Testing the utility of the porphyroclast hyperbolic distribution method of kinematic vorticity analysis

Kinematic vorticity (Wk) is a dimensionless measure of rotation relative to finite stretching and is essential for complete understanding of flow in ductile shear zones. The porphyroclast hyperbolic distribution (PHD) method is a widely used technique for estimating Wk based on the acute angle between the flow eigenvectors determined from the orientations of back-rotated a porphyroclasts. The utility of the PHD was tested with Tertiary ultramytonites from the Santa Catalina Mountains, Arizona, Proterozoic ultramylonites from the Virgin Mountains, Nevada, and a Paleozoic ultramylonite from the Hylas zone in the Virginia Piedmont. Bootstrapping statistics and a computational sieving process were used to analyze the PHD data set. Average standard deviations of the bootstrapped data sets yield a 1 sigma standard error of +/- 9% for a W-k-value measured as a % simple shear. Sieving results imply that back-rotated porphyroclasts may not orient parallel to the extensional eigenvector. The PHD method is useful for discerning between deformations that are pure shear dominated, general shear, or simple shear dominated, but is not accurate enough to report precise Wk-values. When performed on multiple sections the PHD method can identify zones of monoclinic versus triclinic shear. (c) 2007 Elsevier Ltd. All rights reserved.