Variable quartz lattice preferred orientation patterns within a mylonite from the Rockfish Valley Shear Zone of the Central Virginia Blue Ridge
Abstract
The lattice preferred orientation (LPO) patterns that develop during strain reflect deformation conditions, such as strain magnitude, shear sense, and deformation temperature. We present quartz LPO results from a suite of samples collected from a single mylonitic outcrop in the Rockfish Valley Shear Zone in the Blue Ridge of Virginia. In these rocks, quartz occurs both within elongate recrystallized ribbons and as isolated porphyroclasts, typically 250-500 μm long, surrounded by a mica-rich matrix. The quartz LPO patterns reveal a wide range of fabrics despite origination from a single outcrop. In the highly recrystallized ribbons and in some isolated porphyroclasts, well-developed c-axis fabrics are consistent with basal <a> slip. In contrast, other isolated porphyroclasts show lower degrees of crystallization and LPO fabrics are strong but randomly oriented with respect to the macroscopic foliation. We interpret these fabrics to represent the orientation of the porphyroclast prior to deformation. In still other porphyroclasts, fabrics are less well-developed and appear transitional between an initial crystal orientation and the well-developed fabric observed in the highly recrystallized ribbons. Together, these observations suggest that quartz LPO patterns within individual porphyroclasts evolve towards a steady state fabric at different rates, even within samples that share a deformational and temperature history. Possible explanations for the variable recrystallization among these individual porphyroclasts are heterogeneous strain within the sample and/or orientation effects. We infer that ductile shear zones with abundant quartz and mica may require extremely large shear strains in order to reach rheological steady state.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2013
- Bibcode:
- 2013AGUFM.T53A2555R
- Keywords:
-
- 8163 TECTONOPHYSICS Rheology and friction of fault zones;
- 8012 STRUCTURAL GEOLOGY High strain deformation zones;
- 8030 STRUCTURAL GEOLOGY Microstructures;
- 8108 TECTONOPHYSICS Continental tectonics: compressional