The influence of transversal crustal discontinuities on the development of continental rift systems remains poorly constrained, especially because their connection with shallow normal faults is often unclear. Nonetheless, these basement faults likely affect the dynamics and the kinematics of the rifting, especially during the early stages of extension. Our study focusses on the Porcupine Basin, offshore west of Ireland, an aborted rift propagator that experienced a 220 Myr-long geological evolution with several rifting episodes. Detailed seismic analysis, integrated with exploration well data, illustrates the regional complexity of the structural patterns across the basin, with faults running subparallel or transverse to its axis. This tectonic framework controlled the northward migration of the crustal stretching during the Late Jurassic, followed by crustal thinning during the Early Cretaceous. Pre-existing, orogenic-derived structures bound crustal terranes that control deformation pulses when rifted apart. This suggests structural barriers that either slowed the northward rifting migration during the Oxfordian, Kimmeridgian and Tithonian when crosscutting through the Variscan and Caledonian fold-and-thrust belts, or stopped the rifting by the end of the Barremian when it encountered Caledonian and Grenvillian crystalline basements. We propose that this structural inheritance led to the formation of a typical rift propagator of continental nature, and that the Porcupine Basin constitutes a remarkable example of a termination of rifting processes in a well-formed oceanic rift system.

Convergence between African and European plates generates compressional strain, primarily concentrated along the northern African margin. This is testified on the Algerian margin by numerous earthquakes (e.g. Boumerdès, 2003) and by the presence of active folds and thrusts. Multi-channel seismic reflection profiles from the MARADJA I survey reveal north-verging thrusts rooted below the Messinian units, and the geometries of the Messinian salt structures. This study examines the characteristics of salt tectonics offshore Algiers and Dellys, focusing on the effect of the positive structural inversion of the former passive margin on geometries, timing, and mechanisms of salt deformation. The interpretation of seismic reflection and multi-beam bathymetric data of the MARADJA I survey, along with its comparison with analogue models, allowed us to reconstruct the salt tectonics processes on the margin and to identify the predominant role of a plateau uplift on salt deformation. Early and ubiquitous salt deformation by downbuilding was followed by a major phase of plateau uplift (end of Messinian Crisis), leading to westward gravity gliding and a slowdown of the salt deformation above the plateau. Km-tall salt structures were developed and thick minibasins deposited. Salt tectonics is nowadays active only where the relationship between salt and overburden thickness is favorable.

Due to the complexity and high dimensionality of seismic catalogues, the dimensional reduction of raw seismic data and the feature selections needed to decluster these catalogues into crisis and non-crisis events remain a challenge. To address this problem, we propose a two-level analysis. First, an unsupervised approach based on an artificial neural network called self-organising map (SOM) is applied. The SOM is a machine learning model that performs a non-linear mapping of large input spaces into a two-dimensional grid, which preserves the topological and metric relationships of the data. It therefore facilitates visualisation and interpretation of the results obtained. Then, agglomerative clustering is used to classify the different clusters obtained by the SOM method as containing background events, aftershocks and/or swarms. To estimate the classification uncertainty and confidence level of our declustering results, we developed a probabilistic function based on the feature representation learned by the SOM (spatiotemporal distances between events, magnitude variations and event density). We tested the two-level analysis on synthetic data and applied it to real data: three seismic catalogues (Corithn Rift, Taiwan and Central Italy) that differ in area size, tectonic regime, magnitude of completeness, duration and detection methods. We show that our unsupervised machine learning approach can accurately distinguish between crisis and non-crisis events without the need for preliminary assumptions and that it is applicable to catalogues of various sizes in time and space without threshold selection.

Many applications in seismology require to isolate earthquake clusters from a background activity. Relative declustering methods essentially find a 2D representation of an earthquake catalogue that distinguishes between two classes of events: crisis and non-crisis events. However, the number of statistical and/or physical parameters to be used is often limited due to the difficulty of concatenating the information onto a physically meaningful 2D grid. In this study, we propose to alleviate the declustering task by using the ability of unsupervised artificial intelligence to model complex spatio-temporal relationships directly from data. Through a data-driven approach, we define an easily transferable declustering model that provides declustering results with fewer assumptions and no prior selection of thresholds. We first obtain this model by training a self-organising neural network (SOM) that learns to cluster data points according to their feature similarity on a 2D map. We then assign each SOM cluster a label (crisis or non-crisis class) using an agglomerative clustering procedure. We quantify the classification uncertainty by developing a probabilistic function based on the projection learned by SOM. Our method is applied to a synthetic dataset and to real catalogues from the Gulf of Corinth, Central Italy and Taiwan. We discuss the validity of the method by estimating its classification accuracy. For real data, we qualitatively compare our results to previous declustering attempts. We show that our approach is easy to handle, provides a fairly new representation of earthquake catalogues and has the potential to reduce classification ambiguities between nearby events.

This chapter describes the tectonic processes at the origin of the Mediterranean Sea and a particular paleo-environmental event (the Messinian salinity crisis) that resulted in the almost complete desiccation of the Mediterranean Sea. It also presents the morphological characteristics of the basin floor and introduces the tectonic and climatic processes that control sedimentation and thus the evolution of the seafloor.

Abstract Transform marginal plateaus (TMPs) are large and flat structures commonly found in deep oceanic domains, but their origin and relationship to adjacent oceanic lithosphere remain poorly understood. This paper focuses on two conjugate TMPs, the Demerara Plateau off Suriname and French Guiana and the Guinea Plateau, located at the junction of the Jurassic Central Atlantic and the Cretaceous Equatorial Atlantic oceans. The study helps to understand (1) the tectonic history of both Demerara and Guinea plateaus and (2) the relationship between the Demerara Plateau and the adjacent oceanic domains, and finally, (3) throws light on the formation of TMPs. We analyse two existing wide-angle seismic-derived velocity models from the MARGATS seismic experiment (Demerara Plateau), and adjacent composite industrial seismic lines covering the Demerara and Guinea plateaus. The Demerara Plateau displays a 30 km thick crust, subdivided into three layers, including a high-velocity lower crust. The velocities and velocity gradients do not fit the values of typical continental crust but instead correspond to volcanic margin- or large igneous province-type crusts. We propose that the, possibly continental, lower crust is intruded by magmatic material and that the upper crustal layer is made from extrusive volcanic rocks of the same magmatic origin, forming thick seaward (westward)-dipping reflectors (SDRs) sequences. This SDR complex extends to the Guinea Plateau as well and was emplaced during hotspot (Sierra Leone)-related volcanic rifting preceding the Jurassic opening of the Central Atlantic and forming the western margin of the plateau. North–south composite lines linking the Demerara and Guinea plateaus reveal the spatial extent of the SDR complex but also a pre-existing basement ridge separating the two plateaus. The entire Demerara–Guinea margin would therefore be an inherited Jurassic volcanic margin bordering the Central Atlantic Ocean to the east, with a possible conjugate being the Bahamas Plateau on the other side of the ocean. This margin was then reworked during a non-coaxial Cretaceous second phase of rifting potentially accompanied by a magmatic event. Opening of the northern margin occurred in a transform mode splitting the Jurassic volcanic margin into two parts (the Guinea and Demerara TMPs), conceivably along a pre-existing basement ridge. Rifting of the eastern part of the Demerara Plateau occurred surprisingly along the eastern limit of the Jurassic SDR complex, forming the present-day eastern divergent margin of the Demerara Plateau. After that stage, the Demerara and Guinea plateaus are individualized on each side of the Equatorial Atlantic. This study also highlights the major contribution of thermal anomalies related to hotspots and superposed tectonic phases in the case of other TMPs that share numerous characteristics with the Demerara Plateau.

We present an overview of the crustal architecture of the continental margins of the oceanic Algerian Basin in the westernmost Mediterranean Sea. During the Cenozoic, and with a variable oblique convergence between the African and Eurasian plates, the Western Mediterranean Sea has experienced thinning and extension behind a tight orogenic arc formed by the Betics, Rif, and Tell Cordilleras. This study is focused on the structural style affecting the Messinian salt layer, which is mostly restricted to the deep domains of the Algerian Basin, where it is floored by a thin oceanic crust of probable Miocene age. Using deep-penetrating seismic profiles and wells from offshore western Algeria to southeastern Spain, we have analyzed the crustal structures affecting the domains close to the oceanic-continent transition on the three margins of the western Algerian Basin. Since the Early Miocene, active shortening in the Tell-Atlas domain has accommodated most of the plate convergence in the basin, whereas the Alboran margin in the west and the Iberian margin in the north experienced eastward and southward crustal extension and thinning, respectively, accompanied by volcanism. The Algerian margin in the south shows incipient thrusting of African continental crust over oceanic crust. This shortening occurred since at least the Late Miocene, also promoting decoupling and contraction of the deep, sub-horizontal Messinian salt layer. The salt exhibits diapir squeezing and suprasalt folding, whereas the presalt sequence preserves partially-inverted half-grabens. Salt tectonic processes along the northern and western margins of the Western Mediterranean Basin show contrasting structural styles formed by narrow extensional and transtensional domains with gentle salt anticlines. This region shows therefore a somewhat unusual salt-tectonic style, departing from the gravity-driven model typical of continental margins that contain an initial continuous, gently-dipping salt layer. In the Algerian Basin, salt is mostly restricted to deep water domain floored by oceanic crust, so it does not participate in significant gravity-driven deformation. Instead, Messinian salt and the suprasalt sequences underwent significant shortening along the southern margin, simultaneous with thick-skinned extension involving the Messinian evaporites in the northern and eastern margins.

The Algero-Balearic Basin (ABB) is an Oligo-Miocene back-arc basin resulting from a polyphase tectonic evolution involving Tethyan subduction retreat and bilateral slab tear propagation. The ABB was fully opened by the Tortonian, while the Gibraltar and Calabria arcs formed by the narrowing of retreating slab fragments. Since then, the Algerian margin has undergone a tectonic inversion, potentially preceding an incipient subduction as shown by the analysis of the on-offshore deformation distribution. In this work, we aim to shed light on the relationships between the large-scale structures inherited from the ABB opening and the recent margin inversion. For this purpose, we rely on two recent analyses, one addressing the ABB opening (Haidar et al., 2021) and the other mapping the inversion-related structures off-Algeria (Leffondré et al., 2021), both being constrained by a set of deep penetration multi-resolution seismic profiles cross-correlated with magnetic, gravimetric and bathymetric data. The deep ABB has been subdivided into 4 zones with relatively distinct geodynamic evolutions, as demonstrated by variations in pre-Messinian sedimentary infill thickness and basement depth : (1) the oldest, fan-shaped oceanic basin to the east (off-Jijel), formed during the Langhian-Serravallian after collision of the Kabylian blocks with the stretched African margin; (2) the shallower and younger Hannibal thinned continental domain (HD), intruded by intense post-collisional magmatic activity during the Upper Serravallian - Lower Tortonian; and ever-younger to the west, (3) the central-western (off-Algiers-Tipaza) and (4) westernmost zones, formed from the Tortonian to the Lower Messinian in response to the westward retreat of the Gibraltar slab and the concomitant migration of the Alboran block by propagation of vertical tears along a STEP (Subduction Transform Edge Propagator) type margins. The tectonic inversion is characterised by long-wavelength of flexure (>100km) of the ABB towards the Algerian margin and/or buckling of shorter wavelengths (≈30km). The central (HD) and central-eastern (off-Jijel) zones are dominated by flexure, whereas buckling is dominant in the central-western zone. Further, the easternmost (off-Annaba) and westernmost zones exhibit a combination of flexure and buckling. Except in the westernmost zone, characterized by low deformation on a single fault, the margin toe consistently displays inversion-related faults systems consisting of 3 to 4 south-dipping and sub-parallel thrust faults. By comparing the zonation of the deep ABB and the zones with different responses to inversion, we evidence a similar zonation of the margin, with only slight differences likely resulting from data density variations. To the east, the old and wide fan-shaped basin has favored the development of a significant flexural response, whereas the young westernmost zones, narrower and bordered by STEP-faults, evidence a combination of buckling and short-wavelength of flexure. The HD is a complex zone with a shorter wavelength of flexure compared to the eastern zone, probably related to magmatic activities affecting the potentially continental crust. Our results suggest that if initial zonation persists, several parameters may be involved in the control of the inversion mode. These parameters may include the opening-related structural inheritance, the oceanic lithosphere composition, as well as the age and former structures of the margin.

In spite of clear fan‐shaped magnetic anomalies in the Eastern Algero‐Balearic Basin (EABB), the way how and the time when seafloor spreading occurred are still debated. In this work, a new seismo‐stratigraphic interpretation based on deep‐penetration reflection seismic data correlated to reduced‐to‐the‐pole magnetic anomalies and on‐offshore litho‐stratigraphic correlation of Pre‐Messinian units brings new constraints on its age and mode of opening. Our results reveal that the seafloor spreading of the EABB occurred at an intermediate half‐spreading rate of 3.7 ± 0.5 cm/yr for 2.45 ± 0.18 Myr during the Langhian‐Serravallian times, that is, after the Corsica‐Sardinia block rotation and the collision of Lesser Kabylia with Africa. We revise the kinematics of the Algero‐Balearic domain into three stages: (a) birth of a highly stretched continental basin accommodating the southern drift of the Kabylies driven by slab rollback between ∼23 and ∼15 Ma, (b) fast opening of a new EABB between 15.2 and 12.7 Ma by clockwise rotation of a Greater Alboran Block (GALB) promoted by slab tearing, and (c) late East‐West opening of the western basin in response to the westward retreat of the Gibraltar slab and westward slab tearing propagation and the concomitant migration of the GALB. The last stages match both the late formation of Subduction‐Transform Edge Propagator faults at the toes of the Algero‐Balearic margins and the post‐collisional volcanic migration along the Algerian margin interpreted as related to slab tearing propagation. This new scenario favors a significant stretching and splitting of the GALB into several continental fragments resulting from the westward propagation of the arcuate subduction front by lateral tearing of a narrow slab.

The Algerian margin is located at the slow-rate convergent boundary between African and Eurasian plates and experienced several strong earthquakes in the last centuries. Among them, the 2003 Mw 6.8 Boumerdès event has triggered large turbidity currents in the slightly concave canyons of the slope and numerous cable breaks in the abyssal plain. In this study, we explore where, how and when the tectonic inversion of the margin off Boumerdès has left witnesses in the seafloor morphology and whether the observed deformation correlates with the 2003 coseismic rupture zone and with the Plio-Quaternary sedimentation. We have performed a careful analysis of the seafloor morphology and subsurface seismic reflectors at the landscape scale and along/between canyons by combining classical GIS-based methods and Virtual Reality techniques. From the mid-slope to the deep basin off the ∼60 km long Boumerdès-Dellys coast, we evidence large knickpoints corresponding to the development of four main cumulative fault scarps and two perched basins which are deeply incised by steep canyons and gullies. We interpret these structures to result from frontal propagation of two main south-dipping thrusts by upper crustal décollement ramping, evidencing an incipient sub-thrust imbrication in a stage of initiation of an accretionary wedge. The flat-ramp thrust geometry and their along-strike segmentation explain the development and shape of the perched basins in the backlimb of fault-related folds. The onset of growth strata is dated at 1.5 ± .5 Ma on the slope and .9 ± .3 Ma in the deep basin. The length, position, strike and segmentation of the older, southern thrust ramp are consistent with the coseismic characteristics of the Mw 6.8 2003 earthquake. The cumulative vertical scarp throw exceeds 1 km, supporting Quaternary shortening rates of 1.6 ± .7 mm/yr, in agreement with geodetic strain rates across the western Mediterranean basin. Virtual Reality offers powerful and promising means to correlate seismic imagery and seafloor morphology and is of great help to improve the robustness of tectonostratigraphic interpretation.

The Eastern Algerian basin (EAB) occupies a key position at the eastern tip of the Western Mediterranean. Although clear triangular-shaped magnetic anomalies (MA) are well identified since long, the way seafloor spreading occurred during the middle Miocene is still debated. In this work, a new seismo-stratigraphic interpretation of deep penetration seismic data correlated to reduced to the pole MA was used in order to specify the distribution of the oldest sedimentary deposits in the oceanic domain, and thus to assess the best opening models matching the MA fan-shaped pattern. According to this pattern, our results reveal that the seafloor spreading of EAB has occurred along an NW–SE-oriented accretion system giving birth giving birth to six magnetic inversions spanning ca. 2 Myr in Langhian Serravallian times. The oldest units overlying the oceanic floor are of Langhian, i.e., younger than the syn-rift deposits identified on the stretched continental crust further south. We discussed the implications of our findings on the current opening models of the Western Mediterranean Sea.

The Algerian margin (Western Mediterranean) reactivated in compression 8 My ago due to the convergence between Africa and Eurasia, and is nowadays subjected to a strain regime of several mm/y, resulting in destructive earthquakes as the M 6.8 Boumerdès event in 2003. The MARADJA I seismic reflection data acquired in 2003 allowed to image in detail the Messinian Salinity Crisis Mobile Unit (mostly halite) and its brittle sedimentary overburden offshore Algiers. Particularly interesting in the area are the salt-related geometries and the presence of crustal tectonic structures -consequence of the compressional setting of this margin- that created an uplifted plateau offshore Algiers (Déverchère et al., 2005; Domzig et al., 2006). The comparison with the MARADJA II (2005) data offshore Béjaia allowed to better distinguish between the regional trends and the local peculiarities. Together with the general analysis of the structures due to the salt tectonics, as well as the influence of crustal tectonics on salt deformation on the Algerian margin, this study is particularly focused on the geometry, position and triggering processes of a localized minibasins field, which started to form very early –possibly before the end of the Messinian Salinity Crisis- and is still active nowadays. This minibasins field position corresponds to both the external limit of a sedimentary body and the western limit of the previously mentioned uplifted plateau, raising the question of the relative influence of these two contributing factors in the formation of the minibasins. The analogue modelling contribution in this analysis is crucial.

The Algerian continental margin in the western Mediterranean formed as a back-arc basin and is today reactivated by the convergence between the African and Eurasian plates. It is one of the very rare examples of passive margins undergoing inversion expressed in a moderate seismicity of the margin and is possibly giving way for future subduction. With the objective to better know the deep structure of the margin and its associated basin, the origin of its seismicity and to study the mechanism of reactivation, five existing wide-angle seismic profiles along the margin are revisited. They were located offshore Mostaganem, Tipasa, Greater Kabylia, Jijel and Annaba. These profiles show that the basin is underlain by a 5 km thick crust of oceanic magmatic origin, possibly created at non-continuous small accretionary ridge segments. The continent-ocean transition zone is narrow, except at the easternmost profile, possibly due to an opening including a shear movement. No high velocity zone in the lower crust corresponding to mantle rocks has been imaged at the Algerian margin. The continental crust is thinned in a narrow and strongly segmented manner. It is widest (70 km) in the central segment offshore Greater Kabylia where there is a wider zone of distal thinned continental crust than on the other margin segments. The thickest crust detected during this survey corresponds to the African continental crust and the Kabylides blocks and is about 22-25 km thick. This reduced thickness in comparison with unthinned continental crust might be due to the influence of earlier subduction at the margin, in form of erosion by the subducting slab.

The Algerian margin, located in the Western Mediterranean basin, is reactivated in compression since 8 My due to the convergence between Africa and Eurasia, and is nowadays subjected to a N45W compression of several mm/y (Jolivet et al., 1995; Noquet et Calais, 2004). While the reactivation is attested by GPS measurements and destructive seismic events, such as the earthquake of Boumerdes in 2003 (M 6.8), the visualization in the seismic data of the deep structures is made difficult by the presence of a thick Messinian salt layer. The seismic reflection profiles acquired on the Algerian margin during the “Maradja I” oceanographic survey (2003) highlighted the presence of north-verging thrusts offshore Algiers (Déverchère et al., 2005; Domzig et al., 2006), as well as the peculiar geometry of the Messinian salt layer (Lofi et al., 2011, Obone Zue Obame, 2011). Between 2 and 4° East, the margin presents particularly complex salt structures, partly associated to the uplift of the plateau as a consequence of the crustal convergence (Déverchère et al., 2005; Domzig et al., 2006). One of the consequences of the uplift of the plateau is the dipping of the base salt horizon towards W to NNW. Moreover, from the analysis of the seismic reflection profiles, the presence of early (syn-UU) salt movement in the profiles parallel to the margin is clear, while the profiles perpendicular to the margin show compressional features mostly active during the Pliocene to Quaternary period. From the observation of the natural example, and from the comparison with different analogue models, we conclude that offshore Algiers we find the major salt structures and minibasins formed through salt spreading, while the area offshore Boumerdès is characterized by gravity gliding due to the uplifted plateau. Although from this point of view the N-S compressional tectonics favors gravity gliding through the plateau uplift, on the other hand it influences the salt structure development direction, which present a mainly E-W development and a minor and delayed N-S one. A partial influence of the sedimentary body from Algerian rivers on the position of the major salt structures is inferred.

The macroseismic and instrumental observations accumulated by the Bureau Central Sismologique Français and other national agencies over the last 100 years show that the northwestern part of metropolitan France is affected by an apparently diffuse and moderate intraplate seismicity. Far from any plate boundary, well-documented inherited structures, such as the Armorican shear zone network, the Sillon Houiller, and the normal faults related to the Atlantic ocean margin, likely exert significant control on the regional seismicity pattern. However, in the absence of a clearly measurable strain field, processes other than far-field tectonic stress loading such as erosion, gravitational potential energy, and/or hydraulic loadings can co-exist, but their respective influence on the current seismicity is debated and remains to be fully addressed. Reliable detection/location of low-to-moderate magnitude events is one of the most important challenges in the near future to better understand the processes that control this intraplate seismicity. As shown here for a limited region, this issue can be achieved positively, thanks to the new Résif-Epos network, in conjunction with sophisticated algorithms for both earthquakes’ detection and discrimination.

Tectonic inversion of passive margins is a common but poorly documented process preceding subduction inception. We perform here a comprehensive land‐sea experimental modeling of this key process by reproducing the morphotectonic and sedimentary evolution of the central Algerian margin over the last 6 Myr. Our approach is based on scaled analog models integrating interactions between crustal shortening and surface processes, including erosion, water transport, sedimentation, gravitational instabilities, and base‐level changes. A challenge was to simulate the effects of the Messinian Salinity Crisis (MSC) through a major sea‐level oscillation and halite deposition. By using realistic boundary conditions, adapted analog material, and robust, first‐order parameters for physiography setups, we successfully reproduce the morphotectonic domains and the time‐dependent geometrical relationships between fluvio‐deltaic sedimentary systems, erosional surfaces, and thrust faults as observed since Messinian times. Our results highlight (1) the key role played by the MSC sea‐level oscillation on an ultra‐fast building, destruction and re‐sedimentation of fans and deltas from the upper slope to the abyssal plain; (2) the development of a large popup structure subparallel to the coastline, with progressive strain migration from the backthrust on land toward a frontal thrust of opposite vergence at mid‐slope and the margin toe; and (3) the importance of lateral changes in initial wedge shape and strain distribution for determining the non‐cylindrical geometry of the margin and progradation of piggy‐back basins during tectonic inversion. Our results support that the central Algerian margin is witnessing the early building of an accretionary wedge combining thin‐skinned and thick‐skinned tectonic styles.

Subduction initiation is an important but still poorly documented process on Earth. Here, we document one of a few cases of ongoing transition between passive and active continental margins by identifying the geometrical and structural signatures that witness the tectonic inversion of the Algerian continental margin and the deep oceanic domain, located at the northern edge of the slow-rate, diffuse plate boundary between Africa and Eurasia. We have analyzed and tied 7900 km of deep seismic reflection post-stacked data over an area of ∼1200 km long and ∼120 km wide. The two-way traveltime lines were converted into depth sections in order to reconstruct and map realistic geometries of seismic horizons and faults from the seafloor down to the acoustic basement. Along the whole length of this young transitional domain, we identify a clear margin segmentation and significant changes in the tectonic signature at the margin toe and in the deep basement. While the central margin depicts a typical thick- and thin-skinned tectonic style with frontal propagation of crustal thrust ramps, the central-eastern margin (Jijel segment) reveals a higher strain focusing at the margin toe together with the largest flexural response of the oceanic lithosphere. Conversely, strain at the margin toe is limited in the western margin but displays a clear buckling of the oceanic crust up to the Spanish margin. We interpret these contrasting, segmented behavior as resulting from inherited heterogeneities in (1) the geometry of the Algerian continental margin from West to East (wrench faulting in the west, stretched margin elsewhere) and (2) the Miocene thermal state related to the diachronous opening of the Algerian basin and to the magmatic imprint of the Tethyan slab tearing at deep crustal levels. The narrow oceanic lithosphere of the Western Algerian basin is assumed to favor buckling against flexure. From the dimension and continuity of the main south-dipping blind thrusts identified at the margin toe, we reassess seismic hazards by defining potential lengths for ruptures zones leading to potential magnitudes up to 8.0 off the central and eastern Algerian margins.