Algerian Basin Research Articles

Sentinel‐3A Views Ocean Variability More Accurately at Finer Resolution

AbstractThis is the first multiplatform evaluation involving data from the new Sentinel‐3A altimeter. An experiment was undertaken in the Algerian Basin, employing an ocean glider and a ship mission, along the same track and synchronous with an overpass of the Sentinel‐3A mission. This provided three independent views of the ocean velocity field, along a section that encompassed three different oceanographic regimes. The results demonstrate the capacity of Sentinel‐3A to retrieve fine‐scale oceanographic features (~20 km). The intercomparison with in situ platforms showed a significant improvement, order 30% in resolution and 42% in velocity accuracy using a synthetic aperture radar mode with respect to lower‐resolution mode of conventional altimetry. In addition, the three‐platform view provided valuable insight into the variability of evolving oceanographic features, in an area of the Mediterranean that remains chronically under sampled.

Monitoring the Algerian Basin through glider observations, satellite altimetry and numerical simulations along a SARAL/AltiKa track

Abstract The Algerian Basin is a key component of the general circulation in the Western Mediterranean Sea. The presence of both fresh Atlantic water and more saline Mediterranean water gives the basin an intense inflow/outflow regime and complex circulation patterns. Energetic mesoscale structures that evolve from meanders of the Algerian Current into isolated cyclonic and anticyclonic eddies dominate the area, with marked repercussions on biological activity. Despite its remarkable importance, this region and its variability are still poorly known and basin-wide knowledge of its meso- and submesoscale features is still incomplete. Studying such complex processes requires a synergistic approach that involves integrated observing systems. In recent years, several studies have demonstrated the advantages of combined use of autonomous underwater vehicles, such as gliders, with a new generation of satellite altimetry. In this context, we present results of an observational program conducted in the Algerian Basin during fall 2014 and 2015 that aimed to advance our knowledge of its main features. The study was carried out through analysis of high resolution glider observations, collected along the Algerian BAsin Circulation Unmanned Survey (ABACUS) chokepoint, in synergy with co-located SARAL/AltiKa altimetric products and CMEMS numerical simulations. Results show that glider-derived dynamic height and SARAL/AltiKa absolute dynamic topography have similar patterns, with RMS of the differences ranging between 1.11 and 2.90 cm. Even though larger discrepancies are observed near the Balearic and Algerian coasts, correlation coefficients between glider and satellite observations seem mostly to be affected by reduced synopticity between the measurements. Glider observations acquired during the four surveys reveal the presence of several water masses of Atlantic and Mediterranean origin (i.e., AW and LIW at different modification levels) with marked seasonal variability.

From Extension to Shortening: Tectonic Inversion Distributed in Time and Space in the Alboran Sea, Western Mediterranean

Abstract2D seismic reflection data tied to biostratigraphical and log information from wells in the central and southeastern Alboran Sea have allowed us to constrain the spatial and temporal distribution of rifting and inversion. Normal faults, tilted basement blocks, and growth wedges reveal a thinned continental crust that formed in response to NW‐SE extension. To the east, a secondary SW‐NE trend of extension affects the transitional crust adjacent to the oceanic Algerian Basin. The maximum thickness of syn‐rift sediments is ~3.5 km, and the oldest recorded deposits are Serravallian. The WNW‐ESE Yusuf fault formed a buttress separating and accommodating variable extension between two different tectonic domains: the thinned continental crust of Alboran and the oceanic spreading of the Algerian Basin. Late Tortonian to present‐day NW‐SE Africa/Eurasia plate convergence drove shortening and reactivation of some of the earlier extensional structures as reverse and strike‐slip faults, forming complex, compartmentalised subbasins. Tectonic inversion coexisted with the formation of new faults and folds. Inversion was partial along the Habibas Basin and Al‐Idrisi fault, but complete along the Alboran Ridge, where some SW‐NE trending faults were perpendicular to the recent NW‐SE plate convergence and were reactivated as thrusts. The WNW‐ESE Yusuf fault is oblique to the convergence vector, and therefore, reactivation is mainly expressed as transpressional deformation. Volcanic rocks intruded along the Alboran Ridge and Yusuf faults during the latest stages of extension formed rheological anisotropies that localised the later inversion.

Offshore wind climate analysis and variability in the Mediterranean Sea

ABSTRACTThe long‐term offshore wind characteristics in the Mediterranean Sea are assessed using the ERA‐Interim dataset for the period 1979–2014. Three main aspects of the wind climate analysis are examined in detail; the spatio‐temporal behaviour (including variability characteristics) of wind speed and direction for the annual and monthly time scale; the joint association of wind speed and direction for the annual and monthly time scale, and; the wind speed trends and wind direction changes. From this analysis, the systematic wind flow patterns are identified and the general features of the wind climatology patterns are revealed. The most typical examples of stable and intense regional winds are the Mistral (throughout the year) in the Gulf of Lion and the Etesians (during summer) in the Aegean Sea. High values of mean annual variability of wind speed occur over the western part of the basin (Balearic, Tyrrhenian and Ligurian sub‐basins) as well as over the Adriatic and N Aegean Seas, and the E Levantine Basin. Moreover, the assessment of the joint association of wind speed and direction indicates that areas with high values of linear–circular correlation are the ones characterized by intense wind climate. As regards, long‐term changes of wind speed and direction, the linear trend for the former and the angular distance for the latter variable are provided. The fastest increasing wind speed trends are observed in the Ionian, the N Tyrrhenian and N Adriatic Seas, the eastern part of the Algerian Basin up to Balearic Isl. and the western part of the S Levantine Basin while the fastest decreasing ones are observed offshore Monaco, the central Aegean and the E Alboran Seas and the N Levantine Basin. The highest values of angular variance and mean angular distances are depicted for the Ligurian, Balearic and Alboran Seas.

Modeling the intense 2012-2013 dense water formation event in the northwestern Mediterranean Sea: Evaluation with an ensemble simulation approach

The northwestern Mediterranean Sea is a well-observed ocean deep convection site. Winter 2012–2013 was an intense and intensely documented dense water formation (DWF) event. We evaluate this DWF event in an ensemble configuration of the regional ocean model NEMOMED12. We then assess for the first time the impact of ocean intrinsic variability on DWF with a novel perturbed initial state ensemble method. Finally, we identify the main physical mechanisms driving water mass transformations. NEMOMED12 reproduces accurately the deep convection chronology between late January and March, its location off the Gulf of Lions although with a southward shift and its magnitude. It fails to reproduce the Western Mediterranean Deep Waters salinification and warming, consistently with too strong a surface heat loss. The Ocean Intrinsic Variability modulates half of the DWF area, especially in the open-sea where the bathymetry slope is low. It modulates marginally (3–5%) the integrated DWF rate, but its increase with time suggests its impact could be larger at interannual timescales. We conclude that ensemble frameworks are necessary to evaluate accurately numerical simulations of DWF. Each phase of DWF has distinct diapycnal and thermohaline regimes: during preconditioning, the Mediterranean thermohaline circulation is driven by exchanges with the Algerian basin. During the intense mixing phase, surface heat fluxes trigger deep convection and internal mixing largely determines the resulting deep water properties. During restratification, lateral exchanges and internal mixing are enhanced. Finally, isopycnal mixing was shown to play a large role in water mass transformations during the preconditioning and restratification phases.

Turbidite chronostratigraphy off Algiers, central Algerian margin: A key for reconstructing Holocene paleo-earthquake cycles

Northern Algeria is threatened by moderate to large magnitude earthquakes resulting from the slow convergence between the African and European plates. Main active faults are located offshore along the Algerian coast, as exemplified by the 2003 Mw 6.8 Boumerdès earthquake. This event triggered numerous and widespread turbidity currents over ∼150km along strike in the Algerian basin (reaching 2800m of water depth) and demonstrates the multi-source and multi-path characteristics of earthquake-triggered turbidity flows along this margin segment.We rely on the sedimentological analysis of five cores located at the toe of the Algiers margin, close to the 2003 cable break sites, to explore the potential for Holocene turbidite paleoseismology. Radiocarbon measurements provide age models for hemipelagic sediments. Based on sedimentary facies identification, analysis of depositional sequences (stacking pattern) and a stratigraphic framework established by age models, a first correlation of turbidites between the 5 cores is attempted. The number of turbidites is constant at the base of the continental slope and decreases seawards (over 80km away from the coast). From turbidite correlations, 36 synchronous events are identified along the Algiers margin segment over the last 9kyr, and are tentatively interpreted as seismically triggered, providing a 250yr mean recurrence interval.The main historical earthquakes in the Algiers area (2003, 1716 and 1365CE) reasonably correlate with three out of the four last turbidites, strengthening the hypothesis that turbidites are suitable markers for Holocene paleoseismology. Recurrence intervals of turbidites range between 50 and 900yr, defining quiescence periods exceeding 450yr. Three quiescence periods lasting about 800, 1400 and 500yr (7–6.2ka BP, 5.4–4ka BP, and 1.5–1ka BP, respectively) support irregular earthquake cycling. Earthquake-triggered turbidites are more frequent in the study area than in the western adjacent margin segment (offshore El Asnam). This higher frequency could arise from the close location of the seismogenic faults beneath the continental slope, whereas they are located several tenths of kilometers onland in the El Asnam area, implying less instabilities of the submarine slope.

Tectonostratigraphic evolution of the eastern Algerian margin and basin from seismic data and onshore-offshore correlation

The eastern Algerian basin, part of the Western Mediterranean domain, is a back-arc basin resulting from the SE drift of the Lesser Kabylia block gave rise to continental extension and birth of a new oceanic domain. We aim here to define the structure and tectonostratigraphic evolution of this basin and the adjoining continental margin. Indeed, the absence of wells and the lack of published MCS data kept this area mostly unknown. The crustal architecture (down to the Moho) and seismo-stratigraphy of the sedimentary infill are detailed, emphasizing particularly the pre-Messinian series, by combining a new set of offshore deep penetrating seismic, conventional MCS sections, and wide angle seismic data. Because no well exist offshore we realized new field descriptions of the Late Oligocene and Miocene deposits on land and propose onshore-offshore extrapolation and age correlations. The pre-Messinian series display four main units (PMSU1 to 4) above which Messinian units are consistent with stratigraphic models already proposed for the Mediterranean basin, the deep basin displays units as old as the OMK (“Oligo-Miocène Kabyle”) series on land. The basement is shaped by conjugate transcurrent faults striking N120°- N40°, forming a series of rhombic troughs resulting from transtensional tectonics. The coeval E-W extension and NW-SE stretching of the continental crust suggest a radial opening of the Algerian basin during Late Oligocene- Lower Miocene, followed by a progressive uplift of the upper margin after the collision of Lesser Kabylia with Africa. Besides, steep slope of the eastern Algerian margin, downward flexure of the oceanic basin near the margin toe and growth of S-dipping thrusts within the margin evidence a significant Quaternary stress inversion. Overall, this study highlights the complex stress evolution of the East Algerian margin since Oligocene times, as evidenced by significant upward/downward vertical motions and crustal extensional/compressional successions. We summarize this evolution into three main stages: (1) rifting (including sea-floor spreading) at Late Oligocene- Burdigalian in the context of back-arc, (2) collision between AlKaPeCa blocks and Africa at 17 Ma, and post-rift sag basin infill, till Late Tortonian to Messinian, and (3) inversion of the margin and development of flexural basins at Quaternary times.

Retrieval of eddy dynamics from SMOS sea surface salinity measurements in the Algerian Basin (Mediterranean Sea)

AbstractThe circulation in the Algerian Basin is characterized by the presence of fresh‐core eddies that propagate along the coast or at distances between 100 and 200 km from the coast. Enhancements in the processing of the Soil Moisture and Ocean Salinity (SMOS) data have allowed to produce, for the first time, satellite sea surface salinity (SSS) maps in the Mediterranean Sea that capture the signature of Algerian eddies. SMOS data can be used to track them for long periods of time, especially during winter. SMOS SSS maps are well correlated with in situ measurements although the former has a smaller dynamical range. Despite this limitation, SMOS SSS maps capture the key dynamics of Algerian eddies allowing to retrieve velocities from SSS with the correct sign of vorticity.

Extensional vs contractional Cenozoic deformation in Ibiza (Balearic Promontory, Spain): Integration in the West Mediterranean back-arc setting

Based on field work and seismic reflection data, we investigate the Cenozoic tectono-sedimentary evolution offshore and onshore Ibiza allowing the proposal of a new tectonic agenda for the region and its integration in the geodynamic history of the West Mediterranean. The late Oligocene–early Miocene rifting event, which characterizes the Valencia Trough and the Algerian Basin, located north and south of the study area respectively, is also present in Ibiza and particularly well-expressed in the northern part of the island. Among these two rifted basins initiated in the frame of the European Cenozoic Rift System, the Valencia Trough failed rapidly while the Algerian Basin evolved after as a back-arc basin related to the subduction of the Alpine–Maghrebian Tethys. The subsequent middle Miocene compressional deformation was localized by the previous extensional faults, which were either inverted or passively translated depending on their initial orientation. Despite the lateral continuity between the External Betics and the Balearic Promontory, it appears from restored maps that this tectonic event cannot be directly related to the Betic orogen, but results from compressive stresses transmitted through the Algerian Basin. A still active back-arc asthenospheric rise likely explains the stiff behavior of this basin, which has remained poorly deformed up to recent time. During the late Miocene a new extensional episode reworked the southern part of the Balearic Promontory. It is suggested that this extensional deformation developed in a trans-tensional context related to the westward translation of the Alboran Domain and the coeval right-lateral strike-slip movement along the Emile Baudot Escarpment bounding the Algerian Basin to the north.

A 17 Ma onset for the post-collisional K-rich calc-alkaline magmatism in the Maghrebides: Evidence from Bougaroun (northeastern Algeria) and geodynamic implications

Bougaroun is the largest pluton (~200km2) in the 1200km-long Neogene magmatic belt located along the Mediterranean coast of Maghreb. New U–Pb dating on zircons and K–Ar ages on whole rocks and separated minerals document its emplacement at 17Ma within the Lesser Kabylian basement, a continental block that collided with the African margin during the Neogene. This Upper Burdigalian intrusion is therefore the oldest presently identified K-rich calc-alkaline massif in the whole Maghrebides magmatic lineament and marks the onset of its activity. The Bougaroun peraluminous felsic rocks display a very strong crustal imprint. Associated mafic rocks (LREE-enriched gabbros) have preserved the “orogenic” (subduction-related) geochemical signature of their mantle source. Older depleted gabbros cropping out at Cap Bougaroun are devoid of clear subduction-related imprint and yielded Ar–Ar hornblende ages of 27.0±3.0Ma and 23.3±3.2Ma. We suggest that they are related to the Upper Oligocene back-arc rifted margin and Early Miocene oceanic crust formation of the nearby Jijel basin, an extension of the Algerian basin developed during the African (Tethyan) slab rollback. The fact that the Bougaroun pluton intrudes exhumed Kabylian lower crustal units, mantle slices and flysch nappes indicates that the Kabylian margin was already stretched and in a post-collisional setting at 17Ma. We propose a tectono-magmatic model involving an Early Miocene Tethyan slab breakoff combined with delamination of the edges of the African and Kabylian continental lithospheres. At 17Ma, the asthenospheric thermal flux upwelling through the slab tear induced the thermal erosion of the Kabylian lithospheric mantle metasomatized during the previous subduction event and triggered its partial melting. We attribute the strong trace element and isotopic crustal signature of Bougaroun felsic rocks to extensive interactions between ascending mafic melts and the African crust underthrust beneath the Kabylie de Collo basement.

Glider and satellite high resolution monitoring of a mesoscale eddy in the algerian basin: Effects on the mixed layer depth and biochemistry

ABSTRACT Despite an extensive bibliography for the circulation of the Mediterranean Sea and its sub-basins, the debate on mesoscale dynamics and their impacts on bio-chemical processes is still open because of their intrinsic time scales and of the difficulties in their sampling. In order to clarify some of these processes, the “Algerian BAsin Circulation Unmanned Survey-ABACUS” project was proposed and realized through access to the JERICO Trans National Access (TNA) infrastructure between September and December 2014. In this framework, a deep glider cruise was carried out in the area between the Balearic Islands and the Algerian coast to establish a repeat line for monitoring of the basin circulation. During the mission a mesoscale eddy, identified on satellite altimetry maps, was sampled at high-spatial horizontal resolution (4 km) along its main axes and from the surface to 1000 m depth. Data were collected by a Slocum glider equipped with a pumped CTD and biochemical sensors that collected about 100 complete casts inside the eddy. In order to describe the structure of the eddy, in situ data were merged with next generation remotely sensed data: daily synoptic sea surface temperature (SST) and chlorophyll concentration (Chl-a) images from the MODIS satellites, as well as sea surface height and geostrophic velocities from AVISO. From its origin along the Algerian coast in the eastern part of the basin, the eddy propagated northwest at a mean speed of about 4 km/day, with a mean diameter of 112–130 km, mean amplitude of 15.7 cm; the eddy was clearly distinguished from the surrounding waters thanks to its higher SST and Chl-a values. Temperature and salinity values over the water column confirm the origin of the eddy from the Algerian Current (AC) showing the presence of recent Atlantic water in the surface layer and Levantine Intermediate Water (LIW) in the deeper layer. The eddy footprint is clearly evident in the multiparametric vertical sections conducted along its main axis. Deepening of temperature, salinity and density isolines at the center of the eddy is associated with variations in Chl-a, oxygen concentration and turbidity patterns. In particular, at 50 m depth along the eddy borders, Chl-a values are higher (1.1–5.2 μg/l) in comparison with the eddy center (0.5–0.7 μg/l) with maximum values found in the southeastern sector of the eddy. Calculation of geostrophic velocities along transects and vertical quasi-geostrophic velocities (QG-w) over a regular 5 km grid from the glider data helped to describe the mechanisms and functioning of the eddy. QG-w presents an asymmetric pattern, with relatively strong downwelling in the western part of the eddy and upwelling in the southeastern part. This asymmetry in the vertical velocity pattern, which brings LIW into the euphotic layer as well as advection from the northeastern sector of the eddy, may explain the observed increases in Chl-a values.

Evidence for transform motion along the South Balearic margin and implications for the kinematics of opening of the Algerian basin

Abstract Two major types of kinematic models have been proposed to explain the opening of the western Mediterranean basins (Liguro-Provençal and Algerian basins, and Valencia trough). In one type of models, all continental blocks bounding the basins drift to the southeast, driven by the rollback of the Tethys subduction slab. In the other type of models, the Alboran domain drifts to the southwest, implying a westward rollback of the broken subducting slab and a NE-SW opening of the Algerian basin. In most models, however, the structure of the Balearic promontory was not taken into account, despite its key location at the boundary of the three major basins. We used the interpretation of a large seismic database coupled to gravity and magnetic anomaly analyses to characterize the nature and structure of the South Balearic margin. The constraints brought by the new analyses allow us to suggest a new scenario for the opening of the Algerian basin. Seismic profiles show that the South Balearic margin is composed of four segments with different morphologies and crustal structures. Two segments, the Mazarron and the Emile Baudot escarpments, are characterized by steep scarps and sharp crustal thinning. Two other segments, the South Ibiza and South Menorca margins, have a smoother bathymetry and crustal thinning. We interpret the former in terms of transform margins, and the latter as divergent margins. The distribution of faults on the passive margin segments suggests that they have recorded at least two phases of deformation. A first phase of opening, probably in a NW-SE direction, affected the south Balearic margin, and possibly created some oceanic floor. The existence of the transform margin segments and the prominent NW-SE orientation of the magnetic lineations in the eastern Algerian basin suggest that most of this basin opened in a NE-SW direction, in different oceanic corridors. The two eastern corridors formed by the southwestward drift of the Kabylies. The western corridor, bounded by the transform segments of the South Balearic margin and the Algerian margin, results from the southwestward drift of the Alboran domain, as suggested by previous studies.

Pasiphaeid shrimps in the western Mediterranean: geographical variability in distribution and population patterns

This work aimed to analyse the main characteristics of the bathymetric and geographic distribution, population size structure and size at maturity of the continental slope caridean shrimps Pasiphaea sivado and Pasiphaea multidentata in the western Mediterranean, as well as to try to relate the patterns obtained with geomorphologic and hydrographic characteristics. The study area encompassed the Mediterranean coast of the Iberian Peninsula. In both species, marked differences in depth distribution, density, and population size structure were found between Algerian basin populations, particularly in the Alboran Sea, and those in the Catalano-Balearic basin. Both shrimps showed a shallower bathymetric range in the westernmost region of the Algerian basin than in the rest of geographic sectors, linked to the occurrence of upwelling areas on the northwestern edges of the Alboran Sea. Populations of P. sivado reached larger densities, sizes and maturity size in the Algerian basin. No recruitment of P. multidentata was detected in the Algerian basin, whereas it was present in the Catalano-Balearic basin. In both species, the window range of bottom temperature and salinity was larger in juveniles than in adults. These different distribution and population dynamics imply that ecological functioning of these species differs between the two geomorphological basins of the western Mediterranean Sea.

Flexural behaviour of the north Algerian margin and tectonic implications

The Algerian margin formed through back-arc opening of the Algerian basin (Mediterranean Sea) resulting from the roll-back of the Tethyan slab. Recent geophysical data acquired along the Algerian margin showed evidence of active or recent compressive deformation in the basin due to the ongoing Africa–Eurasia convergence. Published data from four wide-angle seismic profiles have allowed imaging the deep structure of the Algerian margin and its adjacent basins. In this study, we converted these velocity models into density models, then into isostatic anomalies. This allowed us to image an isostatic disequilibrium (relative to a local isostasy model) reaching a maximum amplitude at the margin toe. Converting isostatic anomalies into Moho depth variations shows that the Moho extracted from wide-angle seismic data is deeper than the one predicted by a local isostasy model in the oceanic domain, and shallower than it in the continental domain. These anomalies can be interpreted by opposite flexures of two plates separated by a plate boundary located close to the margin toe. We use a finite element model to simulate the lithospheric flexure. The amplitude of the equivalent vertical Moho deflection is larger in the central part of the study area (6–7 km) than on the easternmost and westernmost profiles (3 km). The effective elastic thickness used to best match the computed deflection is always extremely low (always less than 10 km) and probably reflects the relatively low strength of the lithosphere close to the plate boundary. Comparison with other wide-angle seismic profiles across an active and a passive margin show that the North Algerian margin displays isostatic anomalies close to that of an active margin. Finally, plate flexure is highest at the southern tip of the ocean-continent transition, possibly indicating that a former passive margin detachment is reactivated as a crustal scale reverse fault pre-dating a future subduction.

Strong population genetic structure and contrasting demographic histories for the small-spotted catshark (Scyliorhinus canicula) in the Mediterranean Sea.

Coastal and demersal chondrichthyans, such as the small-spotted catshark, are expected to exhibit genetic differentiation in areas of complex geomorphology like the Mediterranean Basin because of their limited dispersal ability. To test this hypothesis, we used a fragment of the mitochondrial cytochrome c oxidase subunit I gene and 12 nuclear microsatellite loci in order to investigate the genetic structure and historical demography of this species, and to identify potential barriers to gene flow. Samples were collected from the Balearic Islands, the Algerian Basin, the Ionian Sea, the Corinthian Gulf and various locations across the Aegean Sea. Additional sequences from the Atlantic and the Levantine Basin retrieved from GenBank were included in the mitochondrial DNA analysis. Both mitochondrial and nuclear microsatellite DNA data revealed a strong genetic subdivision, mainly between the western and eastern Mediterranean, whereas the Levantine Basin shared haplotypes with both areas. The geographic isolation of the Mediterranean basins seems to enforce the population genetic differentiation of the species, with the deep sea acting as a strong barrier to its dispersal. Contrasting historical demographic patterns were also observed in different parts of the species' distribution, most notably a population growth trend in the western Mediterranean/Atlantic area and a slight decreasing one in the Aegean Sea. The different effects of the Pleistocene glacial periods on the habitat availability may explain the contrasting demographic patterns observed. The current findings suggest that the small-spotted catshark exhibits several genetic stocks in the Mediterranean, although further study is needed.

Fueling plankton production by a meandering frontal jet: a case study for the Alboran Sea (Western Mediterranean).

A three dimensional biophysical model was employed to illustrate the biological impacts of a meandering frontal jet, in terms of efficiency and persistency of the autotrophic frontal production, in marginal and semi-enclosed seas. We used the Alboran Sea of the Western Mediterranean as a case study. Here, a frontal jet with a width of 15–20 km, characterized by the relatively low density Atlantic water mass, flows eastward within the upper 100 m as a marked meandering current around the western and the eastern anticyclonic gyres prior to its attachment to the North African shelf/slope topography of the Algerian basin. Its inherent nonlinearity leads to the development of a strong ageostrophic cross-frontal circulation that supplies nutrients into the nutrient-starved euphotic layer and stimulates phytoplankton growth along the jet. Biological production is larger in the western part of the basin and decreases eastwards with the gradual weakening of the jet. The higher production at the subsurface levels suggests that the Alboran Sea is likely more productive than predicted by the satellite chlorophyll data. The Mediterranean water mass away from the jet and the interiors of the western and eastern anticyclonic gyres remain unproductive.

Metamorphic diamonds in a garnet megacryst from the Edough Massif (northeastern Algeria). Recognition and geodynamic consequences

We report for the first time the discovery of diamond inclusions (5–30μm in size), identified by Raman spectroscopy and the characteristic sharp band at 1332cm−1 for crystalline diamond, in a garnet megacryst (≥5cm) from the Edough Massif (NE Algeria). The garnet is adjacent to actinolite and ultramafic boudins and slices of marbles that are inserted within a major mylonite–ultramylonite band. This tectonic contact sharply delineates the Kef Lakhal oceanic unit from the granite-gneiss core below. The host garnet is almandine-dominant and is rich in exsolution of acicular rutile needles. Major and trace elements show a gradual but significant zonation from core to rim, and a sharp increase in grossular component in the rim. Trace element analyses of prismatic rutile inclusions up to 300μm in size indicate that the host metamorphic rock was a mafic protolith of MORB affinity. The Zr-in-rutile thermometry indicates a temperature range of 724–778°C that we relate to rutile growth, either during prograde metamorphism or under peak UHP metamorphic conditions of ≥3.6GPa that were reached during subduction of the UHP-rock precursors. We suggest that the mafic protolith originates from the subducted retreating Calabrian branch of the Tethyan slab, that broke or tore, and which fragments were dragged upward and thrust onto the North African margin along with the Kef Lakhal unit, shortly followed by formation of the Edough dome and opening of the Algerian basin.

Floating debris in the Mediterranean Sea

Results from the first large-scale survey of floating natural (NMD) and anthropogenic (AMD) debris (>2cm) in the central and western part of the Mediterranean Sea are reported. Floating debris was found throughout the entire study area with densities ranging from 0 to 194.6items/km2 and mean abundances of 24.9 AMD items/km2 and 6.9 NMD items/km2 across all surveyed locations. On the whole, 78% of all sighted objects were of anthropogenic origin, 95.6% of which were petrochemical derivatives (i.e. plastic and styrofoam). Maximum AMD densities (>52items/km2) were found in the Adriatic Sea and in the Algerian basin, while the lowest densities (<6.3items/km2) were observed in the Central Tyrrhenian and in the Sicilian Sea. All the other areas had mean densities ranging from 10.9 to 30.7items/km2. According to our calculations, more than 62 million macro-litter items are currently floating on the surface of the whole Mediterranean basin.

Early-stage rifting of the Southern Tyrrhenian region: The Calabria–Sardinia breakup

The Southern Tyrrhenian Sea is an extensional basins linked to the Neogene evolution of the Calabria subduction zone located in the western Mediterranean realm where controversial kinematic and geodynamical models have been proposed. Our study provides a key to unravel timing and mode of extension of the upper plate and the breakup of Calabria from Sardinia. By combining original stratigraphic analysis of wells and seismic profiles off Calabria with a stratigraphic correlation to onshore outcrops, we re-assess the tectonic evolution that controlled the sedimentation and basement deformation of the Southern Tyrrhenian basin during Serravallian–Tortonian times. We document the tectono-stratigraphic evolution of adjacent extensional basins characterized by 3rd order depositional sequences (Ser1, Tor1 and Tor2) and different modes of extension, subsidence and opposite dipping faults. Episodic basin development is recorded by a coarsening-up and fining-up trend of the sedimentary succession and by tectonically enhanced unconformities that reflect three episodes of fault activity. We reconstruct Serravallian–Tortonian paleogeographic maps and propose a block faulting model for the evolution of the Sardinia–Calabria area. Sardinia was disconnected from Calabria through N–S normal faults forming Tyrrhenian extensional basins that formed contemporaneously to the E–W opening of the Algerian basin. Unlike published Serravallian–Tortonian reconstructions of the western Mediterranean realm, our results support a geodynamic model characterized by rapid trench retreat, trench-normal extension in the entire overriding plate and very weak coupling between plates.

Seismic imaging of the eastern Algerian margin off Jijel: integrating wide-angle seismic modelling and multichannel seismic pre-stack depth migration

This study presents the results of a deep seismic survey across the north Algerian margin, based on the combination of 2-D multichannel and wide-angle seismic data simultaneously recorded by 41 ocean bottom seismometers deployed along a north-south line extending 180 km off Jijel into the Algerian offshore basin, and 25 land stations deployed along a 100-km-long line, cutting through the Lesser Kabylia and the Tellian thrust-belt. The final model obtained using forward modelling of the wide-angle data and pre-stack depth migration of the seismic reflection data provides an unprecedented view of the sedimentary and crustal structure of the margin. The sedimentary layers in the Algerian basin are 3.75 km thick to the north and up to 4.5-5 km thick at the foot of the margin. They are characterized by seismic velocities from 1.9 to 3.8 km s(-1). Messinian salt formations are about 1 km thick in the study area, and are modelled and imaged using a velocity between 3.7 and 3.8 km s(-1). The crust in the deep sea basin is about 4.5 km thick and of oceanic origin, presenting two distinct layers with a high gradient upper crust (4.7-6.1 km s(-1)) and a low gradient lower crust (6.2-7.1 km s(-1)). The upper-mantle velocity is constrained to 7.9 km s(-1). The ocean-continent transition zone is very narrow between 15 and 20 km wide. The continental crust reaches 25 km thickness as imaged from the most landward station and thins to 5 km over a less than 70 km distance. The continental crust presents steep and asymmetric upper- and lower-crustal geometry, possibly due to either asymmetric rifting of the margin, an underplated body, or flow of lower crustal material towards the ocean basin. Present-time deformation, as imaged from three additional seismic profiles, is characterized by an interplay of gravity-driven mobile-salt creep and active thrusting at the foot of the tectonically inverted Algerian margin.