Peggy Ridge Research Articles

A new geological map of the Lau Basin (southwestern Pacific Ocean) reveals crustal growth processes in arc-backarc systems

A 1:1,000,000-scale lithostratigraphic assemblage map of the Lau Basin (southwestern Pacific Ocean) has been created using remote predictive mapping (RPM) techniques developed by geological surveys on land. Formation-level geological units were identified in training sets at scales of 1:100,000–1:200,000 in different parts of the basin and then extrapolated to the areas where geological data are sparse. The final compilation is presented together with a quantitative analysis of assemblage-level crustal growth based on area-age relationships of the assigned units. The data sets used to develop mapping criteria and an internally consistent legend for the compilation included high-resolution ship-based multibeam, satellite- and ship-based gravity, magnetics, seafloor imaging, and sampling data. The correlation of units was informed by published geochronological information and kinematic models of basin opening. The map covers >1,000,000 km2 of the Lau-Tonga arc-backarc system, subdivided into nine assemblage types: forearc crust (9% by area), crust of the active volcanic arc (7%), backarc rifts and spreading centers (20%), transitional arc-backarc crust (13%), relict arc crust (38%), relict backarc crust (8%), and undivided arc-backarc assemblages (<5%), plus oceanic assemblages, intraplate volcanoes, and carbonate platforms. Major differences in the proportions of assemblage types compared to other intraoceanic subduction systems (e.g., Mariana backarc, North Fiji Basin) underscore the complex geological makeup of the Lau Basin. Backarc crust formed and is forming simultaneously at 12 different locations in the basin in response to widely distributed extension, and this is considered to be a dominant pattern of crustal accretion in large arc-backarc systems. Accelerated basin opening and a microplate breakout north of the Peggy Ridge has been accommodated by seven different spreading centers. The result is an intricate mosaic of small intact assemblages in the north of the basin, compared to fewer and larger assemblages in the south. Although the oldest rocks are Eocene (~40 m.y. old basement of the Lau and Tonga Ridges), half of the backarc crust in the map area formed within the last 3 m.y. and therefore represents some of the fastest growing crust on Earth, associated with prolific magmatic and hydro-thermal activity. These observations provide important clues to the geological evolution and makeup of ancient backarc basins and to processes of crustal growth that ultimately lead to the emergence of continents.

Geology and kinematics of the Niuafo'ou microplate in the northern Lau Basin

AbstractWe present a revised three‐plate kinematic model for the Lau Basin, focusing on new interpretations of the geologic characteristics of the Niuafo'ou microplate boundaries. It builds upon the three‐plate model by Zellmer and Taylor (2001) while resolving some geologic discrepancies in that model. Along the southern branch of the Mangatolu Triple Junction and the Fonualei Rift and Spreading Center, which together form the Niuafo'ou‐Tonga plate boundary, our model predicts slow to ultraslow opening rates (~32–8 mm/yr southward), consistent with axial morphology and magnetic profile modeling. Our interpretation of the seafloor magnetization pattern and flank morphology along the Central Lau Spreading Center (CLSC) indicates that opening rates along the Australia‐Niuafo'ou boundary are higher than predicted in the previous model, thus compensating for slower rates along the Niuafo'ou‐Tonga boundary. Lastly, we show geologic evidence that Peggy Ridge (PR), interpreted previously as an active transform fault and therefore a strong azimuthal constraint on Niuafo'ou plate motion, is inactive and that the active plate boundary is the adjacent Lau Extensional Transform Zone, extending from the CLSC to its intersection with the Northwest Lau Spreading Center. The removal of the PR azimuthal constraint allows us to determine an Euler pole for Australia‐Niuafo'ou opening that satisfies three‐plate closure criterion and develop a geologically consistent model of Lau Basin kinematics. While the final model focuses on constraining current plate velocities assuming rigid plate Eulerian rotations, the geologic analyses presented herein also provide insight into long‐term basin evolution, which appears to involve significant nonrigid plate kinematics.

Tracing mantle sources and Samoan influence in the northwestern Lau back‐arc basin

Helium isotopes are commonly used as a diagnostic fingerprint of Samoan mantle in the northern Lau Basin, but the extent of input from Samoan sources can only be clearly resolved by coupling 3He/4He with other geochemical tracers such as trace elements and other isotope systems. We present new major element, dissolved volatile (H2O, CO2, S, Cl, F), trace element, and Sr‐Nd‐Pb‐Hf isotope data for new samples from the NW Lau Basin from five distinct regions (Rochambeau Rifts (RR), Northwest Lau Spreading Center (NWLSC), Peggy Ridge (PR), Lau Extensional Transform Zone (LETZ), and Central Lau Spreading Center (CLSC)) that range from distinctively elevated to normal mid‐ocean ridge basalt3He/4He. Helium isotopes variations are not correlated with radiogenic isotopes or trace element abundances. Our new data suggest two‐component mixing of MORB‐like mantle with an enriched mantle source, similar to Samoa, although consideration of a complete regional data set suggests there may be other sources of heterogeneity in the mantle beneath NW Lau. Models of mantle potential temperature (Tp) and primary melt equilibration temperatures indicate similar Tp of ∼1400°C for NW Lau, suggesting no strong temperature gradient. The pressure of melt equilibration deepens toward the north (∼1.1–1.2 GPa at PR, LETZ, CLSC; ∼1.3–1.4 GPa at RR, NWLSC), consistent with melting mantle of a constant Tp but variable H2O content. Samoan and MORB‐like sources are clearly present beneath the NW Lau basin, but geochemical diversity among the existing data suggest that more than two mantle sources may contribute to mantle enrichment beneath NW Lau.

An overview of the volatile systematics of the Lau Basin – Resolving the effects of source variation, magmatic degassing and crustal contamination

The Lau Basin erupts lavas with a range of geochemical features reflecting a complex history of interaction involving different mantle sources. The Valu Fa Ridge (VFR) and Mangatolu Triple Junction (MTJ) region have lavas with arc-like characteristics, Niuafo’ou Island (NV), Peggy Ridge and Central and Eastern Lau Spreading Centers (PR, CLSC and ELSC) erupt mid-ocean ridge basalt (MORB)-like volcanics, whereas the Rochambeau Bank (RB) has features akin to ocean island basalt (OIB). To characterize the volatile systematics of these various regions, we report a comprehensive study of 39 submarine lavas from these various eruptive centers encompassing analyses of the noble gases (He, Ne, and Ar) and carbon (CO2) – both isotopes and abundances – together with other major volatile phases (H2O, S, Cl, and F).Helium isotope ratios of the NV, MTJ, CLSC, and ELSC are MORB-like for the most part except for differentiated lavas that tend to have lower, more radiogenic 3He/4He values. The RB has considerably higher 3He/4He ratios (up to 23 RA in this work) which extend as far south as the PR. The influence of ‘plume-like’ sources in the RB is also apparent in Ne isotopes: RB samples follow a trend similar to Hawaiian basalts in 3-isotope neon space. However, RB lavas have lower 40Ar/36Ar (300–730) and higher [36Ar] than CLSC and ELSC, suggesting greater air contamination. Elemental He/Ne ratios (3He/22NeS and 4He/21Ne∗ where S=solar and *=nucleogenic) are high throughout the Lau Basin and identify the Lau mantle as one of only two high 3He/4He provinces worldwide with such an enrichment of He relative to Ne.Magmatic CO2 and δ13C fall in the range 7–350ppm and −28‰ to −6‰, respectively. RB lavas have less [CO2] and slightly lower δ13C than CLSC and ELSC. The lowest values are found among MTJ lavas. These lavas also have the highest [H2O], [F], [Cl], and [S] whereas the PR, ELSC and CLSC have the lowest. RB has intermediate [H2O]. We estimate primary [CO2] in primary melts using [CO2]–δ13C relationships, and find that RB lavas have higher [CO2] (∼935±168ppm) than ELSC/CLSC (638±115ppm). They also possess higher initial δ13C values, consistent with observations at other hotspot-related localities. However, there are no discernible differences in primary CO2/Nb ratios between mantle sources characterized by high 3He/4He and MORB-like ratios. On the other hand, reconstructed values are considerably higher than that envisaged for depleted MORB mantle based on olivine-hosted melt inclusions.

Major and trace element and Sr-Nd isotope signatures of the northern Lau Basin lavas: Implications for the composition and dynamics of the back-arc basin mantle

[1] We present new major element, trace element and Sr-Nd isotope analyses of volcanic glasses from Mangatolu Triple Junction (MTJ), Peggy Ridge (PR), Rochambeau Bank (RB), and Niuafo'ou Island (NF) within the northern Lau Basin (NLB). Lavas from MTJ range from tholeiitic basalts to basaltic andesites and andesites: such a lava series can be ascribed to fractional crystallization. Lavas from NF, RB and PR are mainly tholeiitic basalts save for two transitional basalts from RB. The lavas came from a compositionally heterogeneous mantle that exhibits compositional features similar to those of the mantle source of Indian mid-ocean basalt and bears the influence of both subduction and ocean island basalt (OIB) components. The subduction components consist mainly of fluid dehydrated from subducted oceanic crust and a minor amount of sediment melt. The geochemically enriched signature of enriched RB, PR and NF lavas comes from two OIB end-member components, most likely derived from enriched Samoan mantle plume materials leaking into NLB. The Rochambeau Rifts (RR)-RB corridor receives the greatest, although still variable, influence from the mantle source of Samoan shield magmatism whereas the outlying PR and NF regions experience a Samoan plume post-erosional type of magmatism. The relatively recent mixing of Samoan plume materials with the subduction-metasomatized Indian-type mantle may be responsible for some of the observed complex relationships between noble gases and other geochemical tracers in some NLB lavas.

Helium isotope variations in seafloor basalts from the Northwest Lau Backarc Basin: Mapping the influence of the Samoan hotspot

New helium isotope results for seafloor basalts from the northern Lau Basin indicate that the signature of the Samoan hotpot has penetrated beneath the Rochambeau Rifts (RR) and the Northwest Lau Spreading Center (NWLSC) all the way to the Peggy Ridge (PR). Helium isotope ratios in 29 samples from the NWLSC – RR range from 10.9 to 28.1 Ra, all clearly higher than typical depleted mantle or MOR‐type values. Comparison with other helium results from the northern Lau Basin show the Samoan hotspot influence is confined to the NWLSC – RR extensional zone, and is absent in the northeast Lau Basin and along the PR. The absence of high 3He/4He ratios in the northeast Lau Basin may delimit the eastward extent of the Pacific Plate tear which allowed southward intrusion of the Samoan plume, or indicate deep mantle flow carrying the Samoan plume signal westward from the Samoan Islands.

Refractory Magmas in Back-Arc Basin Settings--Experimental Constraints on the Petrogenesis of a Lau Basin Example

We present the results of an experimental study on a refractory back-arc basin glass composition 123 95-1 recovered close to the intersection of the North Western Lau Spreading Centre and the Peggy Ridge in the Central Lau Basin. We used both the inverse and forward experimental approaches to determine that a picrite composition of ~16 wt %MgO was parental to the refractory back-arc basalt composition and that this picrite was in equilibrium with a residual lherzolite assemblage at ~2.1 GPa, ~1460°C. Our experimental results suggest that this primary picrite represents neither an aggregate of small melt fractions collected over a depth interval in a melting column nor a small melt fraction. Instead, the primary picrite composition represents a significant melt fraction (15% melting) of a lherzolite source, and its composition is closely modelled by equilibrium batch melting. Our preferred model of magma genesis involves three-dimensional diapiric flow during which significant solid-melt re-equilibrium occurs. Melts are trapped within an ascending diapir until melting reaches a moderate to high fraction (15-25%); at which point the primary picrite magmas segregate from the diapir.

Northern Lau Basin: Backarc extension at the leading edge of the Indo-Australian Plate

GLORIA imagery of the Lau Basin north of 17°S shows several morphotectonic terrains: a basement ridge and sedimented inter-ridge area in the SE; a nascent triple junction in the NE; a deeply sedimented basinal terrain in the central area; a linear neovolcanic zone striking NNESSW in the NW; and the northern flank of a leaky transform, the Peggy Ridge. Extension is now being accommodated at two main areas of spreading, but as no site of persistent long-term backarc crustal accretion is evident in this 250-km-wide portion of the basin, we conclude that past extension was largely by formation of pull-apart basins and local magmatism.

Tectonic activity and plate boundaries along the northern flank of the Fiji Platform

Recent volcanic activity along the northern flank of the Fiji Platform, revealed for the first time from new GLORIA imagery, suggests that the loci of interplate motion in this region have migrated rapidly since the switch from Vitiaz to New Hebridean subduction at 5–8 Ma. At present the plate boundaries along the northern flank of the Fiji Platform consist of two major strike-slip faults of opposing sense: the sinistral Fiji Transform Fault along the northwest flank of the platform, and at least one (or possibly two) zones of dextral strike slip (including Peggy Ridge) along the northeast flank. The tectonic relation-ships of these two fault systems lies north of Fiji and is not determined.

He and Sr isotopes in the Lau Basin mantle: depleted and primitive mantle components

Helium isotope ratios in Lau Basin back-arc basalts range from 7 to 22 times the atmospheric value ( R A ), i.e. from ratios typical of MORB (Depleted Mantle) helium (R/R A = 8 ± 1) to ratios similar to ‘high- 3He’ hotspots as observed in the Hawaiian, Icelandic, and nearby Samoan plume ( R/R A = 24 ). Along the Central Lau Basin spreading axis and its northward extension in the region around Niuafo'ou Volcano, 3He/ 4He ratios have typical MORB values (range = 7.5–8.6), but on Rochambeau Bank, the southern flank of a large seamount, ratios up to 22 R A occur. These high 3He/ 4He ratios are extrema of linear arrays (11–22 R A ) of He vs. Sr, Nd and Pb isotope ratios, between a Depleted Mantle (MORB) end-member and a Primitive Helium Mantle component (PHEM). PHEM is the Enriched Mantle end-member for the ‘depleted’ array formed with the DM component, and at the same time the Depleted end-member for the ‘enriched’ array formed with ‘EM’ the EM2-type end-member for Masefau Bay, Samoan basalts, as these two binary arrays intersect at its composition. Sr and Nd isotopic arrays vs. each other and vs. 3He are consistent with these binary ‘mirror arrays’ for Lau and Masefau basalts. The 3He data show unequivocally that deep-mantle plume material is present at Rochambeau Bank, and to some extent in the leaky transform/spreading axis along Peggy Ridge. We suppose that the Samoan plume component regards itself as an ‘off-ridge’ hotspot relative to the nearby Lau spreading axis, and that some of its material is channeled toward Peggy Ridge in a manner similar to the channeling we observe at the Galapagos and Pascua (Easter Island) hotspots.

Microearthquakes and tectonics in an active back-arc basin: the Lau Basin

Abstract An Ocean Bottom Seismograph (OBS) array was deployed for 20–22 days in late 1984 to investigate the precise locations of microearthquakes and their tectonic implications for active back-arc opening in the northern Lau Basin. Using P- and S-wave arrival times from four or more OBSs, the hypocenters of ∼ 300 shallow earthquakes were located with a high confidence level. The magnitudes of most OBS-located earthquakes were estimated to be less than four. In the northern half of the survey area, a narrow, linear zone of microearthquakes, trending NNW-SSE, has been identified. The northern part of the narrow seismic zone is within a central axial depression at the southern end of the Peggy Ridge. Further south, the trend of the seismic zone becomes more N-S. The narrow seismic zone seems to be composed of at least six seismic segments, offset by short aseismic zones. Most of the seismic segments trend NNW-SSE, suggesting a system of left-stepping en echelon spreading ridges, where the spreading ridge segment is seismically inactive and the transform fault is active. The spreading ridges appear to strike N-S or NNW-SSE, but the direction of the back-arc opening is considered to be NW-SE. No hypocenters were located with a high level of precision in the area south of latitude 18°S, except a small isolated zone of shallow earthquakes at the southeastern part of the survey area. We suggest that the shallow earthquakes in this isolated seismic zone were intraplate events in the Tonga platelet. This platelet is separated from the major Indo-Australian plate by the back-arc opening system in the Lau Basin.

Seismotectonics and present-day relative plate motions in the Tonga-Lau and Kermadec-Havre region

An updated compilation of shallow seismicity in the Tonga-Kermadec arc and back-arc region, including especially 304 focal mechanisms from centroid moment tensor solutions files, provides new insights on the present-day tectonics and allows the relative motions between the Pacific, Indo-Australian and Tonga-Kermadec plates to be quantified. Azimuths of slip vectors along the trench change at 19°S. In the southern domain (35-19°S) the mean azimuth ranges from N280°E to N285°E, while in the northern domain it trends N276–277°E. This 19°S boundary also exits in the back-arc domain. South of 19°S, from 35°S to 20°S the direction of back-arc extension is respectively N135°E, N122°E and N111°E within the southern Havre trough, northern Havre trough and southern Lau basin. In the southern Lau basin the N111°E opening direction is compatible with the trend of the newly mapped Valu Fa ridge. North of 19°S, in the northern Lau basin (16–18°S), a N93°E trending crustal extension occurs along the N135°E Peggy ridge and along an inferred N5–10°E spreading zone. Between these two extensional features a fourth plate, the northern Lau microplate, is present. At 18–19°S a ridge-ridge-ridge triple junction, accommodated by N95°-115°E right-lateral strike-slip fracture zones, is proposed at the complex transitional zone between the southern and the northern parts of the Lau basin. The Fiji-North Tonga fracture zone which bounds northward the Lau back-arc basin is composed of two segments: a N95°E eastern segment extending from the northern end of the Tonga trench to 178°30′W, and a N75°E western segment lying north of the Fiji platform. A ridge-fracture-fracture triple junction is inferred at 178°30′W–15°45′S between the northern end of the Peggy ridge and the two segments of the Fiji-North Tonga fracture zone. Using the model RM-2 of Minster and Jordan (1978) and the directions of back-arc spreading and convergence at the trench, relative plate motions are estimated. The extension rates in the back-arc domain increase northward, and are respectively 0.8, 2.1 and 8 cm/y at 33°S, 28°S and 24°S. Farther north parallelism between directions of different motions does not allow the calculation of any accurate result. Therefore north of 24°S we used a spreading rate of 8 cm/y which corresponds to the 24°S result and also to the value deduced from magnetic anomalies. The consumption rates along the Tonga-Kermadec trench are respectively 7.2, 9.6, 16.4, 17.1 and 17.8 cm/y at 33°S, 28°S, 24°S, 20°S and 17°S. Both the back-arc opening rate and the consumption rate of the Pacific plate sharply increase between the northern Havre trough and the southern Lau basin. This limit coincides with the Louisville ridge-trench junction. Along the N95°E eastern segment of the Fiji-North Tonga fracture zone, a pure left-lateral strike-slip motion, parallel to the N93°E opening tectonics within the northern Lau basin occurs. In contrast along the N75°E western segment of the Fiji-North Tonga fracture zone, a left-lateral strike-slip motion of 7 cm/y should be accompanied by a N135°E extensional motion of 3.5 cm/y.

南太平洋の背弧海盆における海底拡大系の微細構造 ビスマルク海およびラウ海盆での海底地震観測研究

To identify the detailed nature of the extensional opening system of two active back-arc basins in south Pacific, i.e. the Bismarck Sea and Lau Basin, we deployed the local seismic array of newly developed pop-up type Ocean-Bottom-Seismometers (OBS's). Using P- and S-wave arrival time data, hypocenters of more than two hundred small earthquakes were located with high resolution in both of the two back-arc basins.In the western survey area in the Bismarck Sea, a linear N65°W zone of OBS-located earthquakes defines the major transform fault narrower than 3km. In the eastern survey area, P-wave first motions of the events along stepping WNW-ESE seismic segments suggest the left-lateral strike-slip type focal mechanisms with N20°W T-axes. Therefore, it is inferred that small spreading ridges of en echelon type exist connecting the stepping WNW-ESE seismic segments.In the Lau Basin, an NNW linear trend of OBS-located seismic events has identified, which is continuous to the NW-SE seismic zone along the Peggy Ridge. The seismic trend is composed of several short seismic segments, suggesting a left-stepping en echelon crustal opening system with the NW-SE spreading.Thus we identified the fine-scaled structure of the back-arc spreading systems in the basins. Our result provides useful information of the location of hydro-thermal activity associated with the obliquely extensional spreading.

Lau Basin basalts (LBB): trace element and Sr [sbnd]Nd isotopic evidence for heterogeneity in backarc basin mantle

Diverse 87Sr/ 86Sr and 143Nd/ 144Nd isotopic compositions among basalts from the Lau Basin (LBB), an active backarc basin in the southwest Pacific, indicate heterogeneity in the underlying mantle. Isotopic compositions display bimodal distributions which are related to geographic location. Type I LBB ( 87/Sr 86Sr ⩽ 0.70366; 143Nd/ 144Nd ⩾ 0.51297) include tholeiites from the central basin, Peggy Ridge, and Rochambeau Bank, while Type II basaltic and andesitic glasses from the northeastern portion of the basin, near Niua Fo'ou island, have higher 87Sr/ 86Sr ( ⩾ 0.7038) and lower 143Nd/ 144Nd ( ⩽ 0.51288). Both depleted (e.g. N-MORB) and enriched (e.g. E-MORB) trace element abundances occur among Type I and Type II LBB. Covariation between trace element and isotopic ratios among Type I LBB is consistent with mixing between depleted mantle similar to the source for MORB and relatively enriched peridotite similar to the source for E-MORB. Relative to MORB, uniformly high 87Sr/ 86Sr ( +0.0005) among all Type I LBB for given Nd isotopic compositions ( ε Nd = +8 to +12) may reflect a lithospheric component, such as ancient recycled altered ocean crust. Type II LBB have Sr Nd isotopic compositions which are gradational between enriched mantle similar to the source of OIB and a component with distinct Sr isotopic composition such as that observed in Samoan post-erosional basalts. Isotopic and geographic discontinuity between Type I and Type II LBB, and isotopic affinity of Type II and Niua Fo`ou island basalts with those from Samoa suggests that volcanism in the northeastern portion of the basin is tapping deeper mantle beneath the adjoining Pacific plate, as well as Indo-Australian mantle overlying the Pacific lithosphere that is subducted into the Tonga Trench.