Abrupt change in mantle fabric across northern Apennines detected using seismic anisotropy

Abstract

We estimated SKS splitting parameters for 18 earthquakes observed over 2 years on a dense linear seismometer array crossing the Apennines orogen (Northern Italy). An abrupt (within 30 km) change in fast polarization occurs near the crest of the Apennines. Southwest of the crest, fast polarization parallels the Apennines strike (WNW-ESE), rotating to EW orientation near the Tyrrhenian Sea. Northeast of the crest, toward the Po Plain, fast polarization is NNE-SSW, roughly normal to the orogen's strike. Delay times (0.6-2.9s) do not show a remarkable pattern. Splitting parameters beneath the Apennines argue against anisotropy restricted to the crust, and suggest that the mantle region deformed by Apennines slab rollback is spatially limited. We hypothesize that the rollback process has been uneven, evolving to a contortion of the northern edge of the Apennines slab, and perhaps incipient detachment. Lithospheric anisotropy beneath the Po-Plain may also be significant. Copyright 2007 by the American Geophysical Union.