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: 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.

Beschreibung: 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.

Beschreibung: 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.

Beschreibung: Wide-angle reflection/refraction seismic data were acquired on a 450-km-long transect in southern Baffin Bay extending from Baffin Island to Greenland. Dense airgun shots were recorded on 22 ocean bottom seismometers. A P wave velocity model was developed from forward and inverse modeling of the observed travel times. Beneath the Baffin Island shelf, a three-layered continental crust is observed with velocities of 5.5 to 6.9 km/s. Typical for transform margins, there is a sharp transition between continental and oceanic crust. Off Baffin Island, 7-km-thick oceanic crust is interpreted to lie in a major transform fault identified on the gravity map. Beneath the deep Baffin Bay basin, 9-km-thick oceanic crust is encountered but thins to 6 km within an assumed fracture zone. The thicker than normal oceanic crust indicates an ample magma supply, possibly related to melt extracted from a mantle plume. Seaward of the Greenland continental crust, 20-km-thick igneous crust (6.3 to 7.3 km/s) is encountered in a 25 km-wide zone interpreted as a leaky transform fault that can be correlated southward through Davis Strait. The igneous crust is bounded by a 20-km wide basin to the west, underlain by 4-km thick crust of unknown affinity. This structure is probably associated with transform movements. A high-velocity lower crustal layer (7.1 km/s) of 8 km thickness is indicated beneath the Greenland crust and can be correlated into the adjacent thick igneous crust. Both the thick igneous and Greenland crust are covered by up to 4 km of Paleogene volcanics (5.2 to 5.7 km/s).

Beschreibung: Davis Strait is a bathymetric high, located between Canada and Greenland. With a water depth of only 500 m, it acts as a gateway for the exchange of polar water from Baffin Bay in the north to the Labrador Sea and the Atlantic in the south. The Davis Strait region has undergone a complex tectonic evolution and the nature of crust is disputed. In a first stage the strait was characterized by extension due to the separation of the North American plate and Greenland. In a second stage transpression was the dominating force. The most prominent geologic feature is the Ungava Fault Complex, a major transform fault that cuts Davis Strait. To investigate the role of Davis Strait as a polar gateway during the opening of the Labrador Sea and Baffin Bay rift system, we analyse recent seismic reflection and refraction data in combination with potential field data. This information is incorporated into a plate tectonic model. On a 230-km-long east west line in central Davis Strait, a P-wave velocity and a density model were obtained by forward modelling. The models show several blocks of continental crust that are separated by major faults of the Ungava Fault Complex. High velocities in the lower crust indicate intense intrusions of mafic material, which we relate to the arrival of the Iceland mantle plume beneath Greenland in the Paleocene. Seismic reflection data were used to model the complex basement morphology and to develop a sediment stratigraphy. The tectonic modelling reveals that an overlap of 70 km of stretched continental crust needs to be compensated for in the transpressional stage of Davis Strait. We are now working on displaying the evolution of Davis Strait from the initial opening of the Labrador Sea to today and will present new results of this study.