Natural deformation fabrics of plagioclase: implications for slip systems and seismic anisotropy

Abstract

The optical and TEM microstructures and fabrics of twelve naturally deformed plagioclase-containing rocks of compositions from An 0-An 80 have been studied with a view to determining the deformation mechanisms. The optical indicatrix fabrics (α, β, γ) have been recorded in all samples with respect to the foliation and lineation. In general α and β form a girdle in the foliation plane with a high concentration of γ normal to the foliation. In some cases the orientation of (010) and (001) cleavages or albite and pericline twins has allowed the construction of pole figures. In all cases (010) has been shown to be the dominant slip plane with either [001] or [100] the dominant slip direction. TEM shows that the dominant slip system is (010) [001] with secondary (010) [100]. The seismic P-wave anisotropy has been calculated for the first time from the LPO data of plagioclase; the anisotropy coefficient is high, at around 15%. The minimum P-velocity is normal to the lineation in the foliation plane and the maximum is sub-parallel to the foliation normal. The combined information on deformation kinematics and seismic anisotropy is valuable for interpretations of deep crustal seismic sections.