Abstract Reconnaissance 2D seismic reflection data intended to investigate the petroleum potential of New Zealand’s marine territories have contributed many insights into the geological evolution of the large continental block that surrounds New Zealand. These include: definition of a back-thrust system to the Mesozoic Gondwana subduction margin along the Northland–Reinga Basin and the transition to back-arc rifting; the development of a Mesozoic back-arc rift system through the present New Caledonia and probably the Bounty troughs; the Early Cretaceous cause, at least locally, of the cessation of subduction along the New Zealand sector of the Gondwana margin; evidence for anticlockwise rotation of eastern New Zealand relative to the west in Late Eocene time; an explanation for the development of the Alpine Fault and the South Island compressional strike-slip margin between the Pacific and Australian plates through South Island.

Abstract The Reinga Basin occupies a northwest-southeast bathymetric d epression between the West Norfolk and Reinga ridges and has an area of about 100,000 sq. km. Rock samples have been dredged from surrounding ridges, but no boreholes have been drilled. We present a seismic stratigraphy developed using 5,135 line km of new 2D seismic-reflection data and 20,000 line km of older data, and we tie this stratigraphy to boreholes in the nearby Northland and Taranaki basins. We identify six phases of basin evolution. The first phase involved extension across northwest-trending normal faults. The region subsided passively during phase 2, and we infer from regional considerations that this phase lasted from Late Cretaceous until middle Eocene time. Phase 3 was late Eocene compression, which we interpret to be related to the initiation of the Tonga-Kermadec subduction. This led to uplift and erosion of the West Norfolk and Reinga ridges and deposition of detrital material at the center of the Reinga basin. Oligocene to early Miocene regional subsidence (phase 4) resulted in flooding of structures created during phase 3. Uplift of the Wanganella Ridge, in the northwest part of the Reinga Basin, occurred at the end of the early Miocene (phase 5). The last phase is tectonically passive, but with ongoing sedimentation up until the present day (phase 6). Upper Cretaceous units in the nearby Taranaki Basin contain coaly source rocks, and coal has been dredged from the ridge on the southwest margin of the Reinga Basin. Maturation models of three sites in the Reinga Basin predict that Cretaceous type III coaly source rocks within basal strata would begin to generate and expel petroleum in early Cenozoic time and expulsion would continue to the present day. The top of the oil expulsion window is modeled at 4.0 +/- 0.5 km below the sea bed, implying a potential kitchen area of approximately 15,000 sq km for Cretaceous source rocks, or a broader area if Jurassic source rocks are present. Most oil and gas expulsion is predicted to be later than the Eocene to Miocene folding and reverse faulting events that created structural traps. It is outside the scope of our study to develop play concepts or analyze direct hydrocarbon indicators, but our regional stratigraphic and tectonic study, combined with a consideration of petroleum system components that may be present, indicates that the Reinga Basin is prospective for oil and gas.

A spectacular, dense network of cataclastic faults characterizes the Late Cretaceous Ngatuturi Claystone, a massive and mechanically almost isotropic siliceous mudstone. It is part of a Cretaceous to late Oligocene shelf sequence deposited NE of New Zealand that was translated SW in the late Oligocene with the Northland Allochthon in an obduction event associated with southward propagation of a new convergent plate boundary. The allochthon was reactivated in the Miocene, forming the southward-moving substrate of the Waitemata piggyback basin. The cataclasites are submillimeter- to several centimeters–thick black seams that were formed without contemporaneous open tensile fractures, because any fault asperities were immediately ground away. Riedel shear patterns are prominent at all scales, due to multiple reactivation of preexisting fault surfaces. Some fault arrays are so closely spaced that they resemble a cleavage compatible with large-scale folds in the Ngatuturi Claystone. Movement on such faults has allowed formation of structures that appear mesoscopically ductile. More than twenty phases of cross-cutting structures (events E1–E22) are part of the following stages of tectonic development: (I) northeastward thrusting in an accretionary prism; (II) southward transport in the Northland Allochthon; (III) southwestward movement during the main phase of allochthon emplacement; (IV) renewed southward movement of the allochthon; (V) sliding during sedimentation of the Miocene Waitemata Group; and (VI) further intrabasinal thrusting to the south. During the pre-Miocene phases (I–IV), the cataclasites fault network allowed the Ngatuturi Claystone to deform in a macroscopically ductile manner, simultaneously acting as a dynamic aquiclude, thereby facilitating high fluid pressures in the surrounding rocks.

Rare garnet phenocrysts and garnet-bearing xenoliths occur in high-silica, metaluminous to peraluminous andesites and dacites (and their high-level intrusive quartz diorite equivalents) from a Miocene calc-alkaline province in Northland, New Zealand. These garnets are among the most Ca-rich (17–28 mol% grossular) garnets of igneous origin so far recorded in calc-alkaline suite rocks. Associated minerals are dominant hornblende and plagioclase and minor augite, occurring as phenocrysts in xenoliths and as inclusions in the garnet. This mineralogy points to the I-type character of the garnet-bearing host magma compositions, and contrasts this garnet occurrence with the more frequently recorded grossular-poor (3–10 mol%) garnets with hypersthene, plagioclase, biotite and cordierite, found in S-type volcanic and intrusive host rocks. Detailed experimental work on a glass prepared from one of the garnet-bearing dacites closely constrains the conditions under which the natural phenocryst and xenolith mineral assemblages formed. This work was conducted over a pressure-temperature range of 8–20 kbar, 800–1050°C with 3–10 wt% of added H 2 O, defining overall phase relationships for these conditions. Importantly, amphibole only appears at temperatures of 900°C or less and clinopyroxene at >900°C (with 3wt% H 2 O). Orthopyroxene occurs with garnet at lower pressure (∼15 kbar with 3wt% H 2 O; ∼< 10 kbar with 5 wt% H 2 O). Absence of orthopyroxene from the natural garnet-bearing assemblages indicates pressures above these limits during crystallisation. Plagioclase is markedly suppressed (with respect to temperature) with increasing H 2 O content, and for pressures of 10–15 kbar, the maximum H 2 O content possible in the magma with retention of clinopyroxene and plagioclase together (as evident in xenoliths) is 5–6 wt%. Finally, the lack of quartz in any of the xenoliths suggests magma H 2 O content higher than 3% (where quartz appears with amphibole at 900°C), since the quartz liquidus temperature decreases with increasing H 2 O content, and with decreasing pressure. In experiments with 5 wt% H 2 O, a quartz-free field of crystallisation of garnet–clinopyroxene–amphibole–plagioclase occurs between 10 and 15 kbar and temperatures between 850 and 900°C. In addition, detailed experimentally-determined garnet compositional trends, together with ferromagnesian mineral compositional data for specific experiments with 5wt% H 2 O added and run at 10–13 kbar and ∼900°C, suggest that the natural assemblages formed at these conditions. This implies that the parental dacitic magma must have been derived at mantle depths (the Northland crust is ∼25 km thick), and any basaltic or basaltic andesite precursor must have contained ∼2–3 wt% H 2 O. The unique nature of the Northland volcanics and high-level intrusives, preserving evidence of relatively grossular-rich garnet fractionation in the high-pressure crystallisation history of an originally mantle-derived magma, is attributed to a combination of unusually hydrous conditions in the source region, complex tectonic history involving obduction and subduction, possible incorporation of crustal slivers in a mantle-crust interaction zone, and relatively thin (∼25 km) crust.