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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Abstract

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
Date Deposited:	02 Feb 2012 09:28
Last Modified:	23 Sep 2019 19:48
URI:	https://oceanrep.geomar.de/id/eprint/15546

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