The Vema Transverse Ridge (Central Atlantic)

Transverse ridges are elongate reliefs running parallel and adjacent to transform/fracture zones offsetting mid-ocean ridges. A major transverse ridge runs adjacent to the Vema transform (Central Atlantic), that offsets the Mid-Atlantic Ridge by 320 km. Multibeam morphobathymetric coverage of the entire Vema Transverse ridge shows it is an elongated (300 km), narrow (<30 km at the base) relief that constitutes a topographic anomaly rising up to 4 km above the predicted thermal contraction level. Morphology and lithology suggest that the Vema Transverse ridge is an uplifted sliver of oceanic lithosphere. Topographic and lithological asymmetry indicate that the transverse ridge was formed by flexure of a lithospheric sliver, uncoupled on its northern side by the transform fault. The transverse ridge can be subdivided in segments bound by topographic discontinuities that are probably fault-controlled, suggesting some differential uplift and/or tilting of the different segments. Two of the segments are capped by shallow water carbonate platforms, that formed about 3–4 m.y. ago, at which time the crust of the transverse ridge was close to sea level. Sampling by submersible and dredging indicates that a relatively undisturbed section of oceanic lithosphere is exposed on the northern slope of the transverse ridge. Preliminary studies of mantle-derived ultramafic rocks from this section suggest temporal variations in mantle composition. An inactive fracture zone scarp (Lema fracture zone) was mapped south of the Vema Transverse ridge. Based on morphology, a fossil RTI was identified about 80 km west of the presently active RTI, suggesting that a ridge jump might have occurred about 2.2 m.a. Most probable causes for the formation of the Vema Transverse ridge are vertical motions of lithospheric slivers due to small changes in the direction of spreading of the plates bordering the Vema Fracture Zone.     

Mantle peridotites from the Bouvet Triple Junction Region, South Atlantic

ABSTRACT The Bouvet Triple Junction (BTJ) region in the South Atlantic, where the African, South American and Antarctica plates meet, is affected by several topographic/melting anomalies. Causes of these anomalies were investigated through a study of mantle-derived serpentinized peridotites sampled from three sites in the BTJ region: (1) the Inner Corner High at the intersection of the America Antarctic Ridge (AAR) with the Conrad transform; (2) the south wall of the Bouvet transform (South West Indian Ridge, SWIR); and (3) the eastern Bouvet SWIR Transform Intersection. The degree of melting undergone by these rocks was estimated from relic mineral major- and trace-element composition. Geochemical profiles from residual peridotites and associated basalts show a > 1000-km-wide melting anomaly centred on the Bouvet and Spiess topographic anomalies.     

Oceanic tectonic islands

The origin of oceanic islands has been the subject of much speculation, starting with Darwin almost two centuries ago. Two classes of oceanic islands can be identified: ‘volcanic islands’, which form due to excess volcanism caused by melting anomalies in the suboceanic mantle, and ‘tectonic islands’, which form due to transpressive and/or transtensional tectonics of blocks of oceanic lithosphere along transform faults. Modern and sunken tectonic islands from the Atlantic Ocean and Indian Ocean and the Caribbean Sea and Red Sea expose mantle and lower‐crust lithologies and display an elongated narrow morphology; in contrast, volcanic islands expose basalts and have near‐circular morphology. Both are often capped by carbonate platforms. The life cycle of tectonic islands tends to be more complex than that of most volcanic islands; their elongated narrow morphology, together with their tectonic instability and high seismicity, affect the architecture of the carbonate platforms capping them, limiting coral reef development and favouring rhodalgal–foramol biota associations.     

Aragonite from deep sea ultramafic rocks

Aragonite mineralization was observed in serpentinized peridotites from the Romanche and Vema Fracture Zones in the Atlantic and the Owen Fracture Zone in the Indian Ocean, either in veins or as radial aggregates in cavities within the serpentinites. Evidence of incipient dissolution of the aragonite crystals was observed in one case. The aragonites tend to have lower Mg content (< 0.03%) and higher Sr content (> 0.95%) relative to other marine aragonites. Their ${}^{18}\text{O}{}^{16}\text{O}$, ${}^{13}\text{C}{}^{12}\text{C}$ and ${}^{87}\text{Sr}{}^{86}\text{Sr}$ isotopic ratios suggest the aragonite was deposited at ocean floor temperatures from solutions derived from sea water circulating in fissures and fractures within the ultramafic rocks. The ${}^{18}\text{O}{}^{16}\text{O}$ ratios of the serpentines indicate serpentinization occurred at higher temperatures, probably deeper in the crust. Low-T reactions between circulating seawater and Mg-silicates (primarily serpentine and pyroxenes) caused high pH and enrichment of Mg and Ca in the solution, conditions favoring carbonate precipitation. Aragonite was formed rather than calcite presumably because the high Mg²⁺ concentration in the solution inhibited calcite precipitation. The high Sr content of the aragonites is probably related, at least in part, to their low temperature of formation. Opaque mineral grains containing over 8% NiO and over 40% MnO were observed concentrated along the margins of some of the aragonite veins, suggesting that Ni is one of the elements mobilized during reactions between ultramafic rocks and circulating seawater.     

Land use dynamics in Brazilian La Plata Basin and anthropogenic climate change

The La Plata Basin (LPB) is one of the most important regions for agriculture and livestock production in South America, playing a central role in the world food production and food security. Within its borders is also located the whole Brazilian Pantanal region. Identifying the most important land use sectors in LPB as well as the changes observed in the past years is fundamental to recognize which areas of the basin might be more vulnerable to climate change in order to design adaptation strategies. A general characterization of land use and livestock production of Brazilian LPB was done by using the System of Automatic Retrieving (SIDRA) of Brazilian Institute of Geography and Statistics (IBGE) platform as the major source of data. It was observed expressive increases in land areas used for temporary crops, such as soybean, sugarcane, and maize, as well as increases in poultry and swine production. These important changes in agricultural land use and livestock production are currently associated to non-climatic drivers, but this dynamic might be strongly affected by the consequences of climate change and variability, with negative socio-economic impacts for the whole region.     

The MAR-Vema Fracture Zone intersection surveyed by deep submersible Nautile

One of the two objectives of the Vemanaute cruise of the French deep submersible Nautile, was the geological study of the eastern intersection area between the Mid-Atlantic Ridge (MAR) and the Vema Fracture Zone in the equatorial Atlantic. Fourteen dives were conducted that allowed detailed geological survey and sampling of the main morphostructural units of this area: the northern and southern walls of the fracture zone, the median ridge, the northern and southern troughs and the nodal basin. In situ observations of recent tectonic features such as furrows, ridges and circular depressions, concentrated within the southern trough, allowed us to establish the location and the size of the present-day displacement zone. Geological investigations have shown that the nodal basin is entirely floored by basalts thus contrasting with other equivalent areas such as the Kane and Oceanographer fracture zone-MAR eastern intersections. Finally, this study stresses the great opposition between the relatively old and tectonically inactive northern part of the fracture, and the southern part which shows active tectonics and recent volcanic activity.     

Plate tectonics at Lamont: the first year-1966

During the period from the mid 1950s to the 1960s it seemed as if the community of earth scientists was slowly being dragged toward accepting continental drift. But resistance was often steadfast. Each new ideal or set of data produced a trickle of new converts, but also generated heated debate. The concept of sea-floor spreading and the corollary hypothesis that the process of spreading would generate a set of symmetrical magnetic anomalies was particularly disputatious. The discovery of the Eltanin 19 magnetic anomaly profile ended the dispute and the trickle became a flood.     

Serpentinite protrusions in the oceanic crust

Serpentinite may be a significant component of the oceanic crust, not as a continuous layer, but as vertical tectonic protrusions and sills emplaced from the upper mantle into fault zones parallel to the axis of spreading ridges. The diapiric emplacement of serpentinite bodies occurs within 100–200 km of ridge axis, with a rate of ascent on the order of 1 mm/year. Serpentinite protrusions may cause small-scale linear magnetic anomalies parallel to ridge axis. Serpentinites are distributed in the oceanic crust according to an orthogonal pattern, with large serpentinite protrusions aligned along major fracture zones, and smaller serpentinite bodies emplaced in bands parallel to ridge axis.     

Neogene crustal emersion and subsidence at the Romanche fracture zone,equatorial Atlantic

Partly phosphatized, oolitic-biogenic limestones were recovered 950–1300 m below sea level from two sites near the crest of a transverse ridge running parallel and adjacent to the Romanche fracture valley (equatorial Atlantic). Some of the biogenic contituents of the limestones (the benthonic foraminiferal genusAmphistegina; corals of the genusStylophora sp., scaphopods, etc.); their paleofacies assemblage (including echinoderms, gastropods, calcareous algae, etc.); and the presence of a well-developed oolitic facies, indicate that the limestones formed in very shallow water close to sea level. In addition, several features of the limestones (including the presence of stromatolite-like laminae, and dissolution features typical of subaerial diagenesis) suggest that the summit of the transverse ridge might have undergone episodes of emersion. The limestones were formed on a shallow carbonate bank at around the Miocene-Pliocene boundary, i.e., 5 ± 1 Myr ago, as determined by the age of fossil planktonic foraminifera and corals. The transverse ridge must have subsided since that time at an average rate of 0.2 mm/yr. It is unlikely that the vertical motions of the Romanche transverse crustal block were caused solely by accreting plate margin- or mantle plume-related volcanic and/or tectonic processes. It is suggested instead that such motions are related to vertical tectonism typical of large oceanic fracture zones.