Seamount Group Research Articles

Origin of submarine volcanism at the eastern margin of the central atlantic: Investigation of the alkaline volcanic rocks of the carter seamount (Grimaldi Seamounts)

This paper addresses the composition, geochemistry, isotopic characteristics, and age of rocks from the Carter Seamount of the Grimaldi seamount group at the eastern margin of the Central Atlantic. The age of the seamount was estimated as 57–58 Ma. Together with other seamounts of the Grimaldi system and the Nadir Seamount, it forms a “hot line” related to the Guinea Fracture Zone, which was formed during the late Paleocene pulse of volcanism. The Carter Seamount is made up of olivine melilitites, ankaramites, and analcime-bearing nepheline tephrites, which are differentiated products of the fractional crystallization of melts similar to an alkaline ultramafic magma. The volcanics contain xenoliths entrained by melt at different depths from the mantle, layer 3 of the oceanic crust, which was formed at 113–115 Ma, and earlier magma chambers. The rocks were altered by low-temperature hydrothermal solutions. The parental melts of the volcanics of the Carter Seamount were derived at very low degrees of mantle melting in the stability field of garnet lherzolite at depths of no less than 105 km. Anomalously high Th, Nb, Ta, and La contents in the volcanics indicate that a metasomatized mantle reservoir contributed to the formation of their primary melts. The Sr, Pb, and Nd isotopic systematics of the rocks show that the composition of the mantle source lies on the mixing line between two mantle components. One of them is a mixture of prevailing HIMU and the depleted mantle, and the other is an enriched EM2-type mantle reservoir. These data suggest that the formation of the Carter Seamount volcanics was caused by extension-related decompression melting in the Guinea Fracture Zone of either (1) hot mantle plume material (HIMU component) affected by carbonate metasomatism or (2) carbonated basic enclaves (eclogites) ubiquitous in the asthenosphere, whose isotopic characteristics corresponded to the HIMU and EM2 components. In the former case, it is assumed that the melt assimilated during ascent the material of the metasomatized subcontinental mantle (EM2 component), which was incorporated into the oceanic lithospheric mantle during rifting and the breakup of Pangea.

Hot spot activity and tectonic settings near Amsterdam–St. Paul plateau (Indian Ocean)

[1] The Amsterdam–St. Paul (ASP) plateau is located in the central part of the Indian Ocean and results from the interaction between the ASP hot spot and the Southeast Indian Ridge (SEIR). It is located near the diffuse boundary between the Capricorn and Australian plates. The seamount chain of the Dead Poets (CDP) is northeast of the ASP plateau and may represent older volcanism related to the ASP hot spot; this chain consists of two groups of seamounts: (1) large flat-topped seamounts formed 8–10 Ma and (2) smaller conical seamounts formed during the last 2 Myr. The ASP hot spot has produced two pulses of magmatism that have been ponded under the ASP plateau and erupted along the divergent boundary between the Capricorn and Australian plates. The N65° orientation of the CDP as well as the seamount's elongated shapes support an opening motion between the Capricorn and Australian plates along a suture oriented in the N155° direction. This motion compared to the Antarctic plate amounts to an apparent velocity of 7.7 cm/yr northeastward for the Capricorn-Australian block. This motion does not fit with a fixed plume model. We suggest, therefore, that the ASP plume experienced a motion of about 1–2 cm/yr to the SW, which is opposite to the asthenospheric flow in this region and suggests a deep-seated plume.

Incongruent patterns of genetic connectivity among four ophiuroid species with differing coral host specificity on North Atlantic seamounts

AbstractSeamounts are considered to play a defining role in the evolution and diversity of marine fauna, acting as ‘stepping‐stones’ for dispersal, regional centers of genetic isolation and speciation, and refugia for deep‐sea populations. This study focused on the patterns of dispersal and genetic connectivity of four seamount ophiuroid species (Asteroschema clavigera, Ophiocreas oedipus, Ophioplinthaca abyssalis, and Ophioplinthaca chelys) displaying differing levels of associative (epifaunal) specificity to cold‐water coral hosts inhabiting the New England and Corner Rise Seamount chains, and Muir Seamount in the Northwestern Atlantic. Analyses of mt16S and mtCOI revealed evidence for recent population expansion and high gene flow for all four species. However, species‐specific genetic differentiation was significant based on seamount region and depth. Significant differences were found among regional seamount groups for A. clavigera, within seamount regions and seamounts for O. chelys, among 250‐m depth intervals for A. clavigera, among 100‐m depth intervals for O. oedipus, and there were indications of isolation by distance for A. clavigera and O. oedipus. In addition, A. clavigera and O. oedipus, broadcast spawners with high fidelity to specific coral hosts, displayed predominantly westward historical migration, whereas the ophioplinthacids, with lower host‐specificity, displayed predominantly eastward migration. No congruent patterns of historical migration were evident among species and seamounts, yet these patterns can be correlated with species‐specific host specificity, specific depth strata, and dispersal strategies. Conservation efforts to protect seamount ecosystems should promote multi‐species approaches to genetic connectivity, and consider the impact of the ‘dependence’ of biodiversity on host fauna in these vulnerable marine ecosystems.

Effects of oceanographic retention on decapod and gastropod community diversity on seamounts

Using global physical and biological datasets, we tested oceanographic retention (fac- toring out effects of seamount depth and age) as one possible mechanism structuring seamount ben- thic decapod and gastropod communities. We first determined the relative oceanographic retentive potential (such as from Taylor caps or columns) for individual seamounts based on steady-state the- ory. This was then separately compared to decapod and gastropod taxonomic distinctness, our met- ric for the community's response to oceanographic retention. Taxonomic distinctness is a metric based on phylogenetic relatedness between species. Therefore, between-seamount variability in tax- onomic distinctness may reflect the effects of long-term oceanographic retention at the seamount spatial scale. Taxonomic distinctness and retention potential varied between seamounts, but reten- tion did not explain variation in taxonomic distinctness. Among decapod communities, variation in taxonomic distinctness was partly explained by seamount summit depth. With respect to previously suggested causal relationships between seamount retention and biogeographic patterns (divergent seamount communities, high rates of endemism, and inferred speciation at the scale of the seamount), we concluded that seamount-scaled oceanographic retention is weak compared to other ecological drivers of community diversity on seamounts. Alternative processes (such as those related to depth), or alternative spatial scales (within each seamount or between seamount groups and chains) must be considered to explain divergent patterns among seamounts for these taxa.

Reverse propagation of T waves from the Emperor seamount chain

Analysis of seismo‐acoustic T waves associated with an intermediate‐depth earthquake beneath northern Japan reveals that the detected T waves are secondary waves scattered by distant seamounts. The timing of T‐wave onset, 3000 s after occurrence of the earthquake, indicates that the detected T waves did not follow a direct path from the solid‐liquid conversion point near the Japan Trench to the observation stations in northeast Japan. It is hypothesized that the original T waves were reverse‐scattered back toward Japan by peaks of the Emperor seamount chain after propagation in the northern Pacific Ocean. The synthesized envelope patterns for acoustics scattered at the seamounts considering this multi‐path effect reproduce the travel time and amplitude pattern of the observed T waves well, with each seamount group producing a discrete amplitude peak of the scattered wave. These observations suggest that topographical obstructions act as a secondary source for T‐wave propagation in the ocean.

Evolution of the Dok Do seamounts, Ulleung Basin, East Sea: constraints based on the reconstruction of virtual geomagnetic poles using paleomagnetic data

In the Ulleung Basin, East Sea, the Dok Do seamount group comprises Dok Do (Dok Island), consisting of very small islets/rocks and a large submerged volcanic edifice, and two voluminous tablemounts, Simheungtaek and Isabu. We attempted to reconstruct the evolution of these seamounts, using virtual geomagnetic poles (VGPs) determined by the least-squares and the seminorm magnetization methods, with 1,500 m upward continued magnetic anomalies. The VGPs of Dok Do with normal dipole anomaly, and of Simheungtaek with normal dipole anomaly are located near the present magnetic pole. The VGP of Isabu with normal dipole anomaly is located at low latitude, presumably due to overprints of reversals in the Tertiary, and the distortion of magnetization and structures associated with volcanism after its formation. In contrast to the tablemounts, magnetic anomalies over Dok Do are a combination of both normal polarity and reversed polarity dipoles in the northern hemisphere, indicating that Dok Do has had at least two major eruptions, one during normal and another during reversed polarity intervals. From these results, and information on the ages of the seamounts (either published radiometric ages of subaerial volcanic rocks, or ages reconstructed in terms of reported elastic thickness incorporated into an existing cooling plate model), we tentatively propose that (1) Isabu formed first, during a normal polarity interval after the opening of the East Sea had ceased; (2) this was followed by an initial and subsequent large eruption of Dok Do during a normal polarity and a reversed polarity interval after about 5 Ma; and (3) the formation of Simheungtaek occurred in between that of Isabu and Dok Do in a normal polarity interval. The pattern of normal/reversed magnetization is not inconsistent with the geomagnetic polarity timescale for at least the last 5 Ma. Nevertheless, precise ages of formation would need verification by additional geophysical/geochemical constraints. Evaluating various possible models explaining the successive formation of the Dok Do seamounts, we currently favor fracturing and volcanism related to compression-induced weakening of the extensional field from the late Miocene to Pliocene after the opening of the East Sea.

One hundred million years of mantle geochemical history suggest the retiring of mantle plumes is premature

Linear chains of intraplate volcanoes and their geochemistry provide a record of mantle melting through geological time. The isotopic compositions of their lavas characterize their mantle sources, and their ages help backtrack these volcanoes to their original, eruptive source regions. Such data may shed light on a much-debated issue in Earth Sciences: the origin of intraplate volcanism and its underlying mantle and lithosphere dynamics. We show here that three major Western Pacific Seamount groups, ∼ 50–100 million years in age, display distinct Sr, Nd, Hf, and Pb isotopic signatures that can be traced back in time, both geographically and geochemically, to three separate, recently-active intraplate volcanoes in the South Pacific Cook–Austral Islands. Their unique 100 million year history, which shows a persistent geochemical fingerprint, suggests formation from large volumes of laterally fixed, long-lived source regions. Such longevity is unlikely to be attained in the relatively dynamic upper mantle. Therefore, these sources are likely anchored deep in the mantle, isolated from homogenization by mantle convection, and imply a primary origin from deep mantle plumes rather than resulting from lithosphere extension.

Geochemical evolution of intraplate magmatism in the Paleo-Asian Ocean from the Late Neoproterozoic to the Early Cambrian

A group of oceanic islands and/or seamounts (hereafter, paleoseamounts) was produced by oceanic hot-spot magmatism in the Late Proterozoic-Early Cambrian in the southwestern margin of the Paleo-Asian Ocean. They were accreted to the Kuznetsk-Altai island arc in the Late Cambrian and were subsequently incorporated during the closing of the paleocean into the accretionary complexes of the western part of the Altai-Sayan area (southwestern Siberia, Russia). The major-and trace-element compositions and Sr and Nd isotopic systematics of pillow lavas and basalt flows from the Kurai (600 Ma) and Katun’ (550–530 Ma) paleoseamounts of Gorny Altai characterize the evolution of Hawaiian-type magmatism in the Paleo-Asian Ocean during that period. The obtained data show a significant change in lava composition between 600 and 550–530 Ma. The tholeiitic basalts of the Kurai Paleoseamount (600 Ma) from the southern part of Gorny Altai have lower incompatible element contents and higher 147Sm/144Nd values compared with the younger tholeiitic and alkali basalts of the Katun’ Paleoseamount (550–530 Ma), whose rocks are exposed in northern Gorny Altai. The trace-element compositions of the Katun’ lavas are similar to those of the Hawaiian tholeiites, and their 147Sm/144Nd ratios are lower than those of the Kurai basalts. It was suggested that the older Kurai Paleoseamount was formed above a thinner oceanic lithosphere, i.e., closer to a paleospreading axis compared with the younger Katun’ Paleoseamount. The observed temporal variations in the chemical and isotopic characteristics of lavas are probably related to differences in the degree of melting of the heterogeneous mantle owing to the different thickness of the oceanic lithosphere above which the Kurai and Katun’ paleoseamounts were formed. During the Ediacaran, a plume developed beneath the younger and, consequently, thinner lithosphere of the Paleo-Asian Ocean. The higher degree of melting in the mantle column resulted in a more considerable contribution from the refractory depleted material of the upper mantle. After 50–70 Ma, i.e., in the Early Cambrian, the plume affected a thicker lithosphere, its mantle column became shorter, and the degree of melting was lower. Owing to this, the basaltic melt was more contributed by incompatible element enriched less refractory material of the lower mantle.

Can seamounts provide a good habitat for polychaete annelids? Example of the northeastern Atlantic seamounts

Two seamount groups in the northeastern Atlantic were investigated during the 1980s and 1990s: the first was located along the Iberian and African coasts (Galicia, to the north of Portugal; and the Ampere, Gorringe, Josephine and Seine banks near the Madeira–Canary Islands) and the second was located offshore of the southern part of the Azores Islands, included the Atlantis, Hyeres, Irving, Meteor and Plato banks. Among the invertebrates, Annelida, specifically Polychaeta, were studied as surrogates for the biogeographical relationship between coastal and mid-oceanic seamounts in the northeastern Atlantic and the fauna in these areas. The dominant families were Onuphidae (27.46%), Syllidae (18.23%), Eunicidae (15.65%), Amphinomidae (11.45%) and Nereididae (5.61%), representing 78.4% of the total fauna. Data analyses clearly distinguished two seamount groups, one for coastal seamounts and the other offshore. Although the species distinguished and the family composition at the sampled sites were different (i.e., most diversified at the Josephine site and most impoverished at the Irving site), no coastal/offshore faunal impoverishment gradient could be identified. Thus, it seems that seamount environments do not favour any relationship between planktotrophic development and the direct development of polychaetes. Though the number of apparent endemic species was low (<7%), it remained in keeping with other invertebrate groups. Still, while seamounts may well encourage oceanic biodiversity in some zoological groups, this was clearly not the case for the polychaetes. We offer two explanations for this paradox: pelagic productivity and local environmental conditions.

Revised definition of Large Igneous Provinces (LIPs)

Much has been learned about Large Igneous Provinces (LIPs) and their database greatly expanded since their first formal categorization in the early 1990s. This progress provides an opportunity to review the key characteristics that distinguish LIP events from other melting events of the upper mantle, and to reassess and revise how we define LIPs. A precise definition is important to correctly recognize those LIP events with regional to global effects, and to aid in refining petrogenetic models of the origin of LIPs. We revise the definition of LIPs as follows: “Large Igneous Provinces are magmatic provinces with areal extents > 0.1 Mkm 2, igneous volumes > 0.1 Mkm 3 and maximum lifespans of ∼ 50 Myr that have intraplate tectonic settings or geochemical affinities, and are characterised by igneous pulse(s) of short duration (∼ 1–5 Myr), during which a large proportion (> 75%) of the total igneous volume has been emplaced.” They are dominantly mafic, but also can have significant ultramafic and silicic components, and some are dominated by silicic magmatism. In this revision, seamounts, seamount groups, submarine ridges and anomalous seafloor crust are no longer considered as LIPs. Although many of these are spatially-related features post-dating a LIP event, they are constructed by long-lived melting anomalies in the mantle at lower emplacement rates, and contrast with the more transient, high magma emplacement rate characteristics of the LIP event. Many LIPs emplaced in both continental and oceanic realms, are split and rifted apart by new ridge spreading centres, which reinforce the link with mid-ocean ridges as a post-LIP event. Three new types of igneous provinces are now included in the LIP inventory, to accommodate the recognition of a greater diversity of igneous compositions, and preserved expressions of LIP events since the Archean: 1) giant diabase/dolerite continental dyke swarm, sill and mafic–ultramafic intrusion-dominated provinces; 2) Silicic LIPs; and 3) tholeiite–komatiite associations, which may be Archean examples of LIPs. A revised global distribution of LIPs for the Phanerozoic is presented. Establishing the full extent of LIPs requires well-constrained plate reconstructions, and at present, plate reconstructions for the Precambrian are poorly known. However, the possibility of reconstructing the LIP record back to and into the Archean and using this expanded LIP record to better constrain the origins and effects of LIPs is an exciting frontier, and our revised definition is a contribution to that effort.

Rissoidae (Mollusca: Gastropoda) from northeast Atlantic seamounts

The gastropod family Rissoidae is revised at the species level for the Lusitanian seamounts, situated between Portugal and Madeira, and the Meteor group of seamounts, situated south of the Azores in the northeast Atlantic Ocean. Based on material obtained by dredging and trawling, 48 species are reported, of which 30 are described as new. There is very little overlap between the assemblages found on both groups of seamounts, with only two shared species. On the Lusitanian seamounts, 24 species were collected. Seven species (six with planktotrophic development) are shared with the mainland shelf or slope and are represented in low numbers. Eleven species are endemic to this seamount group as a whole and of these, three account for 75% of individuals. Of the species not shared with the mainland, only one is found on the four seamounts, eight (three new) are found on two or three seamounts and/or neighbouring islands, and six (all new) are endemic to a single seamount. On the Meteor group, 26 species were found, of which five are shared with the Azores and 20 (all new) are endemic to the seamount group as a whole. Most species are found on only two or three seamounts, whereas nine species are endemic to only one of the seamounts, and of these five are concentrated on Atlantis seamount. Eight endemic species of the Meteor group included in Porosalvania n. gen. have very different shapes and occupy discrete bathymetric intervals, but are best interpreted as a local radiation originating from a relatively old colonization of this seamount group. The endemic species, and among them the successful ones, all have a paucispiral protoconch denoting non‐planktotrophic development. As in the Macaronesian archipelagos, the Rissoidae are the most species‐rich molluscan family on the northeast Atlantic seamounts.

Lead isotope planetary profiling (LIPP): Summation–depiction of Earth's many reservoirs

An approach aiming at a summation–depiction of the isotopic characteristics of a planet is attempted. The approach, Lead Isotope Planetary Profiling (LIPP), is a thematic derivation from the general field of planetary geochemistry. Specifically, the recently developed methodology of Pb isotope synchronism, which often allows the accurate dating of disturbed geologic systems, has been applied to a large body of terrestrial Pb isotopic data in the literature. The exercise resulted in 26 Pb / Pb lines most of which appear to be highly precise isochrons. The isochrons appear amenable to mutual organization so that they intersect each other (as subgroups) in six distinct and generally well-resolved foci. These potentially infer the present-day Pb isotopic composition of six different reservoirs, from which all terrestrial surface rocks were derived. If so, a fundamental global feature of Earth would have been discovered and summed up in a simple linear image, which constitutes an isotope-profile of the planet. As the profile is governed by the mathematics of the Pb isotope systematics, it is amenable to interpretive elaboration into events, processes and mechanisms. In this report, the six reservoirs were used to construct a model for Earth's Early Episodic Evolution ( EEEE): The layout of the reservoirs suggests that they resulted from four, global differentiation events: The first which occurred at 4.42 Ga or earlier, involved the whole mantle and resulted in the formation of a homogeneous post accretion mantle, PAM (with a present-day 206Pb / 204Pb = 14.0 ± 1 and 207Pb / 204Pb = 14.4 ± 0.3), and a post accretion crust, PAC, with unknown average Pb composition, which was subsequently consumed and obliterated except rare altered vestiges. Operationally, PAM is represented by the first and most ancient reservoir #1, with a lower limit-age of 4.42 Ga. This is the parent-reservoir from which all other reservoirs were derived in three subsequent global differentiation events. Each time, the formed reservoir (i.e., isolated after it was homogenized in U and Pb), survived for > 1 Ga. The second event marking the insetting of global differentiation, mediated by subduction, occurred at 4.13 Ga. It was followed by two other events at 3.4 and 2.96 Ga. In each of the four global events, Pb was irreversibly lost (as a sulfide?) to an efficient hidden sink (the core?). Thus by default, the resulting reservoirs appear enriched in U. The familiar Earth's history, marked by continuous geologic processes, pertains to the last 2.96 Ga of Earth's existence. Inferring the encompassing global nature of the observation, the field of Ocean Island Basalt (OIB) shows affinity to some reservoirs. Also, the filtered isochrons of an ocean island and a group of Seamounts appear to be associated with two of the reservoirs. In addition, the untreated Pb / Pb data on surface rocks, collectively define a ‘flow-pattern’ to which OIB appears to belong. Because they were subducted, OIBs of ancient epochs appear to have left behind recognizable extinction gaps within the global pattern. This suggests episodic initiation of planet-wide hot-spot events, possibly in association with the formation of the reservoirs. Building on the model, the isotopic composition and temporal parameters for ‘the’ source of ocean island basalt ( SOIB) are proposed.

Pb isotopic variability in melt inclusions from the EMI–EMII–HIMU mantle end-members and the role of the oceanic lithosphere

Melt inclusions from four individual lava samples representing the HIMU (Mangaia Island), EMI (Pitcairn Island) and EMII (Tahaa Island) end member components, have heterogeneous Pb isotopic composition larger than that defined by the erupted lavas in each island. The broad linear trend in 207Pb/206Pb–208Pb/206Pb space produced by the melt inclusions from Mangaia, Tahaa and fPitcairn samples reproduces the entire trend defined by the Austral chain, the Society islands and the Pitcairn island and seamount groups. The inclusions preserve a record of melt composition of far greater isotopic diversity than that sampled in whole rock basalts. These results can be explained by mixing of a common depleted component with the HIMU, EMI and EMII lavas, respectively. We favor a model that considers the oceanic lithosphere to be that common component. We suggest that the Pb isotopic compositions of the melt inclusions reflect wall rock reaction of HIMU, EMI and EMII melts during their percolation through the oceanic lithosphere. Under these conditions, the localized rapid crystallization of olivine from primitive basalt near the reaction zone would allow the entrapment of melt inclusions with different isotopic composition.

Miocene changes in bottom current regime recorded in continental rise sediments on the Pacific margin of the Antarctic Peninsula

A Fossil Mounded Sedimentary Body (MB) has been identified in the Miocene sedimentary record on the central continental rise west of Adelaide Island, Antarctic Peninsula, using multichannel seismic reflection profiles. The MB has an elongated NE trend away from a group of seamounts, and it developed between two troughs. We interpret it as a patch drift plastered against the NE (lee) side of an obstacle. The MB's depositional patterns provide the first clear evidence of Early Miocene bottom current activity on the central rise, and they suggest that flow was towards the NE, probably as part of the Antarctic Circumpolar Current. This segment of the rise is, however, presently affected by a SW‐flowing branch of Lower Circumpolar Deep Water. The change in bottom water flow took place during the Middle or Late Miocene, and we suggest that it is probably indicative of more widespread palaeoceanographic changes during this period.

Seismic investigations of the O'Higgins Seamount Group and Juan Fernández Ridge: Aseismic ridge emplacement and lithosphere hydration

The O'Higgins Seamount Group is a cluster of volcanic domes located 120 km west of the central Chilean Trench on the crest of the Juan Fernández Ridge. This aseismic hot spot track is subducting under South America triggering a belt of intraslab earthquake hypocenters extending about 700 km inland. The Juan Fernández Ridge marks the southern boundary of a shallow subduction segment. Subduction of oceanic basement relief has been suggested as a cause for the “flat” slab segments characterizing the Andean trench system. The Juan Fernández Ridge, however, shows only moderate crustal thickening, inadequate to cause significant buoyancy. In 2001, wide‐angle seismic data were collected along two perpendicular profiles crossing the O'Higgins Group. We present tomographic images of the volcanic edifices and adjacent outer rise‐trench environment, which indicate a magmatic origin of the seamounts dominated by extrusive processes. High‐resolution bathymetric data yield a detailed image of a network of syngenetic structures reactivated in the outer rise setting. A pervasive fault pattern restricted to the hot spot modified lithosphere coincides with anomalous low upper mantle velocities gained from a tomographic inversion of seismic mantle phases. Reduced uppermost mantle velocities are solely found underneath the Juan Fernández Ridge and may indicate mineral alterations. Enhanced buoyancy due to crustal and upper mantle hydration may contribute an additional mechanism for shallow subduction, which prevails to the north after the southward migration of the Juan Fernández Ridge.

Polychaetes from the Irving, Meteor and Plato seamounts, North Atlantic ocean: origin and geographical relationships

Data came from the Irving, Meteor and Plato seamounts and, were obtained during the Seamount 2 Expedition on board the RV ‘Le Suroît’ in 1993. On the three seamounts, polychaetes were sampled at 44 sites from 260 to 1800 m: 8 sites on Irving, 27 on Meteor and 9 on Plato. In all five seamounts, 1753 individuals were collected: 403 on Atlantis, 346 on Hyeres, 523 on Irving, 387 on Meteor and 94 on Plato. The species richness from the five seamounts is 53: 31 on Atlantis, 24 on Hyeres, 20 on Irving, 28 on Meteor and 21 species on Plato. Only nine species are present on all five seamounts. Three families Onuphidae, Amphinomidae and Eunicidae are dominant. Two groups of seamounts were identified by cluster analysis: Atlantis and Hyeres and, Irving, Meteor and, Plato. The distribution of the polychaetes collected on these five seamounts are compared with the polychaete fauna from the North Atlantic ocean.

Holocene forearc block rotation in response to seamount subduction, southeastern Península de Nicoya, Costa Rica

The southeastern tip of the Peninsula de Nicoya, Costa Rica, on the Caribbean plate margin lies inboard of the rough bathy- metric terrain on the subducting Cocos plate and along the land- ward projection of the convergence vector for the Fisher seamount group. The southern tip of the peninsula has nearly orthogonal coastlines and extensive, well-preserved, Holocene marine terraces,

Tectonic control of the subducting Juan Fernández Ridge on the Andean margin near Valparaiso, Chile

Near the latitude of Valparaiso, Chile, a fundamental change in configuration of the Benioff Zone, volcanic arc activity, and the structure of the continental margin occurs opposite the subducting Juan Fernández Ridge. Upper plate tectonics related to subduction of the ridge were studied by an international group of geoscientists in the two‐degree segment offshore Valparaiso, extending from the shelf edge seaward across the eastern end of Juan Fernández Ridge. Near the O'Higgins group of seamounts, the Juan Fernández Ridge strikes northeast rather than continuing its east‐west trend across the Pacific Basin. The ridge uplifts the upper plate, and sediments of the Valparaiso Basin are deformed against its southern flank. This deformation is consistent with the southward migration required by an oblique trending ridge and the nearly trench‐normal vector of plate convergence. In the trench axis, the ridge forms a basement barrier behind which sediments 2.5 km deep have ponded. The lower slope over the ridge appears eroded, whereas the margin not yet affected by ridge subduction is fronted by an accretionary prism about 25 km wide. Nazca Plate relief clearly influences tectonism of the margin where it subducts beneath thin continental crust; its relation to deeper processes segmenting the Andean Orogen appears to involve prior tectonic events.

The Darwin Rise demise: the western Pacific guyot heights trace the trans-Pacific Mendocino fracture zone

The Darwin Rise has been proposed so many times and in so many forms and places that the time has come to make a more comprehensive examination of the region. Lying on the NW Pacific Plate between the Geisha Guyots, the Mid-Pacific Mountains, the equator, and the trenches, the region is roughly bounded by magnetic anomaly M20 (147 Ma). It was subjected to a massive outpouring of lava about 105 to 120 Ma, which created the guyots and seamounts in that region. Guyots are excellent tools for studying events of long ago because they eroded in the same lowstand in the Cretaceous and guyot relief, therefore, is a surrogate for paleo-sealevel. The relief is derived by subtracting the break depth of the summit plateau of a guyot from the regional depth. Guyot relief would necessarily be less in the center than to the periphery if the feature formed on a pre-existing rise, as has been postulated. The existence of a paleo-Darwin Rise would give concentric contours for the region in question. Of the sixty guyots used in this study, thirty-seven of these guyots were surveyed using SASS multibeam in the Marcus-Wake seamount group. Twenty-three guyots were surveyed using random track single-beam sonar surveys. An entirely different scenario is shown. Data revealed a major fracture passing through the area coevally or after the guyots formed. Because the depths to the summit are not the same now, vertical tectonics occurred after subaerial erosion. This means the fracture formed during and after the erosion (roughly 105 Ma) and influenced the normal sequence of events in guyot formation. Depending on how one deciphers trends through the Hess Rise morass, SASS bathymetry shows a continuation of the Surveyor/Mendocino fracture zone swarm inside the M20 region to the NE of these data. The fracture swarm continues to the western Pacific trench system. Based on this information, if the Darwin Rise ever existed, it had to have done so elsewhere.

The effect of seamount subduction on seismic coupling

It has long been speculated that the subduction of seamounts may have profound mechanical consequences, particularly on the generation of large subduction zone earthquakes. We consider this problem as a special case of the seismic coupling model of Scholz and Campos. The model predicts that the subduction of a large seamount will increase the normal stress across the subduction interface and hence will enhance seismic coupling. In the case of coupled arcs, this increased coupling will result in a large increase in the recurrence intervals of earthquakes. Such events will therefore appear rare in the instrumental record, as is observed. For decoupled arcs, the subduction of a large seamount should cause them to become locally coupled. Three of the latter localities have been found with satellite altimetry data, two on the Tonga-Kermadec Trench and one on the Izu-Bonin Trench. The diagnostic feature is a shallowing or obstruction of the trench axis adjacent to a chain or group of large seamounts on the subducting plate. In each case, the small to moderate plate-interface seismicity typical of decoupled seismic zones is almost completely absent in a 100–200 km arc segment, indicating that the subduction zone is locally locked, as predicted by the model. The occurrence of large interplate earthquakes at both Tonga sites confirms that they are seismically coupled.