Description: Earthquake locations along the southern Mid-Atlantic Ridge have large uncertainties due to the sparse distribution of permanent seismological stations in and around the South Atlantic Ocean. Most of the earthquakes are associated with plate tectonic processes related to the formation of new oceanic lithosphere, as they are located close to the ridge axis or in the immediate vicinity of transform faults. A local seismological network of ocean-bottom seismometers and land stations on and around the archipelago of Tristan da Cunha, allowed for the first time a local earthquake survey for one year. We relate intra-plate seismicity within the African oceanic plate segment north of the island partly to extensional stresses induced by a bordering large transform fault and to the existence of the Tristan mantle plume. The temporal propagation of earthquakes within the segment reflects the prevailing stress field. The strong extensional stresses in addition with the plume weaken the lithosphere and might hint at an incipient ridge jump. An apparently aseismic zone coincides with the proposed location of the Tristan conduit in the upper mantle southwest of the islands. The margins of this zone describe the transition between the ductile and the surrounding brittle regime. Moreover, we observe seismicity close to the islands of Tristan da Cunha and nearby seamounts, which we relate to ongoing tectono-magmatic activity.

Description: Ultraslow spreading ridges are poorly understood plate boundaries consisting of magmatic and amagmatic segments that expose mostly mantle peridotite and only traces of basalt and gabbro. The slowest part of the global spreading system is represented by the eastern Gakkel Ridge in the Central Arctic Ocean, where crustal accretion is characterized by extreme focusing of melt to discrete magmatic centers. Close to its eastern tip lies the unusual 5,310 m deep Gakkel Rift Deep (GRD) with limited sediment infill, which is in strong contrast to the broader sediment‐filled rift valleys to the east and west. Here, we report an 40Ar/39Ar age of 3.65±0.01 Ma for a pillow basalt from a seamount located on the rim the GRD confirming ultraslow spreading rates of ~7 mm/yr close to the Laptev Sea as suggested from aeromagnetic data. Its geochemistry points to an alkaline lava, attributed to partial melting of a source that underwent prior geochemical enrichment. We note that the GRD extracts compositionally similar melts as the sparsely magmatic zone further west but at much slower spreading velocities of only ~6‐7 mm/yr, indicating the widespread occurrence of similarly fertile mantle in the High Arctic. This enriched source differs from sub‐continental lithospheric mantle that influences magmatism along the Western Volcanic Zone (Goldstein et al. 2008) and is similar to metasomatized mantle ‐ shown to influence melt genesis along the Eastern Volcanic Zone.

Description: Understanding the enigmatic intraplate volcanism in the Tristan da Cunha region requires knowledge of the temperature of the lithosphere and asthenosphere beneath it. We measured phase-velocity curves of Rayleigh waves using cross-correlation of teleseismic seismograms from an array of ocean-bottom seismometers around Tristan, constrained a region-average, shear-velocity structure, and inferred the temperature of the lithosphere and asthenosphere beneath the hotspot. The ocean-bottom data set presented some challenges, which required data-processing and measurement approaches different from those tuned for land-based arrays of stations. Having derived a robust, phase-velocity curve for the Tristan area, we inverted it for a shear wave velocity profile using a probabilistic (Markov chain Monte Carlo) approach. The model shows a pronounced low-velocity anomaly from 70 to at least 120 km depth. VS in the low velocity zone is 4.1-4.2 km/s, not as low as reported for Hawaii (∼4.0 km/s), which probably indicates a less pronounced thermal anomaly and, possibly, less partial melting. Petrological modeling shows that the seismic and bathymetry data are consistent with a moderately hot mantle (mantle potential temperature of 1,410-1,430°C, an excess of about 50-120°C compared to the global average) and a melt fraction smaller than 1%. Both purely seismic inversions and petrological modeling indicate a lithospheric thickness of 65-70 km, consistent with recent estimates from receiver functions. The presence of warmer-than-average asthenosphere beneath Tristan is consistent with a hot upwelling (plume) from the deep mantle. However, the excess temperature we determine is smaller than that reported for some other major hotspots, in particular Hawaii.

Description: A new seismostratigraphic model has been established within the Arctic Ocean adjacent to the East Siberian Shelf on the basis of multichannel seismic reflection data acquired along a transect at 81°N. Ages for the sedimentary units were estimated via links to seismic lines and drill site data of the US Chukchi Shelf, the Lomonosov Ridge, and the adjacent Laptev Shelf. Two distinct seismic units were mapped throughout the area and are the constraints for dating the remaining strata. The lower marker unit, a pronounced high-amplitude reflector sequence (HARS), is the most striking stratigraphic feature over large parts of the Arctic Ocean. It indicates a strong and widespread change in deposition conditions. Probably, it developed during Oligocene times when a reorientation of Arctic Plates took place, accompanied by the gradual opening of the Fram Strait, and a widespread regression of sea level. The top of the HARS likely marks the end of Oligocene/early Miocene (23Ma). An age estimate for the base of the sequence is less clear but likely corresponds to base of Eocene (˜56Ma). The second marked unit detected on the seismic lines parallels the seafloor with a thickness of about 200ms two-way travel time (160 m). Its base is marked by a change from a partly transparent sequence with weak amplitude reflections below to a set of continuous high-amplitude reflectors above. This interface likely marks the transition to large-scale glaciation of the northern hemisphere and therefore is ascribed to the top Miocene (5.3 Ma).

Description: During the Jurassic, the Falkland Plateau was part of Gondwana and occupied a position between the African and Antarctic plates. Several contrasting models exist for the breakup of Gondwana that depend on assumptions about the currently unknown crustal structure of the Falkland Plateau. Here, we present the results of recently acquired wide-angle seismic data along the entire plateau that provide sound constraints on its role in geodynamic reconstructions. In contrast to published crustal models, the new data show that the Falkland Plateau Basin consists of up to 20 km thick oceanic crust, which is bounded to the east by a continental fragment, the Maurice Ewing Bank. In a refined geodynamic model, rifting started between the Falkland Islands and the Maurice Ewing Bank at ~178 Ma and ceased at around ~154 Ma. The plateau's exceptionally thick oceanic crust likely results from its position in an extensional back-arc-regime situated over a mantle thermal anomaly that was also responsible for the extensive onshore Karoo-Ferrar and Chon Aike volcanic provinces.

Description: The initial opening of the Africa-Antarctica Corridor, in the heart of Gondwana, is still enigmatic due to missing information on the origin of major crustal features and the exact timing of the onset of the first oceanic crust in the Jurassic. Therefore, in 2014, new ship-borne magnetic data were systematically acquired in the northern Mozambique Basin and across Beira High, which we merged with all accessible magnetic data in the Mozambique Basin. Herein, distinct magnetic lineations are observed, which allow a refined identification of a whole set of Jurassic magnetic spreading anomalies, constraining the timing of the onset of oceanization, beginning at M38n.2n (164.1 Ma). In combination with high-resolution potential field data from the conjugate Antarctic margin, well-expressed fracture zones can be traced throughout the Africa-Antarctica Corridor and allow the precise rotation of Antarctica back to Africa. The initial fit depicts striking continuations of onshore tectonic features across the plate boundaries taking onshore aeromagnetic data of both margins into account. Within a tight Gondwana fit, the Beira High can be restored along the major sinistral Namama-Orvin Shear Zone of the East African-Antarctic Orogen. The Beira High represents a continental block, which was detached from Antarctica, by 157 Ma at the latest. Simultaneously, the Antarctic plate cleared the area of the MCP. However, the crustal nature of the southern MCP remains ambiguous. The Northern Natal Valley and the Mozambique Ridge consist of thick oceanic crust, being emplaced between M26r-M18n (157.1–144 Ma) and M18n-M6n (144–131.7 Ma), respectively. About the half of this crust was won from the Antarctic plate by a series of southwards directed ridge jumps to the northern boundary of the Explora Wedge. A refined kinematic break-up model constrained by the most extensive magnetic dataset is presented describing consistently the initial opening of the Africa-Antarctica Corridor and the Somali Basin.

Description: The submarine Zambezi Channel is the deep, stable, north-south orientated, lower portion of a channel system draining the continental slope of central Mozambique; transporting material southwards into the Mozambique Channel and Basin, southwest Indian Ocean. Using recently collected Multi Beam Echo Sounder and PARASOUND data we discuss the geomorphology of the Zambezi Channel. This system is enigmatic in that the main channel is stable, with low sinuosity despite being at a low latitude where rivers seasonally deliver fine grained sediment. A further enigma is that system does not now continue upslope to the Zambezi River, the largest river in southern Africa. Instead this river flows into the northern Mozambique basin to the south-west of the small channels. The Zambezi Channel is compared to small-scale physical models in an attempt to better understand the geomorphology of the channel. The geomorphological features of the main channel show a quite remarkable resemblance to an analogue model produced within a purely erosive environment. To explain these enigmas, it is proposed that geomorphology of the main Zambezi Channel was produced by periodic, high-volume pulses of flood water, and associated sediment, from the Zambezi River, the second largest river in Africa. These events are considered to be due to minor tectonic movements along the Chobe Fault in the Kalahari that permitted the draining of several palaeo-lake systems between the Early Pleistocene through to the early Mid-Pleistocene. Such repetitive draining of palaeo-lakes would have produced flooding comparable to glacial dam bursts. Such events would deliver significantly more sediment laden flood water to the region than “normal” flow conditions. We hypothesise that these significant flood events have influenced the geomorphology of the Zambezi River to the extent that it is no longer comparable to other low-latitude systems, and exhibits characteristics akin to high-latitude systems with highly variable sediment input.

Description: The 1500 km long Falkland Plateau is the most prominent morphological structure in the southern South Atlantic Ocean, which crustal composition and development is mainly unknown. At the westernmost boundary of the plateau, the Falkland Islands' Precambrian geology provides the only insight into basement structure and age. The question of whether continental basement of a similar age and origin underlies the Falkland Plateau further east is strongly disputed. We present new high quality constraints on the crustal fabric of the plateau east of the Falkland Islands, based on wide-angle seismic and potential field data acquired in 2013. The P-wave velocity model, supported by amplitude and density modelling, shows that the Falkland Plateau Basin is filled with 8 km of sediments. Continental crust of 34 km thickness underlies the Falkland Islands. The eastern continental margin of the Falkland Islands can be classified as a volcanic rifted margin. The Falkland Plateau Basin is floored by up to 20 km thick oceanic crust. The exceptionally thick igneous crust and its high lower crustal velocities (up to 7.4 km/s) indicate the influence of a regional thermal mantle anomaly during its formation, which provided extra melt material. The wide-angle model revises published crustal models, which predicted thin oceanic or thick extended continental crust below the Falkland Plateau Basin. Our results provide a sound basis for future tectonic interpretations of the area.

Description: The Weddell Gyre (WG) is one of the main oceanographic features of the Southern Ocean south of the Antarctic Circumpolar Current which plays an influential role in global ocean circulation as well as gas exchange with the atmosphere. We review the state‐of‐the art knowledge concerning the WG from an interdisciplinary perspective, uncovering critical aspects needed to understand this system's role in shaping the future evolution of oceanic heat and carbon uptake over the next decades. The main limitations in our knowledge are related to the conditions in this extreme and remote environment, where the polar night, very low air temperatures and presence of sea ice year‐round hamper field and remotely sensed measurements. We highlight the importance of winter and under‐ice conditions in the southern WG, the role that new technology will play to overcome present‐day sampling limitations, the importance of the WG connectivity to the low‐latitude oceans and atmosphere, and the expected intensification of the WG circulation as the westerly winds intensify. Greater international cooperation is needed to define key sampling locations that can be visited by any research vessel in the region. Existing transects sampled since the 1980s along the Prime Meridian and along an East‐West section at ~62°S should be maintained with regularity to provide answers to the relevant questions. This approach will provide long‐term data to determine trends and will improve representation of processes for regional, Antarctic‐wide and global modeling efforts – thereby enhancing predictions of the WG in global ocean circulation and climate.

Description: Some of the oldest surviving oceanic basins in the world, the Mozambique and West Somali basins, were created during the breakup of Gondwana, starting around 180 Ma. Between the two basins, relative movements of West Gondwana and East Gondwana, including Madagascar, created a shear zone, the Davie Fracture Zone (DFZ) with a topographic elevation (Davie Ridge - DR) marking its centre. The crustal composition of the DFZ and DR is a subject of speculation and debate. In this study, we present seismic refraction data across the prominent topography of the southern DR. Ray tracing of the wide-angle data as well as additional seismic amplitude modelling and 2.5D density modelling constrain its crustal structure and architecture. The data indicate that in the Mozambique Channel the DR consists of fragments of continental crust with a thickness of 10 to 12 km. An oceanic crust indenter extends northward from the Mozambique Basin into the area between the DR and the East African margin at 16.5°S. Northeast of the DR, at 41.8°W/14.5°S, the Somali Basin is probably floored by 6 km thick oceanic crust. Hence, the continental DR separates oceanic crust of the Somali and Mozambique basins. The transitional crustal area at the central Mozambican margin is underlain by high velocity lower crust (HVLC). The HVLC has velocities up to 7.3 km/s and extents along the margin, vanishing northward between 16.5° and 14.5°S. At the Madagascan side of the DR, at 16.5°S, the highly intruded stretched continental crust is 9 km thick and possibly underlain with a smaller HVLC of 2.9 km thickness and an E-W extent of 120 km. The oceanic crust at 14.5°S represents the oldest part the Somali Basin, which formed after the initial NW-SE rifting between East and West Gondwana.