Integrated EBSD Modelling of Seismic Anisotropy for a Major Deep Crustal Shear Zone
Abstract
Seismic anisotropy in the middle and lower continental crust is a product of aggregate mineral physical properties and crystallographic alignment over seismic wavelength scales, with hydrous minerals such as amphibole and biotite commonly dominating the bulk anisotropy. However, much of the lower crust may be too dry to stabilize these hydrous minerals in significant quantities, instead being dominated volumetrically and rheologically by feldspar. Quantifying the seismic anisotropy of deformed lower crust remains a challenge due to uncertainties in extrapolating deformation experiments and the rarity of lower crustal exposures. The limitations of informing km-scale models using compositional and mineral orientation data gathered using sample-specific techniques like electron-backscatter diffraction (EBSD) or ultrasound, which are limited to the cm- or mm-scale, add further challenges. This study presents a model of seismic anisotropy for a 5 km wide, hot and dry deep-crustal shear zone in the Canadian Shield that is an analog for lower crustal extensions of several active major continental strike-slip fault systems. EBSD data were collected across the shear zone from the major lithologies and general magnitudes of accumulated strain, and elastic tensors were calculated using the free Matlab toolbox MTEX. Lithologies range from extremely quartz-poor anorthosite and gabbroic lithologies to quartz-rich meta-igneous rocks. The calculated tensors range from 2 - 8% VP and 2 - 6% VS anisotropy individually. These data were then homogenized into km-scale map unit tensors following a detailed geologic field map, and using the texture-sensitive finite element mesh technique of the free toolkit Elastic and Seismic Properties (ESP). The result is that the deep-crustal shear zone has 4% VP and 3.5% VS anisotropy, apparently controlled by strong crystallographically-preferred orientations of dynamically recrystallized plagioclase feldspar and also significantly influenced by lithologic spatial distributions. While the bulk anisotropy of the shear zone has relatively low magnitude, it may still constitute a structure capable of being detected at the multi-kilometer scale by seismic techniques depending on the nature and degree of homogeneity of the surrounding volumes of crust.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2017
- Bibcode:
- 2017AGUFM.S53E..05O
- Keywords:
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- 7205 Continental crust;
- SEISMOLOGY;
- 8025 Mesoscopic fabrics;
- STRUCTURAL GEOLOGY;
- 8030 Microstructures;
- STRUCTURAL GEOLOGY;
- 8110 Continental tectonics: general;
- TECTONOPHYSICS