Velocity anisotropy and shear-wave splitting in rocks from the mylonite belt along the Insubric Line (Ivrea Zone, Italy)

Abstract

Velocity anisotropy and shear-wave splitting has been measured up to 600 MPa on seventeen mylonitic samples from the mylonite belt along the Insubric Line. The rocks, which range from felsic to mafic in composition, have experienced large ductile strain. Seismic anisotropy ( A) and shear-wave splitting (ΔV s) is significantly pressure-dependent. Velocity anisotropies of P- and S-waves range from 9 to 27% ( V p) and 10 to 25% ( V s) at 10 MPa and from 4 to 15% and 6 to 17% at 600 MPa, respectively. Oriented microcracks, in addition to preferred orientation of anisotropic rock-forming minerals, contribute largely to velocity anisotropy and shear-wave splitting in the low-pressure range. At high pressure (600 MPa), where most of the cracks are closed, velocity anisotropy and shear-wave splitting is dominated by preferred mineral orientation (texture). The experimental data at 600 MPa are in fairly good agreement with velocity calculations (Voigt averages) based on volume percentages of major minerals, the stiffness tensor of respective rock forming minerals and the measured crystallographic orientation. Texture-related reflection coefficients ( R c) at the protolith/mylonite interfaces (normal incidence, no density change) are 0.04 ( V p) and 0.03 ( V s) for extremely anisotropic mylonites ( A ≈ 12%). Calculations of the variation of reflection coefficients due to velocity anisotropy of P-waves in mylonites of different lithologies (literature data included) give 90% less than 0.1, 66% less than 0.05 and 44% less than 0.03. These results suggest, that the directional variation of wave velocities in anisotropic mylonites alone does not provide the impedance contrast necessary to give rise for strong seismic reflections.