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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

Dempsey, Edward D.; Prior, Dave J.; Mariani, Elisabetta; Toy, Virginia G.; Tatham, Daniel J.

Authors

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Dr Edward Dempsey E.Dempsey@hull.ac.uk
Lecturer in Structural Geology, Programme Director (Geology and GPG)

Dave J. Prior

Elisabetta Mariani

Virginia G. Toy

Daniel J. Tatham



Contributors

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%.

Journal Article Type Article
Publication Date 2011
Journal Geological Society, London, Special Publications
Print ISSN 0305-8719
Electronic ISSN 2041-4927
Publisher Geological Society
Peer Reviewed Peer Reviewed
Volume 360
Issue 1
Pages 33-47
Institution Citation Dempsey, E. D., Prior, D. J., Mariani, E., Toy, V. G., & Tatham, D. J. (2011). Mica-controlled anisotropy within mid-to-upper crustal mylonites: an EBSD study of mica fabrics in the Alpine Fault Zone, New Zealand. Geological Society Special Publications, 360(1), 33-47. doi:10.1144/SP360.3
DOI https://doi.org/10.1144/SP360.3
Publisher URL http://sp.lyellcollection.org/content/360/1/33