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Institute of Geophysics, Victoria University of Wellington, P.O. Box 600, Wellington, New Zealand (e-mail: tim.stern{at}vuw.ac.nz)
Geodetic, palaeomagnetic and andesite-age data all point to a fan-like opening of central North Island, New Zealand, in the past 5 Ma. Palaeomagnetic rotation rates for the eastern North Island are c. 6° Ma–1 and this is accompanied by back-arc extension of up to 19 mm a–1 at the Bay of Plenty coast and lesser values further south. Although the geodetic observations only span a decade, they show a remarkable consistency with the palaeomagnetic and volcanic arc migration data that span 5 Ma. A fan-like pattern of extension in central North Island is implied, similar to that seen in other continental back-arcs. When rapid fan-like openings do occur, they are likely to be accompanied by extreme thermal events. Heat flux from the central North Island occurs at one of the highest continental rates recorded: c. 26 MW per km of strike of volcanic zone, or 4.3 GW in total. Prior to the Pliocene extension and rotation, central North Island had a c. 20 Ma history of compression and overthrusting. It is proposed that thickening, then subsequent detachment of mantle lithosphere during this phase had a role to play in the Pliocene back-arc opening and the consequent extreme heat flux.
