Plate boundary deformation in South Island, New Zealand, is related to inherited lithospheric structure

The Alpine Fault is the main active structure in the oblique continental collision zone of South Island, New Zealand. It is continuous at the surface for ∼800 km and accommodates ∼70% of current plate motion. A 460 km offset of basement rocks suggests it has accommodated >50% of plate displacement since 45 Ma. Geophysical, geological, and contemporary kinematic data are successfully modeled by slip on a narrow Alpine Fault zone extending into the lower crust. The data are consistent with plate boundary deformation occurring on either northwest- or southeast-dipping shear zones in the upper mantle, or with widely distributed strain in the upper mantle. We propose that the position of the Alpine Fault and any associated lithospheric discontinuity is controlled by an Eocene passive margin that separated Palaeozoic continental lithosphere of the Challenger Plateau from much younger oceanic lithosphere. Strike-slip motion since ∼25 Ma has translated the passive margin into the continental collision zone, resulting in subduction of oceanic lithosphere beneath South Island and progressive localisation of shear strain near the Alpine Fault. The Eocene rift boundary formed by exploiting Cretaceous oceanic transform faults and an older discontinuity within the New Zealand continent. We propose that a precursor to the Alpine Fault could be as old as Palaeozoic. Inherited structure has clearly controlled the first-order deformation pattern through New Zealand since at least 100 Ma and demonstrates that ancient faults can play an important role in determining lithospheric-scale patterns of deformation at continental plate boundaries.