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: The transfer of sediment from the upper continental slope to rise is poorly documented along the southeast African passive margin. New swath bathymetric and sub-bottom data collected in the Natal Valley, southwest Indian Ocean, provide insight into the evolution of the Tugela canyon and fan system. Several distinct downslope changes in canyon morphology are noted. The canyon increases in relief and widens with depth. Basement outcrop is restricted to the head of the canyon becoming less prominent with depth. Step-like terracing of the canyon walls and floor becomes prominent in the mid-slope portions of the canyon and is related to a marked increase in the cross sectional asymmetry of the canyon profile. The contemporary Tugela canyon rests within a depression of the last phase of infilling. The canyon is the product of downslope erosion, and incision, caused by several phases of hinterland uplift in the mid Oligocene, mid Miocene and late Pliocene. Each phase was followed by pelagic infilling of the palaeo-canyon form. Downslope, the uplift phases are preserved in the cut-terraces and axial incisions within the main canyon thalweg. The contemporary canyon is a moribund feature, sediment starvation of the shelf area by current sweeping of the Agulhas current has decreased the material available for canyon incision and fan development. Additional current sweeping by the North Atlantic Deep Water current has stunted the development of the associated fan complex.

Description: The Mozambique Basin is one of the oldest extensional sedimentary basins developed along the eastern African margin. The basin hosts a continuous record of sediments since the Jurassic separation of Antarctica from Africa. The objectives of this study were to extend the regional stratigraphic framework north of the Zambezi Delta into the deep abyssal plains and review the early evolution of the Mozambique Basin using nine multi-channel seismic reflection profiles. We identify six major stratigraphic units that were deposited in Jurassic, Early Cretaceous, Late Cretaceous, Paleogene, Neogene and Quaternary times. Mesozoic sedimentation rates of 5-10 cm/kyr and 1-3 cm/kyr during the Paleogene are calculated in the deeper basin. The presence of shales in neighbouring wells on the shelf implies an euxinic environment in the rapidly subsiding basin until Early Cretaceous times. The Mesozoic sediments have a high seismic velocity that exceeds 4.5 km/s, except in a distinct Early Cretaceous low-velocity (3.7 km/s) zone that may indicate the presence of undercompacted, overpressured shales. In spite of the fact that the Zambezi catchment was much smaller in pre-Miocene times, the high Late Cretaceous sedimentation rates can be attributed to rapid denudation of the African continent after a major tectonic uplift episode at approximately 90 Ma. Increased sediment influx into the basin from the Zambezi in Late Cretaceous times resulted in the formation of an elongated submarine fan lobe into the Mozambique Channel north of Beira High. Strong north-south bottom currents commenced within the channel in Late Cretaceous times, forcing the aggradation of sediments on the southern flank of the lobe. In addition, we observe several current-controlled sediment deposits in the deeper basin that are influenced by north-south bottom currents. Low Paleogene sedimentation rates are attributed to a sediment-starved basin during a relative quiet tectonic phase onshore.

Description: During the RV Polarstern cruise ARK-XXIII-3 in the summer of 2008, seismic reflection and refraction data along an almost 1200 km long transect along 81�N latitude were acquired across the Amundsen Basin, Lomonosov Ridge, Makarov Basin, Mendeleev Ridge and parts of the Canada Basin. The seismic data are dominated by an unconformity/reflector band that is observed along the entire transect that separates a flat-lying well-stratified upper unit from the underlying sediment sequences. In our interpretation this reflector band spans a time interval from breakup of the Lomonosov Ridge from the Siberian/Barents shelves around 56 - 65 Ma to the top of the Oligocene. The velocity-depth functions indicate total sediment thicknesses ranging from 1200 to 2000 m on the Lomonosov Ridge, to 5500-6300 m within the deepest part of the Makarov Basin around 168�E, to 1000 e1500 m on the western flank of the Mendeleev Ridge, and, finally, to ~4000 m within the Canada Basin. The data show that stretched continental crust of the Lomonosov Ridge extends farther into the Makarov Basin than previously known. Horst and graben structures indicate that approximately 50% of the Makarov Basin along the 81�N transect is underlain by stretched continental crust. These structures most likely formed during a rift phase which is older than 56 Ma. Thick Mendeleev Ridge crust (up to 33 km) occupies the remaining portion of the basin. It is likely that the formation of this magmatic crust overprinted older oceanic crust during the Cretaceous Quiet Period (84e120 Ma).