A sensitive vorticity gauge using rotated porphyroblasts, and its application to rocks adjacent to the Alpine Fault, New Zealand

Variable aspect ratio porphyroblasts deformed in non-coaxial flow. and inte rnally containing rotated relicts of an external foliation, can be used to characterise plane strain flow regimes. The distribution obtained by plotti ng the orientation of the long axis of such grains, classified by aspect ra tio, against the orientation of the internal foliation is potentially a sen sitive gauge of both the bulk shear strain (as previously suggested) and ki nematic vorticity number. We illustrate the method using rotated biotite porphyroblasts in the Alpine Schist: a sequence of mid-crustal rocks that have been ramped to the surfa ce along the Alpine Fault. a major transpressional plate boundary. Results indicate that, at distances greater than or equal to similar to1 km from th e fault, the rocks have undergone a combination of irrotational fattening a nd dextral-oblique, normal-sense shear, with a bulk shear strain of similar to0.6 and kinematic vorticity number of similar to0.2. The vorticity analy sis is compatible with estimates of strongly oblate bulk strain of similar to 75% maximum shortening. Dextral-reverse transpressional flow characteris es higher strain S-tectonite mylonite within similar to1 km of the Alpine F ault. These relationships provide insight into the kinematics of flow and d istribution of strain in the hangingwall of the Alpine Fault and place cons traints on numerical mechanical models for the exhumation of these mid-crus tal rocks. (C) 2001 Elsevier Science Ltd. All rights reserved.