Upper-mantle fabrics beneath the Northern Apennines revealed by seismic anisotropy

Abstract

We image anisotropic structure of the upper mantle beneath the Northern Apennines based on body-wave data collected during the RETREAT experiment (2003-2006). Joint analysis of anisotropic parameters evaluated from independent data sets - teleseismic P-wave travel times and shear-wave splitting - allows us to identify regions of different fabrics both in the mantle lithosphere and in the sublithospheric mantle. We recognize three regions - the Tyrrhenian, Adriatic, and Transition - with their own anisotropic characteristics. The slab-parallel flow prevails in the sublithospheric mantle beneath the thin Tyrrhenian plate, while nearly slab-perpendicular orientation of the high velocities dominates on the Adriatic side of the region. The asthenospheric-flow pattern excludes a simple corner-flow model to fit the fabric of the upper mantle in the syn-convergent extensional tectonics and suggests the end of the subduction roll-back. We map fossil anisotropy with inclined symmetry axes within two domains of the thick continental Adriatic lithosphere. We estimate the lithosphere thickness of the Tyrrhenian and Adriatic plates at ~50 km and ~80 km, respectively, the latter being subducted down to no more than ~200 km with indications of inherited frozen-in anisotropic fabric. If a potential detachment at the northern end of the Apennine slab exists, then it would have to be narrow and in its initial stage. Synthetic tests of the well-known trade-off between isotropic heterogeneity and evaluated anisotropic parameters, along with combining independent data sets, document a sufficient separation of both effects. © 2013. American Geophysical Union. All Rights Reserved.