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

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 evolution of Davis Strait, a polar gateway – seismic data and tectonic modelling

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

The separation of North America and Greenland: tectonic and palaeobathymetric reconstruction of southern Baffin Bay and Davis Strait

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

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