Volcanic Seamounts Research Articles

Hydrothermal vents supporting persistent plumes and microbial chemoautotrophy at Gakkel Ridge (Arctic Ocean).

Hydrothermal vents emit hot fluids enriched in energy sources for microbial life. Here, we compare the ecological and biogeochemical effects of hydrothermal venting of two recently discovered volcanic seamounts, Polaris and Aurora of the Gakkel Ridge, in the ice-covered Central Arctic Ocean. At both sites, persistent hydrothermal plumes increased up to 800 m into the deep Arctic Ocean. In the two non-buoyant plumes, rates of microbial carbon fixation were strongly elevated compared to background values of 0.5-1 μmol m-3 day-1 in the Arctic deep water, which suggests increased chemoautotrophy on vent-derived energy sources. In the Polaris plume, free sulfide and up to 360 nM hydrogen enabled microorganisms to fix up to 46 μmol inorganic carbon (IC) m-3 day-1. This energy pulse resulted in a strong increase in the relative abundance of SUP05 by 25% and Candidatus Sulfurimonas pluma by 7% of all bacteria. At Aurora, microorganisms fixed up to 35 μmol IC m-3 day-1. Here, metal sulfides limited the bioavailability of reduced sulfur species, and the putative hydrogen oxidizer Ca. S. pluma constituted 35% and SUP05 10% of all bacteria. In accordance with this data, transcriptomic analysis showed a high enrichment of hydrogenase-coding transcripts in Aurora and an enrichment of transcripts coding for sulfur oxidation in Polaris. There was neither evidence for methane consumption nor a substantial increase in the abundance of putative methanotrophs or their transcripts in either plume. Together, our results demonstrate the dominance of hydrogen and sulfide as energy sources in Arctic hydrothermal vent plumes.

CORE-VTRCC Cruise Report: Geophysical Acquisition and Seafloor Sampling along the Vitória-Trindade Ridge

This work reports the data obtained by the multidisciplinary cruise CORE-VTRCC surveying on the region around the Vitória-Trindade Ridge (VTR) aboard the vessel NHo Cruzeiro do Sul (October 18th to November 6th, 2021). The main objectives of the VTRCC Cruise were (1) to assess the role of the VTR as an oceanographic barrier for bottom currents along the Brazilian margin; and (2) to characterize the morphology of the volcanic seamounts and its relationships with the carbonate sediments that cover it. For that, we performed multibeam bathymetry, magnetometry, high-resolution multichannel seismic survey, subbottom profiling survey, and seafloor sampling along the VTR and along the Columbia Deepwater Channel. The raw dataset was processed for quality control and organized for public access. When required, additional geophysical processing occurred to improve data quality. Seafloor samples are characterized based on the concentration of its biofacies, the sediment grain-sizes and the morphology of rhodoliths sampled. The dataset reveals comprehensively the geological and oceanographic heterogeneity of the region around the VTR.

Inhomogeneous Magnetization of Tyrrhenian Seamounts Revealed From Gravity and Magnetic Correlation Analysis

AbstractWe perform a joint analysis of gravity and magnetic data sets in the Tyrrhenian Sea region to infer the rock physical properties of several volcanic seamounts. We propose a moving‐window application using Poisson's theorem, which relates the total gradient of the magnetic field to the total gradient of the first‐order vertical derivative of the gravity field data. In volcanic environments, where strong intensity of remanent magnetization is expected, the total gradient of the magnetic field is particularly useful since it is almost independent on the direction of the total‐magnetization. The moving‐window approach resulted necessary due to the heterogeneous magnetization distribution of the volcanoes. First, we perform synthetic tests based on realistic seamount models which exhibit inhomogeneous magnetization intensity and orientation. Using the total gradients, we demonstrate that our approach can provide an appropriate magnetization‐to‐density ratio in different subareas of seamounts. The results of the correlation analysis for the Palinuro, Marsili, Vavilov, and Magnaghi seamounts provide interesting information on the variability of magnetization associated with different epochs of formation and demagnetization effects due to hydrothermal alteration processes.

An early Eocene fish assemblage associated with a barite deposit in the lower part of the Crescent Formation, Olympic Peninsula, Washington State, USA

AbstractAbundant shark and rare actinopterygian teeth are reported from a locality within the early Eocene (Ypresian) lower part of the Crescent Formation exposed in the Hamma Hamma River valley on the eastern Olympic Peninsula, Washington State, USA. This part of the Crescent Formation is predominantly submarine volcanic basalt with some sedimentary interbeds deposited in deep water. The teeth are derived from sediments that appear to directly overlay and in places interfinger with the margins of an anomalous lenticular barite deposit; one tooth was found in the barite. Genera represented include deep-water taxa (aff. Chlamydoselachus, Mitsukurina, Notorynchus, Odontaspis) and open marine, epipelagic sharks (Alopias, Brachycarcharias, Jaekelotodus, Macrorhizodus, Otodus, Striatolamia). The only other fossils found were two fragmentary shark vertebrae, numerous shark dermal ossicles, a single teleost tooth (Egertonia) and abundant, minute valves of a discinid brachiopod. This is the first report of macrofossils from the lower part of the Crescent Formation and the only early Eocene shark assemblage described from the North Pacific Basin. The shark assemblage also corroborates paleodepositional interpretations of the lower Crescent Formation as being in part ancient volcanic seamounts during early Eocene time.

Geohazard features of the Central-Southern Tyrrhenian seamounts

ABSTRACT The Magic project (MArine Geohazard along Italian Coasts), a large coordinated initiative that involved the whole marine geological research community in Italy in 2007–2013, produced a series of maps of the Geohazard features of the Central-Southern Tyrrhenian Seamounts. The features derive from multibeam surveys and therefore mainly rely on the morphological expression of seafloor and shallow sub-surface processes and events. Potential geohazards related to eruption and flank instability of the volcanic seamounts of the Tyrrhenian Sea are discussed and compared.

The Variety of the Relief of the Northeastern Atlantic Volcanic Seamounts

The Variety of the Relief of the Northeastern Atlantic Volcanic Seamounts

The Bottom Relief and the Anomalous Magnetic Field of the Ninetyeast Ridge in Near 5° N

The geophysical survey in cruise# 42 of R/V Akademik Boris Petrov (2017) at the polygon near Site 758 ODP clarified significantly knowledge about the bottom topography of the northern segment of the Ninetyeast Ridge. It is crossed by E–W transversal linear troughs, apparently being young tectonic fractures. The morphology of the narrow elevated block in the central part of the deep depression between neighboring large volcanic edifices is characterized in detail. Along with linear tectonic structures, the main features of this block are two volcanic seamounts. The detail magnetic survey at the polygon identified for the first time the nature of the sources of magnetic anomalies on the Ninetyeast Ridge. Intense local magnetic anomalies are observed here, both confined to local structures of the bottom and basement relief, and clearly unrelated to them. These anomalies are of a complex interfering nature caused by the superposition of fields from sources separated by magnetization, spatial position and age. According to the results of paleomagnetic analysis of the anomalies, the age of the structures here is very wide: the youngest features were formed about 20 million years ago, the oldest ones–over 80 million years ago.

Timing of carbon uptake by oceanic crust determined by rock reactivity

Abstract Carbonate formation during the alteration of oceanic crust is a global CO2 sink. Its timing and controls are not well understood, particularly in volcanic seamounts, which react with seawater over tens of millions of years. We report in situ U-Pb age dates of carbonate vein and void fill in 50–74 Ma basaltic basement of the Louisville Seamount Chain. More than 90% of the carbonate formed <20 m.y. after seamount emplacement. Vesicle carbonate precipitated within 8 m.y. (median = 2.9 m.y.) whereas vein carbonate grew over longer time spans (median = 8.1 m.y.). The duration of carbonation was hence limited despite the basement’s long-term exposure to seawater. The age dates imply a rapid infill of vesicles by alteration of confined domains around vesicles. Carbonate formation in veins extended for longer periods of time, likely due to the late opening of fractures, which exposed fresh reactive rock surface to circulating seawater long after the formation of the basement. We suggest that carbonate growth ceased after the volcanic rocks were too altered to liberate sufficient Ca2+ and generate the alkalinity required for carbonate precipitation. A critical extent of rock reactivity is required to sustain carbonate formation. Carbonate precipitation likely ends after much of the exposed basaltic substrate has been altered and the rock reactivity drops below this critical threshold. These findings help to explain the generally short duration of carbonation in the flanks of mid-ocean ridges.

Oxygen and hydrogen isotopic evidence that Kamaʻehuakanaloa (Lōʻihi) Seamount hydrothermal systems are recharged by deep Pacific seawater

We observed negative and positive δ18O and δ2H deviations from fluids collected at Kamaʻehuakanaloa (previously known as Lōʻihi) seamount relative to Pacific seawater from the same depth. Hydrothermal vents on the crater floor of Pele's Pit at Kamaʻehuakanaloa, at a depth of 1320 m, had δ18O and δ2H values as low as −0.19‰ and −0.3‰, respectively. Seawater collected within the caldera, within a zone 45 m above the crater floor, had δ18O and δ2H values as high as 1.15‰ and 6.5‰, respectively. In comparison, Pacific seawater at 1200 m and 1400 m at nearby station ALOHA exhibit intermediate δ18O and δ2H values of 0.2‰ and 0.34‰. The high δ18O and δ2H values observed in the caldera water-column may be explained by isotopic modification processes, including water-rock reactions. However, we did not observe a vent source with similar isotopic composition; this suggests that substantial hydrothermal flow is entering the caldera through unidentified sources, potentially individually small but collectively important. Further, the low δ18O and δ2H values of the crater floor vents cannot be readily explained by isotopic modification processes alone. We conclude the crater floor vents predominantly reflect the isotopic composition of the Pacific seawater entrained into the hydrothermal system. Our observations suggest recharging seawater is entrained from below >4500 m, in the zone of Pacific Bottom Water. These findings illustrate the heterogeneity of hydrothermal transport processes at a volcanic seamount and how seamount hydrothermal convection can provide a mechanism that may contribute to vertical ocean mixing by transporting deep bottom waters to intermediate ocean depths.

Ancient hydrocarbon slicks recorded by a coral atoll in the South China Sea

The intensifying oil slicks exert deleterious effects on coral reefs, hence attracting many investigations on coral's tolerance and resilience to anthropogenic oil pollution. We hypothesize that intense seafloor hydrocarbon seepages in the past could also produce oil slicks and subsequently contaminate coral reefs nearby, which may provide clues on their responses to natural oil contamination. Here, a long coral reef core (NK1) drilled in the isolated Meiji Atoll in the southern South China Sea is analyzed for aliphatic hydrocarbons to test this hypothesis. Two depth intervals, centered at 45–65 m and 95–110 m, corresponding to ages of ca. 0.3–0.6 Ma and 1.3–1.5 Ma, respectively, are characterized by geological-configured hydrocarbons indicative of ancient natural oil pollution. Because the Meiji Atoll stands on a volcanic seamount and in isolation in the sea, we deem that natural oils were incorporated into contemporary coral reefs by surface oil slicks, which were derived from hydrocarbon seepages at the seafloor of the adjacent petroliferous basins. The natural hydrocarbon seepages should result from regional tectonic movements that occurred extensively in the middle and late Pleistocene, likely in association with the arc-continent collision in the Taiwan region since 2 Ma. Albeit the two significant contamination events, it appears that they exerted insignificant disturbances on the growth and development of coral reefs, perhaps protected by certain holobiotic oil-degrading bacteria as indicated by excess production of bacterial branched fatty acids during the events. Our study thus suggests a high resilience of coral reefs to oil pollution under natural conditions, which, of course, does not mean that they would survive readily from the abrupt and intense anthropogenic pollution.

Разнообразие рельефа подводных вулканических гор Северо-Восточной Атлантики

The structure of the volcanic seamounts of the Atlantis-Great Meteor group, as well as areas of the archipelagos of the Cape Verde Islands, the Canary Islands and the Azores was analyzed. Their simplest type is a cone-like structure with one peak (Mt. Maio). A more complex type is represented by two closely spaced volcanic edifices (Mt. Nola). Seamounts were also formed as a result of the activity of a central-type volcano and a fissure system (Mt. Te-Paps). A more complex morphology is represented by mountains that were formed as a result of the activity of an underwater volcano of the central type, which was combined with the activity of 3–4 radial fissure volcanic systems (Mt.Tropic). Seamounts were also formed only during fissure eruptions (Mts. Kondor, Kadamosto). They are ridges, tens of kilometers long without a clearly defined peak. All of the above types of mountains, with their rise above sea level, subsequent abrasion and subsidence, lead to the creation of flat-topped structures (guyots) (Mt. Sengor).

Petrogenetic insights on tephritic magmatism from Davis Bank (South Atlantic Ocean - Vitória-Trindade Ridge (VTR), Brazil): The role of a CO2-enriched mantle source

Davis Bank is one of the largest physiographic features in the Vitória-Trindade Ridge (VTR), which is located offshore of the Eastern Brazilian margin, midway between the coastline and the South Atlantic mid-ocean ridge. The VTR is related to a linear volcanic track of ca. 1200 km in length, composed of several volcanic seamounts that are lithologically characterized by nephelinitic/tephritic alkaline rocks. The Davis Bank lies between the parallels 20°29′ and 20°32′ S and the meridians 29°17′ and 29°21′ W, and is located southeast of the Abrolhos Volcanic Complex (AVC), at the middle of the volcanic ridge, far from the Brazilian coast. This work focuses on the 40Ar/39Ar age determination of the rocks recovered from the seamount and characterization of Sr–Nd isotopes, mineral chemistry and whole-rock composition, aiming to access information about physical properties of magmas, source composition, magmatic evolution of the volcanic ridge, and the regional tectonic interpretation in the context of other seamounts along the Vitória-Trindade Ridge (VTR). Davis Bank presents 40Ar/39Ar age of ca. 21 Ma and is in an oceanic lithosphere of ca. 100 km thickness, which represents a recharging area in the oceanic lithospheric mantle for CO2-metasomatism enrichment. It is associated with mantle anomalies caused by a hotspot or mantle plume that was influenced by the E-W direction of leaking transform fracture zones in the South Atlantic Ocean. Age constraints indicate decreasing ages eastward of the AVC, favoring the hotspot hypothesis for the generation of magmatic rocks in the Vitória-Trindade Ridge.

Spatiotemporal distribution of seamount volume along the Kyushu-Palau Ridge: Implications for rejuvenated volcanism

• Accurate quantification of seamount volcanism reveals multiphase volcanism along the Kyushu–Palau Ridge. • The proximity of Pre-Oligocene lithospheric weak zones to high concentrations of seamount volume indicate tectonic inheritance facilitated magma migration. • Rejuvenated volcanism related to the propagated back-arc spreading during Oligocene contributed to the development of the Kyushu-Palau Ridge. The Kyushu-Palau Ridge (KPR) is a remnant arc that initially formed with the subduction of the Pacific plate beneath the West Philippine Sea. The KPR split from the proto-Izu-Bonin-Mariana (IBM) arc with the back-arc spreading at ∼ 30 Ma. Even though most of the seamounts have been dated, the factors that drive the volcanic arc evolution along the KPR remain unknown. In this study, we quantify the spatiotemporal distribution of the seamount volume along the KPR, representing a quantitative indicator of erupted volcanism. A spatial filtering method based on the White Top-Hat Transform is used to identify seamounts from the bathymetry. Combining seismic data interpretation and gravity data inversion, we estimate the seamount volume distribution along the KPR. Results show that high concentrations of the seamount volume are in proximity to pre-Oligocene lithospheric weak zones, indicating that tectonic inheritance has facilitated magma migration during the early-stage volcanic arc generation. Subsequently, the younger than 30 Ma age of the majority of the seamounts suggests a late-stage rejuvenated volcanism in the Oligocene. In addition, the spatiotemporal variations of the seamount volume in the northern and central segments are synchronous with the propagated spreading of the Perce Vela and Shikoku basins, further indicating that the rejuvenated volcanism was driven by the propagated back-arc spreading. Rejuvenated volcanism can thus be an important contributive factor to remnant volcanic arc formation.

Relative Timing of Off‐Axis Volcanism From Sediment Thickness Estimates on the 8°20’N Seamount Chain, East Pacific Rise

AbstractVolcanic seamount chains on the flanks of mid‐ocean ridges record variability in magmatic processes associated with mantle melting over several millions of years. However, the relative timing of magmatism on individual seamounts along a chain can be difficult to estimate without in situ sampling and is further hampered by Ar40/Ar39 dating limitations. The 8°20’N seamount chain extends ∼170 km west from the fast‐spreading East Pacific Rise (EPR), north of and parallel to the western Siqueiros fracture zone. Here, we use multibeam bathymetric data to investigate relationships between abyssal hill formation and seamount volcanism, transform fault slip, and tectonic rotation. Near‐bottom compressed high‐intensity radiated pulse, bathymetric, and sidescan sonar data collected with the autonomous underwater vehicle Sentry are used to test the hypothesis that seamount volcanism is age‐progressive along the seamount chain. Although sediment on seamount flanks is likely to be reworked by gravitational mass‐wasting and current activity, bathymetric relief and Sentry vehicle heading analysis suggest that sedimentary accumulations on seamount summits are likely to be relatively pristine. Sediment thickness on the seamounts' summits does not increase linearly with nominal crustal age, as would be predicted if seamounts were constructed proximal to the EPR axis and then aged as the lithosphere cooled and subsided away from the ridge. The thickest sediments are found at the center of the chain, implying the most ancient volcanism there, rather than on seamounts furthest from the EPR. The nonlinear sediment thickness along the 8°20’N seamounts suggests that volcanism can persist off‐axis for several million years.

Discovery of Late Mesozoic volcanic seamounts at the ocean-continent transition zone in the Northeastern margin of South China Sea and its tectonic implication

The Northern South China Sea (SCS) margin experienced the switch from Andean-type active subduction to passive extension during late Mesozoic era. However, the mechanism of cessation of paleo-Pacific subduction in the SCS region is unclear, and the location of suture zone between paleo-Pacific plate and South China block is obscure. In this study, we report new 40Ar/39Ar ages and whole-rock Hf isotopes of basalts from two poorly studied volcanic seamounts (namely, Puyuan and Beipo) at the ocean-continent transition zone in the Northeastern SCS margin, with an emphasis on the existence of exotic, buoyant oceanic plateau and its geodynamic effect on subduction cessation. Results indicate that the OIB-type Puyuan and E-MORB-type Beipo volcanic seamounts were erupted at 154.1 Ma and 93.2 Ma, respectively. Whole-rock trace element, Sr-Nd-Pb-Hf isotopic compositions, and subduction polarity point out that these samples were oceanic intraplate basalts formed within the paleo-Pacific plate, independent of the coeval, Late Mesozoic paleo-Pacific subduction system in the region. The ages and geochemical data of the two volcanic seamounts, combined with geochemical data of Late Mesozoic Pacific oceanic plateaus and seismic data of the Northeastern SCS margin, show that there may exist a Late Mesozoic exotic, buoyant oceanic plateau in the region. The collision between the oceanic plateau and the South China block could lead to the cessation of paleo-Pacific plate subduction, and changes in tectonic regime during the Late Cretaceous era (ca. 100–65 Ma). The pre-existing, Late Mesozoic suture zone has been well constrained by the remnant oceanic plateau and continental arc, and may play a key role in the formation of Cenozoic rifting tectonic units in the region.

The Marsili Seamount Offshore Geothermal Reservoir: A Big Challenge for an Energy Transition Model

Renewable energies have been the only sources recording a clear increase in total installed capacity, setting a record in new power capacity in 2020, despite the pandemic. The European Union Green Deal represents a strategy towards a sustainable economic model. In this framework, land-based geothermics has seen very limited development; however, offshore geothermics is almost completely absent in the discussion on energy source alternatives, even though it represents a real challenge for energy transition, including the production of green hydrogen. This article discusses an excursus on the activities carried out on offshore geothermal areas worldwide. We focused on the energy potential capacity of the Marsili volcanic seamount located over the bathial plain of the Tyrrhenian Basin, describing the detailed geological, geochemical, and geophysical investigations that have been carried out on that seamount since the 2000s. All the collected data have shown evidence supporting the existence of an exploitable geothermal system in the Marsili seamount consisting of a reservoir of supercritical geothermal fluids of about 100 km3. We discuss and evaluate the actual consistence of the impacts associated with the occurrence of potential risks. We also describe the necessary further steps towards the pilot well. An important breakthrough in the short-medium term that allows for an exit from the predominance of fossil sources may come from the development of energy production derived from offshore high-enthalpy geothermal fields, especially in areas such as the Southern Tyrrhenian Sea. There is a natural clear predisposition for its exploitation combined with a low ecological footprint, which is the target objective of international agreements in the context of a blue economy strategy.

Giant sponge grounds of Central Arctic seamounts are associated with extinct seep life

The Central Arctic Ocean is one of the most oligotrophic oceans on Earth because of its sea-ice cover and short productive season. Nonetheless, across the peaks of extinct volcanic seamounts of the Langseth Ridge (87°N, 61°E), we observe a surprisingly dense benthic biomass. Bacteriosponges are the most abundant fauna within this community, with a mass of 460 g C m−2 and an estimated carbon demand of around 110 g C m−2 yr−1, despite export fluxes from regional primary productivity only sufficient to provide <1% of this required carbon. Observed sponge distribution, bulk and compound-specific isotope data of fatty acids suggest that the sponge microbiome taps into refractory dissolved and particulate organic matter, including remnants of an extinct seep community. The metabolic profile of bacteriosponge fatty acids and expressed genes indicate that autotrophic symbionts contribute significantly to carbon assimilation. We suggest that this hotspot ecosystem is unique to the Central Arctic and associated with extinct seep biota, once fueled by degassing of the volcanic mounts.

Sedimentology and biostratigraphy of the upper Triassic carbonates from the Shiriya cape, North Kitakami Belt, Japan

AbstractRelics of the Panthalassa Ocean occur today in accretionary complexes and terranes on the entire Circum‐Pacific region. Among them, remains of shallow‐water carbonate systems provide valuable information about the ecology and environmental conditions that prevailed in this immense ocean. In this regard, the Shiriya Cape of the North Kitakami Belt (Aomori Prefecture, Honshu Island, Japan) is an essential place to study. Characterized by the presence of numerous limestone deposits embedded in typical accretionary mélange, including pluri‐kilometric massive slabs and metric bedded deposits, the cape is indeed a key vestige of carbonate deposition in the Panthalassa. All the limestone outcrop form the Shiriya Cape were sampled and studied in detail for the first time. Despite a general poor preservation, macroscopic sedimentary features such as burrows and large megalodontids patches as well as four microfacies were identified and are presented in detail in this work. On the basis of foraminifers and conodonts biostratigraphy, the age of the limestone deposits is defined as Norian. Similarities with analogous and synchronous systems from Panthalassan and Tethyan realms are extensively discussed. The closest analog to the Shiriya limestone appears to be the Panthalassan Dalnegorsk limestone (Russian Far East), both regarding microfacies and depositional setting. Sedimentary successions similar to Lofer‐cycles, well known in Tethyan synchronous systems, were also discovered in the Shiriya Cape. Along with the large limestone slabs, countless limestone‐bearing conglomerates also crop out on the Shiriya Cape. Their study and comparison with similar deposits in Japan highlighted their importance in the understanding of the dismantling of mid‐oceanic carbonates. The observations and comparison with well‐known carbonate systems permitted to establish a hypothetical depositional model of the Shiriya limestone corresponding to a Norian isolated carbonate system developed on an immerged volcanic seamount.

Separation of Gagua Rise from Great Benham Rise in the West Philippine Basin during the Middle Eocene

The West Philippine Basin (WPB) has started opening at ~ 58 Ma and ceased spreading at ~ 33 Ma, developing a fast spreading (~ 44 mm/yr half-spreading rate) magmatic episode between 58 and 41 Ma and the second amagmatic episode between 41 and 33 Ma. The occurrence of the first stage of spreading is closely related to the Oki-Daito mantle plume and related Benham Rise (BR) and Urdaneta Plateau (UP) activity. To the east of the Luzon–Okinawa Fracture Zone (LOFZ), BR was the most active volcanism from 48 to 41 Ma. The geomagnetic ages on both sides of the LOFZ have been determined; however, their causal relationship and evolution in the WPB remain unclear. In this study, we performed integrated analyses of multichannel seismic data and swath bathymetry data for the area to the west of the LOFZ. To the west of the LOFZ, the Gagua Rise (GR), is identified by a high residual free-air gravity anomaly, volcanic seamount chains and an overlapping spreading center. The GR is located at magnetic isochrons C20/C22 (50 to 44 Ma) and shows a thick oceanic crust of at least 12.7 km. We first propose an oceanic plateau named Great Benham Rise (GBR) which includes GR, UP and BR. We infer that the GR was a portion of the GBR since ~ 49 Ma and was separated from the GBR at ~ 41 Ma by the right-lateral LOFZ motion. Later, the relict GBR magmatism only continued in the area to the east of the LOFZ. Overall, the GBR dominates the spreading history of the WPB.

Active tectonics and volcanism in the southernmost Okinawa Trough back-arc basin derived from deep-towed sonar surveys

The southernmost Okinawa Trough back-arc basin is an active and young basin formed just after the collision of the Philippine Sea Plate against the Eurasian continental margin. The back-arc extension occurs intensively because of the southward or southeastward migration of the southernmost Ryukyu Arc, or the roll-back of the Philippine Sea Plate. To better understand the active tectonics and volcanism of the southernmost Okinawa Trough, we have conducted deep-tow sub-bottom profiler and side-scan sonar surveys across the back-arc basin. Our results show that the volcanism of the southernmost Okinawa Trough is distributed in the southern half of the back-arc basin and occurs along some linear or branched zones roughly parallel to the trough axis. Volcanic seamounts are obviously located along the central depression of the basin and their sizes show lateral variation. On the other hand, the northern half of the southernmost Okinawa Trough back-arc basin has almost no volcanic activity, but contains more brittle normal faults. It is noted that gas plumes out of seafloor are generally associated with hydrothermal mounds or activities, instead of volcanic seamounts. We suggest that the more complete rifting of the southernmost Okinawa Trough back-arc is limited to the east of ~122o30′E. To the west of ~122o30’E, the back-arc extension could be still influenced by the inherited NE-SW trending structures of the continental crust created during the former Taiwan orogeny in this area.