ALBERT

Description: Seismic data was acquired to study the transition from rifted continental crust to oceanic crust at the Deep Galicia Margin from June to August 2013. 3D Multichannel reflection and coincident wide-angle seismic data were acquired simultaneously as part of a seismic experiment over an area of 80 km long and 25 km wide in the Deep Galicia margin. The multichannel reflection seismic volume was acquired by the R/V Marcus G. Langseth, which provided a source for the ocean bottom seismic data. A total of 86 ocean bottom hydrophones/seismometer deployments were carried out by F/S Poseidon. Two airgun arrays with total gun volumes of 3,300 cu.in. were deployed as seismic sources. Shots were fired alternately using two source arrays every 37.5 m (shot interval of ~ 16 s with ship speed of 4.5 knots). Data were converted into SEGY format. Further details are available at https://doi.org/10.1038/NGEO2671.

Description: Raw multibeam bathymetry data were recorded on RV Meteor during M62/4 using an Atlas Hydrosweep DS2 multibeam echosounder. The cruise took place between 2004-09-30 (Mindelo) – 2004-11-04 (Recife) in the Southern Atlantic. Data were recorded in international waters outside EEZs as well as within the Ascension Island EEZ with an approximate average depth of around 1500m. There is no information about sound velocity correction made. There are no extra SVP files provided in this publication. Data are unprocessed and can therefore contain incorrect depth measurements (artifacts) if not further processed. Note that refraction errors can be expected due to the lack of proper SVP. The aims of M62-4 were to investigate the tectonics and structure of the Ascension transform inside-outside corner pair and to constrain the processes of segmentation occurring at the Mid-Atlantic Ridge just south of the Ascension FZ. The data are archived at the Federal Maritime and Hydrographic Agency of Germany (Bundesamt für Seeschifffahrt und Hydrographie, BSH) and provided to PANGAEA database for data curation and publication. Data publication is accomplished within work package 2 of the EU Horizon 2020 project iAtlantic- Integrated Assessment of Atlantic Marine Ecosystem in Space and Time (https://www.iatlantic.eu/).

Description: Seismic data was acquired to study the transition from rifted continental crust to oceanic crust at the Deep Galicia Margin from June to August 2013. 3D Multichannel reflection and coincident wide-angle seismic data were acquired simultaneously as part of a seismic experiment over an area of 80 km long and 25 km wide in the Deep Galicia margin. The multichannel reflection seismic volume was acquired by the R/V Marcus G. Langseth, which provided a source for the ocean bottom seismic data. A total of 86 ocean bottom hydrophones/seismometer deployments were carried out by F/S Poseidon. Two airgun arrays with total gun volumes of 3,300 cu.in. were deployed as seismic sources. Shots were fired alternately using two source arrays every 37.5 m (shot interval of ~ 16 s with ship speed of 4.5 knots). Data were converted into SEGY format. Further details are available at https://doi.org/10.1038/NGEO2671.

Description: Seismic data was acquired to study the transition from rifted continental crust to oceanic crust at the Deep Galicia Margin from June to August 2013. 3D Multichannel reflection and coincident wide-angle seismic data were acquired simultaneously as part of a seismic experiment over an area of 80 km long and 25 km wide in the Deep Galicia margin. The multichannel reflection seismic volume was acquired by the R/V Marcus G. Langseth, which provided a source for the ocean bottom seismic data. A total of 86 ocean bottom hydrophones/seismometer deployments were carried out by F/S Poseidon. Two airgun arrays with total gun volumes of 3,300 cu.in. were deployed as seismic sources. Shots were fired alternately using two source arrays every 37.5 m (shot interval of ~ 16 s with ship speed of 4.5 knots). Data were converted into SEGY format. Further details are available at https://doi.org/10.1038/NGEO2671.

Description: The dataset contains the raw .segy files of the ocean bottom seismometers/hydrophones (OBS/H) that recorded wide-angle seismic data along 6 profiles in the Porcupine Basin. The active-source seismic survey was conducted by GEOMAR in 2004. The cruise report, navigation files for each profile, and geographical coordinates of each OBS/H are also included in this dataset.

Description: Seismic data was acquired to study the transition from rifted continental crust to oceanic crust at the Deep Galicia Margin from June to August 2013. 3D Multichannel reflection and coincident wide-angle seismic data were acquired simultaneously as part of a seismic experiment over an area of 80 km long and 25 km wide in the Deep Galicia margin. The multichannel reflection seismic volume was acquired by the R/V Marcus G. Langseth, which provided a source for the ocean bottom seismic data. A total of 86 ocean bottom hydrophones/seismometer deployments were carried out by F/S Poseidon. Two airgun arrays with total gun volumes of 3,300 cu.in. were deployed as seismic sources. Shots were fired alternately using two source arrays every 37.5 m (shot interval of ~ 16 s with ship speed of 4.5 knots). Data were converted into SEGY format. Further details are available at https://doi.org/10.1038/NGEO2671.

Description: R/V METEOR Cruise No. 61 was divided into three different legs, which all focused on the NEAtlantic to the west of Ireland from the Porcupine Seabight towards the Rockall Bank. Legs 1 and 3 concentrated on geo-biological studies on the carbonate mounds in this region, which are covered by a unique cold water coral fauna. Leg 2 dealt with seismic investigations in order to investigate the extension processes that led to the development of the Porcupine rift basin. The foci of the individual legs were on the following themes. M61-1 was a multidisciplinary cruise addressing biological, paleo-geological and hydrographical scientific objectives in the carbonate mound provinces west of Ireland in the eastern Porcupine Seabight and on the Rockall Bank. The cruise started in Lisbon (Portugal) and ended in Cork (Ireland). M61-1 activities were embedded within the ESF-DFG MOUNDFORCE project of the EUROMARGINS Programme. Together with the succeeding M61-3 cruise, these Meteor activities document Germany´s strong scientific and logistic support for the success of this challenging programme. Investigations are also designed as a preparatory cruise for the EUproject HERMES (Hotspot Ecosystem Research on the Margins of European Seas; start April 2005). All institutions participating in M61-1 are partners in HERMES Work package 2 "Coral Reef and Carbonate Mound Systems". M 61-2 was directed at researching the earth's crust in the vicinity of the Porcupine rift basin. During this leg, seismic research has been undertaken in the Porcupine Basin west of Ireland, an area that represents a natural laboratory for the investigation of extensional processes. Firstly, both sides of a rift basin occurring in close proximity to each other could have been studied here, allowing questions about the symmetry of extension to be addressed by several east-west profiles parallel to the direction of extension. Secondly, the amount of extension increases from north to south, so a series of east-west cross sections on different latitudes has provided information on crustal structure during variable extension. The spatial changes between these sections also represent the temporal development of the rift through continued extension. In order to achieve these research goals, a series of east-west oriented wide angle reflection profiles in the Porcupine Basin has been acquired. These profiles aid in the explanation of extensional processes and their development through continued extension. They also address insufficiently explained questions about the initiation of large scale magmatism and intrusion, the onset of mantle serpentinisation and the development of detachment faults. M61-3 During this leg, the only recently discovered 'carbonate mounds' on the NWEuropean continental margin have been investigated, which represent unique geo- and ecosystems for European waters. The broad scientific interest that is directed at these mounds is reflected in three EU-projects, which until recently almost exclusively concentrated their efforts on the mounds, as well as the currently operating ESF-EUROMARGINS project MOUNDFORCE M 61-3 focused on the use of a 'Remotely Operated Vehicle' (ROV) for the investigation of the carbonate mounds. The primary tasks of Bremen's QUEST ROV were a detailed characterization of individual mound structures, selective sample collection and the retrieval of sensor systems placed at the seafloor one year before. These ROV tasks have been supplemented by hydro-acoustic measurements and conventional sediment sampling in order to work - in close collaboration with M61-1 - on the main research focuses of the MOUNDFORCE project: (a) analysis of the environmental factors that drive the development of the 'carbonate mounds', (b) surveying the benthic communities in dependence of changing environmental factors and (c) investigations to the stabilization and lithification of the mound sediments.

Description: We present depth images, from portions of profiles that are close to flow-lines, of Cretaceous oceanic crust in the eastern Central Atlantic. Compared with post-stack time migrations, the images illustrate the improvement resulting from the application of pre-stack depth migration. The images document the scale and geometry of normal faulting in oceanic crust formed over 25 Myr at a half-spreading rate of less than 10 mm yr−1, and the variation in extensional style with position within the spreading segment. Away from major fault zones (FZs), most faults are subplanar, dip more than 35°, are associated with moderate basement relief (0.2–1 km relief) and may penetrate to deep crustal levels. These faults could be related to the lifting of the lithosphere out of the median valley to the flanking mountains. Also observed away from FZs are gently dipping to subhorizontal reflections in the upper crust, which resemble detachment faults. In contrast, the inside corner crust is more rugged, with basement highs rising up to 2 km above the intervening basins. This larger-scale topography is associated with a different style of faulting: the depth images reveal gently dipping (〈35°) faults that are rooted in the deep crust and that project to the ridgeward flank of the dome-shaped large basement highs (1–2 km vertical relief). The faults seem to continue as the ridge-facing flank of these highs and some may extend over the crest of the high to breakaways beyond. In this case, the domal highs that form the exhumed footwall to the faults can be described as oceanic core complexes. These controlling faults are up to 20 km long and have a heave of ∼10 km, sufficient to have accommodated up to 50 per cent extension and to have exhumed deep crustal and perhaps even mantle rocks. We suggest that similar faults can explain the structure and lithologies found at megamullion structures (oceanic core complexes) at inside corners near the present-day spreading ridge.

Description: We present results of a seismic refraction experiment which determines the crustal and upper-mantle structure of an oceanic core complex (OCC) and its conjugate side located south of the 5°S ridge–transform intersection at the Mid-Atlantic Ridge. The core complex with a corrugated surface has been split by a change in location of active seafloor spreading, resulting in two massifs on either side of the current spreading axis. We applied a joint tomographic inversion of wide-angle reflected and refracted phases for five intersecting seismic profiles. The obtained velocity models are used to constrain the magmatic evolution of the core complex from the analysis of seismic layer 3 and crustal thickness. An abrupt increase of crustal velocities at shallow depth coincides with the onset of the seafloor corrugations at the exposed footwall. The observed velocity structure is consistent with the presence of gabbros directly beneath the corrugated fault surface. The thickness of the high-velocity body is constrained by PmP reflections to vary along and across axis between 〈3 and 5 km. The thickest crust is associated with the central phase of detachment faulting at the higher-elevated northern portion of the massif. Beneath the breakaway of the OCC the crust is 2.5 km thick and reveals significantly lower velocities. This implies that the fault initially exhumed low-velocity material overlying the gabbro plutons. In contrast, crust formed at the conjugate side during OCC formation is characterized by an up to 2-km-thick seismic layer 2 overlying a 1.7-km-thick seismic layer 3. Obtained upper-mantle velocities range from 7.3 to 7.9 km s−1 and seem to increase with distance from the median valley. However, velocities of 7.3–7.5 km s−1 beneath the older portions of the OCC may derive from deep fluid circulation and related hydrothermal alteration, which may likely be facilitated by the subsequent rifting. Our velocity models reveal a strongly asymmetric velocity structure across the ridge axis, associated with the accretion of gabbros into the footwall of the detachment fault and upper-crustal portions concentrated at the conjugate side. Our results do not support a substantial increase in the axial ridge's melt supply related to the final phase of detachment faulting. Hence, the footwall rifting at 5°S may be a generic mechanism of detachment termination under very low melt conditions, as predicted by recent numerical models of Tucholke et al.

Description: Hyperextension of continental crust at the Deep Galicia rifted margin in the North Atlantic has been accommodated by the rotation of continental fault blocks, which are underlain by the S reflector, an interpreted detachment fault, along which exhumed and serpentinized mantle peridotite is observed. West of these features, the enigmatic Peridotite Ridge has been inferred to delimit the western extent of the continent-ocean transition. An outstanding question at this margin is where oceanic crust begins, with little existing data to constrain this boundary and a lack of clear seafloor spreading magnetic anomalies. Here we present results from a 160 km long wide-angle seismic profile (Western Extension 1). Travel time tomography models of the crustal compressional velocity structure reveal highly thinned and rotated crustal blocks separated from the underlying mantle by the S reflector. The S reflector correlates with the 6.0–7.0 km s−1 velocity contours, corresponding to peridotite serpentinization of 60–30%, respectively. West of the Peridotite Ridge, shallow and sparse Moho reflections indicate the earliest formation of an anomalously thin oceanic crustal layer, which increases in thickness from ~0.5 km at ~20 km west of the Peridotite Ridge to ~1.5 km, 35 km further west. P wave velocities increase smoothly and rapidly below top basement, to a depth of 2.8–3.5 km, with an average velocity gradient of 1.0 s−1. Below this, velocities slowly increase toward typical mantle velocities. Such a downward increase into mantle velocities is interpreted as decreasing serpentinization of mantle rock with depth.