Louisville Seamount Research Articles

A critical evaluation of recent models for Lau–Tonga arc–backarc basin magmatic evolution

New trace element, Sr-, Nd-, Pb- and Hf isotope data provide insights into the evolution of the Tonga–Lau Basin subduction system. The involvement of two separate mantle domains, namely Pacific MORB mantle in the pre-rift and early stages of back-arc basin formation, and Indian MORB mantle in the later stages, is confirmed by these results. Contrary to models proposed in recent studies on the basis of Pb isotope and other compositional data, this change in mantle wedge character best explains the shift in the isotopic composition, particularly 143Nd/ 144Nd ratios, of modern Tofua Arc magmas relative to all other arc products from this region. Nevertheless, significant changes in the slab-derived flux during the evolution of the arc system are also required to explain second order variations in magma chemistry. In this region, the slab-derived flux is dominated by fluid; however, these fluids carry Pb with sediment-influenced isotopic signatures, indicating that their source is not restricted to the subducting altered mafic oceanic crust. This has been the case from the earliest magmatic activity in the arc (Eocene) until the present time, with the exception of two periods of magmatic activity recorded in samples from the Lau Islands. Both the Lau Volcanic Group, and Korobasaga Volcanic Group lavas preserve trace element and isotope evidence for a contribution from subducted sediment that was not transported as a fluid, but possibly in the form of a melt. This component shares similarities with that influencing the chemistry of the northern Tofua Arc magmas, suggesting some caution may be required in the adoption of constraints for the latter dependent upon the involvement of sediments from the Louisville Ridge. A key outcome of this study is to demonstrate that the models proposed to explain subduction zone magmatism cannot afford to ignore the small but important contributions made by the mantle wedge to the incompatible trace element inventory of arc magmas.

Geoarchaeology of Tonga: Geotectonic and geomorphic controls

AbstractAncient settlement patterns in central Tonga, at the southeastern limit of Lapita expansion into Remote Oceania ˜3 ka, were conditioned by island geomorphology as controlled by spatial geotectonic features and temporal changes in relative sea level on island coasts. Volcanic islands provided lithic resources, but human populations were concentrated on nonvolcanic forearc islands underlain by limestone covered by airfall tephra blankets that weathered to form rich agricultural soils and eroded to provide terrigenous sand for ceramic temper. The forearc islands lie along the Tonga platform, a linear tract of shoals uplifted diachronously by subduction of the buoyant Louisville Ridge at the Tonga Trench. Multiple transverse structural discontinuities break the forearc into discrete structural blocks, some tectonically stable during late Holocene time but others undergoing postuplift subsidence. Understanding the paleoenvironmental settings of Tongan archaeological sites requires reconstructing the contrasting geologic histories of different forearc island clusters. © 2007 Wiley Periodicals, Inc.

Initiation of a plume‐ridge interaction in the South Pacific recorded by high‐precision Pb isotopes along Hollister Ridge

The southern Pacific Ocean offers the rare possibility to study a situation where a spreading ridge (the Pacific‐Antarctic Ridge (PAR)) migrates toward a fixed hot spot (the Louisville hot spot) (Small, 1995). Hollister Ridge is a 450 km long linear structure whose position, between the PAR axis and the most recent edifices of the Louisville hot spot trail, led some authors to suggest that the ridge is genetically related to the hot spot (Small, 1995; Wessel and Kroenke, 1997). Mapping and sampling of the ridge in 1996 revealed, however, that the contribution of the Louisville plume material to its mantle source is minor and suggested that it might be the result of intraplate deformation (Géli et al., 1998; Vlastélic et al., 1998). We report new, highly precise Pb isotopic data from Hollister Ridge, which (1) confirm that the maximal contribution of the Louisville plume, in the centrally, volcanic active part of the ridge, probably does not exceed 20% (15 and 35% for lower and upper limits) and (2) reveal through time an increasing plume influence. The initiation of the Louisville plume involvement in the source of Hollister Ridge is estimated to have occurred between 1.04 and 0.77 Myr ago. It thus followed closely the most recent volcanic activity reported along the Louisville trail (1.11 Ma (Koppers et al., 2004)). This suggests that Hollister Ridge has recorded the dispersion of the Louisville plume as the spreading ridge approached the hot spot. Assuming that the Louisville hot spot is located near the youngest seamount dredged along the Louisville seamount chain, Hollister Ridge lies along the shortest path of pressure release connecting the hot spot to the spreading axis. This path involves, first, an abrupt upwelling across the Eltanin fault system and, subsequently, a more progressive migration toward the spreading axis. Because Hollister Ridge is older than 2.5 Ma, the structure might not be the consequence of the plume‐ridge flow. Instead, Hollister Ridge most likely emplaced through a lithospheric crack (Géli et al., 1998), which, subsequently, may have captured the plume‐ridge flow.

Implications of a nonlinear 40Ar/39Ar age progression along the Louisville seamount trail for models of fixed and moving hot spots

The Louisville seamount trail has been recognized as one of the key examples of hot spot volcanism, comparable to the classic volcanic Hawaiian‐Emperor lineaments. The published total fusion 40Ar/39Ar data of Watts et al. [1988] showed an astonishing linear age progression, firmly establishing Louisville as a fixed hot spot in the South Pacific mantle. We report new 40Ar/39Ar ages based on high‐resolution incremental heating 40Ar/39Ar dating for the same group of samples, showing a marked increase in both precision and accuracy. One of the key findings in our reexamination is that the age progression is not linear after all. The new data show a significantly decreased “apparent” plate velocity for the Louisville seamount trail older than 62 Ma but confirm the linear trend between 47 Ma and the present day (although based on only three samples over 2150 km). The most recent volcanic activity in the Louisville seamount trail has now been dated at 1.11 ± 0.04 Ma for the most southeastern seamount located at 50°26′S and 139°09′W. These results indicate that the Louisville age progression should be interpreted on the basis of both plate and hot spot motion. In this paper we examine our new results in conjunction with the numerical mantle flow models of Steinberger et al. [2004] that also predict marked deviations from simple linear age progressions. With these models we can achieve a good fit to the geometry of both the Hawaiian and Louisville seamount trails and their age progressions as well as the ∼15° paleolatitudinal shift observed by Tarduno et al. [2003] for the Hawaiian hot spot between 80 and 47 Ma. If the model is restricted to Pacific hot spots only, we can improve the fit to the nonlinear age trend for the Louisville seamount trail by allowing an additional rotation change of the Pacific plate around 62 Ma and by decreasing the initiation age of the Louisville plume from 120 to 90 Ma. This improved model features a significant eastward hot spot motion of ∼5° between 80 and 30 Ma for the Louisville hot spot, which is quite dissimilar to the southward motion of the Hawaiian hot spot during the same time interval, followed by a minor ∼2° latitudinal shift over the last 30 Myr. If hot spot tracks are considered globally, the age trend observed for the oldest part of the Louisville seamount trail does not entirely follow the numerical model predictions. This may indicate some remaining inaccuracies in the global plate circuit, but it may also indicate that the Louisville hot spot experienced a motion somewhat different than in the numerical model: faster in the interval between 62 and 47 Ma but slower before that.

Cenozoic oceanographic evolution of the SW Pacific gateway: introduction

The region from the southern tip of the Campbell Plateau to the northern Hikurangi Plateau spans 21‡ of latitude, equivalent to the distance between southern Florida and Newfoundland in the North Atlantic. From New Zealand eastwards along the Chatham Rise to the Louisville Seamounts is 22‡ of longitude, about 2000 km. This region, equivalent in area to a major part of the North American Basin of the North Atlantic now contains 11 stratigraphically useful drill holes, seven more than the four of the DSDP (Sites 277, 278, 279 and 594). The latest holes, Ocean Drilling Program Sites 1119-1125 were drilled on ODP Leg 181, the ¢rst visit of the drilling ship JOIDES Resolution to the region, in August-October 1998 under arduous southern winter conditions (Fig. 1).

From rifting to active spreading in the Lau Basin – Havre Trough backarc system (SW Pacific): Locking/unlocking induced by seamount chain subduction

Associated with Pacific‐Australia plate convergence, the Lau Basin – Havre Trough is an active back‐arc basin that has been opened since ∼5.5 Ma by rifting and southward propagating oceanic spreading. Current back‐arc opening rates decrease from 159 mm yr−1 in the northern Lau Basin to 15 mm y−1 in the southern Havre Trough. Major tectonic changes occur at the transition between Havre Trough rifting and full oceanic spreading of the Eastern Lau Spreading Center (ELSC), where the oblique‐to‐trench, westward subducting Louisville Seamount Chain (LSC) sweeps southwards along the Tonga trench. New swath bathymetry, seismic reflection data, and limited rock sampling in this area constrain a tectonic and kinematic back‐arc model that incorporates the effects of LSC subduction. The ELSC, which extends southward to 24°55′S, forms a deep rift valley propagating southward through older, rifted arc basement. Present‐day seismicity and fresh and fractured pillow lavas at 23°42′S are consistent with rift valley neovolcanism. Conversely, the northern Havre Trough has low seismicity and rifted volcanic basement ridges trending 25–45° oblique to the basin axis consistent with low levels of extensional tectonism and volcanism. This latter structural fabric is interpreted as an early stage of rifting that is now “locked” due to compression on the arc exerted by LSC subduction, while in the Lau Basin such effects have passed as the LSC swept along the Tonga Trench. It is proposed that the Lau‐Havre back‐arc opening is controlled by tectonic constraints exerted at the limits of the system by the LSC subduction, which determines the southward migration of the Tonga Arc pole of rotation and associated Lau Basin opening. A discrete three‐stage back‐arc opening evolution is proposed, comprising: (1) an initial phase of back‐arc rifting along the whole length of the plate boundary, beginning at ∼6–5 Ma; (2) a subsequent phase, mostly present in the southern part of the back‐arc domain and still active in the Havre Trough, of transpression and transtension, starting at ∼4 Ma in the north, as the LSC starts to subduct and sweeps southward along the Tonga trench; and (3) a renewed opening phase in the northern segment of the back‐arc domain, with rifting and spreading, starting at ∼3.5 Ma, as subduction of the LSC along the northern Tonga trench is progressively completed.

Southern right whales: A reassessment of their former distribution and migration routes in New Zealand waters, including on the Kermadec grounds

Historical and other documents have been re‐analysed to provide new insights into the former abundance, distribution, and seasonal movements of southern right whales (Eubalaena australis (Desmoulins, 1822)) in New Zealand waters. In 1935 Charles Haskins Townsend recorded a puzzling concentration of right whale captures south‐east of the Kermadec Islands. The priority he accorded this area can now be explained by the limitations within his sample. However, a close examination of relevant whaling journals and logbooks has confirmed that both sperm whales and right whales were killed there, sometimes in large numbers. This evidence of many right whales so far north in October and November has prompted a reassessment of the former distribution and migration routes followed by right whales throughout the wider New Zealand region. A seasonal migration is now suggested that includes waters well to the east of New Zealand up to, but not beyond, the Louisville Ridge.

Eastern New Zealand Drifts, Miocene-Recent

Abstract Sediment from New Zealand passes into the north-flowing SW Pacific deep western boundary current (DWBC) to form widespread drifts. South of 49¼S, where the DWBC is reinforced by the Antarctic Circumpolar Current, drifts are small and, in the case of Campbell 'skin' Drift, reduced to a veneer over oceanic sediments. North of 49¼S deposition prevails with (1) Chatham Terrace Drifts - 350 km long and 320 m thick deposits around eastern Chatham Rise at 3000 m depth, (2) Chatham Deep Drift - 400 m thick, 300 km long ridge around the 4500-5000 m deep rise base, (3) Louisville Moat Drift -400 m of sediment within an isostatic depression along Louisville Seamount Chain and (4) Rekohu Drift - a 250 km long, 480 m thick ridge of Oligocene to Recent sediment. These drifts are sparsely surveyed, but two others have sufficient detail to be case studies. North Chatham Drift plasters northern Chatham Rise at 2000-4500 m depth. Drilling recovered 587 m of nannofossil ooze/chalk with cyclical amounts of terrigenous detritus and reworked subantarctic microfossils. Deposition has been almost continuous since the early Miocene. Hikurangi Fan-drift is a 300 km-long lobe, developed down-current of Hikurangi Channel/Fan through redirection of turbidity current plumes by the DWBC at 4850->5500 m depth. Seismically, the drift core comprises 200-360 m of probable Miocene palaeo-drift formed prior to the Pleistocene inception of Hikurangi Channel when 400 m of fan-drift accumulated.

The Rapuhia Scarp (northern Hikurangi Plateau) — its nature and subduction effects on the Kermadec Trench

Multibeam bathymetric data, seismic reflection and magnetic data are used to examine the nature of the 1-km-high Rapuhia Scarp and the tectonic effects of its subduction on the Kermadec margin. The Rapuhia Scarp is the boundary between the Hikurangi Plateau and >85Ma oceanic crust. Its intersection with the Kermadec Trench moves south at 4cm/yr. Magnetic modelling indicates that the northern Hikurangi Plateau may be formed of 5km of relatively strongly-magnetised extrusive and intrusive volcanics rocks. Uplift of the Rapuhia Scarp near the trench is consistent with vertical motion on a tear-fault in the subducting plate.The coincident intersection of the subducting Rapuhia Scarp with a NNW-trending volcanic ridge (Intersection Ridge) and the Kermadec Trench results in an abrupt 1.5km increase in the trench depth and inner margin topography. The Rapuhia Scarp boundary sweeps south along the margin. Slumping and subsidence of the inner margin resulting from this transition occur on a variety of scales, from 5-km-wide, 1-km-high arcuate frontal erosion slumps to regional subsidence of up to 4km across the 50–100-km-wide mid-upper inner trench margin as it adjusts to the new subduction geometry. The pattern of inner margin subsidence involving the entire forearc is broadly similar to that observed 1200km further north in the wake of subduction of the Louisville Ridge seamount chain.Subsidence of the inner trench margin results in the loss of approximately 400km3 per kilometre of margin as the subducting Hikurangi Plateau support is removed. We estimate however only one fifth of the above subsidence is frontal or basal erosion highlighting the risks of estimating tectonic erosion from margin subsidence if the cause of subsidence is mis-identified. The remaining 320km2 of subsidence is due to the steeper dip of the subducting plate associated with subduction of the comparatively dense >85Ma oceanic crust.

The Effect of the Mesoparasitic Barnacle Anelasma on the Development of Reproductive Organs of Deep-sea Squaloid Sharks, Centroscyllium and Etmopterus

Specimens of the mesoparasitic barnacle, Anelasma, were found attached to several host deep-sea squaloid sharks, Centroscyllium fabricii from off Greenland, Etmopterus princeps from the Canary Islands, and Etmopterus unicolor and Etmopterus granulosus from Three Kings Ridge and Louisville Ridge near New Zealand. One to three parasites were partially embedded in each host shark in the head, mouth, bases of 1st and 2nd dorsal fins, pectoral and pelvic fins, caudal fin, abdomen or claspers. Testes and claspers of parasitized mature-sized males were less developed than those of non-parasitized mature males. The ova of parasitized females of mature size were small and whitish-yellow compared to the large yellow ova of non-parasitized mature females. The number of mature ova in a parasitized specimen was lower than expected in relation to a non-parasitized individual of the same size. Attachment of parasitic barnacles appeared to retard the development of reproductive organs of host sharks.

Three‐dimensional estimation of elastic thickness under the Louisville Ridge

A three‐dimensional approach to estimating elastic thickness is presented which uses dense satellite altimetry and sparse ship bathymetry. This technique is applied to the Louisville Ridge system to study the tectonic history of the region. The inversion is performed as both a first‐order approximation and a nonlinear relationship between gravity and topography based on Parker's [1973] equation. While the higher‐order effect on the gravity anomaly is nearly zero for most of the region, the magnitude is significant over the summits of the ridge. Nevertheless, the inclusion of the nonlinear terms has only a minor influence on the elastic thickness estimate within each region, lowering the value by ∼1–2 km compared with the linear result. The incorrect assumption of two dimensionality for circular features exhibits a marked effect on the gravitational anomaly, resulting in false sidelobe structure of nearly 20 mGal for large seamounts. Our elastic thickness estimates are compared with the contradictory values obtained in previous studies by Cazenave and Dominh [1984] and Watts et al.. [1988]. We find an increasing elastic thickness along the chain from southeast to northwest, with a discontinuity along the Wishbone scarp. The jump in elastic thickness values northwest of the scarp appears to be an indication of an age discontinuity caused by an extinct spreading center north of the ridge.

Bathymetry of the Tonga Trench and Forearc: a map series

Four new bathymetric maps of the Tonga Trench and forearc between 14S and 27S display the important morphologic and structural features of this dynamic convergent margin. The maps document a number of important geologic features of the margin. Major normal faults and fault lineaments on the Tonga platform can be traced along and across the upper trench slope. Numerous submarine canyons incised in the landward slope of the trench mark the pathways of sediment transport from the platform to mid- and lower-slope basins. Discontinuities in the trench axis and changes in the morphology of the landward slope can be clearly documented and may be associated with the passage and subduction of the Louisville Ridge and other structures on the subducting Pacific Plate. Changes in the morphology of the forearc as convergence changes from normal in the south to highly-oblique in the north are clearly documented. The bathymetric compilations, gridded at 500- and 200-m resolutions and extending along500 km of the landward trench slope and axis, provide complete coverage of the outer forearc from the latitude of the Louisville Ridge-Tonga Trench collision to the northern terminus of the Tonga Ridge. These maps should serve as a valuable reference for other sea-going programs in the region, particularly the Ocean Drilling Program (ODP) and the National Science Foundation MARGINS initiative.

Slow rates of subduction erosion estimated from subsidence and tilting of the Tonga forearc

The convergent margins of the western Pacific are marked by widespread tectonic erosion of the overriding plate by the subducting oceanic lithosphere. At Ocean Drilling Program Site 841 on the mid-trench slope of the Tonga forearc, rates of tectonic trenchward tilting and subsidence of the basement appear to be linked, supporting models of basal erosion focused near the trench. Rates of subsidence significantly exceed those predicted from plate cooling models and require crustal thinning to account for the difference in this extensional environment. Estimated extension at Site 841 is 280%. Tilting and subsidence rates peaked ca. 32–34 Ma, just after spreading in the South Fiji Basin at 35 Ma. However, the trenchward direction of tilt and higher extension on the trench slope than on the Tonga platform suggest a link to tectonic erosion near the trench rather than extension focused in the arc edifice. Since this time, ca. 135 km has been eroded from the edge of the plate, of which 80 km is related to the ongoing collision of the Louisville Ridge. A probable paleo–collision event at 16 Ma caused the forearc at Site 841 to be uplifted and tilted arcward, forming a major unconformity due to mass wasting. Steady-state erosion averages <1.5 km/m.y. There is no compelling evidence to suggest loss of several hundred kilometers of arc crust since the Eocene and thus no reason to suspect that subduction initiation and related magmatism may have predated the ca. 45 Ma boninitic magmatic activity.

Tectonic controls on sedimentation and diagenesis in the Tonga Trench and forearc, southwest Pacific

Sedimentation in the Tonga forearc is dominated by the redeposition of volcaniclastic sediment from the arc volcanic front by mass flows and turbidity currents onto the adjacent Tonga Platform, the shallowest, flattest part of the forearc region. The greatest sediment thicknesses accumulate in debris aprons close to the volcanic front. Collision of seamounts, notably the Capricorn Seamount and the Louisville Ridge, with the forearc radically shortens and steepens the adjacent modern trench slope, allowing sediment to be redeposited deep into the trench. Rotation, usually arcward, of existing basins on the midslope during collision generates angular unconformities, while synchronous uplift of the outer forearc high results in canyon development and downcutting along the eastern edge of the Tonga Platform. Collision also reactivated major across-strike fault zones on the forearc; the zones are subsequently exploited by canyons depositing sediment into the trench. Collapse and renewed extension of the forearc in the wake of collision result in the development of small perched basins, measuring approximately 5 km by 15 km in the midslope area. This morphology is especially developed at 18°30′S to 20°S, implying a 2–3 m.y. interval for their formation following Louisville Ridge collision. Trenchward of these depocenters sedimentation is slow, resulting in manganese crust formation and localized mass wasting along fault scarps. Over longer periods of time (>5 m.y.) tectonic erosion reestablishes a wide, gently sloping forearc into which canyons incise the shallow Tonga Platform by headwall erosion and collapse.

Geochemical Evolution within the Tonga-Kermadec-Lau Arc-Back-arc Systems: the Role of Varying Mantle Wedge Composition in Space and Time

New trace element and Sr, Nd, and Pb isotope data for lavas from the active Tonga–Kermadec arc in the southwest Pacific, the volcano of Niua fo'ou in the back-arc Lau Basin, and Pacific Ocean sediments from DSDP Sites 204 and 275, and ODP Site 596, are integrated with existing geochemical data for lavas from the Lau Basin, Samoa, the Louisville Ridge Seamount Chain (LR-SMC) and the extinct Lau Ridge arc, giving new insights into the petrogenesis of lavas in an active arc–back-arc system. Geochemical variations in Tonga–Kermadec arc lavas are the result of (1) differences in the amount and composition of the material being subducted along the arc, and (2) pre-existing heterogeneities in the upper mantle. Differences in the material being subducted beneath the arc have an important influence on the chemistry of the arc lavas. At the Kermadec Trench, ∼1 km thick layer of sediment is being subducted beneath the arc, compared with ∼200 m at the Tonga Trench. This results in the high Th/U and more radiogenic Pb isotope compositions of Kermadec lavas compared with Tonga lavas. The latter have Pb isotope compositions intermediate between those of Pacific sediments and Pacific mid-ocean ridge basalt (MORB), suggesting that much of the Pb in these lavas is derived from subducting Pacific Ocean crust. This is supported by the Pb isotope signatures of the subducting LR-SMC, which are also observed in lavas from the northern Tongan islands of Tafahi and Niuatoputapu. High field strength element (HFSE) and heavy rare earth element (HREE) concentrations are generally lower in Tongan lavas (particularly those from northern Tongan islands) than in Kermadec lavas. The Tonga Ridge basement, the proto-Tonga arc lavas (ODP Site 839) and the older Lau Ridge arc lavas are generally less depleted than the modern arc lavas. In the back-arc region, upper-mantle depletion as inferred from HFSE and HREE contents of the lavas broadly increases eastwards across the Lau Basin, whereas the subduction signature and volatile (CO2 and F) contents increase eastwards towards the modern arc. These observations suggest that depletion is due to melt extraction during back-arc extension and volcanism, together with a long ‘residence time’ of mantle material within the mantle wedge. The upper mantle beneath the northernmost end of the Tonga arc and Lau Basin contains an ocean-island basalt (OIB) component derived from the Samoa plume to the north. This is reflected in high concentrations of Nb relative to other HFSE in lavas from Niua fo'ou, and Tafahi and Niuatoputapu islands at the northern end of the Tonga arc. Pb isotopes also suggest an LR-SMC contribution into Tafahi and Niuataputapu. Trace element and isotope modelling is used to investigate the combined effects of varying mantle source depletion and subduction on the geochemistry of the arc lavas. The results suggest that the arc lava geochemistry can be explained largely by the balance between a relatively constant subduction input of Pb, Th, U, Cs, Ba, Sr, Rb, K and Sc [corresponding to 0.001–0.005 weight fraction of the Stolper & Newman (1994, Earth and Planetary Science Letters, 121, 293–325] ‘H2O-rich component’ composition), into the overlying, but variably depleted mantle wedge.

Geochemical Evolution within the Tonga-Kermadec-Lau Arc-Back-arc Systems: the Role of Varying Mantle Wedge Composition in Space and Time

New trace element and Sr, Nd, and Pb isotope data for lavas from the active Tonga-Kermadec arc in the southwest Pacific, the volcano of Niua fo'ou in the back-arc Lau Basin, and Pacific Ocean sediments from DSDP Sites 204 and 275, and ODP Site 596, are integrated with existing geochemical data for lavas from the Lau Basin, Samoa, the Louisville Ridge Seamount Chain (LR-SMC) and the extinct Lau Ridge arc, giving new insights into the petrogenesis of lavas in an active arc - back-arc system. Geochemical variations in Tonga-Kermadec arc lavas are the result of (1) differences in the amount and composition of the material being subducted along the arc, and (2) pre-existing heterogeneities in the upper mantle. Differences in the material being subducted beneath the arc have an important influence on the chemistry of the arc lavas. At the Kermadec Trench, ∼1 km thick layer of sediment is being subducted beneath the arc, compared with ∼200 m at the Tonga Trench. This results in the high Th/U and more radiogenic Pb isotope compositions of Kermadec lavas compared with Tonga lavas. The latter have Pb isotope compositions intermediate between those of Pacific sediments and Pacific mid-ocean ridge basalt (MORB), suggesting that much of the Pb in these lavas is derived from subducting Pacific Ocean crust. This is supported by the Pb isotope signatures of the subducting LR-SMC, which are also observed in lavas from the northern Tongan islands of Tafahi and Niuatoputapu. High field strength element (HFSE) and heavy rare earth element (HREE) concentrations are generally lower in Tongan lavas (particularly those from northern Tongan islands) than in Kermadec lavas. The Tonga Ridge basement, the proto-Tonga arc lavas (ODP Site 839) and the older Lau Ridge arc lavas are generally less depleted than the modern arc lavas. In the back-arc region, upper-mantle depletion as inferred from HFSE and HREE contents of the lavas broadly increases eastwards across the Lau Basin, whereas the subduction signature and volatile (CO and F) contents increase eastwards towards the modern arc. These observations suggest thai depletion is due to melt extraction during back-arc extension and vokanism, together with a long 'residence time' of mantle material within the mantle wedge. The upper mantle beneath the northernmost end of the Tonga arc and Lau Basin contains an ocean-island basalt (OIB) component derived from the Samoa plume to the north. This is reflected in high concentrations of Nb relative to other HFSE in lavas from Niua fo'ou, and Tafahi and Niuatoputapu islands at the northern end of the Tonga arc. Pb isotopes also suggest an LR-SMC contribution into Tafahi and Niuataputapu. Trace element and isotope modelling is used to investigate the combined effects of varying mantle source depletion and subduction on the geochemistry of the arc lavas. The results suggest that the arc lava geochemistry can be explained largely by the balance between a relatively constant subduction input of Pb, Th, U, Cs, Ba, Sr, Rb, K and Sc [corresponding to 0.001-0.005 weight fraction of the Stolper & Newman (1994, Earth and Planetary Science Letters, 121, 293-325] 'HO-rich component' composition), into the overlying, but variably depleted mantle wedge.

Subduction obstruction and the crack-up of the Pacific plate

We consider two hypotheses for the origin of a linear trough extending ∼800 km eastward from the intersection of the Louisville seamount chain with the Tonga-Kermadec trench. An extinct spreading center origin is suggested by the trough's eastward termination at a depth discontinuity that resembles a fracture zone. The morphology of the trough, however, is inconsistent with the morphology of other extinct spreading centers, and these two features do not appear to be connected to a larger paleo-plate boundary on their eastern end. We consider an alternative hypothesis in which the trough may be a recent rift in the Pacific plate. Coupling between a large subducting seamount and the overriding Australian plate may serve as a mechanism to focus stresses and nucleate a rift in the subducting Pacific plate. In this model, the seamount would act as a rigid indenter producing tangential extension in the subducting plate about the region where it is coupled to the overriding plate. Impeded subduction is suggested by a prominent seismicity gap around the subducting seamount. The graben morphology of the trough is also consistent with an extensional origin. If hotspot volcanism can produce obstructions and local weaknesses conducive to plate coupling, stress localization, and nucleation of fractures, it may provide a mechanism for subduction-induced plate reorganizations.

238U 230Th disequilibria, magma petrogenesis, and flux rates beneath the depleted Tonga-Kermadec island arc

The highly depleted intra-oceanic Tonga-Kermadec island arc forms an endmember of arc systems and a unique location in which to isolate the effects of the slab flux. High precision TIMS uranium, thorium, strontium, neodymium, and lead isotopes, along with complete major and trace element data, have been obtained on an extensive sample set comprising fifty-eight lavas along the arc as well as nineteen samples of the subducting sediments at DSDP site 204 just to the east of the Tonga-Kermadec trench. Ca/Ti and Al/Ti ratios extend from values appropriate to an N-MORB source in the southern Kermadecs to very high ratios in Tonga interpreted to reflect increasing degrees of depletion of the mantle wedge due to backarc basalt extraction. The isotope data emphasize the need for four components in the petrogenesis of the lavas: (1) the mantle wedge; (2) a component with elevated 207Pb/ 204Pb towards which the Kermadec and southern Tongan lavas extend; (3) a component characterised by high 206Pb/ 204Pb, Ta/Nd, and low 143Nd/ 144Nd observed only in the northernmost Tongan islands of Tafahi and Niuatoputapu; (4) a fluid component characterised by strong enrichments of Rb, Ba, U, K, Ph, and Sr, relative to Th, Zr, and the REE and producing large 211U excesses (( 230Th/ 238U) = 0.8−0.5) in the more depleted lavas. The mantle wedge (Component 1) is isotopically similar to the source of the Lau BABB. Component 2 is average pelagic sediment on the downgoing Pacific plate as observed at DSDP sites 595/596 and in the upper sections of the sediment pile at DSDP site 204. Mass balance calculations indicate that less than 0.5% is recycled into the arc lavas; essentially all the subducted sediment is returned to the upper mantle (∼0.03 km' yr −1). Exceptionally low concentrations of Ta and Nb relative to Th and the LREE requires that this sediment component is added as a partial melt which was in equilibrium with residual rutile or ilmenite. Component 3 is identified as volcaniclastics from the Louisville Ridge which comprise the lower 44 m of the sediment section intersected at DSDP site 204. These volcaniclastics are spatially restricted to the vicinity of the Louisville Ridge and provide a unique sediment tracer which can be used to show that it takes ∼4 Myr from the time of subduction to its first appearance in the arc lava signature. Component 4, the fluid contribution to the lava source is inferred to contribute ∼1 ppm Rb, 10 ppm Ba, 0.02 ppm U, 600 ppm K, 0.2 ppm Ph, and 30 ppm Sr. It has 87Sr/ 86Sr = 0.7035 and 206Pb/ 204Pb = 18.5 and thus it is inferred to have been derived from dehydration of the subducting altered oceanic crust. UTh isotope disequilibria reflect the time since fluid release from the subducting slab and a reference line through the lowest ( 230Th/ 232Th) lavas constrains this to be 30,000–50,000 yr. The UTh and ThRa isotope systematics are decoupled, and it is suggested that ThRa isotope disequilibria record the time since partial melting and thus indicate rapid channelled magma ascent. Olivine gabbro xenoliths from Raoul are interpreted as cumulates to their host lavas with which they form zero age UTh isochrons indicating that minimal time was spent in magma chambers. The subduction signature is not observed in lavas from the backarc island of Niuafo'ou. These were derived from partial melting of fertile peridotite at 130–160 km depth with melt rates around 2 × 10 −4 kg m −3 yr −1.

Trace element transport rates in subduction zones: evidence from Th, Sr and Pb isotope data for Tonga-Kermadec arc lavas

Trace element and Th, Sr and Pb isotope data for young lavas from the Tonga-Kermadec arc in the southwest Pacific suggest that geochemical variations in the lavas along the arc are linked to differences in the material being subducted beneath the arc. Lavas from the southern (Kermadec) segment of the arc have relatively radiogenic Pb isotope compositions, which reflects a contribution from subducted sediment. In contrast, much of the Pb in Tonga lavas is derived from the altered oceanic crust in the subducting Pacific Plate, and lavas from the northernmost Tonga islands of Tafahi and Niuatoputapu contain Pb and Sr derived from the subducted part of the Louisville Seamount Chain. The origin of the Pb in the lavas from these two islands can thus be traced to a point on the subducting slab, and this observation is used to estimate the rate at which trace elements are transported beneath the arc. Our calculations suggest that fluid-soluble elements such as U, Sr and Pb are transported from the subducted slab, across the mantle wedge and back to the surface in lavas over a period of approximately 2–3 Ma, and that magmas are erupted at the surface less than 350 ka after the melts are generated in the mantle wedge.

Recent sedimentation beneath the Deep Western Boundary Current off northern New Zealand

The triangular shaped region defined by Chatham Rise in the south, the Louisville Seamount Chain in the east and the Pacific/Australian plate boundary in the west is one of the most active sedimentary regions of the world ocean. The SW Pacific Deep Western Boundary Current (DWBC), with a transport of 20 Sv, flows through the area producing regions of scour, nepheloid layers and typical depositional bedforms. Sediment is supplied to the region by turbidity currents directly via Hikurangi Channel and by the DWBC, which removes material from Bounty Fan south of Chatham Rise, as well as by fallout of volcanic ash and pelagic biogenic material. The CaCO 3 content of the sediments is very much controlled by terrigenous dilution, with Chatham Rise having ∼ 60%, but Hikurangi Plateau (near the Channel), sited well above the 4750 m-deep CCD, having only ∼ 20%. Although there is much evidence of scour around pinnacles and in scoured, possibly furrowed, mud deposits seen on 3.5 kHz profiles, dilute nepheloid layers, slow geostrophic and measured velocities, photographic and sediment grain-size evidence do not indicate fast flows. Mudwaves are of the irregular, vertically migrating type (rather than progressive antidunes), suggesting flows < ∼ 0.10 m s −1. In common with other Southern Ocean areas, flows may have been strongest during glacial times. Hikurangi Channel delivers its turbidity currents into the path of the DWBC, which sweeps them along to create a deep-sea fan with the characteristics of a contourite drift, here termed a fan-drift. This is mantled with migrating, climbing mudwaves, which are ascribed to deposition from episodic turbidity currents travelling at more than 0.1 m s −1. The fan-drift forms on the right-hand side of a boundary channel along the foot of the 1000 m-high Rapuhia Scarp. It is interpreted as a large right-bank levee formed by turbidity currents overspilling the Channel, which also owes its origin to scour by the DWBC. The turbidite deposits are dominantly mud and the evidence of scour/deposition patterns around a swath-mapped seamount indicate the depositing flows to be consistent with the DWBC flow direction, demonstrating entrainment of the turbidity currents by the deep geostrophic flow. Patterns of sedimentation around the Louisville Seamount Chain indicate action of a strong filament of the DWBC on the east side of the seamounts at least as far north as 37°S, and a relative intensification of the current on the west side of the seamounts north of 38°S.