Beschreibung: High-velocity lower crust (HVLC) and seawarddipping reflector (SDR) sequences are typical features of volcanic rifted margins. However, the nature and origin of HVLC is under discussion. Here we provide a comprehensive analysis of deep crustal structures in the southern segment of the South Atlantic and an assessment of HVLC along the margins. Two new seismic refraction lines off South America fill a gap in the data coverage and together with five existing velocity models allow for a detailed investigation of the lower crustal properties on both margins. An important finding is the major asymmetry in volumes of HVLC on the conjugate margins. The seismic refraction lines across the South African margin reveal cross-sectional areas of HVLC 4 times larger than at the South American margin, a finding that is opposite to the asymmetric distribution of the flood basalts in the Paraná–Etendeka Large Igneous Province. Also, the position of the HVLC with respect to the SDR sequences varies consistently along both margins. Close to the Falkland–Agulhas Fracture Zone in the south, a small body of HVLC is not accompanied by SDRs. In the central portion of both margins, the HVLC is below the inner SDR wedges while in the northern area, closer to the Rio Grande Rise-Walvis Ridge, large volumes of HVLC extend far seaward of the inner SDRs. This challenges the concept of a simple extrusive/intrusive relationship between SDR sequences and HVLC, and it provides evidence for formation of the HVLC at different times during the rifting and breakup process. We suggest that the drastically different HVLC volumes are caused by asymmetric rifting in a simple-shear-dominated extension.

Beschreibung: The interpretation of seismic refraction and gravity data acquired in 2010 gives new insights into the crustal structure of the West Greenland coast and the adjacent deep central Baffin Bay basin. Underneath Melville Bay, the depth of the Moho varies between 26 to 17 km. Stretched continental crust with a thickness of 25 to 14 km and deep sedimentary basins are present in this area. The deep Melville Bay Graben contains an up to ~11km thick infill of consolidated and unconsolidated sediments with velocities of 1.6 to 4.9 km/s. Seawards, at the ~60 km wide transition between oceanic and stretched continental crust, a mount-shaped magmatic structure is observed, which most likely formed prior to the initial formation of oceanic crust. The up to 4 km high magmatic structure is underlain by a ~2 km thick and ~50 km wide high velocity lower crust. More to the west, in the oceanic part of the Baffin Bay basin, we identify a 2-layered, 3.5 to 6 km thin igneous oceanic crust with increasing thickness toward the shelf. Beneath the oceanic crust, the depth of the Moho ranges between 11.5 and 13.5 km. In the western part of the profile, oceanic layer 3 is unusually thin (~1.5 km) A possible explanation for the thin crust is accretion due to slow spreading, although the basement is notably smooth compared to the basement of other regions formed by ultra-slow spreading. The oceanic crust is underlain by partly serpentinized upper mantle with velocities of 7.6 to 7.8 km/s.

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 crustal nature and tectonic development of the Baffin Bay and Davis Strait were enigmatic for a long time due to the lack of unequivocal data. Although it was proposed in earlier studies that oceanic crust underlies the Baffin Bay, no clear magnetic spreading anomalies were detected. Stretched continental crust underlying the basin could be another possibility. The nature of the Davis Strait crust has been discussed as being of continental or oceanic origin. In 2008 and 2010, we collected new geophysical data in the Davis Strait and Baffin Bay as part of a German-Danish-Canadian cooperation project. The aim of this study is to reveal the tectonic reconstruction of the Canada-Greenland separation in the southern Baffin Bay and to provide new insight into the role of the Davis Strait as a polar ocean gateway. We present a 710-km-long crustal model in southern Baffin Bay and a 315-km-long model in the central Davis Strait. We developed P-wave velocity models from ocean-bottom seismograph data and corresponding density models from free-air gravity data. Additional seismic reflection and magnetic anomaly data were evaluated. We find oceanic crust in southern Baffin Bay with an average thickness of 7.5 km. The margins exhibit large volcanic affinity. The Davis Strait crust consists mainly of continental blocks that are divided by a 45-km-long section of highly intruded or new igneous crust. This section coincides with the location of the Ungava Fault Complex. With these new data we developed a new tectonic model and conclude that the Ungava Fault Complex acted as a plate boundary in pre-Eocene times. With a direction change of plate motion during the opening, the Hudson Fracture Zone developed with major strike-slip motion and acted as subsequent boundary. We further compiled published and new seismic stratigraphy data with drill site information and calculated palaeobathymetric grids for the ocean gateway between southern Baffin Bay and northern Labrador Sea. The grids reveal that a water transport between the Labrador Sea and Baffin Bay was not possible in pre-Eocene times. A cyclonic current similar to today probably existed in the early Labrador Sea since the Paleocene. Our palaeobathymetric reconstruction can be used in global palaeocean and palaeoclimate models.

Beschreibung: The Baffin Bay between Greenland and Baffin Island (Canada) opened during the separation of Greenland and Canada in the Palaeocene and Eocene. The Melville Bay is situated in its northeastern part. The crustal composition of Northern and Southern Baffin Bay has been studied in detail: Southern Baffin Bay is underlain by oceanic crust with volcanic margins, while the margins of northern Baffin Bay are characterized by serpentinized mantle material. In contrast, the nature of crust in the deep, central Baffin Bay and the Melville Bay was still unclear due to a lack of deep seismic sounding lines. In 2010 a joint geophysical experiment in the Greenlandic part of Baffin Bay acquired seismic, magnetic and gravity data. We present three velocity and density models derived from seismic refraction and gravity data. Two of the three profiles are located within the Melville Bay and extend in a SW - NE direction from the deep sea area of central Baffin Bay to the shelf area of the Melville Bay. The third profile crosses the northern profile in the Melville Bay and extends in a N - S direction into the Northern Baffin Bay. The profiles in the Melville Bay can be divided in three crustal sections. The deep-sea area reveals a 3.5 - 7 km thick, 2-layered oceanic crust with increasing thickness towards the shelf and up to 6 km thick sediments. The crust is underlain by serpentinized upper mantle with velocities of 7.6 - 7.8 kms-1. A transition zone, which is affected by volcanism, connects the oceanic crust with stretched continental crust underneath the Melville Bay. Basement highs and deep sediment basins characterize the stretched and rifted continental crust. The Melville Bay Graben, the deepest rift basin in Melville Bay, contains up to 10 km thick, possibly metamorphosed sediments with unusually high velocities of up to 4.9 kms 1. Well-constrained reflections of the crust-mantle boundary can be found in many seismic sections indicating a maximum crustal thickness of ~ 26 km in the northern profile and ~ 32 km in the southern profile. In the southern part of the third, N-S extending profile, a 2-layered oceanic crust is covered by up to 5 km thick sediments. Underneath the shelf edge, the crust thickens towards the north in several steps and reaches a maximum thickness of ~ 40 km. The northern part of the profile is characterized by faulted end eroded basement, which crops out at the seafloor.

Beschreibung: The high velocity lower crust HVLC (Vp 〉 7km/s) together with seaward dipping reflectors (SDRs) and continental flood basalts are specific characteristics of volcanic rifted margins. The nature and origin of the HVLC is still under discussion. Here we provide a comprehensive study of the deep crustal structure of the South Atlantic rifted margins in which we focus on the HVLC. We also assess the size and variations along and across the margins. Two new and five existing refraction lines complemented by gravity models cover the area between the Rio Grande Rise - Walvis Ridge to the Falkland Agulhas Fracture Zone. Three seismic lines on the South American margin outline the change from a non-magmatic margin (lacking seaward dipping reflectors) in the south to a well-developed volcanic rifted margin off Uruguay in the north. While the HVLC exhibit a consistent increase in the cross-sectional area along both margins from South to North, we can observe a major asymmetry across the margins. The African margin reveals about two-three times thicker and four times more voluminous HVLC than the South American margin. The distribution of the HVLC stands in a sharp contrast to the one of Etendeka-Paraná flood basalt provinces, which shows the opposite asymmetry. Also the spatial position of the HVLC with regard to the inner SDRs varies consistently from south-to-north along the margins. A simple extrusive/intrusive relationship SDRs and HVLC is questioned. Further it provides evidence for the formation of the HVLC during different times in the rifting and break-up process. We conclude that the HVLC is predominantly a magmatic feature that is related to break-up. Melt generation scenarios based on variations in thickness and average Vp suggest that the greater thickness of HVLC on the African margin is due to active upwelling combined with elevated mantle potential temperatures while the model predicts passive upwelling and a thick lithospheric lid for the South American HVLC. This contrast in upwelling rate and lithospheric thickness can be explained by a model of asymmetric rifting with a simple shear dominated extension. Our estimates for the volume of HVLC bodies imply a total magma production about 4 x 106 km3 on the rifted margins of the South Atlantic.