ALBERT

Description: SUMMARY Baffin Bay represents the northern extension of the extinct rift system in the Labrador Sea. While the extent of oceanic crust and magnetic spreading anomalies are well constrained in the Labrador Sea, no magnetic spreading anomalies have yet been identified in Baffin Bay. Thus, the nature and evolution of the Baffin Bay crust remain uncertain. To clearly characterize the crust in southern Baffin Bay, 42 ocean bottom seismographs were deployed along a 710-km-long seismic refraction line, from Baffin Island to Greenland. Multichannel seismic reflection, gravity and magnetic anomaly data were recorded along the same transect. Using forward modelling and inversion of observed traveltimes from dense airgun shots, a P -wave velocity model was obtained. The detailed morphology of the basement was constrained using the seismic reflection data. A 2-D density model supports and complements the P -wave modelling. Sediments of up to 6 km in thickness with P -wave velocities of 1.8–4.0 km s −1 are imaged in the centre of Baffin Bay. Oceanic crust underlies at least 305 km of the profile. The oceanic crust is 7.5 km thick on average and is modelled as three layers. Oceanic layer 2 ranges in P -wave velocity from 4.8 to 6.4 km s −1 and is divided into basalts and dykes. Oceanic layer 3 displays P -wave velocities of 6.4–7.2 km s −1 . The Greenland continental crust is up to 25 km thick along the line and divided into an upper, middle and lower crust with P -wave velocities from 5.3 to 7.0 km s −1 . The upper and middle continental crust thin over a 120-km-wide continent–ocean transition zone. We classify this margin as a volcanic continental margin as seaward dipping reflectors are imaged from the seismic reflection data and mafic intrusions in the lower crust can be inferred from the seismic refraction data. The profile did not reach continental crust on the Baffin Island margin, which implies a transition zone of 150 km length at most. The new information on the extent of oceanic crust is used with published poles of rotation to develop a new kinematic model of the evolution of oceanic crust in southern Baffin Bay.

Description: Hotspot trails in the South Atlantic controlled by plume and plate tectonic processes Nature Geoscience 5, 735 (2012). doi:10.1038/ngeo1583 Authors: John M. O’Connor, Wilfried Jokat, Anton P. le Roex, Cornelia Class, Jan R. Wijbrans, Stefanie Keßling, Klaudia F. Kuiper & Oliver Nebel The origin of hotspot trails is controversial. Explanations range from deep mantle plumes rising from the core–mantle boundary (CMB) to shallow plate cracking. However, these mechanisms cannot explain uniquely the scattered hotspot trails distributed across a 2,000-km-wide swell in the sea floor of the southeast Atlantic Ocean. This swell projects down to one of the two largest and deepest distinct regions at the CMB, the Africa Low Shear Wave Velocity Province. Here we use 40Ar/39Ar isotopic analyses to date lava samples erupted at several hotspot trails across the Atlantic swell. We combine the eruption ages with an analysis of the structure and age of the sea floor, and find that the trails formed synchronously, in a pattern consistent with movement of the African Plate over plumes rising from the edge of the Africa Low Shear Wave Velocity Province. However, we also find that the seamounts initially formed only at the edge of the swell, where the oceanic crust was spreading apart. Later, about 44 million years ago, the hotspot trails began to cross the swell, but only in locations where the lithosphere was sufficiently young and thin that magma could reach the surface. We conclude that the distribution of hotspot trails in the southeast Atlantic Ocean is controlled by the interplay between deep-sourced mantle plumes and the motion and structure of the African Plate.

Description: The origin of hotspot trails is controversial. Explanations range from deep mantle plumes rising from the core-mantle boundary (CMB) to shallow plate cracking. However, these mechanisms cannot explain uniquely the scattered hotspot trails distributed across a 2,000-km-wide swell in the sea floor of the southeast Atlantic Ocean. This swell projects down to one of the two largest and deepest distinct regions at the CMB, the Africa Low Shear Wave Velocity Province. Here we use 40 Ar/ 39 Ar isotopic analyses to date lava samples erupted at several hotspot trails across the Atlantic swell. We combine the eruption ages with an analysis of the structure and age of the sea floor, and find that the trails formed synchronously, in a pattern consistent with movement of the African Plate over plumes rising from the edge of the Africa Low Shear Wave Velocity Province. However, we also find that the seamounts initially formed only at the edge of the swell, where the oceanic crust was spreading apart. Later, about 44 million years ago, the hotspot trails began to cross the swell, but only in locations where the lithosphere was sufficiently young and thin that magma could reach the surface. We conclude that the distribution of hotspot trails in the southeast Atlantic Ocean is controlled by the interplay between deep-sourced mantle plumes and the motion and structure of the African Plate.

Description: The Boreas Basin is located in Norwegian Greenland Sea bordered by the Greenland Fracture Zone in the south and the Hovgard Ridge in the north, respectively. In the east it adjoins the ultraslow mid-ocean Knipovich Ridge. Previous seismic reflection studies in the Boreas Basin have shown that the basement topography has a roughness, which is typical for ultraslow spreading ridges. This observation supports assumptions that the basin was formed at ultraslow spreading rates during its entire geological history. However, the detailed crustal structure remained unresolved. In summer 2009 new seismic refraction data were acquired in the Boreas Basin during the expedition ARK-XXIV/3 with the research vessel Polarstern. The deep seismic sounding line has a length of 340 km. Forward modelling of the data of 18 ocean bottom seismometers deployed along the NW-SE trending profile reveal an unusual 3.2 km thin oceanic crust. The crustal model is further constrained by S-wave and 2D gravity modelling. The P-wave velocity model shows a layered oceanic crust without oceanic layer 3 and with velocities less than 6.3 km/s except beneath a nearly 2000 m high seamount. Beneath the seamount velocities of up to 6.7 km/s were observed. The mantle velocities range between 7.5 km/s in the uppermost mantle and 8.0 km/s in almost 15 km depth. A serpentinisation of approximately 13% in the uppermost mantle decreasing downwards can explain the low mantle velocities. In summary, the transect confirms earlier models that the entire Boreas Basin was formed at ultraslow spreading rates. Indications for this are the basement roughness and the overall thin oceanic crust. Both observations are typical for ultraslow spreading systems.

Description: The reconstruction of large ice masses in the past is a crucial element for current climate models as correct input and base line parameter as well as for the implementation of associated ice sheet dynamics. For a long time, the ice sheet extent of the Greenland Ice Sheet (GIS) was reconstructed mainly on the basis of terrestrial work. Accordingly, the outer limit of the GIS during the Late Glacial Maximum (LGM) was placed close to the current coastline. Advances in our understanding on the dynamic behaviour of the GIS,especially offshore NE-Greenland, came from hydro-acoustic surveys which indicated a much larger extent of GIS during the LGM. Here, we present hydro-acoustic data acquired with RV “Polarstern” from fjord systems to the shelf edge of NE-Greenland, including the first hydro-acoustic data of Dijmphna and Hekla Sunds. We found morphological evidence for fast-flowing ice filling the fjords, extending onto the shelf as ice stream and reaching the shelf break. Mega-scale glacial lineation, recessional moraines and grounding line wedges document a highly dynamic behaviour of this Westwind Ice Stream of the GIS on NE-Greenland. The ice advance was followed by a rapid retreat to a mid-shelf position where the ice margin repeatedly deposited sets of recessional moraines. A second rapid retreat, probably accompanied by a lift-off of the ice followed and placed the ice margin at the mouth of Dijphna Sund. A last retreat established the modern ice margin in the area. Post-glacial sedimentation was affected by mud diapirism, neo-tectonic activity and submarine mass-wasting inside Dijphna and Hekla Sunds.

Description: Finding the best fit for East- and West-Gondwana requires a detailed knowledge of the initial Jurassic movements between Africa and Antarctica. This study presents results of systematic and densely spaced aeromagnetic measurements, which have been conducted in 2009/2010 across the Astrid Ridge (Antarctica) and in the western Riiser-Larsen Sea to provide constraints for the early seafloor spreading history between both continents. The data reveal different magnetic signatures of the northern and southern parts of the Astrid Ridge, which are separated by the Astrid Fracture Zone. The southern part is weakly magnetised, corresponding to the low amplitude anomaly field of the southwestern Riiser-Larsen Sea. The northern Astrid Ridge bears strong positive anomalies. Several sets of trends are visible in the data. In the Mozambique Channel, we extended the existing magnetic spreading anomaly identifications close to the Mozambique margin. Based on these and on spreading anomalies in the conjugate Riiser-Larsen Sea, we established a new model of the early relative movements of Africa and Antarctica in Jurassic times, and introduce a detailed model for the emplacement of the Mozambique Ridge. The model postulates a tight fit between Africa and Antarctica and two stages of breakup, the first of which lasting until ~159 Ma (M33n). During this stage, Antarctica rotated anticlockwise with respect to Africa. The Grunehogna Craton cleared the Coastal Plains of Mozambique and occupied a position east of the Mozambique Fracture Zone. The southern Astrid Ridge is interpreted to consist of oceanic crust that was formed prior to the Riiser-Larsen Sea during this first stage. During the second stage, Antarctica moved southward with respect to Africa forming the Mozambique Basin and the conjugate Riiser-Larsen Sea. The Mozambique Ridge and the Northern Natal Valley were formed at different spreading centers being active subsequently.