Hebrides Arc Research Articles

Magnetostratigraphic dating of the uplifted atoll of Maré: Geodynamics of the Loyalty Ridge, SW Pacific

The Loyalty Islands (SW Pacific) are uplifted as they are progressively affected by the lithospheric flexure of the Australian plate, before its subduction under the New Hebrides Arc. These geodynamic changes are constrained by magnetostratigraphically dating two sections from Maré Island, where mineral extractions coupled with rock‐magnetic experiments suggest that the magnetic remanence is mostly carried by a mixture of single‐domain to multidomain magnetite/maghemite. With the help of faunal determinations and radiometric dating, the sequences of polarity reversals, correlated to the geomagnetic polarity timescale, range from the top of Chron C4n (late Miocene) to the top of the Gauss Chron (late Pliocene). This new chronostratigraphy refines the timing of two distinctive carbonate units (rhodolith platform/coral reefs) separated by a hardground whose transition is known to coincide approximately with a regional event around the Miocene/Pliocene boundary. The magnetostratigraphic dating indicates that the hardground represents about a 1.9 m.y. hiatus and suggests variable sedimentation rates ranging from 4.7 to 65.4 m/m.y. during the atoll construction. The lithospheric bulge seems to have influenced the evolution of Maré Island some 3.1 m.y. ago, leading to a diachronous emersion of the northeast and southwest rim of the atoll with a mean uplift rate of the order of 4 cm/kyr.

Geodetic measurements of convergence across the New Hebrides Subduction Zone

Between 1990 and 1994, geodetic measurements (GPS observations) have been conducted across the New Hebrides subduction zone where the Australia plate subducts under the New Hebrides Arc. This paper establishes convergence rate variations along the trench. In the South, at Tanna, the relative motion is oriented N244 ±4 and has a uniform rate of 11.7±0.8 cm/yr. The rate at Efaté is 10.3±0.9 cm/yr, oriented N242 ±4. Both azimuths very well compare with slip vectors of the last major earthquakes. In the North, the rate at Santo is only 3.6 ±1.2 cm/yr, oriented N253 ± 26. The difference in the convergence rates between Santo on the one hand and Efaté and Tanna in the other hand points to a right lateral shear zone between Santo and Efaté. At Santo where the plate coupling is very high, the very low convergence rate might be related to the absence of recent strong earthquakes. No significant variations are detected for the baselines within the Australia plate.

Recent Emerged Reef Terraces of the Yenkahe Resurgent Block, Tanna, Vanuatu: Implications for Volcanic, Landslide and Tsunami Hazards

Emerged reef terraces record the extraordinarily rapid Holocene uplift rate of the fault-bounded Yenkahe resurgent block (YRB), located within a partially submerged Quaternary caldera on Tanna Island, Vanuatu, southwest Pacific. The presently active volcano, Yasur, is located at the western end of the YRB. Episodic uplift of the YRB is probably associated with the movement of magma below the Yenkahe area. A historically recorded uplift at Port Resolution Bay in a.d. 1878 raised the shoreline ~ 15 m. This a.d. 1878 event was accompanied by a local earthquake and a tsunami that reached ~ 12 m elevation. Coral samples from terraces at mean altitudes of 155 m and 15 m above present sea level yield $$^{230}Th/^{234}U$$ dates of a.d. $$1002 \pm 10$$ and a.d. $$1868 \pm 4$$, respectively. These ages imply a mean uplift of 156 mm/yr for the YRB since a.d. 1002 until a.d. 1992. This volcanically related uplift occurs within the context of regional uplift of the southern New Hebrides arc. The late Holocene uplift rate, ~ 1 mm/yr, was determined from emerged terraces on the north and west coasts of Tanna. There have been several large volcanic eruptions in the geological history of Tanna; continued emplacement of magma at shallow levels below the YRB poses a possible volcanic hazard to the population there. Numerous faults in the weakly consolidated YRB strata and relief created by rapid uplift increase the probability of hazards from landslides and tsunamis.

Formation of the mid-fifteenth century Kuwae caldera (Vanuatu) by an initial hydroclastic and subsequent ignimbritic eruption

In the mid-fifteenth century, one of the largest eruptions of the last 10 000 years occurred in the Central New Hebrides arc, forming the Kuwae caldera (12x6 km). This eruption followed a late maar phase in the pre-caldera edifice, responsible for a series of alternating hydromagmatic deposits and airfall lapilli layers. Tuffs related to caldera formation (≈ 120 m of deposits on a composite section from the caldera wall) were emitted during two main ignimbritic phases associated with two additional hydromagmatic episodes. The lower hydromagmatic tuffs from the precaldera maar phase are mainly basaltic andesite in composition, but clasts show compositions ranging from 48 to 60% SiO2. The unwelded and welded ashflow deposits from the ignimbritic phases and the associated intermediate and upper hydromagmatic deposits also show a wide compositional range (60–73% SiO2), but are dominantly dacitic. This broad compositional range is thought to be due to crystal fractionation. The striking evolution from one eruptive style (hydromagmatic) to the other (magmatic with emission of a large volume of ignimbrites) which occurred either over the tuff series as a whole, or at the beginning of each ignimbritic phase, is the most impressive characteristic of the caldera-forming event. This strongly suggests triggering of the main eruptive phases by magma-water interaction. A three-step model of caldera formation is presented: (1) moderate hydromagmatic (sequences HD 1–4) and magmatic (fallout deposits) activity from a central vent, probably over a period of months or years, affected an area slightly wider than the present caldera. At the end of this stage, intense seismic activity and extrusion of differentiated magma outside the caldera area occurred; (2) unhomogenized dacite was released during a hydromagmatic episode (HD 5). This was immediately followed by two major pyroclastic flows (PFD 1 and 2). The vents spread and intense magma-water interaction at the beginning of this stage decreased rapidly as magma discharge increased. Subsequent collapse of the caldera probably commenced in the southeastern sector of the caldera; (3) dacitic welded tuffs were emplaced during a second main phase (WFD 1–5). At the beginning of this phase, magma-water interaction continued, producing typical hydromagmatic deposits (HD 6). Caldera collapse extended to the northern part of the caldera. Previous C14 dates and records of explosive volcanism in ice from the south Pole show that the climactic phase of this event occurred in 1452 A.D.

Ignimbrites of basaltic andesite and andesite compositions from Tanna, New Hebrides Arc

Tanna island is part of a large volcanic complex mainly subsided below sea-level. On-land, two series of hydroclastic deposits and ignimbrites overlie the subaerial remains of a basal, mainly effusive volcano. The ‘Older’ Tanna Ignimbrite series (OTI), Late Pliocene or Pleistocene in age, consists of ash flows and ash- and scoria-flow deposits associated with fallout tephra layers, overlain by indurated pumice-flow deposits. Phreatomagmatic features are a constant characteristic of these tuffs. The ‘younger’ Late Pleistocene pyroclastics, the Siwi sequence, show basal phreatomagmatic deposits overlain by two successive flow units, each comprising a densely welded layer and a nonwelded ash-flow deposit. Whole-rock analyses of 17 juvenile clasts from the two sequences (vitric blocks from the phreatomagmatic deposits, welded blocks, scoriaceous bombs and pumices from the ignimbrites) show basaltic andesite and andesite compositions (SiO2=53–60%). In addition, 296 microprobe analyses of glasses in these clasts show a wide compositional range from 51 to 69% SiO2. Dominant compositions at ∼54, 56, 58.5 and 61–62% SiO2 characterize the glass from the OTI. Glass compositions in the lower — phreatomagmatic — deposits from the Siwi sequence also show multimodal distribution, with peaks at SiO2=55, 57.5, 61–62 and 64% whereas the upper ignimbrite has a predominant composition at 61–62% SiO2. In both cases, mineralogical data and crystal fractionation models suggest that these compositions represent the magmatic signature of a voluminous layered chamber, the compositional gradient of which is the result of fractional crystallization. During two major eruptive stages, probably related to two caldera collapses, the OTI and Siwi ignimbrites represent large outpourings from these magmatic reservoirs. The successive eruptive dynamics, from phreatomagmatic to Plinian, emphasize the role of water in initiating the eruptions, without which the mafic and intermediate magmas probably would not have erupted.

Ignimbrites of basaltic andesite and andesite compositions from Tanna, New Hebrides Arc

Tanna island is part of a large volcanic complex mainly subsided below sea-level. On-land, two series of hydroclastic deposits and ignimbrites overlie the subaerial remains of a basal, mainly effusive volcano. The ‘Older’ Tanna Ignimbrite series (OTI), Late Pliocene or Pleistocene in age, consists of ash flows and ash- and scoria-flow deposits associated with fallout tephra layers, overlain by indurated pumice-flow deposits. Phreatomagmatic features are a constant characteristic of these tuffs. The ‘younger’ Late Pleistocene pyroclastics, the Siwi sequence, show basal phreatomagmatic deposits overlain by two successive flow units, each comprising a densely welded layer and a nonwelded ash-flow deposit. Whole-rock analyses of 17 juvenile clasts from the two sequences (vitric blocks from the phreatomagmatic deposits, welded blocks, scoriaceous bombs and pumices from the ignimbrites) show basaltic andesite and andesite compositions (SiO2=53–60%). In addition, 296 microprobe analyses of glasses in these clasts show a wide compositional range from 51 to 69% SiO2. Dominant compositions at ∼54, 56, 58.5 and 61–62% SiO2 characterize the glass from the OTI. Glass compositions in the lower — phreatomagmatic — deposits from the Siwi sequence also show multimodal distribution, with peaks at SiO2=55, 57.5, 61–62 and 64% whereas the upper ignimbrite has a predominant composition at 61–62% SiO2. In both cases, mineralogical data and crystal fractionation models suggest that these compositions represent the magmatic signature of a voluminous layered chamber, the compositional gradient of which is the result of fractional crystallization. During two major eruptive stages, probably related to two caldera collapses, the OTI and Siwi ignimbrites represent large outpourings from these magmatic reservoirs. The successive eruptive dynamics, from phreatomagmatic to Plinian, emphasize the role of water in initiating the eruptions, without which the mafic and intermediate magmas probably would not have erupted.

North Fiji Basin basalts and their magma sources: Part I. Incompatible element constraints

A systematic compilation of the geochemical data collected during the starmer programme is presented, in addition to all published data gathered along the North Fiji Basin (NFB) spreading system. From the 194 samples selected, the geochemical variations observed along the different spreading segments can be related to uni- or multi-modal origin of distinctive magma sources. These variations are interpreted as directly linked to the geodynamical features which surround the NFB. In the southern NFB, on the N174°E segment, N-MORB are present, but the low rate (or dead) subduction along the Hunter ridge still marks its influence as several basalts show a significant subduction-related contamination with negative Nb anomaly and high LOI. Several others are marked by a weak E-MORB source contribution, that may be related to subducted-OIB seamounts from the South Fiji Basin. In the central NFB, along the N-S segment which represents the most active and morphologically regular spreading ridge of the NFB, the magma source produces only N-MORB, with depleted LILE, HFSE, and LREE patterns, except in its northernmost part where magmatic signatures similar to that of the N15° segment appear. Along the N15° segment, three distinctive sources coexist and produce a wide range of geochemical signatures on the basalts collected; (1) a N-MORB source signature; (2) a transitional towards E-MORB source signature with a negative Nb anomaly indicating that some subduction related contamination still exists beneath the NFB (the basalts derived from this source might also have been called BABB previously); and (3) a source signature transitional towards E-MORB or OIB marking the start of an influence that increases towards the northern NFB as the Rotuma-Samoan hot spot lineament is approched. Around 17°S, on the Kaiyo station 4 site explored and sampled by the Nautile deep-sea submersible, as well as in the triple junction area, the same variability derived from three distinctive mantle sources is observed. Along the N160° segment, the three sources still coexist, but the influence of the E-MORB or OIB source (hot spot-related) increases, whereas the influence of the subduction-related source decreases. Along the Pandora-Rotuma ridge, the OIB-derived lava type is the only one present, the eventual contribution from other sources being completely diluted. Thus, the geochemistry of the NFB basalts is directly influenced by (1) the regional geodynamic environment, such as subduction zones and/or hot spot trails, (2) the geodynamical regime and stability of this type of back-arc system. The influence of the New Hebrides subduction, located some 500 km west of the active spreading ridge, is still perceptible, although weak, in the whole northern half of the NFB. However, this infuence is not directly linked to the presently active subduction, but originates rather from the partial melting of an upper mantle source that suffered subduction contamination during the clockwise rotation of the New Hebrides arc leading to the opening of the NFB during the past 12 Myr. In the northern NFB, many basalts result from the mixing of an N-MORB and a OIB source, similar to transitional alkalic lavas from oceanic intra-plate magmatism. This sources mixing increases northwards from 18°20′S to 12°S.

Large-scale deformation associated with ridge subduction

SUMMARY Continuum models are used to investigate the large-scale deformation associated with the subduction of aseismic ridges. Formulated in the horizontal plane using thin viscous sheet theory, these models measure the horizontal transmission of stress through the arc lithosphere accompanying ridge subduction. Modelling was used to compare the Tonga arc and Louisville ridge collision with the New Hebrides arc and d’Entrecasteaux ridge collision, which have disparate arc-ridge intersection speeds but otherwise similar characteristics. Models of both systems indicate that diffuse deformation (low values of the effective stress-strain exponent n) are required to explain the observed deformation. Deformation is somewhat insensitive to the vertically integrated strength of the arc (inversely proportional to the Argand number Ar), but indicates that the arc lithosphere is not extremely weak (Ar < 100). Low values of both Ar and n suggest that the thermal structure is typical of ‘cold’ or ‘normal’ arcs and that deformation is dominated by flow in the lower crust and mantle. In addition, low values of n (approaching Newtonian flow) may indicate that specific deformation mechanisms dictate deformation of the arc lithosphere. Possible mechanisms include low-stress, grain-size dependent creep, pyroxenite-controlled rheology and mechanisms associated with water weakening. Changes in the boundary conditions greatly affect deformation within island arcs. High rates of arc-ridge intersection speed (Tonga-Louisville system) yield arc-parallel tension and crustal thickening in the wake of ridge subduction. In contrast, low rates of arc-ridge intersection speed (New Hebrides-d’Entrecasteaux system) yield compressional deformation directly arcward of the collision zone and transverse strike-slip faulting adjacent to the region of compressional deformation. Localized regions of extensional deformation along the frontal part of the arc adjacent to the collision zone may contribute to the formation of re-entrants.

High-Mg andesites from the southern termination of the New Hebrides island arc (SW Pacific)

In the southern New Hebrides arc, magmatic and tectonic processes are closely linked. Between 21°S and 22°S, a “normal” broadly arc tholeiitic magmatic suite is essentially similar to those occurring in the main part of the arc, whereas south of 22°S, at the southern termination of the arc, a high-Mg andesite suite appears, with the more mafic endmembers having mineralogical and compositional affinities to high-Ca boninites. During the last 2 Ma, this southern termination propagated southwards and played a continuing role in the transient transform junction between the southern tip of the trench and the N-S spreading axis of the North Fiji Basin. In this region, which has been dominated for several million years by fast transform motions, the low magma production rates in this section of the arc, and the unusual boninite-related affinities of the arc volcanics may be due to a combination of a subducting slab torn by hinge zones, abnormally small depth-to-slab distances beneath volcanoes, and an unusually hot ambient geotherm due to rising diapirs supplying the intersecting back-arc spreading axis. The on-going collision between the Loyalty Ridge and the arc may also contribute to these unusual magmatic characteristics, as presently only a very small amount of convergence occurs along the southernmost segment of the trench. In such a complex tectonic setting, with a spreading axis propagating into an intra-oceanic arc, and major transform plate motions, generation of high-Ca boninitic magmas occurs by melting of a refractory hydrated mantle at a shallow level under the Hunter Ridge, the necessary extra heat being supplied by the rising diapirs supplying magmas to the intersecting spreading ridge axis. The high-Mg andesite suite probably results via a combination of fractional crystallization from such high-Ca boninitic parental magmas and subsequent magma mixing processes, operative all along the southern termination of the arc.

Regional setting of a complex backarc: New Hebrides Arc, northern Vanatu-eastern Solomon Islands

New GLORIA imagery over the complex backarc area of the northern New Hebrides Arc clearly outlines the deeply faulted Jean Charcot Troughs. The troughs are 2400–3000 m deep and are not magmatically active. They appear to be fragmented older crust and are not backarc basins in the usual sense.

Giant tuff cone and 12-km-wide associated caldera at Ambrym Volcano (Vanuatu, New Hebrides Arc)

Ambrym, in the New Hebrides arc, has been considered as an effusive, basaltic volcano. The present paper discusses the general structure of this edifice, which consists of a basal shield volcano topped by an exceptionally large tuff cone surrounding a 12-km-wide summit caldera. Dacitic pyroclastic flow deposits are exposed in the lower part of the tuff series; they grade upward into composite sequences of bedded surtseyan-type hyaloclastites, ash flow deposits, and fallout tephra which are essentially basaltic in composition, in such a way that the tuff cone may be considered as mainly basaltic. The relationship between the eruption of pyroclastics and the collapse event precludes a classical model of caldera formation at a basaltic volcano in which “quiet” subsidence (i.e. Kilauea type) is the dominant mechanism. Interpretation of the tuff series implies intervention of external water and suggests both explosive and collapse mechanisms. A model of caldera formation which assumes an enlargement of the ring fracture during a first plinian and dacitic, then essentially hydrobasaltic eruption is proposed.

Analysis of shallow heat flow measurements on Matthews and Hunter volcanoes (SW pacific)

Temperature measurements on shallow vertical profiles undertaken on Matthews and Hunter volcanoes of the New Hebrides arc (SW Pacific) demonstrate the absence of both unsteady and steady conductive abnormal flux at the location of the studied profiles. The reasons for this absence are explained in terms of limits in depth or magnitude for possible sources of heat inside the volcanoes. It implies that the magma chamber is of rather limited extent. This type of flux measurement has a low cost and it will be possible to implant a series of such temperature profiles on an edifice in order to obtain a map of the flux that could be widely used for the location of heat sources.

Geology of the d'Entrecasteaux-New Hebrides arc collision zone: results from a deep submersible survey

During the SUBPSO1 cruise, seven submersible dives were conducted between water depths of 5350 and 900 m over the collision zone between the New Hebrides island arc and the d'Entrecasteaux Zone (DEZ). The DEZ, a topographic high on the Australian plate, encompasses the North d'Entrecasteaux Ridge (NDR) and the Bougainville guyot, both of which collide with the island-are slope. In this report we use diving observations and samples, as well as dredging results, to analyse the geology of the Bougainville guyot and the outer arc slope in the DEZ-arc collision zone, and to decipher the mechanisms of scamount subduction. These data indicate that the Bougainville guyot is a middle Eocene island arc volcano capped with reef limestones that appear to have been deposited during the Late Oligocene to Early Miocene and in Miocene-Pliocene times. This guyot possibly emerged during the Middle and Late Miocene, and started to sink in the New Hebrides trench after the Pliocene. The rocks of the New Hebrides arc slope, in the collision zone, consist primarily of Pliocene-Recent volcaniclastic rocks derived from the arc, and underlying fractured island-arc volcanic basement, possibly of Late Miocene age. However, highly sheared, Upper Oligocene to Lower Miocene nannofossil ooze and chalk are exposed at the toe of the arc slope against the northern flank of the NDR. Based on a comparison with cores collected at DSDP Site 286, the ooze and chalk can be interpreted as sediments accreted from the downgoing plate. East of the Bougainville guyot an antiform that developed in the arc slope as a consequence of the collision reveals a 500-m-thick wedge of strongly tectonized rocks, possibly accreted from the guyot or an already subducted seamount. The wedge that is overlain by less deformed volcaniclastic island-arc rocks and sediments includes imbricated layers of Late Oligocene to Early Miocene reef and micritic limestones. This wedge, which develops against the leading flank of the guyot, tends to smooth its high-drag shape. A comparison between the 500-m-thick wedge of limestones that outcrops southeast of the guyot and the absence of such a wedge over the flat top of the guyot, although the top is overthrust by island-arc rocks and sediments, can be interpreted to suggest that the wedge moves in the subduction zone with the guyot and facilitates its subduction by streamlining.

Detachment of part of the downgoing slab and uplift of the New Hebrides (Vanuatu) Islands

Several seismological observations suggest that there is a gap within the downgoing slab of Australian lithosphere plunging beneath the New Hebrides, and ages of elevated coral terraces on the New Hebrides Islands suggest that the islands are rising rapidly. We suggest that the creation of the gap in the slab, within the last 1 M.y., occurred by part of the slab detaching from the rest and sinking rapidly into the underlying asthenosphere. This lowered the downward force on the slab at shallower depths and therefore on the overriding plate, and the continued decrease of this force, as the deeper slab sinks freely, has allowed the islands above it to rise. Although this suggestion cannot account for all aspects of the uplift of the islands, it provides a simple mechanism for explaining the fairly young uplift and its distribution along the New Hebrides arc.

The collision zone between the North d'Entrecasteaux Ridge and the New Hebrides Island Arc: 2. Structure from multichannel seismic data

The d'Entrecasteaux zone (DEZ) collides with the central New Hebrides island arc and consists of two subparallel ridges that strike east‐west, stand 1–2 km above the surrounding oceanic plate, and subduct obliquely (15°) northward beneath the arc. Rocks dredged from the north ridge as well as reflections evident in multichannel seismic reflection data indicate that this ridge has a volcanic origin. Crystalline volcanic rocks are common along the lower flank of the ridge, but sedimentary, probably volcaniclastic, rock caps the ridge. Seismic reflection data collected over the lower arc slope reveal that mass wasting deposits locally make up most of the accretionary wedge. These deposits appear to form discrete bodies, suggesting that mass wasting occurred episodically. Large anticlines and thrust faults having large vertical separation are not readily evident where the colliding ridge intersects the arc slope; apparently, slope rocks have low strength so that mass wasting deposits formed instead of large‐relief structures. Mass wasting is thought to occur as the accretionary wedge is uplifted in response to the northward oblique subduction of the north ridge. The toe of the north ridge flank marks an abrupt transition in the lithologies that make up the footwall of the interplate decollement. Footwall lithologies change from ocean basin to volcaniclastic ridge material, and this transition probably marks a discontinuity in friction along the decollement or in rock mechanical properties because north of the transition, thrust faults deform the accretionary wedge whereas south of the transition, steep reverse faults crosscut the wedge and pierce the north flank of the ridge. This piercement means that the decollement at least locally lies within the ridge and that ridge material exotic to the New Hebrides arc may be incorporated into the accretionary wedge.

Mapping of P-wave slab anomalies beneath the Tonga, Kermadec and New Hebrides arcs

Abstract P-wave velocity anomalies of several per cent fast relative to the Jeffreys-Bullen (JB) model are seen around subduction zones under the Tonga, Kermadec and New Hebrides trenches from inversion of International Seismological Centre (ISC) travel-time residuals from intermediate and deep-focus earthquakes. Mantle heterogeneities outside the model are corrected using previous velocity models. Ray paths inside the model region are traced allowing two-dimensional velocity variations. The rays of similar source and receiver locations are combined into summary rays, and the hypocenters are relocated as part of the inversion iterations. The resulting images are interpreted in conjunction with resolution and error estimations. The slab-like anomalies are generally continuous in places of high seismicity, and decrease in amplitude with depth. Most coherent slab anomalies are surrounded by large slow patches between the surface and 350-km depth. The fast slab beneath the Tonga arc appears to end at ∼ 550-km depth, before the seismicity ends, but there is a fast band at 750–1000-km depths below the deepest Tongan earthquakes. The fast slab-like anomaly beneath the Kermadec arc tends to flatten to subhorizontal near the bottom of upper mantle, and it occurs aseismically near New Zealand down to 500–550-km depth. Interpretations in the lower mantle in many places, such as Kermadec and New Hebrides arcs, however, are hampered by poor resolution.

Seismotectonics and present-day relative plate motions in the New Hebrides — North Fiji Basin region

Relative motions between the Pacific plate (P), Indo-Australian plate (IA), New Hebrides (NH) arc microplate and the North Fiji Basin (NFB) microplate are estimated using shallow seismicity, 276 focal mechanism solutions, bathymetry, magnetism and the RM-2 plate model of Minster and Jordan (1978). Within the NFB, we define the western NFB (WNFB), eastern NFB (ENFB) and southern NFB (SNFB) microplates. A 8 cm/y spreading in a N72°E direction occurs along the N-S trending spreading ridge of NFB (WNFB-ENFB boundary). Along the ENFB-IA boundary located at 176°E, a 3 cm/y extension in a N108°E direction is proposed. The WNFB-P boundary which includes the Hazel Holme Extensional Zone (HHEZ), is complex and a general N25° E extension (2 cm/y) is inferred. The Fiji fracture zone (FFZ) is composed of two segments: Along the eastern zone (P-IA boundary) trending N80° E, a left-lateral strike-slip motion occurs (9.6 cm/y) accommodated by minor N109°E extension (pull-apart); while along the western zone (ENFB-P boundary) a N84°E trending left-lateral strike-slip motion (7 cm/y) is inferred. The WNFB-SNFB boundary corresponds to a N70° E broad fracture zone along which an E-W left-lateral strike-slip motion is proposed. The SNFB microplate moves rapidly eastwards (10.5 cm/y at 172°E) and is almost attached to the IA plate. Within the NFB, an improved model is discussed which includes deformations within the IA and P plates and rotations with Euler poles close to the NFB. Along the New Hebrides trench, the consumption rates are respectively 16, 15, 9 and 12 cm/y at 11°, 12.5°, 15.5° and 20°S. The minimum rate occurs where the d'Entrecasteaux ridge collides with the New Hebrides arc. Rates of extension at the rear of the NH arc are 7, 5.5 and 2 cm/y at 11°, 12.5° and 20°S respectively. Back-arc compression occurs between 13°30′S and 17°S. The western end of the Hazel Holme extension zone coincides with the 13°30′S boundary separating the NH back-arc compressive belt and the northern NH back-arc troughs. South of 21°–22° S the trend of the slip vector changes along the NH trench from N70°E to N20°W. This implies a N-S motion of convergence along the E-W trending southern part of the NH trench (motion SNFB-IA of 1.5 cm/y at 172°E) and the existence of a N70° E trending left-lateral fracture zone crossing the southern part of the NFB. This fracture should be considered as a major plate boundary.

Geodynamics of an arc-ridge junction: the case of the New Hebrides Arc/North Fiji Basin

Abstract Detailed surveys (bathymetry, magnetism, seismicity and focal mechanism solutions) recorded on the junction between the southern New Hebrides Arc and the North Fiji Basin enlighten the geodynamic complexity of the area. The presently active c. N-S-oriented spreading structures of the North Fiji Basin are superimposed on ancient ones, which appeared during a N135°E spreading episode active before 3 Ma. A recent southward extension of the New Hebrides Arc occurred c. 2 Ma ago. Structural and geophysical consequences of both events partly obscure the present arc-ridge junction. For example, back-arc troughs do not exist to the south of the former arc termination. N45°E structural directions, present all over the studied area, are interpreted as older transform faults related to the N135°E spreading axis, which also left recognizable NW-SE magnetic anomalies. However, the New Hebrides Arc/North Fiji Basin junction remains geodynamically unstable, due to the concomitance of convergent, strike-slip and divergent movements. Their respective crustal expressions, i.e. arc volcanism, lateral displacements and seafloor spreading, are examined and considered in their regional environment. A provisional model of the arc-ridge junction that accounts for most of the parameters analysed above is presented.

SEISMIC COUPLING AND OUTER RISE EARTHQUAKES

Variation of interplate seismic coupling at subduction zones is a major factor controlling the size of the largest underthrusting events. This variation also has a profound effect on the regional intraplate stresses in the vicinity of the subduction zone. Outer rise seismicity is strongly correlated with variations in interplate coupling, reflecting the stress state of the interplate coupled zone. Over 200 outer rise earthquakes with known focal mechanisms are used to investigate the relationship between stresses in the outer rise and interplate seismic coupling. These events occur within the downgoing (i.e., oceanic) plate near the bathymetric trench axes and generally fall into the categories of tensional (normal) or compressional (thrust) with their tensional or compressional stress axes oriented approximately horizontal and perpendicular to the trench. In uncoupled subduction zones, only tensional outer rise earthquakes occur, which indicates that the outer rise is dominated by tensional stresses associated with plate bending and/or slab pull forces. In strongly coupled subduction zones, both tensional and compressional outer rise events are found. These events are related both spatially and temporally to the distribution of large underthrusting earthquakes and are thus an integral part of the earthquake cycle. In the strongly coupled regions, tensional outer rise events follow large underthrusting events as the outer rise is temporarily in tension due to the underthrusting motion. Compressional outer rise events take place as compressional stress slowly accumulates oceanward of locked sections of the interplate zone. In four instances, compressional outer rise earthquakes have been followed by large underthrusting events which have occurred 2, 4, 7, and 19 years after the associated outer rise event. The remaining compressional outer rise events are located in regions that are either known seismic gaps or in regions where the seismic potential is unknown. The occurrence of compressional outer rise earthquakes suggests that compressional stress is accumulating in the adjacent interplate region and that there is the potential for a future large underthrusting event in the region. Thirty compressional outer rise events have been located in trench segments of Middle and South America, the Kurile Islands, the Tonga and Kermadec islands, the New Hebrides Arc, and the Solomon Islands regions. In both the southern Kamchatka and northern New Hebrides regions the outer rise seismicity indicates that the stress regimes in the outer rise have changed with time from tensional, following a previous large underthrusting event, to compressional at present. Thus three stages of the cycle from underthrusting to tensional outer rise regime to compressional outer rise regime are present, requiring only the occurrence of the next underthrusting event to complete the cycle. The occurrence of compressional outer rise events is useful for assessing the seismic potential of a region on an intermediate time scale.

Diffuse back-arc deformation in the southwestern Pacific

Earthquake distribution and focal mechanisms from the Lau and North Fiji back-arc basins indicate a diffuse and shear-dominated system of deformation, at odds with traditional plate tectonic models of lithospheric accretion in back-arc basins. The entire back-arc region between the Tonga and New Hebrides arcs is more realistically modelled as a giant, internally deformed, pull-apart basin, formed along a left step in the transform boundary between the Pacific and Indo-Australian plates.