CONCEPTS OF VORTICITY IN REGIONAL SHEAR ZONES

Regional shear zones characteristically involve remarkable variations in deformation fabrics related to the involvement of multiple rheological units and multiple periods of deformation. It is also likely that true discontinuities – faults – within the shear zone introduce sharp gradients or truncations in deformation gradients, potentially obscuring interpretations of flow kinematics and rate-of-deformation determinations. In areas of complicated stratigraphy or minimal exposure, shear zone boundaries may be ambiguous, making kinematic vorticity determinations that depend on knowing the shear zone geometry impractical.

It is therefore critical in regional shear zones to build the case for vorticity from rocks relatively isolated from structures unrelated to the strain being described. Kinematics for the later stages of ductile deformation are often easier to assess because the effects of overprinting discrete deformation can be removed by delineating those features with stereonets or careful outcrop-scale mapping. Earlier, pervasive ductile structures in the shear zone can be problematic if the conditions of deformation approximate those of later shearing. Fortunately, in many shear zones the later stages of shearing often involve retrograde metamorphism and structures formed under those conditions are distinctive.

As an example, the Brevard fault zone in the southern Appalachian Mountains contains a rich mixture of stratified and, less commonly, massive rheological units. This regional shear zone contains both high grade (amphibolite facies) and retrograde (greenschist facies) metamorphic minerals. Except where it is modified by late stage contractional faults, kinematics of the shear zone as indicated by finite stretching lineations, porphyroclast rotation, and shear band geometry are remarkably uniform for several hundred km. Even at the scale of a single thin-section of some of the heterogeneous units in the shear zone, however, it is evident that flow was partitioned between discrete ductile deformation zones (shear bands), rotation of porphyroclastic garnet and feldspar, and folding in microlithons. Three-plane orthogonal views of the fabric in these rocks combined with other indicators show a strong preference for marginally triclinic near strike-slip dextral shear.