Beschreibung: The paper presents an overview of recent seafloor long-term single-frame multiparameter platform developed in the framework of the European Commission and Italian projects starting from the GEOSTAR prototype. The main features of the different systems are described as well as the sea missions that led to their validation. The ORION seafloor observatory network recently developed, based on the GEOSTAR-type platforms and engaged in a deep-sea mission at 3300 m w.d. in the Mediterranean Sea, is also described

Beschreibung: Highlights • Crustal thickness of the Lofoten/Vesterålen shelf is greater than old study suggested. • Mafic lower crust of the shelf area explains observed resistance to deformation. • Four Mesozoic-Cenozoic erosion episodes are indicated by sedimentary velocities vs burial. • Extensive erosion episodes are likely to be detrimental to petroleum potential. Abstract The Norwegian continental shelf has been through several rift phases since the Caledonian orogeny. Early Cretaceous rifting created the largest sedimentary basins, and Early Cenozoic continental breakup between East Greenland and Europe affected the continental shelf to various degrees. The Lofoten/Vesterålen shelf is located off Northern Norway, bordering the epicontinental Barents Sea to the northeast, and the deep-water Lofoten Basin to the west. An ocean bottom seismometer/hydrophone (OBS) survey was conducted over the shelf and margin areas in 2003 to constrain crustal structure and margin development. This study presents Profile 8-03, located between the islands of Lofoten/Vesterålen and the shelf edge. The wide-angle seismic data were modeled using forward/inverse raytracing to build a crustal velocity-depth transect. Gravity modeling was used to resolve an ambiguity in seismic Moho identification in the southwestern part. Results show a crustal thickness of ~31 km, significantly thicker than what a vintage land station based study suggested. Profile 8-03 and other OBS profiles to the southwest show high sedimentary velocities at or near the seafloor, increasing rapidly with depth. Sedimentary velocities were compared to the velocity-depth function derived from an OBS profile at the Barents Sea margin, tied to a coincident well log, where there is little erosion. Results from this profile and the crossing Profile 6-03 (Breivik et al. 2017) indicate three major erosion episodes; Late Triassic-Early Jurassic, tentatively mid-Cretaceous, Late Cretaceous–early Cenozoic, and a minor late glacial erosion episode off Vesterålen.

Beschreibung: Seismic refraction investigations along a 440-km long profije on the northern Baltic Shield have resolved the crustal structure in this area of Archaean to Early Proterozoic lithosphere formation. The profile, called the POLAR Profile, extends approximately along a SW-NE-oriented line from the Karelian Province in northern Finland across the Lapland Granulite Belt and the Kola Peninsula Province to the Varanger Peninsula in northeastern Norway. At six shotpoints, large explosions (200–1680 kg), and at three shotpoints, small explosions (80 kg) were detonated and recorded at an average station spacing of 2 km, providing high-quality record sections. A two-dimensional cross section of the crust was obtained by forward modelling using ray-tracing techniques. High-velocity bodies are found in the upper crust related to the Karasjok-Kittilä Greenstone Belt and the Lapland Granulite Belt. They extend to a depth of 6–13 km. In the Karelian Province in the southwest, a low-velocity zone was found between the depths of 8 and 14 km. The middle crust shows a slight increase in the average velocities from the southwest to the northeast, and a small velocity jump is found along a mid-crustal boundary between 18 and 21 km. The thickness of the middle crust varies between 16 and 18 km. The lower crust and the crust-mantle boundary (Moho) show considerable lateral variation. The top of the lower crust lies between 26 and 33 km, while its thickness decreases from 21 km in the southwest to 10–14 km beneath the Lapland Granulite Belt and the Inari Terrain, reaching 20 km again in the extreme northeast. The velocities also change laterally. The thin lower crust is characterized by rather low velocities (6.8–6.9 km/s), whereas in the southwest and northeast the velocities (6.9–7.3 km/s) resemble more typical shield structures. The Moho is found at 47 km in the Karelian Province, rises to 40 km beneath the Lapland Granulite Belt and descends to 46 km in the northeastern part of the Kola Peninsula Province. The upper mantle velocities at the Moho range from 8.1 km/s in the region of the thin crust, to 8.5 km/s and more beneath the Karelian Province. It is tempting to suggest that the anomalous lower crust underlying the Lapland Granulite Belt and the Inari Terrain may represent the remnants of an Early Proterozoic back-arc basin that was active prior to the 2.0 to 1.9 Ga plate convergence event, during which the Lapland Granulite Belt was thrust onto the Archaean basement of the Karelian Province. Another explanation is to assume that the velocity reduction in the anomalous lower crust was caused by a rather pronounced uplift of this region following the 1.9-Ga collision event.

Beschreibung: The eastern Sunda margin off Indonesia (from central Java to Sumba Island) remains a little investigated subduction zone, contrary to its well-studied northwestern segment. Whereas large portions of the Sunda margin are considered a classical accretionary zone, subduction characteristics along the central Java sector indicate erosive processes as the dominant mode of mass transfer. The tectonic framework of the central Java margin, with a convergence rate of 6.7 cm/yr, insignificant sediment input and a pronounced seafloor roughness where the oceanic Roo Rise is subducting underneath Java, facilitates subduction erosion. Evidence for erosion comes from newly acquired geophysical data off central Java: local erosive processes in the wake of seamount subduction are documented by a high-resolution bathymetric survey and result in an irregular trend of the deformation front sculpted by seamount collision scars. Subduction of oceanic basement relief leads to large-scale uplift of the forearc, as recorded on a reflection seismic profile, and to a dismemberment of the previous outer forearc high, giving way to isolated topographic elevations. The broad retreat of the Java Trench and deformation front above the leading edge of the Roo Rise has exposed an area of approximately 25,000 km2 of deeper seafloor formerly covered by the previous frontal prism. Frontal erosion coincides with a steepening of the lower slope angle in the central Java sector compared to the neighbouring segments. In global compilations, the key geological parameters of the central Java margin lie in the erosive regime, reflecting the interplay of basement relief subduction, negligible sediment supply and a high convergence rate on the evolution of the margin.

Beschreibung: Offshore Ecuador, the Carnegie Ridge is a volcanic ridge with a carbonate sediment drape. During the SALIERI Cruise, multibeam bathymetry was collected across Carnegie Ridge with the Simrad EM120 of the R/V SONNE. The most conspicuous features discovered on the Carnegie Ridge are fields of circular closed depressions widely distributed along the mid-slope of the northern and southern flanks of the ridge between 1500 and 2600 m water depth. These circular depressions are 1–4 km wide and typically 100–400 m deep. Most are flat floored and some are so densely packed that they form a honeycomb pattern. The depressions were carved into the ridge sedimentary blanket, which consists of carbonate sediment and has been dated from upper Miocene to upper Pleistocene. Several hypotheses including pockmark origin, sediment creeping, paleo-topography of the volcanic basement, effects of subbottom currents, and both marine and subaerial karstic origins are discussed. We believe that underwater dissolution process merits the most serious consideration regarding the origin of the closed depression.

Beschreibung: The Dalrymple Trough marks part of the transform plate boundary between India and Arabia in the northern Arabian Sea. Oblique extension is presently active across this portion of the boundary at a rate of a few millimetres per year, and seismic reflection profiles across the trough confirm that it is an extensional structure. We present new swath bathymetric and wide-angle seismic data from the trough. The bathymetric data show that the trough is bounded by a single, steep, 3-km-high scarp to the southeast and a series of smaller, en-echelon scarps to the northwest. Wide-angle seismic data show that a typical oceanic crustal velocity structure is present to the northwest, with a crustal thickness of ~ 6 km. There is an abrupt change in crustal thickness and velocity structure at the northwestern edge of the trough, and the trough itself is underlain by 12-km-thick crust interpreted as thinned continental crust. Therefore we infer that Dalrymple Trough is an unusual obliquely extending plate boundary at which continental crust and oceanic crust are juxtaposed. The extensional deformation is focused on a single major fault in the continental lithosphere, but distributed over a region ~ 60 km wide in the oceanic lithosphere

Beschreibung: Continental rifting at the Vøring Margin off mid-Norway was initiated during the earliest Eocene (~54 Ma), and large volumes of magmatic rocks were emplaced during and after continental breakup. In 2003, a marine survey collecting ocean bottom seismometer, single-channel re!ection, and magnetic data was conducted on the Norwegian Margin to constrain continental breakup and early sea!oor spreading processes. The pro"le described here crosses the northern part of the Vøring Plateau, and the crustal velocity model was constructed through a combination of ray-tracing and forward gravity modeling, the latter corrected for the thermal effects remaining from the sea!oor spreading. We found a maximum igneous crustal thickness of 18 km, decreasing to 6.5 km over the "rst ~6 M.y. after continental breakup. Both the volume and the duration of excess magmatism are about twice as large as that of the Møre Margin south of the East Jan Mayen Fracture Zone, which offsets the two margin segments by ~170 km. A similar reduction in magmatism occurs to the north over an along-margin distance of ~150 km to the Lofoten Margin, but without a margin offset. Both the geochemical data and the mean P-wave velocity indicate that there is active mantle upwelling combined with a moderate temperature increase during the earliest mantle melting at the Vøring Margin. The mean P-wave velocity versus crustal thickness also indicates that there is a transition from convection dominated to temperature dominated magma production ~2 M.y. after breakup. The magnetic data were used to derive plate half-spreading rates for the Northern Vøring Margin, which are very similar to that obtained at the Møre Margin. There is a strong correlation between magma productivity and early plate spreading rate, suggesting a common cause. A model for the breakup-related magmatism should be able to explain this correlation, but also the magma production peak at breakup, the along-margin magmatic segmentation, and the active mantle upwelling. Proposed end-member hypotheses comprise elevated uppermantle temperatures caused by a hot mantle plume, or edge-driven small-scale convection !uxing mantle rocks through the melt zone. Edge-driven convection does not easily explain these observations, but a mantle plume model in which buoyant plume material !ows laterally to pond in the rift-topography at the base of the lithosphere close to breakup time is promising: When the continents break apart, the hot and buoyant plume-material can !ow up into the rift zone from surrounding areas as the rift transits to drift, and the excess temperature of this material will then cause excess magmatism which dies off as the rift-restricted material is spent. The buoyancy of the plume-material may in addition cause active upwelling which can increase the melting furthermore, and also increase the force on the plate boundaries to enhance plate spreading rate. This conceptual model explains how both excess magmatism and spreading rate will be reduced similarly with time as the plume material is consumed by plate spreading, and thus correlate.

Beschreibung: Water transported within the subducting oceanic lithosphere into the Earth's interior affects a wealth of subduction zone processes, including intraslab earthquakes and arc magmatism. In recent years growing evidence suggests that much of the hydration of oceanic plates occurs at the trench–ocean slope right before subduction. Here, normal faults are created while the rigid lithosphere bends into the trench. Offshore of Middle America, multi-channel seismic reflection imaging suggests that bending-related faults cut into the uppermost mantle, providing a mechanism for hydration and transformation of mantle peridotites into serpentinites. Seismic wide-angle reflection and refraction data were collected coincident with one of the seismic profiles where the faults have been imaged. Travel time inversion provides evidence that both crustal and uppermost mantle velocities are reduced with respect to the velocity structure found in mature oceanic crust away from deep-sea trenches. If mantle velocity reduction is solely produced by hydration, velocities indicate 10–15% of serpentinization in the uppermost 3 km of the mantle, where seismic data provide enough resolution. A small network of ocean bottom hydrophones, deployed for about a month, detected ∼ 3 local micro earthquakes per day. Earthquake epicentres align with fault scarps at the seafloor and continuous earthquake activity might be an important process to facilitate the percolation of seawater into the upper mantle.

Beschreibung: The structure of the Mid-Atlantic Ridge at 5°S was investigated during a recent cruise with the FS Meteor. A major dextral transform fault (hereafter the 5°S FZ) offsets the ridge left-laterally by 80 km. Just south of the transform and to the west of the median valley, the inside corner (IC – the region bounded by the ridge and the active transform) is marked by a major massif, characterized by a corrugated upper surface. Fossil IC massifs can also be identified further to the west. Unusually, a massif almost as high as the IC massif also characterizes the outside corner (OC) south of the inactive fracture zone and to the east of the median valley. This OC massif has axis-parallel dimensions identical to the IC massif and both are bounded on their sides closest to the spreading axis by abrupt, steep slopes. An axial volcanic ridge is well developed in the median valley both south of the IC/OC massifs and in an abandoned rift valley to the east of the OC massif, but is absent along the new ridge-axis segment between the IC and OC massifs. Wide-angle seismic data show that between the massifs, the crust of the median valley thins markedly towards the FZ. These observations are consistent with the formation of the OC massif by the rifting of an IC core complex and the development of a new spreading centre between the IC and OC massifs. The split IC massif presents an opportunity to study the internal structure of the footwall of a detachment fault, from the corrugated fault surface to deeper beneath the fault, without recourse to drilling. Preliminary dredging recovered gabbros from the scarp slope of the rifted IC massif, and serpentinites and gabbros from the intersection of this scarp with the corrugated surface. This is compatible with a concentration of serpentinites along the detachment surface, even where the massif internally is largely plutonic in nature.

Beschreibung: We examine the micro-earthquake seismicity recorded by two temporary arrays of ocean bottom seismometers on the outer rise offshore southern Chile on young oceanic plate of ages 14 Ma and 6 Ma, respectively. The arrays were in operation from December 2004–January 2005 and consisted of 17 instruments and 12 instruments, respectively. Approximately 10 locatable events per day were recorded by each of the arrays. The catalogue, which is complete for magnitudes above 1.2–1.5, is characterized by a high b value, i.e., a high ratio of small to large events, and the data set is remarkable in that a large proportion of the events form clusters whose members show a high degree of waveform similarity. The largest cluster thus identified consisted of 27 similar events (average inter-event correlation coefficient 〉 0.8 for a 9.5 s window), and waveform similarity persists far into the coda. Inter-event spacing is irregular, but very short waiting times of a few minutes are far more common than expected from a Poisson distribution. Seismicity with these features (high b value, large number of similar events with short waiting times) is typical of swarm activity, which, based on empirical evidence and theoretical considerations, is generally thought to be driven by fluid pressure variations. Because no pronounced outer rise bulge exists on the very young plate in the study region, it is unlikely that melt is accessible from decompression melting or opening of cracks. A fluid source related to processes at the nearby ridge is conceivable for the younger segment but less likely for the older one. We infer that the fluid source could be seawater, which enters through fractures in the crust. Most of the similar-earthquake clusters are within the crust, but some of them locate significantly below the Moho. If our interpretation is correct, this implies that water is present within the mantle. Hydration of the mantle is also indicated by a decrease of Pn velocities below the outer rise seen on a refraction profile through one of the arrays [Contreras-Reyes, E., Grevemeyer, I., Flueh, E.R., Scherwath, M., Heesemann, M., 2007. Alteration of the subducting oceanic lithosphere at the southern central Chile trench-outer rise. Geochem., Geophys. Geosyst. 8, Q07003.]. The deepest events within the array on the 6 Ma old plate occur where the temperature reaches 500–600 °C, consistent with the value observed for large intraplate earthquakes within the mantle (650 °C), suggesting that the maximum temperature at which these fluid-mediated micro-earthquakes can occur is similar or identical to that of large earthquakes.