ALBERT

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: Models of melting accompanying mantle upwelling predict far more melt than is observed at nonvolcanic margins. The discrepancy may be explained if the paradigm of a uniform asthenosphere is incorrect. Work on the velocity structure of the continents has shown that the convecting sublithospheric mantle may have a potential temperature as low as 1200 °C, ∼100 °C cooler than that beneath the oceans. The continental geotherm derived from studies on xenoliths brought up by kimberlites is also compatible with a cool sublithospheric mantle except where perturbed by mantle plumes. Upwelling of such cool mantle during rifting leads to little melt production, even for rapid extension rates, explaining the formation of amagmatic margins away from mantle plumes. However, the transition to seafloor spreading and the development of normal-thickness oceanic crust requires the invasion of the amagmatic rift by hotter oceanic asthenosphere and/or plume material. This influx may cause a transient thermal uplift, recorded as a breakup unconformity. Conversely, at volcanic margins, the onset of seafloor spreading is accompanied by the escape of the hot plume puddle along the mid-ocean ridge system away from the volcanic margin, leading to a pulse of rapid subsidence.

Description: Rifted margins show an apparent discrepancy between the amount of extension measurable from faults and the amount of crustal or lithospheric thinning determined from wide-angle data or subsidence. This extension discrepancy has been interpreted in terms of depth-dependent stretching in which the upper crust is extended and thinned far less than the rest of the lithosphere. Here I show that at those nonvolcanic margins where the velocity structure is well known, the upper and lower crust appear to thin equally toward the margin, implying that the discrepancy is not due to depth-dependent stretching, but more likely due to unrecognized polyphase and top basement faulting.