R. J. Holcombe* and T.A. Little**

The Alpine Schist in New Zealand is an east-tilted sequence of mid-crustal rocks that has been ramped to the surface along the SE-dipping Alpine Fault. Within about 1.5 km of the present surface trace of the active Fault in the Franz Josef and Fox glaciers area, the youngest fabrics in the schist are mylonitic and broadly reflect the known kinematics of the fault (dextral with a slight reverse component, and with transpressional flow kinematics). Further away from the fault, this youngest ductile fabric is no longer pervasive and the dominant foliation is an older SE-dipping, intensely lineated fabric (D2). This high-strain fabric is locally overprinted by zones of strong asymmetric folding (F3) that result in only a modest bulk shortening but which control the orientation of the older fabric.

Both biotite and garnet are developed as abundant porphyroblasts in the graphitic schist, and the growth of both is syntectonic with, or overprint, F3 folds and fabrics. The size and preservation of both species is such that individual porphyroblasts generally overgrow several graphitic laminae and each internal lamination can easily be correlated with its external parent. Thus, we have elements of a fine-scale displacement gauge in these rocks. Using these relationships, we can show that the contractional stretch across the limbs of the F3 folds is 50% since the growth of garnet, and 25% since the growth of the earliest preserved large biotite porphyroblasts. Both species show evidence of rotation associated with a slight shear and a shortening across the layers. Biotite, in particular, can be used to characterise the flow of this deformation because the rotation (relative to the external laminae) can be precisely measured using the Si/Se relationships. If we assume that the porphyroblasts exhibit a flow-coupling consistent with Ghosh and Ramberg-type flow models then the angle and sense of rotation of a biotite lath is a function of its initial orientation, the aspect ratio, the kinematic vorticity number, and the shear strain. We have compared measured orientation distributions of biotite laths with theoretical deformed distributions generated using this flow model. In particular, plots of the orientation of Si versus the orientation of the long axis of the porphyroblast, have distributions that are very sensitive to differences in kinematic vorticity number, and from which shear strain can also be estimated.

In the Alpine Schist described above, the deformation producing shortening across the F3 limbs has a low vorticity number (~0.2) and a bulk shear strain of about 0.6. The flow in these rocks has been strongly transpressional. The shear sense is consistently east-block-down (normal relative to the current steeply SE-dipping foliation orientation) and is independent of the position in the F3 folds and apparently overprints those structures, resulting in widespread oblique quartz grain-shape fabrics of uniform shear sense. We interpret this shear component to have been imprinted on the F3 fabric during late Cenozoic deformation and uplift of the schists. Remarkably,this widespread, shear deformation in the Alpine schist is dip-slip, in contrast to the strike-slip dominated kinematics of the narrow mylonite zone to the west ¾ a relationship suggestive of marked strain partitioning of oblique motion. The kinematics of dip-slip shear in the schist is strongly transpressive (low kinematic vorticity number), similar to that of strike-slip dominated shear in the mylonite zone.