Simplification of the Southwest Pacific Neogene arcs: inherited complexity and control by a retreating pole of rotation

Abstract

Abstract Neogene arc activity in the Southwest Pacific began simultaneously at 25 Ma on three differently oriented sectors. Two sectors (Norfolk-Three Kings and Colville Ridges), aligned north and north-northeast, were near-orthogonal to subduction, while the intervening Northland-Reinga sector (northwest aligned) was strongly sinistral-oblique and produced twin parallel arcs of differing compositions. This complexity was inherited from a complex late Eocene-Oligocene margin that was convergent but amagmatic, and was due primarily to its proximity to the Pacific-Australia pole of rotation. The inception of arc magmatism at 25 Ma was triggered by a 20° increase in convergence angle between the north moving Australia Plate and the northwest moving Pacific plate (from 44° to 65°), and an increase in convergence rate from c. 20 to 30–40 mm yr −1 . Between 25 and 15 Ma three subduction zones were required. The Pacific Plate was subducted at the Colville Arc, and a South Fiji Basin Plate was subducted at the Norfolk-Three Kings Arc in a manner analogous to the present-day Philippine Sea Plate located between the Mariana Arc and the Japan Trench. During this interval the Norfolk Basin opened as a back-arc basin, while the Three Kings Arc moved eastwards between two fracture zones. Eastward movement of the Three Kings Arc ( c. 350 km) was sufficient to drive a magma-producing subduction rate of c. 35 mm yr −1 independently of Pacific Plate convergence which was therefore taken up at the Colville Subduction Zone. Sinistral-oblique subduction beneath the Northland-Reinga sector was also driven by Pacific Plate convergence of c. 30 mm yr −1 . At 15 Ma the Colville Ridge extended 400 km southwestwards across North Island, New Zealand, to become a Colville-Coromandel-Taranaki Arc, and the two other arc segments died; this was the main simplification event, and it was probably a response to the progressive southeasterly retreat of the pole of Pacific-Australia rotation located to the south of New Zealand and the consequent increase in convergence rate. Between 15 and 5 Ma there was stability and no back-arc basin formed. A big change at 5Ma was probably driven by further movement of the pole, this time to the southwest, and an increase in convergence rate across New Zealand. In response, the arc split to form the still-active Lau-Havre Back-arc Basin and the active Tonga-Kermadec-Taupo Arc, and the New Zealand mountains formed. Factors remaining to be resolved are the time of collision of the oceanic Hikurangi Plateau, which has entered the New Zealand Subduction Zone, and the timing of dextral displacement(s) of the forearc to its present location along the eastern margin of the North Island. Arc simplification was accompanied by further fragmentation and dispersal of New Zealand continental crust which had already been fragmented by late Cretaceous extension. The Vening Meinesz Fracture Zone, 900 km long, originated as an Eocene near-transform sinistral boundary, and had a complex Cenozoic record of strike-slip and transpressional activation and reactivation, with both sinistral and dextral sense.