The effect of oriented microcracks on seismic velocities in an ultramylonite

Siegesmund, S., Kern, H. and Vollbrecht, A. (1991) The effect of oriented microcracks on seismic velocities in an ultramylonite Tectonophysics, 186 (3-4). pp. 241-251. DOI 10.1016/0040-1951(91)90361-u.

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The influence of oriented microcracks on the directional dependence of P- and S-wave velocities has been investigated in an epidote-muscovite-ultramylonite. Optical and SEM studies revealed that the fabric of the mylonite is characterized by oriented intragranular and grain-boundary cracks. Comparison of orientation patterns of cracks with calculated elastic properties (Voigt average) of a model rock (isotropic matrix, ultramylonite crack geometry) and laboratory seismic data indicates that wave propagation is particularly affected by oriented grain-boundary cracks. Microcracks are most effective at low pressure and largely closed above about 200-300 MPa. The residual part of velocity anisotropy observed above this crack-closing pressure is mainly due to preferred mineral orientation, in particular of mica. Maximum concentration of poles to (001) mica planes corresponds to the maximum of poles to oriented grain-boundary cracks so that the effects of texture and oriented cracks on wave velocities line up, giving rise to extreme values of velocity anisotropy and shear-wave splitting at low-pressure conditions.

Document Type: Article
Research affiliation: Kiel University
Refereed: No
DOI etc.: 10.1016/0040-1951(91)90361-u
ISSN: 0040-1951
Date Deposited: 02 Feb 2012 09:28
Last Modified: 08 Oct 2012 07:33

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