Australian-Pacific Plate Boundary Research Articles

Décollement strength and mechanical segmentation along the frontal wedge of the Hikurangi convergent margin (New Zealand): Insights from critical taper analysis

To better understand the mechanics of outer forearcs, which is influenced to a large degree by the frictional properties of the plate interface, is a pre-requisite to address topics like natural hazard such as large earthquakes and slumping. Previous research including geodetic and geophysical measurements as well as critical taper analyses (CTA) has revealed that the mechanics of the Hikurangi margin (HM) offshore New Zealand may vary along the margin. However, the relationship between the morphology of the submarine margin and effective décollement strength of the Pacific-Australian plate boundary is not fully understood. From areal gridded data of surface slope (α) and plate dip (β) we characterise the central HM as an intermediate accretionary wedge and the northern HM as a non-accretionary wedge sensu Lallemand et al. (1994). Our areal CTA, comprising the entire region of the submarine margin from the transition to strike-slip faulting in the south to subduction erosion in the northernmost part of HM reveals significant variation of the effective décollement strength of the Pacific-Australian plate boundary. Altogether the décollement strength shows significant regional variability along the HM (0.02 to 0.21), increasing to the toe of the accretionary wedge, and in regions with seamount subduction. Furthermore, mean values of the décollement strength are increasing from the central (0.05) to the northern HM (0.09). Also, our CTA suggests moderate to high fluid overpressure along the plate interface. Finally, out approach of an areal computation of the décollement strength provides a low-effort and on the same hand helpful tool to characterise convergent margin mechanics.

Sediment provenance in the Murchison and Maruia basins, Aotearoa/New Zealand: a record of Neogene strike-slip displacement, convergence, and basement exhumation along the Australian–Pacific plate boundary

ABSTRACT The Murchison and Maruia basins are situated on the Australian Plate adjacent the Alpine Fault, an ideal place to study Cenozoic Australian–Pacific plate boundary evolution. Sandstone provenance was investigated using petrographic and detrital zircon U–Pb geochronologic methods and primarily varies geographically. Secondary up-section variation tracks spatiotemporal changes in basement exhumation. Eocene–middle Miocene western Murchison Basin sandstone is arkosic, derived locally from Devonian–mid-Cretaceous granitoids. Early Miocene–?Pliocene eastern Murchison and Maruia basin sandstone is lithic-/plagioclase-rich, reflecting a mixed provenance: proximal Carboniferous–mid-Cretaceous granitoids, and distal mid-Cretaceous Median Batholith and Permian–Triassic Eastern Province metasedimentary sources in the Pacific Plate. This confirms early Miocene Pacific Plate exhumation during initial Alpine Fault evolution began in northern Fiordland/western Otago but the contrast between eastern and western Murchison Basin indicates limited westward sediment dispersal. Higher proportions of Brook Street Terrane (BST) detrital zircon in middle Miocene eastern Murchison Basin sandstone suggests accelerated Pacific Plate exhumation at c. 15 Ma. By c. 10 Ma, the eastern Murchison Basin BST source had switched to the Australian Plate due to Waimea–Flaxmore fault reverse reactivation and dextral plate boundary strike-slip displacement, which by the late Miocene/Pliocene led to Rakaia Terrane-affinity Haast Schist replacing Caples Terrane as the main Maruia Basin sediment source.

Electrical Properties and Anisotropy of Schists and Fault Rocks from New Zealand’s Southern Alps under Confining Pressure

Magnetotelluric models spanning the Pacific–Australian Plate boundary in New Zealand’s South Island indicate a localized zone of low electrical resistivity that is spatially coincident with the ductile mid-crustal part of the Alpine Fault Zone (AFZ). We explored the source of this anomaly by measuring the electrical properties of samples collected from surface outcrops approaching the AFZ that have accommodated a gradient of systematic strain and deformation conditions. We investigated the effects of tectonite fabric, fluid saturated pore/fracture networks and surface conductivity on the bulk electrical response and the anisotropy of resistivity measured under increasing confining pressures up to 200 MPa. We find that porosity and resistivity increase while porosity and the change in anisotropy of resistivity with confining pressure (δ (ρ‖/ρ⊥)/δ (peff)) decreases approaching the AFZ, indicating the electrical response is controlled by pore fluid conductivity and modified during progressive metamorphism. Conversely, Alpine mylonites exhibit relatively low resistivities at low porosities, and lower δ (ρ‖/ρ⊥)/δ (peff) than the schists. These findings indicate a transition in both the porosity distribution and electrical charge transport processes in rocks that have experienced progressive grain size reduction and mixing of phases during development of mylonitic fabrics due to creep shear strain within the AFZ.

Detrital zircon provenance of the Cretaceous–Neogene East Coast Basin reveals changing tectonic conditions and drainage reorganization along the Pacific margin of Zealandia

Abstract The Upper Cretaceous–Pliocene strata of New Zealand record ~100 m.y. of Zealandia's evolution, including development of the Hikurangi convergent margin and Alpine transform plate boundary. A comprehensive, new detrital zircon U-Pb data set (8315 analyses from 61 samples) was generated along a ~700 km transect of the East Coast Basin of New Zealand. Age distributions were analyzed and interpreted in terms of published data available for Cambrian–Cretaceous igneous and metasedimentary source terranes using a Monte Carlo mixture modeling approach. Results indicate a widespread Early Cretaceous transition in sediment source from the Gondwana interior to the Median Batholith magmatic arc prior to Late Cretaceous rifting from Antarctica. Submergence of Zealandia during a Late Cretaceous–Paleogene drift phase led to major drainage reorganization and the influx of Eastern Province sediment to the East Coast Basin. A long-lived sediment conduit that transported extraregional Western Province detritus to the south-central East Coast Basin may have developed along a structural precursor to the Alpine Fault. Marked Neogene increase of Upper Jurassic–Lower Cretaceous Torlesse Composite Terrane sediment to the central East Coast Basin resulted from exhumation of the Axial Ranges, convergence along the Hikurangi subduction margin, and concurrent development of the Alpine Fault. Concurrent influx of contemporaneous Neogene zircon in the northern East Coast Basin indicated the onset of subduction-related volcanism of the Northland–Coromandel Volcanic Arc. Middle Miocene–Pliocene exhumation and dextral translation of the Nelson region along the Alpine Fault resulted in the eastward routing of Western Province sediment to the central East Coast Basin. Finally, topography developed across the plate boundary and ultimately partitioned continental drainage of Zealandia, such that sediment from the Murihiku, Caples, and Rakaia Terranes in the Otago region was routed to the southern extent of the East Coast Basin. These results illuminate the evolution of the Zealandia continental drainage divide in response to the initiation of the Pacific-Australian plate boundary and demonstrate the power of mixture modeling and large data sets for deciphering sediment routing in dynamic tectonic environments.

The magma source of small-scale intraplate monogenetic volcanic systems in northern New Zealand

The Auckland intraplate volcanic province consists of four small fields of basaltic monogenetic volcanoes. Each of these was active for a period of ~1 Myr and during the last 3 Myr the locus of magmatic activity migrated northward in discrete ~50 km steps. The most recent of these volcano fields is the Auckland Volcanic Field (AVF) in which individual eruptions have produced discrete magma batches within the composition range nephelinite to sub-alkaline basalt. This range of compositions is explained by partial melting involving variable melt/solid ratios together with modal heterogeneity in their mantle source. Eruptions in the field have been irregularly spaced during its 200 kyr life with repose periods varying between ≤0.1 and 13 kyr. Although most of Auckland's volcanoes represent a single period of eruption, paired eruption sequences in which smaller more alkaline magma batches have been followed by more voluminous less alkalic magmas have also occurred. A significant flare-up in activity at about 30 kyr produced compositionally discrete eruptions from at least five spatially separate locations all within a temporal interval of as little as 100 years. An interpretation of these data is that each event (volcano) in the AVF represents a discrete batch of magma extracted from an source with the capacity of producing a compositional variety of magma. Such a source forms when upwelling mantle undergoes adiabatic rise and exists as a metastable entity at the asthenosphere-lithosphere boundary for prolonged periods (105–106 years) during which time it is tapped to create individual magma batches. The shorter term magma extraction events that produced individual volcanoes in the AVF are suggested to be the result of crustal scale tectonic events. The behaviour of these four spatially separate monogenetic volcanic systems at temporal intervals of <106 years is likely linked to the long term tectonic evolution of the Pacific-Australian Plate boundary beneath the New Zealand region.

Highly localized upper mantle deformation during plate boundary initiation near the Alpine fault, New Zealand

AbstractPeridotite xenoliths entrained in magmas near the Alpine fault (New Zealand) provide the first direct evidence of deformation associated with the propagation of the Australian-Pacific plate boundary through the region at ca. 25–20 Ma. Two of 11 sampled xenolith localities contain fine-grained (40–150 μm) rocks, indicating that deformation in the upper mantle was focused in highly sheared zones. To constrain the nature and conditions of deformation, we combine a flow law with a model linking recrystallized fraction to strain. Temperatures calculated from this new approach (625–970 °C) indicate that the observed deformation occurred at depths of 25–50 km. Calculated shear strains were between 1 and 100, which, given known plate offset rates (10–20 mm/yr) and an estimated interval during which deformation likely occurred (&amp;lt;1.8 m.y.), translate to a total shear zone width in the range 0.2–32 km. This narrow width and the position of mylonite-bearing localities amid mylonite-free sites suggest that early plate boundary deformation was distributed across at least ∼60 km but localized in multiple fault strands. Such upper mantle deformation is best described by relatively rigid, plate-like domains separated by rapidly formed, narrow mylonite zones.

Structure and kinematics of active faulting in the Hope-Kelly and Alpine Fault intersection zone, South Island, New Zealand

Using lidar, photogrammetry and field studies, we investigate the structure and kinematics of the Hope, Kelly, and Alpine faults where they converge and intersect along the Australian-Pacific plate boundary in New Zealand. Approximately 50% of the reverse-dextral slip rate of the Alpine fault in the west transfers to the dextral central Hope Fault in the east via a horsetail splay zone of distributed oblique dextral-normal shear forming the Hope-Kelly fault system. The westernmost portion of the Hope fault is misoriented for dextral slip and is likely an extensional fault that accommodates the Alpine fault slip rate change across the intersection zone. As it transitions to an extensional fault, most of the strike-slip of the Hope fault is transferred southward onto Kelly fault strands via incipient, linking fault domains of coeval NE-SW and EW-striking dextral faults. The transition from the narrow zone defining the central Hope Fault to dispersed horsetail splays of the Hope-Kelly fault system is located immediately above the eastward down-dip projection of the Alpine Fault at the base of the seismogenic zone. This suggests that structural interactions among these faults within seismogenic depths control surface rupture geometries and kinematics. Coeval intermingling of surface rupture traces transecting latest Pleistocene to Holocene landforms indicates ongoing structural interactions in active faulting at the intersection of the Hope-Kelly fault splays with the Alpine fault. The transition from dextral to dextral normal kinematics on the horsetail splay accommodates differential orogenic growth through geometrically and kinematically efficient faulting that allows extension of the Alpine fault hanging wall parallel to strike, enabling the slip rate change between the central and northern Alpine fault sections.

Origin of geochemically heterogeneous mid-ocean ridge basalts from the Macquarie Ridge Complex, SW Pacific

The Macquarie Ridge Complex (MRC), located at the Australian–Pacific plate boundary south of New Zealand, is a rugged bathymetric ridge comprising a series of submarine seamounts and Macquarie Island, the only subaerial portion of the complex. Mid-ocean ridge basalts (MORBs) from Macquarie Island show various enrichments in incompatible elements with compositions ranging from typical normal MORB to enriched MORB. However, these basalts have isotopic compositions trending towards a high μ-like (μ = 238U/204Pb; HIMU) mantle component, which is unusual for MORB-type rocks. The origin of this mantle signature is not understood, and it is unclear whether this isotopic signature is characteristic of the entire MRC or unique to Macquarie Island. Here we report new major and trace element abundances, and Sr, Nd, and Pb isotopes for samples from the MRC seamounts and from new sampling sites on Macquarie Island. The geochemical and isotopic data show that the entire MRC comprises normal to enriched MORB. Mixing modelling indicates that the heterogeneous isotopic signatures of the MRC basalts are not derived from contamination of the nearby Balleny mantle plume but have affinities with that of the Cenozoic Zealandia intraplate HIMU-like basalts. We propose that the heterogeneous geochemical signatures of the MRC basalts are derived from amphibole-bearing garnet pyroxenite veins, which is supported by the rare earth element partial melting modelling and strong correlations between Nd and Pb isotopic ratios vs La/Sm. We posit that the pyroxenite veins were generated in the oceanic lithospheric mantle, which was metasomatised by hydrous and carbonatitic fluids/melts derived either from delaminated, metasomatised Zealandia subcontinental lithosphere mantle, or from subducted material in the asthenosphere. The subducted material could be derived from ancient and/or recent subduction along the former east Gondwana margin.

Isotopic mapping reveals the location of crustal fragments along a long-lived convergent plate boundary

New Guinea has acted as the boundary between the Australian and Pacific plates for hundreds of millions of years. Strike-slip movement and arc–continent collisions along this boundary during the Cenozoic have shuffled rocks of different age and composition in a series of terranes along the plate boundary making mapping them a considerable challenge. Here we report results of SrNd isotopic data obtained from rock samples from western New Guinea that are representative of the different terranes. These isotopic data reveal the crustal affinity of the terranes and we have used these data to map their spatial distribution. The isotopic data show three distinct crustal domains underlying western New Guinea; Palaeozic–Mesozoic Australian continental crust (87Sr/86Sr = 0.719594 to 0.710921; εNd = −13.85 to 1.373); thinned transitional crust intruded by Miocene–Pleistocene magmatic rocks (87Sr/86Sr = 0.706524 to 0.704019; εNd = 6.67 to 2.13); and accreted island arc crust (87Sr/86Sr = 0.704053 to 0.703759; εNd = 6.63 to 4.97). These data, together with crustal contamination models, indicate that the northern-most extent of Australian continental crust exists beneath the northern-most section of western New Guinea. We also combined our isotopic data with existing data across New Guinea and used these to develop an isotopic map that shows the position of the ancient Australian–Pacific Plate boundary, producing results that are also consistent with broad-scale seismic tomography imagery. Our findings provide a framework for mapping other plate boundaries, particularly ancient systems where only fragmentary data exist.

Volcanoes buried in Te Riu-a-Māui/Zealandia sedimentary basins

ABSTRACT Many volcanoes are buried within Te Riu-a-Māui/Zealandia sedimentary basins. Based on a large collection of seismic reflection and drillhole data, this paper outlines the morphology, location and age of over 400 volcanoes buried and preserved in offshore New Zealand basins. We discuss the relationships of these volcanoes to the timing and locations of the main tectonic events that shaped Zealandia in the last 105 Myr. Late Cretaceous volcanism mainly formed large (>20 km3) composite cones and volcanic complexes that erupted along rift faults related to the break-up of Zealandia from south-eastern Gondwana. Cenozoic intraplate volcanic activity was widespread, typically forming clusters of scattered small-volume (<1 km3) craters and cones. In contrast, large Cenozoic volcanic complexes, composite cones and shield volcanoes erupted along linear arcs associated with subduction along the Pacific-Australian plate boundary. In detail, the presence of pre-existing structures and types of host rocks also influence the passage of magma in the shallow (<5 km) crust and the location of eruptive centres, regardless the tectonic setting of eruption. Insights from this work demonstrate the value of characterising volcanoes buried in sedimentary basins for improved understanding of the interplay between volcanism and plate tectonics in Zealandia and further afield.

Structural evolution of the active Waimea-Flaxmore Fault System in the Nelson-Richmond urban area, South Island, New Zealand

ABSTRACTThe Waimea-Flaxmore Fault System (W-FFS) comprises NE-striking, SE-dipping reverse faults superposing Eastern Province terranes on Cenozoic sedimentary units of the Moutere Depression. We present a revised geological map and cross sections of the W-FFS in the Nelson-Richmond urban area, aimed at reconstructing the structural evolution during multiple phases of reactivation. Our data and structural interpretations depict the inversion of the W-FFS from an inherited system of Late Cretaceous-Eocene normal faults to multiple reverse fault splays that propagated through the cover sequence over the last 10 Ma. Fault tip propagation with folding of the cover sequence persisted during the Quaternary, with breaching of the Waimea and Flaxmore faults to the surface and propagation of a splay thrust (Tahunanui Fault) still concealed beneath Quaternary deposits. Ongoing crustal shortening associated with the Pacific-Australian plate boundary in the northern South Island creates favourable conditions for reactivation of the long-lived W-FFS in the contemporary stress field.

Carbon dioxide generation and drawdown during active orogenesis of siliciclastic rocks in the Southern Alps, New Zealand

Collisional mountain building influences the global carbon cycle through release of CO2 liberated by metamorphic reactions and promoting mechanical erosion that in turn increases chemical weathering and drawdown of atmospheric CO2. The Southern Alps is a carbonate-poor, siliciclastic mountain belt associated with the active Australian Pacific plate boundary. On-going, rapid tectonic uplift, metamorphism and hydrothermal activity are mobilising carbon. Here we use carbon isotope measurements of hot spring fluids and gases, metamorphic host rocks, and carbonate veins to establish a metamorphic carbon budget.We identify three major sources for CO2 within the Southern Alps: (1) the oxidation of graphite; (2) consumption of calcite by metamorphic reactions at the greenschist–amphibolite facies boundary, and (3) the dissolution of groundmass and vein-hosted calcite. There is only a minor component of mantle CO2 arising on the Alpine Fault. Hot springs have molar HCO3−/Ca2+ ∼9, which is substantially higher than produced by the dissolution of calcite indicating that deeper metamorphic processes must dominate.The total CO2 flux to the near surface environment in the high uplift region of the Southern Alps is estimated to be ∼6.4 × 108 mol/yr. Approximately 87% of this CO2 is sourced from coupled graphite oxidation (25%) and disseminated calcite decarbonation (62%) reactions during prograde metamorphism. Dissolution of calcite and mantle-derived CO2 contribute ∼10% and ∼3% respectively. In carbonate-rich orogens CO2 production is dominated by metamorphic decarbonation of limestones. The CO2 flux to the atmosphere from degassing of hot springs in the Southern Alps is 1.9 to 3.2 × 108 mol/yr, which is 30–50% of the flux to the near surface environment. By contrast, the drawdown of CO2 through surficial chemical weathering ranges between 2.7 and 20 × 109 mol/yr, at least an order of magnitude greater than the CO2 flux to the atmosphere from this orogenic belt. Thus, siliciclastic mountain belts like the Southern Alps are net sinks for atmospheric CO2, in contrast to orogens involving abundant carbonate rocks, such as the Himalaya, that are net CO2 sources.

The fluid budget of a continental plate boundary fault: Quantification from the Alpine Fault, New Zealand

Abstract Fluids play a key role in modifying the chemical and physical properties of fault zones, which may prime them for repeated rupture by the generation of high pore fluid pressures and precipitation of commonly weak, secondary minerals. Fluid flow paths, sources and fluxes, and the permeability evolution of fault zones throughout their seismic cycles remain poorly constrained, despite their importance to understanding fault zone behaviour. Here we use geochemical tracers of fluid–rock exchange to determine budgets for meteoric, metamorphic and mantle fluids on a major compressional tectonic plate boundary. The Alpine Fault marks the transpressional Pacific–Australian plate boundary through South Island, New Zealand and appears to fail in regular ( 329 ± 68 yrs ) large earthquakes ( M w ∼ 8 ) with the most recent event in 1717 AD. Significant convergent motion has formed the Southern Alps and elevated geothermal gradients in the hangingwall, which drive crustal fluid flow. Along the Alpine Fault the Alpine Schist of the Pacific Plate is thrust over radiogenic metasedimentary rocks on the Australian plate. The absence of highly radiogenic (87Sr/86Sr > 0.7200) strontium isotope ratios of hangingwall hot springs and hydrothermal minerals formed at a range of depths in the Alpine Fault damage zone indicates that the fluid flow is restricted to the hangingwall by a cross-fault fluid flow barrier throughout the seismogenic crust. Helium isotope ratios measured in hot springs near to the Alpine Fault (0.15–0.81 R A ) indicate the fault is a crustal-scale feature that acts as a conduit for fluids from the mantle. Rock-exchanged oxygen, but meteoric water-like hydrogen isotope signatures of hydrothermal veins indicate that partially rock-exchanged meteoric fluids dominate down to the top of the brittle to ductile transition zone at ∼6 km. Geochemical tracer transport modelling suggests only ∼0.02 to 0.05% of total rainfall west of the Main Divide penetrates to depth, yet this recharge flux is sufficient to overwhelm other fluid contributions. Calculated mantle fluid fluxes of CO2 and H2O (0.2 and 3 to 13 mol/m2/yr respectively) and metamorphic H2O fluxes (4 to 750 mol/m2/yr) are considerably lower than the focused meteoric water discharge flux up the Alpine Fault (4 × 103 to 7 × 104 mol/m2/yr), driven by the >3000 m hydrologic head of the Southern Alps. Meteoric waters are primarily responsible for modifying fault zone permeability during fluid–rock interactions and may facilitate the generation of high pore fluid pressures that could assist episodic earthquake rupture.

Effect of strain-weakening on Oligocene–Miocene self-organization of the Australian-Pacific plate boundary fault in southern New Zealand: Insights from numerical modelling

Tectonic inheritance acquired from past geological events can control the formation of new plate boundaries. The aim of this paper is to explore the role of inherited NE and NW trending fabrics and their rheological influence on the propagation of Oligocene–Miocene strike-slip faulting that matured to become the Australian-Pacific plate boundary fault in southern New Zealand. Strain weakening plays a significant role in controlling the formation, growth and evolution of strain localization. In this study, three-dimensional thermo-mechanical models have been used to explore the effect of strain weakening on the Oligocene–Miocene self-organization of strain localization. Strain weakening is simulated through decreasing either the coefficient of friction of upper crust, its cohesion, or the rheological viscosity contrast between the inherited structures and their surrounding wall rocks. Viscosity contrast is obtained by varying the viscosity of inherited structures. Softening coefficient (α) is a measure of strain weakening. Our experiments robustly demonstrate that a primary boundary shear zone becomes mature quicker when softening coefficients are increased. Deformation is focused along narrow high-strain shear zones in the centre of the model when the softening coefficients are high, whereas the strain is more diffuse with many shear zones spread over the model and possibly some high-strain shear zones focused near one border at lower softening coefficients. Varying the viscosity contrast has less effect on the distribution of maximum finite strain.Under simple-shear boundary conditions, NW trending inherited structures make a major contribution to forming early zones of highly focused strain, up to a shear strain of about γ=3.7. During this process, most NE-trending structures move and rotate passively, accommodate less strain, or even be abandoned through time.

The Early-Middle Miocene subduction complex of the Louisiade Archipelago, southern margin of the Woodlark Rift

Field, microstructural, and 40Ar/39Ar thermochronologic data from the Louisiade Archipelago, the southern rifted margin of the Woodlark Basin in SE Papua New Guinea, document an accretionary wedge that formed during Early-Middle Miocene N-dipping subduction of the Australian margin and transpression along the Australian-Pacific plate boundary. Metasedimentary rocks of the Calvados Schist and the metagabbros that intrude them were metamorphosed at up to greenschist-facies conditions. Three tectonic foliations (S1–S3) are present and F1–F3 fold hinges plunge ESE or WNW, parallel to mineral, stretching, and intersection lineations. Fold vergence is dominantly to the SW, and top-to-the-SW thrusting of ultramafic rocks over the Calvados Schist is documented locally on Rossel Island. The data suggest progressive deformation associated with NNE-SSW shortening and ESE-WNW extension via dissolution-precipitation creep and, more locally, dislocation creep. 40Ar/39Ar step-heating analyses of three white mica separates yield Middle Miocene plateau or plateau-like segments that are affected by variable Pliocene argon loss and are interpreted as syntectonic mica growth during metamorphism and deformation followed by partial resetting just prior to the onset of seafloor spreading in the Woodlark Basin. A ∼12 Ma 40Ar/39Ar age from a dacite sill of the Panarora Volcanics provides a minimum age constraint for the termination of northward subduction and a maximum age for crosscutting brittle strike-slip faults. These data are critical to constraining the subduction-exhumation history of the world's youngest high-ultrahigh-pressure terrane and further support analogies between SE Papua New Guinea and the Early Oligocene Western Alps.

New c. 270 kyr strike-slip and uplift rates for the southern Alpine Fault and implications for the New Zealand plate boundary

Along 100 km of the Alpine Fault, major valleys and glacial deposits can be matched across an 8000 m dextral offset. We use paleontologic and stratigraphic age constraints to date c. 270 ka marine sediments uplifted to 600 m elevation and overlying c. 270 ka glacial deposits related to the 8000 m dextral offset. These constraints yield a fault-proximal Australian plate uplift rate of 2.6 (−0.5/+0.4) mm/yr and an Alpine Fault dextral slip rate of 29.6 (−2.5/+4.5) mm/yr. Our rates resolve an apparent along-strike drop in strike-slip rate and instead support a relatively constant along-strike dextral slip rate of ∼28 mm/yr (∼80% of current Australian-Pacific plate boundary motion). We argue that the rate of dextral slip on the southern Alpine Fault has been relatively constant over the last ≥3.5 myr, and that ductile fault processes may rate-limit the fault from accommodating a progressively higher percentage of plate boundary motion through time (i.e., the fault reached maturity long ago). The spatiotemporally constant strike-slip rate of the southern Alpine Fault and a previously published paleoseismic record of near-regular earthquake recurrence both characterize the Alpine Fault as a mature plate boundary fault zone that behaves in a constant way with behavior predictable over timescales of thousands and hundreds of thousands of years.

Temperate carbonate debrites and short-lived earliest Miocene yo-yo tectonics, eastern Taranaki Basin margin, New Zealand

This study examines two unconformity-bound earliest Miocene temperate shallow-marine limestones at remote Gibson Beach, western North Island, on the eastern margin of Taranaki Basin, New Zealand's only producing hydrocarbon province. The local geology comprises an upper limestone of up to 6m of spectacular conglomeratic limestone (rudstone; Papakura Limestone) whose pebble- to boulder-sized clasts were derived from cannibalisation of the lower skeletal limestone (bryomol grainstone; Otorohanga Limestone). Multiple lines of evidence require the Otorohanga Limestone to have been well indurated before erosion and clast generation. Cementation occurred from pressure dissolution of calcitic skeletons at burial depths of probably 400m or more. Subsequent uplift led to local subaerial exposure and karstification of a fault-block cliffed coastline where physical erosion sourced talus limestone debris onto adjacent high-energy pocket beaches. Storm and/or seismic events periodically triggered mass flows of the limestone clasts offshore onto the contemporary mixed siliciclastic-carbonate shelf as channelised carbonate debrites. Lithification of these Papakura Limestone debrites occurred via pressure dissolution, and required a second burial episode. Karst pinnacles preserved on the eroded upper surface of the Papakura Limestone demand uplift again and erosion in a subaerial setting, before subsequent deep burial by Early Miocene siliciclastic shelf sand and turbidite deposits. Strontium isotope dating of brachiopod fossils suggests the two postulated burial-uplift cycles, involving movements up to 400±100m, occurred very rapidly within several 100kyr. Speculative drivers of the yo-yo tectonic events are earliest Miocene movements on the nearby major Taranaki Fault thrust in association with regional changes in subduction tectonics at the Australian-Pacific plate boundary, along with pulsed emplacement of obducted gravity slide deposits (Northland Allochthon) onto northern North Island. The two limestones record a previously undocumented degree of eastern Taranaki Basin margin tectonic mobility relevant to a fuller appreciation of petroleum system development in that basin.

Thermal regime in the central Alpine Fault zone, New Zealand: Constraints from microstructures, biotite chemistry and fluid inclusion data

The thermal state within plate boundary fault zones is likely to influence the dimensions of large co-seismic fault ruptures, and strain localisation in underlying mylonitic shear zones. Vein microstructures, biotite chemistry and fluid inclusion data offer new insights into the thermal structure of the mylonitic shear zone down dip of the Alpine Fault, the main Pacific–Australian plate boundary structure through southern New Zealand. Quartz veins within the fault rocks were formed and deformed at temperatures ranging from > 500 °C to immediately above that of the brittle–viscous transition (∼ 325 °C), illustrating that fluids are present throughout the fault zone. Microthermometric data from primary, mixed CO 2–H 2O fluid inclusions in boudin-neck quartz–carbonate veins indicate that these inclusions were trapped at temperatures of 325 ± 15 °C and pressures of c. 40 MPa. Vein microstructures indicate that fluid temperatures were equal to rock temperatures. Assuming hydrostatic fluid pressure, the veins were trapped at depths of < 8 km for an average density of mixed H 2O–CO 2 fluids in the shallower crust of > 500 kg m − 3 . Biotite grains in the mylonites are a mixture of porphyroclastic grains with high R = Ti/Mg# inherited from the parent Alpine Schist, and smaller, low R grains that recrystallised during mylonitisation, particularly those that are found within syn-mylonitic shear bands. Assuming a simple step-function model approximating diffusive equilibration, the changes in biotite chemistry indicate Δ T during mylonitic deformation of ∼−340 °C according to the Ti-in-biotite thermometer of Henry et al. (2005). A best-fit uplift P– T path to existing data and these new results indicate that the uplift path P– T gradient of fault zone rocks above the brittle–viscous transition is of the order of 40 °C km − 1 . Below the brittle–viscous transition, the uplift path P– T gradient is 10 °C km − 1 . The P– T profile within the fault rocks is comparable to that in the immediate hanging wall Alpine Schist, indicating there is no peak in the crustal isotherms in the fault hanging wall, as was indicated by previous numerical models, and analyses that did not consider data from the fault rocks themselves.

Holocene subsidence at the transition between strike-slip and subduction on the Pacific-Australian plate boundary, Marlborough Sounds, New Zealand

This paper provides the first solid evidence in support of a century-old hypothesis that the mountainous Marlborough Sounds region in central New Zealand is subsiding. More recent hypotheses suggest that this may be a result of southward migration of a slab of subducted Pacific Plate causing flexural downwarping of the overlying crust in the vicinity of the transition between subduction and strike-slip on the Pacific-Australian plate boundary. The proxy evidence for gradual Holocene subsidence comes from micropaleontological study of seven intertidal sediment cores from the inner Marlborough Sounds (at Havelock, Mahau Sound and Shakespeare Bay). Quantitative estimates (using Modern Analogue Technique) of former tidal elevations based on fossil foraminiferal faunas provide evidence of tectonic (not compaction-related) subsidence in all cores. Estimates of subsidence rates for individual cores vary within the range 0.2–2.4 m ka −1. The wide variation within subsidence rate estimates are related to a combination of the accuracy limits of radiocarbon dates, elevation estimates, and particularly our poor knowledge of the New Zealand Holocene sea-level curve. The most consistent subsidence rate at all three sites for the mid-late Holocene (last 6–7 ka) is ∼0.7–0.8 m ka −1. This rate is consistent with the average subsidence rate in the adjacent 4-km thick Wanganui sedimentary basin for the last 5 myr. Subsidence is inferred to have migrated southwards from the Wanganui Basin to impinge on the inner Marlborough Sounds in just the last 100–200 ka.

Backarc rifting, constructional volcanism and nascent disorganised spreading in the southern Havre Trough backarc rifts (SW Pacific)

High resolution multibeam (EM300 and SEABEAM) data of the Southern Havre Trough (SHT), combined with observations and sample collections from the submersible Shinkai6500 and deep-tow camera, are used to develop a model for the evolution and magmatism of this backarc system. The Havre Trough and the associated Kermadec Arc are the product of westward subduction at the Pacific–Australian plate boundary. Detailed studies focus on newly discovered features including a seamount (Saito Seamount) and a deep graben (Ngatoroirangi Rift, > 4000 m water depth floored with a constructional axial volcanic ridge > 5 km in length and in excess of 200 m high), both of which are characterised by pillow and lobate flows estimated at < 20,000 years old based on sediment cover, high reflectivity and thin Mn crusts on recovered glassy olivine basalts and basaltic andesites. Elongate volcanic ridges at 35°15′S and 34°30′S, and backarc seamounts (35°30′S, 178°30′E) occur at the eastern margin of the SHT. Similar seafloor morphology is observed in the central and western portions of the basin, suggesting that recent volcanism may be broadly distributed across the backarc. Mass balance modelling indicates a maximum crustal thickness of ~ 11 km to < 6 km, similar to estimates of crustal thickness in the Lau Basin to the north. Given such high crustal attenuation and extensive backarc mafic magmatism within deep SHT rifts, we propose that the SHT is in an incipient phase of distributed and “disorganised” oceanic crustal accretion in multiple, ephemeral, and short but deep (> 4000 m) spreading systems. These discontinuous spreading systems are characterised by failed rifts, rift segmentation, and propagation. Successive episodes of magmatic intrusion into thinned faulted arc basement results in defocused asymmetrical accretion. Cross-arc volcanic chains, isolated volcanoes and underlying basement plateaus are interpreted to represent a “cap” of recent extrusives. However, they may also be composed entirely of newly accreted crust and the spatially extensive basement fabric of elongated volcanic ridges may be the surface expression of pervasive dike intrusion that has thoroughly penetrated and essentially replaced the original arc crust with newly accreted intrusives.