Lau Ridge Research Articles

Body wave attenuation in the Tonga Subduction Zone

Attenuation P and S waves of in the Tonga subduction zone is estimated from locally recorded data using a spectral ratio method. The data consist of three‐component, digital seismograms of 77 earthquakes recorded at two stations in Fiji and one in Tonga. The stations in the Fiji Islands lie above the deep earthquakes in the subduction zone while the Tonga station is close to the top of the subducted slab. Variations in the high‐frequency spectral decay rate define four attenuation provinces. Paths from earthquakes deeper than 350 km to stations on Viti Levu (Fiji) and the Lau ridge traverse mantle with normal attenuation (Qp=570, Qs=200–400). The subducting Tonga slab has very low attenuation (Qp>2000, Qs≥1800) relative to the surrounding mantle. The wedge of mantle above the slab at depths shallower than 350 km and east of the Lau ridge is extremely attenuating for P waves (Qp=240). Several paths from main thrust zone events to the station in Tonga show intermediate attenuation (Qp=300–500) in the forearc region. The observed spectra do not require Q to be frequency dependent in the 1.0 Hz to 7.6 Hz band in this region.

Late Cenozoic tectonic history of Lau Ridge, Southwest Pacific, and associated shoreline displacements: Review and analysis

Abstract The islands of the Lau Ridge in eastern Fiji have long been a focus for the attention of geologists owing to their spectacular sequences of preserved shorelines. A review of past work is given, and 107 shoreline levels are recognised across 32 Lau islands. The data are tabulated and plotted against their longitudes and latitudes. No shoreline groups are recognised from a plot of height against longitude, which appears to repudiate claims that the Lau Ridge was tilted eastwards during the opening of the back-arc Lau Basin, east of the ridge. In the height/latitude plot, three groups of shorelines (24) are clearly identified between 21 m and 80 m above present mean sea level. Above the level of shoreline 4, another shoreline group (5a) is recognised. Below 27 m, three shoreline groups (lb-Id) are identified with slightly less certainty. The average correlation coefficient for the seven shoreline groups (lb-5a) is −0.809. The gradients of regression lines show a consistent decrease with decreasing height through shoreline group 4 (1 m/12.58 km) to shoreline group lb (1 m/30.72 km). A belief that uplift alone has been responsible for relative shoreline emergence is weakened by the absence of any clear geotectonic cause. It is possible that contributory causes included (1) residual movements of the Lau Ridge resulting from Pliocene collision with the Viti Levu-Vanua Levu arc segment, (2) interaction between the spreading Lau Basin and the remnant arc (Lau Ridge) after this time, and (3) uplift associated with late Cenozoic thermal events in Lau. The reasonably consistent decrease in gradient exhibited by shoreline groups 1b-5a is partially explained by sea-level changes resulting principally from geoid deformation. A decrease in oceanic geoid gradient in Lau could have been caused by the contraction or movement northwestwards of the geoid high presently centred over New Guinea. It is concluded that both uplift and geoidal-eustatic change may have contributed to the displacement of shorelines in Lau since the early Pliocene. The cause of neither movement is clear.

Petrology and Geochemistry of Igneous Rocks from the Tonga Trench: A Non-Accreting Plate Boundary

Petrologic and geochemical examination of a varied suite of intermediate, mafic, and ultramafic rocks dredged from the deep flanks of the Tonga Trench between 20°S and 21°S show that the landward slope has not developed by accretion of material from the subducted Pacific plate. The lowermost trench slopes (> 9000 m) are part of a graben in the Mesozoic Pacific plate; the west and east walls of this graben expose normal- and enriched-type ocean-ridge basalts. The distribution of recovered rock types suggests that the shoaler (< 9000 m) nearshore flank is crudely layered from peridotite (up to 8500 m), to gabbro (to 7000 m), to volcanic rocks (7000-5000 m). Peridotites are fresh to moderately serpentinized harzburgites, with some dunite and minor lherzolite. They are distinctly more depleted in clinopyroxene and have more magnesian mineral compositions than are characteristic of tectonite periedotites from ocean-ridges, fracture zones, and many ophiolites. Volcanics from the upper nearshore slope are basalts, andesites, and dacites. Unlike the N- and E-type basalts being subducted, they are most similar to low-Ti, low-Ba arc volcanics from the Lau Ridge and Mariana forearc. Overall, the coherent 4 + km crustal section of the nearshore flank exposed above the structural plate boundary at 9000+ m is geochemically unlike crust generated in mid-ocean. It may be primitive island-arc crust, older oceanic crust modified by island-arc volcanism, or atypical ocean-ridge crust upon which the Tonga arc was constructed. There is no evidence of accretion of material like that being subducted-non-accretion or tectonic erosion are inferred to be the principal processes which have shaped this margin.

Shear-wave splitting in the upper-mantle wedge above the Tonga subduction zone

Summary. Intermediate-period seismograms of shear waves from deep earthquakes are examined for shear-wave splitting diagnostic of seismic velocity anisotropy in the upper-mantle wedge between the subducting Tonga slab and stations in the Fiji Islands. Earthquakes with near-vertical incidence angles are selected to minimize complications of converted phases and the free surface. Horizontal-component seismograms for 25 deep earthquakes recorded at station LAK on the Lau Ridge show splitting of up to 0.9 s with an average of 0.45 * 0.25 s between fast and slow S-wave components. Anisotropy in the upper mantle is the most likely cause of the splitting. The ESE orientation of the fast velocity direction at station LAK corresponds to an axis of symmetry of the anisotropic mantle and is approximately perpendicular to magnetic lineations in the Lau Basin and the northeastern portion of the South iiji Basin. The observations are consistent with a model of flow alignment of olivine (100) and orthopyroxene (001) axes parallel to the direction of basin extension. Seismograms for eight deep earthquakes at station SVA on Viti Levu show up to 1.0 s splitting with an average of 0.65 * 0.29 s and a weak NNE preferred orientation of fast velocity. Polarizations of shear-wave particle motions corrected for anisotropy are within 36 5 26 of polarixations predicted for nine moment tensor solutions. The magnitude of shearwave splitting and estimates of the thickness of the lithosphere in the Fiji region imply that the anisotropy cannot be contained in the lithosphere alone. Either the anisotropy is distributed in both the lithosphere and asthenosphere, or the asthenosphere alone is anisotropic.

Living Pleurotomariidae (Mollusca: Gastropoda) from the South Pacific

Abstract Perotrochus caledonicus n.sp. is described from the upper bathyal south of New Caledonia, an old area that has remained stable over the past few million years, and P. tangaroana n.sp. is described from the bathyal of Lau Ridge. These represent the first occurrence of living Pleurotomariidae in the South Pacific; other representatives of the family are restricted to Southeast Asian seas or the Atlantic.

Plate tectonics background to biogeographic development in the southwest Pacific over the last 100 million years

About 170 million years ago Greater India, an unknown bit of continent, Greater Australia, Greater New Zealand (includes submarine New Zealand Plateau) an Antarctica were all of a piece and part of Gondwana. The lines of pending dismemberment were present as long-established rift zones. The northern margin of this part of Gondwana (New Guinea), the eastern one (New Caledonia—Norfolk Ridge) and southeastern one (New Zealand—New Zealand Plateau) were active margins with considerable volcanism and both shallow- and deep-water sedimentation. At about 165 m.y. the unknown piece of continent was carried off with the creation of the Wharton Basin. In the Early Cretaceous (140-100 m.y.). Greater India departed to the northwest, a brief marine invasion occurred along the line of the present south margin of Australia and there was a great spread of epeiric seas over the remainder of Greater Australia and possibly also Greater New Zealand. The northern, New Guinea edge, was a volcanic island arc setting; a bordering arc system along the New Caledonia—New Zealand eastern edge made up the Inner Melanesian Arcs. The Rangitata Orogeny culminated in the Early Cretaceous, and was followed by a time of uplift, metamorphism, and regression, over much of Greater New Zealand. Regression of the seas was also a feature of Australia in the Late Cretaceous (100-65 m.y.). The major event of the Late Cretaceous was the creation of the Tasman Sea (78-56 m.y.). The the separation of Greater New Zealand from Antarctica. A large continental block, Tasmantis (Lord Howe and New Caledonia Ridges, New Zealand), was thus detached from Austro-Antarctica. This was itself split by the formation of the New Caledonia Basin at about 65 m.y. The Coral Sea was opened by spreading and sinistral strike-slip of a part of New Guinea north of the Papuan Mobile Belt at roughly the same time. The Inner Melanesian Arc System was disrupted. An Outer Melanesian Arc system now developed from New Guinea to Tonga and a young marginal sea began to form on the east side of the New Caledonia—Norfolk Ridge. The major event, however, was the separation of Australia from Antarctica. Obduction of seafloor and crust took place at the New Guinea and New Caledonia margins. Greater New Zealand was split by a dextral fracture system. The New Zealand Plateau remained on the Pacific Plate. Greatly increased convergence between Australia and Pacific Plates began at about 40 m.y. By mid-Oligocene (30 m.y.) the circum-Antarctic Current had begun its action. At the same time the New Hebrides—South Fiji marginal sea grew very large. By 6 m.y. (end of the Miocene) the northern boundary between Indo-Australian and Pacific Plates (the Melanesian Borderlands) became increasingly a zone of intense sinistral strike-slip and many island blocks moved WNW. Strong vertical movements have been a feature of the Borderlands. The Fiji platform rotated and the New Hebrides arc back-tracked to its present position. The old marginal sea was one-half consumed, the North Fiji Basin created by complex local spreading. The Lau Ridge became a remnant arc with the creation of the Lau—Havre interarc basin. New Zealand was joined to Tonga by way of the Kermadec Islands and subduction zone. Movement along the Alpine Fault has continued, conveying a strike-slip component to Kermadec—Tonga subduction.

Geophysical events

Tonga Islands eruption(s) and pumice rafts, South Pacific Ocean (all times are local, equal to GMT + 12 hours). The pumice described in last month's Bulletin continued to drift in a generally westward direction in June. In mid‐June, patches of pumice were reported near Vanua Mbalavu, Katafanga, and Yathata, small islands and atolls at the north end of the Lau Ridge (see map). The pumice at Yathata formed a strip 5 km long but only about 5 m wide. Large quantities of pumice were seen during the same period around Late Island (18.81°S, 1 74.65°W), about 400 km ESE of the Lau Group.The source of the pumice remains uncertain. In mid‐May, streaks of pumice extended back to Late Island from the main raft and the island's cone was covered with white material, which was thought to be fresh ash (see last month's Bulletin). However, activity has been reported at 3 other sites along the Tonga Arc in May and June.

Evolution of a simple Island arc: The Lau-Tonga Ridge

The Pacific Plate is being consumed under the Lau-Tonga Ridge down a subduction zone dipping at 45°. The trench inner wall has ultramafic rocks exposed, possibly within the accretionary wedge or prism. The fore-arc basin has a pre-Late Eocene basement overlain by 3?5 km of Late Eocene to Recent sediments. Plio-Pleistocene volcanics occur at the rear of the present fore-arc zone. The Lau basin was formed by rifting apart of the former Lau-Tonga Ridge between the fore-arc and the volcanic arc in the Early Pliocene. The Lau Ridge (the old volcanic arc) is capped by a line of volcanic piles certainly as old as Miocene but probably extending back to the Late Eocene. The size of the accumulated pile increases towards the north. The western flank of the Lau Ridge was built by a volcanic accumulation of over 3 km thickness on oceanic crust of the South Fiji Basin formed during the Oligocene. This sea floor postdates the inception of arc volcanism. In the discussion it is suggested that (a) the region has had high heat flow since the volcanism started and most of the magma has come from the mantle wedge overlying the downgoing slab, (b) the volcanism was initiated in the Tonga region over a steeply dipping Benioff Zone, (c) the locus of volcanism moved rearwards during the Late Eocene as the dip of the Benioff Zone became gentler, (d) the crust of the region reached its present thickness by (i) addition of sediment to the Tonga Ridge after the first volcanic phase and volcanic activity on the Lau Ridge and (ii) intrusions into the deeper crust, (e) the crustal thickening by both processes increased to the north as subduction rates increased away from the Pacific-India plate poles of relative rotation.

Composition and age of Lau Basin and Ridge volcanic rocks: Implications for evolution of an interarc basin and remnant arc: Discussion and reply

Composition and age of Lau Basin and Ridge volcanic rocks: Implications for evolution of an interarc basin and remnant arc: Discussion and reply

Composition and age of Lau basin and ridge volcanic rocks: implications for evolution of an interarc basin and remnant arc

Composition and age of Lau basin and ridge volcanic rocks: implications for evolution of an interarc basin and remnant arc

The Melanesian Borderlands and India — Pacific plates' boundary

The Melanesian Borderlands extend from New Guinea to Tonga and, as the name implies, occupy a border position between the India and Pacific plates. They include island chains with small land surface, deep trenches, basinal seas and real and apparent fracture systems. As morphotectonic elements, most show anomalous features in terms of today's models. The complex as a whole is treated as made up of seven subdivisions, most of which are Cainozoic tectonic unities. These are: Bismarck Sea and surrounds; Solomon Block; Coral Sea and eastern extension; New Hebrides and South Fiji Basins; New Hebrides Block; Fiji Plateau and Lau Basin; Fiji Platform, Lau and Tonga Ridges. The Bismarck Sea is not the model of a marginal sea. The surrounding land areas have arc-like features and bear the impress of one or more reversals in polarity but some features, especially geochemical ones, are difficult to relate to arc development: the Solomons, New Hebrides and Fiji-Lau-Tonga land areas echo, with varying emphases, this same condition. The Solomons appear to have been shaped, for at least the last half of their history, by sinistral shearing movement between the two major plates and by collision with the Ontong Java Plateau. The New Hebrides, following apparent polarity reversal, has followed a subduction zone retreating to the southwest and is now on the point of collision with the Loyalty Ridge. This movement was coincident with creation of the Fiji Plateau and accommodated along the Hunter Fracture Zone. The Fiji Platform is peculiar in that it shows evidence of bodily rotation. It developed as an arc until this rotation became a dominant factor at least five million years ago. The Lau-Tonga ridges were part of the Fiji are until about the same time. Since then the Lau Basin has arisen as an interarc basin so that the Lau Ridge is now a remnant arc. The Coral Sea is an enigmatic basin the genesis of which is unknown although it is linked to the break-up of the northerly part of an old continental-cum-arc system bordering the Australian continent — Owen Stanley Metamorphics, Louisiade Archipelago, Rennell Ridge and New Caledonia. On the Pacific side of this Inner Melanesian system, the New Hebrides and South Fiji Basins are the remnants of a former Palaeogene marginal sea; the creation of the Fiji Plateau over the last 5–10 m.y. has effectively halved it. The northern boundary of the Fiji Plateau includes the Cape Johnson Trough and Vitiaz Trench; the latter can be extrapolated east to the northwestern termination of the Tonga Trench. This boundary has been viewed as a relic subduction zone. Much more data are needed, especially from the basinal seas, before a really convincing reconstruction can be attempted of the events which led to the present disposal of elements within the Melanesian Borderlands.

Tectonic framework of the New Hebrides island arc

Although the New Hebrides island arc system has a number of anomalous characteristics, it conforms to the tectonic pattern observed in other western Pacific island arc systems. The two island chains represent the frontal and third arcs and are separated by a narrow extensional zone, or inter-arc basin, in which the troughs and ridges trend northerly rather than parallel to the arc system. Transverse lineations break the arc system into sections with different characteristics. The section including Santo, Malekula, Maewo and Pentecost is strongly uplifted, lacks a morphologic trench, and has a wider but inactive extensional zone. Despite the anomalous trench-frontal arc pair, the Benioff zone and volcanic chain pass through this section unchanged. South and east of 21 °S the arc system appears to be much younger with poorly developed features and no extensional zone. The 90–130 km trench-volcanic chain separation in the New Hebrides arc system is markedly smaller than in other, older arc systems. Lack of a mid-slope basement high and of the associated sediment apron suggest that oceanic crust and sediment are accreted to the forward edge of the frontal arc with time. The New Hebrides arc system apparently faced northeast before mid-Miocene time and was part of a continuous series of arcs from the Lau Ridge through the Solomon Islands. In the Late Tertiary, the New Hebrides arc reversed its polarity and began to move westward relative to Fiji, creating a trench-trench transform fault along the Matthew-Hunter Ridge, consuming a part of the South Fiji Basin, and opening the Fiji Plateau. In the Quaternary the Matthew-Hunter Ridge west of 175 °E became an arc system, and the inter-arc basin of the New Hebrides opened. The en-echelon pattern within the extensional zone is thought to reflect extension perpendicular to the arc system together with right lateral shear parallel to it.

Lateral variations of seismic-wave attenuation in the upper mantle above the inclined earthquake zone of the Tonga Island Arc: Deep anomaly in the upper mantle

Large variations of the predominant frequencies and amplitudes of the seismic phases Pn and Sn occur for hundreds of paths that cross much of the region between the Tonga island arc and the Fiji islands. The data clearly delineate a region of high attenuation in the uppermost mantle behind the Tonga arc. The region of high attenuation, a zone about 300 km wide, is between the Tonga ridge (the frontal arc) and the Lau ridge (the third arc). The alignment of active volcanoes on the Tonga ridge seems to mark the eastern boundary of this high-attenuation zone. If low attenuation, or high Q, correlates with strength, our observations indicate that the uppermost mantle is considerably weaker beneath the Lau basin, and a gap in or a substantial thinning of the lithosphere exists between the frontal arc and the third arc. This is in excellent agreement with the hypothesis that the Lau basin is the product of crustal extension. A detailed study of the variations of predominant frequencies of P and S waves that traverse the wedge of mantle above the inclined seismic zone of intermediate and deep earthquakes of Tonga shows that material with anomalously high attenuation is present throughout the prism of mantle above depths of about 150 to 300 km along the inclined seismic zone. That is, the data imply an anomalous increase in attenuation in the asthenosphere near the inclined seismic zone; Q may be as small as 50 for P waves and less than 20 for S waves. Regions of anomalously high attenuation also exist behind several other island arcs in a position similar to that behind the Tonga arc. These zones seem compatible with models in which the asthenosphere material is mobilized by the descending lithospheric slab.

Propagation ofPwaves and Rayleigh waves in Melanesia: Structural implications

The records of the Orstom seismological stations in New Caledonia and New Hebrides were used for a study of the propagation of P waves and Rayleigh waves in the South Pacific seismic belts, particularly in the New Hebrides area. After a brief study of the seismicity of the New Hebrides arc, it is shown that P arrivals from deep-focus earthquakes in Fiji and from the Longshot explosion are late in Melanesia, and that local differences exist between stations. A study of P-wave propagation from normal earthquakes of the New Hebrides implies an important difference between the stable areas of New Caledonia in the Loyalty Islands and the seismic area of New Hebrides. It is shown that the propagation has an azimuthal influence, and this influence is interpreted as a lithospheric plate effect and as a root of mountain effect. In the second part of the paper, Rayleigh-wave dispersion is studied by a systematic investigation of influence coefficients of layer parameters. The structural framework of the area between Australia and the andesite line is presented. In this area the crust is thicker than in oceanic regions, being about 20 km on the average, about 15 km in basins, and 25–30 km under the Coral Sea platform and ridges. Theoretical models to explain body-wave and Rayleigh-wave propagation give results in fair agreement with observations in the New Hebrides anomalous area. It is found that there is a crust where P-wave velocity is high on a low-velocity upper mantle. Speed increases with depth from 7.4 km/sec at a depth of 20 km to 8.1 km/sec at a depth of 120 km. In the third part of this paper the results and geological implications are discussed in the sea-floor spreading and continental-drift concepts. There is fair agreement with the hypothesis that spreading occurs from the Indian Antarctic and Pacific Antarctic ridges. The area to the east of New Hebrides (North Fiji basin, Fiji Islands, and Lau ridge) would be a large stable plate between two mobile plates of lithosphere converging and thrusting under the Tonga-Kermadec and New Hebrides arcs. According to the propagation of seismic waves under New Caledonia and the Loyalty Islands and to geological observations, it is suggested that New Caledonia and Norfolk ridge would be an island arc whose activity ceased in the Oligocene.

The evolution of the Lau Ridge, Fiji Islands

The Lau Ridge lies about 150 miles west of the Tonga Ridge and is sub-parallel to it. It may be an example of a “dead” island arc, activity having transferred to the Tonga-Kermadec region after an Upper Miocene change in the pattern of sea floor spreading in the Pacific. As a consequence of this change the Fiji Plateau has become detached from the main Pacific lithospheric plate. The continental-type Fiji Islands are seen as having evolved inside a sharp bend in the former arc.