Petrofabric-derived seismic properties of a mylonitic quartz simple shear zone: implications for seismic reflection profiling

Abstract

Abstract The link between petrofabric (LPO) and seismic properties of an amphibolite-facies quartzo-feldspathic shear zone is explored using SEM/EBSD. The shear-zone LPO develops by a combination of slip systems and dauphine twinning, with a -maximum parallel to lineation ( X ) and c -maximum normal to mylonitic foliation ( XY ). The LPO are used to predict elastic parameters, from which the three-dimensional seismic properties of different shear-zone regions are derived. Results suggest that LPO evolution is reflected in the seismic properties but the precise impact is not simple. In general, the P-wave velocity ( V P ) minimum is parallel to the a -axis maximum; the direction of maximum shear-wave splitting ( AV S ) is parallel to mylonitic foliation; and the V P maximum and AV S minimum are parallel to the c -axis maximum. The seismic anisotropy predicted is significant and increases from shear zone wallrock to mature mylonite. The P-wave anisotropy ranges from 11 to 13%, fast and slow shear waves’ anisotropies range from 6 to 15% and the magnitude of shear-wave splitting ranges from 9 to 16%. Nevertheless, such anisotropy requires a considerable thickness of rock with this LPO before it becomes seismically visible (i.e. 100s of m for local earthquakes; 10s of m for controlled source experiments). However, reflections and mode conversions provide much better resolution, particularly across tectonic boundaries. The low symmetry and strong anisotropy due to the LPO suggest that multi-azimuth wide-angle reflection data may be useful in the determination of the deformation characteristics of deep shear zones.