Flat-topped Seamounts Research Articles

Origin of Subaqueous Dunes on a Flat-topped Seamount, Dokdo in the East (Japan) Sea, Korea

Origin of Subaqueous Dunes on a Flat-topped Seamount, Dokdo in the East (Japan) Sea, Korea

Preliminary Results on the Composition and Structure of Soft-Bottom Macrobenthic Communities of a Seamount: the Galicia Bank (NE Atlantic Ocean)

The Galicia Bank (Atlantic Ocean, NW Iberian Peninsula) is a flat-topped seamount located about 200 km from the northwest coast of Spain. This bank is included in the LIFE+ “INDEMARES” project, who aims to identify priority areas of conservation in the Spanish seas. In 2010 and 2011 two oceanographic surveys were carried out to investigate species and habitats of this deep ecosystem. A total of 28 stations were sampled using a box corer with a sampling area of 0.09 m2, in order to evaluate the distribution and biodiversity of the macroinfauna of the Galicia Bank. An additional sediment sample was taken at each station to analyse the granulometric composition and the organic matter content. Sediments were mainly sandy in nature (medium, fine and very fine sands), with low levels of organic matter content. The samples analysed yielded a total of 1189 individuals, belonging to 89 families. Polychaetes were the best represented group in total number of individuals (67%), followed by molluscs (14%). Echinoderms (9.7%), crustaceans (6%), and the Others group (Nemertea and Sipuncula) (3.9%) were less abundant. Two major macrobenthic assemblages were determined through multivariate analyses. Sedimentary composition was the main factor structuring the benthic community, separating medium sand stations from those that had finer sediments.

FEEDING TRACES OF RECENT RAY FISH AND OCCURRENCES OF THE TRACE FOSSIL PISCICHNUS WAITEMATA FROM THE PLIOCENE OF SANTA MARIA ISLAND, AZORES (NORTHEAST ATLANTIC)

The bowl-shaped trace fossil Piscichnus waitemata Gregory 1991 appears in Pliocene sandstones from Santa Maria Island (Azores Archipelago), extensively excavated during a stage of island evolution when the volcanic edifice was a guyot (flat-topped seamount) isolated in the NE Atlantic. The host sediments were deposited at depths from the intertidal zone to fair-weather wave base in a tropical climate under the influence of periodic storms and hurricanes. The traces were produced by ray fishes hunting for polychaetes, crustaceans and bivalves living in the sediment, similar to present-day nearshore, warm waters in the Azores, Baja California Sur (Mexico), and New Zealand, from which examples of feeding depressions are drawn (incipient Piscichnus). While P. waitemata is abundantly present in planar sediments on top of the guyot, far fewer trace fossils occur in sandstone deposited on the guyot's margins. Presumably, the different densities of ray holes in the two sedimentary bodies were a response to lesser availability of prey, lower seawater temperatures (due to greater depths), and a more dynamic environment in which life conditions were less favorable. Moreover, the potential preservation of bowl-shaped depressions was lower in this setting, given the steepness of the seafloor, stronger currents, and constant sediment mobility. The top of the guyot was a more favorable habitat, refuge and/or nursery ground for many ray fishes. Measurement of the diameters of the ray holes indicate three distinct size classes, which may suggest that several species were responsible for their formation.

Abnormally high mercury contents in hydrogenic ferromanganese crusts from Seth Guyot (Northwestern Pacific)

Variations in mercury contents in marine sediments have implications for hydrothermal activity, paleoclimate, depositional environments, and primary bioproduction. Mercury contents reach 148 ppb in hydrogenic ferromanganese crusts on flat-topped seamounts. Such crusts, with up to 4120 ppb Hg, were dredged from the slopes of Seth Guyot in the western Marcus-Wake Seamounts in 1982, during the 13th cruise of RV Vulkanolog. The Seth Fe-Mn crusts are of the same origin as hydrogenic Co-rich ferromanganese deposits from seamounts in other oceanic regions. Mercury accumulated in the Cenozoic as Fe-Mn oxyhydroxides in the crusts adsorbed Hg from bottom water. The process was especially rapid during the Pliocene volcano-tectonic rejuvenated stage.

Growth and demise of Cenozoic isolated carbonate platforms: New insights from the Mozambique Channel seamounts (SW Indian Ocean)

Although long-term evolutions of isolated shallow-water carbonate platforms and demise episodes leading to guyot formation have been the subject of numerous studies during the last decades, their driving processes are still the subject of active debates. The Mozambique Channel (SW Indian Ocean) is characterized by several flat-topped seamounts ranging from 11°S to 21°S in latitudes. Based on a comprehensive geomorphologic study and on dredged samples analysis, we show that these features correspond to tropical isolated shallow-water carbonate platforms. Coupling strontium isotopy and foraminifera biostratigraphy, well-constrained chronostratigraphy results indicate that shallow-water carbonate production started in the Mozambique Channel during distinct Cenozoic periods ranging from Paleocene to Early Miocene. Our data also demonstrate that these carbonate platforms were subsequently characterized by different evolutions locally marked by tectonic and rejuvenated volcanism. While some of them kept developed until present days, forming modern carbonate systems, some others were drowned during Late Neogene and subsided to form guyots. Although different factors can be discussed, tectonic and volcanism appear as good potential triggers for demise episodes during Late Miocene-Early Pliocene times. Chronology and location of this geodynamical activity tend to emphasize influence of East African rift system until southern Mozambique Channel.

서태평양 해저산의 망간각 자원평가를 위한 해저지형 특성 분석

서태평양 공해 해저산의 다중빔 음향측심자료와 해저면 영상관측 자료를 활용하여 해저산 정상부와 경사면에 피복된 망간각의 공간 분포 변화 양상을 파악하였다. 다중빔 음향측심기를 이용하여 구분된 해저산의 지형 특성은 정상부 면적의 약 70% 이상이 경사가 <TEX>$5^{\circ}$</TEX> 미만으로 비교적 평평한 지대로 이루어져 있으나 후방산란강도는 해저면의 매질변화를 지시하는 이봉분포를 보였다. 이 이봉분포에서 -30 dB이상의 높은 최빈값은 경사면과 정사부 가장자리 지역에서 우세하였으며 -30 dB이하의 낮은 최빈값은 정상부 중앙지역에서 우세하였다. 해저면 표층의 영상자료와 후방산란 자료의 연관성을 분석한 결과, 정상부 중앙지역은 경사면에 비해 상대적으로 완만한 기울기와 미교결 퇴적물만 존재하여 후방산란이 낮게 나타난다. 반면, 정상부 가장자리 및 경사면은 퇴적물이 없어 기반암이 노출되거나 망간각이 피복되어 후방산란이 높게 나타남을 보여준다. 따라서 다중빔 음향측심조사를 통해 획득된 후방산란 자료와 해저면의 퇴적물 및 망간각 분포 사이의 상관관계가 높다는 것을 알 수 있다. 이 연구결과는 다중빔 음향측심조사를 통해 획득된 후방산란 자료를 활용하면 기요형태의 해저산에 피복된 망간각의 전체적인 분포 규모를 확인할 수 있음을 의미한다. 따라서 해저산 지역의 망간각 개발 유망구역을 선별하는데 후방산란 자료가 유용하게 활용될 수 있을 것이다. We characterize the spatial distribution of Cobalt-rich ferromanganese crusts covering the summit and slopes of a seamount in the western Pacific, using acoustic backscatter from multibeam echo sounders (MBES) and seafloor video observation. Based on multibeam bathymetric data, we identify that ~70% of the summit area of this flattopped seamount has slope gradients less than <TEX>$5^{\circ}$</TEX>. The histogram of the backscatter intensity data shows a bi-modal distribution, indicating significant variations in seabed hardness. On the one hand, visual inspection of the seafloor using deep-sea camera data exhibits that the steep slope areas with high backscatter are mainly covered by manganese crusts. On the other hand, the visual analyses for the summit reveal that the summit areas with relatively low backscatter are covered by sediments. The other summit areas, however, exhibit high acoustic reflectivity due to coexistence of manganese crusts and sediments. Comparison between seafloor video images and acoustic backscatter intensity suggests that the central summit has relatively flat topography and low backscatter intensity resulting from unconsolidated sediments. In addition, the rim of the summit and the slopes are of high acoustic reflectivity because of manganese crusts and/or bedrock outcrops with little sediments. Therefore, we find a strong correlation between the acoustic backscatter data acquired from sea-surface multibeam survey and the spatial distribution of sediments and manganese crusts. We propose that analyzing acoustic backscatter can be one of practical methods to select optimal minable areas of the ferromanganese crusts from seamounts for future mining.

Morfología y estructura de la zona especial de manejo de Banco Hannibal, Pacífico de Panamá mediante el uso de sensores remotos acústicos

The Hannibal Bank sits within the Coiba UNESCO World Heritage Site in Pacific Panama and is also a fisheries management zone. Despite the protected status of the area and the importance of the Bank for commercial fish species such as snapper and tuna, the seamount has received no detailed survey except some collection of organisms. This study mapped the major topographic features and complexity of the Hannibal Bank seamount using acoustic remote sensing. A survey area of around 125 km2 was defined using existing charts and side-scan sonar data were collected during July 2008. A bathymetric output was imported to ArcGIS where a digital bathymetric model and slope map were created. The Benthic Terrain Modeler (BTM) extension for ArcGIS was used to calculate bathymetric position index and rugosity, and used to create a map of zones representing the various seabed morphology zones. The Hannibal bank is an elongated, triangular guyot (flat topped seamount), which ranges in depth from 53m to 416m, covers an area of 76 km2 and is 14.4 km long and 7.1 km wide. Hannibal bank is composed of steep slopes, more gentle slopes, top of the seamount, crests (elevated ridges at the top of the pinnacles), rugose areas (on crests, top of seamount and slope), gullies and pinnacles. The bank is asymmetric in nature with the Northerly side having a relatively gentle slope with gullies across the surface compared to the SouthWest side which is far steeper and more rugose. There are two pinnacles to the North and South East of the bank that range in depth from 180 to 333 m. Rocky substrate makes up 22.6 km2 of the bank and sediment 37.8 km2. The bank and its steeply sided, rugose areas and pinnacles provide upright structures which can disrupt and topographically enhance currents, increasing productivity. The rugose areas of Hannibal Bank should be primary targets for further research efforts as they may contain corals and their rugosity indicates that these should be some of the highest faunal diversity areas of the bank. Hannibal Bank is likely to come increasing pressure in the future through climate change and fishing and this study has produced valuable information to assist in the future mapping and management of habitats, associated species and fisheries.

Sill to surface: Linking young off-axis volcanism with subsurface melt at the overlapping spreading center at 9°03′N East Pacific Rise

No young, off-axis, mid-ocean ridge lavas have yet been directly linked to underlying off-axis melt bodies. In this study, we present new measurements of 238U–230Th–226Ra–210Pb isotope compositions for a suite of lavas from the overlapping spreading center (OSC) at 9°03′N on the East Pacific Rise (EPR). These lavas span a large range of compositions, from basalt to dacite, and include both axial and off-axis samples recovered from a prominent, axis-parallel pillow ridge and a flat-topped seamount that overlie the westernmost extent of a 4-km-wide melt lens (Kent et al., 2000). We report 210Pb excesses in axial basalts and basaltic andesites, which we suggest results from gas-magma fractionation of 222Rn from 226Ra beneath dacite magmas. In addition, our U-series ages agree with visual observations, indicating that while most recent volcanic activity occurs at the spreading axis, active volcanism also occurs away from the axis. Specifically, the off-axis pillow ridge and seamount samples overlying the off-axis subsurface melt body have eruption ages of less than 8ka, and likely as young as 1ka, despite being located on crust that has a spreading age of ~75ka. The young ages of these lavas, combined with existing geological, geochemical and geophysical constraints, provide evidence for a genetic link between the pillow ridge and seamount lavas and the seismically imaged, underlying off-axis melt lens. This link demonstrates that off-axis volcanism does not necessarily come from a sub-axial magma body and can be sourced directly from off-axis magma bodies.

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.

Structure and development of an axial volcanic ridge: Mid-Atlantic Ridge, 45°N

We describe the most comprehensive and detailed high resolution survey of an axial volcanic ridge (AVR) ever conducted, at 45°N on the Mid-Atlantic Ridge. We use 3 m resolution sidescan sonar, deep-towed magnetic field measurements, video observations from eleven ROV dives, and two very-high-resolution bathymetry and magnetic surveys. The most recently active AVR has high topographic relief, high acoustic backscatter, high crustal magnetization and little faulting. It is sharp-crested, 25 × 4 km in extent and 500 m high, and is covered by approximately 8000 volcanic “hummocks” whose detailed nature is revealed for the first time. Each is an individual volcano ≤ 450 m in diameter and ≤ 200 m high, ranging from steep-sided (45°) cones to low domes. Many have suffered significant flank collapse. Hummocks tend to align in rows parallel to the AVR axis, parallel to its NE-trending spurs or, on its lower flanks, sub-normal to the AVR trend. These latter are spaced 1–2 km apart and comprise 1–2 km-long rows of single volcanoes. We infer that their emplacement is controlled by down-flank magma transport, possibly via lava tubes. The AVR contains only one large flat-topped seamount. The flanking median valley floor consists of either older hummocky volcanic terrain or flat-lying, mostly sediment-covered lavas. These typically have low-relief lobate surfaces, inflation and collapse structures, and occasional lava tubes and tumuli. The AVR displays open fissures, mostly along its crest. There is direct evidence for only a few small faults on the AVR, though steep, outward-facing slopes draped by elongate pillows may be small normal faults covered by lava. The surrounding median valley floor is heavily fissured. Normal faults cut it and an older AVR, the latter displaying significant outward facing faults. High crustal magnetization, an approximate proxy for crustal age within the Brunhes, is confined to the active AVR. Magnetic palaeointensity measurements are consistent with ages up to ~ 12 ka for several samples from the active AVR and ≥ 12 ka for one sample from the median valley floor. This is much less than the predicted spreading age, implying distribution of melt off-axis or episodic AVR growth.

Solutions of barotropic trapped waves around seamounts

In this paper, solutions of free, barotropic waves around axisymmetric seamounts are derived. Even though this type of oscillation has been studied before, we revisit this problem for two main reasons: (i) the linear, barotropic, shallow-water equations with a rigid lid are now solved with no further approximations, in contrast with previous studies; (ii) the solutions are applied to a wide family of seamounts with profiles proportional to exp(rs), with r being the radial distance from the centre of the mountain and s any positive real number. (Most previous works are restricted to the special case s = 2.) The resulting dispersion relation possesses a remarkable simplicity that reveals a number of wave characteristics, for instance, the discrete wave frequencies and the angular phase speed of the waves around the seamount are easily derived as a function of the seamount shape. By varying the shape parameter one can study trapped waves around flat-topped seamounts or guyots (s &gt; 2) or sharp, cone-shaped topographies (s &lt; 2).

Marine Macroalgal Diversity Assessment of Saba Bank, Netherlands Antilles

BackgroundLocated in the Dutch Windward Islands, Saba Bank is a flat-topped seamount (20–45 m deep in the shallower regions). The primary goals of the survey were to improve knowledge of biodiversity for one of the world's most significant, but little-known, seamounts and to increase basic data and analyses to promote the development of an improved management plan.Methodology/Principal FindingsOur team of three divers used scuba to collect algal samples to depths of 50 m at 17 dive sites. Over 360 macrophyte specimens (12 putative new species) were collected, more than 1,000 photographs were taken in truly exceptional habitats, and three astonishing new seaweed community types were discovered. These included: (1) “Field of Greens” (N 17°30.620′, W 63°27.707′) dominated by green seaweeds as well as some filamentous reds, (2) “Brown Town” (N 17°28.027′, W 63°14.944′) dominated by large brown algae, and (3) “Seaweed City” (N 17°26.485′, W 63°16.850′) with a diversity of spectacular fleshy red algae.Conclusions/SignificanceDives to 30 m in the more two-dimensional interior habitats revealed particularly robust specimens of algae typical of shallower seagrass beds, but here in the total absence of any seagrasses (seagrasses generally do not grow below 20 m). Our preliminary estimate of the number of total seaweed species on Saba Bank ranges from a minimum of 150 to 200. Few filamentous and thin sheet forms indicative of stressed or physically disturbed environments were observed. A more precise number still awaits further microscopic and molecular examinations in the laboratory. The expedition, while intensive, has only scratched the surface of this unique submerged seamount/atoll.

Cavalli Seamount, Northland Plateau, SW Pacific Ocean: a Miocene metamorphic core complex?

Cavalli Seamount (34°06′S, 174°10′E) is an irregular-shaped, flat-topped seamount on the Northland Plateau, SW Pacific Ocean. In contrast to Miocene lavas recovered from other seamounts in the area, the rocks dredged from Cavalli Seamount consist of biotite schist, with minor calc-silicate bands. Mineralogy, whole-rock chemistry, U–Pb dating of detrital zircons, and Sr and Nd isotopic composition collectively indicate a siliciclastic–carbonate protolith of latest Cretaceous to Paleogene stratigraphic age. Metamorphic index minerals are sillimanite, andesine, garnet, ilmenite and K-feldspar, and indicate peak P – T conditions of c . 650 °C and c . 0.4 GPa. U–Pb, Ar/Ar and fission-track dating of minerals, and micropalaeontological dating of associated limestones, indicate rapid cooling in the interval 23–21 Ma. The schist contains chloritized and hematized slickenslides parallel to a penetrative lineation, s–c planes and conjugate microfractures, and has limestone geopetal infillings parallel to foliation. These features are consistent with the interpretation of Cavalli Seamount as part of the lower plate of an Early Miocene metamorphic core complex. Although other explanations are possible, the core complex interpretation fits with regional Early Miocene events including crustal thickening as a result of allochthon emplacement in northern New Zealand, crustal thinning associated with rapid extension in the South Fiji Basin, Pacific trench rollback, and abundant Early Miocene igneous activity.

The marine podocopid Ostracoda of Easter Island: a paradox in zoogeography and evolution

From a total of 20 samples, of which 8 were collected in the eulittoral and 12 by SCUBA diving in the sublittoral, more than 11,000 ostracods were recovered. These comprise 30 species belonging to 20 genera. Of the 30 species, one is a cladocopid, one a platycopid and the remainder podocopids, of which three are bairdiids, three cyprids and the remainder (22) are cytherids. Twenty-one of the species are both new and endemic as are two new subspecies. Only four species have been described from elsewhere ( Peripontocypris magnafurcata, Neonesidea tenera, Triebelina bradyi, Dentibythere dentata), although two others ( Cytherelloidea keiji, and Loxoconchella honoluluensis) are represented at the island by new subspecies. However, an additional three species are known elsewhere only in manuscript; two of these are only known from the nearest inhabited land, the Pitcairn/Henderson group some 1400 miles to the west. The remaining 22 are endemic to Easter Island and are monographed elsewhere. There are no species common with the Galapagos Islands nor with the South or Central American coast or Clipperton Island. The three described species and the four known, but as yet undescribed species, all occur to the west and it is in the Indo-Pacific and Australasia that the closest relatives of the endemic species are encountered. The Easter Island fauna seems clearly to be the product of a migration or series of migrations from the west. However, that this or these events were not simple is attested to by the fact that, although the seven species known at Easter Island and elsewhere are all from tropical environments, none of the pan-tropical Tethyan taxa are recorded there. Three of this group occur at Pitcairn/Henderson, however, although none are among the four species common to Pitcairn/Henderson and Easter islands. Easter Island and the other volcanic islands to the west and west-northwest are all situated on a submarine ridge together with numerous guyots. Many of these today have relatively shallow summits and all flat-topped seamounts must have been emergent at times in their history. It is suggested that the eastwards migration (migrations) took place at those times of lowest sea level and greatest emergence of the summits by normal island-hopping sweepstakes routes.

Comparison of flat-topped stellate seamounts on Earth's seafloor with stellate domes on Venus using side-scan sonar and Magellan synthetic aperture radar

The diameters of many domes on Venus imaged by the Magellan synthetic aperture radar (SAR) are an order of magnitude larger than terrestrial subaerial domes; however, surveys of the seafloor using sonar imaging systems such as GLORIA (Geological Long Range Inclined Asdic) and SeaBEAM have revealed flat-topped volcanic features with similar diameters, volumes and slope angles. Suggested analogies between domes on Venus and seamounts on the northern East Pacific Rise and off Hawaii have been tested by D.K. Smith [J. Volcanol. Geotherm. Res. 73 (1996) 47–64] using comparisons of size and shape and significant differences identified. GLORIA images of flat-topped seamounts near Hawaii show them to have stellate planforms. Over half of the total population of 375 domes on Venus have been modified and many have stellate planforms. We have focused on the similarities in stellate planforms between flat-topped seamounts and modified domes on Venus. We have used GLORIA sonar and Magellan radar data to constrain the formation and evolution of both flat-topped seamounts on the ocean floor and volcanic domes on Venus to see if the processes that formed them are similar. We propose that flat-topped seamounts on the Hawaiian Ridge are built on sediments overlying the oceanic crust and that available evidence supports three processes that can form stellate margins: (1) radial dike intrusions related to flank rift zones; (2) dike intrusions which cause slope failures; and (3) gravitational slope failures unrelated to intrusions. For Venus, where low erosion rates and the absence of a sediment layer results in lavas being emplaced directly onto the crust, the available evidence supports stellate margins originating by gravitational slope failures unrelated to diking, though intrusions cannot be ruled out with existing data.

Upper plate deformation associated with seamount subduction

In many active margins, severe deformation is observed at the front of the overriding plate where seamounts or aseismic ridges subduct. Such deformation appears to be a main tectonic feature of these areas which influences the morphology and the seismicity of the margin. To better understand the different stages of seamount subduction, we have performed sandbox experiments to study in detail the evolution of deformation both in space and time and thus complement seismic images and bathymetry interpretation. We focus, in this paper, on the surface deformation directly comparable with seafloor morphology. Two types of subducting seamounts were modelled: relatively small conical seamounts, and larger flat-topped seamounts. The indentation of the margin by the seamount inhibits frontal accretion and produces a re-entrant. The margin uplift includes displacement along backthrusts which propagate from the base of the seamount, and out-of-sequence forethrusts which define a shadow zone located on the landward flank of the seamount. When the seamount is totally buried beneath the margin, this landward shielded zone disappears and a larger one is created in the wake of the asperity due to the elevated position of the décollement. As a consequence, a section of the margin front follows behind the seamount to greater depth. A `slip-line' network develops concurrently above the subducting seamount flanks from the transtension along the boundaries of the shadow zone. In a final stage, normal faults, controlled by the shape of the seamount, develop in the subsiding wake of the asperity. Swath-bathymetric data from the Costa Rica margin reveal detailed surface deformation of the margin above three subducting seamounts. Shaded perspective views highlight the detailed structure of the seafloor and compare well with surface deformation in the sandbox experiments. The good correlation between the marine data and experimental results strengthen a structural interpretation of the Costa Rican seamount subduction.

The paradox of drowned carbonate platforms and the origin of Cretaceous Pacific guyots

Geochemical, stratigraphic and palaeolatitudinal data from deep boreholes drilled through Pacific guyots—flat-topped seamounts—help to explain the drowning of these Cretaceous shallow-water carbonate platforms that once thrived through the accumulation of biogenic and inorganic calcium carbonate sediment in mid-oceanic regions. The platforms drowned sequentially over a 60-million-year interval while they were being transported northward by Pacific plate motion through a narrow equatorial zone (∼0–10° S). Such platforms were apparently resistant to the effects of Cretaceous oceanic anoxic events. Although the mechanism responsible for drowning remains unknown, the tropics have not always been the refuge for atolls that they are today.

Fault-controlled magma transport through the mantle lithosphere at slow-spreading ridges

Abstract At the ends of spreading segments at slow-spreading ocean ridges the axial valleys are usually asymmetric and bounded by large valley-wall faults, in contrast to segment centres, which are usually symmetric, with relatively small faults. These morphological variations are believed to reflect differences in the thermal structure of the lithosphere, caused by focused upwelling of partially molten mantle beneath segment centres. This gives rise to a thinner crust but thicker, and consequently stronger, lithosphere at segment ends, which is likely to provide a barrier to melt migration in comparison with segment centres. High-resolution sidescan sonar images of parts of the Mid-Atlantic Ridge reveal that flattopped seamounts occur preferentially at segment ends. Many of these seamounts are situated asymmetrically within the axial valley, and show a spatial association with the large segment-end faults, raising the possibility that melt may be channelled through the lithospheric mantle by such faults. If this is the case, then one might expect to encounter gabbros within mantle shear zones. We here document one such case in the Lizard Ophiolite, Cornwall, Southwest England: the Carrick Luz shear zone, a 100m wide dykelike body of gabbro and gabbro mylonite occurring within mantle ultramafic rocks. The shear zone shows a progression from penetrative mylonite fabrics, discrete ultramylonitic shear planes, through to cataclasites and fault gouges. The penetrative mylonite fabrics themselves deform, and are in turn cut by, mafic dykes. Shear direction and sense is the same for all of the fault rock types, ductile and brittle, and after correction for the regional tilt of the ophiolite section, are consistent with normal faulting. Gabbro mylonitic shear zones such as this within the lithospheric mantle provide a mechanism for weakening the lithospheric mantle. They might be expected to have a high acoustic impedance contrast with the surrounding ultramafic rocks, and are strong candidates for dipping seismic reflectors observed in old oceanic crust.

Weakly nonlinear oscillatory flow over an isolated seamount

Abstract Numerical models can show large errors in the modelling of flow around tall topography. We present a weakly nonlinear analytical solution of barotropic oscillatory flow over a tall axisymmetric seamount and compare the results to a numerical tidal model. The comparison is formulated for a flat topped seamount with parabolic sides and for a standing wave boundary condition in order to make the problem analytically tractable and to reduce it to a series of ordinary differential equations. The comparison shows reasonable agreement within the confines of the analytical formulation confirming that the numerical model can exploit tall topography. Comparisons to a Gaussian seamount and Kelvin wave forcing illustrate the generality of the results. The analytical formulation produces an equation relating relative errors in the comparison to discontinuities in topographical slope; the numerical model should have smaller errors over smoother topography. Residual velocities are found to be proportional to th...

Detailed volcanic geology of the MARNOK area, Mid-Atlantic Ridge north of Kane transform

Abstract We present the results of a detailed study of the volcanic geology of the median valley floor of the Mid-Atlantic Ridge in two adjacent but contrasting spreading segments immediately north of the Kane transform fault, based on a high-resolution deep-towed sidescan sonar (TOBI) survey, near-bottom photographic traverses, and geochemical analyses from 28 precisely sited sets of dredge samples. This has allowed us to assess the relative importance of small (intersegment) and large (transform) boundaries on magmatic processes at slow spreading ridges. We find that the petrological effect of the transform (so-called ‘transform fault effect’) is minor compared to the manifestation of the second-order segmentation. The southern, ‘narrowgate’-type segment has a poorly developed neovolcanic zone, which tapers towards its bathymetric centre where continuous faulting causes rapid dismemberment. Flat-topped seamounts are preferentially located at the ends of the segment, particularly near the non-transform offset. There is a striking variation in the degree of fractionation along the length of this segment: primitive lavas were sampled at the centre, while more fractionated basalts, showing a greater range of parental compositions, form discrete volcanic edifices at the ends. In contrast, the northern segment has a wider inner valley, and syn-magmatic faults extend up to 15 km along the crest of its robust axial volcanic ridge. The along-segment trend of increasing MgO towards the bathymetric crest of the axial volcanic ridge is similar to, but less well-defined than, that for the southern segment. There is no variation in the bulk degree of melting along the segments as determined from the major element geochemistry; hence, melt migration is favoured over focused mantle upwelling as the main cause of the crustal thickness variations which define second-order segmentation. Radigenic isotopes show only very small variations with no systematic pattern emerging within or between the ridge segments. The isotopic irregularities are not related to the variations in incompatible elements, implying that the latter result from dynamic melting processes rather than from long-lived source heterogeneity. A model is developed which relates the volcanology, tectonic style and the geochemical trends to the episodicity of magma supply and eruption.