacking. Shorter splitting delay times and nulls in central Anatolia suggest weaker azimuthal anisotropy in the asthenosphere, supporting models that invoke vertical mantle flow patterns (lithospheric dripping/asthenospheric upwelling). Thus, we conclude that the signal of mantle anisotropy more closely reflects the lithospheric deformation, complex slab architecture and geodynamic diversity of the region than previously recognized.