The calculated seismic properties of quartz mylonites with typical fabrics: relationship to kinematics and temperature

Abstract

SUMMARY Quartz mylonites produced by intense ductile deformation in shear zones are often invoked as seismic reflectors in the lower continental crust. The seismic properties (Vp, Vsl, V., AV, and polarization planes of V., and V=) have been calculated for five quartz mylonites that display typical lattice preferred orientations (LPOs). All specimens display considerable seismic anisotropy with V, anisotropy coefficients of between 8 and 12 per cent. There are good correlations between the LPO of the a-axes and the minimum V, direction, and the c-axes and the maximum V, direction. The other seismic properties are not reliable indicators of LPO. As the a-axis is the major flow direction in quartz, a correlation between the ductile flow direction and the V, minimum can be made. With increasing temperature and pressure alpha-quartz transforms to beta-quartz. In the beta field quartzites are seismically transverse isotropic. Furthermore there is a considerable increase in V, from 6 km s-l in the alpha field to nearly 7 km s-' in the beta field. The transition is instantaneous and hence will result in a sharp seismic discontinuity with a reflection coefficient of -0.08 between pure aggregates of alpha and beta phases. However, under typical conditions of the continental crust the seismological importance of this phase transition will be limited by the presence of secondary minerals and partial melting.