Computer Science
Scientific paper
Nov 2011
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011e%26psl.311..212n&link_type=abstract
Earth and Planetary Science Letters, Volume 311, Issue 3, p. 212-224.
Computer Science
Seismic Anisotropy, Microstructure, Crenulation Cleavage, Ebsd, Sample Preparation
Scientific paper
The anisotropy of seismic wave propagation is strongly influenced by the mineralogy and microstructure of rocks. Phyllosilicates are elastically highly anisotropic and are therefore thought to be important contributors to seismic anisotropy in the continental crust. Crenulation cleavage is one of the most common microstructural fabrics found in multiply-deformed, phyllosilicate-rich, crustal rocks. We calculated the bulk elastic properties and resulting wave velocities for rock samples that preserved three different stages of crenulation cleavage development: an initial planar foliation, a moderately-developed crenulation cleavage, and a well-developed crenulation cleavage. Mineral orientation maps were obtained using electron backscatter diffraction and calculations were made using asymptotic expansion homogenization combined with the finite element method. The difficulties involved with sample preparation and data acquisition of phyllosilicate-rich rock samples are also discussed. We compare our results to more conventional methods for calculating an aggregate stiffness matrix from a mineral orientation map, namely Voigt and Reuss averages. These averages do not account for grain-scale interactions and therefore deviate from the results calculated using asymptotic expansion homogenization. Our results show that the rocks characterized by a planar foliation and a moderately developed crenulation cleavage are highly anisotropic, with P-wave anisotropies up to 30.9% and S-wave anisotropies up to 34.2%, whereas the rock characterized by a well developed crenulation cleavage is only mildly anisotropic, with a P-wave anisotropy of 15.5% and S-wave anisotropy of 10.7%. Progressive development of the fabric also causes the orientations of P- and S-wave velocity maxima and S-wave polarization directions to change markedly. Despite the high anisotropy imparted by a planar schistosity, the variety of folds and fabrics typically found in phyllosilicate-rich rocks within larger-scale crustal volumes will tend to mute the anisotropy, possibly to the point of appearing nearly isotropic.
Gerbi Christopher
Goupee Andrew J.
Johnson Scott E.
Naus-Thijssen Félice M. J.
Vel Senthil S.
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