An estimate of kinematic vorticity from rotated elongate porphyroblasts

Internal fabric in strain shadows around elongate rutile porphyroblasts indicates rotation of porphyroblasts relative to the incremental stretching direction during deformation within a regional-scale shear-zone in south-central Alaska. For porphyroblasts of various aspect ratios and initial orientations, this fabric is used to assess the sense of rotation relative to the direction of maximum incremental stretching, the total rotation, and the vorticity. The total rotation of porphyroblasts relative to the external fabric based on these parameters varies as a function of porphyroblast orientation. All 111 porphyroblasts, regardless of orientation, have rotated in a clockwise sense relative to the external fabric. Comparison of these results with previous models suggests that the deformation which affected the porphyroblasts took place in the field of continuous forward rotation with a maximum ratio of the rate of pure shearing to the rate of simple shearing (S(r)) of 0.21 or a kinematic vorticity number W(k) > 0.92. The model curves showing rotation vs final orientation do not vary significantly over the range of S(r) and R that is relevant to this study. The data have significant scatter, yet there is remarkable agreement between the data and the model for a γ of 1.5. These results illustrate how strain shadows around porphyroblasts with large aspect ratios and variable initial orientations provide a more complete assessment of kinematics than inclusion trails within more equant porphyroblasts.