Volcanic Seamounts Research Articles

Introduction to the special issue on SFB 574 “Volatiles and fluids in subduction zones: climate feedback and trigger mechanisms for natural disasters”

combine field work on land with marine cruises probing the seafloor. During the course of the SFB 574, many different aspects of subduction zone processes were investigated. Geophysical investigations identified and quantified the input of water through hydration of the bend-faulted subducting plate as far as mantle depths. This process and water release from the subducting slab deeper in the subduction zone was investigated by numerical modeling. These studies highlighted the important role of hydration and dehydration of mantle rocks for the global water cycle. Subducted oceanic fracture zones are another conduit for water transported deep into the subduction zone. Compactional and thermal dehydration of the subducted sediment layer affects the strength of interplate coupling and the depth and lateral extension of the seismogenic zone where disastrous earthquakes are generated. The fluids, largely generated by clay mineral transformation, are expelled through the forearc by splay faults. At the seafloor, the sites of cold seeps, often associated with mud volcanoes, are populated by biota that control the carbon transfer to the ocean. The most prominent manifestations of cold seeps are authigenic carbonates that form from anaerobic oxidation of methane and which serve as archives of forearc processes. The largest cold seep emissions occur at faults generated by the subduction of volcanic seamounts. Petrological analyses of ancient, exhumed subduction zone metamorphic rocks revealed the important role of pervasive, typically channelized fluids in high-pressure metamorphic reactions of dewatering subducted igneous crust and mantle. Slab-derived fluids hydrate the mantle wedge along the slab–wedge interface and form a subduction channel in which mixing of different fluids and fluid–rock interaction causes metasomatic overprinting. The element redistributions associated with the liberation of fluids from Sonderforschungsbereiche (SFBs) are a successful funding model in use by the German Science Foundation (DFG) for over 30 years to strengthen basic research first locally at universities and later also supra-regionally by including academic institutions at different cities and states. Literally translated, SFB means “special research area” that comprises research that complements but does not duplicate research at participating institutions and departments. The English terminology used by the DFG is “Collaborative Research Centre,” which better describes the expected approach by emphasizing collaboration and interdisciplinary efforts in such a way that the overall result is better than the sum of individual results. The SFB 574 had united more than 70 scientists with expertise in structural geology, geophysics, sedimentology, geochemistry, empirical and experimental petrology, volcanology, and biology for 11 years (2001–2012). The overarching theme addressed the role of volatiles in subduction zone tectonic, hydrological, metamorphic and magmatic processes, and resulting hazards. The main areas of research were the subduction zones of Central America and southern Chile. Both extend across shorelines from deepsea trenches to arc-volcano summits and thus required to

Constraining the age and origin of the seamount province in the Northeast Indian Ocean using geophysical techniques

The breakup of western margin of Australia from Greater India started around 155 Ma and progressed southwards. After the separation, the interceding intraplate region experienced large volumes of submarine volcanism, extending over 100 Myrs. The Christmas Island Seamount Province (CHRISP, as it has been dubbed) lies south of the Java-Sunda Trench, and contains numerous submerged volcanic seamounts, and two sub-aerially exposed island groups—Cocos (Keeling) Islands, and Christmas Island. While recent geochronological investigations have shed light on the diverse eruption ages of the volcanics of this region, some islands/seamounts have demonstrated protracted volcanic histories, and it is not clear how the volcanic loading, tectonic subsidence, and subsequent emergence history of the islands relates to these discrete volcanic episodes. This study utilises a number of geophysical techniques to determine the crustal structure, loading and subsidence history, and last sub-aerial exposure age for the CHRISP. The study shows that flexural and subsidence modelling are reliable techniques in constraining the age of the seamounts when geochronological techniques are not possible. Utilising regional gravity signatures, we model the crustal structure underneath the Cocos (Keeling) Island, and constrain the thickness of the limestone cover between 900 and 2,100 m. Using age-depth subsidence curves for oceanic lithosphere the time since these seamounts were exposed above sea-level was determined, and a trend in exposure ages that youngs towards the west is observed. Two episodes of volcanism have been recorded at Christmas Island and they are of different origin. The younger phase in the Pliocene is a manifestation of flexure induced cracks produced in the lithosphere as it rides the subduction fore-bulge, whereas a low velocity seismic zone rising from the lower mantle, and tectonic reorganization, may be associated with the older Eocene volcanic phase, as well as much of the rest of the province. Our modelling also supports the existence of an older, undated volcanic core to Christmas Island, based on the loading ages from flexural modelling.

Geology of Martin Vaz Island, South Atlantic, Brazil

Martin Vaz Island is one of the oceanic islands of the homonym archipelago linked to the Vitoria-Trindade volcanic seamount ridge, an important South Atlantic physiographic feature. It is located at 20°28′28″S latitude and 28°51′05″W longitude, approximately 1200 km away from the Brazilian coast. Its geology is an important preserved indicator of Pleistocene alkaline volcanic activities.This work presents lithologic mapping and microscopic observations of Martin Vaz Island rocks. The geological map was developed and prepared at the 1:5000 scale using Esri ArcGIS. Some geological features and structures can be highlighted: two necks, several phonolite dykes and sub-horizontal layers formed by phonolitic pyroclastic rocks and basanites.

A volcanic province near the western termination of the Charlie-Gibbs Fracture Zone at the rifted margin, offshore northeast Newfoundland

A mid-Cretaceous to Late Cretaceous volcanic province, named here the Charlie-Gibbs Volcanic Province, is described near the western termination of the Charlie-Gibbs Fracture Zone, against the rifted continental margin northeast of Newfoundland. We used seismic data to map 14 volcanic seamounts, now buried below younger sediments. They rise 0.7 to 2 s two-way time (twt) above the surrounding basement level and are about 8–30 km wide. Some are conical while others are more flat-topped. Underlying igneous units resembling flows and sills are also observed. Based on magnetic modeling of the large positive magnetic anomalies associated with the seamounts, the total thickness of igneous rocks can locally reach about 8 km. This magmatism occurred in the vicinity of the Charlie-Gibbs Fracture Zone and extends about 150 km to the north along the rifted continental margin. The volcanic province also forms the northern boundary of the Jurassic-Early Cretaceous Orphan Basin, along a major transform margin there. Truncation of rift-related structures which extend to deep crustal levels is observed at the transform, along trends similar to those of prerift Appalachian terrane boundaries on the adjacent shelf. This suggests the existence of a preexisting weak zone in the continental lithosphere within which a complex strike-slip fault system developed and may have controlled the location of final continental breakup between the Rockall and North American plates in the Late Cretaceous.

Electrical conductivity during incipient melting in the oceanic low-velocity zone.

A low viscosity layer in the upper mantle, the Asthenosphere, is a requirement for plate tectonics1. The seismic low velocities and the high electrical conductivities of the Asthenosphere are attributed either to sub-solidus water-related defects in olivine minerals2-4 or to a few volume percents of partial melt5-8 but these two interpretations have shortcomings: (1) The amount of H2O stored in olivine is not expected to be higher than 50 ppm due to partitioning with other mantle phases9, including pargasite amphibole at moderate temperatures10, and partial melting at high temperatures9; (2) elevated melt volume fractions are impeded by the too cold temperatures prevailing in the Asthenosphere and by the high melt mobility that can lead to gravitational segregation11,12. Here we determined the electrical conductivity of CO2-H2O-rich melts, typically produced at the onset of mantle melting. Electrical conductivity modestly increases with moderate amounts of H2O and CO2 but it dramatically increases as CO2 content exceeds 6 wt% in the melt. Incipient melts, long-expected to prevail in the asthenosphere10,13-15, can therefore trigger its high electrical conductivities. Considering depleted and enriched mantle abundances in H2O and CO2 and their effect on the petrology of incipient melting, we calculated conductivity profiles across the Asthenosphere for various plate ages. Several electrical discontinuities are predicted and match geophysical observations in a consistent petrological and geochemical framework. In moderately aged plates (>5Ma), incipient melts most likely trigger both the seismic low velocities and the high electrical conductivities in the upper part of the asthenosphere, whereas for young plates4, where seamount volcanism occurs6, higher degree of melting is expected.

New insights on volcanic and tectonic structures of the southern Tyrrhenian (Italy) from marine and land seismic data

We present results from the first crustal seismic tomography for the southern Tyrrhenian area, which includes ocean bottom seismometer (OBS) data and a bathymetry correction. This area comprises Mt. Etna, the Aeolian Islands, and many volcanic seamounts, including the Marsili Seamount. The seismicity distribution in the area depends on the complex interaction between tectonics and volcanism. The 3‐D velocity model presented in this study is obtained by the inversion of P wave arrival times from crustal earthquakes. We integrate travel time data recorded by an OBS network (Tyrrhenian Deep Sea Experiment), the SN‐1 seafloor observatory, and the land network. Our model shows a high correlation between the P wave anomaly distribution and seismic and volcanic structures. Two main low‐velocity anomalies underlie the central Aeolian Islands and Mt. Etna. The two volumes, which are related to the well‐known active volcanism, are separated and located at different depths. This finding, in agreement with structural, petrography, and GPS data from literature, confirms the independence of the two systems. The strongest negative anomaly is found below Mt. Etna at the base of the crust, and we associate it with the deep feeding system of the volcano. We infer that most of the seismicity is generated in brittle rock volumes that are affected by the action of hot fluids under high pressure due to the active volcanism in the area. Lateral changes of velocity are related to a transition from the western to the central Aeolian Islands and to the passage from continental crust to the Tyrrhenian oceanic uppermost mantle.

A low‐relief shield volcano origin for the South Kaua‘i Swell

The South Kaua‘i Swell (SKS) is a 110 km x 80 km ovoid bathymetric feature that stands >2 km high and abuts the southern flank of the island of Kaua‘i. The origin of the SKS was investigated using multibeam bathymetry and acoustic backscatter, gravity data, radiometric ages, and geochemistry of rock samples. Most of the SKS rock samples are tholeiitic in composition with ages of 3.9–5.4 Ma indicating they were derived from shield volcanism. The ages and compositions of the SKS rocks partially overlap with those of the nearby Ni‘ihau, Kaua‘i and West Ka‘ena volcano complexes. The SKS was originally described as a landslide; however, this interpretation is problematic given the ovoid shape of SKS, its relatively smooth, flat‐to‐convex surface, and the lack of an obvious source region that could accommodate what would be one of Earth's most voluminous (6 x 103 km3) landslides. The morphology, size, and the surrounding gravity anomaly are more consistent with the SKS being a low‐relief shield volcano, which was partially covered with a small volume of landside debris from south Kaua‘i and later with some secondary volcanic seamounts. A shield origin would imply that Hawaiian and possibly other hotspot shield volcanoes can take on a wider variety of forms than is commonly thought, ranging from tall island‐building shields, to smaller edifices such as Ka‘ena Ridge and Mahukona, to even lower‐relief volcanoes represented by the SKS and possibly the South West O‘ahu Volcanic Field.

Arsenian pyrite-bearing altered volcanics dredged SE of Cheshire Seamount, western Woodlark Basin, Papua New Guinea

In many subaerial hydrothermal ore deposits arsenian pyrite is an important host for Au, however, arsenian pyrite is rare on the modern seafloor. During a recent survey for submarine hydrothermal mineralization in the western Woodlark Basin volcanic breccias containing abundant arsenian pyrite were dredged from the flanks of a volcanic seamount in a water depth of 2000 m. This area is particularly interesting because it is located at the transition from continental splitting to oceanic spreading where enhanced heat flow and deep crustal faults may fertilize mineralization. The sulfidic breccia is essentially monomictic and matrix-supported containing altered dacitic clasts. Mineralogical investigation of the breccia reveals silicification and sulfidation as the main alteration types. Quartz occurs in fragments and also constitutes the breccia matrix attesting to silicification as a significant alteration process. Pyrite is the dominant ore mineral with only minor amounts of Fe-oxyhydroxide and goethite. Bulk geochemistry shows a slight enrichment of Au (0.12 ppm) in association with elements such as As-Ag-Hg-Zn-Pb-Sb, key elements indicative of a low sulfidation environment. Three generations of pyrite are recognized on the basis of morphology. Arsenic-free, early framboidal pyrite (py1) is overgrown by arsenian colloform (py2) or massive pyrite (py3) containing up to 3.93 wt% As. Arsenic speciation in the pyrite is in the form of As1- and As3+. The presence of arsenian pyrite in hydrothermal breccias at this seamount indicates the potential for Au mineralization in the area.

A Precambrian microcontinent in the Indian Ocean

Ridges of thick, raised crust on the Indian Ocean floor were thought to be mostly volcanic seamounts formed above the Reunion mantle plume. Dating of zircon minerals in Mauritian lavas, however, indicates that fragments of an ancient microcontinent may be preserved beneath the seamounts, contributing to the thickened crust.

Crustal manifestations of a hot transient pulse at 60°N beneath the Mid-Atlantic Ridge

Since its inception at 62Ma, mantle convective upwelling beneath Iceland has had a significant influence on Cenozoic vertical motions, magmatism and paleoceanography in the North Atlantic Ocean. Crucially, intersection of the Reykjanes Ridge with the Icelandic Plume provides us with a useful window into the transient activity of this plume. Here, the spreading ridge acts as a linear sampler of plume activity, which is recorded as a series of time-transgressive V-shaped ridges and troughs. We present the results of a detailed study of the spreading ridge close to 60°N, where the youngest V-shaped ridge of thickened oceanic crust is forming today. A combination of multibeam bathymetry and seismic reflection profiles, acquired along and across the ridge axis, is used to map the detailed pattern of volcanism and normal faulting. Along the ridge axis, the density of volcanic seamounts varies markedly, increasing by a factor of two between 59°N and 62°N. Within this zone, seismic imaging shows that there is enhanced acoustic scattering at the seabed. These observations are accompanied by a decrease in mean fault length from ∼12km to ∼6km. A 1960–2009 catalog of relocated teleseismic earthquake hypocenters indicates that there is a pronounced gap in seismicity between 59°N and 62°N where the cumulative moment release is two orders of magnitude smaller than that along adjacent ridge segments. A steady-state thermal model is used to show that a combination of increased melt generation and decreased hydrothermal circulation accounts for this suite of observations. The predicted decrease in the thickness of the brittle seismogenic layer is consistent with geochemical modeling of dredged basaltic samples, which require hotter asthenospheric material beneath the spreading axis. Thus, along-axis variation in melt supply caused by passage of a pulse of hot material modulates crustal accretion processes and rheological properties.

The International Court of Justice and the Territorial Dispute between Indonesia and Malaysia in the Sulawesi Sea

In the early 1990s, a long-running and bitter dispute over the ownership of two small islands in the Sulawesi Sea, and to the waters and seabed near those islands, was placing ever greater strain on the relationship between Indonesia and Malaysia. In October 1996, just when it appeared that there was no way of resolving the dispute, the governments of Malaysia and Indonesia suddenly agreed to refer the question of ownership of the islands to the International Court of Justice (ICJ). By this time the ICJ had ruled on many disputed sovereignty cases since its establishment under the United Nations (UN) Charter in 1945. The remarkable aspect of this case was that the ICJ was very much a new actor as far as maritime security governance in Southeast Asia was concerned. (1) In fact, the only time Southeast Asian countries had submitted a dispute of any kind to the ICJ was in 1959, when Thailand and Cambodia referred their competing claims to ownership of the Temple of Preah Vihear. As a general rule, states are reluctant to hand over the settlement of disputes to an outside body, especially when--as is the case when states refer disputes to the ICJ--they must agree to be bound by the ruling of that body no matter what the outcome. Southeast Asian states, however, have been particularly protective of their sovereignty, especially when resolving territorial disputes. All Southeast Asian states experienced colonial rule or interference, all had their borders shaped to some extent--entirely in the case of Malaysia and Indonesia--by the colonizing powers, and two among them--Indonesia and Vietnam--underwent wars of independence. The ICJ's decision in the Preah Vihear case had done little to diminish the reluctance of states in this region to refer their disputes to an outside body: when the court ruled in favour of Cambodia in 1962, the Thai government's first reaction was to declare that the temple would be defended to the last drop of blood. (2) What, then, made the Indonesian and Malaysian governments refer the question of ownership of the two islands to the ICJ? Did the ICJ's ruling resolve the dispute between the two countries? Have the two governments become, as a result of their experience in this case, more willing to have their disputes settled by the ICJ or by any other outside body? Before trying to answer these questions we must examine how the dispute arose in the first place. The Origins of the Dispute Sipadan and Ligitan, the two islands at the centre of the dispute, are mere specks in the Sulawesi Sea. Sitting atop a steep extinct volcanic seamount and occupying 12 hectares, (3) Sipadan lies about 14 miles south of mainland Sabah and 42 miles east of the island of Sebatik, which is divided between Malaysia and Indonesia by the 4[degrees] 10' North parallel as agreed by the United Kingdom and the Netherlands in a treaty signed in 1891. Ligitan, which is even smaller, lies 12 miles to the east of Sipadan and is part of an extensive system of coral reefs. At the time the dispute arose neither island was inhabited. Sipadan was regularly visited by collectors of turtle eggs but otherwise neither island was valued its natural However, islands can have an importance that transcends their intrinsic value. Despite their size they can be seen as worth fighting when they are regarded as an indivisible part of a state's territory. When, as is often the case, they are located on the fringes of a state's land possessions, they can also serve as the starting point delimiting the state's maritime boundaries. It was because this second feature applied to both islands that the dispute between Indonesia and Malaysia first arose. In 1969 the Malaysian and Indonesian governments began negotiations on the continental shelf boundary between their two countries. There was a great deal to negotiate, both because the two countries shared extensive continental shelves in the Straits of Malacca, South China and the Sulawesi Sea, and because the 1958 UN Convention on the Continental Shelf, which both countries had signed and ratified, gave coastal states sovereign rights over their continental shelves for the purpose of exploring it and exploiting its natural resources. …

Estimating the age of volcanism in Seamount Provinces of the Northeast Indian Ocean

The breakup of Australian landmass from Greater India, part of the late dispersal of Gondwana, started around 136 Ma. Concurrent with this breakup was the eruption of significant volumes of volcanics, on both the continental margins of Australia and India, including the North-West shelf, and in the ocean basin separating the two. Later submarine volcanism occurred within the Christmas Island Seamount Province (ChrISP), or which Christmas and Cocos (Keeling) Islands are two subareal examples. These volcanic events significantly impacted the thermal evolution of these margins, but the ultimate cause for this disparate volcanism, and the relationship between margin volcanics and later submarine volcanic events, remains unclear. The study tries to establish a relation between the volcanic activities and look for any evidence of these volcanic episodes. The study uses gravity, magnetic, and subsidence modelling to attempt to constrain the structure and ages of seamount volcanism within the Christmas Island Seamount Province, including the Wharton Basin and Argo Abyssal Plain. Gravity modelling helps to determine the crustal structure and constrains the depth of limestone cap, which further helps in calculating when the seamount was sub aerially exposed and approximates the time since it was last exposure. The relationship between the volcanism observed in the ChrISP (Late Cretaceous to Eocene) and that recorded earlier on the NW shelf continental margin, remains ambiguous, and will be better constrained with forthcoming geochemical analysis. But the results presented here point to a rich and complex history of volcanism within the Indo-Australian plate.

Limited latitudinal mantle plume motion for the Louisville hotspot

Hotspots that form above upwelling plumes of hot material from the deep mantle typically leave narrow trails of volcanic seamounts as a tectonic plate moves over their location. These seamount trails are excellent recorders of Earth’s deep processes and allow us to untangle ancient mantle plume motions. During ascent it is likely that mantle plumes are pushed away from their vertical upwelling trajectories by mantle convection forces. It has been proposed that a large-scale lateral displacement, termed the mantle wind, existed in the Pacific between about 80 and 50 million years ago, and shifted the Hawaiian mantle plume southwards by about 15° of latitude. Here we use 40Ar/39Ar age dating and palaeomagnetic inclination data from four seamounts associated with the Louisville hotspot in the South Pacific Ocean to show that this hotspot has been relatively stable in terms of its location. Specifically, the Louisville hotspot—the southern hemisphere counterpart of Hawai’i—has remained within 3–5° of its present-day latitude of about 51° S between 70 and 50 million years ago. Although we cannot exclude a more significant southward motion before that time, we suggest that the Louisville and Hawaiian hotspots are moving independently, and not as part of a large-scale mantle wind in the Pacific.

Hot, cracking rocks deep down

Most volcanism on Earth takes place under water, yet little is known about submarine eruptions. Monitoring of two volcanic seamounts beneath the Pacific Ocean reveals the pulsed nature of their eruption cycles.

Magnetic recording of the Cenozoic oceanic crustal accretion and evolution of the South China Sea basin

We review and discuss some of the recent scientific findings made on magnetic data in the South China Sea (SCS). Magnetic anomalies bear extremely rich information on Mesozoic and Cenozoic tectonic evolution. 3D analytical signal amplitudes computed from magnetic anomalies reveal very precisely relict distributions of Mesozoic sedimentary sequences on the two conjugate continental margins, and they are also found very effective in depicting later-stage magmatism and tectonic transitions and zonation within the SCS oceanic crust. Through integrated analyses of magnetic, gravity and reflection seismic data, we define the continent-ocean boundary (COB) around the South China Sea continental margin, and find that the COB coincides very well with a transition zone from mostly positive to negative free-air gravity anomalies. This accurate outlining of the COB is critical for better tracing magnetic anomalies induced by the oceanic crust. The geometrically complex COB and inner magnetic zonation require the introduction of an episodic opening model, as well as a transform fault (here coined as Zhongnan Fault) between the East and Southwest Sub-basins, while within the East and Southwest Sub-basins, magnetic anomalies are rather continuous laterally, indicating nonexistence of large transform faults within these sub-basins. We enhance magnetic anomalies caused by the shallow basaltic layer via a band-pass filter, and recognize that the likely oldest magnetic anomaly near the northern continental margin is C12 according to the magnetic time scale CK95. Near the southern continental margin, magnetic anomalies are less recognizable and the anomaly C12 appears to be missing. These differences show an asymmetrical opening style with respect to the relict spreading center, and the northern part appears to have slightly faster spreading rates than to the south. The magnetic anomalies C8 (M1 and M2, ∼26 Ma) represent important magnetic boundaries within the oceanic basin, and are possibly related to changes in spreading rates and magmatic intensities. The magnetic evidence for a previously proposed ridge jump after the anomaly C7 is not clear. The age of the Southwest Sub-basin has yet to be further examined, most favorably with deep-tow magnetic surveys and ocean drilling. Our magnetic spectral study shows that the shallowest Curie points are located around the eastern part of the Southwestern Sub-basin, whereas within the East Sub-basin Curie depths are smaller to the north of the relict spreading center than to the south. This pattern of Curie depths is consistent to regional heat flow measurements and later-stage volcanic seamount distributions, and we therefore reason that Curie-depth variations are closely associated with later-stage magmatism, rather than with crustal ages. Although magnetic anomalies located around the northern continent-ocean transition zone (COT) are relatively quiet, this area is not a typical magnetic quiet zone since conceptually it differs markedly from an oceanic magnetic quiet zone. The relatively quiet magnetic anomalies are seemingly associated with a shallowing in Curie isotherm and thinning in magnetic layer, but our comprehensive observations suggest that the well-preserved thick Mesozoic sedimentary rocks are major causes for the magnetically quiet zone. The high similarities between various low-pass filtered marine and air-borne magnetic anomalies and satellite magnetic anomalies clearly confirm that deeper magnetic sources (in the lower crust and the uppermost mantle) have contributions to long-wavelength surface magnetic anomalies in the area, as already inferred from magnetically inversed Curie depths. The offshore south China magnetic anomaly (SCMA) becomes more prominent on low-pass filtered marine and air-borne magnetic anomalies and satellite magnetic anomalies, indicating very deeply-buried magnetic sources beneath it.

Age and Geochemical Features of Dredged Basalts from Offshore SW Taiwan: The Coincidence of Intra-Plate Magmatism with the Spreading South China Sea

This study reports age and geochemical analyses of basaltic rocks dredged from volcanic seamounts offshore SW Taiwan. 40Ar/39Ar dating results of these rocks show them to be of the early Miocene age of ~22 -21 Ma. They are evolved alkali basalts that show OIB-type geochemical features similar to post-spreading seamount basalts (14-3.5 Ma) in the South China Sea (SCS) and Miocene intraplate basalts on the Penghu Islands (16-8 Ma) and NW Taiwan (23-9 Ma). Their Sr-Nd-Pb isotope data plot within the range of the SCS seamount basalts that show an EM2-like component in the mantle source. The age and overall geochemical characteristics of the dredged basalts are comparable to those of the Kungkuan basalts, NW Taiwan and Baolai basalts, SW Taiwan, suggesting an extensive alkali basaltic volcanism along the southeastern Eurasian continental margin during the early Miocene that resulted from regional lithospheric extension in association with seafloor spreading in the South China Sea.

Fore-arc mantle wedge seismicity under northeast New Zealand

Relocated upper mantle seismicity, using a 3D velocity model, delineates three tightly clustered (about 12 km diameter) zones of seismicity that occur in the mantle wedge under the fore-arc region of the northern Hikurangi margin, North Island, New Zealand. These clusters extend from the subducting plate at about 55 km depth up to about 35 km, and coincide with strong seismic reflectivity recorded active source seismic reflection measurements, suggesting a fluids association. They have maximum magnitudes of about Mw 4.5. The clusters correspond closely with a change from low to high Vp and Qp, and with average Vp/Vs that is interpreted to be caused by the transition from serpentinised to normal peridotite. The location of this transition appears to correspond to the locus of dehydration of peridotite, suggesting that the seismicity is caused by dehydration embrittlement. The release of these fluids where the serpentinised mantle wedge reaches the locus of dehydration may provide a mechanism for transferring additional fluids or fluid fluxed partial melt into the adjacent back-arc basin and facilitate the generation of voluminous magmas that are erupted there. The occurrence of serpentinite in the fore-arc mantle under northeastern North Island may influence the shallow extent of interplate coupling under the region. The location and spacing of the clusters is consistent with thermally driven diapiric or fluid upwellings, but they also show a spatial relationship with proposed lithospheric fractures and associated volcanic seamounts on the subducting Hikurangi plateau.

First data on the age of rocks from the central part of the Vitoria-Trindade Ridge (Brazil Basin, South Atlantic)

Micropaleontological and isotope-geochronological investigations of calcareous sedimentary rocks and volcanites dredged out from the central portion of the submarine Vitoria-Trindade Ridge during the 24th cruise of R/V Akademik Vavilov have been conducted. It has been established based on micropaleontological analysis, which included determination of the species composition of foraminifera and nannoplankton, that the sequence of sedimentary rocks having a pelagic nature formed on the slopes of the volcanic seamounts in the central portion of the Vitoria-Trindade Ridge from the Early to Mid-Miocene to the Holocene; a good correlation between the degree of lithification of these rocks and their age is observed. It has also been established that the carbonate platforms on the abraded tops of the Davis Seamount and the Dogaressa Bank, which are located in the east-central portion of the Vitoria-Trindade Ridge, started forming in the Early Miocene (19–24 Ma). It has been determined using local U-Pb dating of zircon grains with a SHRIMP-II high resolution secondary ion mass spectrometer that the volcanites forming the upper portion of the volcanic rock sequence of the Jaseur Seamount (29.8 ± 6.6 Ma) located in the west-central portion of the Vitoria-Trindade Ridge date to the Oligocene. The investigations conducted have confirmed the opinion that the Vitoria-Trindade Ridge formed in general because of the activity of the hot spot located under the volcanic Trindade and Martin Vaz Islands. However, separate extended lenticular segments of this ridge existed for a long time as single structures, within which the age of the seamounts was not linearly dependent on the distance from the location of the hot spot. Lenses of hot mantle matter that form at the sublithospheric level as a result of impulses of plume activity and move along with the lithospheric plate play a defining role in the development of individual segments forming the Vitoria-Trindade Ridge.

Upper Triassic foraminifers from Panthalassan carbonate buildups of Southwestern Japan and their paleobiogeographic implications

Detailed sampling of the Upper Triassic atoll-type carbonates of the Sambosan Accretionary Complex throughout Southwest Japan yielded highly abundant and diversified porcelaneous, microgranular, agglutinated and hyaline foraminifers of Carnian-Rhaetian age, as well as some microproblematica and ostracods. The foraminiferal assemblages were collected from shallow-water carbonates originated upon volcanic seamounts surrounded by deep-water radiolarian chert in a mid-open oceanic realm of the Panthalassan Ocean during Triassic time. Because most studies of the Upper Triassic microfauna come from the former Tethys, counterparts of the Panthalassan Ocean are pivotal to decipher the micropalaeontological biodiversity of the western circum Pacific, as well as to evaluate the distribution patterns of organisms and their evolution trends throughout the Tethys and Panthalassa. This study reports on 42 genera and 60 species whose associations can be used as sedimentary facies indicators of carbonate buildup environments. Japanese specimens show a strong Tethyan affinity, and especially with the Peri- and Southern Tethyan forms. A palaeobiogeographic distribution analysis using a large foraminiferal database is led, in order to evaluate the extraordinary spreading of these Upper Triassic foraminifers between the Neo-Tethys and the Panthalassa. Data are finally integrated in a new plate tectonic model, where six faunistic provinces are defined, each containing a characteristic foraminiferal assemblage. This map provides for the first time a useful and visual synthesis of the Upper Triassic foraminifer palaeobiogeographic distribution.

First record of Hyalascus (Hexactinellida: Rossellidae) from the Indian Ocean, with description of a new species from a volcanic seamount in the Andaman Sea

First record of Hyalascus (Hexactinellida: Rossellidae) from the Indian Ocean, with description of a new species from a volcanic seamount in the Andaman Sea