Development of shape and lattice preferred orientations: application to the seismic anisotropy of the lower crust

We review the physical basis of the development of fabrics in plastic and viscous flow and illustrate the typical fabrics formed by these processes in the main rock-forming silicates of the lower crust (feldspar, quartz, pyroxene and amphibole). The orientation process in plastic deformation where a single slip is dominant is recalled and the role of the constraint of neighbouring grains is emphasized. The fabric development of anisometric crystals in viscous flow is discussed as a function of the main controlling parameters: shear strain, aspect ratio and interference between crystals. The same sense of fabric asymmetry is introduced by plastic and viscous flow between the flow plane and the shape preferred orientation and hence coherent kinematic analysis can be undertaken in both modes of flow. In order to assess the role of such fabrics in the seismic laminations of the lower continental crust we have calculated the seismic P-wave properties of typical fabrics for hypothetical monomineralic and polymineralic rocks. The calculations show that the strongest anisotropies develop in monomineralic rock with values between 5 and 16%, compared with 5 and 8% for typical rock compositions. The strongest anisotropies for layered monomineralic rocks generated by fabrics is only 6% compared to the 14% suggested by model studies of the observed seismic laminations. We suggest that other effects, such as compositional layering and/or constructive interference of seismic waves are responsible for augmenting the apparent anisotropy.