THE ROLE OF UPPER MANTLE INSTABILITY FOR ROCK UPLIFT, POTASSIC VOLCANISM AND CRUST-MANTLE THINNING WITHIN WESTERN NORTH ISLAND, NEW ZEALAND

Western North Island of New Zealand underwent nearly 30 my of compression and shortening during the Oligocene – Miocene period. At the close of the Miocene geological evidence from the marine sedimentary record shows a broad (300 km wide) surface and rock uplift ( max 2.5 km uplift) that began just south of Auckland City. The uplift moved southwards through the Pliocene leaving a series of potassic andesite/basalt volcanoes in its wake. A step in the Moho and mantle lid is now observed at the southern limit of potassic volcanism. Also through the Pliocene, back-arc spreading developed in the adjacent central North Island to the west. Thus on the eastern margin of the western North Island hot asthenosphere was rapidly juxtaposed against thickened crust and mantle lithosphere.

The juxtaposition of asthenosphere against thickened lithosphere is unstable. We develop preliminary 2D models for convective removal of mantle lithosphere where the boundary conditions include both an initial thickening, and a free edge in the mantle portion of the lithosphere. A drip-like instability readily grows from the free-edge of our model and migrates inwards. Although the migrating drip is similar to delamination, the process entails continuous viscous deformation. We find that the mantle lithosphere will laterally migrate ~ 200 km in 20 my if the viscosity at the top of the mantle lid is ~ 10 ²¹ Pa s. This migration rate provides a reasonable fit to the western North Island data, and the value of viscosity is similar to that required for other global localities where convective instabilities in the mantle have developed. An additional important finding is that if we want to replicate sedimentary basin development which is observed above the proposed downwelling mantle lithosphere, then the crustal viscosity ~ 10 x mantle viscosity.