Retrograde mica in deep crustal granulites: Implications for crustal seismic anisotropy

The primary contributor to seismic anisotropy in deep continental crust is commonly thought to be crystallographically preferred orientation of anisotropic mica and amphibole. The abundance of these hydrous phases is very sensitive to temperature, pressure, and fluid content. Consequently, the thermodynamic stability of these phases provides important information with which to interpret seismic anisotropy in crustal studies. An example is given of the evolution of anhydrous, mica-absent, lower crustal felsic granulite to a mica-rich (20-40% mode) tectonite during exhumation along a crustal-scale shear zone, which should significantly influence the potential seismic anisotropy of the structure. This phenomenon may occur in the Himalayan orogen where a seismically anisotropic layer has been recently interpreted as the Main Himalayan Thrust. The apparent southward strengthening of anisotropy along the thrust may be due to an increase in mica content as rocks in the mid-crustal channel are hydrated and cool through the granulite-amphibolite transition.