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Taupo volcanic zone and central volcanic region; backarc structures of North Island, New Zealand



Taupo volcanic zone and central volcanic region; backarc structures of North Island, New Zealand



Pages 1-28 1995



Volcanism in North Island, New Zealand, is related to subduction within the Taupo-Hikurangi arc-trench system. The central volcanic region (CVR) is a wedge-shaped basin, approximately 4 m.y. old, which is defined by geophysical parameters. It corresponds to an area of low density, low-velocity volcanic rocks and anomalous seismic properties within the upper mantle. Seismic data also provide evidence for a 7.4-7.5 km/s layer at the unusually shallow depth of about 15 km beneath CVR; this material is inferred to be partially melted upper mantle. The youngest and active part of CVR is the Taupo volcanic zone (TVZ), the currently active backarc basin of the subduction system where volcanism is 2 m.y. old. TVZ is an area of high-convective-heat output (4 x 10 (super 9) W). This output is equivalent to an average heat flow of about 800 mW/m (super 2) , about 13 times greater than the continental norm and one of the highest on record for a backarc basin. Rates of extension in the currently active TVZ are about 7 mm/yr in the north and 18 mm/yr in the south, derived from shear strain rates varying between 0.18 and 0.5 x 10 (super -6) /yr shear. Subsidence rates are 1-2mm/yr. Focal mechanisms from recent small earthquakes are inconclusive, but most appear to be transcurrent. The larger 1987 Edgecumbe (m=6.3) earthquake, in the Bay of Plenty, had a normal focal mechanism. There are three volcanic assemblages in TVZ: high-Al basalt, basaltic andesite-andesite-dacite, and rhyolite-ignimbrite, in order of increasing volume. High-Al basalt and rhyolite occur in a bimodal assemblage associated with caldera structures in the extensional backarc basin, while the basaltic andesite-andesite-dacite assemblage occurs principally at the northern and southern ends of TVZ and a narrow frontal arc along the eastern margin. The andesitic assemblage is undoubtedly slab related and was formed in a multistage process involving dehydration of the slab, anatexis of the overlying mantle, fractionation of mineral phases, and minor crustal assimilation. The high-Al basalts are considered to be derived from a hot mantle wedge above the subduction zone with variation due to fractionation and minor assimilation. Origin of rhyolites and ignimbrites is more controversial. They are most likely to be the result of either melting of earlier-formed rocks or sedimentary/metamorphic rocks of a restricted composition from within the crust, perhaps with a minor upper mantle source component, or partial melting of a mantle-derived source with significant crustal contamination.

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