Western Woodlark Basin Research Articles

S- and Sr-isotopic compositions in barite–silica chimney from the Franklin Seamount, Woodlark Basin, Papua New Guinea: constraints on genesis and temporal variability of hydrothermal fluid

Isotopic ratios of strontium and sulfur in six layers across a horizontal section of a hydrothermal barite–silica chimney from Franklin Seamount of western Woodlark Basin have been investigated. Sr-isotopic ratios in barite samples (87Sr/86Sr = 0.70478–0.70493) are less radiogenic than seawater (87Sr/86Sr = 0.70917) indicating that substantial leaching of sub-seafloor magma was involved in the genesis of hydrothermal fluid. The SO2 of magma likely contributed a considerable amount of lighter S-isotope in fluid and responsible for the formation of barite, which is isotopically lighter (δ34S = 19.4–20.5 ‰) than modern seawater (δ34S ~ 21 ‰). The systematic changes in isotopic compositions across the chimney wall suggest temporal changes in the mode of mineral formation during the growth of the chimney. Enrichment of heavy S- and Sr-isotopes (δ34S = 20.58 ‰; 87Sr/86Sr = 0.70493) in the outermost periphery of the chimney indicates that, at the initial stage of chimney development, there was a significant contribution of seawater sulfate during barite mineralization. Thereafter, thickening of chimney wall occurred due to precipitation of fluid carrying more magmatic components relative to seawater. This led to a gradual enrichment of lighter isotopes (δ34S = 20.42–19.48 ‰; 87Sr/86Sr = 0.70491–0.704787) toward the inner portion of the chimney wall. In contrast, the innermost layer surrounding the fluid conduit is characterized by heavier and more radiogenic isotopes (δ34S = 20.3 ‰; 87Sr/86Sr = 0.7049). This suggests there was increasing influence of percolating seawater on the mineral paragenesis at the waning phase of the chimney development.

Texture, mineralogy and geochemistry of hydrothermally altered submarine volcanics recovered southeast of Cheshire Seamount, western Woodlark Basin

The western Woodlark Basin lies within a gold-rich metallogenic province. This area is characterized by detachment faults that record ongoing extension and act as major pathways for the circulation of hydrothermal fluids. Dredging from the flanks of a submarine ridge southeast of Cheshire Seamount, western Woodlark Basin retrieved hydrothermally altered monomictic to polymictic crackle, mosaic and chaotic breccias with at least 30% clasts N2 mmin diameter. The precursor rocks are andesitic to rhyolitic in composition, but have been intensely hydrothermally altered, with about 90% of the volcanic glass replaced by secondaryminerals. The breccias show five generations of quartz growth, with the first being related to magmatic processes and the remaining four to alteration stages including silicification, chloritization, illitization, sericitization, albitization, and sulfidation. Needle-like crystals ofmordenite (zeolite)withmultiple growth centers growon the fourth generation of quartz. Notable textural variants in the breccias are vesicles, perlitic cracks, and zoned alteration halos that mantle the rims of clasts. Electron microprobe analyses on chlorite from breccia samples have identified clinochlore as the main chlorite type and indicate a formation temperature in the range of 210–304 °C. This and the elevated Au–As–Ag–Hg–Zn–Pb–Sb contents of a mineralized sample indicate hydrothermal alteration temperatures N200 °C suggesting that these breccias may represent the upflow zone of a hydrothermal system and highlight the potential for seafloor massive sulfides in the area. The breccias show elevated contents of immobile trace elements and LREE as well as a depletion in Ta and Nb suggesting that the precursor rocks were formed in a rift-related suprasubduction environment.

Arsenian pyrite-bearing altered volcanics dredged SE of Cheshire Seamount, western Woodlark Basin, Papua New Guinea

In many subaerial hydrothermal ore deposits arsenian pyrite is an important host for Au, however, arsenian pyrite is rare on the modern seafloor. During a recent survey for submarine hydrothermal mineralization in the western Woodlark Basin volcanic breccias containing abundant arsenian pyrite were dredged from the flanks of a volcanic seamount in a water depth of 2000 m. This area is particularly interesting because it is located at the transition from continental splitting to oceanic spreading where enhanced heat flow and deep crustal faults may fertilize mineralization. The sulfidic breccia is essentially monomictic and matrix-supported containing altered dacitic clasts. Mineralogical investigation of the breccia reveals silicification and sulfidation as the main alteration types. Quartz occurs in fragments and also constitutes the breccia matrix attesting to silicification as a significant alteration process. Pyrite is the dominant ore mineral with only minor amounts of Fe-oxyhydroxide and goethite. Bulk geochemistry shows a slight enrichment of Au (0.12 ppm) in association with elements such as As-Ag-Hg-Zn-Pb-Sb, key elements indicative of a low sulfidation environment. Three generations of pyrite are recognized on the basis of morphology. Arsenic-free, early framboidal pyrite (py1) is overgrown by arsenian colloform (py2) or massive pyrite (py3) containing up to 3.93 wt% As. Arsenic speciation in the pyrite is in the form of As1- and As3+. The presence of arsenian pyrite in hydrothermal breccias at this seamount indicates the potential for Au mineralization in the area.

Analysis of cetacea vocalizations using ocean bottom seismic array observations, western Woodlark Basin, Papua New Guinea.

Over a 6-month period in 1999 an array of seven ocean bottom seismometers and seven bottom mounted hydrophones covered a 30-km2 area at 2–3-km water depth in the western Woodlark Basin. The array’s main task was the detection of microearthquakes associated with nearby active tectonics. However, it also made fortuitous broadband recordings of Cetacea vocalizations that we associate with finback whales. Each of these signals has a 10–15-s duration and they exhibit a repetitive pattern with 10–15-min period. The signals are well recorded across both the seismometers and hydrophones, which were sampled at 64 and 125 Hz, respectively. Time-varying spectral analysis demonstrates that the signals are frequency modulated in the 20–30-Hz band with most energy occurring at ∼22 Hz. We employ array-based methods (e.g., optimal beam forming techniques) and acoustic transmission loss simulations to determine range and bearing of the acoustic sources. Preliminary analysis indicates that the acoustic sources originate near the array, and secondary signals may represent back-scattered energy from the short-wavelength, high-topographic features associated the active tectonics with in the basin.

Late Cenozoic volcanism in the western Woodlark Basin area, SW Pacific: the sources of marine volcanic ash layers based on their elemental and Sr–Nd isotope compositions

Tephra fallout layers and volcaniclastic deposits, derived from volcanic sources around and on the Papuan Peninsula, form a substantial part of the Woodlark Basin marine sedimentary succession. Sampling by the Ocean Drilling Program Leg 180 in the western Woodlark Basin provides the opportunity to document the distribution of the volcanically-derived components as well as to evaluate their chronology, chemistry, and isotope compositions in order to gain information on the volcanic sources and original magmatic systems. Glass shards selected from 57 volcanogenic layers within the sampled Pliocene–Pleistocene sedimentary sequence show predominantly rhyolitic compositions, with subordinate basaltic andesites, basaltic trachy-andesites, andesites, trachy-andesites, dacites, and phonolites. It was possible to correlate only a few of the volcanogenic layers between sites using geochemical and age information apparently because of the formation of strongly compartmentalised sedimentary realms on this actively rifting margin. In many cases it was possible to correlate Leg 180 volcanic components with their eruption source areas based on chemical and isotope compositions. Likely sources for a considerable number of the volcanogenic deposits are Moresby and Dawson Strait volcanoes (D'Entrecasteaux Islands region) for high-K calc-alkaline glasses. The Dawson Strait volcanoes appear to represent the source for five peralkaline tephra layers. One basaltic andesitic volcaniclastic layer shows affinities to basaltic andesites from the Woodlark spreading tip and Cheshire Seamount. For other layers, a clear identification of the sources proved impossible, although their isotope and chemical signatures suggest similarities to south-west Pacific subduction volcanism, e.g. New Britain and Tonga–Kermadec island arcs. Volcanic islands in the Trobriand Arc (for example, Woodlark Island Amphlett Islands and/or Egum Atoll) are probable sources for several volcaniclastic layers with ages between 1.5 to 3 Ma. The Lusancay Islands can be excluded as a source for the volcanogenic layers found during Leg 180. Generally, the volcanogenic layers indicate much calc-alkaline rhyolitic volcanism in eastern Papua since 3.8 Ma. Starting at 135 ka, however, peralkaline tephra layers appear. This geochemical change in source characteristics might reflect the onset of a change in geotectonic regime, from crustal subduction to spreading, affecting the D'Entrecasteaux Islands region. Initial 143Nd/144Nd ratios as low as 0.5121 and 0.5127 for two of the tephra layers are interpreted as indicating that D'Entrecasteaux Islands volcanism younger than 2.9 Ma occasionally interacted with the Late Archean basement, possibly reflecting the mobilisation of the deep continental crust during active rift propagation.

Isotopic evidence (B, C, O) of deep fluid processes in fault rocks from the active Woodlark Basin detachment zone

We report results from boron, carbon and oxygen stable isotope analyses of faulted and veined rocks recovered by scientific ocean drilling during ODP Leg 180 in the western Woodlark Basin, off Papua New Guinea. In this area of active continental extension, crustal break-up and incipient seafloor spreading, a shallow-dipping, seismically active detachment fault accommodates strain, defining a zone of mylonites and cataclasites, vein formation and fluid infiltration. Syntectonic microstructures and vein-fill mineralogy suggest frictional heating during slip during extension and exhumation of Moresby Seamount. Low carbon and oxygen isotope ratios of calcite veins indicate precipitation from hydrothermal fluids (δ 13C PDB down to −17‰; δ 18O PDB down to −22‰) formed by both dehydration and decarbonation. Boron contents are low (<7 ppm), indicating high-grade metamorphic source rock for the fluids. Some of the δ 11B signatures (17–35‰; parent solutions to calcite vein fills) are low when compared to deep-seated waters in other tectonic environments, likely reflecting preferential loss of 11B during low-grade metamorphism at depth. Pervasive devolatilization and flux of CO 2-rich fluids are evident from similar vein cement geochemistry in the detachment fault zone and splays further updip. Multiple rupture-and-healing history of the veins suggests that precipitation may be an important player in fluid pressure evolution and, hence, seismogenic fault movement.

Microbial and hydrothermal aspects of ferric oxyhydroxides and ferrosic hydroxides: the example of Franklin Seamount, Western Woodlark Basin, Papua New Guinea

Deposits of Fe-Si-Mn oxyhydroxides are commonly found on the seafloor on seamounts and mid-ocean spreading centers. At Franklin Seamount located near the western extremity of Woodlark Basin, Papua New Guinea, Fe-Si-Mn oxyhydroxides are being precipitated as chimneys and mounds upon a substrate of mafic lava. Previous studies have shown that the vent fluids have a low temperature (20–30°C) and are characterized by a total dissolved iron concentration of 0.038 mM kg-1, neutral pH (6.26) and no measurable H2S. The chimneys have a yellowish appearance with mottled red–orange patches when observed in situ from a submersible, but collected samples become redder within a few hours of being removed from the sea. The amorphous iron oxyhydroxides, obtained from active and inactive vents, commonly possess filamentous textures similar in appearance to sheaths and stalks excreted by the iron-oxidizing bacteria Leptothrix and Gallionella; however, formless agglomerates are also common. Textural relationships between apparent bacterial and non-bacterial iron suggest that the filaments are coeval with and/or growing outwards from the agglomerates. The amorphous iron oxyhydroxides are suggested to precipitate hydrothermally as ferrosic hydroxide, a mixed-valence (Fe2+-Fe3+) green–yellow iron hydroxide compound. Consideration of the thermodynamics and kinetics of iron in the vent fluid, suggest that the precipitation is largely pH controlled and that large amounts of amorphous iron oxyhydroxides are capable of being precipitated by a combination of abiotic hydrothermal processes. Some biologically induced precipitation of primary ferric oxyhydroxides (two-XRD-line ferrihydrite) may have occurred directly from the fluid, but most of the filamentous iron micro-textures in the samples appear to have a diagenetic origin. They may have formed as a result of the interaction between the iron-oxidizing bacteria and the initially precipitated ferrosic hydroxide that provided a source of ferrous iron needed for their growth. The processes described at Franklin Seamount provide insight into the formation of other seafloor oxyhydroxide deposits and ancient oxide-facies iron formation.

3‐D crustal velocity structure at the rift tip in the Western Woodlark Basin

We present a three‐dimensional (3‐D) crustal velocity model for part of the western Woodlark Basin in the SW Pacific, where rifting of a continental arc transitions to seafloor spreading. Velocities to ∼14 km depth are constrained by regularized inversion of first arrival traveltimes recorded at 14 ocean bottom instruments from a network of airgun profiles within an 80 km × 80 km area. The 3‐D velocity model images the axis and flanks of the rift basin adjacent to the westernmost spreading segment. Velocity contours that dip north at ∼10° parallel seismic reflectors from within a thick ophiolitic basement. High velocities (7.5 km/s) at the base of our model and preliminary analysis of earthquake P‐wave arrival times are consistent with an average crustal thickness of 15–20 km. Anomalously shallow upper mantle (8 km/s) velocities occur 6 km south of the spreading center at a depth of 3 km below the seafloor, and in the western part of the model, ∼30 km east of the D'Entrecasteaux Islands, at a depth of 6 km below the seafloor.

Permeability variation across an active low-angle detachment fault, western Woodlark Basin (ODP Leg 180), and its implication for fault activation

Abstract In the western Woodlark Basin, off Papua New Guinea, the variation from continental rifting to sea-floor spreading has profound effects on the mechanical response of the lithosphere. The extension is well expressed in a seismically active, shallow-dipping detachment fault. Recent Ocean Drilling Program drilling (ODP Leg 180) in the area obtained cores from the hanging wall, detachment fault gouge, and footwall, of which samples underwent permeability testing in the laboratory. Permeability variation was found to be critically dependent on (1) flow direction, i.e. fabric anisotropy of the rocks, and (2) deformational structures in the hanging wall to the fault. Regarding the first, a slight but distinct increase in permeability has been recorded parallel to the fabric (compared with flow normal to this, as indicated by anisotropy indices of k horizontal / k vertical of &gt;1.7). This phenomenon appears most profound directly above fault zones in the hanging-wall block, which are interpreted to represent splays to the main detachment fault plane at depth. Here, shear-enhanced compaction seals fluid flow to the sea floor, so that conductive flow parallel to the fault plane is favoured (in general one order of magnitude higher). The fault gouge, mainly consisting of highly serpentinized basalt and chlorite, exhibits an increase in permeability relative to the clay- and siltstones of the hanging-wall block. In the metamorphic basalt from the tectonic footwall, permeability decreases again by three orders of magnitude ( k is c. 6e—17 to 5e—18 m 2 ). Consequently, the detachment and synthetic splays related to it are zones of enhanced fluid migration in the fault plane direction. Fluid overpressure, and hence fault activity, is suggested to be trigered by seal of the top of the fault zone, owing to both shear fabrics and cementation processes.

Microbial and hydrothermal aspects of ferric oxyhydroxides and ferrosic hydroxides: the example of Franklin Seamount, Western Woodlark Basin, Papua New Guinea

Microbial and hydrothermal aspects of ferric oxyhydroxides and ferrosic hydroxides: the example of Franklin Seamount, Western Woodlark Basin, Papua New Guinea

Deformation fabrics of faulted rocks, and some syntectonic stress estimates from the active Woodlark Basin detachment zone

Abstract Ocean Drilling Program (ODP) Leg 180 investigated, in the western Woodlark Basin off Papua New Guinea, the nature and evolution of continental extension, eventually leading to crustal break-up and sea-floor spreading. At Moresby Seamount, the rift-related extension is localized at a recently active low-angle (30°) detachment fault, partly buried beneath a Pliocene-Pleistocene sedimentary synrift sequence. Data from three drillsites sample the detachment fault itself, secondary faults in its hanging wall and a steep normal fault cutting the footwall. The fault plane itself is manifested as a strongly altered fault gouge. Deformation of turbiditic sediments in several fault zones in the hanging wall is dominated by brittle mechanisms, and accompanied by intensive veining and pervasive diagenetic cementation. The metabasic rocks of the footwall below the detachment show an unusual transition from ductile to brittle deformation fabrics with increasing depth. Many fracture systems show evidence of repeated opening and healing during multistage hydrothermal mineralization. Syn-mylonitic microstructures and vein fill mineralogy suggest exhumation of the detachment footwall from considerable depth in the crust. Two palaeo-piezometers were applied to calcite-filled veins that show evidence of plastic deformation. Differential stress values of similar magnitude and probably close to the rock failure strength are found in both the hanging wall and footwall.

Micropalaeontological and palaeomagnetic approaches to stratigraphic anomalies in rift basins: ODP Site 1109, Woodlark Basin

Abstract The stratigraphic succession of the western Woodlark Basin is examined in detail relative to ODP Site 1109, where c. 55 m of Pliocene sediment lies anomalously shallow owing to slumping between c. 1.10 and 0.65 Ma in the tectonically active rift basin. Palaeomagnetic reversals and varying percentages of the characteristic microfossils, Globigerinoides fistulosus and discoasters, within the deformed sediment defining the slump indicate that the Pliocene sediment was periodically introduced by a number of slumps rather than being emplaced by a single slump event. Palaeomagnetic and biostratigraphic events in the remainder of the hemipelagic section of Site 1109 to c. 4 Ma appear undisturbed and consistent with those at adjoining sites. Globorotalia truncatulinoides first appeared between 2.65 and 2.71 Ma at these low-latitude sites, which extends the previously reported area of evolution of the species in the southwestern Pacific toward the Equator. Seven of the nine coiling changes in Pulleniatina through time are recognized and dated using sedimentation rates. Along with palaeomagnetic events and microfossil species datum levels, these coiling changes can be used to correlate between sites in the area.

40 Ar/ 39 Ar ages of fallout tephra layers and volcaniclastic deposits in the sedimentary succession of the western Woodlark Basin, Papua New Guinea: the marine record of Miocene-Pleistocene volcanism

Abstract Five fallout tephra layers and 13 heterolithological volcaniclastic deposits drilled at Holes 1115A, 1115B, 1115C, 1109C, 1109D and 1118A, during Leg 180 on the downflexed northern margin of the western Woodlark Basin, have been dated by single-crystal laser 40 Ar/ 39 Ar analyses. The fallout tephra layers range in age from 0.135 ± 0.008 Ma to 2.84 ± 0.03 Ma. Sedimentation ages determined for the volcaniclastic deposits range from 1.75 ± 0.29 Ma to 3.79 ± 0.01 Ma, closely matching the nannofossil, planktonic foraminifer and palaeomagnetic chronostratigraphies of the holes. However, two volcaniclastic deposits from 516.91 m below seafloor (mbsf) and 632.5 mbsf in Hole 1118A are significantly older than indicated by biostratigraphic and palaeomagnetic data, probably because of the presence of older reworked volcanic crystals. The youngest ash layer is derived from explosive eruptions in the Dawson Strait area of the D’Entrecasteaux Islands, whereas the four older tephra layers are attributed to explosive eruptions in the Moresby Strait area of the D’Entrecasteaux Islands. The 40 Ar/ 39 Ar ages of volcaniclastic sand layers in Holes 1115C and 1118A indicate a transition from a shallow-water succession (&lt;150m) to a deeper-water succession (150–500m) with rapid deposition of volcaniclastic sands, mainly by turbiditic currents, at 3.8 Ma. This transition is related to the subsidence of the margin during rifting of the Woodlark Basin. Two volcaniclastic deposits with ages of 13.84 ± 0.02 Ma and 14.04 ± 0.03 Ma, respectively, provide important time markers in the middle Miocene sedimentary sequence at Hole 1115C, where biostratigraphic ages are scarce. Our 40 Ar/ 39 Ar ages represent the first marine record of Miocene to Pleistocene volcanism in the area of eastern Papua.

Evolution of the basalts from three back-arc basins of southwest Pacific

Petrography, petrochemistry, and mineral chemistry of basalts from the Woodlark, Manus, and Lau basins from the southwest Pacific, have been studied to understand their magmatic evolution. Basalt from the western Woodlark Basin (Dobu Seamount) indicates mixing of a near-primitive magma with fractionated basaltic melt in shallow magma chambers. Basalts from Manus Basin and Central Lau Spreading Center (Lau Basin) are typical N-MORBs, and they exhibit olivine fractionation under high oxidizing conditions while basalts from Mangatolu Triple Junction (Lau Basin) are enriched in Al2O3, K2O, and Zr indicative of a contribution from a subducting plate.

Late Quaternary palaeoceanography of the western Woodlark Basin (Solomon Sea) and Manus Basin (Bismarck Sea), Papua New Guinea, from planktic foraminifera and radiocarbon dating

Foraminiferal assemblages and radiocarbon dates from three cores collected in the western Woodlark Basin and one from the Manus Basin have provided information on late Quaternary sedimentation rates and characteristics of water masses. In the western Woodlark Basin, where turbidites and slumps are present, sedimentation rates vary from a few tens of cm/1000 year to 140 cm/1000 year, with background rates decreasing eastwards from New Guinea from 7 cm/1000 year to 3.7 cm/1000 year. In the Manus Basin during the past 16,000 years the sedimentation rate was 15.5 cm/1000 year. In the western Woodlark Basin the lysocline has deepened by at least 900 m over the past 23,500 years. It was above 2680 m before 23,500 years ago; at 3145 m about 13,000 years ago and is now at 3500 m. During the past 30,000 years variations in the mean annual temperatures of surface waters in the western Woodlark Basin were about 3 °C, and about 4.5 °C in the Manus Basin. There were cool intervals at 19,500 and 13,000 years ago. A poorly developed early Holocene warm interval 10,000 − 8500 years ago was followed by a cooler interval with a minimum 8000 years ago. The Holocene climatic optimum occurred 5000 years ago with temperatures higher than at present.

Sediments of the active rift zone in the western Woodlark Basin and the development of hydrothermal and volcanic activity

In the western Woodlark Basin within the neovolcanic zone sediments form ponds in fissures and between individual pillows and tubes. A continuous thin sediment cover appears only at the base of the neovolcanic zone. Its thickness increases with the distance from the zone. Data on the lithology, mineralogy and chemistry of sediment cores from the spreading centre in the western Woodlark Basin and adjacent areas show that these calcareous sediments (avg. 44.1 % CaCO 3) contain significant amounts of terrigenous, arc volcanic, hyaloclastic and hydrothermal material, the latter constituting up to 15% of the non-carbonate fraction. Two types of sedimentary sequences occur in the studied area. Those of the first type were formed by particle-by-particle sedimentation; horizons of redeposited sediments occur in sequences of the second type. Redeposition of sediments is widespread and is tentatively correlated with cycles of volcanic activity at swells along the spreading centre. These alternate with hydrothermal cycles, which were determined by the presence of hydrothermal matter in sediments. Radiocarbon dating of sediments showed that both cycles are of similar duration, of the order of 10,000 years. They are not synchronous along the spreading axis, each swell having a specific evolution. Based on these determinations we can say that within the studied spreading centre only the Franklin Seamount swell is prospective for contemporary hydrothermal activity.

Geochemistry of basalts from the western Woodlark, Lau and Manus basins: implications for their petrogenesis and source rock compositions

Geochemical compositions are reported for back-arc and marginal basin basalts sampled during the 1990 cruise of R/V Akademik Mstislav Keldysh in the Lau Basin (North Lau, LN; King's Triple Junction, LK; Central Lau, LC; East Lau, LE), Manus Basin (Red Star region, MRS), and western Woodlark Basin (Dobu Seamount region, WD, continental rifting segment; East Basin region, WE, the boundary between continental and spreading segments; and Franklin Seamount region, WF, spreading rifting segment). Two main geochemical types of basalts have been distinguished within the Lau and Manus back-arc marginal basins: (i) depleted basalts comparable with N-MORB lavas (LC, LE, MRS); and (ii) enriched basalt lavas (LN, LK vesicular pillow basalts: LK, VPB) correspond to typical BABB. The sources of melts were depleted spinel or plagioclase lherzolite (N-MORB type) for LC, LE, MRS basalts; the N-MORB source was metasomatized by adding LILE and volatiles (‘arc-source’ components) for enriched lavas. Direct observations from Mir submersibles in the area of the King's triple junction (Lau Basin) established that LILE-depleted massive sheet-flow basalts overlie vesicular pillow basalts. The western Woodlark Basin is a tectonic transition zone between typical oceanic spreading in the central part of the basin and continental rifting in easternmost Papua-New Guinea. The WD basalts show strong LILE, LREE, P, and Zr enrichment and are spatially associated with alkaline rhyolites. The WE and WF lavas demonstrate moderate enrichment in these elements. The western Woodlark lavas formed from multiple sources, including N-MORB mantle, ‘arc-source’ components and enriched mantle like OIB.

Geophysical and morpho-tectonic study of the transition between seafloor spreading and continental rifting, western Woodlark Basin, Papua New Guinea

Two major morpho-tectonic domains, separated by a major transfer zone, are described at the transition between seafloor spreading and continental rifting in the western Woodlark Basin, off-shore eastern Papua New Guinea. The oceanic domain comprises new oceanic crust formed during the Bruhnes Epoch, older transitional crust and the rifted continental margins. Two rift branches are recognized within the continental domain. The southern rift branch has failed while the northern branch is a locus of maximum extension with initial development of oceanic crustal accretion. Based on magnetic data, seafloor spreading in the western part of the Woodlark Basin commenced between 3.5 and 2.5 Ma along the northern margin of the basin and maintained its position adjacent to this margin up to 0.8 Ma (Jaramillo event). Just prior to the Bruhnes Epoch, the seafloor spreading axis jumped southward to its current position. Frequent jumps of seafloor spreading centers and relocation of paleoaxes indicate the instability of the Woodlark extensional system. The transition between the two domains is characterized by differential localization of extensional strain. Variations in the localization can result from progressive change of extensional mode when the initial rifting evolves into seafloor spreading and/or from differences in lithospheric rheology as the rift propagates into the continental margin. A consequence of such variations is the massive production of new oceanic crust during seafloor spreading which is being balanced across the transfer zone by broadly distributed deformation within continental lithosphere.

Late Quaternary diatom assemblages and their stratigraphic significance, western Woodlark Basin, Papua New Guinea

Forty-nine genera and 85 diatom species are reported from western Woodlark Basin Late Pleistocene and Holocene bathyal sediments. Although planktic pelagic diatoms predominate throughout, shallow-water benthic and neritic forms, with some brackish and fresh-water forms, are common in the upper (Holocene) part of the section. This implies sediment redeposition by slumps and turbidity currents. The redeposited assemblages are absent from underlying Upper Pleistocene sediments deposited during a cold interval, but reoccur in warmer water (Würm) sediment deeper in the section.

Analogies to oceanic behaviour in the continental breakup of the western Woodlark basin

THE Woodlark basin/D'Entrecasteaux Island region off northeast Papua New Guinea (Fig. 1) offers a rare glimpse of the propagation of active rifting and continental breakup1 into orogenically thickened (and subsequently extended) lithosphere2–10. Rifting, continental breakup, and the subsequent formation of oceanic lithosphere by sea-floor spreading often involve rupture controlled by propagation of a rift tip11–20. We recognize several tectonic elements in the evolving western Woodlark intracontinental system that are close geometric analogues of wholly oceanic propagators, and a kinematic development that can be viewed as an extrapolation of oceanic tectonics. The response to deformational stresses in the final stages of breakup in a thick, relatively hot, and hence weak continental lithosphere appears akin to that of oceanic lithosphere.