Melanesian Arc Research Articles

The Petrology and Geochemistry of REE-Enriched, Alkaline Volcanic Rocks of Ambitle Island, Feni Island Group, Papua New Guinea

Ambitle in the Feni Island Group is located within the NW trending Tabar–Lihir–Tanga–Feni (TLTF) volcanic island chain, Melanesian Arc, northeastern Papua New Guinea. The TLTF chain is renowned for its alkaline magmatism, geothermal activity, copper–gold mineralization, and world-class gold mining. Although its geochemical patterns indicate island arc signatures (i.e., high LILE and depleted HFSE), TLTF volcanism is not directly related to the older Melanesian Arc subduction system. However, it may have been influenced by source mantle metasomatism linked to the older subduction. The purpose of this study is to (1) present and interpret the petrographic, mineralogical, and geochemical data from Feni within the context of the tectonic evolution of the TLTF and (2) propose a geodynamic, petrogenetic model for the Feni volcanic rocks. The key methodologies used in this study are field mapping and sampling, petrographic analysis using the optical microscope, whole-rock geochemical analysis via XRF and ICP MS, and mineralogical analysis using an electron microprobe. The main rock types sampled in this study include feldspathoid-bearing basalt, trachybasalt, phonotephrite, trachyandesite, and trachydacite. Minerals identified include forsteritic olivine, diopside, augite, labradorite, andesine, anorthitic plagioclase, nepheline, and leucite in the primitive mafic suites, whereas the more evolved intermediate and felsic hypabyssal suites contain amphibole, albite, orthoclase, biotite, and either rare quartz or feldspathoids. Amphibole composition is primarily magnesiohastingsite with minor pargasite formed under polybaric conditions. Accessory minerals include apatite, titanite, and Ti-magnetite. We propose that limestone assimilation followed by fractional crystallization are plausible dominant processes in the geochemical evolution of the Ambitle volcanics. Clinopyroxene fractionation is dominant in the mafic volcanics whereas hornblende fractionation is a major petrologic process within the intermediate suites proven by the enrichment of LREE and depletions in MREE and HREE. Feni magmas are also highly enriched in REEs relative to neighboring arcs. This study is globally significant as alkaline magmas are important sources of Cu, Au, and REE as critical elements for green energy and modern technology.

The Geological and Tectonic Evolution of Feni, Papua New Guinea

Feni is located at the southeastern end of the NW-trending Tabar–Lihir–Tanga–Feni (TLTF) volcanic island chain, in northeastern Papua New Guinea. This island chain is renowned for hosting alkaline volcanics, geothermal activity, copper–gold mineralization, and mining. There is no agreed consensus on the tectonic and petrogenetic evolution of Feni. Thus, the purpose of our paper is to present the geology of Feni within the context of the regional tectonic evolution of the TLTF chain and offer a succinct and generic geodynamic model that sets the stage for our next paper. The methodologies used in this study include a critical review of published and unpublished literature in conjunction with our geological observations on Feni. The Pliocene-to-Holocene TLTF chain is a younger arc situated within the greater Eocene-to-Oligocene Melanesian Arc bounded by New Ireland to the west, the Kilinailau Trench and Ontong Java Plateau in the east, and the New Britain Trench to the south. The geological units mapped on Feni include a large volume of basaltic lava flow and trachyandesite stocks intruding a limestone and siltstone basement. Younger units include the trachyte domes, pyroclastic flow, and ash fall deposits. The major structures on Feni are normal or extensional faults such as the Niffin Graben. Feni magmatism is attributed to the petrogenetic processes of polybaric or decompression melting and crystal fractionation of magmas previously influenced by sediment assimilation, mantle wedge metasomatism, slab tears, slab melts, and subduction. Deep lithospheric normal faults provide the fluid pathways for the Feni alkaline magmas.

National-Scale Geochemical Baseline and Anomalies of Chromium in Papua New Guinea

Papua New Guinea (PNG) is located at the convergence edge of the Pacific Plate and the Indo-Australian Plate, consisting of three units. There are three chromium mineralization types in PNG. Based on national-scale geochemical mapping in PNG during 2015–2018, 1399 samples of stream sediments were collected from Highland Region, Papua Peninsula, and New Guinea Islands. This paper preliminarily studied chromium’s geochemical background, spatial distribution characteristics, and geochemical anomalies. The chromium concentration ranged from 3 ppm to 74,600 ppm, with a median value of 145 ppm, which was higher than the upper crustal abundance of chromium and the chromium geochemical baseline of Europe, Australia, North America, and China. In terms of stream sediment samples in different tectonic units, as mafic–ultramafic magmatic rocks are widely developed, the median chromium values of the New Guinea Orogen, including the Papuan Fold Belt, the New Guinea Thrust Belt, the Finisterre Terrane, the Aure Fold Belt, the Eastern Fold Belt, and the Eastern Papuan Composite Terrane, were higher than the value of the Melanesian Arc. The ophiolitic complexes, such as the April ophiolite, the Marum ophiolite, and the Papua ultramafic belt, significantly correlated with the higher chromium concentration. Eleven chromium high anomalies with mineralization potential were delineated, including three laterite and podiform prospecting areas and eight placer prospecting areas. Based on the chromium–nickel integrated anomaly map, comprehensive exploration and exploitation of nickel and chromium can be carried out in 1 and 11 high anomaly areas related to lateritic mineralization.

Origin of microplates under oblique subduction system in New Guinea: Inferences from gravity and magnetic data

The New Guinea Island is formed under the oblique subduction of the Pacific Plate. The formation and evolution of the microplates around the New Guinea Island are complex and still poorly understood. Here we use the published gravity, magnetic datasets and the CRUST1.0 model, combined with the wavelet multi-scale decomposition and residual crustal gravity anomaly separation, to analyze the crustal structure and boundary characteristics of different microplates. The results show that the gravity and magnetic anomalies have good responses to microplate boundaries, such as deep-seated large-scale faults and subduction zones. Seven microplates (including the Caroline, the North Bismarck, the Manus, the South Bismarck, the Solomon Sea, the Woodlark and the Ontong Java Plateau microplates) have been identified based on their different geophysical properties. Combined with other geological records and plate reconstruction, we analyze the regional tectonic activity, dynamics and microplate evolution in this region. The New Guinea Island was formed and shaped by the oblique subduction of the South Pacific since the Late Cretaceous. Several key events are recorded at 45 Ma, 25 Ma and 5 Ma, which are highly related to the formation of the microplates in the convergent margin. The key events mainly involve the formation and termination of the Melanesian Arc, the collision between the Ontong Java Plateau and the Solomon Islands, and the arc-continent collision at the New Britain Island Arc. These events contributed to a series of island arcs and back-arc basins that further evolved into relatively independent microplates. By analyzing the formation patterns and boundaries of microplates, we classify these microplates into the accretion-derived, collision-derived and back-arc rifting-derived types. The evolution history of each type of microplate is reconstructed in details, which provides additional constraints on the Cenozoic tectonic reconstruction of the Southwest Pacific region.

In search of Gondwana heritage in the Outer Melanesian Arc: no pre-upper Eocene detrital zircons in Viti Levu river sands (Fiji Islands)

Volcanic arcs of the Southwest Pacific, collectively referred to as the Outer Melanesian Arc, are generally thought to result from subduction of the Pacific Plate since the Late Cretaceous. Meanwhile, it is largely accepted that eastward roll-back of the old and dense oceanic plate allowed opening of marginal basins, which isolated large blocks of the former Gondwana margin. Incidentally, some ‘intra-oceanic’ volcanic arcs may have been nucleated on small continental fragments. Detrital zircons collected from sand banks in the mid-reaches of rivers from Viti Levu Island have been analysed for U–Pb geochronology and geochemistry, in order to search for a possible ancient continental arc basement, remnants of a Late Cretaceous arc, and determine the timing and evolution of Fiji arc magmatism. In contrast with some other places of the Outer Melanesian Arc (Solomon, Vanuatu), no pre-upper Eocene zircons have been found. Thus, Gondwana-derived fragments or Late Cretaceous–Paleocene arc remnants are unlikely to form the basement of Viti Levu. Zircon geochemistry confirms the purely intra-oceanic character of volcanic-arc magmatism as well. Variations in some trace-element ratios closely reflect the evolution of Viti Levu Arc from upper Eocene inception to upper Miocene climax and finally Pliocene intra-arc rifting and abandonment.

Tectonic evolution and copper-gold metallogenesis of the Papua New Guinea and Solomon Islands region

Papua New Guinea and the Solomon Islands are in one of the most prospective regions for intrusion-related mineral deposits. However, because of the tectonic complexity of the region and the lack of comprehensive regional geological datasets, the link between mineralization and the regional-scale geodynamic framework has not been understood. Here we present a new model for the metallogenesis of the region based on a synthesis of recent studies on the petrogenesis of magmatic arcs and the history of subduction zones throughout the region, combined with the spatio-temporal distribution of intrusion-related mineral deposits, and six new deposit ages. Convergence at the Pacific-Australia plate boundary was accommodated, from at least 45 Ma, by subduction at the Melanesian trench, with related Melanesian arc magmatism. The arrival of the Ontong Java Plateau at the trench at ca. 26 Ma resulted in cessation of subduction, immediately followed by formation of Cu-Au porphyry-epithermal deposits (at 24–20 Ma) throughout the Melanesian arc. Late Oligocene to early Miocene tectonic reorganization led to initiation of subduction at the Pocklington trough, and onset of magmatism in the Maramuni arc. The arrival of the Australian continent at the Pocklington trough by 12 Ma resulted in continental collision and ore deposit formation (from 12 to 6 Ma). This is represented by Cu-Au porphyry deposits in the New Guinea Orogen, and epithermal Au systems in the Papuan Peninsula. From 6 Ma, crustal delamination in Papua New Guinea, related to the prior Pocklington trough subduction resulted in adiabatic mantle melting with emplacement of diverse Cu and Au porphyry and epithermal deposits within the Papuan Fold and Thrust Belt and Papuan Peninsula from 6 Ma to the present day. Subduction at the New Britain and San Cristobal trenches from ca. 10 Ma resulted in an escalation in tectonic complexity and the onset of microplate tectonics in eastern Papua New Guinea and the Solomon Islands. This is reflected in the formation of diverse and discrete geodynamic settings for mineralization within the recent to modern arc setting, primarily related to upper plate shortening and extension and the spatial relationship to structures within the subducting slab.

Evolutionary biogeography and tectonic history of the ghost moth familiesHepialidae, Mnesarchaeidae, and Palaeosetidae in the Southwest Pacific (Lepidoptera: Exoporia).

The biogeographic history of Exoporia (Lepidoptera) in the Southwest Pacific is reconstructed for genera and species that show distributional boundaries corresponding to tectonic structures in the region. Correlations with tectonic formations of Mesozoic origin such as the Whitsunday Volcanic Province and Otway-Bass-Gippsland Basin system in Australia, the Vitiaz Fracture Zone in northern Melanesia, and the Western Province-Eastern Province boundary, Waitaki Fault Zone, and Waihemo Fault Zone of New Zealand are presented as evidence of an East Gondwana origin for genera and species before the geological separation of Australia and New Zealand. The correlated boundaries also suggest that many extant species retain at least parts of their original East Gondwana distribution ranges. The presence of Exoporia on the northern Melanesian Arc, New Caledonia, and New Zealand is attributed to the tectonic isolation of these areas when East Gondwana expanded into the Pacific following retreat of the Pacific Plate subduction zone. Local endemism of Mnesarchaeidae in New Zealand is interpreted as the result of an original vicariance from a widespread ancestor ('Exoporia') resulting in two allopatric descendants -a narrowly distributed Mnesarchoidea and a widely distributed Hepialoidea. The current overlap of these two groups in New Zealand is explained as the result of subsequent range expansion by the Hepialoidea prior to geological fragmentation of East Gondwana. The potential impact of Cretaceous geography on modern distributions is also considered for Exoporia in southern Africa and northern America. Along with lateral displacement of Exoporia, tectonic processes also contributed to the origin of high elevation endemics through a process of passive tectonic uplift.

Late Miocene Pacific plate kinematic change explained with coupled global models of mantle and lithosphere dynamics

AbstractThe timing and magnitude of a Pacific plate motion change within the past 10 Ma remains enigmatic, due to the noise associated with finite‐rotation data. Nonetheless, it has been hypothesized that this change was driven by the arrival of the Ontong Java Plateau (OJP) at the Melanesian arc and the consequent subduction polarity reversal. The uncertainties associated with the timing of this event, however, make it difficult to quantitatively demonstrate a dynamical association. Here, we first reconstruct the Pacific plate's absolute motion since the mid‐Miocene (15 Ma), at high‐temporal resolution, building on previous efforts to mitigate the impact of finite‐rotation data noise. We find that the largest change in Pacific plate‐motion direction occurred between 10 and 5 Ma, with the plate rotating clockwise. We subsequently develop and use coupled global numerical models of the mantle/lithosphere system to test hypotheses on the dynamics driving this change. These indicate that the arrival of the OJP at the Melanesian arc, between 10 and 5 Ma, followed by a subduction polarity reversal that marked the initiation of subduction of the Australian plate underneath the Pacific realm, were the key drivers of this kinematic change.

Signatures of invasion: using an integrative approach to infer the spread of melon fly, Zeugodacus cucurbitae (Diptera: Tephritidae), across Southeast Asia and the West Pacific

Invasion into new areas by already widespread pest organisms often occurs through non-obvious routes, with the origins of such invasions difficult to determine. Understanding population structure using multiple datatypes can help untangle past dispersal events and reveal putative contemporary invasion pathways. The tephritid fruit fly, Zeugodacus cucurbitae (Coquillett), is a serious pest of cucurbits and other commercial crops and is considered native to the Indo-Oriental region, but is invasive in both Africa and the Pacific. Here, we combine molecular (microsatellites and COI) and morphological (male genetalia length and wing shape geometric morphometrics) data within an integrative taxonomic framework to test hypotheses concerning levels of Z. cucurbitae population variation observed in Southeast Asia (native range, 10 sites, ~200 individuals) versus the West Pacific (invasive range, 4 sites, ~80 individuals), and whether single or multiple introductions of Z. cucurbitae have occurred into the West Pacific. We also use this case to explicitly test if using an integrative approach provides more information about hypothesized invasion pathways than either genetic or morphological approaches would do alone. All datasets support Z. cucurbitae as being more variable in Southeast Asia than the West Pacific, and within these regions populations appear to be structured geographically. In particular, mainland and Sundaic Southeast Asian locations formed separate clusters, and New Guinea and Solomon Islands were not closely related to Guam and Hawaii. Evidence supports a separate single origin for New Guinea from the Melanesian arc, the Solomon Islands from Malaysia/Singapore, and Guam from mainland Asia, but multiple introductions into Hawaii from mainland Asia. Taken together, we argue that there is great value in integrating evidence from multiple sources as it can provide finer resolution of population relationships than any single data source alone.

Short-lived subduction and exhumation in Western Papua (Wandamen peninsula): Co-existence of HP and HT metamorphic rocks in a young geodynamic setting

Understanding processes of formation of accretionary wedges requires studying young and well preserved examples. The Lengguru wedge (West Papua) is younger than 10Myr and is a result of oblique and fast subduction (11cm/yr) of the Bird's Head (a part of the Australian margin beneath the Melanesian Arc). Thus, the rapid formation of this wedge (actually a Fold and Thrust Belt) may be attributed to a single tectonic event. High pressure (HP) metabasic and metasedimentary rocks have been discovered in the core of the wedge in the Wandamen peninsula. Locally, these rocks are overprinted by migmatization during the decompression. Field relationships indicate that migmatites and anatectic leucogranite crosscut the HP metamorphic rocks. This paper reports characterization of petrology and thermobarometry of metasediments and metabasic samples, all included in a former mélange-type rock association, as well as geochronology of metasediments. Pressure–Temperature (P–T) conditions highlight two stages of metamorphism within a small time bracket. In metasediments, a first stage of relatively HP (~13–17kbar) is observed as attested by a first paragenesis of garnet–kyanite–phengite. Garnet, kyanite and phengite are then destabilized in favor of biotite during the decompression (5–12kbar and >550°C). Late fractures filled with chlorite and a second generation of white mica crosscut the foliation (<7kbar and 500–600°C). In metabasic rocks, the peak of pressure is reached from 17 to 23kbar and from 700 to 800°C. Geochronological study was conducted using U–Pb dating (LA-MC-ICPMS) on zircons from the metasedimentary and migmatitic rocks. Some zircon cores often present high Th/U ratio typical for magmatic origin, as confirmed by trace element signature of metasediments, and recorded ages older than 300Ma, witness of the activity of a volcanic arc on the former margin of the Australian craton. Most other zircons present a low Th/U ratio attesting their metamorphic origin, confirmed by inclusions of kyanite. The metamorphic zircons are dated between 5.61±0.04Ma and 8.1±1.1Ma. These very young metamorphic ages attest for a rapid subduction and exhumation event.

Melanesian arc far-field response to collision of the Ontong Java Plateau: Geochronology and petrogenesis of the Simuku Igneous Complex, New Britain, Papua New Guinea

Abstract Understanding the evolution of the mid-Cenozoic Melanesian arc is critical for our knowledge of the regional tectonic development of the Australian-Pacific plate margin, yet there have been no recent studies to constrain the nature and timing of magmatic activity in this arc segment. In particular, there are currently no robust absolute age constraints at the plate margin related to either the initiation or cessation of subduction and arc magmatism. We present the first combined U–Pb zircon geochronology and geochemical investigation into the evolution of the Melanesian arc utilizing a comprehensive sample suite from the Simuku Igneous Complex of West New Britain, Papua New Guinea. Development of the embryonic island arc from at least 40 Ma and progressive arc growth was punctuated by distant collision of the Ontong Java Plateau and subduction cessation from 26 Ma. This change in subduction dynamics is represented in the Melanesian arc magmatic record by emplacement of the Simuku Porphyry Complex between 24 and 20 Ma. Petrological and geochemical affinities highlight genetic differences between ‘normal’ arc volcanics and adakite-like signatures of Cu–Mo mineralized porphyritic intrusives. The contemporaneous emplacement of both ‘normal’ arc volcanics and adakite-like porphyry intrusives may provide avenues for future research into the origin of diverse styles of arc volcanism. Not only is this one of few studies into the geology of the Melanesian arc, it is also among the first to address the distant tectono-magmatic effects of major arc/forearc collision events and subduction cessation on magmatic arcs, and also offers insight into the tectonic context of porphyry formation in island arc settings.

Quantifying the forces needed for the rapid change of Pacific plate motion at 6 Ma

Studies have documented several rapid changes along the Pacific–North American, Pacific–Antarctic and Pacific–Australian plate boundaries in latest Miocene to earliest Pliocene time consistent with a sudden clockwise rotation of Pacific plate velocity relative to hotspots during this period. We test the hypothesis that this change in plate motion was initiated by cessation of subduction along the northern Melanesian arc due to the collision between the arc and the Ontong Java plateau. This hypothesis has long been formulated but never tested quantitatively. We use a geomechanical model of the lithosphere to determine the changes in plate boundary forces that are necessary to obtain the observed change in the Pacific plate motion. Our model results show that the change in motion can be explained by a clockwise rotation of the slab-related (basal-strength) component of plate driving force. The change of slab-related force from a post-6 Ma to a pre-6 Ma setting is perpendicular to the arc and points towards the Australian plate. The force per unit length is in the range of currently accepted values for subduction zones. Since there have been no other relevant changes at subduction zones along the Pacific plate boundary during the latest Miocene, we relate this change in slab-related force to the former southward-dipping Pacific plate slab along the northern Melanesian arc system which is now detached. Our model results suggest that rapid changes in plate motion can be triggered by slab detachment, with consequences for plate boundary processes even at great distances from the event.

Relative excitation of the seismic shear waves Sn and Lg as a function of source depth and their propagation from Melanesia and Banda arcs to Australia

SUMMARY. - Seismic activity associated with the collision of the continentalpart of the Australian plate with the oceanic Melanesian arcs along Papua NewGuinea and the Banda arc provides an unusual opportunity to study the relativeexcitation of the seismic shear waves Sn and Lg. These waves are produced byearthquakes located along the arcs in the upper 200 km of the earth and arerecorded by the Australian WWSSN Stations at Charters Towers (CTA) and AliceSprings (ASP). The paths to these stations are predominantly continental. The dataclearly show that for events located at crustal depths, Lg is the predominant phaseon the records and Sn is either absent or very weak. For events deeper than about50-70 km, Sn becomes the predominant phase on the records. These observationsarc in qualitative agreement with the explanations of Sn and Lg as highermodes of surface waves, for the particle displacement amplitudes are maximumwithin the crust for Lg and maximum within the lid of the lithospheric mantlefor Sn. The data suggest that either the crustal wave guide for Lg is moreefficient than that for Sn, or that Lg is more easily excited than Sn. No clearLg is observed from shallow earthquakes when the length of the segment of thepath crossing oceanic structure is greater than about 200 km. Also, widespreadQuaternary volcanism within the « stable » area of central Papua New Guineato the south of the mobile belt does not seem to affect the efficient transmissionof high-frequency (1 Hz) shear energy.The paths from events located along the New Hebrides, Solomon, and NewBritain arcs to Australia traverse oceanic structure, and no Lg is observed fromthese paths. The inefficient propagation of Sn along these paths from bothshallow and intermediate-depth events can be explained as follows: 1) Forthe New Hebrides case, the inefficiency of Sn propagation for paths exceedingabout 20° distance is related to the relatively young age (Lower to MiddleEocene) of the suboceanic lithosphere of the Coral Sea. As proposed by CHINN,ISACKS and BARAZANGI (1979), such relatively thin lithosphere is probably not anefficient wave guide for shear energy of about 1 Hz 2). The inefficiency of Snpropagation from events located along the northern Solomon and the New Britainarcs is probably due to anomalous attenuation in the uppermost mantle beneaththe Woodlark basin and beneath southeastern Papua New Guinea. 3) Theinefficiency of Sn propagation from events deeper than about 150 km locatedin the westernmost part of the northerly-dipping Benioff zone of the NewBritain arc could be due either to structural isolation of the nearly verticaldescending segment of the plate in which the events occur, or could be due tostructural complexity of the plates in the region which interrupts the waveguide for Sn to the Australian stations. This structural feature could be theresult of the collision of the Australian plate and the New Britain arc.

The terrestrial herpetofauna of Torres and Banks Groups (northern Vanuatu), with report of a new species for Vanuatu

A recent herpetological field trip to the Torres Group, an island group located at the northernmost border of Vanuatu, about 150 km from the southernmost Solomon Islands, allowed the collection of about 300 reptile specimens. Among these, Lepidodactylus guppyi is a new species record for Vanuatu. I also provide many new species records for the Torres Group, including two recently introduced species. The terrestrial herpetofauna of the islands of the Torres Group is reviewed for the first time and compared (1) to that of the Solomon Islands and particularly the southern Solomon island groups (Santa Cruz Group) bordering the Torres Group in the north, (2) to the remainder of Vanuatu and particularly Espiritu Santo Island which I recently surveyed, and (3) to a neighbouring group of islands in northern Vanuatu, the Banks Group. The Banks and Torres Groups share the same herpetofauna and their affinities are much stronger to the remainder of Vanuatu than to the southern Solomon Islands, thus suggesting their similar paleopositions during Melanesian arc movements.

Intra- and extra-caldera volcaniclastic facies and geomorphic characteristics of a frequently active mafic island–arc volcano, Ambrym Island, Vanuatu

Ambrym is one of the most voluminous active volcanoes in the Melanesian arc. It consists of a 35 by 50 km island elongated east–west, parallel with an active fissure zone. The central part of Ambrym, about 800 m above sea level, contains a 12 kilometre-wide caldera, with two active intra-caldera cone-complexes, Marum and Benbow. These frequently erupting complexes provide large volumes of tephra (lapilli and ash) to fill the surrounding caldera and create an exceptionally large devegetated plateau “ash plain”, as well as sediment-choked fluvial systems leading outward from the summit caldera. Deposits from fall, subordinate base surge and small-volume pyroclastic (scoria) flows dominate the volcaniclastic sequences in near vent regions. Frequent and high-intensity rainfall results in rapid erosion of freshly deposited tephra, forming small-scale debris flow- and modified grain flow-dominated deposits. Box-shaped channel systems are initially deep and narrow on the upper flanks of the composite cones and are filled bank-to-bank with lapilli-dominated debris flow deposits. These units spill out into larger channel systems forming debris aprons of thousands of overlapping and anastomosing long, narrow lobes of poorly sorted lapilli-dominated deposits. These deposits are typically remobilised by hyperconcentrated flows, debris-rich stream flows and rare debris flows that pass down increasingly shallower and broader box-shaped valleys. Lenses and lags of fines and primary fall deposits occur interbedded between the dominantly tabular hyperconcentrated flow deposits of these reaches. Aeolian sedimentation forms elongated sand dunes flanking the western rim of the ash-plain. Outside the caldera, initially steep-sided immature box-canyons are formed again, conveying dominantly hyperconcentrated flow deposits. These gradually pass into broad channels on lesser gradients in coastal areas and terminate at the coast in the form of prograding fans of ash-dominated deposits. The extra-caldera deposits are typically better sorted and contain other bedding features characteristic of more dilute fluvial flows and transitional hyperconcentrated flows. These outer flank volcaniclastics fill valleys to modify restricted portions of the dominantly constructional landscape (lava flows, and satellite cones) of Ambrym. Apparent maturity of the volcanic system has resulted in the subsidence of the present summit caldera at a similar rate to its infill by volcaniclastic deposits.

Trois millnaires de migrations et de mtissages en Nouvelle-Caldonie. Ralit biologique et dficit culturel

Diversity took a variety of forms during the colonial period. This is why biological mtissage actually gave rise to a plurality of mixed communities. The Austronesian society that took shape in New Caledonia around 1100 BC was no exception. It did not coalesce around the arrival of a single pirogue or a single group from Vanuatu, but rather around the arrival of successive waves of humans from the northwest islands of the Melanesian arc. However, if biological mtissage remained more or less constant during the first three millennia of a human presence in New Caledonia, its examplars were progressively integrated into the dominant culture. Each new wave of human colonization brought with it technological innovations or new social practices. Ultimately, while we can observe a gradual retreat in the perception of mtissage as colonial society's preferred solution, native groups increasingly took a back seat by comparison withf other communities.

Rapid change in drift of the Australian plate records collision with Ontong Java plateau

The subduction of oceanic plateaux, which contain extraordinarily thick basaltic crust and are the marine counterparts of continental flood-basalt provinces, is an important factor in many current models of plate motion and provides a potential mechanism for triggering plate reorganization. To evaluate such models, it is essential to decipher the history of the collision between the largest and thickest of the world's oceanic plateaux, the Ontong Java plateau, and the Australian plate, but this has been hindered by poor constraints for the arrival of the plateau at the Melanesian trench. Here we present (40)Ar-(39)Ar geochronological data on hotspot volcanoes in eastern Australian that reveal a strong link between collision of the Greenland-sized Ontong Java plateau with the Melanesian arc and motion of the Australian plate. The new ages define a short-lived period of reduced northward plate motion between 26 and 23 Myr ago, coincident with an eastward offset in the contemporaneous tracks of seamount chains in the Tasman Sea east of Australia. These features record a brief westward deflection of the Australian plate as the plateau entered and choked the Melanesian trench 26 Myr ago. From 23 Myr ago, Australia returned to a rapid northerly trajectory at roughly the same time that southwest-directed subduction began along the Trobriand trough. The timing and brevity of this collisional event correlate well with offsets in hotspot seamount tracks on the Pacific plate, including the archetypal Hawaiian chain, and thus provide strong evidence that immense oceanic plateaux, like the Ontong Java, can contribute to initiating rapid change in plate boundaries and motions on a global scale.

Cenozoic tectonics of New Guinea

Major hydrocarbon discoveries have been made in eastern and westernmost New Guinea, and there is great potential for additional discoveries. Although the island is a type locality for arc-continent collision during the Cenozoic, the age, number, and plate kinematics of the events that formed the island are vigorously argued. The northern part of the island is underlain by rocks with oceanic island arc affinities, and the southern part is underlain by the Australian continental crust. Based on regional sedimentation patterns, it is argued herein that the Cenozoic tectonic history of the island involves two distinct collisional orogenic events.The first Cenozoic event, the Peninsular orogeny of Oligocene age (35–30 Ma), was restricted to easternmost New Guinea. Emergent uplifts that shed abundant detritus resulted from the subduction of the northeastern corner of the Australian continent beneath part of the Inner Melanesian arc. This collision uplifted the Papuan ophiolite and formed the associated mountainous uplift that was the primary source of siliciclastic sediments that largely filled the Aure trough. Between the Oligocene and Miocene, the paleogeography of the region was similar to present-day New Caledonia. The continental crust under central and western New Guinea remained a passive margin.The second event, the Central Range orogeny, began in the latest middle Miocene, when the bulldozing of Australian passive-margin strata first created emergent uplifts above a north-dipping subduction zone beneath the western part of the Outer Melanesian arc. The cessation of carbonate shelf sedimentation and widespread initiation of siliciclastic sedimentation on top of the Australian continental basement is dated at about 12 Ma. This collision emplaced the Irian ophiolite and created the present mountainous topography forming the spine of the island.

Arc dynamics and tectonic history of Fiji based on stress and kinematic analysis of dikes and faults of the Tavua Volcano, Viti Levu Island, Fiji

Stress analysis of dikes and kinematic analysis of faults within the Tavua Volcano of Viti Levu, Fiji, indicate an apparent migration of the causative maximum principal stress axis (σ1) from NW toward north between ∼5.0 and &lt;3.9 Ma. The dikes and faults are part of successive structural and magmatic events at the arc scale. We present data on the chronological succession of stress indicators in the Tavua Volcano, based on overprinting relationships and published geochronology, which also equate with a counterclockwise rotation of Fiji, assuming an approximately constant N‐S regional compressive stress orientation during this time. The latter interpretation is consistent with paleomagnetism studies that require substantial counterclockwise rotation of the Fiji Platform, since fragmentation of the formerly continuous Outer Melanesian Arc during the latest Miocene to earliest Pliocene. The stress data suggest that the Fiji platform has undergone an ∼50° rotation since ∼5 Ma. Given that rotation apparently stopped at 3 Ma, then particularly fast rotation of ∼25° Myr−1 occurred from ∼5 Ma to 3 Ma. Recent seismicity data indicate the platform is currently subject to N‐S compression. The history of late Miocene and Pliocene events in the region can be related to the consequences arising from collision of an island arc with an oceanic plateau.

Subducted slabs beneath the eastern Indonesia–Tonga region: insights from tomography

Tomographic images of mantle structure beneath the region north and northeast of Australia show a number of anomalously fast regions. These are interpreted using a recent plate tectonic reconstruction in terms of current and former subduction systems. Several strong anomalies are related to current subduction. The inferred slab lengths and positions are consistent with Neogene subduction beneath the New Britain and Halmahera arcs, and at the Tonga and the New Hebrides trenches where there has been rapid rollback of subduction hinges since about 10 Ma. There are several deeper flat-lying anomalies which are not related to present subduction and we interpret them as former subduction zones overridden by Australia since 25 Ma. Beneath the Bird’s Head and Arafura Sea is an anomaly interpreted to be due to north-dipping subduction beneath the Philippines–Halmahera arc between 45 and 25 Ma. A very large anomaly extending from the Papuan peninsula to the New Hebrides, and from the Solomon Islands to the east Australian margin, is interpreted to be the remnant of south-dipping subduction beneath the Melanesian arc between 45 and 25 Ma. This interpretation implies that a flat-lying slab can survive for many tens of millions of years at the bottom of the upper mantle. In the lower mantle there is a huge anomaly beneath the Gulf of Carpentaria and east Papua New Guinea. This is located above the position where the tectonic model interprets a change in polarity of subduction from north-dipping to south-dipping between 45 and 25 Ma. We suggest this deep anomaly may be a slab subducted beneath eastern Australian during the Cretaceous, or subducted north of Australia during the Cenozoic before 45 Ma. The tomography also supports the tectonic interpretation which suggests little Neogene subduction beneath western New Guinea since no slab is imaged south of the New Guinea trench. However, one subduction zone in the tectonic model and many others, that associated with the Trobriand trough east of Papua New Guinea and the Miocene Maramuni arc, is not seen in the tomographic images and may require reconsideration of currently accepted tectonic interpretations.