Beschreibung: The crustal structure and continental margin between southern Nares Strait and northern Baffin Bay were studied based on seismic refraction and gravity data acquired in 2010. We present the resulting P wave velocity, density and geological models of the crustal structure of a profile, which extends from the Greenlandic margin of the Nares Strait into the deep basin of central northern Baffin Bay. For the first time, the crustal structure of the continent-ocean transition of the very northern part of Baffin Bay could be imaged. We divide the profile into three parts: continental, thin oceanic, and transitional crust. On top of the three-layered continental crust, a low-velocity zone characterizes the lowermost layer of the three-layered Thule Supergroup underneath Steensby Basin. The 4.3–6.3 km thick oceanic crust in the southern part of the profile can be divided into a northern and southern section, more or less separated by a fracture zone. The oceanic crust adjacent to the continent-ocean transition is composed of 3 layers and characterized by oceanic layer 3 velocities of 6.7–7.3 km/s. Toward the south only two oceanic crustal layers are necessary to model the travel time curves. Here, the lower oceanic crust has lower seismic velocities (6.4–6.8 km/s) than in the north. Rather low velocities of 7.7 km/s characterize the upper mantle underneath the oceanic crust, which we interpret as an indication for the presence of upper mantle serpentinization. In the continent-ocean transition zone, the velocities are lower than in the adjacent continental and oceanic crustal units. There are no signs for massive magmatism or the existence of a transform margin in our study area.

Beschreibung: Up to Jurassic times the Antarctic and African continents were part of the supercontinent Gondwana. Since some 185 Ma the rifting in our research area caused the dispersal of Gondwana and Eastern Africa. The timing and geometry of the break-up as well as the amount of volcanism connected to the Jurassic rifting are still controversial. In the southern part of the Mozambique channel a prominent basement high, the Beira High, forms a specific crustal anomaly along the margin. It is still controversial if this high is a continental fragment or was formed during a period of enhanced magmatism. Therefore a deep seismic profile with 37 OBS/H was acquired from the deep Mozambique Channel, across the Beira High and terminating on the shelf. The main objectives are to provide constraints on the crustal composition and origin of the Beira High as well as the amount of volcanism and the continent-ocean transition below the Zambezi Delta. To obtain a P-wave velocity model of this area the data was forward modelled by means of 2D-Raytracing. Furthermore, potential field data acquired in parallel to the seismic data were used to calculate a 2D gravity model. Preliminary results indicate a 20-24 km thick crust for the Beira High. In good agreement to the adjacent oceanic crust in the Mozambique Channel the upper crust has velocities between 5.5-5.9 km/s. The middle crust is characterised by velocities between 6.2-6.7 km/s and the lower crust higher than 6.7 km/s and a density of 3.0 g/cm3. However, these velocities are only constrained by Moho reflections, since no diving waves are observed for the lower crust. In the area of the Zambezi Delta Depression the top of the acoustic basement is at 11.5 km depth and the crust thickness thins to 7 km. The basement here is overlain by a 2 km thick layer of 4.9-5.1 km/s, which we interpret as pre-rift sediments (Karoo-Belo-Group, including Lava Flows on top). Furthermore, evidence for the presence of a high velocity body (HVB) at below the western part of Beira High with a velocity of 7.2-7.4 km/s and 3 km thickness is found. Below the shelf our results indicate evidences for an increased volcanism during the initial break-up. The location of the continent-ocean boundary as well as the geometry of the break-up depend strongly on the tectonic classification of Beira High. Future work will provide further constraints by amplitude modelling, a 3D gravity model of Beira High and by means of interpretation of the magnetic anomalies.

Beschreibung: Up to Jurassic times the Antarctic and African continents were part of the supercontinent Gondwana. Some 185 Ma the onset of rifting caused the dispersal of this vast continent into several minor plates. The timing and geometry of the initial break-up between Africa and Antarctica as well as the amount of volcanism connected to this Jurassic rifting are still controversial. In the southern part of the Mozambique Channel a prominent basement high, the Beira High, forms a distinct crustal anomaly along the Mozambican margin. It is still controversial if this area of shallow basement is a continental fragment or was formed during a period of enhanced magmatism and is of oceanic origin. Therefore, a wide-angle seismic profile with 37 OBS/H was acquired starting from the deep Mozambique Channel, across the Beira High and terminating on the shelf off the Zambezi River. The main objectives are to provide constraints on the crustal composition and origin of the Beira High as well as the amount of volcanism and the position of the continent-ocean transition below the Zambezi Delta. To obtain a P-wave velocity model of this area the data were forward modeled by means of the 2D-Raytracing method. Preliminary results indicate a clear thickening of the crust below the Beira High up to 20-24 km. Evidences for a high velocity body are found in the area below the Zambezi shelf with velocities of 7.2-7.4 km/s and up to 5 km thickness. Oceanic basement velocities at the very eastern part of the line start with values of 5.5 km/s, and increase to 6.9 km/s at lower crustal levels, that are typical for Jurassic oceanic crust. Across the Beira High the starting velocity and its gradient slightly change, presenting typical values for continental fragments. However, due to a sparse ray coverage of diving waves for the Beira High lower crust, these velocities still have to be proved. Thus, we will introduce the final results of a Finite Difference amplitude modeling, which will constrain the lowermost velocity gradients to allow a sound interpretation of the Beira High origin. The acquired shipborne, magnetic data show a complex magnetic pattern and strong influences by the presence of lava flows and intrusions and require further investigations. We will introduce the latest results of the joint interpretation of seismic and potential field data sets.

Beschreibung: We study the basement configuration in the slow-spreading Eurasia Basin, Arctic Ocean. Two multichannel seismic (MCS) profiles, which we acquired during ice-free conditions with a 3600 m long streamer, image the transition from the North Barents Sea Margin into the southern Eurasia Basin. The seismic lines resolve the up to 5000 m thick sedimentary section, as well as the crustal architecture of the southern Eurasia Basin along 120 km and 170 km, respectively. The seismic data show large faulted and rotated basement blocks. Gravity modeling indicates a thin basement with a thickness of 1–3 km and a density of 2.8*103 kg/m3 between the base of the sediments and the top of the mantle, which indicates exhumed and serpentinized mantle. The Gakkel spreading ridge, located in northern prolongation of the seismic lines is characterized by an amagmatic or sparsely magmatic segment. From the structural similarity between the basement close to the ultra-slow spreading ridge and our study area, we conclude that the basement in the Eurasia Basin is predominantly formed by exhumed and serpentinized mantle, with magmatic additions. An initial strike-slip movement of the Lomonosov Ridge along the North Barents Sea Margin and subsequent near-orthogonal opening of the Nansen Basin is supposed to have brought mantle material to the surface, which was serpentinized during this process. Continuous spreading thinned the serpentinized mantle and subsequent normal faulting produced distinct basement blocks. We propose that mantle exhumation has likely been active since the opening of the Eurasia Basin.

Beschreibung: Main objective of the project is the investigation of the crustal structure of the margin of Mozambique. This will improve our understanding of the driving forces and processes leading to the initial Gondwana break-up. Some 185 Ma the onset of rifting caused of the opening of the Mozambique and Somali Basin and the dispersal of this vast continent into several minor plates. The timing and geometry of the initial break-up between Africa and Antarctica as well as the amount of volcanism connected to this Jurassic rifting are still controversial. However, the conjugated margin in the Riiser-Larsen Sea is covered by an up to 400 m thick ice cap, precluding the set-up of a deep seismic experiment in this area. Consequently, the investigations focus on the continental margin of central Mozambique. Here, a prominent basement high, the Beira High, forms a critical geological feature of uncertain crustal fabric. It is still controversial if this area of shallow basement is a continental fragment or was formed during a period of enhanced magmatism and is of oceanic origin. Therefore, a wide-angle seismic profile with 37 OBS/H was acquired starting from the deep Mozambique Channel, across the Beira High and terminating on the shelf off the Zambezi River (Fig. 1). The main objectives are to provide constraints on the crustal composition and origin of the Beira High as well as the amount of volcanism and the position of the continent-ocean transition along the margin of central Mozambique. To obtain a P-wave velocity model of this area the data were forward modelled by means of the 2D-Raytracing method, supported by an amplitude and gravity modelling. In the Mozambique Basin mainly normal oceanic crust of 5.5–7 km thickness with velocities of 6.5–7.0 km/s in the lower crust is present (Fig. 2). A sharp transition towards Beira High marks the continent-ocean boundary. Here the crust thickens to 23 km at maximum. A small velocity-depth gradient and a constant increase in velocity with basal velocities of maximum 7.0 km/s are in good agreement with typical velocities of continental crust and continental fragments. The density model indicates the existence of felsic material in greater depths and supports a fabric of stretched, but highly intruded continental crust below Beira High. A gradual decrease in crustal thickness characterizes the transition towards the Mozambican shelf area. Here, in the Zambezi Delta Depression 11 km of sediments cover the underlying 7 km thick crust. The presence of a high-velocity lower crustal body with velocities of 7.1–7.4 km/s indicates underplated, magmatic material in this part of the profile. However, the velocity structure in the shelf area allows no definite interpretation because of the experimental setup. Thus, the crustal nature below the Zambezi Delta remains unknown. The difference in stretching below the margins of Beira High suggests the presence of different thinning directions and a rift jump during the early rifting stage. The acquired shipborne magnetic data complement our dataset in the Mozambique Basin and reveal clear evidence for the presence of lava flows and intrusions, pointing to an increased break-up related magmatism.

Beschreibung: Up to Jurassic times the Antarctic and African continents were part of the supercontinent Gondwana. Since some 185 Ma the rifting in our research area caused the dispersal of Gondwana and Eastern Africa. The timing and geometry of the break-up as well as the amount of volcanism connected to the Jurassic rifting are still controversial. In the southern part of the Mozambique channel a prominent basement high, the Beira High, forms a specific crustal anomaly along the margin. It is still controversial if this high is a continental fragment or was formed during a period of enhanced magmatism. Therefore a deep seismic profile was acquired from the deep Mozambique Channel, across the Beira High and terminating on the shelf. The main objectives are to provide constraints on the crustal composition and origin of the Beira High as well as the amount of volcanism and the continent-ocean transition below the Zambezi Delta. To obtain a P-wave velocity model of this area the data was forward modeled by means of 2D-Raytracing. Furthermore, potential field data acquired in parallel to the seismic data were used to calculate a 2D gravity model. Preliminary results indicate a 20-24 km thick crust for the Beira High. In good agreement to the adjacent oceanic crust in the Mozambique Channel the upper crust has velocities between 5.5-5.9 km/s. The middle crust is characterized by velocities between 6.2-6.7 km/s and the lower crust higher than 6.7 km/s and a density of 3.0 g/cm3. However, the velocities for lower crust are only constrained by Moho reflections, since no diving waves are observed for this layer. In the area of the Zambezi Delta Depression the top of the acoustic basement is at 11.5 km depth and the crust thickness thins to 7 km. The basement here is overlain by a 2 km thick layer of 4.9-5.1 km/s, which we interpret as pre-rift sediments (Karoo-Belo-Group, including Lava Flows). Furthermore, evidence for the presence of a high velocity body (HVB) at below the western part of Beira High with a velocity of 7.2-7.4 km/s and 3 km thickness is found. Below the shelf our results indicate evidences for an increased volcanism during the initial break-up. The location of the continent-ocean boundary as well as the geometry of the break-up depend strongly on the tectonic classification of Beira High. Future work will provide further constraints by amplitude modelling, a 3D gravity model of Beira High and by means of interpretation of the magnetic anomalies.

Beschreibung: The rifting of Gondwana started some 180 million years ago. The continental drift created some of the oldest ocean basins along Eastern Africa, the Somali and the Mozambique basins. As a consequence of the relative movements between Africa and Antarctica-India-Madagascar a shear margin developed along the present day coastline of northern Mozambique and Tanzania. In addition, the N-S oriented offshore Davie Ridge is believed to have formed during the shear movements between both parts of Gondwana. However, if the Davie Ridge is of continental origin or has been formed by magmatic processes during the continental drift is unknown, since any crustal information is missing so far. Previous studies in this area are rare and only few seismic reflection data sets from the 1970s and 1980s are available. In 2014 four seismic refraction data along east-west-orientated profiles as well as gravity and magnetic field data across the Davie Ridge with RV Sonne were collected to determine its crustal composition as well as the position of the continent-ocean-transition. Here, we present a first P-wave velocity model across the Mozambican sheared margin at 13º S. The profile is situated in a region, where the ridge topography vanishes. In total, 20 OBS/OBH systems were used on profile 20140130 over the Davie Ridge. Most of the instruments recorded data with a very good quality. In the best records, P-wave phases can be observed at a source-receiver offset of 130 km. The total thickness of the sediments is about 5 km in the Comores Basin and about 3 km offshore Mozambique. The sediments show at 3.5 and 5 km depth unusual high seismic velocities of 4.0-4.6 km/s. Our results indicate a shallow Moho close to the shelf break. Here, the crust thins to 4 km. This area is assumed to be the western part of the Davie-Ridge and might represent a sharp transition (50 km) from continental to oceanic crust, which is typical for a sheared margin. East of the Davie Ridge the data indicate a crustal thickness of 6 km, which is most likely of oceanic origin.