Preferred orientation and anisotropy of seismic and magnetic properties in gabbronorites from the Bushveld layered intrusion

The preferred orientation of plagioclase and pyroxene in gabbronorites from the Bushveld layered igneous intrusion (South Africa) was investigated using electron backscatter diffraction. In two localities 200 km apart (Belfast and Rustenburg) strong preferred orientation was observed with the principal fabric features aligned in the subhorizontal foliation plane (defined by (010) planes of plagioclase and (100) planes of pyroxene). The pattern appears to result from the orientation of tabular crystals during the dynamic intrusion events that formed the layered structure. Primary magmatic oxides are also influenced by the preferred orientation of plagioclase, generating an oblate magnetic susceptibility fabric parallel to the foliation plane. Plagioclase preferred orientation gives rise to other directionally dependent geophysical properties including a large seismic anisotropy with p-waves traveling 4-8% faster in the vertical than in the horizontal direction.