Nordost-Atlantik = Northeast Atlantic : Forschungsschiff Meteor, Reise Nr. M61 ; 19.04.2004 - 21.06.2004

Abschnitte, die sich in erster Linie auf das Seegebiet westlich von Irland von der Porcupine Seabight bis zur Rockall Bank konzentrieren. Im Mittelpunkt des ersten und des dritten Abschnittes stehen dabei geo-biologische Untersuchungen an den carbonate mounds“ in diesem Gebiet, die von einer einzigartigen Tiefwasserkorallenfauna bewachsen sind. Der zweite Abschnitt befasst sich mit seismischen Untersuchungen zu den Extensionsprozessen, die zur Entstehung des Porcupine Riftbeckens geführt haben.METEOR Cruise No. 61 is divided into three different legs, which focus on the NE-Atlantic to the west of Ireland from the Porcupine Seabight towards the Rockall Bank. Legs 1 and 3 focus on geo-biological studies on the carbonate mounds in this region, which are covered by a unique cold water coral fauna. Leg 2 deals with seismic investigations in order to investigate the extension processes that led to the development of the Porcupine rift basin

Northeast Atlantic 2004 – Cruise No. M61, April 19 – June 6, 2004 – Lisbon (Portugal) – Ponta Delgada (Azores)

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

Microearthquake seismicity of the Mid-Atlantic Ridge at 5°S: A view of tectonic extension

We report measurements made with an ocean bottom array which was operated for 10 days on the Mid-Atlantic Ridge just south of the 5°S transform fault/fracture zone. A total of 148 locatable earthquakes with magnitudes ∼0.5–2.8 were recorded; seismic activity appears to be concentrated within the western half of the median valley. The median valley seismic zone is bounded in along-axis direction by the transform fault to the north and the tip of the axial volcanic ridge to the south. A few scattered events occurred within the inside corner high, on the transform fault, and in the western sidewall close to the segment center. Earthquakes reach a maximum depth of 8 km below the median valley floor and appear to be predominantly in the mantle, although a few crustal earthquakes also occurred. The presence of earthquakes in the mantle indicates that it is not strongly serpentinized. We infer the median valley seismic activity to primarily arise from normal faulting

Flipping detachments: The kinematics of ultraslow spreading ridges

Although the seafloor spreading Hess initially proposed was a virtually amagmatic process, little attention has been paid to that possibility since. We construct a kinematic framework for virtually amagmatic and magma-poor Hess-style seafloor spreading, and successfully apply it to processes operating at the Southwest Indian Ridge (SWIR). The kinematic model is based on symmetric divergence about a rift axis at depth, with a repeating cycle in which a fault propagates up from the rift axis, develops into a detachment fault accommodating the plate divergence, migrates beyond the rift axis and is abandoned when a new fault propagates up through the footwall from the rift axis. We rigorously explore the controls on the depth, dip and timing of fault initiation and abandonment and use the kinematic framework to reconstruct the evolution of smooth mantle-dominated seafloor at the SWIR through symmetric divergence about a fixed rift axis. The model predicts the development of successive detachments of flipping polarity, as observed, each rooting along a narrow and fixed rift axis at 20 km depth, the base of the seismically defined brittle lithosphere. The detachments root at 80° (consistent with constraints on seismicity-defined detachment orientation at oceanic core complexes), and exhume mantle. Based on the continuity of basement ridges, of magnetic anomalies and of the seismic activity at the base of the lithosphere, it appears that these exhumation detachments transition laterally into rafting detachments, transporting fault-bounded volcanic slices up and away from the spreading axis to form the rougher volcanic seafloor found between mantle-dominated domains. The kinematic framework shows that increased magmatic divergence requires the detachments to root at shallower depths, consistent with the seismicity-defined shallowing of the base of the brittle lithosphere moving along the ridge axis towards the volcanic centres. Only in the immediate vicinity of volcanic centres, where the seismicity dies out, may magmatism dominate. We conclude that detachment tectonics dominate the process of ultraslow seafloor spreading as well as much of slow seafloor spreading, totalling about one third of the global ridge system, and present the first 30 tectonic model for ultraslow seafloor spreading

Insights into exhumation and mantle hydration processes at the Deep Galicia margin from a 3D high-resolution seismic velocity model

High-resolution velocity models developed using full-waveform inversion (FWI) can image fine details of the nature and structure of the subsurface. Using a 3D FWI velocity model of hyper-thinned crust at the Deep Galicia Margin (DGM) west of Iberia, we constrain the nature of the crust at this margin by comparing its velocity structure with those in other similar tectonic settings. Velocities representative of both the upper and lower continental crust are present, but there is no clear evidence for distinct upper and lower crustal layers within the hyper-thinned crust. Our velocity model supports exhumation of the lower crust under the footwalls of fault blocks to accommodate the extension. We used our model to generate a serpentinization map for the uppermost mantle at the DGM, at a depth of 100 ms (~340m) below the S-reflector, a low-angle detachment that marks the base of the crust at this margin. We find a good alignment between serpentinized areas and the overlying major block bounding faults on our map, suggesting that those faults played an important role in transporting water to the upper mantle. Further, we observe a weak correlation between fault heaves and serpentinization beneath the hanging-wall blocks, indicating that serpentinization was controlled by a complex faulting during rifting. A good match between topographic highs of the S and local highly serpentinized areas of the mantle suggests that the morphology of the S was affected by the volume-increasing process of serpentinization and deformation of the overlying crust