Seismic structure of the volcanic apron north of Gran Canaria

High-resolution reflection seismic profiles through the volcanic apron north of Gran Canaria collected during Meteor Cruise 24 were interpreted in the light of results from Leg 157 (Sites 953 and 954). The shape of the submarine island flanks of Gran Canaria and the two adjacent islands of Fuerteventura to the east and Tenerife to the west were reconstructed by interpretating seismic profiles that penetrated the sediments covering the deeper portions of the volcanic pedestals. The ~4750-m-deep flank of Fuerteventura is the oldest submarine island flank, influencing the subsequent shield-building of Gran Canaria to the east, whose 16- to 15-Ma shield is ponded against Fuerteventura, forming a topographic barrier between the islands. The associated reduction of the current cross section has caused strong bottom currents, indicated by erosional features and contourites. To the north, the flank of Gran Canaria extends 60 km seaward to a depth of ~4500 m. The shield of the Anaga massif on northeast Tenerife onlaps the flank of Gran Canaria to the east. Seismic correlation of the feathered edge of the Anaga shield (~50 km off Tenerife at a depth of 4000 m) to the bio- and magnetostratigraphy at Site 953 results in an age of ~6 Ma. The surrounding sedimentary basin is characterized by chaotic and discontinuous reflection patterns of the slope facies, turning into well-stratified basin facies ~30–40 km off the coast. The westward decrease of reflectivity in the northern apron is interpreted to be caused by the submarine ridge off Galdar at the western limit of the north coast of Gran Canaria, through which mass flows from Gran Canaria entering the sea in the north were diverted to the northeastern part of the apron. The volcanic activity correlates with the sedimentation rates in the apron. The lowest rate corresponds to the volcanic hiatus on Gran Canaria (9–5 Ma) with 3–4 cm/k.y., and the highest rate (up to 12 cm/k.y.) was found during the voluminous Miocene volcanism on the island. A number of large mass-wasting events could be identified, interbedded with the pelagic background sedimentation. The basaltic breccia drilled at Site 954 (lithologic Unit IV) is interpreted to represent the deposits associated with a slope failure at the northern flank of Gran Canaria at 12 Ma. The seismic mapping reveals >60 km3 of debris advanced at least 70 km into the apron. The volume fits well with the dimensions of an amphitheater at the northern flank of Gran Canaria. The Quaternary volcanism on La Isleta at northeast Gran Canaria extends further seaward, where the seismic data show young lava flows. Other submarine volcanism occurred in the channel between Gran Canaria and Fuerteventura

Comparison of seismic reflection data to a synthetic seismogram in a volcanic apron at Site 953

The volcanic apron of Gran Canaria at Site 953 is characterized by numerous, closely spaced reflectors, allowing a highresolution stratigraphic correlation. The calibration of the presite survey seismic data (during the Meteor Cruise 24) with regard to the lithology and stratigraphy found at the drill site was achieved by computing a synthetic seismogram serving as the link between seismic and borehole data. Because logging data were available for only 53% of the hole, velocity and density measurements taken from the recovered cores were used in the missing intervals to obtain a complete synthetic seismogram. Most reflectors in the upper ~900 m of the sequence (lithologic Units I–V) turned out to be thin volcaniclastic layers intercalated to the nonvolcanic background sediments. Their thicknesses are generally <2 m, and the reflections from their tops and bases overlap, forming a single reflection. The limit of the seismic detection of such interbeds is on the order of several decimeters and thus requires special care for the processing of the velocity and density data to avoid destruction of the signal from these thin layers

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

Effect of C282Y genotype on self-reported musculoskeletal complications in hereditary hemochromatosis

Objective Arthropathy that mimics osteoarthritis (OA) and osteoporosis (OP) is considered a complication of hereditary hemochromatosis (HH). We have limited data comparing OA and OP prevalence among HH patients with different hemochromatosis type 1 (HFE) genotypes. We investigated the prevalence of OA and OP in patients with HH by C282Y homozygosity and compound heterozygosity (C282Y/H63D) genotype. Methods A total of 306 patients with HH completed a questionnaire. Clinical and demographic characteristics and presence of OA, OP and related complications were compared by genotype, adjusting for age, sex, body mass index (BMI), current smoking and menopausal status. Results In total, 266 of the 306 patients (87%) were homozygous for C282Y, and 40 (13%) were compound heterozygous. The 2 groups did not differ by median age [60 (interquartile range [IQR] 53 to 68) vs. 61 (55 to 67) years, P=0.8], sex (female: 48.8% vs. 37.5%, P=0.18) or current smoking habits (12.4% vs. 10%, P=0.3). As compared with compound heterozygous patients, C282Y homozygous patients had higher median serum ferritin concentration at diagnosis [1090 (IQR 610 to 2210) vs. 603 (362 to 950) mu g/L, P<0.001], higher median transferrin saturation [80% (IQR 66 to 91%) vs. 63% (55 to 72%), P<0.001]) and lower median BMI [24.8 (22.1 to 26.9) vs. 26.2 (23.5 to 30.3) kg/m2, P<0.003]. The overall prevalence of self-reported OA was significantly higher with C282Y homozygosity than compound heterozygosity (53.4% vs. 32.5%; adjusted odds ratio [aOR] 2.4 [95% confidence interval 1.2-5.0]), as was self-reported OP (25.6% vs. 7.5%; aOR 3.5 [1.1-12.1]). Conclusion Patients with C282Y homozygosity may be at increased risk of musculoskeletal complications of HH.Association Rhumatisme et Travail (Centre Viggo Petersen, Hospital Lariboisiere, Paris

A review of the NE Atlantic conjugate margins based on seismic refraction data

The NE Atlantic region evolved through several rift episodes, leading to break-up in the Eocene that was associated with voluminous magmatism along the conjugate margins of East Greenland and NW Europe. Existing seismic refraction data provide good constraints on the overall tectonic development of the margins, despite data gaps at the NE Greenland shear margin and the southern Jan Mayen microcontinent. The maximum thickness of the initial oceanic crust is 40 km at the Greenland–Iceland–Faroe Ridge, but decreases with increasing distance to the Iceland plume. High-velocity lower crust interpreted as magmatic underplating or sill intrusions is observed along most margins but disappears north of the East Greenland Ridge and the Lofoten margin, with the exception of the Vestbakken Volcanic Province at the SW Barents Sea margin. South of the narrow Lofoten margin, the European side is characterized by wide margins. The opposite trend is seen in Greenland, with a wide margin in the NE and narrow margins elsewhere. The thin crust beneath the basins is generally underlain by rocks with velocities of >7 km s−1 interpreted as serpentinized mantle in the Porcupine and southern Rockall basins; while off Norway, alternative interpretations such as eclogite bodies and underplating are also discussed

Moho and basement depth in the NE Atlantic Ocean based on seismic refraction data and receiver functions

Seismic refraction data and results from receiver functions were used to compile the depth to the basement and Moho in the NE Atlantic Ocean. For interpolation between the unevenly spaced data points, the kriging technique was used. Free-air gravity data were used as constraints in the kriging process for the basement. That way, structures with little or no seismic coverage are still presented on the basement map, in particular the basins off East Greenland. The rift basins off NW Europe are mapped as a continuous zone with basement depths of between 5 and 15 km. Maximum basement depths off NE Greenland are 8 km, but these are probably underestimated. Plate reconstructions for Chron C24 (c. 54 Ma) suggest that the poorly known Ammassalik Basin off SE Greenland may correlate with the northern termination of the Hatton Basin at the conjugate margin. The most prominent feature on the Moho map is the Greenland–Iceland–Faroe Ridge, with Moho depths >28 km. Crustal thickness is compiled from the Moho and basement depths. The oceanic crust displays an increased thickness close to the volcanic margins affected by the Iceland plume