Displays

The illumination of the seafloor through Marie Tharp’s lens was instrumental in the plate tectonics revolution and fundamentally transformed our understanding of earth processes. Rather than creating traditional contour maps from isolated soundings, her work yielded physiographic diagrams based on sparse echo-sounding profiles that were complemented by stylized views based on her interpretation of the trends and texture of the seafloor. These maps showed the fabric of seafloor in ways that could not have been achieved or communicated with traditional contour plots. Despite the sparseness of the input data, Marie Tharp and Bruce Heezen’s early seafloor maps are remarkably consistent with modern bathymetric maps that are based on orders of magnitude more observations.

An important part of the legacy of Tharp’s work is codified in the evolution of bathymetric data synthesis efforts led by several of her contemporaries and successors at the Lamont-Doherty Earth Observatory (LDEO). After Tharp’s seminal work transforming bathymetric profiles into the first maps of the global seafloor, efforts were undertaken to digitize echo-sounding profiles creating new opportunities for analysis and integration as well as the development of new software tools and approaches for working with those data. In the 1980s, the availability of multibeam sonars in the academic sector ushered in a new era of mapping by extending the data coverage from profiles to swaths that revealed spatially-continuous areas of the seafloor. The Ridge Multibeam Synthesis (RMBS) Project, which began in the 1990s, built upon Tharp’s early work and sought to advance understanding of the global mid-ocean ridge system by integrating swath data from multiple ships and cruises to create detailed bathymetric grids served online via the early web. Just over a decade later, the Global Multi-Resolution Topography (GMRT) Synthesis emerged as the next generation product under the inspiration of William Haxby. GMRT shifted the focus of effort from the mid-ocean ridges to the global ocean, and presented an efficient scalable solution using a tiled approach for providing access to global bathymetric data at native resolution. GMRT continues today and includes a curated collection of bathymetry data from over 1,100 research expeditions covering more than 9% of the global ocean at 100m spatial resolution. This presentation will describe the legacy of Marie Tharp in the context of the continuity of seabed mapping work at LDEO, including the evolution of bathymetry data synthesis and integration projects and how they connect to complementary efforts in the international arena including the Nippon Foundation – GEBCO Seabed 2030 Project.

How to cite: Ferrini, V., Ryan, W., Carbotte, S., and O'Hara, S.: Marie Tharp’s Ongoing Legacy in Global Seabed Mapping Efforts, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11736, https://doi.org/10.5194/egusphere-egu2020-11736, 2020.

The International Bathymetric Chart of the Southern Ocean (IBCSO), part of the Nippon Foundation – GEBCO – Seabed 2030 project, is a collaborative effort to create high-resolution bathymetric compilations off Antarctica. Detailed knowledge of seafloor morphology is fundamental to almost all marine and maritime scientific activities. For example, it can be used to understand past glacial development, to create habitat models and maps, and to identify ocean current pathways that may contribute to increased basal melt of the Antarctic ice sheets. In comparison to IBCSO V1.0, which extended to 60° south, the new version now extends up to 50° south increasing the ocean area by a factor of approximately 2.5. With this extension, the new bathymetric model will include important submarine features like the Drake Passage, the South Sandwich Arc, and the southern parts of the Kerguelen Plateau and Campbell Plateau. IBCSO continues to build on the on the largest database of bathymetric soundings for the Southern Ocean that was gathered by a variety of international institutions. We will present the new IBCSO V2.0 data set for the first time and will highlight its improvement in comparison to its predecessor.

How to cite: Hehemann, L., Arndt, J. E., and Dorschel, B.: IBCSO V2.0: An updated Antarctic bathymetry product of Seabed 2030, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3529, https://doi.org/10.5194/egusphere-egu2020-3529, 2020.

Marie Tharp (1920-2006) was a pioneer of modern oceanography. She was an American geologist and oceanographic cartographer who, together with his husband Bruce Heezen, generated the first bathymetric map of the Atlantic Ocean floor. Tharp's work revealed the detailed topography and geological landscape of the seafloor. Her work revealed the presence of a continuous rift valley along the Mid-Atlantic Ridge axis, causing a paradigm in earth sciences that led to the acceptance of plate tectonics and continental drift theories. Piecing maps together in the late 1940s and early 1950s, Marie and his partner Bruce Heezen discovered the 75.000 km underwater ridge bounding around the globe. By this finding, they laid the conclusion from geophysical data that the seafloor spreads from mid-ocean ridges and that continents are in motion with respect to one another—a revolutionary geological theory at that time. Many years later, satellite images demonstrate that Tharp’s maps were accurate. In this contribution, we focus on detailed bathymetric maps collected from year 1992 to today, which include bathymetric maps from diverse parts of the world. For instance, we will show a) Back-arc basins (i.e. the Bransfield Basin, Antarctica; and the North Fiji Basin, SW Pacific); b) Mid-ocean ridges and fracture zones (i.e. the MAR at the South of Azores, the MAR at the Oceanographer-Hayes, and the St. Paul Fracture Zone at the Equator), and c) Active tectonic structures from the Gulf of Cadiz and Alboran Sea, located at the Africa-Eurasia plate boundary (Gibraltar Arc). Regarding this last area, we will characterize the seafloor expression of the fault systems, as well as the subsurface structure of the faults in the Gulf of Cadiz and Alboran Sea. This zone is characterized by a moderate seismicity, mainly reverse and strike-slip focal mechanisms; although large historical (AD1755, AD1829) and instrumental earthquakes or large/great magnitude also occurred, such as the earthquakes of 1969, 1994, 2004 and 2016. In addition, the Gulf of Cadiz-Alboran Sea area is compartmentalized in different crustal domains, bounded by active strike-slip fault systems. We adopted a multi-scale approach, including morphological analysis of shipboard multibeam bathymetry, near-bottom bathymetry obtained with Autonomous Underwater Vehicles (AUVs) at a resolution of 1-2 m, and medium to deep penetration multi-channel seismic (MCS) data. Finally, we will also show a couple of videos from recent marine cruises in the Gibraltar Arc (SHAKE-2015 and INSIGHT-2018), both using state-of-the-art high-resolution marine technologies.

How to cite: Gràcia, E., Martínez Loriente, S., Diez, S., Gómez de la Peña, L., Serra, C. S., Bartolome, R., Sallarès, V., Lo Iacono, C., Perea, H., Roger, U., Ingo, G., and Ranero, C. R.: A tribute to Marie Tharp: Mapping the seafloor of back-arc basins, mid-ocean ridges, continental margins and plate boundaries, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3676, https://doi.org/10.5194/egusphere-egu2020-3676, 2020.

The pioneering seafloor mapping by Marie Tharp played a key role in the acceptance of the plate tectonic theory. Her physiographic maps,  published with Bruce Heezen,  covered the Earth’s oceans and revealed with astonishing accuracy the submarine landscape. She exposed the full extent of the global mid-ocean ridge system, documented features such as seamounts and volcanic chains, trenches, and transform faults. Marie Tharp co-authored the first papers describing the major fracture zones in the Central Atlantic (Chain, Romanche, Vema). In 1952, she also discovered that the Atlantic ridge has a central valley (the axial valley), and convinced her colleague Bruce Heezen that it, which corresponds to sustained seismicity (highlighted by other researchers at the same time thanks to the worldwide networking of seismological stations), is a rift that separates the eastern and western provinces of the Atlantic Ocean. Tharp and Heezen were not yet talking about plate tectonics at this time. But when, at the beginning of the 1960s, the first magnetic anomaly maps showed that the oceans were "young", and that the age of the seabed increased with the distance from the ridges, their physiographic map became an essential element in understanding the role that these ridges play, as well as the distribution of the main current terrestrial plates. In this poster, we present original maps and sketches that document this key contribution to the understanding of the Earth's tectonics.

How to cite: Cannat, M., Smith, D., Fornari, D., Ferrini, V., and Escartin, J.: Marie Tharp: Seafloor mapping and ocean plate tectonics, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12641, https://doi.org/10.5194/egusphere-egu2020-12641, 2020.

The Reykjanes Ridge is a 900 km long oblique slow-spreading ridge, formed by individual “en echelon” axial volcanic ridges (AVR), directly under the influence of the Iceland hotspot. From the Reykjanes Peninsula to the Bight fracture zone, the Reykjanes Ridge shows variations in lava chemistry, crustal thickness, thermal structure and ridge morphology, which has been attributed to this influence. Our study focuses on four areas of the ridge mapped and sampled during the cruise MSM75 in 2018. The northern area is characterized by the only known hydrothermal field discovered along the Reykjanes Ridge. The two central areas are located in a region of increasing magma supply. Finally, the southernmost area is underlined by the only magma body ever detected seismically below the Reykjanes Ridge. The analysis combines 15 m resolution ship-based bathymetry, ground-truthing from ROV dives and geochemical analysis of glass samples to look at variations of magma composition, fault density, seamount density and morphology along the ridge axis. Two major parameters influence the distribution and geometry of faults and seamounts: the distance from the hotspot and the accretion state of individual AVR (i.e., magmatic extension vs. tectonic extension). Fracture geometry is highly influenced by the depth of the brittle-ductile boundary that deepens with distance from the plume center, while fault density at the axis reflects different development stages of individual AVR. Seamount morphologies may also reflect individual AVR development, but we also show morphological variation with distance from the hotspot, correlated with the average variation in lava composition and mantle temperature.

How to cite: Le Saout, M., Devey, C., Palgan, D., Lux, T., Petersen, S., Þórhallsson, D., Tomkowicz, A., and Brix, S.: Variation of the tectono-magmatic activity along the Reykjanes Ridge: Influence of the Iceland hotspot on the accretionary processes, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5873, https://doi.org/10.5194/egusphere-egu2020-5873, 2020.

A sizeable portion of oceanic lithosphere has been produced at the Carlsberg Ridge, one of the three major ridge branches that shaped the Indian Ocean. Accretion started in the Paleocene with the ultra-fast widening of the Arabian Sea to the North and the Eastern Somali Basin to the South (full spreading rate of ~130 mm/yr between 61 Ma and 49 Ma), both basins opening in the wake of the rapid migration of India towards Eurasia. Spreading rate abruptly dropped to ultra-slow after 47 Ma and a long period of accretion stagnation prevailed (full rate <12 mm/yr) until the establishment of the slow present-day regime at ~20 Ma (mean 24 mm/yr rate since Chron 6 at the western end of the Carlsberg Ridge). Mode and rate of production of ocean floor at the Carlsberg Ridge seem to have interacted with a number of regional tectonic events since the beginning of the Himalayan orogeny, including the early Indian continent collision, the westward propagation of the Sheba Ridge into the Africa/Arabia continent and the coeval initiation of the Owen transform and opening of the Gulf of Aden. Here we report the results of the recent CARLMAG survey (Spring 2019) conducted at the westernmost edge of the Carlsberg Ridge close to its intersection with the active Owen transform fault. The cruise was conducted aboard BHO Beautemps-Beaupré operated by the French Naval Hydrographic and Oceanographic Service. We explored the post-50 Ma ocean floor along a set of long profiles crossing both sides of the ridge using multibeam bathymetry, bottom reflectivity, mud penetrator, magnetic and gravimetric measurements. For the first time, semi-complete multibeam coverage allows detailed mapping of the seafloor until it gets buried below the sediments of the Indus fan, at least over the northern limb. The southern limb, devoid of sediments, shows clear rotation of the main fault trends towards older ages, which we attribute to changes in India-Somalia kinematics. The region close to the ridge axis and close to the Owen transform is rich in oceanic core complexes, some of them known from patchy previous acquisitions, and others discovered in the course of our survey. Their highly corrugated surfaces show a wide variety of shapes at various distances from the ridge axis that may be seen as snapshots through time, bearing important information regarding their formation and progressive erosion as they move away. A clear pattern of Miocene oceanic magnetic lineations is recognized, as well as a few older anomalies at the extreme Northern and Southern limits of the survey. This dataset allows us to build a new structural and kinematic scenario for the evolution of this segment of the Carlsberg Ridge and frame it into a more regional geodynamic framework.

How to cite: Chamot-Rooke, N., Janin, A., Rodriguez, M., Delescluse, M., Dyment, J., Fournier, M., Huchon, P., Olive, J.-A., Rabaute, A., and Vigny, C.: Geophysical investigation of the western end of the Carlsberg Ridge: preliminary results of the CARLMAG cruise, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8997, https://doi.org/10.5194/egusphere-egu2020-8997, 2020.

Ultraslow-spreading oceanic ridges (<20 mm/yr) constitute about 35% of the global ridge system and yet the lithospheric structure that accretes at these spreading rates is little understood. At these ridges, the interplay between plate- and mantle-driven processes produces complex relationships between intermittent volcanic seafloor and extensive non-volcanic seafloor domains, with a subsurface structure that differs significantly from the traditional 3-layer crust topping the uppermost mantle that forms at faster-spreading rates. We present new constraints on the velocity and reflectivity structure of the oceanic lithosphere at the ultraslow-spreading Southwest Indian Ridge (SWIR) at 64°30’E. The eastern SWIR has a full-spreading rate of <14 mm/yr and represents a magma-poor endmember. In this area, broad serpentinized mantle domains are exposed with little interference of igneous rocks that can make their identification and geophysical characterization challenging. We use coincident wide-angle ocean bottom seismometer (OBS) and multichannel seismic (MCS) data collected during the SISMOSMOOTH 2014 survey along two long (~150 km) orthogonal profiles, one along the ridge valley (EW direction) and one across it (NS direction). We first run traveltime tomography using picks of first arrivals recorded by 16 OBS placed on the NS profile and 16 OBS on the EW profile. The computed models show that seismic velocities increase rapidly with depth, changing from 3.5-4 km/s at the seafloor to 7 km/s at 2-5 km and that the vertical gradient reduces for velocities greater than 7 km/s. We suggest that the changes in velocity with depth are related to changes in the degree of serpentinization and interpret the subsurface structure to be composed of highly fractured and fully serpentinized peridotites at the top with a gradual decrease in pore space and serpentinization to unaltered peridotites at depth. The NS velocity model shows greater lateral velocity variations than the EW profile, which indicates a more complex structure for the former. Next we perform MCS data analysis to produce reflection sections for the two profiles. Time-migrated sections are converted to depth using the velocities derived from the tomographic models. We observe steep south-dipping reflections around the highest topographic feature on the NS profile, coincident with a sharp lateral change in the velocities and a high vertical gradient in the velocity model, which we interpret as the seismic expression of an active axial detachment fault. Clear Moho arrivals are not identified either in the OBS or the MCS record sections, consistent with our interpretation of the subsurface being composed of a gradual transition from serpentinized peridotites to fresh mantle peridotites.

How to cite: Corbalan, A., Nedimović, M., Grevemeyer, I., Louden, K., and Watremez, L.: Structure of the ultraslow-spreading Southwest Indian Ridge at 64°30’E from coincident multichannel and wide-angle seismic data, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10256, https://doi.org/10.5194/egusphere-egu2020-10256, 2020.

Terrain classification at slow-spreading ridges has been a topic of interest since the significant discovery of mantle rocks exhumed by detachment faults in various segments of the Mid-Atlantic spreading axis. These rocks commonly form domed massifs, so-called core complexes, in contrast to the linear fault-bounded abyssal hills of magmatic spreading terrains. However, there is still limited quantitative description of these two distinct structures. We present analysis of high-resolution bathymetry data 21-24 N over the Mid-Atlantic Ridge and its derivatives to highlight the shapes and directionality of the two oceanic crust features. We assign an optimized 8 arc-minute (~14.8 km) window, mimicking the average size of core complexes, in which we compute the Eigenvalues from each cell within the window based on its directionality and slope. We use the two most dominant Eigenvalues – representing the window’s overall horizontal directionality – to compute eccentricity values and weight them with the sine of the slope. From the computation, we found that areas with weighted eccentricity of 0-0.6 represent the omnidirectional terrains that result from tectonic activities; 0.6-0.9 represents the extended terrain or the buffer zone between the tectonic and magmatic terrains; values >0.9 highlight bidirectional magmatic terrains. Based on this classification, we found significantly more evidence of detachment faulting west of the spreading axis compared to the eastern side. This analysis also highlights neo-volcanic activity that started at around 2 Ma that propagates to the south, cutting a fracture zone before it became inactive. The result contributes to a new approach in mining information from high-resolution bathymetry data to assess oceanic spreading type and its symmetry at a slow-spreading ridge through time.

How to cite: Alodia, G., Green, C., McCaig, A., and Paton, D.: A novel approach for oceanic spreading terrain classification at the Mid-Atlantic Ridge using Eigenvalues of high-resolution bathymetry, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-337, https://doi.org/10.5194/egusphere-egu2020-337, 2020.

The marked increase in seawater Mg/Ca during the Cenozoic is poorly understood, due to the limited availability of proxy data and uncertainty in elucidating the respective contributions of Mg sources and sinks through geological time¹. Though established as a potentially large source of dissolved Mg over twenty years ago, the weathering of abyssal peridotites² is a largely unexplored potential source of Mg to oceanic budgets. The release of magnesium from peridotite weathering can occur in high temperature environments, during serpentinisation near the ridge axis³, as well as low temperature off-axis environments where peridotite and serpentinite are altered to clays, carbonates and silicates⁴. The relative magnitude of Mg fluxes from these sources are poorly constrained. Recent studies, however, now provide a general method for estimating bulk crustal lithologies of mid-ocean ridges based on spreading rate (i.e. proportion and mass of basalts, gabbros, peridotites and serpentinised peridotite) through time⁵—enabling us to quantitatively assess potential Mg contributions from these different environments.

We constructed a model for oceanic crustal weathering (proportional to depth below the seafloor) to develop estimates of the mass and isotopic composition of magnesium loss from peridotite during alteration in both high- and low-T environments. As Mg fractionation occurs predominantly in low-T reactions, the primary serpentinisation reaction in near-ridge environments is unlikely to result in isotopic differentiation. Comparably, the secondary low-T alterations, of both remaining peridotites (to clays and iron hydroxides) and serpentinite (e.g. to talc and dolomite) are likely to result in the fractionation of Mg. We extend our analysis to incorporate the fractionation of these systems⁴ and their release of Mg into the ocean. We completed our analysis by presenting a compilation of fluid data for magnesium concentrations in ultramafic bodies from hydrothermal systems, in order to evaluate our model.

References

(1) Staudigel, H. "Chemical fluxes from hydrothermal alteration of the oceanic crust." (2014): 583-606.

(2) Snow, J.E. and Dick, H.J., 1995. Pervasive magnesium loss by marine weathering of peridotite. Geochimica et Cosmochimica Acta, 59(20), pp.4219-4235.

(3) Seyfried Jr, W.E., Pester, N.J., Ding, K. and Rough, M., 2011. Vent fluid chemistry of the Rainbow hydrothermal system (36 N, MAR): Phase equilibria and in situ pH controls on subseafloor alteration processes. Geochimica et Cosmochimica Acta, 75(6), pp.1574-1593.

(4) Liu, P.P., Teng, F.Z., Dick, H.J., Zhou, M.F. and Chung, S.L., 2017. Magnesium isotopic composition of the oceanic mantle and oceanic Mg cycling. Geochimica et Cosmochimica Acta, 206, pp.151-165.

(5) Merdith, A.S., Atkins, S.E. and Tetley, M.G., 2019. Tectonic controls on carbon and serpentinite storage in subducted upper oceanic lithosphere for the past 320 Ma. Frontiers in Earth Science, 7, p.332.

How to cite: Merdith, A., Andreani, M., Daniel, I., and Gernon, T.: A magnesium budget for serpentinisation of abyssal peridotite during the Cenozoic, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19115, https://doi.org/10.5194/egusphere-egu2020-19115, 2020.

Observations of the apparent links between plate speeds and the global distribution of plate boundary types have led to the suggestion that subduction may provide the largest component in the balance of torques maintaining plate motions. This would imply that plate speeds should not exceed the sinking rates of slabs into the upper mantle. Instances of this ‘speed limit’ having been broken may thus hint at the existence of driving mechanisms additional to those resulting from plate boundary forces. The arrival and emplacement of the Deccan-Réunion mantle plume beneath the Indian-African plate boundary in the 67-62 Ma period has been discussed in terms of one such additional driving mechanism. Its spatial and temporal coincidence with an abrupt speed-up of the Indian plate has led to suggestions that the arrival of plumes at the base of the lithosphere can introduce a push force capable of overwhelming entire circuits of plates and triggering plate tectonic reorganizations.

We challenge the occurrence of a pulse of anticorrelating accelerations and decelerations in seafloor spreading rates around the African and Indian plates and, with it, the proposal that plume-related forces in the Indian Ocean had a significant impact on the Indo-Atlantic plate circuit in late Cretaceous and Paleogene times. Using existing and newly-calculated high-resolution models of plate motion based on seafloor spreading data, we show that the increase in divergence rates previously documented for ridges bordering the Indian plate is artefactual. Records from spreading centers throughout the Indo-Atlantic plate circuit show an ubiquitous increase in plate divergence rates at 67-64 Ma, which is best explained in terms of a timescale error affecting chrons 29-28. Corrected for this error, the motion of the circuit’s plates show little change around Deccan times. Furthermore, we find that Post-Deccan reorganization of the Indo-Atlantic plate circuit can be explained in terms of long-term plate boundary evolution without the need to invoke a large additional plume push force in the 70-60 Ma period.

How to cite: Perez-Diaz, L., Eagles, G., and Sigloch, K.: Indo-Atlantic plate accelerations and tectonic reorganisations in the Late Cretaceous: no need for plume-push forces, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5076, https://doi.org/10.5194/egusphere-egu2020-5076, 2020.

The plate tectonic revolution and decipherment of magnetic isochrons that followed the pioneering work of Marie Tharp and coworkers in visualizing the seafloor has led to a near-complete understanding of the first-order evolution of global seafloor spreading.  However, lagging behind in exploration and understanding are areas of seafloor formed during the Cretaceous Normal Superchron (CNS, 121-83 Ma) when no magnetic reversals were recorded to guide investigators.  Thus, for such regions tectonic interpretations are largely driven by mapping and identifying seafloor fabric indicators such as fracture zones, abyssal hills, rift propagators, and extinct spreading centers.  Here, we focus on the relict spreading system of the Cretaceous Ellice Basin that was apparently formed by seafloor spreading that split the world’s single largest oceanic plateau Ontong Java Nui, composed of present day Ontong Java, Manihiki and Hikurangi plateaus and other (now subducted) fragments.  We examine what was known about this basin from historical single and multibeam bathymetry, what was revealed by the advent of satellite altimetry, and why bathymetric mapping is still required to infer short-length-scale tectonic fabric.  High-resolution bathymetric data from the central basin were recently acquired by the University of Hawaii’s vessel R/V Kilo Moana. Evolution of the spreading system is characterized by three main stages of spreading based on directional and morphological analyses of the seafloor fabric indicators identified from bathymetry.  Spatially conjugate points symmetric about the spreading central zone were identified at the establishment and cessation of each spreading stage and were assumed to be of the same age to form pseudo-isochrons. Pseudo-isochrons were then utilized in reconstructing the basin through time.  The earliest Stage 1 fracture zones trend E-W and consist of multiple closely spaced, parallel fault strands that were indistinguishable in satellite altimetry.  A clockwise rotation of the spreading direction led to Stage 2 NW-SE trending fracture zones, which splayed from Stage 1 multistrands. An offset between Stage 2 fracture zones indicates a short-lived late Stage 3 that appears to be the result of a counter-clockwise rotation of the spreading direction shortly before spreading ceased.  Seafloor evidence for the initial breakup and rifting between Ontong Java and Manihiki plateaus prior to Stage 1 has yet to be mapped.  Basaltic rocks dredged from selected locations along the survey track promise to provide tighter temporal constraints on the evolution of Ellice Basin.

How to cite: Wessel, P., Benyshek, E., and Taylor, B.: Mapping Cretaceous Seafloor Fabric: A Changing View of the Equatorial Ellice Basin, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2244, https://doi.org/10.5194/egusphere-egu2020-2244, 2020.

The early development and growth of seamounts are poorly known as the birth of a volcano on the sea bottom has been rarely observed. The on-going Mayotte seismo-volcanic crisis is associated with the formation of a new seafloor volcano at a water depth of 3300 m and provides the opportunity to study its early development.

Four oceanographic cruises, MAYOBS 1 to 4, were carried out between May and July 2019 aboard the French R/V Marion Dufresne. High resolution bathymetry and backscatter data as well as sub-bottom profiler, gravity and magnetic profiles were collected during each cruise. A dense network of profiles has been achieved over the new volcano at different epochs, allowing to assess its detailed morphology and the evolution through time. During MAYOBS4, a deep-towed underwater camera provided sea bottom videos and photos on the volcano.

First results indicate that the new volcano is still growing at the end of July 2019. Repetitive surveys in May, June and July 2019 allow to document the morphological evolution of the volcano, to estimate the volume of material emplaced between each epoch and to discuss the emitted lava rate.

The new volcano has a starfish shape and is now 820 m high. Steep slopes are observed close to the summit and several radial ridges developed from its central part, displaying hummocky morphology similar to the ones observed along mid oceanic axial volcanic ridges. At the bottom, flat areas with high backscatter could indicate channelized lava flows emplaced at higher effusion rates. The morphological analysis combined with video imagery brings constraints to the eruptive processes yielding to the formation of a nascent volcano.

How to cite: Deplus, C., Feuillet, N., Thinon, I., Jorry, S., Fouquet, Y., Bachèlery, P., Bermell, S., Besson, F., Bickert, M., Gaillot, A., Guérin, C., Le Friant, A., Paquet, F., Pierre, D., and Pitel-Roudaut, M.: Early development of a deep sea volcano offshore Mayotte revealed by seafloor mapping : first results from the MAYOBS cruises, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11026, https://doi.org/10.5194/egusphere-egu2020-11026, 2020.

The Mangatolu Triple Junction (MTJ) is an intraoceanic back-arc spreading center that is host to at least 3 distinct hydrothermal systems. It is located in the NE Lau Basin, which opened due to rollback of the Pacific plate along the Tonga-Kermadec trench. At the MTJ, three spreading centers meet in a ridge-ridge-ridge (RRR)-type triple junction separating the Tonga plate in the east, the Niuafo’ou microplate in the southwest, and an unnamed microplate in the north. The MTJ is directly linked to the formation and evolution of the Northeast Lau microplate mosaic, as plate fragmentation inevitably results in the formation of triple junctions, but it remains unclear whether the spreading centers are the drivers of plate fragmentation or a consequence of stress relocation related to microplate rotation. Detailed investigation of the geology and structural setting of the MTJ therefore provides valuable insight into the development in the northeast Lau Basin. Here we present the first comprehensive 1:200,000 geological map of the MTJ, based on a compilation of marine geophysical data (hydroacoustics, magnetics, and gravity) derived from 7 research cruises that have investigated the region between 2004 and 2018. Analysis of the mapped geological formations at the MTJ shows the importance of relict arc crust originating from the Tofua Arc in the architecture of the triple junction, which includes three stages of back-arc crust development and extensive off-axis volcanism. The spreading centers along each arm of the MTJ exploit pre-existing crustal weaknesses, interpreted to have formed during initial Lau Basin opening. A reconstruction of the basin opening, based on the mapped features and published spreading rates, revealed that initiation of the MTJ commenced approximately 180,000 years ago, consistent with the very recent and ongoing dynamic evolution of the NE Lau Basin and emerging microplate mosaic. Intersecting fabrics indicate sequential evolution of the 3 arms of the triple junction, with extension along the northeast arm dominant in the early history and more recent extension along the southern and western arms. The results of this study contribute to our growing understanding of the tectonic framework of the northeast Lau Basin and the role of triple junctions in microplate formation.

How to cite: Mensing, R., Stewart, M., Hannington, M., Baxter, A., and Mertmann, D.: The tectonic and volcanic evolution of the Mangatolu Triple Junction, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19897, https://doi.org/10.5194/egusphere-egu2020-19897, 2020.

We produced the first geological map of the Scotia Sea area based on the available geophysical and geological data. Combining magnetic, Bouguer gravity anomaly and high-resolution bathymetric data with geological data from dredged samples allowed us to map lithologies and structural features in this mostly submerged and complex tectonic area. This geological map allowed us to integrate a very inter-disciplinary dataset, thereby reviewing the available data and addressing some of the still persisting geological challenges and controversies in the area.

One of the most important and persistent discussions is the nature and age of the Central Scotia Sea. We mapped this part of the Scotia Sea as basaltic-andesitic lithology partly covered by thick, oceanic sediments. This differs in lithology from the West and East Scotia Sea, which we mapped as a basaltic lithology. Based on our lithological map, its unusual thickness and the presence of the Ancestral South Sandwich Arc (ASSA, early Oligocene-late Miocene) we argue that Central Scotia Sea has an Eocene to earliest Oligocene age.

Cross-sections combining the geology, crustal structure and mantle tomography reveal high velocity anomalies and colder mantle material below the structural highs along the South Scotia Ridge (Terror Rise, Pirie Bank, Bruce Bank and Discovery Bank) and below the South Sandwich Islands. We interpreted those as the southern, stagnated part of the subducting slab of the South Sandwich Trench, following the geometry of Jane Basin and the currently active subducting slab at the South Sandwich Trench. Low velocity anomalies are observed below Drake Passage and the East Scotia Sea, which are interpreted as warmer toroidal mantle flow around the slab edges below the Chilean trench and the South Sandwich trench.

Based on our geological map and integrated cross-sections we propose a multi-phase evolution of the Scotia Sea area with Eocene or older oceanic crust for the Central Scotia Sea. A first wide-rift-phase initiated before 30 Ma in the West Scotia Ridge, Protector Basin, Dove Basin and Jane Basin either as a result of the diverging South American and Antarctic continents and/or due to subduction rollback that commenced soon after subduction initiation that eventually caused the ASSA to form. The first full spreading center developed in the West Scotia Sea, aided by the warmer toroidal mantle flow causing spreading to be abandoned in the other basins (~30 Ma). A second rift phase in the fore-arc, in between the ASSA and the South Sandwich trench (~20 Ma), initiated through a redistribution of far-field forces as a result of continuous trench retreat. The warmer toroidal mantle concentrated on the East Scotia Ridge resulting in the second spreading system (15 Ma), abandoning the West Scotia Ridge spreading system 6-10 Ma.

We show that it is possible to create a geological map in a very remote area with an extreme environment with the available geological and geophysical data. This new way of producing geological maps in the offshore domain provides a better insight into the geological history of geologically complex areas that are largely submerged.

How to cite: Beniest, A. and Schellart, W. P.: Geological mapping in the offshore domain: unravelling the tectonic history of the Scotia Sea, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2083, https://doi.org/10.5194/egusphere-egu2020-2083, 2020.