CRITICAL ASPECT RATIO VORTICITY GAGE IN TRANSPRESSION

Vorticity analyses provide information on the relative plate motions recorded by ancient fault zones. The critical aspect ratio vorticity gage (Passchier, 1987; Wallis et al., 1993, Jessup et al., 1997) is one of the most popular vorticity tools because the input data are easy to collect: aspect ratio of a rigid clast and the orientation of the long axis of the clast relative to the shear plane. In this contribution we attempt to calibrate the original critical aspect ratio vorticity gage for the special case of transpression. Our approach is to reproduce the original aspect ratio vs. vorticity relationship using the equations describing rigid clast rotation in plane strain general shear with no volume loss (Ghosh and Ramberg, 1976). In the same manner we develop the aspect ratio vs. vorticity relationship for transpression using the equations describing rigid clast rotation in transpression (Tikoff and Teyssier, 1994). Our results indicated application of the general shear based vorticity gage to zones characterized by transpressional kinematics consistently underestimated the simple shear component of deformation. For most deformations the difference between the two models is minor. However, if a transpressional zone is characterized by a high pure shear component of deformation (i.e. Wk < 0.4) the difference becomes significant.