Relationship between crustal finite strain and seismic anisotropy in the mantle, Pacific–Australia plate boundary zone, South Island, New Zealand

Summary The relationship between crust and mantle deformation in plate boundary zones is an outstanding problem in geodynamics. New Zealand provides a rare opportunity to examine the way strike‐slip faults relate to deep‐seated zones of lower crustal and mantle flow. A conspicuous bend deflects elongated terranes such as the Dun Mountain ophiolite through >70° in the continental crust, which we interpret as being the result of distributed dextral shear between the Pacific and Australian plates in the Cenozoic. We utilized variations in the strike of two different geological markers (ophiolite terrane and fold belts) towards the Alpine fault to calculate finite strains in three crustal domains. The deflection is best matched by a transpressional, rather than a simple shear deformation. These transpressional models predict maximum horizontal finite strain azimuths (±10°) that trend anticlockwise ∼30° to ∼10° from the Alpine fault. These azimuths match published fast polarization azimuths of SKS and local (<100 km deep) shear waves. This coincidence indicates that lithospheric mantle and upper crustal deformation are broadly coupled, although the former is more widely distributed. As shear wave splitting results from mineral fabrics that are the result of finite strain, they are appropriately compared with the finite strain recorded by the deformed crustal markers. Using the geodetic data to characterize the infinitesimal displacement field, and the deformed pattern of markers to characterize the finite displacement field, one concludes that deformation in New Zealand was not steady state. The observed seismic anisotropy was probably the product of ∼45 Myr of PAC–AUS motion, not just 5 Myr as some have suggested. Our results contradict those of recent experimental studies of olivine deformation at high temperature, by suggesting that seismic anisotropy fabrics in naturally deformed mantle rocks can track finite strain at shear strains of >2.1, and strains (ɛ s ) > 1.9 without being reoriented towards the shear plane by recrystallization. In agreement with our modelling of mantle flow, the large SKS shear wave splitting delays (>2 s) on the South Island suggest that the direction of maximum finite stretch in the mantle is more likely horizontal than vertical. This inference is consistent with the 3‐D strain calculated from deflected markers in the crust.