Some new aspects of kinematic vorticity analysis in naturally deformed quartzites

Abstract

Two vorticity methods have been proposed to estimate kinematic vorticity number in naturally deformed quartzites. These combine the degree of quartz c-axis-fabric asymmetry either with the strain-ratio in the XZ-plane of finite strain ( R XZ ) or with the angle, δ, between the oblique-grain-shape fabric and the main foliation. Here, it is suggested that the incorporation of R XZ data with angle δ values can also serve as an additional method of vorticity analysis. The use of a new nomogram that provides a rapid means of evaluating the consistency of vorticity estimates obtained by the three methods is also discussed. A comparison of vorticity estimates from low-grade test samples using all methods yields that three methods may record different parts of deformation history. This hypothesis was then used to discuss the spatial and temporal variation of kinematic vorticity in a crustal-scale shear zone. This natural example showed that the new method is able to record late-stage changes in the flow regime, which are slightly or not detectable by previous methods. Also, it is noteworthy that vertical vorticity profiles based on results of rigid porphyroclast analysis tend to ‘average’ profiles obtained by the estimates of quartz based methods.