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Mica-controlled anisotropy within mid-to-upper crustal mylonites: an EBSD study of mica fabrics in the Alpine Fault Zone, New Zealand

By
Edward D. Dempsey
Edward D. Dempsey
School of Environmental Sciences, University of Liverpool, 4 Brownlow Street, Liverpool L69 3GP, UK
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Dave J. Prior
Dave J. Prior
School of Environmental Sciences, University of Liverpool, 4 Brownlow Street, Liverpool L69 3GP, UK
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Elisabetta Mariani
Elisabetta Mariani
School of Environmental Sciences, University of Liverpool, 4 Brownlow Street, Liverpool L69 3GP, UK
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Virginia G. Toy
Virginia G. Toy
Geology Department, University of Otago, P.O. Box 56, Dunedin 9054, New Zealand
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Daniel J. Tatham
Daniel J. Tatham
School of Environmental Sciences, University of Liverpool, 4 Brownlow Street, Liverpool L69 3GP, UK
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Published:
January 01, 2011

Abstract

The lattice preferred orientation (LPO) of both muscovite and biotite were measured by electron backscatter diffraction (EBSD) and these data, together with the LPOs of the other main constituent minerals, were used to produce models of the seismic velocity anisotropy of the Alpine Fault Zone. Numerical experiments examine the effects of varying modal percentages of mica within the fault rocks. These models suggest that when the mica modal proportions approach 20% in quartzofeldspathic mylonites the intrinsic seismic anisotropy of the studied fault zone is dominated by mica, with the direction of the fastest P and S wave velocities strongly dependent on the mica LPOs. The LPOs show that micas produce three distinct patterns within mylonitic fault zones: C-fabric, S-fabric and a composite S–C fabric. The asymmetry of the LPOs can be used as kinematic indicators for the deformation within mylonites. Kinematic data from the micas matches the kinematic interpretation of quartz LPOs and field data. The modelling of velocities and velocity anisotropies from sample LPOs is consistent with geophysical data from the crust under the Southern Alps. The Alpine Fault mylonites and parallel Alpine schists have intrinsic P-wave velocity anisotropies of 12% and S-wave anisotropies of 10%.

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Contents

Geological Society, London, Special Publications

Deformation Mechanisms, Rheology and Tectonics: Microstructures, Mechanics and Anisotropy

David J. Prior
David J. Prior
University of Otago, New Zealand
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Ernest H. Rutter
Ernest H. Rutter
University of Manchester, UK
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Daniel J. Tatham
Daniel J. Tatham
University of Liverpool, UK
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Geological Society of London
Volume
360
ISBN electronic:
9781862394483
Publication date:
January 01, 2011

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