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GEOPHYSICAL RESEARCH LETTERS, VOL. 33, L24301, doi:10.1029/2006GL028130, 2006

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

K. Mahan

Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California, USA


Abstract

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.

Received 15 September 2006; accepted 14 November 2006; published 16 December 2006.

Keywords: anisotropy; lower crust; shear zones; mica; granulites.

Index Terms: 7205 Seismology: Continental crust (1219); 8159 Tectonophysics: Rheology: crust and lithosphere (8031); 3660 Mineralogy and Petrology: Metamorphic petrology; 8012 Structural Geology: High strain deformation zones; 8045 Structural Geology: Role of fluids.

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Citation: Mahan, K. (2006), Retrograde mica in deep crustal granulites: Implications for crustal seismic anisotropy, Geophys. Res. Lett., 33, L24301, doi:10.1029/2006GL028130.