Lau Ridge 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.

Significant Impact of Hydrothermalism on the Biogeochemical Signature of Sinking and Sedimented Particles in the Lau Basin

AbstractIron (Fe) is an essential micronutrient for diazotrophs, which are abundant in the Western Tropical South Pacific Ocean (WTSP). Their success depends on the numerous trace metals, particularly Fe, released from shallow hydrothermal vents along the Tonga Arc. This study aimed to explore the spatio‐temporal impact of hydrothermal fluids on particulate trace metal concentrations and biological activity. To identify the composition of sinking particles across a wide area of the WTSP, we deployed sediment traps at various depths, both close and further west of the Tonga Arc. Seafloor sediments were cored at these deployment sites, including at a remote location in the South Pacific Gyre. The sinking particles were composed of a large amount of biological material (up to 88 mg d−1), indicative of the high productivity of the region. A significant portion of this material (∼21 ± 12 wt.%) was lithogenic of hydrothermal origin, as revealed through Al‐Fe‐Mn tracing. The sinking material showed similar patterns between lithogenic and biogenic fractions, indicating that hydrothermal input within the photic layer triggered surface production. A hydrothermal fingerprint was suggested in the sediments due to the high sedimentation rates (>47 cm kyr−1) and the presence of large, heterogeneous, metal‐rich particles. The presence of nearby active deep hydrothermal sources was suspected near the Lau Ridge due to the large particle size (1–976 μm) and the significant excess of Fe and Mn (2–20 wt.%). Overall, this study revealed that hydrothermal sources have a significant influence on the biogeochemical signature of particles in the region.

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.

Crustal and lithospheric structure of inactive volcanic arc terrains in Fiji

The crustal and lithospheric velocity structure of the Fiji area may place important constraints on the evolution of island arcs in the region. Here, we obtain receiver functions from one permanent seismic station and three temporary networks on the Fiji Platform, the Lau Ridge, and other small islands to develop point estimates of crustal thickness under the networks. We obtain preliminary estimates of crustal thickness and Poisson's ratio from H-κ stacking of the receiver functions. We then perform an inversion for shear-wave velocity structure using a joint inversion of receiver functions and Rayleigh wave dispersion curves from ambient seismic noise and teleseismic events. The joint inversion results show an average crustal thickness of ~30 km beneath Viti Levu, the largest Fiji island, suggesting an extensive period of arc crustal formation and thickening. In contrast, we find shallower Moho depths averaging ~22 km beneath the smaller Fiji islands and the Lau Ridge. The limited thickness of the high velocity lid obtained by inversion indicates that the lithosphere is thinned beneath the marginal Fiji islands, which may be due to the early rifting event forming the Lau Basin together with the crustal thinning of this area as well, and to thermal lithospheric erosion processes associated with adjacent back-arc spreading. A relatively slow velocity middle crustal layer, roughly of 11–16 km thickness, exists throughout Fiji, suggesting that remnant arcs contain significant volumes of felsic crustal rocks necessary for continental crust formation.

Seismic evidence of the lithosphere-asthenosphere boundary beneath the Tonga area, southwestern Pacific

We study the lithosphere-asthenosphere boundary (LAB) in the Tonga subduction zone using four deep focal events recorded by the Capital Seismic Network (CSN) of China and the Northeast China Extended Seismic Array (NECESSArray). The existence and depths of the LAB beneath the Lau Ridge are clearly revealed by the linear slant stacking of observed sP precursors (sdP). As illustrated by the reflected points of robust sdP phases from LAB, the depth of LAB in the northern Lau Ridge is about 63km with a range of 63–64km, in the northwest is about 77km with a range of 76–78km and in the south is about 72km. The uncertainty of determined depths is no more than 4km given the effects of the picking errors of arrival times and velocity heterogeneities. Our results suggest that the oceanic lithosphere of the Lau Ridge is thinner in the northern part than in the northwestern and the southern parts. According to the previous geodynamic and petrological studies, the oceanic lithospheric thinning in the northern Lau Ridge should be related to the strong erosion of the active mantle convection in the back-arc mantle wedge with the enrichment of the volatiles and melts.

Upper mantle structure of the Tonga‐Lau‐Fiji region from Rayleigh wave tomography

AbstractWe investigate the upper mantle seismic structure in the Tonga‐Lau‐Fiji region by jointly fitting the phase velocities of Rayleigh waves from ambient‐noise and two‐plane‐wave tomography. The results suggest a wide low‐velocity zone beneath the Lau Basin, with a minimum SV‐velocity of about 3.7 ± 0.1 km/s, indicating upwelling hot asthenosphere with extensive partial melting. The variations of velocity anomalies along the Central and Eastern Lau Spreading Centers suggest varying mantle porosity filled with melt. In the north where the spreading centers are distant from the Tonga slab, the inferred melting commences at about 70 km depth, and forms an inclined zone in the mantle, dipping to the west away from the arc. This pattern suggests a passive decompression melting process supplied by the Australian plate mantle from the west. In the south, as the supply from the Australian mantle is impeded by the Lau Ridge lithosphere, flux melting controlled by water from the nearby slab dominates in the back‐arc. This source change results in the rapid transition in geochemistry and axial morphology along the spreading centers. The remnant Lau Ridge and the Fiji Plateau are characterized by a 60–80 km thick lithosphere underlain by a low‐velocity asthenosphere. Our results suggest the removal of the lithosphere of the northeastern Fiji Plateau‐Lau Ridge beneath the active Taveuni Volcano. Azimuthal anisotropy shows that the mantle flow direction rotates from trench‐perpendicular beneath Fiji to spreading‐perpendicular beneath the Lau Basin, which provides evidence for the southward flow of the mantle wedge and the Samoan plume.

Sourcing of Miocene accretionary lapilli on ‘Eua, Tonga; atypical dispersal distances and tectonic implications for the central Tonga Ridge

ABSTRACTVolcaniclastics hosting accretionary lapilli on the Tonga Ridge were sourced from the remnant Lau Ridge, prior to Lau back-arc basin opening. For the ‘Eua occurrences, an atypical dispersal distance of not less than 70 km is estimated, partly arising from the anomalous easterly position of ‘Eua. Dispersal within ocean-surface pyroclastic density currents is supported but strike-slip movement in a fault zone south of ‘Eua, post-middle Miocene but pre-ridge-splitting, can account for part of the dispersal distance by vertical axis block rotation, a tectonic process common on the southern Tonga–Kermadec–Hikurangi trend. In this model, the volcano which sourced the ‘Eua tephra was on a subjacent block, rather than the block which hosts ‘Eua. After deposition but prior to the opening of the Lau Basin, the accretionary lapilli on ‘Eua became displaced by block rotation c. 40 km towards the Tonga trench and away from source.

Oligocene-Miocene spreading history of the northern South Fiji Basin and implications for the evolution of the New Zealand plate boundary

A tectonic model of the evolution of the northern half of the South Fiji Basin, including the Minerva Triple Junction and Cook Fracture Zone, is developed from regional gravity, multibeam bathymetry, and a new interpretation of magnetic anomalies pinned to radiometric dates of oceanic crust in the basin. The geometry and age of a portion of the Minerva Triple Junction and the Cook-Minerva spreading center (the connection from the triple junction to the Cook Fracture Zone, which accommodated coeval opening of the Norfolk Basin), are resolved with multibeam bathymetry and magnetics. The South Fiji Basin opened from about 34 to 15 Ma in an anticlockwise sweep about an Euler pole located at the northern end of the present Lau Ridge. This rotation and a rigidly straight southeastward motion of the Three Kings Ridge were accommodated by the configuration of the triple junction changing from ridge-fault-fault to ridge-ridge-fault to ridge-ridge-ridge. During this evolution the southeastern arm of the system, the Julia Fracture Zone, underwent several transformations and the Cook-Minerva spreading center experienced repeated ridge jumps. The kinematics of the northern South Fiji Basin dictate, to a large extent, the evolution of the southern South Fiji Basin and the Norfolk Basin. This in turn leads to the interpretation of a complex trench-trench-double transform fault framework at the northern New Zealand margin, which explains most aspects of the geology, structure, and arc volcanic history of the margin and provides a radical new setting for the origin of the Northland Allochthon.

Seismic structure beneath the Tonga arc and Lau back‐arc basin determined from joint Vp, Vp/Vs tomography

The Tonga arc and associated Lau basin exhibit many geologically important processes that link subduction and mantle flow with plate separation and crustal production. We create seismic tomograms of the Tonga‐Lau region by jointly inverting for Vp and Vp/Vs structure using data from the LABATTS ocean bottom seismograph experiment and several island deployments to better constrain dynamic processes in the mantle wedge. Jointly using P and S data can help distinguish between the various mechanisms responsible for seismic velocity anomalies such as temperature and the presence of melt and/or volatiles. Because high attenuation in the wedge limits the S wave data set, we focus on 2‐D inversions beneath the linear OBS array where resolution is best and also parameterize the solution in terms of the Vp/Vs ratio. As expected, the subducting slab has fast Vp and Vs and a low Vp/Vs ratio, consistent with the cold downgoing plate. The Central Lau Spreading Center (CLSC) exhibits stronger anomalies in Vp/Vs than in Vp, with the anomalies larger than would be predicted purely by temperature variations. The CLSC anomaly extends >100 km to the west of the axis, suggesting a broad region of melt production driven by passive upwelling from plate separation rather than active upwelling mechanisms. The anomaly is asymmetric about the axis, suggesting that slab‐induced corner flow possibly influences mantle dynamics several hundred kilometers away from the arc. There is a strong anomaly beneath the volcanic arc that gradually deepens as it trends toward the back arc, likely outlining a hydrated region of melt production that feeds the volcanic front. Hydration possibly continues throughout the wedge to at least 400 km depth. The Lau ridge exhibits a thicker lithosphere relative to the rest of the Basin, while the Fiji platform likely has a thinner lithosphere than the Lau Ridge from more recent extension. There is also a reasonable likelihood of a small degree of partial melt in the uppermost mantle beneath the platform.

Tonga Ridge and Lau Basin crustal structure from seismic refraction data

The crustal structure across the Tonga‐Lau arc‐back arc system from the Lau Ridge to the Pacific Plate (178°–170°W, 18°19°S) is modeled, using data from an 840‐km‐long air gun refraction line over 19 ocean bottom seismometers and one land station. The data reveal that the Pacific Plate crust is 5.5 km thick, with a velocity structure similar to that found at the present‐day East Pacific Rise (EPR). Beneath Tonga Ridge, an intermediate velocity layer (6–7 km/s) is up to 7.5 km thick and has a velocity‐depth distribution similar to andesitic rocks found in continental crust. The crust is abnormally thin (4 km) at the boundary between the Tonga Ridge and the Lau Basin. At the east end of Lau Basin, the crust is 5.5–6.5 km thick and resembles crust formed at the EPR except for a thicker sheeted‐dike section (2–3 km) and thinner lower crust (2 km). The Lau Basin crust thickens to 7–8 km near the Central Lau Spreading Center (CLSC), mostly through thickening of the lower crust. The crust thickens again to 8.5–9.5 km at 50 km west of the CLSC, mostly through thickening of the midcrust. In the thick westernmost section, the crustal structure is uniform even though one part of this section formed through extension of arc‐type crust while the rest was created at an oceanic spreading center. The relative homogeneity of these rocks suggests that their petrology may be dominated by postemplacement magmatic infilling from a mantle source west of the spreading center.

Seismic attenuation tomography of the Tonga‐Fiji region using phase pair methods

The anelastic structure of the region surrounding the Tonga slab and Lau back arc spreading center in the southwest Pacific is studied using data from 12 broadband island stations and 30 ocean bottom seismographs. Two differential attenuation methods determine δt* over the frequency band 0.1 to 3.5 Hz for earthquakes in the Tonga slab. The S‐P method measures the difference in spectral decay between P and S waves arriving at the same station. The P‐P method measures the difference in spectral decay for P waves with different paths through the upper mantle. Eight hundred sixty phase pairs are used to invert for two‐dimensional 1/Qα structure using a nonnegative least squares algorithm. A grid search method determines the Qα/Qβ ratio most compatible with both the S‐P and P‐P differential measurements. The highest attenuation (Qα = 90) is found within the upper 100 km beneath the active portions of the Lau Basin extending westward to the Lau Ridge. These regions probably delineate the source region for the back arc spreading center magmas, expected to be within the upper 100 km based on petrological considerations. The high attenuation regions also correlate well with zones of low P wave velocity determined by regional velocity tomography. Somewhat lower attenuation is found beneath the Fiji Plateau than beneath the Lau Basin. The entire back arc is characterized by a gradual decrease in attenuation to a depth of 300 to 400 km. The slab is imaged as a region of low attenuation (Qα > 900) material. A Qα/Qβ ratio of 1.75 provides the best fit between the S‐P and P‐P data sets upon inversion. Spectral stacking shows no frequency dependence within the frequency band analyzed.

Magnitude and timing of New Hebrides Arc rotation: Paleomagnetic evidence from Nendo, Solomon Islands

Paleomagnetic results from Nendo record a total rotation of the northwestern corner of the New Hebrides Arc of 52° ± 12° clockwise. Reversed magnetic polarity and a nannofossil assemblage constrain the age of the Nendo pole to 4.29 ‐ 3.58 Ma. Paleomagnetic data from Malekula, farther south in the same arc, imply a rotation of only about 28° since the late Miocene, probably because these poles have been affected by rotation of Malekula in response to collision of the arc with the D'Entrecasteaux Zone aseismic ridge. A new late Miocene reconstruction of the New Hebrides and Vitiaz arcs based on the paleomagnetic evidence requires a rotation of 39° around the point of intersection of the two arcs. The same rotation restores both the northwestern and southeastern segments of the New Hebrides Arc, although rotation of the southeastern segment commenced earlier. A series of sequential movements of segments of the arc, occurring at different times but acting to keep the arc roughly collinear, may be driven by a mechanism which minimizes deformation of the subducted plate. Propagation of spreading into the northwestern corner of the North Fiji Basin after 4.3 Ma is implied by the paleomagnetic results, and is consistent with spreading anomaly patterns in the basin. When the New Hebrides and Vitiaz Arcs are restored to their position relative to the Lau Ridge at 10 Ma, the Fiji Platform can be accommodated after clockwise rotation of 140°, to a position in line with the New Hebrides Arc. The large rotation for Fiji is consistent with paleomagnetic evidence indicating at least 115° of rotation.

Geochemical Evolution within the Tonga-Kermadec-Lau Arc-Back-arc Systems: the Role of Varying Mantle Wedge Composition in Space and Time

New trace element and Sr, Nd, and Pb isotope data for lavas from the active Tonga–Kermadec arc in the southwest Pacific, the volcano of Niua fo'ou in the back-arc Lau Basin, and Pacific Ocean sediments from DSDP Sites 204 and 275, and ODP Site 596, are integrated with existing geochemical data for lavas from the Lau Basin, Samoa, the Louisville Ridge Seamount Chain (LR-SMC) and the extinct Lau Ridge arc, giving new insights into the petrogenesis of lavas in an active arc–back-arc system. Geochemical variations in Tonga–Kermadec arc lavas are the result of (1) differences in the amount and composition of the material being subducted along the arc, and (2) pre-existing heterogeneities in the upper mantle. Differences in the material being subducted beneath the arc have an important influence on the chemistry of the arc lavas. At the Kermadec Trench, ∼1 km thick layer of sediment is being subducted beneath the arc, compared with ∼200 m at the Tonga Trench. This results in the high Th/U and more radiogenic Pb isotope compositions of Kermadec lavas compared with Tonga lavas. The latter have Pb isotope compositions intermediate between those of Pacific sediments and Pacific mid-ocean ridge basalt (MORB), suggesting that much of the Pb in these lavas is derived from subducting Pacific Ocean crust. This is supported by the Pb isotope signatures of the subducting LR-SMC, which are also observed in lavas from the northern Tongan islands of Tafahi and Niuatoputapu. High field strength element (HFSE) and heavy rare earth element (HREE) concentrations are generally lower in Tongan lavas (particularly those from northern Tongan islands) than in Kermadec lavas. The Tonga Ridge basement, the proto-Tonga arc lavas (ODP Site 839) and the older Lau Ridge arc lavas are generally less depleted than the modern arc lavas. In the back-arc region, upper-mantle depletion as inferred from HFSE and HREE contents of the lavas broadly increases eastwards across the Lau Basin, whereas the subduction signature and volatile (CO2 and F) contents increase eastwards towards the modern arc. These observations suggest that depletion is due to melt extraction during back-arc extension and volcanism, together with a long ‘residence time’ of mantle material within the mantle wedge. The upper mantle beneath the northernmost end of the Tonga arc and Lau Basin contains an ocean-island basalt (OIB) component derived from the Samoa plume to the north. This is reflected in high concentrations of Nb relative to other HFSE in lavas from Niua fo'ou, and Tafahi and Niuatoputapu islands at the northern end of the Tonga arc. Pb isotopes also suggest an LR-SMC contribution into Tafahi and Niuataputapu. Trace element and isotope modelling is used to investigate the combined effects of varying mantle source depletion and subduction on the geochemistry of the arc lavas. The results suggest that the arc lava geochemistry can be explained largely by the balance between a relatively constant subduction input of Pb, Th, U, Cs, Ba, Sr, Rb, K and Sc [corresponding to 0.001–0.005 weight fraction of the Stolper & Newman (1994, Earth and Planetary Science Letters, 121, 293–325] ‘H2O-rich component’ composition), into the overlying, but variably depleted mantle wedge.

Geochemical Evolution within the Tonga-Kermadec-Lau Arc-Back-arc Systems: the Role of Varying Mantle Wedge Composition in Space and Time

New trace element and Sr, Nd, and Pb isotope data for lavas from the active Tonga-Kermadec arc in the southwest Pacific, the volcano of Niua fo'ou in the back-arc Lau Basin, and Pacific Ocean sediments from DSDP Sites 204 and 275, and ODP Site 596, are integrated with existing geochemical data for lavas from the Lau Basin, Samoa, the Louisville Ridge Seamount Chain (LR-SMC) and the extinct Lau Ridge arc, giving new insights into the petrogenesis of lavas in an active arc - back-arc system. Geochemical variations in Tonga-Kermadec arc lavas are the result of (1) differences in the amount and composition of the material being subducted along the arc, and (2) pre-existing heterogeneities in the upper mantle. Differences in the material being subducted beneath the arc have an important influence on the chemistry of the arc lavas. At the Kermadec Trench, ∼1 km thick layer of sediment is being subducted beneath the arc, compared with ∼200 m at the Tonga Trench. This results in the high Th/U and more radiogenic Pb isotope compositions of Kermadec lavas compared with Tonga lavas. The latter have Pb isotope compositions intermediate between those of Pacific sediments and Pacific mid-ocean ridge basalt (MORB), suggesting that much of the Pb in these lavas is derived from subducting Pacific Ocean crust. This is supported by the Pb isotope signatures of the subducting LR-SMC, which are also observed in lavas from the northern Tongan islands of Tafahi and Niuatoputapu. High field strength element (HFSE) and heavy rare earth element (HREE) concentrations are generally lower in Tongan lavas (particularly those from northern Tongan islands) than in Kermadec lavas. The Tonga Ridge basement, the proto-Tonga arc lavas (ODP Site 839) and the older Lau Ridge arc lavas are generally less depleted than the modern arc lavas. In the back-arc region, upper-mantle depletion as inferred from HFSE and HREE contents of the lavas broadly increases eastwards across the Lau Basin, whereas the subduction signature and volatile (CO and F) contents increase eastwards towards the modern arc. These observations suggest thai depletion is due to melt extraction during back-arc extension and vokanism, together with a long 'residence time' of mantle material within the mantle wedge. The upper mantle beneath the northernmost end of the Tonga arc and Lau Basin contains an ocean-island basalt (OIB) component derived from the Samoa plume to the north. This is reflected in high concentrations of Nb relative to other HFSE in lavas from Niua fo'ou, and Tafahi and Niuatoputapu islands at the northern end of the Tonga arc. Pb isotopes also suggest an LR-SMC contribution into Tafahi and Niuataputapu. Trace element and isotope modelling is used to investigate the combined effects of varying mantle source depletion and subduction on the geochemistry of the arc lavas. The results suggest that the arc lava geochemistry can be explained largely by the balance between a relatively constant subduction input of Pb, Th, U, Cs, Ba, Sr, Rb, K and Sc [corresponding to 0.001-0.005 weight fraction of the Stolper & Newman (1994, Earth and Planetary Science Letters, 121, 293-325] 'HO-rich component' composition), into the overlying, but variably depleted mantle wedge.

Holocene tectonic histories for five islands in the south-central Lau group, south Pacific

Low-level (<30 m) evidence for emergence is reported for five islands along the south-central Lau Ridge, which has a well-documented history of late Cenozoic uplift. Most of the evidence is erosional, some of the lowest is emerged reef and beachrock: all can be linked to higher relative sea levels. Correlations are established between islands and most of the series of emerged shorelines found to exhibit characteristics of coseismically uplifted series elsewhere. Dates for various shorelines allow ages and recurrence intervals of coseismic-uplift events to be tentatively established for all islands. By reconciling this with contemporary sealevel changes, magnitudes of coseismic uplift are obtained for the middle and late Holocene. The islands of Moce and Oneata have experienced fewest coseismic-uplift events with small uplift magnitudes. Four coseismic-uplift events are identified for Lakeba with an average magnitude of 2.63 m and an assumed recurrence interval of 1045 years: no uplift has apparently occurred since 3800 BP. Five coseismic-uplift events are identified for Namuka-i-Lau with an average magnitude of 2.08 m and an assumed recurrence interval of 1045 years: no uplift has apparently occurred since 2360 BP. Three coseismic-uplift events occurred on Nayau, averaging 1.53 m in magnitude and recurring every 1045 years: no uplift has occurred since 2240 BP. The approach used here has implications not only for understanding tectonic history but may eventually prove a useful tool for isolating Holocene sealevel history in tectonically active areas.

Geodetic observations of very rapid convergence and back-arc extension at the Tonga arc

THE Earth's most active zone of mantle seismicity arises from the subduction of the Pacific plate at the Tonga trench1. It is not known why this slab generates so many more earthquakes than other subducting slabs worldwide. Above the subduction zone the active Tofua (Tonga) volcanic arc is separated by the V-shaped Lau basin from a remnant arc, the Lau ridge, located at the eastern edge of the Australian plate2. The irregular and discontinuous magnetic lineations within the basin have proven difficult to interpret3,4, and so the regional kinematic framework has been obscure. We report geodetic measurements of crustal motion within the Tonga-Lau system, which reveal the fastest crustal motions yet observed. The Lau basin is opening at a rate which increases northwards to a maximum of ∼160 mm yr−1 No straining is observed within the northern Tonga ridge, suggesting that it comprises part of a rigid microplate. Convergence rates across the Tonga trench increase northwards to a maximum of ∼240 mm yr−1. The extraordinary seismic activity of the subducting slab is probably related to this unusually rapid subduction.

Volcanism and sedimentation in a rifting island-arc terrain: an example from Tonga, SW Pacific

Abstract Scientific drilling of narrow sub-basins within the Lau back-arc basin system of the SW Pacific has recovered uppermost Miocene to Recent volcaniclastic sediment and pelagic nannofossil oozes. Pliocene sediment gravity flow and turbidite sands from the western Lau Basin indicate a local source for the sediment, probably intrabasinal seamount volcanoes active during the initial stages of arc rifting. Derivation of abundant material from either the remnant volcanic arc (Lau Ridge) or the new Tofua arc is ruled out by the rugged topography of the basin and the proximal nature of the facies. Sediments from the Tonga platform, adjacent to the present day Tofua arc, indicate a peak in volcanic activity prior to and during the generation of the first back-arc basin crust at 5.25 Ma. A 2.0 Ma hiatus in arc volcanism on the trench side of the basin after rifting was brought to an end by the foundation of the Tofua arc at 3.0 Ma (Late Pliocene). On the basis of the sedimentary, geochemical and seismic data it is suggested that basin rifting involved an initial stage of extension of the original island arc, accompanied by volcanism in the form of major seamount volcanoes within the basin. These produced volcanic ash by submarine eruption, which was then reworked into adjacent sub-basins by slumping, gravity flow or turbidity current. Basin opening proceeded with a trenchward migration of extension and volcanism with time. This system was disrupted by the southward propagation of the Eastern Lau Spreading Centre into the southern Lau Basin at 1.5–1.0 Ma. This resulted in extension and volcanism being concentrated along the median valley of the spreading centre and a cessation in explosive volcanism of wide compositional range. Sedimentation in the Lau Basin since that time has been principally pelagic with minor ash layers mostly derived from the Tofua arc.

Petroleum geology of Fiji

Most petroleum exploration in Fiji has been directed towards the shallow-water sedimentary basins around and extending onto Viti Levu, the Bligh Water and Bau Waters Basins. A large amount of commercial multi-channel seismic-reflection data has been acquired across these basins since 1971 and five deep wells were drilled offshore and on Viti Levu from 1980 to 1982. All the wells were dry, although some had minor shows of gas and oil fluorescence. The wells penetrated over 2500 m of sedimentary rocks largely of Miocene and younger age, but some Oligocene or older volcanogenic rocks were also intersected. The well sections comprise mainly volcaniclastics, and Lower to Middle Miocene shallow-water limestones, which were the primary targets, were not encountered. A large amount of recently released commercial seismic data from around Viti Levu is interpreted here to enable an assessment of petroleum potential to be made. Up to about 4 km and 2 km of Late Miocene and younger strata are interpreted in the Bligh Water and Bau Waters Basins, respectively, but strata underlying an angular unconformity, believed to be of Late Miocene age, are not seismically resolved and thus the total sedimentary thickness is unknown. The sedimentary sequences off Viti Levu are cut by large faults, and pre-Pliocene rocks are affected by folding which is locally quite intense. No source rocks have been identified in the deep wells nor in tests of outcrop and stratigraphic borehole material from Viti Levu and vanua Levu, but anomalous amounts of pentane in seabed sediments off northern Viti Levu suggest that some thermogenic hydrocarbons have been generated. Information on subsurface reservoirs is sparse, but limestones are generally considered to have better reservoir potential than coarse volcaniclastics and are the more desirable targets. Because of the limited seismic resolution, no traps with Lower to Middle Miocene shallow-water limestone facies can presently be identified in the basins offshore Viti Levu. Late Miocene and younger sequences contain numerous structurally formed potential traps, and several Pliocene reef-like seismic mounds also occur in the western Bligh Water Basin and form potential traps. Mudstones are widespread in the offshore basins and seals are likely to be good, although some relatively recent faulting may have breached some potential traps. Commercial and non-commercial reconnaissance geophysical surveys have also been conducted across other sedimentary basins in Fiji, including the deep-water Suva and Baravi Basins and basins around vanua Levu and on the Lau Ridge.

Attenuation structure beneath the Lau Back Arc Spreading Center from teleseismic S phases

Teleseismic sS and sScS waveforms provide constraints on the attenuation structure above deep seismic rones. We estimate the average Qβ along sS and sScS paths beneath the Lau back arc by comparison with S and ScS waveforms. Q is determined using both time and frequency domain analyses of transverse long period records assuming Q is frequency independent. In the frequency domain technique, spectra from reflected (sS or sScS) and direct (S or sScS) waveforms are analyzed using least squares. The time domain method uses a grid search to determine the attenuation operator that, when convolved with the direct phase, produces the best fit to the reflected phase. In both methods the direct phase is convolved with a crustal response function to simulate the crustal interaction of the reflected phase. Large events subject to source directivity effects and source‐station combinations involving ray propagation down the subducting slab are not used. The resulting attenuation estimates show low Q values which increase with focal depth, suggesting high attenuation in the upper mantle and a rapid decrease in attenuation below 200–300 km. Lateral variations are also indicated with very high attenuation (Q ∼ 20–35) in the upper mantle near the active spreading centers and lower attenuation west of the Lau Ridge. Q‐temperature relationships extrapolated from laboratory experiments suggest that the low Q regions represent zones of asthenospheric partial melting.

Magmatic evolution of Late Cenozoic volcanic rocks of the Lau Ridge, Fiji

Three main groups of lavas are exposed on islands of the Lau Ridge: the Lau Volcanic Group (LVG), 14.0–5.4 Ma, are predominantly andesite; Korobasaga Volcanic Group (KVG), 4.4–2.4 Ma, are predominantly basalt and Mago Volcanic Group (MVG), 2.0–0.3 Ma, are basalt-hawaiite. LVG and KVG lavas are mostly medium-K tholeiitic rocks with high LILE/HFSE ratios characteristic of islands ares, while MVG lavas are ne-normative alkalic rocks with high LILE and HFSE, characteristic of ocean island basalts. LVG lavas have high ɛNd (+8.0–+8.4) and low 87Sr/86Sr (0.70273–0.70349) similar to N-MORB, whereas KVG lavas have slightly more radiogenic values (ɛNd=+7.5−+8.4; 87Sr/86Sr=0.70323-0.70397). MVG lavas form an isotopically distinct group having lower ɛNd (+4.6–+4.9) and (87Sr/86Sr ranging from 0.70347–0.70375). LVG lavas were erupted in a primary oceanic island arc (Vitiaz arc) during the Miocene. Basaltic lavas were derived by approximately 19% partial melting of mantle wedge peridotite with only minor subduction component. Andesites and dacites were produced by low-pressure plagioclase-pyroxene-titanomagnetite dominated crystal fractionation. KVG lavas were erupted during the period immediately prior to or during the initial stages of rifting in the Lau Basin, and, like LVG lavas, show significant chemical differences at the northern and southern ends of the Lau Ridge. Lavas at the northern end (type (ii)) appear to be derived from a more depleted source than LVG but with a greater amount of subduction component. Those at the southern end (type (i)) probably came from a slightly more enriched source with less subduction component. MVG basalts and hawaiites were derived from an enriched mantle with little or no subduction input. The hawaiites (type (i)) could not have been derived from the basalts (type (ii)), and the two magma types must have come from different sources, indicating mantle heterogeneity. The lack of subduction influence indicates the MVG lavas are tectonically unrelated to the present-day Tonga arc, and the lack of depletion indicators suggests they have tapped a different (new?) part of the mantle wedge. This may reflect introduction of sub-Pacific mantle through the present Tonga-Lau subduction system.