ASYMMETRIC QUARTZ CRYSTALLOGRAPHIC FABRICS PRODUCED DURING CONSTRICTIONAL DEFORMATION

Numerical simulations predict unique quartz crystallographic preferred orientation (CPO) patterns for plane-strain, flattening-strain, and constrictional-strain geometries. Multiple studies of naturally and experimentally deformed quartzites support the predicted patterns for plane strain and flattening strain. This study tests the predictions for constrictional strain.

Quartz-rich samples from the Pigeon Point high-strain zone (PPHSZ), Klamath Mountains, California, were deformed under amphibolite-facies conditions in a 1-km-wide domain of L tectonites. Independent finite-strain analyses show that the L tectonites record true constrictional strain. The gently ESE-dipping PPHSZ records subordinate top-to-the-WNW-directed simple shear and dominant coaxial zone-normal contraction coupled with transport-parallel elongation. L tectonites likely record an additional coaxial component of transport-perpendicular, strike-parallel contraction.

Quartz CPO's from three samples were measured using electron backscatter diffraction (EBSD) methods, and from one thin section using a universal stage. For each sample multiple domains were analyzed from separate lineation-parallel and lineation-normal sections. As predicted, quartz c-axes form diffuse double girdles about elongation lineations. However, the c-axis girdles also contain distinctly asymmetric point maxima not previously reported in the literature. Quartz a-axis plots are also asymmetrical. Data sets from the same sample exhibit very similar CPO patterns with nearly identical point maxima. Fabric asymmetry is not consistent between samples when the fabrics are viewed either parallel with the lineation and straight down into the ground or parallel with the lineation and perpendicular to the gently dipping upper boundary of the PPHSZ. Hence, if CPO asymmetry is from a simple-shear component, it is not consistent throughout the L tectonite domain.

In the L tectonite samples, basal planes of biotite and muscovite are subparallel with the lineation, but appear randomly oriented in lineation-normal sections. Thus far we have been unable to accurately measure mica CPO's to quantitatively evaluate this observation. We expect plots of mica should form girdles about the lineation.