Abstract

Laboratory measurements of compressional- and shear-wave velocities, and shear-wave splitting have been carried out on a set of upper greenschist – lower amphibolite facies of metasediments and metavolcanics and plutonic rocks from two ductile shear zones in the Flin Flon Belt (FFB) of the Trans-Hudson Orogen (THO). Selected metamorphic rocks vary in composition from felsic to mafic. Test sites with outcrops of sheared metamorphic rocks were correlated with a series of inclined seismic reflectors possibly extending from the midcrust and intersecting a well-mapped shear zone at the surface. Determination of the lithological and physical properties of highly deformed metamorphic rocks is essential for proper interpretation of the nature of observed seismic reflectors. To investigate the anisotropic properties of the rocks, compressional velocity was measured at confining pressure up to 300 MPa in three mutually orthogonal directions aligned with respect to visible textural features. In addition, on selected samples, shear-wave velocity was measured at two orthogonal polarizations for each of three propagation directions to determine shear-wave splitting. The seismic heterogeneity of hand specimens was also investigated by measuring P- and S-wave velocities on several cores cut in the same direction. Observed compressional-wave anisotropy varied from quasi-isotropic to 24%. Maximum observed shear-wave splitting reaches a value of 0.77 km/s at confining pressure of 300 MPa. The pressure invariance of observed P-wave anisotropy and shear-wave splitting indicates that intrinsic anisotropy due to the lattice-preferred orientation (LPO) of highly anisotropic minerals, such as mica and hornblende, is mainly responsible for the measured seismic anisotropy.

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