206 Strain and Displacement: Patterns, Gradients, Partitioning, and Reconstructions

This session aims to bring together researchers from structure-related disciplines to facilitate discussions of approaches that inform our understanding of strain and displacement. We welcome contributions from observations, experiments, theory, and modeling that characterize deformation from micro to tectonic scales.

Tuesday, 21 October 2025
1:30 PM - 5:30 PM
HBGCC, 217C

Crystallographic preferred orientation in quartz within brittle cataclastic rocks of the Iron Mountain fault, southwest Virginia

Paper number: 206-1
Program: Technical Sessions
Time: 1:50 PM – 2:05 PM

Authors

Abstract

Interpretation of quartz crystallographic preferred orientation (CPO) has long served as a critical tool in determining a range of kinematic and metamorphic conditions in mylonites. Quartz CPO development is largely controlled by temperature dependent crystal plastic deformation mechanisms and is unexpected in lower temperature brittle cataclastic rocks. A CPO has been documented in calcite within several natural and experimental brittle cataclastic fault zones, dominantly in normal faults, but a quartz CPO has only been documented in brittle cataclastic fault zones, with various fault types, in one natural and one experimental study. The CPOs from both quartz and calcite in the natural and experimental studies have been variously attributed to preferential mineral growth due to pressure solution, growth in the extension direction, shear heating, plastic behavior in very fine-grained minerals, and either aseismic or seismic activity. 

We present similar observations to those described in the literature of a CPO in quartz grains of brittle cataclastic rocks, but in a contractional setting. These samples are from the Iron Mountain thrust fault at the base of the Blue Ridge thrust sheet in northeastern Tennessee and southwestern Virginia. The Iron Mountain fault is characterized by a series of cataclastic fracture zones within quartz-rich sandstones. These samples also contain pressure solution seams, and the majority of grains have diameters of less than 10 microns and jagged boundaries. We performed electron backscatter diffraction analysis on oriented samples of quartz from within several cataclastic zones of the Iron Mountain fault and found that the grains within these zones contain a weak but pervasive CPO. Our observations are consistent with fabric development associated with pressure solution and preferential quartz growth parallel to the extension direction on the Iron Mountain fault. More broadly the presence of a quartz CPO in these rocks implies there is a greater range of rock types and tectonic settings in which these fabrics can develop than previously recognized and may provide a means to quantify grain scale fabrics associated with seismic and aseismic brittle deformation processes. 

Geological Society of America Abstracts with Programs. Vol. 57, No. 6

doi: 10.1130/abs/2025AM-413038

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