Changxing Long

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 (Quebec, 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.

Plagioclase preferred orientation and induced seismic anisotropy in mafic igneous rocks

Fractional crystallization and crystal segregation controlled by settling or floating of minerals during the cooling of magma can lead to layered structures in mafic and ultramafic intrusions in continental and oceanic settings in the lower crust. Thus, the seismic properties and fabrics of layered intrusions must be calibrated to gain insight into the origin of seismic reflections and anisotropy in the deep crust. To this end, we have measured P and S wave velocities and anisotropy in 17 plagioclase-rich mafic igneous rocks such as anorthosite and gabbro at hydrostatic pressures up to 650 MPa. Anorthosites and gabbroic anorthosites containing >80 vol% plagioclase and gabbros consisting of nearly equal modal contents of plagioclase and pyroxene display distinctive seismic anisotropy patterns: Vp(Z)/Vp(Y) ≥ 1 and Vp(Z)/Vp(X) ≥ 1 for anorthosites while 0.8   Vp(Y) than the gabbros. Laminated anorthosites with Vp(X) ≈ Vp(Y) ≪ Vp(Z) display a strong crystal preferred orientation (CPO) of plagioclase whose (010) planes and [100] and [001] directions parallel to the foliation. For the gabbros and amphibolites characterized by Vp(X) ≈ Vp(Y) > Vp(Z) and Vp(X) > Vp(Y) > Vp(Z), respectively, pyroxene and amphibole play a dominant role over plagioclase in the formation of seismic anisotropy. The Poissons ratio calculated using the average P and S wave velocities from the three principal propagation-polarization directions (X, Y, and Z) of a highly anisotropic anorthosite cannot represent the value of a true isotropic equivalent. The CPO-induced anisotropy enhances and decreases the foliation-normal incidence reflectivity at gabbro-peridotite and anorthosite-peridotite interfaces, respectively.