RELATIONSHIP BETWEEN CRUSTAL FINITE STRAIN AND SEISMIC ANISOTROPY IN THE MANTLE, PACIFIC-AUSTRALIA PLATE BOUNDARY ZONE, SOUTH ISLAND, NEW ZEALAND

The nature of upper mantle deformation in plate boundary zones is an outstanding problem in geodynamics, in particular the relationship between crust and mantle deformation, the way in which strike- or oblique-slip faults may relate to deep-seated zones of lower crustal and mantle flow, and the width of such zones. A conspicuous bend deflects elongate terranes such as the Dun Mountain ophiolite (DMOB) through >70° in the continental crust of New Zealand, a deformation which we interpret as the result of distributed dextral shear between the Pacific and Australian plates in the Cenozoic. We use changes in strike of two different markers towards the Alpine fault, including the DMOB, to calculate the finite deformation (including principal strains) in several crustal domains. Bulk simple shear cannot explain the observed marker deflections, whereas a transpressive shear zone or a “rotating slat” model can. These assume negligible length changes parallel to the fault. Both yield similar maximum horizontal finite strain azimuths (±10°), trending anticlockwise of the Alpine fault, at an angle decreasing from ~30° to ~10° in proximity to that structure. The azimuths match published fast directions of SKS and local (<100 km deep) S waves, supporting the view that deformation is in the lithospheric mantle, that it is distributed over a >100 km width and that it is approximately coherent with that of the overlying crust. Our results suggest that seismic anisotropy fabrics in the mantle can track finite strain at horizontal strain ratios of 6-12 and shear strains of >2.1 without being substantially reoriented towards the shear plane by recrystallization. That fast polarization directions of SKS waves at two stations are subparallel to the Alpine fault, however, does not rule out that recrystallization may cause a reorientation of [100] axes of olivine towards the shear direction at shear strains of ~2.0-2.5. If so, the apparently restricted distribution of this process beneath the Southern Alps remains unexplained.