A critique of vorticity analysis using rigid clasts

Estimating the kinematic vorticity numbers from rock fabrics presents many problems. In this paper we use numerical modeling to investigate the reliability of those widely-used vorticity analysis methods using rigid clasts. We use very simple flows (steady state, homogeneous, and having monoclinic symmetry) to represent plane-straining zones and transpressional zones and assume that the rotation of rigid clasts perfectly obeys the theory of Jeffery (1922, Proceedings of the Royal Society of London A102, 161–179). These are assumptions made in current vorticity analysis using rigid clasts. Even with these simple assumptions, our modeling shows that the current methods have intrinsic uncertainties so large that it is pointless to use the estimated vorticity numbers to constrain shear zone boundary conditions and kinematics. It is perfectly consistent with numerical modeling results if the currently reported vorticity numbers estimated from rigid clasts (in the range of 0.50–0.85) are all interpreted as being from natural shear zones with close-to-simple-shearing flows. The large uncertainties arise because the motion of rigid clasts is intrinsically a three dimensional problem.

Research highlights
► Vorticity analysis using rigid clasts has intrinsic uncertainties too large to be useful.
► Reported vorticity numbers from clasts could be from close-to-simple-shear flows.
► Rigid clast rotation should be dealt with by the complete 3D Jeffery theory.