Subduction zone retreat and recent tectonics of the South Island of New Zealand

Abstract

The central South Island of New Zealand is a zone of recent oblique continent‐continent convergence between the Australian and Pacific plates. Convergence is localized on the Alpine Fault, and its broader expression has created the Southern Alps. We propose that many of the strike variations in orogenic style of this zone (e.g., physical extent and style of crustal shortening, argon release patterns, and fission track observations) can be explained by differential amounts of subduction zone retreat beneath the orogen. The observed patterns suggest greater eastward retreat of the subducting Pacific plate in the south than in the north. To support this hypothesis, we compare geodynamic models having varying amounts of subduction zone retreat with geological and geophysical observations. Incorporating subduction zone retreat not only creates tectonic variations similar to those observed but also provides a geodynamically consistent explanation for the size, shape, and location of the broad negative Bouguer gravity anomaly associated with the orogen. In addition, these geodynamic models suggest that subduction zone retreat may result in local topographic lows which may serve as preferential locations for the formation of intermontane basins and associated lakes, possibly explaining the chain of South Island lakes. Our preferred interpretation of the geologic history of the central South Island convergence has an initial phase of Pacific plate lower crust and mantle subduction (underthrusting) with no retreat followed by a second phase of subduction zone retreat in the southern part of the zone. This retreat may be a consequence of oblique convergence, which creates interference beneath Fiordland between the south east dipping Australian plate and the underthrust west dipping Pacific plate to the north and forces retreat.