Seismic Velocities of the Whitestone Anorthosite and its Mylonitized Equivalents in the Parry Sound Shear Zone

Abstract

<p>Compressional wave velocities of the Whitestone anorthosite and its mylonitic equivalent in the Parry Sound shear zone have been measured in the field and to two kilobars in a laboratory confining pressure vessel. Mylonitiztion of the anorthosite has resulted in a preferred orientation of constituent minerals and retrograde mineral assemblages. A seimic anisotropy is consequently developed in the mylonite such that the P-wave velocitites are lower for propagation directions perpendicular to mylonite than for its anorthosite protolith.</p> <p>In the field, rock weathering and surface fractures control velocity variations.</p> <p>At low confining pressure (shallow depth) the P-wave velocity anisotropy is controlled by fracturing which is in turn related to the mylonitic fabric. At approximately one kilobar pressure (depths close to five kilometres) where fractures and porosity are insignificant, the P-wave anisotropy is due solely to the aggregate mineral velocities and their solution and their orientations. The undeformed Whitestone anorthosite has an average P-wave velocity of 7.02 km/s measured in three perpendicular directions at 2 kilobars confining pressure. The mylonite has the following P-wave velocities at 2 kilobars confining pressure: 6.83 km/s parallel to both foliation and lineation, 6.70 km/s parallel to foliation and perpendicular to lineation, and 6.57 km/s perpendicular to foliation.</p>