104 Structural Geology II: From Fractures to Shear Zones to Mantle Fabrics

Monday, 8 November 2004
1:30 PM - 5:30 PM

KINEMATIC VORTICITY ANALYSIS RECONSIDERED

Paper number: 104-11
Program: Technical Sessions
Time: 4:00 PM – 4:15 PM

Authors

Abstract

Quantitative understanding of displacement and thinning or thickening across high-strain zones requires knowledge of the rotational component of strain or vorticity. As originally defined the kinematic vorticity number (Wk) relates instantaneous rotation to instantaneous stretching. Structural geologists more commonly confront the end product of cumulative deformation and thus the mean kinematic vorticity number (Wm) is appropriate. The merits of kinematic vorticity analysis are a source of ongoing debate, however we view a vorticity estimate as an important deformation parameter that should be combined with information about strain symmetry, three-dimensional strain, and volume change.

The Rs/Θ method provides meaningful estimates of Wm in high-strain zones developed from originally isotropic rocks that experienced strains of <20:1. At very large strain ratios the Θ angle is too low to resolve simple versus general shear deformation. The Rs/Θ method is valid for steady state and non-steady state deformations, constrictional and flattening strains, and volume changes. In mylonitic rocks with ‘relatively low’ strains (<10:1), vorticity gauges that are based on the stable position of rigid clasts overestimate the simple shear component relative to the Rs/Θ and quartz c-axis methods. The porphyroclast hyperbolic distribution (PHD) method yields an estimate of the neutral kinematic vorticity number (Wn) and provides a measure of the last increments of rotational strain. PHD analysis of ultramylonitic rocks from a variety of tectonic settings reveal significant rotational strain along planes both parallel and normal to the elongation lineation. These data suggest triclinic deformation symmetries may be common.