Foundation Seamounts Research Articles

The geodynamics of plume-influenced mid-ocean ridges: insights from the Foundation Segment of the Pacific-Antarctic Ridge

The intersection of the Foundation Plume with the Pacific-Antarctic Ridge is a key location in global geodynamics where a mantle plume is approached by and interacting with a fast-spreading mid-ocean ridge. Here, we discuss a comprehensive major and trace element and Sr-Nd-Pb isotope dataset of new and existing samples from the young Foundation Seamount Chain (<5 Ma) and adjacent section of the Pacific-Antarctic Ridge. We use the geochemistry of axial, off-axis and intraplate lavas to map the spatial extent of plume dispersal underneath the ridge as well as the internal zonation of the upwelling plume. We show that the unusual length, increased crustal thickness and occurrence of silicic rocks on the axis of the Foundation Segment are the direct result of plume being tapped by the axial melting zone. We demonstrate that the plume is not homogeneous but shows a HIMU-like (high time-integrated 238U/204Pb) OIB (Ocean Island Basalt) component characterized by 206Pb/204Pb of up to 20.5 in its center and a more EM1-like (Enriched Mantle one) OIB component characterized by low U/Pb and 206Pb/204Pb but high Rb/Nb and 87Sr/86Sr towards its edges. Plume entrainment leads to a high magma supply rate that fosters the formation of silicic rocks and triggers the lengthening of the segment over time. However, plume dispersal is not symmetric as the geochemical tracers for the OIB component are extending <100 km northwards but >300 km southwards. We relate this to the current plate tectonic framework in which the obliquity between the migrating ridge and the absolute plate motions induces a sub-axial asthenospheric flow that preferentially channels plume material southwards.

Accuracy and Resolution of SWOT Altimetry: Foundation Seamounts

AbstractWe assess the accuracy and spatial resolution of the surface water and ocean topography (SWOT) swath altimeter for deriving marine gravity anomalies. The analysis is performed at the Foundation Seamounts in the South Pacific where we developed a highly accurate gravity field by combining the long‐wavelength (>40 km) gravity field derived from previous nadir altimeters with the shorter wavelength gravity field from the seafloor topography as constrained by the ship gravity. In this region, the sea surface slope (SSS) of the ocean variability is 50–100 times smaller than the slope signal of the seamounts so can be ignored in the analysis. Each SWOT cycle can deliver SSS accurately with a standard deviation of 2.6 µrad and a spatial resolution of 14 km at typical sea state (significant wave height 4 m), with accuracy diminishing at high significant wave heights (e.g., exceeds 6 m). Averaging repeated SWOT measurements improves the accuracy and resolution. For example, we expect that averaging just 10 repeats (7 months) results in accuracy/resolution that matches the best marine gravity maps based on 230 months of nadir altimetry. With a mission lasting over a year, SWOT promises a substantial leap in marine gravity accuracy and resolution, uncovering previously uncharted details of the seafloor, including thousands of uncharted seamounts.

Oxygen isotope evidence for the formation of andesitic–dacitic magmas from the fast-spreading Pacific–Antarctic Rise by assimilation–fractional crystallisation

Andesitic to dacitic lavas occur along a 300km long portion of the Pacific–Antarctic Rise close to the intersection of the spreading axis with the Foundation seamount chain. The fresh silicic glasses have low δ18O isotope values between 5.6‰ and 5.1‰ whereas basaltic glasses from the same ridge section have normal MORB δ18O values. Additionally, two FeTi basaltic and all silicic glasses have high Cl (up to 1.1wt.%) and K2O (up to 1.6wt.%) contents, indicating assimilation of hydrothermally altered material. Modelling suggests that the fractionating magma assimilated up to 30% of hydrothermally altered material after 57% fractional crystallisation of the basaltic magma in a melt lens at less than 2km depth. In contrast, the basaltic glasses show little assimilation and clinopyroxene-melt barometry indicates crystal fractionation in deeper melt sills. Relatively low H2O/Ce and Li/Ce ratios as well as the low δ18O values and high Cl and K concentrations in the silicic glasses suggest assimilation of altered crustal rock rather than a brine. While some of the variability in highly incompatible element ratios is best explained by crystal fractionation processes of FeTi-oxides (e.g., decreasing Nb/U) others require a reaction of the melt with residual amphibole and clinopyroxene (Cl/K, Tb/Yb, Hf/Sm, Ce/Yb). The initial onset of FeTi oxide crystallisation is associated with a reduced oxygen fugacity causing sulphide saturation and significant loss of S, Cu, and Co from the evolved melts. This change to more reducing melts as a result of oxide crystallisation is supported by the strong development of a negative Eu anomaly in the andesites and dacites indicating stronger partitioning of Eu into plagioclase. Reverse mineral zoning in the evolved lavas also indicates that replenishment by mafic melts occurs in the shallow melt lens.

Shallow genetic and morphological divergence among seaperches in the South Pacific (family Scorpaenidae; genus Helicolenus)

The phylogenetic relationships among populations of seaperch, Helicolenus spp., in the south-west Pacific were examined with mtDNA markers. Parts of the cytochrome b gene [459 base pair (bp)] and the control region (448 bp) were sequenced in 58 specimens from the south-west Pacific and four specimens of Helicolenus lengerichi from Chile. Only one clade was recognized in New Zealand coastal waters, despite a wide range of colour morphs. This clade also occurred in the mid Tasman Sea on the Norfolk Ridge and around Tasmania and Victoria. A second sympatric clade was identified around Tasmania and Victoria and to the west of New Zealand. A third allopatric clade was identified to the north of New Zealand and in deep water on the Chatham Rise and a fourth clade on the Foundation Seamounts and the Louisville Ridge. Helicolenus lengerichi from Chile formed a fifth clade. Assuming a molecular clock, the clades were estimated to have diverged c. 0.7-2.6 million years ago. Only two clades, around Tasmania and Victoria, were separated using morphology, colour (in live) and dorsal-fin soft ray counts and were confirmed as Helicolenus percoides and Helicolenus barathri. Two characters, orbit diameter and colour variation, previously used to identify two species in New Zealand waters were unreliable characters for species discrimination. Principle component analyses of 11 morphological measures from 67 individuals did not delineate the clades. A canonical discriminant analysis was able to separate four of the five clades, but mean discriminate probabilities were low (77.6%), except for the five Chilean specimens of H. lengerichi (100%).

Plume–ridge interaction revisited: Evidence for melt mixing from He, Ne and Ar isotope and abundance systematics

Submarine volcanic glass He, Ne and Ar data from the Pacific–Antarctic-Rise at its intersection with the Foundation Seamount Chain have been obtained to further elucidate one of the main outstanding problems in plume–ridge interaction studies: the physical state in which mantle flow and mixing occur in those settings. Mixing of plume mantle and mid-ocean ridge basalt source material may either be in the form of two solids, of solid-melt or of two melt phases only. The positive correlation of 4He/ 40Ar ⁎ with 206Pb/ 204Pb as well as the negative correlation of 4He/ 40Ar ⁎ with 40Ar ⁎ and 4He concentrations, combined with a lack of correlation between 4He/ 40Ar ⁎ and ridge depth or MgO concentration, indicate that the enriched mantle plume material has been extensively degassed prior to mixing with the mid-ocean ridge source material. Degassing of the plume source and mid-ocean ridge source material prior to mixing is also suggested by the He versus Pb isotope systematics. However, the occurrence of degassing processes requires the existence of melts, implying that mixing between the plume and the mid-ocean ridge material occurs in the physical form of melts.

Global gravity, bathymetry, and the distribution of submarine volcanism through space and time

The seafloor is characterized by numerous seamounts and oceanic islands which are mainly volcanic in origin. Relatively few of these features (<∼0.1%), however, have been dated, and so little is known about their tectonic setting. One parameter that is sensitive to whether a seamount formed on, near, or far from a mid‐ocean ridge is the elastic thickness, Te, which is a proxy for the long‐term strength of the lithosphere. Most previous studies are based on using the bathymetry to calculate the gravity anomaly for different values of Te and then comparing the calculated and observed gravity anomaly. The problem with such an approach is that bathymetry data are usually limited to single‐beam echo sounder data acquired along a ship track and these data are too sparse to define seamount shape. We therefore use the satellite‐derived gravity anomaly to predict the bathymetry for different values of Te. By comparing the predicted bathymetry to actual shipboard soundings in the vicinity of each locality in the Wessel global seamount database, we have obtained 9758 Te estimates from a wide range of submarine volcanic features in the Pacific, Indian, and Atlantic oceans. Comparisons where there are previous estimates show that bathymetric prediction is a robust way to estimate Te and its upper and lower bounds. Te at sites where there is both a sample and crustal age show considerable scatter, however, and there is no simple relationship between Te and age. Nevertheless, we are able to tentatively assign a tectonic setting to each Te estimate. The most striking results are in the Pacific Ocean where a broad swath of “on‐ridge” volcanism extends from the Foundation seamounts and Ducie Island/Easter Island ridge in the southeast, across the equator, to the Shatsky and Hess rises in the northwest. Interspersed among the on‐ridge volcanism are “flank ridge” and “off‐ridge” features. The Indian and Atlantic oceans also show a mix of tectonic settings. Off‐ridge volcanism dominates in the eastern North Atlantic and northeast Indian oceans, while flank ridge volcanism dominates the northeastern Indian and western south Atlantic oceans. We have been unable to assign the flank ridge and off‐ridge estimates an age, but the on‐ridge estimates generally reflect, we believe, the age of the underlying oceanic crust. We estimate the volume of on‐ridge volcanism to be ∼1.1 × 106 km3 which implies a mean seamount addition rate of ∼0.007 km3 yr−1. Rates appear to have varied through geological time, reaching their peak during the Late/Early Cretaceous and then declining to the present‐day.

A new species of trumpeter (Teleostei; Percomorpha; Latridae) from the central South Pacific Ocean, with a taxonomic review of the striped trumpeter Latris lineata

The silver trumpeter Latris pacifica sp. nov. is described based on 18 specimens collected at 100–300 m depth from the Foundation Seamounts area in the central South Pacific Ocean. It can be distinguished from all other latrids and cheilodactylids in having the following counts: D. XIX‐XX,40–44; A. III,32–37; vertebrae 18+24–25 = 42–43; and a near uniform silvery grey to silver‐brown body. This new latrid is most closely related to Latris lineata and is placed in the genus Latris which is rediagnosed. The striped trumpeter Latris lineata is taxonomically reviewed based on 22 specimens collected from Tristan da Cunha, Amsterdam Island, Tasmania, New Zealand, and the Foundation Seamounts. Its presence off Chile, based on specimens identified as L. hecateia but no longer extant, is uncertain. A key to the species of Latridae is provided.

Silicic volcanism and hydrothermal activity documented at Pacific‐Antarctic Ridge

How well do we know the composition of oceanic crust? Countless studies have described the occurrence of mid‐ocean ridge basalt (MORB) at spreading centers, and few would argue that the bulk composition of oceanic crust is other than basaltic. Nevertheless, silicic volcanism (>55 wt.% SiO2) does occur along part of the northern East Pacific Rise (10.5°N; Thompson et al. [1989]),on the 095° propagator of the Galápagos Spreading Center [Clague et al., 1981],and was recently discovered on the Pacific‐Antarctic Ridge (PAR) near its intersection with the Foundation seamount chain [Hekinian et al., 1997, 1999]. Silicic lavas were recovered from a 290‐km‐long section of the northern PAR adjacent to the active Foundation plume (Figure 1) during cruise 157 of the F/S Sonne, which took place in June and July 2001. Furthermore, widespread hydrothermal activity indicates that the volcanogenic massive sulfidesilicic lava association is not only restricted to subduction and back arc settings.

The support mechanism of the young Foundation Seamounts inferred from bathymetry and gravity

SUMMARY The youngest volcanoes of the Foundation volcanic chain were built on a plate less than 5 Myr near the Pacific‐Antarctic ridge (PAR) axis. The progressive approach of the spreading ridge to the hotspot during the last 10 Myr resulted in the building of the edifices upon a progressively younger plate. Estimates of the effective elastic thickness, Te, using two independent methods show that the elastic thickness of the plate diminishes towards the spreading ridge, from 5‐3 km to 2‐0 km. The spatial variation of Te correlates with a significant change in both the morphology and the volume of the volcanoes, suggesting an important control of the elastic thickness on the volcanic processes. The reduction in Te also explains conveniently the reduction in the distance between the North and the South Systems, the double volcanic line that characterizes the young part of the Foundation chain, in the direction of young loading ages. The South System formed at the top of a flexural bulge created by the emplacement of the North System, the main volcanic line. The low Te estimate for the South System and the shape of the Bouguer anomalies confirm this model. The effective elastic thicknesses obtained in this study are well matched by isotherms between 200 ◦ C and 300 ◦ C and support the hypothesis of a weak plate. Three mechanisms possibly acted to lower the plate rigidity: Re-heating of a ‘normal’ plate by the hotspot (North System); local fracturing (South System) and the creation of a thin lithosphere at the PAR axis by the coupling of a hotspot and the ridge-related melting zones (near-ridge volcanoes).

Contrasted interactions between plume, upper mantle, and lithosphere: Foundation chain case

The Foundation Seamounts are a volcanic chain formed during the last 21 m.y. by the action of a hot spot, presently located near the axis of the Pacific‐Antarctic Ridge. The western part of the chain is formed by volcanoes with ages ranging from 21 to 16 m.y., spatially distributed along two parallel lines roughly 200 km apart. The shape and distribution of the edifices are strongly suggestive of an emplacement along preexisting fissures or fractures, resulting from lithospheric deformation during the major kinematic reorganization of 25 m.y. Their chemical and isotopic composition becomes enriched toward young volcano ages, resulting from the progressive contamination of the upper mantle by the arrival of a plume pulse. The eastern part of the chain is formed by volcanoes younger than 5 m.y., also distributed along two subparallel lines. The distance between these two lines diminishes toward the Pacific‐Antarctic Ridge axis. A geoid high is located very near the Pacific‐Antarctic Ridge axis, over the volcanoes of the north line, which lies in the prolongation of the older Foundation Seamounts, and possibly marks the location of the Foundation hot spot. Both the geoid anomaly and the morphology of the edifices show that the north line is the main locus of the volcanism. The southern line was probably formed on top of the flexural arch resulting from the emplacement of the north line. The chemical and isotopic compositions of the basalts reveal a growing influence of the ridge on the off‐axis plume magmatism. The pattern is coherent with a mixing between two sources, occurring when the two melting zones merge and overlap.

Ridge–hotspot interaction: the Pacific–Antarctic Ridge and the foundation seamounts

The study of different magmatic provinces between the Resolution fracture zone (33°S–131°W) and the Pacific–Antarctic Ridge (PAR) axis (37°S–111°W) suggests that similar processes of interaction between hotspot and spreading axial magmatism occurred 20–25 Ma and 0–5 Ma ago. There is evidence of this process from the changes in composition observed in the lavas erupted near 400–300 km between the present day PAR axis (37°S–111′W) and the eastern tip of the Foundation Seamount (FS) hotspot near 36°20′S–114°W where the last alkali enriched volcanics [K/Ti>0.30, Zr/Y>6 and (Ce/Yb) N>4] have erupted. This transitional province between the PAR and the FS consists of volcanic cones built on several volcanic ridges (<200 km in length) which have erupted less enriched volcanics such as E-[K/Ti=0.25–0.33, Zr/Y=5–6 and (Ce/Yb) N=3–4] and T-[K/Ti=0.11–0.25, Zr/Y=2–4 and (Ce/Yb) N=1–2] MORBs than those from the FS. It is also noticed that there is a general decrease in the degree of the basalt alkalinity (more T-MORBs) towards the PAR axis. The limit of the FS hotspot influence corresponds to the area where the VR intersect the PAR axis for a distance of about 100 km along its strike between 37°10′S and 38°20′S. Indeed, the lava erupted further to the north and to the south of these latitudes contains N-MORBs [K/Ti=0.05–0.11, Zr/Y<3 and (Ce/Yb) N=<2]. Many Old Pacific Seamounts (OPS, >20 Ma) also built on volcanic ridges are identified west (>1200 km from the PAR axis) of a Failed Rift Propagator (FRP) forming the eastern boundary of the ancient Selkirk microplate. Some of these seamounts made of alkali basalts were built during the initiation of the FS hotspot 20–23 Ma ago. The interaction and the influence (thermal) of mantle plume magmatism with the ancient spreading ridge of the Farallon–Pacific plates was responsible for the eruptions of the T-MORBs and andesitic lavas. This situation is comparable to that presently observed on the PAR axis where silicic lavas are also erupted in association with T-MORBs.

Migration rate of volcanism along the Foundation Chain, SE Pacific

Volcanism has migrated along the NW–SE-trending Foundation Chain, SE Pacific, at a rate of 91±2 mm/y for the past 21 million years (My). The Foundation Chain is at least 1900 km long, and was erupted mostly in the interior of the Pacific plate. The linear migration of intraplate, often geochemically enriched, volcanism along the Foundation Chain can be explained by the theory of tectonic plate drift over a narrow, stationary plume of hot mantle material upwelling from depth. The present hotspot (or at least its eastern extent) generated by this Foundation plume is now located on, or close to, the Pacific–Antarctic spreading-axis. Local spreading/microplate boundaries have migrated systematically towards the Foundation plume for at least the past 21 My, a process that was, however, interrupted by the creation of the Selkirk microplate. The Foundation Chain has erupted, therefore, onto ocean crust ranging in age from ∼17 Ma to ≥0 Ma, with distances between the Foundation plume and local spreading/microplate boundaries varying from ∼900 km to ≥0 km. Such interplay between the Foundation plume and local seafloor, spreading and microplate boundaries — probably in combination with changing stresses in the lithosphere — apparently induced major changes in the fabric (and composition) of the Foundation Chain, e.g., the switch from individual seamounts to elongated, en echelon, ridges at ≥5 Ma. It can also explain why ages for a few Foundation seamounts located at the edges of the Selkirk microplate do not lie along the overall linear trend of migrating volcanism. The linear velocity of the Pacific plate over the Foundation plume, calculated on the basis of the migration rate of volcanism along the Foundation Chain, is predicted by a number of rotation poles for the motion of the Pacific plate in a fixed hotspot reference frame. The fact that the measured migration rates of volcanism along the Foundation and Hawaiian chains are predicted by these rotation poles (1) confirms that the distribution of Foundation Chain ages records the absolute motion of the Pacific plate, and (2) supports the fundamental assumption of stationary mantle plumes in the case of Hawaii versus Foundation for at least the past 21 My. The Foundation Chain was created by a weak plume, primarily as relatively small, probably rapidly erupted (≤1 My) individual seamounts or seamount clusters at a rate of approximately one every My. Linear migration rates of volcanism are most likely defined unequivocally along such types of hotspot chain, on the basis of age–distance data. It should also be easier to identify and characterize volcanism created as a result of plume interactions with local spreading/microplate boundaries. Rotation poles for Pacific plate drift show that the initial stages of volcanism along the southern Austral Chain (≥34 Ma) could have been created by the Foundation plume, suggesting that the path of Pacific plate drift over the stationary Foundation plume could be at least 3000 km long.

Global Sea Floor Topography from Satellite Altimetry and Ship Depth Soundings

A digital bathymetric map of the oceans with a horizontal resolution of 1 to 12 kilometers was derived by combining available depth soundings with high-resolution marine gravity information from the Geosat and ERS-1 spacecraft. Previous global bathymetric maps lacked features such as the 1600-kilometer-long Foundation Seamounts chain in the South Pacific. This map shows relations among the distributions of depth, sea floor area, and sea floor age that do not fit the predictions of deterministic models of subsidence due to lithosphere cooling but may be explained by a stochastic model in which randomly distributed reheating events warm the lithosphere and raise the ocean floor.

Intraplate versus ridge volcanism on the Pacific‐Antarctic Ridge near 37°S–111°W

Exploration of the Foundation Volcanic Chain (33°S–131°W; 37°S–111°W) revealed the existence of different magmatic provinces with relation to their geological settings. (1) The Pacific‐Antarctic Ridge (PAR) is made up of several en echelon segments where both glassy midocean ridge basalts (MORBs) with low incompatible elements (K2O&lt;200 ppm, Zr&lt;120 ppm and Ce&lt;20 ppm) as well as andesites and dacites have erupted, (2) Oblique Ridges located up to 300 km from the PAR axis are topped with seamounts made up essentially of transitional (T) and enriched (E) MORBs with intermediate incompatible elements (K2O=0.11–0.40%, Zr=70–140 ppm and Ce =15–30 ppm), (3) the Foundation Seamounts (FS) consisting essentially of isolated volcanoes which have erupted alkalic lavas (alkali basalt, trachybasalt and trachyandesite) with high incompatible elements (K2O (0.50–1.1%, Zr (&gt;150 ppm) and Ce (&gt;48 ppm)) at about 306–1300 km from the PAR axis, (4) The Old Pacific Seamounts built on a crust older than 23 m. y. located west of longitude 124°W (&gt;1300 km from the PAR axis) consist of T and EMORB. On the PAR axis, extensive crystal fractionation (&gt;65%) produced the silicic lavas. On the basis of Pacific plate reconstruction using a half spreading rate of about 50 mm/yr and integrating the observed compositional changes with respect to the structural settings, it is inferred that the last volcanic events giving rise to the FS took place at about 110 km from the PAR axis about 5 m. y. ago. The Oblique Ridges built between 5 m. y. and &lt;1 m. y. are believed to represent ancient leaky transforms and/or large discontinuities between accreting ridge segments filled by volcanic cones during the interaction (mixing) of the enriched plume components of the FS with PAR depleted (MORB type) magmatism. The Old Pacific Seamounts built on ancient crust (&gt;23 m. y.) with MORB volcanics comparable to those of the the Oblique Ridge‐PAR provinces, could also have been formed by an interaction between the Foundation Seamount (dredge site 28) hotspot magmatism and that of an ancient accreting ridge magmatism precursor of the PAR.

The Foundation Seamount chain: a first survey and sampling

The Foundation Seamounts form a 1400 km-long chain on the Pacific plate from 32 °S, 127 °W to the Pacific-Antarctic spreading axis at 38 °S, 111 °W. Previously only known from sparse single-beam echosoundings and satellite altimetry, we present here the first multibeam bathymetric survey and geological sampling results. We confirm that the submarine topography correlates with the altimetry, and that the chain is volcanic rather than tectonic or microcontinental in origin. The chain can be divided up morphologically and geochemically into three section: (1) west of 125 °W large flat-topped volcanoes composed of incompatible-element depleted lavas ( Nb Zr N ≈ 1) of a near-ridge origin with little or no plume influence, (2) between 125 and 115 °W true intraplate volcanoes with incompatible element enrichment ( Nb Zr N > 1.9) generated over the Foundation plume, (3) east of 115 °W E-W-trending volcanic ridges with compositions ( Nb Zr N 2.0-0.3) suggestive of interaction between the plume and the Pacific-Antarctic spreading axis. On the spreading axis moderate incompatible element enrichments ( Nb Zr N ≈0.8, cf. ≈ 0.3 outside the Foundation area) also suggest plume influence. It appears that the activity of the Foundation plume in the last few million years has (1) significantly waned and (2) become wholly channeled towards the spreading axis. The Foundation plume may be in the process of “dying”.

The Foundation Seamounts: tectonic setting of a newly discovered seamount chain in the South Pacific

Large areas of the South Pacific remain unsurveyed because they are so remote. A major volcanic chain, the Foundation Seamounts lies between 33°S and 39°S and between 111°W and 125°W. Poorly defined on conventional bathymetric charts, it becomes very apparent when combining an analysis of conventional geophysical data with the satellite altimeter data. The length of the seamount chain is 1350 km and the volcanoes are clustered within a 180 km wide band, trending approximately N70W, which is the direction of absolute motion of the Pacific plate over a fixed reference system. Some of these volcanoes are so shallow that they may have emerged above the sea surface at an earlier time. No dates are known for the seamounts. Surrounding the Foundation Seamounts chain, the tectonic setting is that of the Pacific plate at the time when the strike of the spreading ridge changed from a northwesterly direction to the present northeasterly one. This episode of spreading reorganization involved the existence of a short-lived microplate. West of the Pacific Antarctic ridge, a captured segment of Nazca plate accounts for the large discrepancies in the magnetic anomalies offsets along the Agassiz fracture zone. Large propagators like the Adventure and Kurchatov troughs are described. The newly discovered western segment of the Chile fracture zone is the Pacific plate match of the Chile fracture zone on the Nazca plate.