Search

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.

The rupture of continents and creation of new oceans is a fundamental yet primitively understood aspect of the plate tectonic cycle. Building upon past achievements by ocean drilling and geophysical and geologic studies, we propose “The Continental Breakup and Birth of Oceans Mission (COBBOOM)” as the next major phase of discovery, for which sampling by drilling will be essential.In September 2006, fifty-one scientists from six continents gathered in Pontresina, Switzerland to discuss current knowledge of continental breakup and sedimentary basin formation and how the Integrated Ocean Drilling Program (IODP) can deepen that knowledge Coffin et al., 2006). Workshop participants discussed a global array of rifted margins (Fig. 1), formulated the critical problems to beaddressed by future drilling and related investigations, and identified key rift systems poised for IODP investigations.

Energy levels of internal waves are estimated from seismic reflection data. Three legacy seismic sections from 1993 and 1997 obtained off the Iberian Peninsula have been analyzed for acoustic reflections within the water column. The reflections are aligned continuously for up to several kilometers over large parts of the sections and in the depth interval from 200 to 2000 m. Depth variations of these reflections are thought to be caused by the background internal wave field. From the variations we derive horizontal wave number spectra of normalized internal wave displacement. The general slope of the power density spectra is remarkably consistent for all sections and agrees well with model spectra for internal waves. Significant differences within the sections can be found when sufficiently large subsections are averaged. The spatial variation of the energy level indicates increasing internal wave activity with shallower water depths as well as near a subsurface eddy.

[1] The existence of normal faults that moved at low angles (less than 20°) has long been debated. One possible low-angle fault is the S detachment at the west Galicia (Spain) margin and thought to occur at the top of serpentinized mantle. It is unlikely that S was a large submarine slide as it was probably active over several million years without the development of any compressional features such as toe thrusts, it appears to have rooted beneath the conjugate Flemish Cap margin, and it is similar to structures elsewhere that also appear to be rooted detachments. Here we analyze depth images to identify synrift sediment packages above S and use the geometry of these synrift packages to constrain the angle at which S both formed and remained active. We find that S must have remained active at angles below 15°, too low to be explained simply by the low friction coefficient of partially serpentinized peridotites. Instead, we suggest that transient high fluid pressures must have developed within the serpentinites and propose a model in which anastomosing fault strands are alternately active and sealed, enabling moderately high fluid pressures to develop.

Wide–angle reflection seismic experiments were performed at the Storegga slide offshore Norway in 2002 with the goal to quantify the amount of gas hydrate and free gas in the sediment. Twenty‐two stations with Ocean Bottom Hydrophones (OBH) and Seismometers (OBS) were deployed for a 2D and a 3D experiment. Kirchhoff depth migration is used to transform the seismic wide–angle data into images of the sediment layers and to obtain P wave velocity–depth functions. The gas hydrate and free gas saturations are estimated from the elastic properties of the sediment on the basis of the Frenkel–Gassmann equations. There is 5–15% gas hydrate in the pore space of the sediment in the gas hydrate stability zone (GHSZ). The free gas saturation takes the value of 0.8% for a homogeneous distribution of gas in the pore water and 7% for the model of a patchy gas distribution.

The S reflection west of Iberia has been interpreted as a low‐angle detachment fault separating crustal fault blocks from partially serpentinized mantle. We apply full waveform inversion to investigate the fine structure of S. Our results confirm that S is largely a step increase in velocity (and density), probably from crustal rocks to partially serpentinized mantle peridotites. A ∼50 m thick low velocity zone above S might represent a main fault zone of highly serpentinized peridotites or hydrofractured and altered crustal rocks above the main fault zone. Both interpretations imply focused fluid flow along S, raising the possibility that low‐angle movement along S was aided by the development of local, transient high fluid pressures.

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.

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 (

GEOMAR-Report No 103

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.

Discharge of cold fluids from the seafloor into the ocean is known from many continental margins. The amount and mechanism of this fluid and volatile recycling are important factors for global mass balances. This work investigates cold vents and seeps in the forearc region of the active continental margin offshore Nicaragua with geoacoustic methods (multibeam bathymetry, deep-towed sidescan sonar, subbottom profiler). Ground truthing is available from camera system surveys and coring on several of the structures. The area is characterized by a high number of so-called mound structures visible in the multibeam bathymetry, and various intensity anomalies without a significant topographic expression but showing high backscatter in the sidescan sonar image. The increased backscatter on mounds and intensity anomalies is mainly related to an increased seafloor roughness, and attributed to hardgrounds like authigenic carbonate formations. A significant influence of the seafloor topography on the sidescan sonar signal is largely excluded. This is shown by a new processing algorithm, developed in this work. The detailed analysis of the seep sites suggests a classification into three end members and explains their formation through the interaction of several processes: Episodic fluid venting and associated authigenic carbonate formation, background sedimentation with subsidence, and erosion.

The structure of the oceanic crust at the Mid-Atlantic Ridge at 5° South is investigated using wide-angle seismic data. Here, two morphologically contrasting spreading segments are separated by a 70 km offset transform fault. Applying a first-arrival tomography and a joint refraction and reflection tomography the obtained models show strong lateral heterogeneity in both velocities and Moho depths. In the segment north of the 5°S fracture zone (FZ) the along-axis crustal thickness thins from 9 km at the segment center to 5-6 km at the segment ends. In contrast to the magmatically robust northern segment, spreading south of the 5°S FZ rather seems to be tectonically controlled. Here, an inside corner high has been split by a change in location of active seafloor spreading. Most parts of this segment are underlain by anomalously thin crust with the thinnest portions associated with the flanks of the inside corner high and a fossil second order ridge-axis discontinuity. The crust formed at the inside corner reveals throughoutly higher velocities than corresponding outside corner crust, which is consistent with models of asymmetric crustal accretion along a detachment fault.

(Keine deutschsprachige Zusammenfassung vorhanden.) This work constrains the structure of the active margin off Nicaragua and Costa Rica through the analysis of two coincident seismic reflection and refraction profiles. The modelling of the combined seismic datasets verified the structure and the P-wave velocities of the subducted oceanic Cocos Plate and the caribbean continental margin for an area ranging from central Nicaragua to neighboring Costa Rica. The existence of a shallow mantle sliver lying under the nicaraguan shelf is confirmed. Comparing the data presented in this study with gravity data and magnetic anomalies serves as a strong indication of an accreted terrain stretching as far as Guatemala. The imaging of deep cutting faults into the mantle, the modelling of low P-wave velocities of 7.2-7.8 km/s and a high Poisson ratio of 0.3 in the mantle, and gravity modelling support the assumption of a partially serpentinised mantle at the outer rise. Refraction data indicate a thickness of 3-13 km for this layer.

This work studies the contribution and significance of mud mounds in the dewatering of the continental fore-arc in an erosive subduction system. Through the analysis of multibeam bathymetric, narrow-beam sediment echosounder, and multi-channel seismic data, the main goals are to determine the structure and mechanism of mud domes investigated at two study areas, and to identify and locate alternative fluid pathways through the subsurface. Structural investigations near a single large mound reveal it to be of diapiric origin. Based on the observed deformation and sediment deposition patterns in its vicinity, a local tectonic model of the Mound Culebra area is constructed. Thermal modelling in the area, allowed by a widespread presence of clathrates in the subsurface, helps identify probable locations of heat and fluid advection as well as evaluate their relative importance. These results are compared to those from a second study area, the differences revealing a significant effect of subducting seafloor topography on the deformation and dewatering patterns of the fore-arc. A possible fate of mud mounds on the Central American margin is also outlined.

The purpose of this work has been to study the processes of continental extension and break-up at non-volcanic margins. In particular, we studied how extension halts leading to failed rifts. For this purpose, I have processed multichannel seismic reflection and modelled wide angle data from the Galicia Interior Basin (GIB), a failed rift located offshore west Iberia (Chapter 1, 2 and 3). Our studies reveal the structure, amount of thinning and timing of extension at the GIB (Chapter 2 and 3). We have integrated the new results on the GIB with those already existent at the rest of the margin and suggested possible causes for rift failure at the basin in connection with propagation of seafloor spreading at the final break-up margins (Chapter 3 and Outlook). Additionally, we studied how extension leads to continental break-up at non-volcanic margins of the west Iberia type. We developed a numerical model based on the seismic, petrological and chronological observations made at the segments of the west Iberia margin where continental break-up occurred (Chapter 4). We have applied this model successfully to other non-volcanic margins such as the south-west Greenland margin, the Rockall Trough and the Porcupine Basin (both located offshore west Ireland)(Chapter 5). We have also found that the differences in the initial thermal structure of the margins may lead to the differences in the structural style of continental break-up between non-volcanic margins of the west Iberia type and those showing more magmatic activity during extension (e.g. the Woodlark basin, located at the eastern tip of Papua New Guinea). Our modelling stresses the importance of the rheological consequences of the process of serpentinisation at non-volcanic margins of the west Iberia type in contrast to those of a more robust magmatic production.