No abstract

SUMMARY 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 to 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 to 7.0 km s−1. The upper and middle continental crust thin over a 120‐km‐wide continent–ocean transition 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.

King George Island is the largest of the South Shetland Islands, close to the tip of the Antarctic Peninsula. The annual mean temperature on the island has increased by 1 • • C during the past three decades, and the ice cap that covers the majority of the island is sensitive to climatic change. We present data from two field campaigns (1997 and 2007): 700 km of global positioning system (GPS) and ground-penetrating radar (GPR) profiles were collected on Arctowski Icefield and on the adjacent central part. The data were analysed to determine the surface and bed topography and the thermal regime of the ice. Average ice thickness is 250 m and maximum thickness is 420 m. The GPR profiles show isochrones throughout the ice cap which depict the uparching of Raymond bumps beneath or close to the ice divides. A water table from percolation of meltwater in the snowpack shows the firn-ice boundary at ∼ ∼35 m depth. The firn layer may be temperate due to the release of latent heat. In the area below 400 m a.s.l., backscatter by water inclusions is abundant for ice depths below the watertable. We interpret this as evidence for temperate ice. Scatter decreases significantly above 400 m. Ice temperatures below the water table in this part of the ice cap are subject to further field and modelling investigations.

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 this model 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.

[1] Cold ice within a polythermal ice body controls its flow dynamics through the temperature dependence of viscosity, and affects glacier hydrology by blocking water flow paths. Lakes on the surface, linked by persistent, deeply incised meltwater streams, are hallmark features of cold ice in the ablation zone of a glacier or ice sheet. Ice radar is a convenient method to map scattering from internal water bodies present in ice at the pressure melting temperature (PMT). Consequently, lack of internal scatters is indicative of cold ice. We use a helicopter-borne 30 MHz ice radar to delineate the extent of cold ice within Grenzgletscher (Zermatt, Swiss Alps). The inferred thermal structure is validated with temperature measurements in 15 deep boreholes, showing excellent agreement. The cold ice occupies 80-90 % of the total ice thickness in a 400 m wide flow band along the central flow line. Quantitative interpretation of ice radar scattering power indicates a decrease of ice water content between PMT and 0.5 K below PMT, as predicted by theory, and observed in the laboratory. The cold ice which emerges at the surface in the lower ablation zone is impermeable to water, and is thus devoid of moulins if not crevassed. The surface water from melt and rain is routed through deeply incised, persistent streams and lakes, and cryoconite holes are frequent, in stark contrast to the adjacent temperate ice from other tributaries. The cold ice thus has a strong control on glacier hydrology, but is likely to change due to continued warming.