Havre Trough Research Articles

Geodynamic Evolution of the Lau Basin

AbstractThe formation of Lau Basin records an extreme event of plate tectonics, with the associated Tonga trench exhibiting the fastest retreat in the world (16 cm/yr). Yet paleogeographic reconstructions suggest that seafloor spreading in the Lau Basin only initiated around 6 Ma. This kinematics is difficult to reconcile with our present understanding of how subduction drives plate motions. Using numerical models, we propose that eastward migration of the Lau Ridge concurrent with trench retreat explains both the narrow width and thickened crust of the Lau Basin. To match the slab geometry and basin width along the Tonga‐Kermadec trench, our models suggest that fast trench retreat rate of 16 cm/yr might start ~15 Ma. Tonga slab rollback induced vigorous mantle flow underneath the South Fiji Basin which is driving the extension and thinning of the basin and contributing to its observed deeper bathymetry compared to neighboring basins.

Hadal zones of the Southwest Pacific and east Indian oceans

The hadal zone (water depths > 6000 m) are unlike the overlying shallower marine regions (bathyal and abyssal) as it does not follow a continuum from the continental shelves to abyssal plains, but rather exhibits a globally disjunct series of discrete deep-sea habitats confined within geomorphological features. From an ecological perspective, hadal communities are often endemic to individual or adjacent features and are partitioned and isolated by geomorphological structures. To examine the size, shape, depth and degree of isolation of features where hadal fauna inhabit, this study explores the broad seafloor geomorphology, and distinctly partitioned hadal areas, across the Southwest Pacific and East Indian oceans using global bathymetric datasets. This research revealed the area occupied by hadal depths to be 716,915 km2 of which 58% are accounted for by trenches, 37% in basins and troughs, and 5% fracture zones. The largest feature in terms of area > 6000 m depth is the Wharton Basin with 218,030 km2 spanning 376 discrete areas. The largest continuous hadal habitats were the Kermadec and Tonga trenches at 145,103 and 111,951 km2 respectively, whereas features such as the Java Trench comprise two hadal components partitioned by a bathymetric high. Conversely, no physical barrier exists between the New Britain and Bougainville trenches thus any literature pertaining to hadal species or habitats from these trenches can be merged. This study highlights that the hadal zone mainly comprises two main geomorphological features (trenches and basins) that differ in size, depth and complexity. Hadal basins cover vast, generally shallower areas, comparable to abyssal plains, whereas trenches, despite a lesser footprint, represent greater depth ranges and complexity. As such, sampling designs and interpretation of ecological data must differ and hadal basins likely play an increasingly important role in understanding ecological shifts from abyssal to hadal ecosystems.

Dynamical Modeling of Fault Slip Rates at the New Zealand Plate Boundary Indicates Fault Weakness

AbstractWe construct a thin‐sheet dynamical model of the New Zealand plate boundary that includes faults. Our model fits fault slip rates, style of distributed deformation, and is constrained by relative plate boundary motion. We assume a pseudo‐plastic rheology and achieve a best fit to slip rate observations with a deviatoric stress magnitude of 20 MPa. Modeled local forces are significant at Puysegur and Hikurangi subduction zones, and smaller forces are related to mantle downwelling beneath South Island and Havre Trough mantle upwelling. Modeled tractions on faults are mostly 5–20 MPa, similar to or slightly smaller than stress magnitudes adjacent to faults. Modeled shear tractions are generally 2–10 MPa, comparable to stress drops during earthquakes. Modeled stress orientations and fault dips suggest that many faults are not optimally oriented for their style of faulting. Notably small traction magnitudes of <5 MPa and shear tractions of <0.5 MPa are modeled for faults in the central North Island Dextral Fault Belt (NIDFB), which we infer to be very poorly oriented. Friction coefficients on faults (ratio of shear stress to effective normal stress) are in the range 0.1–0.3 for major crustal faults such as the Alpine Fault and Marlborough faults, but subduction zones and the NIDFB have values <0.1. We propose that low values of long‐term fault strength, shear stress resolved onto the fault, and overall magnitudes of deviatoric stress in the crust are a consequence of dynamic weakening of faults during fault slip.

Monitoring volcanic activity with distributed acoustic sensing using the Tongan seafloor telecommunications cable

AbstractThe devastation caused by the January 2022 eruption of Hunga Tonga-Hunga Ha’apai volcano (HTHH) in the Tongan archipelago reminded us of the importance of monitoring shallow-sea volcanic activity. Seismic observations are essential for such monitoring, but there were no operational seismic stations in Tonga at the time of the eruption. There are only a few islands near Tongan volcanoes, and installation and maintenance of seismic stations on remote islands are expensive. Seismic observations based on distributed acoustic sensing (DAS) using a seafloor cable may provide a more practical and economical solution. To investigate the potential of this approach, we made preliminary DAS observations for 1 week using the seafloor domestic broadband telecommunications cable in Tonga. DAS equipment was installed at the landing station of the seafloor cable at Nuku’alofa on Tongatapu, the main island of Tonga. To provide reference data, we installed several seismometers on Tongatapu. The DAS data we obtained showed high noise levels in areas of shallow coral reef, but noise levels decreased greatly in deeper water areas, indicating that DAS is suitable for seismic observations of the deep seafloor. We detected many local and regional earthquakes during our week of observation and determined 17 earthquake hypocenters by picking P- and S-wave arrival times from the DAS and onshore seismic data. Although most of these were tectonic events related to the subduction of the Pacific plate along the Tonga trench, several events were detected around the volcanic chain of the Tongan archipelago including one event beneath the HTHH crater, implying that activity at HTHH has continued since the 2022 eruption. The much lower cost of installation of DAS equipment compared to that for pop-up type ocean-bottom seismometers and the ability of DAS systems to monitor seismic activity in real-time make it an attractive option for monitoring the activity of HTHH and other volcanoes near seafloor cables in the Tongan archipelago. Graphical Abstract

Four distinct pulses of volcanism built the Melanesian Border Plateau: Implications for oceanic mid-plate superstructure formation

The ocean basins contain numerous volcanic ridges, seamounts and large igneous provinces (LIPs). Numerous studies have focused on the origin of seamount chains and LIPs but much less focus has been applied to understanding the genesis of large volcanic structures formed from a combination or series of volcanic drivers. Here we propose the term Oceanic Mid-plate Superstructures (OMS) to describe independent bathymetric swells or volcanic structures that are constructed through superimposing pulses of volcanism, over long time periods and from multiple sources. These sources can represent periods when the lithosphere drifted over different mantle plumes and/or experienced pulses of volcanism associated with shallow tectonic drivers (e.g. plate flexure; lithospheric extension). Here we focus on the Melanesian Border Plateau (MBP), one example of an OMS that has a complex and enigmatic origin.The MBP is a region of shallow Pacific lithosphere consisting of high volumes of volcanic guyots, ridges and seamounts that resides on the northern edge of the Vitiaz Lineament. Here we reconcile recently published constraints to build a comprehensive volcanic history of the MBP. The MBP was built through four distinct episodes: (1) Volcanism associated with the Louisville hotspot likely generating Robbie Ridge and some Cretaceous seamounts near the MBP. (2) Construction of oceanic islands and seamounts during the Eocene when the lithosphere passed over the Rurutu-Arago hotspot. (3) Reactivation of previous oceanic islands/seamounts and construction of new volcanos in the Miocene when the lithosphere passed over the Samoa hotspot. (4) Miocene to modern volcanism driven by lithospheric deformation and/or westward entrainment of enriched plume mantle due to toroidal mantle flow driven by the rollback of the Pacific plate beneath the Tonga trench. The combination of these processes is responsible for ∼222,000 km2 of intraplate volcanism in the MBP and indicates that this OMS was constructed from multiple volcanic drivers.

Geodetic Strain Rates for the 2022 Update of the New Zealand National Seismic Hazard Model

ABSTRACT Geodetic data in plate boundary zones reflect the accrual of tectonic strain and stress, which will ultimately be released in earthquakes, and so they can provide valuable insights into future seismic hazards. To incorporate geodetic measurements of contemporary deformation into the 2022 revision of the New Zealand National Seismic Hazard Model 2022 (NZ NSHM 2022), we derive a range of strain-rate models from published interseismic Global Navigation Satellite Systems velocities for New Zealand. We calculate the uncertainty in strain rate excluding strain from the Taupō rift–Havre trough and Hikurangi subduction zone, which are handled separately, and the corresponding moment rates. A high shear strain rate occurs along the Alpine fault and the North Island dextral fault belt, as well as the eastern coast of the North Island. Dilatation rates are primarily contractional in the South Island and less well constrained in the North Island. Total moment accumulation derived using Kostrov-type summation varies from 0.64 to 2.93×1019 N·m/yr depending on method and parameter choices. To account for both aleatory and epistemic uncertainty in the strain-rate results, we use four different methods for estimating strain rate and calculate various average models and uncertainty metrics. The maximum shear strain rate is similar across all methods, whereas the dilatation rate and overall strain rate style differ more significantly. Each method provides an estimate of its own uncertainty propagated from the data uncertainties, and variability between methods provides an additional estimate of epistemic uncertainty. Epistemic uncertainty in New Zealand tends to be higher than the aleatory uncertainty estimates provided by any single method, and epistemic uncertainty on dilatation rate exceeds the aleatory uncertainty nearly everywhere. These strain-rate models were provided to the NZ NSHM 2022 team and used to develop fault-slip deficit rate models and scaled seismicity rate models.

Trace elements in abyssal peridotite olivine record melting, thermal evolution, and melt refertilization in the oceanic upper mantle

Trace element concentrations in abyssal peridotite olivine provide insights into the formation and evolution of the oceanic lithosphere. We present olivine trace element compositions (Al, Ca, Ti, V, Cr, Mn, Co, Ni, Zn, Y, Yb) from abyssal peridotites to investigate partial melting, melt–rock interaction, and subsolidus cooling at mid-ocean ridges and intra-oceanic forearcs. We targeted 44 peridotites from fast (Hess Deep, East Pacific Rise) and ultraslow (Gakkel and Southwest Indian Ridges) spreading ridges and the Tonga trench, including 5 peridotites that contain melt veins. We found that the abundances of Ti, Mn, Co, and Zn increase, while Ni decreases in melt-veined samples relative to unveined samples, suggesting that these elements are useful tracers of melt infiltration. The abundances of Al, Ca, Cr, and V in olivine are temperature sensitive. Thermometers utilizing Al and Ca in olivine indicate temperatures of 650–1000 °C, with variations corresponding to the contrasting cooling rates the peridotites experienced in different tectonic environments. Finally, we demonstrate with a two-stage model that olivine Y and Yb abundances reflect both partial melting and subsolidus re-equilibration. Samples that record lower Al- and Ca-in-olivine temperatures experienced higher extents of diffusive Y and Yb loss during cooling. Altogether, we demonstrate that olivine trace elements document both high-temperature melting and melt–rock interaction events, as well as subsolidus cooling related to their exhumation and emplacement onto the seafloor. This makes them useful tools to study processes associated with seafloor spreading and mid-ocean ridge tectonics.

Estimating Plate Tectonic Forces at the New Zealand Plate Boundary Using Strain Rates Derived From a Kinematic Model of Fault Slip

AbstractWe construct a model of forces at the New Zealand plate boundary from dynamical thin‐sheet modeling. Stress magnitudes estimated are 10–50 MPa and effective viscosities are 0.5–5 × 1021 Pa s within actively deforming regions. Models that include only far‐field forces and forces from variations in topography and bathymetry can fit observations in most of the plate boundary, but basal tractions are required to fit extension in Havre Trough. For models that include nontopographic forces, we specify a rheology to acquire a unique solution for each of three rheologies: power‐law rheology with n = 3, power law with n = 5, and a pseudo‐plastic equal‐stress rheology. The inclusion of nontopographic forces allows these models to fit observations very well. We predict forces in Havre Trough equivalent to basal tractions of 7–10 MPa at 20‐km depth. Models with n = 3 and n = 5 require antiparallel forces on opposite sides of the plate boundary to drive deformation in South Island which would imply a plate boundary zone that is more localized at depth. The equal‐stress pseudo‐plastic model drives deformation with plate motion boundary conditions and localizes it with variations in effective viscosity. The effective rheology of South Island is most realistically modeled by an equal‐stress pseudo‐plastic rheology. The n = 3 or n = 5 power‐law rheology models require a highly localized boundary zone in the lower lithosphere, but a broader zone in the upper lithosphere.

On Tsunami Waves Induced by Atmospheric Pressure Shock Waves After the 2022 Hunga Tonga‐Hunga Ha'apai Volcano Eruption

AbstractEmploying a linear shallow water equation (LSWE) model in the spherical coordinates, this paper investigates the tsunami waves generated by the atmospheric pressure shock waves due to the explosion of the submarine volcano Hunga Tonga–Hunga Ha'apai on 15 January 2022. Using the selected 59 atmospheric pressure records in the Pacific Ocean, an empirical atmospheric pressure model is first constructed. Applying the atmospheric pressure model and realistic bathymetric data in the LSWE model, tsunami generation and propagation are simulated in the Pacific Ocean. The numerical results show clearly the co‐existence of the leading locked waves, propagating with the speed of the atmospheric pressure waves (∼1,100 km/hr), and the trailing free waves, propagating with long gravity ocean wave celerity (∼750 km/hr). During the event, tsunamis were reported by 41 Deep‐ocean Assessment and Reporting of Tsunamis (DART) buoys in the Pacific Ocean, which require corrections because of the occurrence of atmospheric pressure waves. The numerically simulated tsunami arrival time and the amplitudes of the wave crest and trough of the leading locked waves compare reasonably well with the corrected DART measurements. The comparisons for the trailing waves are less satisfactory, since free waves could also have been generated by other tsunami generation mechanisms, which have not been considered in the present model, and by the scattering of locked waves over changing bathymetry. In this regard, the numerical results show clearly that the deep Tonga trench (∼10 km) amplifies the trailing waves in the Southeast part of the Pacific Ocean via the Proudman resonance condition.

A Kinematic Model of Quaternary Fault Slip Rates and Distributed Deformation at the New Zealand Plate Boundary

AbstractWe construct a kinematic model of Quaternary long‐term deformation at the New Zealand plate boundary, including slip rates on major faults and strain rates between faults. We use an iterative method based on statistical and physical principles to find a velocity field that fits fault slip rate observations, has consistent off‐fault strain‐rate style, and is constrained by known plate motion. The kinematic model balances on‐fault and off‐fault deformation and provides improved estimates of fault slip rates and uncertainties in a statistically rigorous manner. We predict shortening rates of 45 ± 8, 34 ± 3, and 20 ± 3 mm/yr across the northern, central, and southern portions of the Hikurangi subduction margin, respectively; these rates are lower and better constrained by regional kinematics than previous estimates. In addition, the model predicts large strains adjacent to the Alpine Fault, indicating ∼9 mm/yr of plate motion in central South Island on faults with currently unknown slip rates. Differences between our long‐term velocity field and a contemporary velocity field arise mainly through interseismic locking on major faults. However, we suggest that differences with contemporary deformation in northeastern North Island are due to uncertainty in Havre Trough extension parameters and <5–10 mm/yr of missing dextral slip on unknown onshore faults, which implies seismic hazard is under‐predicted in that region.

Contrasting Roles of DOP as a Source of Phosphorus and Energy for Marine Diazotrophs

The oceanic dissolved organic phosphorus (DOP) pool is mainly composed of P-esters and, to a lesser extent, equally abundant phosphonate and P-anhydride molecules. In phosphate-limited ocean regions, diazotrophs are thought to rely on DOP compounds as an alternative source of phosphorus (P). While both P-esters and phosphonates effectively promote dinitrogen (N2) fixation, the role of P-anhydrides for diazotrophs is unknown. Here we explore the effect of P-anhydrides on N2 fixation at two stations with contrasting biogeochemical conditions: one located in the Tonga trench volcanic arc region (“volcano,” with low phosphate and high iron concentrations), and the other in the South Pacific Gyre (“gyre,” with moderate phosphate and low iron). We incubated surface seawater with AMP (P-ester), ATP (P-ester and P-anhydride), or 3polyP (P-anhydride) and determined cell-specific N2 fixation rates, nifH gene abundance, and transcription in Crocosphaera and Trichodesmium. Trichodesmium did not respond to any DOP compounds added, suggesting that they were not P-limited at the volcano station and were outcompeted by the low iron conditions at the gyre station. Conversely, Crocosphaera were numerous at both stations and their specific N2 fixation rates were stimulated by AMP at the volcano station and slightly by 3polyP at both stations. Heterotrophic bacteria responded to ATP and 3polyP additions similarly at both stations, despite the contrasting phosphate and iron availability. The use of 3polyP by Crocosphaera and heterotrophic bacteria at both low and moderate phosphate concentrations suggests that this compound, in addition to being a source of P, can be used to acquire energy for which both groups compete. P-anhydrides may thus leverage energy restrictions to diazotrophs in the future stratified and nutrient-impoverished ocean.

Distinguishing Volcanic Contributions to the Overlapping Samoan and Cook-Austral Hotspot Tracks

Abstract To deconvolve contributions from the four overlapping hotspots that form the “hotspot highway” on the Pacific plate—Samoa, Rarotonga, Arago-Rurutu, and Macdonald—we geochemically characterize and/or date (by the 40Ar/39Ar method) a suite of lavas sampled from the eastern region of the Samoan hotspot and the region “downstream” of the Samoan hotspot track. We find that Papatua seamount, located ~60 km south of the axis of the Samoan hotspot track, has lavas with both a HIMU (high μ = 238U/204Pb) composition (206Pb/204Pb = 20.0), previously linked to one of the Cook-Austral hotspots, and an enriched mantle I (EM1) composition, which we interpret to be rejuvenated and Samoan in origin. We show that these EM1 rejuvenated lavas at Papatua are geochemically similar to rejuvenated volcanism on Samoan volcanoes and suggest that flexural uplift, caused by tectonic forces associated with the nearby Tonga trench, triggered a new episode of melting of Samoan mantle material that had previously flattened and spread laterally along the base of the Pacific plate under Papatua, resulting in volcanism that capped the previous HIMU edifice. We argue that this process generated Samoan rejuvenated volcanism on the older Cook-Austral volcano of Papatua. We also study Waterwitch seamount, located ~820 km WNW of the Samoan hotspot, and provide an age (10.49 ± 0.09 Ma) that places it on the Samoan hotspot trend, showing that it is genetically Samoan and not related to the Cook-Austral hotspots as previously suggested. Consequently, with the possible exception of the HIMU stage of Papatua seamount, there are currently no known Arago-Rurutu plume-derived lava flows sampled along the swath of Pacific seafloor that stretches between Rose seamount (~25 Ma) and East Niulakita seamount (~45 Ma), located 1400 km to the west. The “missing” ~20-million-year segment of the Arago-Rurutu hotspot track may have been subducted into the northern Tonga trench, or perhaps was covered by subsequent volcanism from the overlapping Samoan hotspot, and has thus eluded sampling. Finally, we explore tectonic reactivation as a cause for anomalously young volcanism present within the western end of the Samoan hotspot track.

Back-Arc Spreading Centers and Superfast Subduction: The Case of the Northern Lau Basin (SW Pacific Ocean)

The Lau Basin is a back-arc region formed by the subduction of the Pacific plate below the Australian plate. We studied the regional morphology of the back-arc spreading centers of the Northern Lau basin, and we compared it to their relative spreading rates. We obtained a value of 60.2 mm/year along the Northwest Lau Spreading Centers based on magnetic data, improving on the spreading rate literature data. Furthermore, we carried out numerical models including visco-plastic rheologies and prescribed surface velocities, in an upper plate-fixed reference frame. Although our thermal model points to a high temperature only near the Tonga trench, the model of the second invariant of the strain rate shows active deformation in the mantle from the Tonga trench to ~800 km along the overriding plate. This explains the anomalous magmatic production along all the volcanic centers in the Northern Lau Back-Arc Basin.

Basalt Geochemistry and Mantle Flow During Early Backarc Basin Evolution: Havre Trough and Kermadec Arc, Southwest Pacific

AbstractThe Havre Trough (HT) backarc basin in the southwest Pacific is in the rifting stage of development. We distinguish five types of basalt there based on their amount and kind of slab component: backarc basalts (BAB) with little or no slab component, modified BAB with slight amounts, reararc (RA) with more, remnants of the preexisting arc (Colville Ridge horsts), and arc front volcanoes within the HT. Previous subarc mantle is quickly removed and replaced by more fertile mantle with less slab component. The ambient mantle is “Pacific” isotopically, and more enriched in Nb/Yb and Nd and Hf isotope ratios north of the Central Kermadec Discontinuity at 32°S than to the south. The contrast may reflect inheritance in the south of mantle that was depleted during spreading that formed the southern South Fiji Basin and a higher degree of melting because of a wetter slab‐derived flux. The slab component also differs along strike, more like a dry melt in the north and a supercritical fluid in the south. The mass fraction of slab component increases southward in the backarc as well as the arc front. RA volcanoes have the most slab component (1%–2%) and form indistinct ridges at high angles to, and <50 km behind, frontal volcanoes. Backarc basalts have less and occur throughout the basin. Slab components are distributed further into the backarc, and more irregularly, during the rifting than spreading stage of backarc basin development. The rifting stage is disorganized geochemically as well as spatially.

Hikurangi Plateau subduction a trigger for Vitiaz arc splitting and Havre Trough opening (southwestern Pacific)

AbstractSplitting of the Vitiaz arc formed the Tonga-Kermadec and Lau-Colville Ridges (southwestern Pacific Ocean), separated by the Lau Basin in the north and Havre Trough in the south. We present new trace element and Sr-Nd-Hf-Pb isotope geochemistry for the Kermadec and Colville Ridges extending ∼900 km north of New Zealand (36°S–28°S) in order to (1) compare the composition of the arc remnants with Quaternary Kermadec arc volcanism, (2) constrain spatial geochemical variations in the arc remnants, (3) evaluate the effect of Hikurangi igneous plateau subduction on the geochemistry of the older arc lavas, and (4) elucidate what may have caused arc splitting. Compared to the Kermadec Ridge, the Colville Ridge has higher more-incompatible to less-incompatible immobile element ratios and largely overlapping isotope ratios, consistent with an origin through lower degrees of melting of more enriched upper mantle in the Vitiaz rear arc. Between ca. 8 and 3 Ma, both halves of the arc (∼36°S–29°S) included a more enriched (EM1-type) composition (with lower 206Pb/204Pb and 207Pb/204Pb and higher Δ8/4 Pb [deviation of the measured 208Pb/204Pb ratio from a Northern Hemisphere basalt regression line] and 87Sr/86Sr) compared to older and younger arc lavas. High-Ti basalts from the Manihiki Plateau, once joined to the Hikurangi Plateau, could serve as the enriched Vitiaz arc end member. We propose that the enriched plateau signature, seen only in the isotope ratios of mobile elements, was transported by hydrous fluids from the western margin of the subducting Hikurangi Plateau or a Hikurangi Plateau fragment into the overlying mantle wedge. Our results are consistent with plateau subduction triggering arc splitting and backarc opening.

On the seawater density in gravity calculations

The bathymetry from the continental margin off the Baja California Peninsula, and across the Tonga Trench, are used to quantify the differences in gravitational attraction, arising by considering a constant density ρw = 1030 kg/m3, typically used in marine geophysical studies, versus two global zonal models that describe the seawater density as a function of depth and geodetic latitude. With precision better than 1 μGal, I found that the maximum difference across the continental margin of the Baja California Peninsula is less than 1.5 mGal in the Cedros Deep away from the Continental Rise, while it is less than 0.05 mGal on the Continental Shelf. The maximum difference across the Tonga trench is about 5.3 mGal. Considering that the overall accuracy achieved in typical marine or satellite gravity surveys is at best of 1 mGal, the calculation of the gravity effect due to the seawater layer by either one of the two zonal models may reduce the error budget in stripping-off the gravity effect in deep waters.

GMT BASED COMPARATIVE ANALYSIS AND GEOMORPHOLOGICAL MAPPING OF THE KERMADEC AND TONGA TRENCHES, SOUTHWEST PACIFIC OCEAN

Current study is focused on the GMT based modelling of the two hadal trenches located in southwest Pacific Ocean, eastwards from Australia: Tonga and Kermadec. Due to its inaccessible location, the seafloor of the deep-sea trench can only be visualized using remote sensing tools and advanced algorithms of data analysis. The importance of the developing and technical improving of the innovative methods in cartographic data processing is indisputable. Automatization in data analysis has been significantly increased over the past years. However, using programming and scripting in cartography still remains lower comparing to the use of the traditional GIS. Therefore, developing GMT-based methods for the geomorphological data processing is crucial for better understanding the landforms of the seafloor. Methodology includes application of the GMT scripting toolset for the automated digitizing of the profiles crossing the trenches in perpendicular direction. A sequence of the GMT codes enabled to visualize raster and vector data, perform geomorphological modelling, descriptive statistical data analysis and quantitative comparison of the two trenches. Using GMT, the bathymetric sample data of the Kermadec and Tonga trenches were modeled, analyzed and compared. The results show deeper bathymetry and more seafloor roughness for the Tonga. Comparing to Kermadec, Tonga Trench has steeper gradient of the profiles. The seafloor geomorphology is strongly affected by a variety of factors that shape actual form of both trenches. The experimental methodology is fully based on the GMT scripting with presented and explained codes.

Early evolution of a young back-arc basin in the Havre Trough

Back-arc basins are found at convergent plate boundaries. Nevertheless, they are zones of significant crustal extension that show volcanic and hydrothermal processes somewhat similar to those of mid-ocean ridges. Accepted models imply the initial rifting and thinning of a pre-existing volcanic arc until seafloor spreading gradually develops over timescales of a few million years. The Havre Trough northeast of New Zealand is a unique place on Earth where the early stages of back-arc basin formation are well displayed in the recent geological record. Here we present evidence that, in this region, rifting of the original volcanic arc occurred in a very narrow area about 10–15 km wide, which could only accommodate minimal stretching for a very short time before mass balance required oceanic crustal accretion. An initial burst of seafloor spreading started around 5.5–5.0 million years ago and concluded abruptly about 3.0–2.5 million years ago, after which arc magmatism dominated the crustal accretion. The sudden transition between these different tectonomagmatic regimes is linked to trench rollback promoted by gradual sinking of the subducting lithosphere, which could have diverted the arc flux outside the region of seafloor spreading and induced the vertical realignment of surface volcanism with the source of arc melts at depth. The Havre Tough back-arc basin, New Zealand, formed rapidly and in two phases: initial, limited seafloor spreading was followed by a transition to arc magmatism, as shown by geophysical data and modelling.

Deep circulation in the Lau Basin and Havre Trough of the western South Pacific Ocean from floats and hydrography

A system of meridional ridges in the western South Pacific Ocean frame the Lau Basin and Havre Trough, and form a barrier to direct communication between the far western South Pacific basins and the interior South Pacific Ocean. The eastern side of this system comprises the Tonga and Kermadec Ridges, the location of the main deep western boundary current entering the Pacific Ocean. Observations from floats released in the Lau Basin as part of the RIDGE2000 program suggested the presence of a western boundary current along the Lau Ridge exiting into the North Fiji Basin. Those observations, together with Argo sub-surface float data and repeat hydrographic sections, confirm and expand the boundary current observations along the Lau Ridge throughout the Lau Basin and into the Havre Trough, along the Colville Ridge. The observations also reveal two previously unrecognized westward flowing jets bisecting the Lau Basin and Havre Trough. Using an extension to the classic Stommel-Arons abyssal circulation model, the predicted strength and location of these boundary currents and their bifurcation is compared with the float observations. The model provides a simplified view of the dynamics controlling the boundary current structure in the deep basins. A comparison of transport within the western boundary current derived from float data, hydrographic sections, and the idealized analytical model indicates that roughly 4 Sv (below 1,000 db) is transported northward through the Lau Basin, exiting into the North Fiji Basin.

Ar-Ar age constraints on the timing of Havre Trough opening and magmatism

ABSTRACTThe age and style of opening of the Havre Trough back-arc system is uncertain due to a lack of geochronologic constraints for the region. 40Ar/39Ar dating of 19 volcanic rocks from across the southern Havre Trough and Kermadec Arc was conducted in three laboratories to provide age constraints on the system. The results are integrated and interpreted as suggesting that this subduction system is young (<2 Ma) and coeval with opening of the continental Taupo Volcanic Zone of New Zealand. Arc magmatism was broadly concurrent across the breadth of the Havre Trough.