KINEMATIC VORTICITY ANALYSIS RECONSIDERED

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 (W_k) relates instantaneous rotation to instantaneous stretching. Structural geologists more commonly confront the end product of cumulative deformation and thus the mean kinematic vorticity number (W_m) 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 R_s/Θ method provides meaningful estimates of W_m 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 R_s/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 R_s/Θ and quartz c-axis methods. The porphyroclast hyperbolic distribution (PHD) method yields an estimate of the neutral kinematic vorticity number (W_n) 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.