Seismic anisotropy of metapelites from the Ivrea-Verbano zone and Serie dei Laghi (northern Italy)

Abstract

Laboratory measurements of compressional and shear-wave velocities up to 600 MPa confining pressure were carried out on amphibolite to granulite facies metapelites from the Serie dei Laghi and Ivrea-Verbano zone, northern Italy. The metapelite samples show moderate to high compressional-wave anisotropy and significant shear-wave splitting for propagation directions parallel to foliation. Serie dei Laghi samples are generally transversely isotropic for compressional-wave propagation, a pattern that, based on calculated velocities, is the consequence of strong preferred orientation of micas. Ivrea-Verbano samples generally have an orthorhombic anisotropy pattern for compressional waves that is caused by the preferred orientation of both mica and sillimanite. Shear-wave anisotropy and shear-wave splitting patterns differ significantly for the two fabric types. Intense static recrystallization can obliterate seismic anisotropy, but partial static recrystallization does not appreciably reduce anisotropy. We conclude that seismic anisotropy may occur in mid to lower continental crust where it is predominantly composed of metapelitic rocks and if there is a consistent regional fabric. This seismic anisotropy may be diminished if there is no consistent regional fabric or if postmetamorphic intrusions have caused static recrystallization.