Havre Trough Research Articles

Hf isotopic evidence for small-scale heterogeneity in the mode of mantle wedge enrichment: Southern Havre Trough and South Fiji Basin back arcs

Magmas from SW Pacific back-arc basins have geochemical and isotopic signatures indicating variable mantle and subduction-derived components. Basalts from South Fiji Basin (SFB) are little influenced by subduction, but come from variably enriched mantle, resulting from mixing between enriched mantle, like FOZO, and depleted mantle, like DMM. The same components are present in the Havre Trough mantle, but Havre Trough basalts come from a mantle wedge to which a greater proportion of subduction-derived components are added. Their slab-derived components are isotopically similar to locally subducting sediment, with variable Sr and Pb from altered oceanic crust. Their compositional diversity correlates with morphology, previously described as contrasting Arc-type and Rift-type back-arc regimes. Geochemical modeling indicates that material is added as both supercritical fluids and slab melts below the back-arc, is locally distinct, and correlate with differences in the predicted slab-surface pressure and temperature conditions. Deeper slab surfaces correspond to higher-temperatures at a given distance from the volcanic front, but not necessarily with an increase in the amount of slab-derived material. Slab fluxes for rift-type basalts are consistent with predicted slab-surface temperatures at or below the water-saturated solidus. However, some are consistent with melting in equilibrium with residual rutile, zircon and monazite, so melting may have occurred by fluid fluxing of the slab surface, requiring external fluids from within the slab. Arc-type basalts are explained by thermal anomalies in the mantle wedge, which may correspond to locally hotter slab-surface temperatures and more fractionated slab-derived component signatures in their source.

Geochemical evolution of Monowai volcanic center: New insights into the northern Kermadec arc subduction system, SW Pacific

We present trace element and Sr-Nd-Pb isotope data on volcanic rocks recovered from the submarine Monowai volcanic center, which marks the midpoint of the ∼2500 km long Tonga-Kermadec arc. The center consists of a large (12 × 9 km) partly hydrothermally active caldera and a 12 km diameter ∼1500 m high volcanically and hydrothermally active stratovolcano. The stratovolcano lavas are tholeiitic basalts, which show variable evidence for plagioclase (±pyroxene) accumulation. The caldera lavas range from basalt to andesite, with the compositional variation being consistent with fractional crystallization as the dominant process. The mafic Monowai magmas were generated by relatively high degrees (12%–20%) of partial melting of a previously depleted MORB-type spinel-peridotitic mantle, metasomatized by slab-derived fluids. Strongly fluid mobile 87Sr/86Sr and 207Pb/204Pb ratios of the Monowai basaltic lavas are similar to those from the Putoto, Raoul, and Macauley volcanic centers 200–400 km to the south, suggesting derivation largely from subducted sediment. In contrast, variably fluid immobile 143Nd/144Nd ratios suggest an isotopically heterogeneous mantle along this segment of the arc. Higher 206Pb/204Pb in Monowai lavas imply some influence from the nearby subducting Louisville seamounts in melt generation. The formation of one of the Earth's largest submarine mafic calderas can best be explained through drainage of a single magma reservoir and subsequent collapse caused by trench-perpendicular extension, probably via southward progressive rifting of the northern Havre Trough.

The 29 September 2009 Samoa Islands Tsunami: Simulations Based on the First Focal Mechanism Solutions and Implications on Tsunami Early Warning Strategies

The tsunamigenic earthquake (Mw = 8.1) that occurred on 29 September 2009 at 17:48 UTC offshore of the Samoa archipelago east of the Tonga trench represents an example of the so-called “outer-rise” earthquakes. The areas most affected were the south coasts of Western and American Samoa, where almost 200 people were killed and run-up heights were measured in excess of 5 m at several locations along the coast. Moreover, tide gauge records showed a maximum peak-to-peak height of about 3.5 m near Pago Pago (American Samoa) and of 1.5 m offshore of Apia (Western Samoa). In this work, different fault models based on the focal mechanism solutions proposed by Global CMT and by USGS immediately after the 2009 Samoan earthquake are tested by comparing the near-field recorded signals (three offshore DART buoys and two coastal tide gauges) and the synthetic signals provided by the numerical simulations. The analysis points out that there are lights and shadows, in the sense that none of the computed tsunamis agrees satisfactorily with all the considered signals, although some of them reproduce some of the records quite well. This “partial agreement” and “partial disagreement” are analysed in the perspective of tsunami forecast and of Tsunami Early Warning System strategy.

Chemistry of hot springs along the Eastern Lau Spreading Center

The Eastern Lau Spreading Center (ELSC) is the southernmost part of the back-arc spreading axis in the Lau Basin, west of the Tonga trench and the active Tofua volcanic arc. Over its 397-km length it exhibits large and systematic changes in spreading rate, magmatic/tectonic processes, and proximity to the volcanic arc. In 2005, we collected 81 samples of vent water from six hydrothermal fields along the ELSC. The chemistry of these waters varies both within and between vent fields, in response to changes in substrate composition, temperature and pressure, pH, water/rock ratio, and input from magmatic gases and subducted sediment. Hot-spring temperatures range from 229° to 363 °C at the five northernmost fields, with a general decrease to the south that is reversed at the Mariner field. The southernmost field, Vai Lili, emitted water at up to 334 °C in 1989 but had a maximum venting temperature of only 121 °C in 2005, due to waning activity and admixture of bottom seawater into the subseafloor plumbing system. Chloride varies both within fields and from one field to another, from a low of 528 mmol/kg to a high of 656 mmol/kg, and may be enriched by phase separation and/or leaching of Cl from the rock. Concentrations of the soluble elements K, Rb, Cs, and B likewise increase southward as the volcanic substrate becomes more silica-rich, especially on the Valu Fa Ridge. Iodine and δ 7Li increase southward, and δ 11B decreases as B increases, apparently in response to increased input from subducted sediment as the arc is approached. Species that decrease southward as temperature falls are Si, H 2S, Li, Na/Cl, Fe, Mn, and 87Sr/ 86Sr, whereas pH, alkalinity, Ca, and Sr increase. Oxygen isotopes indicate a higher water/rock ratio in the three systems on Valu Fa Ridge, consistent with higher porosity in more felsic volcanic rocks. Vent waters at the Mariner vent field on the Valu Fa Ridge are significantly hotter, more acid and metal-rich, less saline, and richer in dissolved gases and other volatiles, including H 2S, CO 2, and F, than the other vent fields, consistent with input of magmatic gases. The large variations in geologic and geophysical parameters produced by back-arc spreading along the ELSC, which exceed those along mid-ocean ridge spreading axes, produce similar large variations in the composition of vent waters, and thus provide new insights into the processes that control the chemistry of submarine hot springs.

Rifting to spreading in the southern Lau Basin: Variations within the transition zone

The Lau Basin and Havre Trough are back-arc basins related to Pacific–Australian plate convergence. Seafloor spreading occurs in the Lau Basin whereas the Havre Trough is in a rifting stage. At present, the spreading propagator's tip lies at the southern end of the Valu Fa Ridge (VFR) at 22°40′S. Studying this propagation process provides an opportunity to characterize the evolution of rifting to the initiation of seafloor spreading which is fundamental to back-arc basin development. New geophysical data of the southern Lau Basin reveals that as spreading propagates south, it evolves in a discrete style south of 22°40′S. The propagation axis lies along the eastern margin of the basin, where the well defined, linear VFR loses its identifying morphology. Topography in this eastern zone is characterized by grabens separated by short narrow ridges. High backscatter intensity indicates tectonic and magmatic activity in this eastern area. Mantle Bouguer anomalies (MBA) increase southwards from the VFR to form an elevated MBA area extending west from the currently active area. This indicates eastward migration of active rifting, during which the arc crust was extremely thinned. High magnetization is observed in a left-stepping pattern south of the VFR. We interpret this pattern as discrete segments that characterize the initiation of the spreading stage. There is no evidence of a single, continuous spreading axis like that which characterizes the central and northern Lau Basin. The magnetization highs are discrete and are observed in areas where deformation and magmatism are focused. They are offset relative to the VFR, though they generally follow the same north–south trend as the VFR.

Sources of constructional cross‐chain volcanism in the southern Havre Trough: New insights from HFSE and REE concentration and isotope systematics

We report new compositional and isotopic data for submarine lavas erupted on the Rumble V Ridge cross chain behind the Kermadec Arc at ∼36°S and for locally subducting sediment. In order to constrain cross‐arc changes in the melt source, Havre Trough ambient mantle wedge isotope and trace element characteristics are interpreted from regional back‐arc basalts that are relatively free of slab‐derived components. They have MORB‐like trace element concentrations and are isotopically “Pacific” but define greater heterogeneity in 206Pb/204Pb and 176Hf/177Hf than previously known within the Havre Trough. In contrast with the ambient mantle, all Rumble V Ridge lavas have trace element and isotopic characteristics consistent with subduction zone contributions despite their rear‐arc setting but are less fluid‐enriched than at the Kermadec volcanic front. A broad trend in Nd‐Hf isotopic space and the correlation between 176Hf/177Hf and Hf concentration anomaly for Rumble V Ridge lavas is explained by across‐arc changes in (1) the mantle wedge component, (2) the nature of the subduction component, and (3) the mass fraction of subduction components added. Samples from the distal cross chain tend to have lower 176Hf/177Hf at similar 143Nd/144Nd compared with samples closer to the arc, suggesting that a low‐176Hf/177Hf component is preferentially removed from the mantle wedge during trenchward advection. Isotope trends suggest that locally subducting sediment is the primary slab component for Rumble V Ridge magmas, but bulk mixing of ambient mantle with sediment or slab‐derived fluids cannot account for cross‐arc trace element ratios. Instead, cross‐chain isotope and trace element characteristics are explained by the addition of 0.05%–2.0% sediment melts where trace zircon, monazite, and rutile are residual. The cross chain tracks an east to west increase in the mass fraction of a common subduction component. A particularly enriched subset of eastern basalts is interpreted as being derived from the addition of an even higher mass fraction of a different subduction component with greater apparent stability of refractory trace phases during sediment melting. An implication of this study is that both Nd and Hf can be mobile in sediment‐rich subduction zones, but the relative mobility depends on the sediment composition and depth of melting, and the absolute mobility is small.

Geochemistry and tectonic setting of Matakaoa Volcanics, East Coast Allochthon, New Zealand: Suprasubduction zone affinity, regional correlations, and origin

In north-eastern New Zealand, the Late Cretaceous to Eocene submarine volcanic rocks of the East Coast Allochthon referred to as Matakaoa Volcanics formed a dominantly mafic melange with minor abyssal sediments during the subduction of oceanic crust. Accreted and melanged pillow lavas and dolerites display the geochemical and isotopic features of tholeiites with some diversity referred to as midocean ridge basalts, island-arc tholeiites and back-arc basin basalts. The association of supra-subduction tholeiitic magmas, abyssal sediments and polymetallic volcanogenic sulphide deposits is typical of many back-arc basins; therefore, a correlation with the alkaline and transitional basalts of the Hikurangi-Manihiki- Ontong Java Plateau may be ruled out. These petrological features and the fossil ages as well, are closely similar to those of the Tangihua Complex (Northland Allochthon) and allow Matakaoa and Tangihua mafic volcanic rocks, now separated by the modern Havre Trough, to be correlated within one single Late Cretaceous to Eocene 2 basin. The consideration of possible younger ages (Oligocene) is also discussed. The occurrence of Late Cretaceous-Eocene oceanic crust with supra-subduction zone affinities to the east of the Norfolk Ridge poses the problem of the intervening southwest-dipping subduction, the missing Late Cretaceous-Eocene arc and their relationship with Southeast Gondwana marginal break-up.

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.

Crustal seismic reflection measurements across the northern extension of the Taupo Volcanic Zone, North Island, New Zealand

A crustal seismic reflection profile across the offshore Taupo Volcanic Zone (OTVZ) has been reprocessed to improve the lower crust–upper mantle image. The OTVZ is underlain by several strong sub-horizontal crustal reflections that may correspond to shear zones or mafic sills. A reflector interpreted to correspond to Moho lies at a depth of about 8–8.5 s twt (seconds two-way-traveltime) – about 20 km – under the western margin of the Raukumara Peninsula and rises to a depth of about 6 s twt (about 16 km) under the active, eastern part of the OTVZ and can be traced at about this depth across most of the OTVZ. Underlying this inferred Moho are two diffuse zones of discontinuous reflectivity, one underlying the active eastern OTVZ and a less well defined one underlying the landward extension of the Havre Trough. They extend from about 15 to 35 km depth, within the mantle wedge, and are inferred to be imaging a zone of depleted upper mantle associated with crustal extension. A localised zone of deep (30–50 km) reflectivity and seismicity occurs under the eastern OTVZ margin and may relate to fluid flow from the Hikurangi plateau being subducted further east.

Seamounts, knolls and petit‐spot monogenetic volcanoes on the subducting Pacific Plate

ABSTRACTOn the western part of the Pacific Plate most seamounts formed during the Cretaceous period in the so‐called West Pacific Seamount Province (WPSP). On the northwestern part of the same plate, the Joban and Japanese Seamount Trail (JJST) are also composed of Early Cretaceous seamounts. However, two new groups of knolls were recently discovered during multibeam surveys on the Pacific Plate along the Japan Trench. One group consists of circular knolls that are flat‐topped in shape and correspond to eruptive ages of approximately 75 Ma. The other group consists of irregularly shaped knolls, also called petit‐spot volcanoes, that are found on the outer‐rise systems of the subducting Pacific Plate. These petit‐spots seem much younger and available age data suggest that they only formed in the last few million years. Acoustic reflective data, which are simultaneously obtained with bathymetrical data, are a most powerful tool to distinguish the petit‐spots from the Cretaceous edifices in the WPSP and JJST. In this paper, we present the results of an exploratory search for these new kind of petit‐spot volcanoes along the trenches in the Pacific Ocean, with an emphasis on the Japan and Tonga trenches. The sizes of these irregularly shaped petit‐spot volcanoes are several orders of magnitude less than the Cretaceous seamounts and circular knolls, yet they appear to be ubiquitous on the ocean floor, in particular, where incipient melts in the asthenosphere can be squeezed out by tectonic forces.

Break-up spots: Could the Pacific open as a consequence of plate kinematics?

The South Central Pacific is the location of an abnormal concentration of intraplate volcanism. Noting that this volcanism is present from the Kermadec Tonga trench to the Easter microplate and forms a wide east–west channel, we propose to explain its occurrence in relation to the Pacific plate geometry and kinematics. We construct 2D numerical models of stress and strain within the Pacific plate using its velocity field and boundary conditions. The models indicate a shear band, associated to a change from compressional stresses to the south to tensional stresses to the north, which develop after 10 Myr between the Australian plate corner and the Easter microplate. We propose that the Central Pacific intraplate volcanism is related to this process, and may represent the first step of a future plate re-organization which will eventually break the Pacific plate in a southern and a northern plate due to intraplate stresses. Present-day intraplate volcanism would define break up spots of the future border.

Multiple mantle plume components involved in the petrogenesis of subduction‐related lavas from the northern termination of the Tonga Arc and northern Lau Basin: Evidence from the geochemistry of arc and backarc submarine volcanics

We report new geochemical data for boninites and backarc basin‐type basalts recovered from the northern termination of the Tonga trench and Lau Basin. Boninitic pillow lavas, ranging from high‐Mg compositions to andesites and dacites, have been erupted within large submarine volcanic edifices (calderas and volcanoes) associated with active rifting of both the northern end of the Tofua volcanic arc and in a backarc position relative to the arc volcanoes on the northern Tonga Ridge. The mantle sources in the area are a complex mixture of (1) the “normal” Tongan mantle wedge source that has “Pacific”‐type isotopic signature with (2) the plume‐related components (EMI, EMII, and HIMU) and (3) an “Indian”‐type source upwelling beneath the backarc spreading. Some of these sources, such as the “normal” mantle wedge and variably depleted residual plume mantle, are fluxed by subduction components from the slab, which produces boninites, tholeiites, and mixtures thereof. Other mantle sources, such as “Indian”‐type backarc mantle and also some of the plume mantle, produce melts due to adiabatic decompression. These melts are variably mixed with each other and with the slab‐fluid fluxed subduction‐related melts to form the observed spectrum of magma compositions.

Axial crustal structure of the Lau back-arc basin from velocity modeling of multichannel seismic data

Located west of the Tonga trench, the Lau back-arc basin is a prime environment for studying the interplay between oceanic spreading systems and an active subduction zone. Within the basin lie two complementary, intermediate-rate spreading systems, the Central Lau Spreading Center (CLSC) and the Eastern Lau Spreading Center/Valu Fa Ridge (ELSC/VFR), that are positioned 170 to 40 km away from the active volcanic arc, respectively. Multichannel seismic (MCS) images of both systems reveal systematic variations in axial crustal structure primarily related to the proximity of the volcanic arc, but also related to spreading rate, morphology, and petrology. Upper crustal refraction data selected from the along-axis seismic lines collected during the 1999 MCS survey were modeled in both the time-versus-range ( t– x) and intercept time-versus-slowness ( τ– p) domains to provide validation and detail to the seismic reflection observations. The results show that as both the proximity to the arc and the spreading rate decrease southward: 1) seismic layer 2A thickens by 0.6 km between the CLSC and VFR, from 0.4 km to 1.0 km, 2) the average depth of the axial magma chamber (AMC) increases from 1.5 km at the CLSC to 2.8 km at the southern VFR, excluding the northern section of the ELSC that shows no continuous AMC reflector, but does show an isolated melt sill, 3) the upper crustal basement velocity decreases from 2.1 km/s at the CLSC to 1.8 km/s at the VFR, and 4) the velocities of both layer 2A and 2B decrease between the CLSC and northern VFR from 3.2 to 2.5 km/s and 5.0 to 3.9 km/s, respectively. Along with a broad axial high morphology, these features of the CLSC — thinner layer 2A, shallower AMC, and faster crustal velocities — correlate best with the fast spreading East Pacific Rise and intermediate spreading Juan de Fuca Ridge. Conversely, the structural characteristics of the central ELSC/VFR have no known counterpart in the global mid-ocean ridge system. We attribute this primarily to the volatile concentration in the magmas coming from the Tonga subduction zone.

Oligocene–Miocene tectonic evolution of the South Fiji Basin and Northland Plateau, SW Pacific Ocean: Evidence from petrology and dating of dredged rocks

We present new analytical data from lavas and associated rocks dredged and/or drilled from the South Fiji Basin, Northland Plateau, Colville Ridge and Havre Trough. These results provide much-needed ground truth about the geology, age and tectonic evolution of the Cenozoic submarine ridges and basins between the active intraoceanic Tonga–Kermadec arc, and rifted continental borderlands of New Zealand, the Norfolk Ridge and New Caledonia. Key results from this study include: (1) Ar–Ar dates on Minerva Abyssal Plain oceanic crust suggest that the ages of magnetic anomalies in the South Fiji Basin have been overestimated by earlier workers; (2) subduction-related lavas are widespread across the region, are not presently organised into arc-like chains, and cluster in the age range 22–18 Ma (Early Miocene); (3) the oldest subduction-related lavas occur in the western part of the region (32–26 Ma: Norfolk and Three Kings Ridge); (4) shoshonites, interpreted as rifted arc lavas, were erupted in a narrow 20–21 Ma interval over a wide area. Put together, these results indicate high magmatic flux and large and rapid horizontal tectonic translations and basin opening from 18–23 Ma in the region immediately north of New Zealand. We explain the Miocene tectonomagmatic development of the region by a model of rapid rollback of a single, east-facing Pacific arc–trench system that became established after Northland Allochthon emplacement. Critical testing of this, versus other, tectonic models must await drilling and dating of thus-far unsampled Kupe Abyssal Plain crust.

Subduction-related magmatism at the southern tip of the North Fiji backarc basin

The North Fiji backarc basin (NFB) began opening ~12 Ma in response to subduction reversal along the Vanuatu section of the Proto-Vitiaz arc. The triangular shape of the basin reflects ongoing (since ~5 Ma) southward propagation of the backarc spreading centre in response to westward roll-back of the Vanuatu trench and eastward roll-back of the Tonga trench. During this time (5-3 Ma), a short-lived intraoceanic arc, the Hunter Ridge, formed along the southeast margin of the NFB. This arc magmatism occurred in response to a short period of NNW-directed subduction of the South Fiji basin crust under the NFB. The southern tip of the North Fiji backarc basin (22°S, 174°E) is a volcanically active submarine triple junction between the backarc basin spreading centre, Vanuatu trench and Hunter Fracture zone. This area has been mapped and sampled during the SS10/2004 cruise of R/V ``Southern Surveyor'. Volcanic rocks erupted within 100 km cover the entire spectrum of subduction-related magmas from backarc basin basalts to boninites, arc tholeiites, calc-alkaline basalts, high-Mg andesites and adakites. Subduction-related lavas in the area are dominated by olivine-clinopyroxene-phyric basalts with high-Mg phenocrysts (Mg# to 94). We will present detailed geology, geochemistry and mineralogy (including melt inclusion compositions) of all volcanic rock from this region and discuss their origin.

Seed plants of Fiji: an ecological analysis

An annotated list of indigenous Fijian seed plant genera is presented and comprises 484 genera and 1315 species in 137 families. The relative diversity of the largest families and genera in Fiji is indicated and compared with floras in New Caledonia and the Upper Watut Valley, Papua New Guinea. Differences and similarities appear to be due to biogeographical/phylogenetic factors rather than ecological differences or means of dispersal. Generic diversity for the seed plants as a whole is greatest between 0–100 m and decreases monotonically with altitude. However, in the largest family, Orchidaceae, maximum diversity occurs between 200–400 m. Fifty percent of the families are recorded from shore habitat. Twenty-seven percent of the families and 80 species occur in or around mangrove, where the most diverse families are Orchidaceae, Rubiaceae, and the legumes. Some of the mangrove-associate species are pantropical or Indo-Pacific but most are locally or regionally endemic. Fifty-six percent of the Fijian families are recorded on limestone. Twenty-nine species are restricted to limestone and 12 species usually occur on limestone. The importance of calcium in reducing the effects of salinity is emphasized and 39 species are recorded from both mangrove and limestone. A plagiotropic habit occurs in 38 species which occur on limestone or around beaches, and 20 of these are Pacific endemics. Genera restricted to higher altitudes include many present elsewhere in Melanesia but absent from Australia despite suitable habitat there, again indicating the importance of biogeographical and historical factors. Altitudinal anomalies in Fiji taxa are cited and include 7 anomalously high records from northern Viti Levu, a site of major uplift, and 22 anomalously low altitudinal records in the Lau Group, a site of subsidence. It is suggested that the Fijian flora has not been derived from immigrants from Asia, but has evolved more or less in situ. Taxa would have survived as metapopulations on the individually ephemeral volcanic islands always found at oceanic subduction zones and hot spots, and the atolls which characterize areas of subsidence. The complex geology of Fiji is determined by its position between two subduction zones of opposite polarity, the Vanuatu and Tonga Trenches, in what is currently a region of transform faulting. The large islands comprise fragments of island arcs that have amalgamated and welded together. There has been considerable uplift as well as subsidence in the islands and it is suggested that both these processes have had drastic effects on the altitudinal range of the taxa. Limestone and mangrove floras could have provided a widespread, diverse ancestral species pool from which freshwater swamp forest, lowland rainforest, dry forest, secondary forest, thickets, and montane forest have been derived during phases of uplift. © 2006 The Linnean Society of London, Biological Journal of the Linnean Society, 2006, 89, 407–431.

A coupled solid–fluid method for modelling subduction

We present a novel dynamic approach for solid–fluid coupling by joining two different numerical methods: the boundary-element method (BEM) and the finite element method (FEM). The FEM results describe the thermomechanical evolution of the solid while the fluid is solved with the BEM. The bidirectional feedback between the two domains evolves along a Lagrangian interface where the FEM domain is embedded inside the BEM domain. The feedback between the two codes is based on the calculation of a specific drag tensor for each boundary on finite element. The approach is presented here to solve the complex problem of the descent of a cold subducting oceanic plate into a hot fluid-like mantle. The coupling technique is shown to maintain the proper energy dissipation caused by the important secondary induced mantle flow induced by the lateral migrating of the subducting plate. We show how the method can be successfully applied for modelling the feedback between deformation of the oceanic plate and the induced mantle flow. We find that the mantle flow drag is singular at the edge of the retreating plate causing a distinct hook shape. In nature, such hooks can be observed at the northern end of the Tonga trench and at the southern perimeter, of the South American trench.

Tephra beds in deep-sea cores off northern New Zealand: implications for the history of Taupo Volcanic Zone, Mayor Island and White Island volcanoes

Twenty piston cores were collected from water depths of ∼ 650–3300 m from offshore Bay of Plenty, North Island, New Zealand. They contain tephra from 11 Okataina Volcanic Centre and 4 Taupo Volcanic Centre rhyolite eruptions during the last ∼ 50 kyr that produce a tephrostratigraphic framework across the tectonically and volcanically complex region of southern Havre Trough, Alderman Trough and the Bay of Plenty continental shelf. This allows correlation between offshore and onshore sequences up to 200 km from source volcanoes, covering much of Marine Isotope Stages 3 and 2. The framework temporally constrains poorly dated and newly recognised volcanic events. Macroscopic tephra layers from the peralkaline Mayor Island volcano are documented for the first time at pre-50, post-50, 40.5, 37.4, 22.2, and 14.2 ka, in addition to the well known 7 ka (Tuhua) event. These macroscopic layers represent some of the most explosive events from this volcano, and provide new marker horizons. They are dispersed up to 90 km north-east to east of the edifice. Minor tephra dispersal is also recorded at 17.8, 25.5, ∼ 35 and pre-50 ka. The tephra contain high SiO 2 (73–75.5 wt.%) glass and subordinate basaltic components, and each tephra represents a distinct magma batch that can be fingerprinted. The Mayor Island tephra form two temporal trends toward less evolved magma compositions, punctuated by a large caldera-forming event at 36 ka. The historically active, andesitic White Island volcano does not have widely dispersed tephra, and the oldest primary deposit found is ∼ 21 ka. Five pre-50 ka rhyolite eruptions from an unknown Taupo Volcanic Zone source provide evidence for explosive activity in a time interval poorly documented on-land. The cores demonstrate the patchy and uneven preservation of large magnitude tephra falls caused by local faulting, bioturbation and ponding in bathymetrically complex regions. Reworked tephra layers are common and often lack indicative lithological features. Such units could easily be misinterpreted as primary events without micro-beam geochemical analyses of glass shards.

Volatile contents of Kermadec Arc–Havre Trough pillow glasses: Fingerprinting slab-derived aqueous fluids in the mantle sources of arc and back-arc lavas

Aqueous fluids and sediment melts from subducting oceanic lithosphere are the driving force for the refertilisation of the mantle overlying subduction zones. Volatile elements in particular play an important role in the generation of aqueous fluids and the transport of fluid mobile elements. Here we report volatile contents of quenched glasses from pillow-lava rims in the Kermadec Arc–Havre Trough subduction system to provide constraints on the source, generation, and composition of slab-derived aqueous fluids. Water (∼1.5 wt.% in glasses, 2.5 wt.% in two melt inclusions), carbon (up to 16 ppm in glasses, 180 ppm in one inclusion) and sulphur (< 700 ppm) contents of all glasses are consistent with degassing of volatiles, which migrated from the melt, through growing vesicles, and into the water column, even in samples collected at depths of up to 3000 m. By contrast, halogen contents have not been affected by degassing or any other secondary processes such as assimilation of seawater. Fluorine contents (250–520 ppm) can be obtained almost entirely from melting of depleted mantle similar to that which generates mid-ocean ridge basalt. However, Cl (650–3000 ppm) is enriched tenfold compared to depleted mantle-derived lavas. A minimum of 70% H 2O, > 80% Ba and Pb, and > 98% of Cl is required to be derived from aqueous slab-derived fluids. We propose a single homogenous reservoir for the aqueous fluid source. This reservoir is most likely to be hydrated serpentinite formed above the subducting slab by fluids derived from altered oceanic crust and subducting sediment. Dehydration of the serpentinite occurs as it descends into the mantle, transporting fluid mobile elements into the overlying mantle and triggering partial melting. The similarity of aqueous fluid compositions derived for other oceanic arcs suggests that this process may occur in arc systems worldwide.

Rocks beneath New Zealand's Central North Island: Mantle or crust?

A long‐standing question in crustal seismology is the nature of rocks with compressional P‐wave speeds of 7–7.6 kilometers per second that are found beneath continental rift zones [e.g., Cook, 1962]. Two interpretations are possible: They are either lower crustal rocks, or upper mantle material with unusually low P‐wave velocities because of the presence of a small percent of partial melt.An April 2005 explosion seismology experiment set out to explore the issue of what is crust and what is mantle in the Taupo Volcanic Zone (TVZ), central North Island, New Zealand. The TVZ is the apparent continuation of oceanic back‐arc spreading within the Havre Trough into the continental lithosphere of New Zealand (Figure 1). Determining whether crust or mantle exists in such settings is fundamental to understanding back‐arc basins and subduction zones, both of which are vital elements in recycling oceanic lithosphere and creating new continental crust