Kermadec Trench Research Articles

Metagenome sequencing and 982 microbial genomes from Kermadec and Diamantina Trenches sediments

Deep-sea trenches representing an intriguing ecosystem for exploring the survival and evolutionary strategies of microbial communities in the highly specialized deep-sea environments. Here, 29 metagenomes were obtained from sediment samples collected from Kermadec and Diamantina trenches. Notably, those samples covered a varying sampling depths (from 5321 m to 9415 m) and distinct layers within the sediment itself (from 0~40 cm in Kermadec trench and 0~24 cm in Diamantina trench). Through metagenomic binning process, we reconstructed 982 metagenome assembled genomes (MAGs) with completeness >60% and contamination <5%. Within them, completeness of 351 MAGs were >90%, while an additional 331 were >80%. Phylogenomic analysis for the MAGs revealed nearly all of them were distantly related to known cultivated isolates. The abundant bacterial MAGs affiliated to phyla of Proteobacteria, Planctomycetota, Nitrospirota, Acidobacteriota, Actinobacteriota, and Chlorofexota, while the abundant archaeal phyla affiliated with Nanoarchaeota and Thermoproteota. These results provide a dataset available for further interrogation of diversity, distribution and ecological function of deep-sea microbes existed in the trenches.

Implications of Surface Heat Flux for Shear Stress and Temperature on the Plate Interface Beneath Northern Honshu

AbstractMeasurements of surface heat flux from boreholes of the NIED Hi‐net network of seismometers are used to determine temperature and shear stress on the plate interface beneath northern Honshu. At the maximum depth of thrust‐faulting earthquakes, about 60 km, the temperature is 660 ± 240°C and the average shear stress during slip on the interface is 100 ± 45 MPa. The equivalent quantities from the oceanic setting of the Kermadec trench are 630 ± 140°C and 115 ± 25 MPa. The shear stresses are substantially higher than commonly assumed in models of convergent plate interfaces, but are consistent with measurements made above other convergent plate interfaces where heat flux data are more sparse. They are also consistent with the level of shear stress required to explain measurements of pressure and temperature from high‐pressure‐low‐temperature terrains that experienced metamorphism on subduction interfaces, in particular those from the Cretaceous Sanbagawa belt of SW Japan.

Distribution and genomic variation of ammonia-oxidizing archaea in abyssal and hadal surface sediments

Ammonia-oxidizing archaea of the phylum Thaumarchaeota play a central role in the biogeochemical cycling of nitrogen in benthic sediments, at the interface between pelagic and subsurface ecosystems. However, our understanding of their niche separation and of the processes controlling their population structure in hadal and abyssal surface sediments is still limited. Here, we reconstructed 47 AOA metagenome-assembled genomes (MAGs) from surface sediments of the Atacama and Kermadec trench systems. They formed deep-sea-specific groups within the family Nitrosopumilaceae and were assigned to six amoA gene-based clades. MAGs from different clades had distinct distribution patterns along oxygen-ammonium counter gradients in surface sediments. At the species level, MAGs thus seemed to form different ecotypes and follow deterministic niche-based distributions. In contrast, intraspecific population structure, defined by patterns of Single Nucleotide Variants (SNV), seemed to reflect more complex contributions of both deterministic and stochastic processes. Firstly, the bathymetric range had a strong effect on population structure, with distinct populations in abyssal plains and hadal trenches. Then, hadal populations were clearly separated by trench system, suggesting a strong isolation-by-topography effect, whereas abyssal populations were rather controlled by sediment depth or geographic distances, depending on the clade considered. Interestingly, genetic variability between samples was lowest in sediment layers where the mean MAG coverage was highest, highlighting the importance of selective pressure linked with each AOA clade’s ecological niche. Overall, our results show that deep-sea AOA genome distributions seem to follow both deterministic and stochastic processes, depending on the genomic variability scale considered.

Sediment Accumulation and Carbon Burial in Four Hadal Trench Systems

AbstractHadal trenches are considered to act as depocenters for organic material, although pathways for the material transport and deposition rates are poorly constrained. Here we assess focusing, deposition and accumulation of material and organic carbon in four hadal trench systems underlying different surface ocean productivities; the eutrophic Atacama and Kuril‐Kamchatka trenches, the mesotrophic Kermadec trench and the oligotrophic Mariana Trench. The study is based on the distributions of naturally occurring 210Pbex, 137Cs and total organic carbon from recovered sediment cores and by applying previously quantified benthic mineralization rates. Periods of steady deposition and discreet mass‐wasting deposits were identified from the profiles and the latter were associated with historic recorded seismic events in the respective regions. During periods without mass wasting, the estimated focusing factors along trench axes were elevated, suggesting more or less continuous downslope focusing of material toward the interior of the trenches. The estimated organic carbon deposition rates during these periods exhibited extensive site‐specific variability, but were generally similar to values encountered at much shallower settings such as continental slopes and margins. Organic carbon deposition rates during periods of steady deposition were not mirrored by surface ocean productivity, but appeared confounded by local bathymetry. The inclusion of deposition mediated by mass‐wasting events enhanced the sediment and organic carbon accumulations for the past ∼150 years by up to a factor of ∼4. Thus, due to intensified downslope material focusing and infrequent mass‐wasting events, hadal trenches are important sites for deposition and sequestration of organic carbon in the deep sea.

Glycerol dialkyl glycerol tetraethers in surface sediments from three Pacific trenches: Distribution, source and environmental implications

Glycerol dialkyl glycerol tetraethers (GDGTs) have been widely used to elucidate sources of sediment total organic carbon (TOC), past temperature and presence of methanogenesis in diverse environments. However, their applicability to hadal trenches with their unique deposition dynamics remains unknown. Here, we analyzed GDGTs and their stable isotope values and content of TOC in surface sediments from the Kermadec Trench region (KT; 6080–10010 m), New Britain Trench region (NBT; 1553–8931 m), and Atacama Trench region (AT; 2560–8085 m). These regions are at very different distances from terrestrial sources and have varying net primary productivity (NPP) in the waters above them. The GDGT concentration was highly variable (54.5–2416 μg g−1 TOC) within and between trench regions and was not directly related to local NPP or apparent terrestrial inputs. This finding is presumably due to complex deposition dynamics within the trench interior. Isoprenoidal GDGTs (isoGDGTs; 75.4–99.1%) were dominant over branched GDGTs (brGDGTs, 0.91–24.6%) in all samples, leading to low levels of Branched versus Isoprenoidal Tetraether (BIT) index (0.01–0.27). Thus, sediment TOC is mainly derived from marine sources. However, compared to adjacent non-hadal sites, trench axis sites have a higher BIT index, lower acyclic hexa-/pentamethylated brGDGT and lower δ13C, supporting relative enrichment of terrestrial organic carbon at the trench axis. The application of TetraEther indeX of tetraethers consisting of 86 carbon atoms (TEX86) resulted in sea surface temperature (SST) estimates of 18.9–23.7 °C in the KT, 28.6–30.2 °C in the NBT, and 17.9–20.4 °C in the AT. The close agreement between TEX86-SST with observed in situ SST suggests that isoGDGTs are not selectively degraded during the transport towards the hadal realm, and that TEX86 from hadal settings robustly records an integrated regional SST signal.

Mapping Internal Tides From Satellite Altimetry Without Blind Directions

AbstractSatellite altimetry is one practical technique for observing internal tides on the global scale. However, it is a great challenge to extract weak internal tide signals. This paper presents a new technique for mapping internal tides from satellite altimeter data. Along‐track high‐pass filtering is needed to remove long‐wavelength nontidal noise and the barotropic tidal residual; however, the filter also removes internal tides having large angles with respect to satellite ground tracks. It thus causes blind directions in mapping internal tides from satellite altimetry: Generally west‐east propagating internal tides are missed. The new technique addresses the blind‐direction issue by replacing the problematic one‐dimensional (1‐D) high‐pass filter with a two‐dimensional (2‐D) band‐pass filter. This mapping technique is able to retrieve ubiquitous westbound and eastbound internal tides not captured in previous estimates. Long‐range westbound and eastbound waves travel over thousands of km from numerous generation sites such as the Emperor seamount chain, the Hawaiian Ridge, and the Kermadec trench. Evaluation using independent Cryosat‐2 data reveals that the new internal tide model may reduce more SSH variance than a model built in 2016 does in regions of strong internal tides. However, this mapping technique makes no improvement in strong boundary current regions, due to the dominance of mesoscale motions. Moreover, the new internal tide model contains leaked noise from westward propagating tropical instability waves (TIWs), which can be suppressed by prior along‐track high‐pass filtering. This paper suggests that better internal tide models may be constructed using both 1‐D and 2‐D filters with optimized parameters.

Flexure and gravity anomalies of the oceanic lithosphere beneath the Louisville seamount

We have calculated the elastic thickness (Te), flexural deflection, and gravity anomaly of the oceanic crust beneath the Louisville seamount (LSC-03), near the Kermadec trench. A regional-residual separation of the bathymetry was performed to remove the effect of other geologic features (e.g., the trench). We used the uniform density and dense core models to approximate the total mass of the seamount, which was defined as the surface load required for flexural deformation. From the flexure modeling results, we found that more flexural depression was predicted by the uniform density model than by the dense core model. However, the uniform density model predicted a significantly smaller gravity anomaly than observed, whereas the dense core model minimized the prediction misfits reasonably. The best flexure model was found with a Te of 16km for the uniform density model and 6km for the dense core model. The flexure computed with the dense core model was consistent with the seismically detected Moho. The flexure modeling for LSC-03, thus, indicates that the dense core model better approximates the inner structure of the LSC-03. Based on the crustal age and geochronology of the given seamount, the age of the oceanic crust at the time of seamount formation (Δt) is 20Ma. If this is the case, however, the Te estimates from both flexure models require some degree of lithospheric reheating by Louisville hotspot activity. Alternatively, considering the tectonic plate motion of the Osbourn Trough, Δt becomes approximately 4Ma. This younger lithosphere model is more consistent with the observed flexural deformation and the Te estimate from the dense core model. Therefore, the time that the seamount-induced lithospheric deformation occurred may be far earlier than the age-dated volcanism.

The distribution of benthic biomass in hadal trenches: A modelling approach to investigate the effect of vertical and lateral organic matter transport to the seafloor

Most of our knowledge about deep-sea habitats is limited to bathyal (200–3000m) and abyssal depths (3000–6000m), while relatively little is known about the hadal zone (6000–11,000m). The basic paradigm for the distribution of deep seafloor biomass suggests that the reduction in biomass and average body size of benthic animals along depth gradients is mainly related to surface productivity and remineralisation of sinking particulate organic carbon with depth. However, there is evidence that this pattern is somewhat reversed in hadal trenches by the funnelling of organic sediments, which would result in increased food availability along the axis of the trenches and towards their deeper regions. Therefore, despite the extreme hydrostatic pressure and remoteness from the pelagic food supply, it is hypothesized that biomass can increase with depth in hadal trenches. We developed a numerical model of gravitational lateral sediment transport along the seafloor as a function of slope, using the Kermadec Trench, near New Zealand, as a test environment. We propose that local topography (at a scale of tens of kilometres) and trench shape can be used to provide useful estimates of local accumulation of food and, therefore, patterns of benthic biomass. Orientation and steepness of local slopes are the drivers of organic sediment accumulation in the model, which result in higher biomass along the axis of the trench, especially in the deepest spots, and lower biomass on the slopes, from which most sediment is removed. The model outputs for the Kermadec Trench are in agreement with observations suggesting the occurrence of a funnelling effect and substantial spatial variability in biomass inside a trench. Further trench surveys will be needed to determine the degree to which seafloor currents are important compared with the gravity-driven transport modelled here. These outputs can also benefit future hadal investigations by highlighting areas of potential biological interest, on which to focus sampling effort. Comprehensive exploration of hadal trenches will, in turn, provide datasets for improving the model parameters and increasing predictive power.

Rapid weakening of subducting plates from trench-parallel estimates of flexural rigidity

The negative buoyancy force of sinking lithosphere (slabs) is the principle driving force for subducting plates, but transmission of this force to the subducting plate depends on the strength of the slab (e.g., Conrad and Hager, 2001). Slab strength has been studied in the context of plate bending within subduction zones for a wide range of rheologies (i.e., perfectly elastic, perfectly viscous, perfectly plastic, brittle-ductile layered). Because the applicability of these rheologic models cannot be distinguished based on trench-perpendicular plate bending models (Forsyth, 1980), a method was developed to directly measure variations in plate strength with distance from the trench and has found significant plate weakening within 100km of the Kermadec Trench (Billen and Gurnis, 2005). Using the same method we show that rapid plate weakening trenchward of the forebulge also exists at the Tonga and Japan-Izu-Bonin subduction zones. The observed plate weakening provides further evidence for a plate rheology that leads to significant yielding (loss of elastic strength and reduction in effective viscosity) within the bending region of the subducting plate. This rapid weakening within the shallow, low curvature, region of the plate may significantly decrease estimates of energy dissipation related to plate bending, compared to recent calculations assuming high plate strength and constant plate curvature.

Living with natural hazards in the Asia–Pacific region

Many might say that it could not be a worse time to live in the Asia–Pacific region. In the past few years we have not only experienced the 2004 Indian Ocean tsunami, but also the 2006 Java, 2007 Solomon Islands, 2009 New Zealand, 2010 Chilean and the 2011 Tōhoku (Japan) earthquakes and tsunamis. We should not forget either the seemingly endless list of other natural hazards such as tropical cyclones and typhoons (e.g. 2010 Fiji and Philippines), volcanic eruptions (2010 Indonesia), river floods (2010 Pakistan), wildfires (2009 Australia), amongst many others. For a variety of reasons, an increasing number of people in the Asia–Pacific region are either choosing to live, or are finding themselves living, at the coast (Sieh 2006). This in itself is not a problem except when they are exposed to hazards such as tsunamis and cyclones. It is in this coastal zone, though, that we have seen the recent effects of devastating natural hazards. The value of the work carried out by natural hazards researchers has recently been brought to the forefront of public awareness. It is not always easy to recognize the hazards that a low-lying coastal area faces either because there may be no previous historic records of large-magnitude, destructive tsunamis from which we might learn valuable lessons (Satake & Atwater 2007) or because people are, for various reasons, unaware of the area’s geological past (Normile 2011). As we write this editorial, the ongoing troubles faced by the Fukushima nuclear power plant are broadcast to us daily. While it is true that the first reactor was completed there in 1971, an expert panel reviewed the plant’s sesimic resistance in 2008 (Normile 2011). By this time Japanese scientists were well aware of the Jogan tsunami of 869AD, an event that they concluded could recur at about 1000 year intervals. This had not been factored into the orginal design plans for the Fukushima plant, but the opportunity existed to address this issue in 2008 – it was not (Normile 2011). The people of Japan and the rest of the world are now living with the aftermath of that decision. If there is one thing that is evident from the work that we do on geological hazards, it is that it is all too easy to base estimates of magnitude and frequency on extrapolations from historical data. These forecasts, based generally on a few centuries of data at best, are completely inadequate for the big events that have simply not occurred in historic time. For some types of climatic and hydrological hazards, homogenous and robust datasets are of much shorter duration, so any trends are difficult to identify with confidence (Terry & Gienko 2010; Terry & Wotling 2011). The implications for such oversights have proven to be catastrophic and it is only through a combination of geological, climatic and numerical modelling research, combined with more traditional historical studies, that we can hope to guard against such events catching us off guard in the future. An interesting case in point relates to palaeotsunami work carried out in New Zealand over the past 10 years or so that has consistently pointed to there having been a large event affecting the northern coasts of the country in the fifteenth century (e.g. Nichol et al. 2003; Walters et al. 2006; Goff 2008; Goff et al. 2010). This signifcant body of literature was largely ignored until recent numerical modelling work found ‘an intriguing similarity in distribution and scale’ of modelled runup heights with those of the palaeotsunami data (Power et al. 2011). Why is this so important? Palaeotsunami researchers have long proposed that the most likely source for this event was from a large subduction zone event on the Tonga–Kermadec trench to the north of New Zealand. Until recently, however, numerical modellers and geophysicists did not feel that this was plausible despite warnings from key researchers that ‘simple geophysical hypotheses about maximum earthquake size at subduction zones, and about patterns of earthquake recurrence, appear to be of limited value in the long-term forecasting of the time and size of great subduction zone earthquakes’ (Satake & Atwater 2007, p. 368). This point has been re-emphasized

Geochemistry of mid-Cretaceous Pacific crust being subducted along the Tonga–Kermadec Trench: Implications for the generation of arc lavas

The Pacific Plate is currently being subducted at a rapid rate beneath the Indo-Australian Plate along the Tonga–Kermadec Trench in the southwestern Pacific. It has long been assumed that the lithosphere being subducted is relatively old and homogeneous in composition. Basaltic lavas dredged from the upper crust of the incoming lithosphere along the length of the trench are mid- to late-Cretaceous in age. Although the samples are mainly N-MORB, they range from tholeiitic to alkalic basalts. Concentrations of incompatible trace elements show a high degree of variability (e.g., Ba=8 to 270 ppm, Rb=0.4 to 39 ppm, Sr=60 to 625 ppm, and Nd=2.3 to 25 ppm). Neodymium, Sr and Pb isotopic data also show wide ranges (εNd(T)=5.7–11.2; 87Sr/86Sri=0.70224–0.70311; 206Pb/204Pbi=17.84–20.22). More importantly, the basaltic crust being subducted displays a latitudinal compositional variation that is similar to that shown by the Tonga–Kermadec arc lavas. Previous studies have proposed that the variably depleted sub-arc mantle, which was preconditioned through a backarc melt extraction or displacement process, is mainly responsible for the latitudinal variation in the Tonga–Kermadec arc lavas. However, our new results suggest a greater role of the lithospheric input into the source of arc lavas. The three end-member possibilities linking the latitudinal variation of the lithospheric input to the source of arc lava output are: (1) the mantle wedge beneath the volcanic arc on the west side of the trench, the main source of Tonga–Kermadec arc lavas, is a western extension of the Cretaceous Pacific upper mantle east of the trench; (2) the altered oceanic crust melts and the resultant slab melt modifies the mantle source of arc lavas; and (3) fluids dehydrated from the altered oceanic crust effectively transfer the compositional signature of the subducted slab into the mantle source of arc lavas.

A CONCEPTUAL MODEL FOR THE ORIGINS OF GEOTHERMAL AND VOLCANIC ACTIVITY IN THE NORTH ISLAND OF NEW ZEALAND

AbstractThe current geothermal and volcanic activity in the North Island of New Zealand is explained as a consequence of Pacific and Australian plate interactions over the last 20 million years. The primary hypothesis is that the Kermadec subduction zone has for the last 20 million years or more been retreating in a south-easterly direction at about five centimetres per year. It is surmised that this motion and interaction with another subduction zone almost at right angles to it under the North Island resulted in plate tearing due to the incompatibility of the plate geometry where these subduction zones interacted. The nature and consequences of this plate tearing are partially revealed in published maps of the plate currently under the North Island. If the subducted parts of this plate, as shown in Eiby’s maps, [G. A. Eiby, “The New Zealand sub-crustal rift”, New Zeal. J. Geol. Geophy.7 (1964) 109–133] are straightened, then the plate edge lies on a curve giving a rough picture of their position before being torn and subducted by the Kermadec trench motion. This map of the tear suggests the shape of the edge of a missing plate segment torn from the plate, and implies a rotation of the upper North Island, clockwise approximately 20 degrees, about a point just south of the Thames estuary. A consequence of this plate tearing is that the solid retreating crustal wave generating magma pressure beneath the crest of the solid wave has the potential to inject significant basaltic magma into the crust through the tears. These intrusive magma fluxes have the ability to generate geothermal fields and rhyolitic lavas from crustal melts. This could explain the geothermal activity along the Coromandel peninsula five to seven million years ago, the ignimbrite outcrops about Lake Taupo and the current geothermal and volcanic activity stretching from Taupo to Rotorua.

Implementation of Routine Regional Moment Tensor Analysis in New Zealand

The active tectonics of New Zealand are dominated by three main features (figure 1). Beneath the North Island there is active subduction of the Hikurangi trough where the Pacific plate is undergoing oblique subduction beneath the Australian plate at ∼ 45 mm/yr ( e.g. , Demets et al. 1990; Demets et al. 1994). The Hikurangi trough extends from the Kermadec trench in the north and terminates in the upper South Island. Large subduction zone earthquakes of around Mw 8.0 in the lower North Island and around Mw 6.9 farther north are expected (Reyners 1998). Slow slip events have also been observed on the Hikurangi subduction interface ( e.g. , Wallace and Beavan 2006) similar to those documented at other Pacific Rim subduction margins ( e.g. , Dragert et al. 2001; Obara et al. 2004; Larson et al. 2004). A number of dextral strike-slip faults are also present throughout the North Island and the upper South Island and have produced a number of M 6.0–7.5 earthquakes in the past century (Doser and Webb 2003). The Wellington region is cut by five active dextral faults that have average recurrence intervals of meter-scale surface rupture that range from ∼ 500 to 5,000 years (Van Dissen and Berryman 1996). The Wairarapa fault is the only one of these to have ruptured in historic times, in 1855, in an event estimated at Mw 8.2 (Darby and Beanland 1992). ▴ Figure 1. Overview of the tectonic setting of New Zealand, which is dominated by the interaction between the Pacific plate and the Australian plate. In the North Island the Pacific plate is subducting beneath the Australian plate at the Hikurangi trough and Kermadec trench. The Alpine fault delineates a dextral strike-slip fault through most of the South Island, and south of the South Island the …

Constraints on subducting plate strength within the Kermadec trench

Four specially designed surveys parallel to the Kermadec trench allow localized estimates of plate strength within the subducting Pacific plate to be made. The transfer function between topography and gravity is estimated for five trench‐parallel ship tracks at distances of 25–110 km from the trench axis. We find a clear reduction in the magnitude and peak wavelength of the transfer function from the outer rise to the trench axis. The change in the transfer function indicates a decrease in plate strength and is consistent with a reduction in flexural rigidity by 3–5 orders of magnitude or a decrease in effective elastic thickness by more than 15 km. Such a large‐magnitude decrease in the effective elastic strength suggests that the plate has little or no elastic strength within the trench and that viscous stresses play an important role in transferring slab‐pull forces to the subducting plate and regulating plate speeds in subduction zones.

A constraint on the shear stress at the Pacific‐Australian plate boundary from heat flow and seismicity at the Kermadec forearc

New heat flow measurements and relocated hypocenters that constrain the subduction geometry of the Pacific plate at the Kermadec trench yield an estimate of shear stress on the thrust fault. With the exception of a few relatively high values (&gt;60 mW m−2) on the upper forearc region near the active volcanic ridge, most of the 64 heat flow values along two profiles across the forearc range from 20 to 40 mW m−2. Corrections for bottom water temperature variations that caused nonuniform temperature gradients were required for most measurements. The means of the most reliable values for the northern and southern profiles are 28.9±5.8 mW m−2 (n = 33) and 28.9±7.2 mW m−2 (n = 19), respectively, and the measurements show no apparent systematic variation along the profiles over the range ∼50 to 150 km distance from the trench. The means of the 10 values at each of two sites on the Pacific plate seaward of the profiles are 57.2±6.3 and 60.2±6.6 mW m−2. Redeterminations of focal depths and fault plane solutions of earthquakes in the vicinity of the heat flow profiles indicate thrust faulting on a plane dipping 17°±2°. Calculated values of heat flow for a two‐dimensional analytical approximation to conduction through the upper plate, diffusion into the downgoing slab, and advection by that slab are consistent with either a uniform stress of ∼40±17 MPa along the thrust fault or stress increasing linearly at ∼0.5±0.2 MPa km−1 with distance from the trench axis. The comparable scatter in the heat flow measurements about those calculated from these simple stress distributions does not show one to be a better approximation than the other.

The Rapuhia Scarp (northern Hikurangi Plateau) — its nature and subduction effects on the Kermadec Trench

Multibeam bathymetric data, seismic reflection and magnetic data are used to examine the nature of the 1-km-high Rapuhia Scarp and the tectonic effects of its subduction on the Kermadec margin. The Rapuhia Scarp is the boundary between the Hikurangi Plateau and >85Ma oceanic crust. Its intersection with the Kermadec Trench moves south at 4cm/yr. Magnetic modelling indicates that the northern Hikurangi Plateau may be formed of 5km of relatively strongly-magnetised extrusive and intrusive volcanics rocks. Uplift of the Rapuhia Scarp near the trench is consistent with vertical motion on a tear-fault in the subducting plate.The coincident intersection of the subducting Rapuhia Scarp with a NNW-trending volcanic ridge (Intersection Ridge) and the Kermadec Trench results in an abrupt 1.5km increase in the trench depth and inner margin topography. The Rapuhia Scarp boundary sweeps south along the margin. Slumping and subsidence of the inner margin resulting from this transition occur on a variety of scales, from 5-km-wide, 1-km-high arcuate frontal erosion slumps to regional subsidence of up to 4km across the 50–100-km-wide mid-upper inner trench margin as it adjusts to the new subduction geometry. The pattern of inner margin subsidence involving the entire forearc is broadly similar to that observed 1200km further north in the wake of subduction of the Louisville Ridge seamount chain.Subsidence of the inner trench margin results in the loss of approximately 400km3 per kilometre of margin as the subducting Hikurangi Plateau support is removed. We estimate however only one fifth of the above subsidence is frontal or basal erosion highlighting the risks of estimating tectonic erosion from margin subsidence if the cause of subsidence is mis-identified. The remaining 320km2 of subsidence is due to the steeper dip of the subducting plate associated with subduction of the comparatively dense >85Ma oceanic crust.

Continent-Derived Vitric Mud and Mafic-Arc Rocks in Deep Kermadec Trench Diamictons

ABSTRACT Bimodal volcanogenic sediments dredged from depths greater than 7 km on the Kermadec Trench inner slope at latitude 31°S comprise diamictons of mudstone, basalt, and dolerite clasts enclosed in noncalcareous mud. The mud and mudstones are medium-to-high-K rhyolitic vitric mud of continental-arc derivation that has been transported 900 km from New Zealand. All contain minor shards of Kermadec arc low-K rhyolitic glass. All mudstones appear to be Quaternary, but one basalt clast gave a latest Miocene K-Ar age (7.84 ± 0.64 Ma). There have been at least three periods of deposition of mud, and two or three episodes of emplacement of diamict. The present trench slope is 10-24°, and diamicts are interpreted to be deposits of debris flows of varying fluidity. Graded mafic volcanic sand beds are interpreted to be deposits of turbidity currents or sandy debris flows. The degree of consolidation of mudstone clasts, along with the presence of compaction faults, suggests burial followed by re-exhumation, implying that redeposition processes may be deep-reaching. Shallow-water fossils and calcareous mudstone clasts originating above the carbonate compensation depth (c. 4.5 km), plus clasts of basalt and dolerite sourced locally on the crest of the Kermadec arc, reached the lower trench slope in gravity flows that bypassed the mid-slope terrace. There was extensive mixing of products from various eruptions and sources during multiple episodes of transportation, and during deposition and redeposition on the lower trench slope. A range of agencies and routes is available to carry New Zealand (Taupo Volcanic Zone)-derived vitric mud to the Kermadec Trench--both low-level and high-level winds plus the north-flowing Deep Western Boundary Current (DWBC) of Antarctic Bottom Water. The latter washes the Raukumara Plain forearc basin and the western trench slope to depths as shallow as 2-3 km. We infer that ashfall tephra carried over the Raukumara Plain by the prevailing low-level southwesterly wind is the principal source of the muds found in the trench. The relative purity of the vitric mud in the trench might be explained by the tendency of rhyolitic material to dominate sediment dispersal systems in the aftermath of very large caldera eruptions. Confinement of the mud to the lower trench slope is probably caused by the tendency of DWBC surges of >20 cm/s to veer 10°-30° clockwise of the trench axis. The continent-derived volcanic ash on the subducting slab may influence oceanic arc magma composition.

Morphology and history of the Kermadec trench–arc–backarc basin–remnant arc system at 30 to 32°S: geophysical profile, microfossil and K–Ar data

Knowledge of the time span of arc activity, essential for correct tectonic reconstructions, has been lacking for the Kermadec arc system, but is supplied in this paper through study of microfossils contained in dredge samples, and K–Ar ages on dredged basalt clasts. The Kermadec system at south latitudes 30 to 32° in the southwest Pacific comprises from west to east the Colville Ridge (remnant arc), Havre Trough (backarc basin), Kermadec Ridge (active arc) and Kermadec Trench (site of west-dipping subduction of Pacific plate lithosphere beneath the Australian plate). Data are presented from two traverses (dredge, magnetic, single-channel seismic) across the whole system. An important transverse tectonic boundary, the 32°S Boundary, lies between the two traverse lines and separates distinct northern (32–25°S) and southern (32–36°S) sectors. The northern sector is shallower and well sedimented with broad ridges and a diffuse backarc basin. The southern sector is deeper with narrow ridges and steep escarpments facing inwards to a little-sedimented, rifted backarc basin. The Kermadec Ridge slopes smoothly trenchward to a mid-slope terrace (forearc basin) with minor sediment fill at 5–6 km water depth. A steeper (10–24°) and more rugged lower trench slope is mantled with New Zealand-sourced rhyolitic vitric mud diamictons containing locally derived mafic volcanic clasts; one clast is of late Miocene age (K–Ar age 7.84 Ma). The arc (Kermadec Ridge) is capped by active volcanoes; very young K–Ar ages (<150 ka) from basalts dredged from the 40-km wide transition zone between the Kermadec Ridge and the Havre Trough, north of the 32°S Boundary, support the concept of arc retreat to the southeast. South of the 32°S Boundary the Kermadec Ridge deepens and narrows, makes a left-step of 10 km, and presents a 2.8-km scarp face to the Havre Trough; K–Ar ages from dredged basalt clasts range from 1.25 to 2.04 Ma and indicate exposure of older arc rocks. The deepest and most sedimented portion of the backarc basin lies on the western side, both north and south of the 32°S Boundary, and the centre of basin opening is inferred to lie on the eastern side. There is foundered arc material and former hydrothermal activity in the centre of the rifted basin. The remnant arc (Colville Ridge) has subsided approximately 700 m. On the northern profile it is broad and has a perched sedimentary basin at 1 km water depth on the eastern flank. On the southern profile it is narrow and presents a 2.5-km scarp face to the Havre Trough. Arc substrate rocks are exposed on both ridges. Derived microfossils, sedimentary clasts with fossil-based depositional ages in the late Miocene and Pliocene, clasts of hypabyssal and plutonic rocks, and dated basalt clasts as old as 2 Ma, together indicate continuing collapse and surficial reworking on both ridges. Derived microfossils establish that the Colville and Kermadec Ridges have existed (initially as one ridge) since at least the earliest Miocene; by inference, ridge volcanism has been active since the same time, about 25 Ma. Rare older microfossils may indicate earlier existence of the ridge. A 25-Ma inception of arc volcanism is synchronous with contiguous arc sectors to the north (Tonga) and south (New Zealand). We have no new data on the age of the age of the Havre Trough, which is generally considered to be less than 5 Ma. A seamount entering the Kermadec Trench is early Eocene or older, and a ridge/seamount in the South Fiji Basin (west of the Colville Ridge) is middle Miocene or older.

Discovery of hydrothermal sulfide mineralization from southern Kermadec arc volcanoes (SW Pacific)

We report the discovery of hydrothermal sulfide mineralization within the summit caldera of two southern Kermadec frontal arc volcanoes (Brothers and Rumble II West) of the ∼1200 km long, Kermadec–Havre arc–back-arc system. The Brothers and Rumble II (West) calderas, both with re-surgent domes and comprising mostly dacite and basalt–andesite host rocks, respectively, rise to water depths of ≤1500 m. These calderas comprise mostly effusive lavas and volcaniclastic deposits, including talus breccias along their inner caldera walls. These two southern Kermadec hydrothermal sites have similar geological settings and mineralogy to other arc front vent sites. Two principle ore assemblages are identified, comprising: (i) chalcopyrite–pyrite–barite, and (ii) sphalerite–marcasite–barite ± pyrite. The most prevalent ore texture consists of massive chalcopyrite + pyrite ± sphalerite, commonly overprinting barite. Early marcasite cores within large, euhedral pyrites, and trails of pyrite inclusions within chalcopyrite are indicative of recrystallization. The ore mineralogy is consistent with postulated venting temperatures of ≥300°C. Sulfide geochemistry is consistent with the ore mineralogy with concentrations of Cu, Fe, and Zn up to 15.3, 19.1, and 18.8 wt%, respectively. The sulfide geochemistry is distinct, however, from comparable western Pacific vent sites, and may reflect source heterogeneity due to proximity to continental New Zealand and sediment recycling along the southern Kermadec trench–arc system. The recovery of two partial caridean vent shrimps suggests that present-day hydrothermal venting is occurring within the Brothers caldera.

The dammed Hikurangi Trough: a channel‐fed trench blocked by subducting seamounts and their wake avalanches (New Zealand–France GeodyNZ Project)

The Hikurangi Trough, off eastern New Zealand, is at the southern end of the Tonga–Kermadec–Hikurangi subduction system, which merges into a zone of intracontinental transform. The trough is mainly a turbidite‐filled structural trench but includes an oblique‐collision, foredeep basin. Its northern end has a sharp boundary with the deep, sediment‐starved, Kermadec Trench.Swath‐mapping, sampling and seismic surveys show modern sediment input is mainly via Kaikoura Canyon, which intercepts littoral drift at the southern, intracontinental apex of the trough, with minor input from seep gullies. Glacial age input was via many canyons and about an order of magnitude greater. Beyond a narrow, gravelly, intracontinental foredeep, the southern trench‐basin is characterized by a channel meandering around the seaward edge of mainly Plio‐Pleistocene, overbank deposits that reach 5 km in thickness. The aggrading channel has sandy turbidites, but low‐backscatter, and long‐wavelength bedforms indicating thick flows. Levées on both sides are capped by tangentially aligned mudwaves on the outsides of bends, indicating centrifugal overflow from heads of dense, fast‐moving, autosuspension flows. The higher, left‐bank levée also has levée‐parallel mudwaves, indicating Coriolis and/or boundary currents effects on dilute flows or tail plumes.In the northern trough, basin‐fill is generally less than 2 km thick and includes widespread overbank turbidites, a massive, blocky, avalanche deposit and an extensive, buried, debris flow deposit. A line of low seamounts on the subducting plate acts as a dam preventing modern turbidity currents from reaching the Kermadec Trench. Major margin collapse probably occurred in the wake of a large subducting seamount; this seamount and its wake debris flow probably dammed the trench from 2 Ma to 0.5 Ma. Before this, similar dams may have re‐routed turbidity currents across the plateau.