IMPLICATIONS OF THE LOCATION OF THE DEEPER EXTENT OF THE PLATE BOUNDARY FOR EXHUMATION ALONG THE ALPINE FAULT, NEW ZEALAND

The Alpine Fault through the South Island, New Zealand serves as one of the primary locations where relatively deep levels of the crust are exposed along a dominantly strike-slip fault system. Although the location and geometry of the plate bounding fault system is well mapped and described in the near surface, less attention has been paid to the nature of the lithospheric plate boundary (shear zone) in the upper mantle beneath the South Island. Defining the location of such a plate boundary zone is difficult, since offset between the near surface and deeper extents of the boundary are possible - in fact expected in transpressional regimes. In the case of the Pacific/Australian plate boundary, the tectonics of the southern end of the Alpine Fault allow us to constrain the eastern extent of the Australian plate at all lithospheric levels at the time it becomes part of the Alpine Fault boundary. Specifically in the transition from the Puysegur/Fiordland subduction to the Alpine Fault transpression, the Australian plate is torn, generating an abrupt eastern edge to the Australian plate as it enters the Alpine fault tectonic regime. This fixed point on the plate boundary structure translates northeast and helps us to define the shear zone geometry likely to accommodate the plate motion at depth along the length of the South Island. The mismatch between the deeper shear zone and upper crustal fault zone implies a complex pattern to the decoupling between the crust and upper mantle of the Pacific lithosphere. Thus the patterns of exhumation observed adjacent to the Alpine fault reflect (at least in part) the consequences of spatial variations in coupling between the crust and lithospheric mantle of the Pacific plate and associated variations in the rheology.