The Davis Strait crust - a transform margin between two oceanic basins

The Davis Strait is located between Canada and Greenland and connects the Labrador Sea and the Baffin Bay basins. Both basins formed in Cretaceous to Eocene time and were connected by a transform fault system in the Davis Strait. Whether the crust in the central Davis Strait is oceanic or continental has been disputed. This information is needed to understand the evolution of this transform margin during the separation of the North American plate and Greenland. We here present a 315-km-long east–west-oriented profile that crosses the Davis Strait and two major transform fault systems—the Ungava Fault Complex and the Hudson Fracture Zone. By forward modelling of data from 12 ocean bottom seismographs, we develop a P-wave velocity model.We compare thismodel with a density model from ship-borne gravity data. Seismic reflection and magnetic anomaly data support and complement the interpretation. Most of the crust is covered by basalt flows that indicate extensive volcanism in the Davis Strait. While the upper crust is uniform, the middle and lower crust are characterized by higher P-wave velocities and densities at the location of the Ungava Fault Complex. Here, P-wave velocities of the middle crust are 6.6 km s−1 and of the lower crust are 7.1 km s−1 compared to 6.3 and 6.8 km s−1 outside this area; densities are 2850 and 3050 kg m−3 compared to 2800 and 2900 kg m−3. We here interpret a 45-km-long section as stretched and intruded crust or as new igneous crust that correlates with oceanic crust in the southern Davis Strait. A high-velocity lower crust (6.9–7.3 km s−1) indicates a high content of mafic material. This mantle-derived material gradually intruded the lower crust of the adjacent continental crust and can be related to the Iceland mantle plume. With plate kinematic modelling, we can demonstrate the importance of two transform fault systems in the Davis Strait: the Ungava Fault Complex with transpression and the Hudson Fracture Zone with pure strike-slip motion. We show that with recent poles of rotation, most of the relative motion between the North American plate and Greenland took place along the Hudson Fracture Zone

Sonnefahrt 230 - MOCOM - Asymmetrisches Aufbrechen von Gondwana im Mosambik Becken

Hauptziel des Projektes ist es den geologischen Bau des Kontinentrandes vor Mosambik zu verstehen um damit einen Beitrag zum Verständnis der Prozesse, die zum Zerbrechen von Gondwana führten, zu liefern. Im fruehen Stadium des Gondwana Zerfalls oeffneten sich sowohl das Mosambik- als auch das Somalibecken. Bisherige geowissenschaftliche Untersuchungen im suedlichen Afrika als auch am konjugierten Kontinentalrand in der Antarktis lieferten deutliche Hinweise, dass das Aufbrechen mit massiven Vulkanismus verbunden war und nicht symmetrisch verlief. Allerdings ist der konjugierende Kontinentrand in der Riiser Larsen See (Antarktis) von bis zu 400 m dicken Eis bedeckt, so dass hier eine Untersuchung mit tiefenseismischen Experimenten nicht möglich ist. Daher konzentriert sich dieses Vorhaben auf den Kontinentrand vor Mosambik, der die Position der initialen Riftbecken markiert. Im ersten Teil dieser Fahrt stand der Kontinentalrand vor Zentralmosambik im Fokus. Zur Ergaenzung der bereits gesammelten Daten in diesem Gebiet (2005 AISTEK II, 2007 MoBaMaSis) wurde ein refraktionsseismisches Profil (20140010) mit 37 OBS/H erhoben. Es verläuft vom Mosambikbecken ueber das Beira High hin auf den Schelf. Das Profil soll Hinweise speziell ueber die Struktur und den Ursprung des Beira Highs liefern, sowie weitere Erkenntnisse liefern ueber die Krustenstruktur des Sambesi Deltas. Ergaenzend zu den seismischen Daten wurden Gravimetrie-, Magnetik und Bathymetrie-Daten entlang des Kontinentalrandes gesammelt. Vorlaeufige Ergebnisse deuten auf eine verdickte Kruste zwischen 20 bis 24 km Maechtigkeit im Bereich des Beira Highs hin. In Uebereinstimmung mit den benachbarten Profilabschnitten weisst die obere Kruste Geschwindigkeiten zwischen 5.3 -5.9 km/s auf. Der bestimmte Geschwindigkeitsverlauf von 6.2 – 7.0 km/s in den tieferen Krustenabschnitten des Beira Highs laesst noch keine endgueltige tektonische Einordnung zu. Grund hierfuer ist unter anderem die geringe Strahlabdeckung innerhalb der unteren Kruste. Hier wird die nachfolgende Potentialfeldmodellierung weiter Hinweise liefern. Im Bereich der Zambezi Delta Depression verlaeuft das Basement in rund 9,5 km Tiefe. Die Kruste verduennt sich hier auf etwa 10 km Maechtigkeit und zeigt einen fuer ozeanische Kruste typischen Geschwindigkeitsverlauf. Hinweise fuer einen verstaerkten Vulkanismus waehrend des Aufbruchs zeigen sich sowohl durch eine etwa 2-3 km dicke Schicht aus Lava Flows mit Geschwindigkeiten um 4,9 – 5,1 km/s und durch einen Hochgeschwindigkeitskoerper mit 7,2 – 7,4 km/s und 3 km Maechtigkeit. Die laterale Ausdehnung dieses Koerpers sowie die Lokalisierung des Kontinent-Ozean-Uebergangs werden ebenfalls mit Hilfe der nachfolgenden Schweremodellierung bestimmt

The crustal structure of Beira High, Central Mozambique – Combined investigation of wide-angle seismic and potential field data

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

The crustal structure of southern Baffin Bay: implications from a seismic refraction experiment

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 - 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 - 7.0 km s−1. The upper and middle continental crust thin over a 120-km-wide continent-ocean transi- tion 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

The crustal structure of Beira High, central Mozambique—Combined investigation of wide-angle seismic and potential field data

The timing and geometry of the initial Gondwana break-up between Africa and East Antarctica is still poorly known due to missing information about the continent-ocean boundaries along the rifted margins. In this context, the Beira High off central Mozambique forms a critical geological feature of uncertain crustal fabric. Based on new wide-angle seismic and potential field data across Beira High a P-wave velocity model, supported by amplitude and gravity modelling, provides constraints on the crustal composition of this area. 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. 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 felsicmaterial 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 12 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 and consequently the landward position of the continentocean boundary 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

SO230 - MOCOM project - The crustal structure of Beira High, Central Mozambique - Combined investigation of wide-angle seismic and potential field data

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