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 singlecrystal 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 516.91m 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 (<150m) to a deeper-water succession (150–500 m) 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.

Petrographic examination of temper sands in prehistoric Oceanian pottery collected by archaeologists from island groups spread across the tropical Pacific Ocean shows that the sands vary compositionally in geographic patterns that are governed by geotectonic setting and vagaries of local bedrock exposure on individual islands. The small islands serve as virtual point sources of sediment derived exclusively from the restricted array of rocks that form each island. Both natural and manually added tempers can be traced to bedrock sources by the same petrographic methodology, but independently sourcing clay bodies requires geochemical comparison of clay pastes with potential clay sources. Oceanian tempers include calcareous as well as terrigenous sands, but only the latter can be associated unequivocally with specific islands or island groups because the nature of reef tracts is similar throughout the tropical Pacific. Exotic tempers can be distinguished from indigenous tempers because their compositions are incompatible with the geology of the islands where the exotic sherds are found. Human migration into islands of the Pacific Ocean was the last main stage in human dispersal over the planet, with no human occupation of the small islands lying beyond island Southeast Asia and Australasia until 1500 B.C. The earliest inhabitants possessed a ceramic culture, and ceramic traditions evolved over subsequent centuries to produce a varied succession of ceramic phases. Lapita pottery, which is the oldest ware in southwest Pacific island groups, is especially notable because its production was limited to a time frame short enough to allow Lapita sherds to serve a role akin to index fossils. Temper sands in Lapita and post-Lapita sherds from the same locales are indistinguishable and show that salient temper contrasts are controlled by island geology rather than habits of ancient potters. Prehistoric collecting sites for temper sand were not necessarily identical to places where modern sand accumulates because of severe environmental changes on many islands. The compositions of terrigenous temper sands in Pacific Oceania reflect the complex pattern of circum-Pacific plate boundaries and intra-Pacific hotspot chains, and define oceanic basalt, andesitic arc, postarc-backarc, dissected orogen, and tectonic highland temper classes composed of different associations of grain types. The geographic distribution of different temper classes reflects not only the current geotectonic setting of each island group but also their paleotectonic settings when exposed rock assemblages were formed. Temper aggregates include beach, stream, and rarely dune sands, as well as grog (brokensherd) and crushed-rock particles in some island groups. Terrigenous grain types in Oceanian temper sands are subdivided by petrographic analysis into three main groups: light mineral grains including quartz and feldspars, heavyferromagnesian mineral grains including opaque iron oxides and ferromagnesian silicates, and a variety of polycrystalline lithic fragments that are dominantly of volcanic derivation in most temper suites. Useful triangular compositional diagrams plot relative proportions of grain types for populations of total terrigenous grains, mineral grains exclusive of lithic fragments, ferromagnesian silicate mineral grains, all non-ferromagnesian grains, only transparent mineral grains, and exclusively quartz and feldspar mineral grains. Supplemental grain parameters or indices express ratios of grain types among quartz and feldspar mineral grains, ferromagnesian grains, and lithic fragments. Oceanic basalt tempers are mineralogically simple volcanic sands derived from basaltic to basanitic volcanic assemblages of intraoceanic hotspot chains erupted in the interior of the Pacific plate in the eastern Caroline Islands, along the northern Melanesian borderland, in Samoa and American Samoa, and in the Marquesas Islands. Andesitic arc tempers are volcanic sands displaying more compositional variability and are the most abundant tempers within the region of Oceanian ceramic cultures, occurring along island arcs flanking the Philippine Sea plate, bounding the Banda Sea in eastern Indonesia, within the Bismarck Archipelago east of New Guinea, along the reversed-polarity Solomon and Vanuatu arcs, on the Fiji platform and the Lau remnant arc, and in Tonga. Postarc and backarc volcanic sand tempers, variously displaying affinities with both oceanic basalt and andesitic arc tempers, are known from the Bismarck Archipelago, the Vanuatu backarc region, the Horne Islands of the northern Melanesian borderland, and both the Fiji platform and the Lau remnant arc. All volcanic sand tempers of Pacific Oceania are composed of phenocrystic mineral grains and volcanic lithic fragments. Most are quartz-free or quartz-poor, but quartzose variants are present locally along island arcs where silicic eruptions accompanied more typical andesitic to basaltic activity, and within backarc settings where bimodal igneous assemblages are exposed. Most quartzose Oceanian temper sands are either dissected orogen tempers containing dominantly igneous but not exclusively volcanic detritus, or tectonic highland tempers containing recycled sedimentary detritus. Dissected orogen tempers with quartz-ose plutonic detritus occur in selected sherd suites from the Torres Strait Islands, the Bismarck Archipelago, and the Solomon Islands, but are especially characteristic from the south coast of Viti Levu in Fiji. Quartzose tectonic highland tempers occur in sherds from the outer Banda arc, the Aru Islands in the Arafura Sea, the D'Entrecasteaux Islands of the Solomon Sea, and New Caledonia. Nonquartzose tectonic highland tempers derived from ophiolitic rocks of uplifted oceanic crust are present in sherds from Yap and New Caledonia. Comparisons of temper compositions among temper classes indicate that oceanic basalt and basaltic backarc tempers contain significantly higher proportions of olivine mineral grains than arc and postarc tempers, which include a varied array of temper types containing different proportions of pyroxenes and hornblendes. Dissected orogen and quartzose andesitic arc tempers display varying proportions of quartz, plagioclase, and K-feldspar within the compositional field typical for circum-Pacific orogenic sands. Tectonic highland tempers contain distinctly higher proportions of nonigneous lithic fragments than other temper classes. The presence of exotic sherds containing temper sands incompatible with the geology of the islands from which they were recovered documents 106 instances of ceramic transfer between different islands, mostly lying within the same island groups, but also between island groups lying far apart. Two-thirds of the instances of ceramic transfer involved interisland distances of less than 200 km, and most of the remainder involved distances in the range of 200–600 km, but a few cases of ceramic transfer for 1000 km or more are known from temper analysis.

Abstract The late Cenozoic arc-type volcanic arc in southeastern Papua New Guinea developed in an environment of complex tectonic processes including obduction, subduction, rifting and sea floor spreading. The volcanic arc extends from the Papuan Peninsula south-eastward through the D'Entrecasteaux Islands into the Louisiade Archipelago. Lithologies are predominantly basaltic andesite and andesite, but include basalt, dacite and rhyolite. The rocks have typical arc-type geochemical features but include a group ranging from basalt to dacite which, although comparable in most other aspects of their compositions, are higher in MgO, Cr and Ni. These high-Mg rocks are less porphyritic and have simple olivine- or clinopyroxene- dominated phenocryst assemblages compared with the associated low-Mg rocks. The low-Mg rocks are plagioclase-phyric and contain augite and hypersthene with or without olivine, hornblende and biotite phenocrysts. Boninites are spatially associated with, but genetically unrelated to the arc-type rocks in Papua. The high-Mg rocks represent magmas derived by partial melting of subduction-modified mantle which rose rapidly from their source. In contrast, the low-Mg lavas represent magmas which were modified by shallow processes. The unusual abundance of high-Mg lavas in southeastern Papua is related to extensional tectonics which allowed deep sourced magmas to rise without significant modification. Supplementary material: Major and trace element analyses of lavas from the Papuan volcanic arc are available at http://www.geolsoc.org.uk/SUP18644

The origin of map-view bends of orogenic belts and arcs remains enigmatic. Here I summarize geological evidence indicating that a bend of the northern end of a ribbon continent extending north from the Northland Peninsula, New Zealand, through New Caledonia and the Loyalty Islands and into the submarine d’Entrecasteaux ridge (the NNNCd’E ribbon continent) is an orocline that has formed as a result of oroclinal orogeny (buckling about vertical axes of rotation due to pinning of the leading edge of a migrating lithospheric beam). An analogue model is used to investigate the geometric relationship between orocline development, the rotation of the Vanuatu–New Hebrides arc, and the origin of the North Fiji basin. The NNNCd’E ribbon continent terminates to the northeast in the Vanuatu–New Hebrides arc. The asymmetric, triangular, northwest-tapering North Fiji lies northeast of (behind) the arc. Paleomagnetic, geological, and geodetic data imply that since 10 Ma the Vanuatu–New Hebrides arc has rotated ~60° clockwise about a pole of rotation located at its northwest end, opening the North Fiji basin. The analogue model demonstrates that the arc was forced to rotate clockwise due to its southward advance being impeded by the NNNCd’E ribbon continent. In this model, the ribbon continent originated as a linear, north-trending feature that ended in the arc, just west of its midpoint. Southward advance of the arc buckled the ribbon continent, giving rise to the orocline. Buckling forced the arc to rotate clockwise, opening the asymmetric North Fiji basin. The lithospheric-scale of buckling is consistent with orocline formation controlling the clockwise rotation of the Vanuatu–New Hebrides arc and formation of the North Fiji basin. Buckling of the NNNCd’E ribbon continent against the migrating Vanuatu–New Hebrides arc explains the curved ribbon continent, the clockwise rotation of the arc, and the origin of the North Fiji basin. This ongoing oroclinal orogeny provides us with an opportunity to further understand the processes responsible for and involved in the buckling of lithospheric beams and to refine interpretations of ancient orogens that are thought to be the products of oroclinal orogeny.

Abstract The New Hebrides island arc strikes northward through the southwestern Pacific Ocean and parallels the trench where the Australia-India plate on the west is subducted beneath the North Fiji basin on the east. Convergence between the oceanic plate and the arc occurs in an easterly direction (N75 − E) as determined from earthquake focal mechanisms (Pascal et al., 1978; Isacks et al., 1981). However, the rate of convergence is poorly constrained because of plate motion beneath the North Fiji basin. Chase (1971) estimates an 8 cm/yr convergence rate at the New Hebrides trench, whereas Taylor et al. (1985) propose a convergence rate between 11 and 20 cm/yr. The d'Entrecasteaux Zone (DEZ) is a large submarine mountain system that extends north and east from New Caledonia to where it collides with the New Hebrides arc. Near the arc the DEZ consists of two subparallel ridges that trend east-west and show considerable topographic relief. Collision of the DEZ with the arc has apparently caused abrupt transitions from a single axial line of volcanic islands south and north of the intersection to triple chains of islands at the intersection. The Aoba basin formed within the arc's summit and lies beneath 2000 to 3000 m of water. Although summit basins are present outside of the collision zone, the Aoba basin has greater bathymetric expressions than any other summit basin. The main objective of this discussion is to describe two seismic lines—-one obtained within and one obtained outside of this collision zone—-to illustrate the contrast in structure