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 co llision zone of South Island, New Zealand. It is continuous at the surface for similar to 800 km and accommodates similar to 70% of current plate moti on. 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 zo ne extending into the lower crust. The data are consistent with plate bound ary deformation occurring on either northwest- or southeast-dipping shear z ones in the upper mantle, or with widely distributed strain in the upper ma ntle. We propose that the position of the Alpine Fault and any associated l ithospheric discontinuity is controlled by an Eocene passive margin that se parated Palaeozoic continental lithosphere of the Challenger Plateau from m uch younger oceanic lithosphere. Strike-slip motion since similar to 25 Ma has translated the passive margin into the continental collision zone, resu lting in subduction of oceanic lithosphere beneath South Island and progres sive localisation of shear strain near the Alpine Fault. The Eocene rift bo undary formed by exploiting Cretaceous oceanic transform faults and an olde r discontinuity within the New Zealand continent. We propose that a precurs or to the Alpine Fault could be as old as Palaeozoic. Inherited structure h as clearly controlled the first-order deformation pattern through New Zeala nd since at least 100 Ma and demonstrates that ancient faults can play an i mportant role in determining lithospheric-scale patterns of deformation at continental plate boundaries. (C) 2000 Published by Elsevier Science B.V. A ll rights reserved.