Kinematic vorticity analysis and evolving strength of mylonitic shear zones: New data and numerical results

Abstract

The kinematic vorticity number is an important quantity in structural geology and tecton- ics, giving a nonlinear ratio of simple shear to pure shear deformation. We use natural obser- vations and numerical models to show how rigid clast methods for determining the kinematic vorticity number (W k ) are compromised where strain localization occurs at the matrix-clast interface. Our numerical results show that the critical shape factor cutoff between perma- nently rotating and stable clasts, used to determine W k , is highly sensitive to coupling between the clast and the matrix. This fi nding provides an elegant explanation for the fact that rigid clast methods tend to underestimate W k relative to other methods. We present numerically determined envelopes for clast behavior across a range of kinematic vorticity numbers, clast shape factors, and matrix-clast coupling. Our numerical models show that the shape-pre- ferred orientations of feldspar clasts trend toward the positions of mica fi sh with increasing localization at the clast boundary, suggesting that mica fi sh behave as highly lubricated clasts. Our data and numerical results show that the clast-matrix interface may be several orders of magnitude weaker than the surrounding matrix and that weak interfaces can lead to a marked drop in the bulk shear strength of faults and shear zones.