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: The crustal structure of the Mesozoic deep Galicia margin and adjacent ocean-continent boundary (OCB) was investigated by seismic reflection (including pre-stack depth migration and attenuation of seismic waves with time). The seismic data were calibrated using numerous geological samples recovered by drilling and/or by diving with submersible. The N-S trending margin and OCB are divided in two distinct segments by NE-SW synrift transverse faults locally reactivated and inverted by Cenozoic tectonics. The transverse faulting and OCB segmentation result from crustal stretching probably in a NE-SW direction during the rifting stage of the margin in early Cretaceous times. The Cenozoic tectonics are related to Iberia-Eurasia convergence in Palaeogene times (Pyrenean event). In both segments of the deep margin, the seismic crust is made of four horizontal layers: (1) two sedimentary layers corresponding to post- and syn-rift sequences, where velocity ranges from 1.9 to 3.5 km s−1, and where the Q factor is low, the two sedimentary layers being separated by a strong reflector marking the break-up unconformity; (2) a faulted layer, where velocity ranges from 4.0 to 5.2 km s−1, and where the Q factor is high. This layer corresponds to the margin tilted blocks, where continental basement and lithified pre-rift sediments were sampled; (3) the lower seismic crust, where the velocity (7 km s−1 and more) and the Q factor are the highest. This layer, probably made of partly serpentinized peridotite, is roofed by a strong S-S’ seismic reflector, and resting on a scattering, poorly reflective Moho. A composite model, based both on analogue modelling of lithosphere stretching and on available structural data, accounts for the present structure of the margin and OCB. Stretching and thinning of the lithosphere are accommodated by boudinage of the brittle levels (upper crust and uppermost mantle) and by simple shear in the ductile levels (lower crust and upper lithospheric mantle). Two main conjugate shear zones may account for the observations and seismic data: one (SZ1), located in the lower ductile continental crust, is synthetic to the tilting sense of the margin crustal blocks; another (SZ2), located in the ductile mantle, accounts for the deformation of mantle terranes and their final unroofing and exposure at the continental rift axis (now the OCB). The S-S′ reflector is interpreted as the seismic signature of the tectonic contact between crustal terranes and mantle rocks partly transformed into serpentinite by syn-rift hydrothermal activity. It is probably related to both shear zones SZ1 and SZ2. The seismic Moho is lower within the lithosphere, at the fresh-serpentinized peridotite boundary.

Description: The regional distribution of mounds, associated bottom-simulating reflectors (BSRs) and submarine landslides of the Pacific margin of Nicaragua suggests a genetic relationship between them. In the landslide-dominated parts of the margin, mud mounds occur in groups upslope behind the scarps and aligned parallel to the headwall. The morphotectonic features associated with the slides suggest that the slope failure could be triggered by slope oversteepening on the trailing flank of subducted seamounts. Geometric analysis of the faults triggering and controlling the mud mounds and associated BSRs also indicates that they were caused by collapses of the uplifted sea floor. Thus we propose a simple conceptual genetic model for the occurrences of the submarine landslides, surrounding mud mounds and associated BSRs in the area. Seamount subduction created locally higher fluid overpressure in the décollement. The uplift and fracturing of the margin wedge above the subducting seamount opened pathways for the overpressured fluid to escape, leading to the formation of numerous mud mounds on the sea floor and the BSR in the subsurface. The higher fluid supply locally reduced the shear strength of the sediments and facilitated failure of these sediments as landslides on the oversteepened slope caused by the subduction of the seamount.

Description: The southern Porcupine Basin is characterized by axial stretching factors that are greater than six and typical of rifted margins. As such, the basin can be regarded as a natural laboratory to investigate the evolution and symmetry of rifting leading towards continental separation and breakup. A bright reflection (here named P) cuts down to the west from the base of the sedimentary section, is overlain by small fault blocks and appears to represent a detachment fault. P may in part follow the top of partially serpentinized mantle: this interpretation is consistent with gravity modelling, and with numerical models of crustal embrittlement and mantle serpentinization during extension. Furthermore, P closely resembles the S reflection west of Iberia, where such serpentinites are well documented. Although overall the basin remains symmetrical, the consistent westward structural dip of the detachment implies that, at high stretching factors, extension became asymmetric. Farther south, the ‘Porcupine Median High’, appearing lens-shaped in cross-section, overlies the tilted fault blocks and is onlapped by postrift sediment. Despite no evidence for synrift magmatism, this high has previously been interpreted as a basaltic structure. However, it develops above the line of intersection of the crust–mantle boundary with the P detachment, and hence may be related to the spatial limit of serpentinization. The median high may represent a serpentinite mud volcano or diapir; we suggest that such structures produce the serpentinite breccias found within the rifted continent–ocean transition of nonvolcanic margins.

Description: The symmetry or asymmetry of the process of continental breakup has been much debated over the last 20 years, with various authors proposing asymmetric simple shear models, others advocating more symmetric, pure shear models and some combinations of the two. The unroofing of vast expanses of sub-continental mantle at non-volcanic margins has led some authors to argue in favour of simple shear models, but supporting evidence is lacking. Subsidence evidence from conjugate margin pairs is equivocal, and the detailed crustal and lithospheric structure of such pairs not generally well enough known to draw firm conclusions. In the Porcupine Basin, where the final stages of break-up are preserved, the development of structural asymmetry is demonstrable, and apparently related to late stage coupling of the crust to the mantle following the complete embrittlement of the crust. This agrees with theoretical modelling results, which predict that asymmetric models can develop only on a lithospheric scale when the crust and mantle are tightly coupled. However, whether such asymmetry is maintained during continued exhumation of the mantle is unclear.