Seismic reflectivity of a finely layered, granulite-facies ductile shear zone in the southern Grenville Province (Quebec)

Abstract

Lithoprobe reflection profile 54 across the granulite-facies Morin shear zone (MSZ) in the Grenville Tectonic Province (Québec, Canada) reveals a west-dipping zone of strong reflections. The origin of these reflections has been investigated by detailed field mapping, laboratory measurements of P-wave velocities and densities of representative rock samples from the main lithological units, and forward synthetic modeling. Petrofabric and microstructural analyses were performed in order to interpret the measured seismic properties. Synthetic seismograms generated from the velocity and density data demonstrate that reflections originate from lithologic thin layers and thin-layer clusters, likely resulted from intensive tectonic transposition and metamorphic differentiation. Lattice-preferred-orientation (LPO)-induced anisotropy of granulite-facies mylonites (except for those containing abundant sillimanite, biotite or amphibole) is very low, and hence unlikely to be the primary cause for reflections. Confining pressure does not significantly affect the reflectivity of the shear zone because the reflection coefficients are not sensitive to pressure up to at least 600 MPa. Since composite reflections of thin-layer clusters can cause high reflectivity, the strong reflectivity from some deep crustal sections with low refraction velocities may be interpreted to be due to the presence of thin-layer, mafic clusters within dominantly felsic rocks.