Volcanic Ridge Research Articles

Estudios Sobre Rubiáceas Méxicanas XXXVI

Two new Rogiera species have been detected in the collections of the Andayu group of Oaxaca (R. oaxacensis) and of the State University of Tabasco (R. tabascensis). Both species are notable because of their pending, creeping, liana-like habit growing on calcareous rocks, which is a rare geological formation in the mountainous systems of México formed mostly by old crystalline cordilleras and young volcanic ridges. Up to now only one species of Rogiera, the R. subscandens from Chiapas and Guatemala has been known of this life-form.

Contrast comparison of differential diagenetic pathways of Lower Tithonian carbonate materials from the Betic Cordillera (S. Spain): Evidence for physico-chemical paleo-seawater properties

The reconstruction of paleoenvironmental conditions is based on the study of ancient rock materials. For this purpose, skeletal materials are often assumed to have higher preservation potential, sometimes favored over the use of matrix micrite. However, factors such as ecology, depositional setting, mineralogy and diagenetic processes may obscure original geochemical signals, and care must be taken when reading such an intricate record. This work provides a thorough comparison between geochemical signatures of different carbonate materials, interpreted in the light of their paleoenvironmental significance. Burial and diagenetic processes are taken into account, contextualized by ammonite and belemnite ecology.Two sections deposited in epioceanic Tethyan areas and corresponding to the same Lower Tithonian ammonite biozone are under scope: a condensed pelagic and cephalopod-rich limestone (the Alamedilla sector) intercalated in thick mafic rocks piles, and a typical Ammonitico Rosso facies (the Cardador section). In contrast to depositional and taphonomic histories typical for a Tethyan top-swell site (i.e., rather calcareous Ammonitico Rosso), events of carbonate-mud deposition, rapid burial and early lithification, synsedimentary sliding and over imposition of firmground horizons characterize pelagic cephalopod-rich limestones. Differential taphonomy of carbonate skeletals agrees with differential preservation, favoring inner-cast preservation in Ammonitico Rosso and shell preservation (neomorphic calcite) in pelagic cephalopod-rich limestones.Stable carbon and oxygen isotope data, together with trace element data (Sr, Mg, Fe, Mn) are presented for a wide range of carbonate materials: matrix micrite, neomorphic ammonite shells, belemnite rostra and carbonate cements. Optical inspection (including cathodoluminescence imaging) and geochemical data suggest a fair preservation of the above mentioned materials, except for diagenetic marine burial carbonate cements. Carbon isotope values fall within the typical Tethyan Late Jurassic range (ca. 2‰), except data from belemnite rostra which are depleted around −1‰, which is interpreted as resulting from metabolic fractionation. Oxygen isotope values are more variable, but the clustering of different carbonate materials within a narrow range of values between −1 and 1‰ is consistent with conservative diagenesis. Warmer burial fluids explain lower values in later diagenetic carbonate cements. No evidence of pervasive diagenetic imprint is revealed by elemental composition. The increased abundance of Mg, Fe, and especially Mn (up to 1500ppm), is recognized as hydrothermal contamination. Proximity to hydrothermal sources (the Mid-Subbetic Volcanic Ridge) accords with higher Mn concentration at the Alamedilla site. Interestingly, the timing of the mineralogical stabilization of each carbonate material reveals a potential record of a post-sedimentary geochemical event.The obtained information demonstrates the complexity of ancient carbonate records, and encourages future research focused on the preservation potential of geochemical proxies throughout time. The studied case hopes to be a representative example to approach the interpretation of potential past and present rare analogs.

Lower bathyal and abyssal distribution of coral in the axial volcanic ridge of the Mid-Atlantic Ridge at 45°N

The deep-sea floor below 3000m occupies 50% of the surface of the planet and is composed mainly of fine sediments. Most studies of deep-sea benthic fauna have concentrated on soft sediments with little sampling in rocky areas and even less on non-vent mid-ocean ridges. To assess the distribution and abundance of coral between 2500m and 3500m depths, video footage from the ROV Isis taken during a cruise to the Axial Volcanic Ridge (AVR) of the Mid-Atlantic Ridge at approx 45°30′ N was analysed. Abundances per 100m were calculated and mapped using Arc GIS, with a maximum of 59 being observed. 20 putative species were identified. Scleractinia were absent from the observed area and the coral fauna was dominated by Octocorallia. The data were separated into four substratum types, sediment, sloped rock, flat rock and mixed substratum, with both abundance and community being compared. Sedimented and rocky areas had different coral communities with sediment having a higher occurrence of Pennatulidae and Chrysogorgidae than rock. Sloped rock had the highest abundance of corals. We suggest that this increase in abundance reflects higher food availability as well as the solid substratum on which coral larvae settle.

Palaeomagnetic study of a subaerial volcanic ridge (São Jorge Island, Azores) for the past 1.3 Myr: evidence for the Cobb Mountain Subchron, volcano flank instability and tectonomagmatic implications

We present a palaeomagnetic study on 38 lava flows and 20 dykes encompassing the past 1.3 Myr on S. Jorge Island (Azores Archipelago-North Atlantic Ocean). The sections sampled in the southeastern and central/western parts of the island record reversed and normal polarities, respectively. They indicate a mean palaeomagnetic pole (81.3°N, 160.7°E, K= 33 and A95= 3.4°) with a latitude shallower than that expected from Geocentric Axial Dipole assumption, suggesting an effect of non-dipolar components of the Earth magnetic field. Virtual Geomagnetic Poles of eight flows and two dykes closely follow the contemporaneous records of the Cobb Mountain Subchron (ODP/DSDP programs) and constrain the age transition from reversed to normal polarity at ca. 1.207 ± 0.017 Ma. Volcano flank instabilities, probably related to dyke emplacement along an NNW-SSE direction, led to southwestward tilting of the lava pile towards the sea. Two spatially and temporally distinct dyke systems have been recognized on the island. The eastern is dominated by NNW-SSE trending dykes emplaced before the end of the Matuyama Chron, whereas in the central/western parts the eruptive fissures oriented WNW-ESE controlled the westward growth of the S. Jorge Island during the Brunhes Chron. Both directions are consistent with the present-day regional stress conditions deduced from plate kinematics and tectonomorphology and suggest the emplacement of dykes along pre-existing fractures. The distinct timing and location of each dyke system likely results from a slight shift of the magmatic source.

Upper Mantle Pollution during Afar Plume–Continental Rift Interaction

New Pb, Sr, Nd, Hf, and He isotope data for Quaternary basalts, erupted from Debre Zeyit, Butajira, and the Wonji Fault Belt of the Main Ethiopian Rift, show systematic mixing relationships involving three distinct mantle sources. The Pb, Sr, Nd, and Hf isotopic arrays converge in a specific region of isotopic multi-space where they define the composition of the Afar mantle plume (centered about (206)Pb/(204)Pb = 19 center dot 5, (87)Sr/(86)Sr = 0 center dot 7035, epsilon(Nd) = +4 center dot 6, epsilon(Hf) = +9 center dot 3, (3)He/(4)He > 15 R(A)). This plume end-member has an identical composition to that observed previously in oceanic basalts. The distinct isotopic arrays for the various volcanic areas in the Main Ethiopian Rift vary spatially in a systematic manner, and may be viewed as pseudo-binary mixing arrays. This further suggests that the Afar mantle plume interacts with the local continental lithosphere and upper mantle asthenosphere (mid-ocean ridge basalt-like source) through an ordered sequence of mixing events. Simple mixing models require that the mass proportions of continental lithosphere and upper mantle involved in magma generation must be nearly constant within each volcanic area, but that the proportion of plume material decreases regularly with distance southwestward along the Main Ethiopian Rift, away from the central axis of the plume. This systematic behavior means that continental lithosphere can become detached and mixed into the shallow mantle prior to the flow of upwelling plume material beneath the developing rift system. Detachment and mixing into the asthenosphere during continental rift evolution is an important process for producing the range of ambient upper mantle compositions sampled by mid-ocean ridge volcanism away from island hotspots.

Evolution of Macquarie Ridge Complex seamounts: Implications for volcanic and tectonic processes at the Australia–Pacific plate boundary south of New Zealand

The Macquarie Ridge Complex (MRC) coincides with the Australia–Pacific plate boundary south of New Zealand and is characterised by a rugged bathymetry that comprises numerous seamounts along its length. Tectonic plate reconstructions show that the Australia–Pacific plate boundary evolved from a spreading centre to a transpressional boundary from ca. 40 to 6Ma. However, limited sampling and high-resolution mapping of the region have meant that the composition and morphology of the MRC remain relatively unknown. Here we present results from multibeam mapping, photographic investigation and petrologic and geochemical analyses from 10 seamounts along the MRC. Dredge samples collected from the seamounts are alkaline to sub-alkaline basalts, containing 45.1–50.8wt.% SiO2, 3.5–12.0wt.% MgO and 1.9–4.9wt.% total alkalis. All samples are enriched mid-ocean ridge basalts that have K/Ti>0.15, which originated as low degree partial melts during the late stages of mid-ocean ridge volcanism. This oceanic crust has been sheared and accreted along the plate boundary since ca. 6Ma by cumulative transpressional relative plate motion, such that lavas from distinct magma chambers have been juxtaposed at individual seamounts along the ridge. MRC seamounts have formed as discrete elevations as a result of dip-slip and strike-slip faulting of the ridge axis. Three guyot-type seamounts have flat-top summit plateaux that were presumably formed by wave erosion via relative sea-level lowering due to uplift of the ridge. These seamounts have since subsided due to extension associated with the step-over fault geometry of the transform plate boundary along the McDougall and Macquarie ridge segments. While the seamounts are of volcanic origin, they are manifestations of tectonic and geomorphic processes. Seamounts distal from the ridge axis, however, are not subjected to plate boundary deformation and instead have morphologies formed through volcanic processes.

Eruptive hummocks: Building blocks of the upper ocean crust

The spreading axis at many slow-spreading mid-ocean ridges is marked by an axial volcanic ridge. In this study, we use a combination of high-resolution remote sensing methods to elucidate the detailed nature of volcanoes in such a ridge. We find that the “hummocks” described in previous sidescan sonar studies are dome- or cone-shaped edifices, 5–150 m high with diameters of 30–330 m. We estimate they form quickly, in single eruptions, each of which may produce several hummocks. Hummock collapse is common and hummocks of all heights are prone to failure. Collapses generally occur down the regional seafloor slope, suggesting control by local topography. Approximately 33% of hummocks lose ∼40% of their volume by collapse, so ∼12% of all material erupted on the axial volcanic ridge is rapidly converted to talus. The higher porosity of these deposits may increase average upper crustal porosity by several percent, contributing >0.5 km s−1 to seismic velocity decrease in the upper oceanic crust, and may be one of the dominant mechanisms for increasing porosity in upper slow-spreading oceanic crust.

Distributed deformation ahead of the Cocos-Nazca Rift at the Galapagos triple junction

The Galapagos triple junction is not a simple ridge-ridge-ridge (RRR) triple junction. The Cocos-Nazca Rift (C-N Rift) tip does not meet the East Pacific Rise (EPR). Instead, two secondary rifts form the link: Incipient Rift at 2°40′N and Dietz Deep volcanic ridge, the southern boundary of the Galapagos microplate (GMP), at 1°10′N. Recently collected bathymetry data are used to investigate the regional tectonics prior to the establishment of the GMP (∼1.5 Ma). South of C-N Rift a band of northeast-trending cracks cuts EPR-generated abyssal hills. It is a mirror image of a band of cracks previously identified north of C-N Rift on the same age crust. In both areas, the western ends of the cracks terminate against intact abyssal hills suggesting that each crack initiated at the EPR spreading center and cut eastward into pre-existing topography. Each crack formed a short-lived triple junction until it was abandoned and a new crack and triple junction initiated nearby. Between 2.5 and 1.5 Ma, the pattern of cracking is remarkably symmetric about C-N Rift providing support for a crack interaction model in which crack initiation at the EPR axis is controlled by stresses associated with the tip of the westward-propagating C-N Rift. The model also shows that offsets of the EPR axis may explain times when cracking is not symmetric. South of C-N Rift, cracks are observed on seafloor as old as 10.5 Ma suggesting that this triple junction has not been a simple RRR triple junction during that time.

Mitigating the consequences of extreme events on strategic facilities: Evaluation of volcanic and seismic risk affecting the Caspian oil and gas pipelines in the Republic of Georgia

In this work we identify and quantify new seismic and volcanic risks threatening the strategic Caspian oil and gas pipelines through the Republic of Georgia, in the vicinity of the recent Abuli Samsari Volcanic Ridge, and evaluate risk reduction measures, mitigation measures, and monitoring. As regards seismic risk, we identified a major, NW-SE trending strike-slip fault; based on the analysis of fault planes along this major transcurrent structure, an about N-S trend of the maximum, horizontal compressive stress (σ1) was determined, which is in good agreement with data instrumentally derived after the 1986, M 5.6 Paravani earthquake and its aftershock. Particularly notable is the strong alignment of volcanic vents along an about N-S trend that suggests a magma rising controlled by the about N-S-directed σ1. The original pipeline design included mitigation measures for seismic risk and other geohazards, including burial of the pipeline for its entire length, increased wall thickness, block valve spacing near recognized hazards, and monitoring of known landslide hazards. However, the design did not consider volcanic risk or the specific seismic hazards revealed by this study. The result of our analysis is that the Baku-Tbilisi-Ceyhan (BTC) oil pipeline, as well as the Baku-Tbilisi-Erzerum South Caucasian natural gas pipeline (SCP) were designed in such a way that they significantly reduce the risk posed by the newly-identified geohazards in the vicinity of the Abuli-Samsari Ridge. No new measures are recommended for the pipeline itself as a result of this study. However, since the consequences of long-term shut-down would be very damaging to the economies of Western Europe, we conclude that the regionally significant BTC and SCP warrant greater protections, described in the final section of or work. The overall objective of our effort is to present the results in a matrix framework that allows the technical information to be used further in the decision-making process, with the goal of reducing the uncertainty in the final decision. This approach is applicable to the study of risks in other pipeline systems.

Helium isotope variations between Réunion Island and the Central Indian Ridge (17°–21°S): New evidence for ridge–hot spot interaction

We report new helium abundance and isotope results for submarine basaltic glasses from the Central Indian Ridge (CIR) between the Marie Celeste (16.7°S) and Egeria fracture zones (FZ) (20.6°S); the adjacent Gasitao, Three Magi, and Rodrigues ridges; and for olivine separates from lavas and cumulate xenoliths from the Mascarene Islands (Reunion, Mauritius, and Rodrigues). Helium isotope ratios in basaltic glasses range from 7.1 to 12.2 R A (where R A = air 3He/4He) and lie between values of Mid‐Ocean Ridge Basalt (MORB) (8 ± 1 R A) and samples from Reunion Island (11.5 to 14.1 R A). The highest 3He/4He values (up to 12.2 R A) are found in glasses recovered off axis from the Three Magi and Gasitao ridges. Along the CIR axis, MORB‐like 3He/4He ratios are found near the Egeria FZ, and there is a marked increase to values of ∼11 R A between ∼19° and 20°S. The lowest 3He/4He values (<8 R A) are found immediately south of the Marie Celeste FZ, where incompatible trace element ratios (e.g., La/Sm) are highest. These low 3He/4He ratios can be explained by closed system radiogenic 4 He ingrowth in either (1) a fossil Reunion hot spot mantle component, embedded into the subridge mantle when the CIR migrated over the hot spot at ∼34 Ma or (2) trace element enriched MORB mantle. In contrast, the high 3He/4He ratios observed on the CIR axis adjacent to the Gasitao Ridge, and along the off‐axis volcanic ridges, are consistent with flow of hot spot mantle material from Reunion (∼1100 km to the west) toward the CIR.

Petrology and Sr-Nd-Pb-He isotope geochemistry of postspreading lavas on fossil spreading axes off Baja California Sur, Mexico

Postspreading volcanism has built large seamounts and volcanic ridges along the short axes of a highly segmented part of the East Pacific Rise crest that ceased spreading at the end of the middle Miocene, offshore Baja California Sur, Mexico. Lava samples from Rosa Seamount, the largest volcano, are predominantly alkalic basalts, mugearites, and benmoreites. This lavas series was generated through fractional crystallization and is compositionally similar to the moderately alkalic lava series in many oceanic islands. Samples from volcanic ridges at three adjacent failed spreading axes include mildly alkalic, transitional, and tholeiitic basalts and differentiated trachyandesites and andesite. The subtle but distinct petrologic and isotopic differences among the four sites may be due to differences in the degree of partial melting of a common, heterogeneous source. Postspreading lavas from these four abandoned axes off Baja California Sur together with those from other fossil spreading axes and from seamount volcanoes that grew on the East Pacific Rise flanks define a compositional continuum ranging from normal mid-ocean ridge basalt (NMORB)-like to ocean island basalt (OIB)-like. We propose that the compositional spectrum of these intraplate volcanic lavas is due to different degrees of partial melting of the compositionally heterogeneous suboceanic mantle in the eastern Pacific. A large degree of partial melting of this heterogeneous mantle during vigorous mantle upwelling at an active spreading center produces NMORB melts, whereas a lesser degree of partial melting during weak mantle upwelling following cessation of spreading produces OIB-like melts. The latter melts have a low (<8 RA) 3He/4He signature indicating their formation is different from that of OIBs from major “hot spot” volcanoes in the Pacific with high 3He/4He ratios, such as Hawaii and Galapagos.

Comments on the paper “Is the track of the Yellowstone hotspot driven by a deep mantle plume? — Review of volcanism, faulting, and uplift in light of new data” by Kenneth Pierce and Lisa Morgan [J. Volcanol. Geotherm. Res. 188 (2009), 1–25

The global mid-ocean ridge (MOR) system represents a major site for outgassing of volatiles from Earth's mantle. The amount of H2O released via eruption of mid-ocean ridge basalts varies along the global ridge system and greatest at sites of interaction with mantle plumes. These deep-sourced thermal anomalies affect approximately one-third of all MORs – as reflected in enrichment of incompatible trace elements, isotope signatures and elevated ridge topography (excess melting) – but the physical mechanisms involved are controversial. The “standard model” involves solid-state flow interaction, wherein an actively upwelling plume influences the divergent upwelling generated by a mid-ocean ridge so that melting occurs at higher pressures and in greater amounts than at a normal spreading ridge. This model does not explain, however, certain enigmatic features including linear volcanic ridges radiating from the active plume to the nearby MOR. Examples of these are the Wolf–Darwin lineament (Galápagos), Rodrigues Ridge (La Réunion), Discovery Ridge (Discovery), and numerous smaller ridge-like structures associated with the Azores and Easter–Salas y Gómez hot spots. An important observation from our study is that fractionation-corrected MORB with exceptionally-high H2O contents (up to 1.3 wt.%) are found in close proximity to intersections of long-lived plume-related volcanic lineaments with spreading centres.New algorithms in the rare-earth element inversion melting (INVMEL) program allow us to simulate plume–ridge interactions by mixing the compositions of volatile-bearing melts generated during both active upwelling and passively-driven corner-flow. Our findings from these empirical models suggest that at sites of plume–ridge interaction, moderately-enriched MORBs (with 0.2–0.4 wt.% H2O) result from mixing of melts formed by: (i) active upwelling of plume material to minimum depths of ∼35 km; and (ii) those generated by passive melting at shallower depths beneath the ridge. The most volatile-rich MORB (0.4–1.3 wt.% H2O) may form by the further addition of up to 25% of “deep” small-fraction plume stem melts that contain >3 wt.% H2O. We propose that these volatile-rich melts are transported directly to nearby MOR segments via pressure-induced, highly-channelised flow embedded within a broader “puddle” of mostly solid-state plume material, spreading beneath the plate as a gravity flow. This accounts for the short wavelength variability (over 10s of km) in geochemistry and bathymetry that is superimposed on the much larger (many 100s of km) “waist width” of plume-influenced ridge.Melt channels may constitute a primary delivery mechanism for volatiles from plume stems to nearby MORs and, in some instances, be expressed at the surface as volcanic lineaments and ridges. The delivery of small-fraction hydrous melts from plume stems to ridges via a two-phase (melt-matrix) regime implies that a parallel, bimodal transport system is involved at sites of plume–ridge interaction. We estimate that the rate of emplacement of deep-sourced volatile-rich melts in channels beneath the volcanic lineaments is high and involves 10s of thousands of km3/Ma. Since mantle plumes account for more than half of the melt production at MORs our findings have important implications for our understanding of deep Earth volatile cycling.

Discovery of a black smoker vent field and vent fauna at the Arctic Mid-Ocean Ridge

The Arctic Mid-Ocean Ridge (AMOR) represents one of the most slow-spreading ridge systems on Earth. Previous attempts to locate hydrothermal vent fields and unravel the nature of venting, as well as the provenance of vent fauna at this northern and insular termination of the global ridge system, have been unsuccessful. Here, we report the first discovery of a black smoker vent field at the AMOR. The field is located on the crest of an axial volcanic ridge (AVR) and is associated with an unusually large hydrothermal deposit, which documents that extensive venting and long-lived hydrothermal systems exist at ultraslow-spreading ridges, despite their strongly reduced volcanic activity. The vent field hosts a distinct vent fauna that differs from the fauna to the south along the Mid-Atlantic Ridge. The novel vent fauna seems to have developed by local specialization and by migration of fauna from cold seeps and the Pacific.

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.

Recent volcanism in relation to plate interaction and deep-level geodynamics

The spatial distribution of recent (under 2 Ma) volcanism has been studied in relation to mantle hotspots and the evolution of the present-day supercontinent which we named Northern Pangea. Recent volcanism is observed in Eurasia, North and South America, Africa, Greenland, the Arctic, and the Atlantic, Indian, and Pacific Oceans. Several types of volcanism are distinguished: mid-ocean ridge (MOR) volcanism; subduction volcanism of island arcs and active continental margins (IA + ACM); continental collision (CC) volcanism; intraplate (IP) volcanism related to mantle hotspots, continental rifts, and transcontinental belts. Continental volcanism is obviously related to the evolution of Northern Pangea, which comprises Eurasia, North and South America, India, Australia, and Africa. The supercontinent is large, with predominant continental crust. The geodynamic setting and recent volcanism of Northern Pangea are determined by two opposite processes. On one hand, subduction from the Pacific Ocean, India, the Arabian Peninsula, and Africa consolidates the supercontinent. On the other hand, the spreading of oceanic plates from the Atlantic splits Northern Pangea, changes its shape as compared with Wegener’s Pangea, and causes the Atlantic geodynamics to spread to the Arctic. The long-lasting steady subduction beneath Eurasia and North America favored intense IA + ACM volcanism. Also, it caused cold lithosphere to accumulate in the deep mantle in northern Northern Pangea and replace the hot deep mantle, which was pressed to the supercontinental margins. Later on, this mantle rose as plumes (IP mafic magma sources), which were the ascending currents of global mantle convection and minor convection systems at convergent plate boundaries. Wegener’s Pangea broke up because of the African superplume, which occupied consecutively the Central Atlantic, the South Atlantic, and the Indian Ocean and expanded toward the Arctic. Intraplate plume magmatism in Eurasia and North America was accompanied by surface collisional or subduction magmatism. In the Atlantic, Arctic, Indian, and Pacific Oceans, deep-level plume magmatism (high-alkali mafic rocks) was accompanied by surface spreading magmatism (tholeiitic basalts).

Large fault fabric of the Ninetyeast Ridge implies near‐spreading ridge formation

Ninetyeast Ridge (NER) is a linear volcanic ridge in the Indian Ocean thought to have formed by hotspot volcanism on the northward‐drifting Indian plate. Geological data from the ridge are sparse, so its tectonic evolution is poorly known. We studied satellite‐derived gravity data, seismic reflection profiles, and multibeam bathymetry to examine NER structure. Gravity data show that the ridge displays a series of nearly E‐W trending lineations with average spacing ∼0.4° (45 km). In seismic and bathymetry data, these lineations correlate with horsts and grabens that probably formed near the time of ridge emplacement. From their extensional nature and trends, we infer that these faulted structures formed near the spreading ridge that separated the Indian and Antarctic plates and their ubiquity implies the hotspot was never far from this spreading ridge.

The influence of small-scale mantle heterogeneities on Mid-Ocean Ridge volcanism: Evidence from the southern Mid-Atlantic Ridge (7°30′S to 11°30′S) and Ascension Island

Volcanism along Mid-Ocean Ridges is known to exhibit significant isotopic and elemental variations, traditionally regarded as reflecting global scale variations in mantle depletion history and refertilization processes. The ∼ 400 km long Mid-Atlantic Ridge (MAR) between the Ascension and Bode Verde Fracture Zones (7°30′S to 11°30′S) has been sampled at high spatial resolution (c. 10 km along-axis scale) and large elemental and isotopic variations have been identified covering almost the entire compositional spectrum of MAR basalts. In this paper we employ a multi-isotope (Sr–Nd–Hf–Pb) and trace element approach in order to explore the geodynamic implications of along-ridge compositional variations and their relationship to off-axis volcanism on nearby Ascension Island, commonly regarded as plume-related. The studied portion of the MAR consists of four major segments. Isotopic data from the deeply incised northern and southern segments A1 and A4 define a trend involving a high-εHf depleted mantle endmember. This isotopic signature is inferred to result from an ancient melting event in the garnet stability field, causing high time-integrated Lu/Hf and elevated 176Hf/ 177Hf. In contrast, a low-εHf depleted mantle endmember is indicated by the compositional trend for samples from the topographically elevated central portion (segments A2 and A3). Both trends converge at a common enriched endmember with least diluted compositions represented by samples from the subaerial eruptive stage of nearby Ascension Island. Because linear co-variations between isotope compositions and the respective parent–daughter ratios cannot be explained as representing “mantle isochrons” we infer that our data reflect arrays related to physical mixing of depleted and enriched mantle domains. This implies that distinctive mantle domains at kilometer scale may survive convection processes over time spans in the order of 1–2 Ga. This finding corroborates the results of recent Os-isotope studies of abyssal peridotite which returned similar conclusions. The systematics of along-axis isotopic variations show that mantle upwelling is highly variable. A systematic decrease of the enriched isotopic signature from the central segment A3 toward the northern termination of the segment A2 suggests a northward flow of enriched mantle material. In marked contrast, the compositional variations along the marginal segments A1 and A4 are random indicating heterogeneous mixing of mantle domains at a spatial resolution of < 50 km to 10 km (along-axis scale). With regard to Ascension Island we show that the submarine volcanic stage, sampled by a 3216 m long drill hole, was fed by a distinctive enriched mantle source currently inaccessible to partial melting. We interpret this observation within the framework of recent local-scale geophysical investigations and infer an on-axis origin of the bulk of the Ascension volcanic edifice at around 5 to 6 Ma, synchronous with the surrounding oceanic crust. Off-axis partial melting of the current common enriched mantle endmember accounts for rejuvenation of volcanism on Ascension generating the volumetrically subordinate subaerial portion of the island.

Three-dimensional gravity and magnetic modelling of the Irish sector of the NE Atlantic margin

A new 3D lithospheric model has been constructed using high-resolution gravity data from the Irish National Seabed Survey. The sedimentary component of the model incorporated density variations due to laterally varying overcompaction associated with Cenozoic denudation. After optimisation based on gravity inversion, regional crustal thickness variations were defined which are in reasonable agreement with the results of wide-angle seismic experiments. High crustal extension factors ( β > 5) characterise the deeper parts of the Rockall and Porcupine basins and in places the model indicates extreme stretching ( β > 10) beneath these basins. This could be because of instability in the gravity inversion, although other recent investigations have independently suggested similarly high extension factors. In contrast, the Hatton Basin is characterised by an apparent extension factor of about 2. The modelling resolves a pattern of NE- to NNE-trending local Mesozoic basins on the margins of the Rockall Trough, helping to delineate structures that were previously only sparsely sampled by seismic surveys. It appears possible that rifts with similar trends underlie the volcanic rocks which obscure the deeper parts of the Hatton Basin. The linear trends of the basins to the south and east of Ireland are interpreted to have been inherited from a basement fabric that was initially established during the Late Precambrian assembly of this basement and subsequently subjected to Caledonian and Variscan reactivation. Magnetic modelling indicates that the variations in the thickness of the crystalline crust predicted by the gravity models can explain the regional magnetic anomaly patterns over the Rockall and Porcupine basins, but that significant additional magnetic material (probably igneous rocks of both Palaeogene and Cretaceous ages) is required to explain the anomalies in the Hatton Basin region. The magnetic signature of the Rockall Basin is distinctly different to that over the basement (of similar apparent thickness) formed during mid-Cretaceous (C34N) opening of the ocean basin to the south. This is an impediment to hypotheses that invoke mid-Cretaceous sea-floor spreading rather than intracontinental rifting to explain the development of the basin. The exception is in the extreme south of the basin where the volcanism associated with the Barra Volcanic Ridges combined with indications of relatively strong lithosphere could be evidence of incipient ocean opening.

Petrogenesis of Davidson Seamount lavas and its implications for fossil spreading center and intraplate magmatism in the eastern Pacific

Seafloor spreading causes abundant magmatism along active ocean spreading centers, but the cause of magmatism along fossil spreading centers is enigmatic. Samples collected from Davidson Seamount, a typical volcanic ridge along an abandoned spreading center in the eastern Pacific, consist of an alkalic basalt to trachyte lava series; transitional basalts were sampled from another part of the abandoned axis, 20 km from the seamount. All samples experienced complex fractional crystallization prior to eruption, but they all share a common, compositionally heterogeneous mantle source. The parental magmas of the transitional basalts were produced from this source at higher degree of melting than those of the alkalic lava series. The composition of Davidson lavas overlaps with those of ridges along other fossil spreading centers and isolated near‐ and off‐ridge seamounts in the eastern Pacific. Together they define a compositional continuum ranging from tholeiitic, normal mid‐ocean ridge basalt (MORB)‐like to alkalic, ocean island basalt (OIB)‐like, similar to lavas that form linear island chains and ridges. We propose that this entire compositional spectrum of intraplate lavas that do not form linear volcanic chains in the eastern Pacific results from variations in the degree of partial melting of a common, compositionally heterogeneous mantle source. This source consists of more easily melted, geochemically enriched components of varying sizes and amounts embedded in a depleted lherzolitic matrix. Large degree of partial melting produces normal MORB–like melts represented by some near‐ridge seamount lavas, whereas small degree of melting produces OIB‐like fossil spreading center lavas. The small degree of partial melting beneath recently abandoned spreading centers results from either buoyancy‐driven decompression melting of the hot lithospheric and asthenospheric mantle material beneath active spreading centers or rapid motion, with respect to the underlying asthenosphere, of abandoned spreading axes that are thickening over a fertile mantle. Mid‐Tertiary volcanic rocks in coastal California, which are compositionally akin to intraplate lavas, are interpreted to be small degree partial melts of the same compositionally heterogeneous sub‐Pacific mantle that has upwelled through windows in a subducted slab.

Evolution of the Grenada and Tobago basins and implications for arc migration

The tectonic mechanisms controlling how volcanic arcs migrate through space and geologic time within dynamic subduction environments is a fundamental tectonic process that remains poorly understood. This paper presents an integrated stratigraphic and tectonic evolution of Late Cretaceous to Recent volcanic arcs and associated basins in the southeastern Caribbean Sea using seismic reflection data, wide-angle seismic refraction data, well data, and onland geologic data. We propose a new tectonic model for the opening of the Grenada and Tobago basins and the 50–250-km eastward jump of arc volcanism from the Late Cretaceous Aves Ridge to the Miocene to Recent Lesser Antilles arc in the southeast Caribbean based on the mapping of three seismic megasequences. The striking similarity of the half-graben structure of the Grenada and Tobago basins that flank the Lesser Antilles arc, their similar smooth basement character, their similar deep-marine seismic facies, and their similar Paleogene sediment thickness mapped on a regional grid of seismic data suggest that the two basins formed as a single, saucer-shaped, oceanic crust Paleogene forearc basin adjacent to the now dormant Aves Ridge. This single forearc basin continued to extend and widen through flexural subsidence during the early to middle Eocene probably because of slow rollback of the subducting Atlantic oceanic slab. Rollback may have been accelerated by oblique collision of the southern Aves Ridge and southern Lesser Antilles arc with the South American continent. Uplift and growth of the southern Lesser Antilles arc divided the Grenada and Tobago basins by early to middle Miocene time. Inversion of normal faults and uplift effects along both edges of the Lesser Antilles arc are most pronounced in its southern zone of arc collision with the South American continent. The late Miocene to Recent depositional histories of the Grenada and Tobago basins are distinct because of isolation of the Grenada basin by growth and uplift of the Neogene Lesser Antilles volcanic ridge.