A New Model for the Origin of New Zealand's Southern Alps

Abstract:

The Southern Alps of New Zealand are widely thought to be a direct consequence of the small component of Late Cenozoic relative plate convergence in continental lithosphere between the Pacific and Australian plates. We show with a simple model of crustal balancing that Cenozoic crustal thickening for the southern part, extending for ~250 km south of Mt Cook, is likely to be significantly more than that expected for this convergence. The detailed lithospheric structure beneath the Southern Alps, combined with seismicity, geodetic deformation, paleomagnetism and Cenozoic plate reconstructions, suggest two additional mechanisms for crustal thickening. Firstly, the Hikurangi margin, forming the northern subducting part of the plate-boundary, has rotated ≥60° clockwise in the Neogene, requiring intense shortening behind the subduction zone. We propose that this shortening was partly accommodated by southward extrusion of crust along the length of the Hikurangi margin and beneath the Southern Alps, during the Early to Late Miocene (25 – 10 Ma), contemporaneous with marked uplift of the offshore eastern part of southern South Island. Secondly, the Alpine Fault appears to root into a subhorizontal detachment at mid-crustal levels, extending for >50 km beneath the Southern Alps. We propose that this detachment is a consequence of sideways underthrusting of a wedge of the extended Australian plate continental margin, which formed during initial asymmetrical Eocene to Oligocene rifting much farther south. This was subducted along the Fiordland margin, to the south at ~10 Ma and then dragged sideways beneath the Southern Alps, as a consequence of ~300 km of strike-slip motion along the Alpine Fault. Both these mechanisms provide an explanation for how the thickness of the lithosphere can be nearly doubled over a wide region in a predominantly strike-slip setting, creating the Southern Alps, yet little distributed shortening is observed at the surface. In either case, lithospheric deformation involves a strong component of motion along the length of the deforming zone, and cannot be interpreted simply in terms of 2-D cross-sections.