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

By
G. E. Lloyd
G. E. Lloyd
School of Earth Sciences, The University
,
Leeds LS2 9JT, UK
g.lloyd@earth.leeds.ac.uk
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J. M. Kendall
J. M. Kendall
School of Earth Sciences, The University
,
Leeds LS2 9JT, UK
g.lloyd@earth.leeds.ac.uk
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Published:
January 01, 2005

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 (VP) minimum is parallel to the a-axis maximum; the direction of maximum shear-wave splitting (AVS) is parallel to mylonitic foliation; and the VP maximum and AVS 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.

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Contents

Geological Society, London, Special Publications

Petrophysical Properties of Crystalline Rocks

P. K. Harvey
P. K. Harvey
University of Leicester, UK
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T. S. Brewer
T. S. Brewer
University of Leicester, UK
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P. A. Pezard
P. A. Pezard
Université de Montpellier II, France
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V. A. Petrov
V. A. Petrov
IGEM, Russian Academy of Sciences, Russia
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Geological Society of London
Volume
240
ISBN electronic:
9781862394889
Publication date:
January 01, 2005

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