North African Margin Research Articles

Geochemistry of metasedimentary rocks from the Eastern Pyrenees (Iberian Peninsula): Implications for correlation of Ediacaran terranes along the Gondwanan margin

The North African margin of Gondwana formed by subduction and accretion during Ediacaran to Cambrian times. Sections of this active margin were incorporated during the late Paleozoic to the Variscan and Appalachian orogens. In the Variscan realm, these sections define in some regions a Cadomian basement comprising thick metasedimentary series associated with coeval subduction-related magmatic rock dated in the range 650–500 Ma. In the Cap de Creus Massif (Eastern Pyrenean domain), the Cadomian metasedimentary rocks formed from sediments derived from mixed felsic and intermediate/mafic sources, intruded by granitoid and minor mafic igneous rocks. These series were formed in a peri-Gondwanan volcanic arc setting, like other Cadomian series described in the SW of the Iberian Massif, such as Serie Negra Group of the Ossa Morena Complex. However, the Nd model ages of the Cap de Creus metasedimentary samples range from 1431 to 1620 Ma and are younger than those of their SW Iberia equivalents. This result can be interpreted as an indication of an eastern location in the Ediacaran - Cambrian peri-Gondwanan magmatic arc. Furthermore, the Cadomian series in the Central Iberian Zone have even younger Nd model ages than those of SW and NE Iberia. We present two end-member paleogeographic models consistent with reconstruction of the initial location of the Cap de Creus Massif and its relations with other domains (Ossa Morena Complex and Central Iberian Zone) in this sector of the North African Gondwanan margin.

Upper Cenomanian caprinulid-radiolitid rudists (Bivalvia) from the Gattar Member of Jebel el Kebar (central Tunisia): Stratigraphical implications and palaeobiogeographical relationships with coeval rudist-assemblages from carbonate platforms of the southern Tethyan margin

The Gattar Member (upper part of the Zebbag Formation) is a prominent dolomitic limestone unit in Tunisia, the age of which was ascribed either to the Turonian, the Cenomanian, or the Cenomanian−Turonian. Here, we present the discovery of a Caprinulidae-Radiolitidae rudist asssemblage in the Gattar Member of Jebel el Kebar (central Tunisia). This consists of Caprinula cf. boissyi, Sauvagesia cf. sharpei, Durania arnaudi, Neocaprina sp. and Eoradiolites sp. The detailed description of two sections of the Gattar Member in Jebel el Kebar allows us to define the sedimentological and palaeoecological attributes of the rudist-bearing carbonates and their sequential relationships with the Zebbag and Rouana stratigraphical units respectively. The caprinulid-radiolitid assemblage is age-calibrated, based on the ammonite standard zonation of the Cenomanian stage from Portugal, South-Eastern France, and Oman, where rudist, benthic foraminifera and ammonite Zones have been correlated. Its age is late Cenomanian. The correlations of the Gattar Member between central Tunisia and the Gafsa-Chotts area are established on the base of ammonite and rudist biostratigraphical markers. The age of the Gattar Member of southern Tunisia and of its stratigraphical counterpart in western Libya is discussed. At last, we analyze the palaeobiogeographical distribution of the upper Cenomanian caprinulid-radiolitid assemblages from coeval carbonate platforms of the North African margin (Algeria, Morocco) and of the North Arabian margin (Levant region: Syria, Jordan, Lebanon, Israel, North Sinai).

Late extension of a passive margin coeval with subduction of the adjacent slab: The Western Alps and Maghrebides files

The evolution of the Alpine Tethys margins during the beginning of the African-Eurasian convergence was little studied compared to their evolution during the post-Pangea rifting and oceanic expansion,i.e., from the Early Jurassic to the early Late Cretaceous. The present work firstly aims to make up for this shortcoming in the case of the distal European margin of the Alpine Tethys, namely the Briançonnais domain of the Western Alps. We show that this margin was affected by strong post-rifting extension mainly in Late Cretaceous-Paleocene times and propose to make it the type of the (rare) “Late Extension Passive Margins”. Remarkably, this extension shortly preceded Lutetian times, when Briançonnais margin encroached the SE-dipping subduction zone under the Adria microplate. Secondly, we assess the post-rifting evolution of the north-Tethyan paleomargin in the Maghrebides transects,i.e., south-west of the Briançonnais transect along the same European-Iberian paleomargin. For this purpose, we consider the Triassic-Eocene series of the “Dorsale Calcaire” in the Alkapeca Blocks located along southeastern Iberia until the Eocene then transported onto the North African margin. Examination of the literature shows that the Tethyan margin of the Alboran block was strongly affected by normal faulting as early as Late Jurassic-Early Cretaceous times whereas post-rifting extension of the Kabylian blocks mainly occurred in the Late Cretaceous-Paleocene like in the Briançonnais. We propose that post-rifting extension of the Alboran block southern margin resulted from the sinistral movement of Africa relative to Iberia while the later extension of the Kabylian blocks can be related to the further convergence kinematics. Subduction of the Ligurian-Maghrebian slab under the North African margin would have occurred at that time in the southward continuation of the Alpine subduction. The overriding Adria and North African margins did not experience significant compression at that time. During the Eocene, a subduction polarity reversal occurred, which was associated with the relocation of the subduction zone along the Alkapeca block. This was the beginning of the Apenninic subduction, which triggered the back-arc opening of the Mediterranean basins.

Evaluación del peligro de tsunami en la costa argelina: estudio de escenario de un tsunami generado por un terremoto en la bahía de Argel

The western Mediterranean coasts have been hit by tsunamis in the past and can be opposed to this risk at any time. The North African margin is the main area prone to destructive earthquakes that sometimes trigger tsunamis and potentially harm human life and coastal city property. The present study consists of numerical modeling of a wave of tsunami acting on the Algerian coasts for an earthquake of magnitude Mw 7.55 with length and width of the fault (73.3 x 29.9 km) and a vertical slip of 3.5 meters. This numerical modeling is based on a code of calculations developed based on the two-dimensional hydrodynamic model sources Telemac-2D, making it possible to map the heights of the waves and maximum speeds generated by the tsunami. The simulation indicates that the maximum values for the surface heights are about 1.2 meters and 30 to 50 m/s for the flow velocities. The results show that our study area is more exposed to the inundation hazard by a tsunami causing significant damage to basic infrastructure. The results obtained are exported to a GIS information platform allowing the development of an illustrative cartography of the hydraulic characteristics of the tsunami's wave generated and the arrival time and displacement of the inundation with the location of vulnerable areas to inundation. This constitutes a decision-making tool for land use planning decision-makers and a means of Risk mitigation in the event of potential earthquakes causing tsunamis in the region.

Middle-Late Pleistocene Eastern Mediterranean nutricline depth and coccolith preservation linked to Monsoon activity and Atlantic Meridional Overturning Circulation

The eastern Mediterranean Sea lies under the influence of high- and low-latitude climatic systems. The northern part of the basin is affected by Atlantic depressions and continental and polar air masses that promote intermediate and deep-water formation. The southern part is influenced by subtropical conditions and monsoon activity. Monsoon intensification results in enhanced freshwater discharge from the Nile River and other (now dry) systems along the North African margin. This freshwater influx into the Mediterranean Sea reduces surface water buoyancy loss. Disentangling the influences of these diverse climatic forcings is hindered by inherent proxy data limitations and by interactions between the climatic forcings. Here we use a wealth of published and new paleoclimate records across Termination II to understand the impacts of the higher latitude and subtropical/monsoon climate influences on coccolithophore ecology and holococcolith preservation in Aegean Sea sediment core LC21. We then use these findings to interpret coccolith assemblage variations at Ocean Drilling Program Site 967 (located nearby LC21, at the Eratosthenes Seamount) during multiple glacial-interglacial cycles across the Middle Pleistocene (marine isotopic stages 14–9). The LC21 analysis suggests that holococcolith preservation was enhanced during Heinrich Stadial 11 (∼133 ka) and cold spell C26 (∼119 ka). These two events have been previously linked to cold conditions in the North Atlantic and Atlantic Meridional Overturning Circulation weakening. We propose that associated atmospheric perturbations over the Mediterranean Sea promoted deep-water formation, and thus holococcolith preservation. Similarly, in the Middle Pleistocene (MIS 14-9) of Site 967, we observe temporal coincidence between ten episodes of enhanced holococcolith preservation and episodes of Atlantic Meridional Overturning Circulation slowdown. In Site 967, we also identified repeated fluctuations in placoliths and in Florisphaera profunda, which indicate nutricline depth variations. The development of a deep chlorophyll maximum is associated with the North Africa and wet phases, as recently observed using elemental proxy records at Site 967, during the deposition of sapropel layers. A further deep chlorophyll maximum development is identified during MISs 12 and 10, as a result of pycnocline and nutricline shoaling within the lower part of the photic zone due to glacial sea-level lowering and water mass transport reduction at both the Gibraltar and Sicily Straits. Finally, enhanced holococcolith preservation during cold/dry events is clearly correlated to weakened monsoon activity in both Africa and Asia.

Present-day overpressure in southern Tunisia: Characterization, possible causes and implications for drilling operations

Formation pore pressure in sedimentary basins is known to vary with depth. These variations are caused by one or more sedimentary, tectonic or diagenetic mechanisms. In Mesozoic basins of southern Tunisia, a detailed study has been carried out in order to characterize the regional pore pressure profile. The results show an over-pressure interval ranging from upper Triassic to lower Jurassic salts series.The study of possible overpressure mechanisms allows us to conclude that the present-day overpressure can be explained by the active compressional tectonics that is affecting the North African margin. Nevertheless, other past overpressures have been interpreted from this study and have been attributed to different mechanisms. One of the most important paleo-overpressure mechanisms was gypsum dehydration. This diagenetic reaction has led to pore pressure approaching lithostatic pressure, demonstrated by hydraulic fractures and beef veins observed in fields.In this study, we established a method based on a depth chart diagram enabling the identification of the likely mechanism involved in reservoir overpressure build-up and/or the prediction of a potential mechanism. This result could be important in the analysis of unexplored basins to predict the anticipated pressure profile and thereby reduce the risk of encountering unexpected overpressured zones and related drilling hazards. This method could be applied to other sedimentary basins with standard geological backgrounds.

Isotopic and geochemical record of the active to passive margin transition in NW Iberia during the Cambrian-Ordovician: vestiges of a waning continental arc

Time-resolved geochemical data from the orthogneisses of the basal units of the Allochthonous complexes of NW Iberian Massif allow us to track the changes in composition of the magmas generated in a subduction-related setting that affects a continental margin, which evolves from the building of a volcanic arc to its dismantlement during a rifting episode. The basal units are interpreted as a peripheral section of the North African margin of Gondwana and record Neoproterozoic to Cambrian arc-related magmatism, followed by an episode of Cambro-Ordovician rifting. The intrusive rocks of the basal units, preserved as orthogneisses, can be grouped into two magmatic suites: calc-alkaline (c. 493–475 Ma) and alkaline to per-alkaline (c. 475–470 Ma). The calc-alkaline association has a chemical composition ranging from magnesian calcic metaluminous types to ferroan calc-alkalic and alkali-calcic peraluminous types. The moderately alkaline association and per-alkaline metagranites have ferroan alkalic and per-alkaline to metaluminous composition. The chemical characteristics of the metaigneous rocks from the basal units indicate that the older members of the calc-alkaline suite (tonalites and granodiorites) were generated in a continental arc environment, while the alkaline and per-alkaline rocks were formed in an intraplate extensional setting. The negative values in eNd (− 7.8 to − 1.5) of the calc-alkaline suite suggest a crustal influence in its generation. The eNd values in the alkaline orthogneisses range between − 2.0 and 2.2, and are positive in the per-alkaline group, suggesting a greater participation of a mantle component. The transition from the calc-alkaline suite to the alkaline suite is represented by alkali-granites, which are coeval with the intrusion of basic magmas, and likely derived from a mantle magma underplating that triggered the rifting episode after the cessation of continental arc magmatism during the Ordovician.

Syn-Collision Folding and Multi-Layers Detachment in the Molassic Basin, Northeastern Tunisia

Tunisia had been the place of various tectonic episodes which have drawn the actual structural map of the country. The Neogene period was the most active period in which the collision contributed to the mountain ranges formation such as the Atlassic chain. The contrast of mechanical behavior between the different sedimentary units of the cover and the substratum is a determining factor that has controlled the deformation style of many folding structures. Detachment folding has been studied on the basis of field and seismic data examples from the Northeastern Tunisia. We discuss different aspects of detachment to constrain the chronology of structural events and understand the cover‒substratum relationship during the major folding phases. In the present paper, we found that the deformation style admitted previously do not reflect adequately the tectonic structure of Kechabta Neogene basin (North of Tunisia). The cover-substratum relationship study at this area revealed two major shortening phases; the first had been happened during the Upper Tortonian and the second in the Lower Quaternary. The resulting folding style is characterized by disharmonic and multi-layers detachment phenomena produced by incompetent evaporitic sequences. The two described folding phases are superimposed and testify at a large scale the continental collision which controls the North African margin during the Late Miocene to the Quaternary.

Mesorif Gabbros (External Rif, North Morocco): Ophiolitic Suture Witness or a “Newly Discovered” Manifestation of the Central Atlantic Magmatic Province?

AbstractThe Rif Mountain located in Northern Morocco represents the westernmost extremity of the Peri‐Mediterranean Alpine orogenic system. This belt is classically subdivided into: 1) Internal zones (or Internal Rif which is an allochthonous metamorphic domain known worldwide thanks to the presence of large peridotite outcrops); 2) flysch zones (sediments corresponding to the original cover of the Maghrebian Tethys); and 3) External zones (or External Rif representing the North African margin remnants).The External Rif is itself subdivided into Prerif, Mesorif and Intrarif domains from the South to the North and is composed by Upper Triassic to Cenozoic sediments structured in a nappe stack. Mafic rock intrusions were reported in this External Rif since the preliminary geological studies even if they were wrongly mapped as “granites”. Later careful fieldwork and mapping studies specified nevertheless their gabbroic nature. A more recent petrological and geochemical study of a few outcrops of those gabbroic intrusions conducted the corresponding authors, encouraged by the presence of ultramafic rocks (Beni Malek serpentines) in the eastern part of the External Rif to consider those gabbros as oceanic and so, associated to “an ophiolitic suture”. Nevertheless, a recent and extended survey on the architecture and the rifting history of the Mesozoic margin of the Maghrebian Tethys focusing mainly on the Mesorif domain favor another interpretation. The detailed petrography of those mafic intrusions (mainly gabbros + dolerites and rarely extrusive rocks); their geochemical fingerprints and mainly their geochronological data (ages around 200 Ma based on U‐Pb zircon LA‐ICP dating) all together indicate clearly that those mafic intrusions are rather a “newly discovered” manifestation of the Central Atlantic Magmatic Province activity “CAMP” in the External Rif belt. The CAMP magmatism has been largely well described and characterized in all the other geological domains of Morocco (Anti‐Atlas, High Atlas, Middle Atlas and Meseta) but never described until the date in the Rif belt which highlights the importance of the results reported here.

The Rif Chain (Northern Morocco) in the Late Tortonian‐Early Messinian Tectonics of the Western Mediterranean Orogenic Belt: Evidence From the Tanger‐Al Manzla Wedge‐Top Basin

AbstractDuring the Neogene, the Western Mediterranean subduction‐related orogen developed under differing modes and senses of subduction, resulting in the formation of the Apennines, Maghrebides, Rif, and Betics. In this work, we present the Neogene kinematic evolution of the Rif, based on literature data and new results from structural‐stratigraphic analyses and biostratigraphic investigations carried out in the External Zone of the Rif. We analyzed three stratigraphic sections: Dar Zhirou, Saf Lahmame, and Seguedla. The results of these analyses allow us to reconstruct a wide Tortonian‐Messinian wedge‐top basin in the Tanger‐Al Manzla area, when the leading edge of the Rif was at the frontal thrust of the Prerif. The presence of this wedge‐top basin points to a Tortonian‐Messinian compressional deformation that affected a wide area of the Rif, including the Prerif, Mesorif, and Intrarif. The late Miocene compressional deformation that affected the Mesorif and Intrarif occurred as out‐of‐sequence thrusting, due to renewed compressional tectonics in the internal zone of the chain triggered by the collision of the Rif accretionary wedge with the North African Margin. In the Neogene evolution of the Western Mediterranean subduction‐related orogen, the late Miocene was a major pulse in the interplay between African and European plates, as evidenced by the increase in migration rates of some segments of the circum‐Mediterranean fold‐and‐thrust belt, the out‐of‐sequence thrusting in the Rif, as well as the occurrence of late Miocene imbricate thrust structures in oceanic fracture zones of the African Plate (Central Atlantic Ocean).

Caractéristiques pétrologiques et géochimiques des roches magmatiques d’El Aouana, NE algérien

[fr] Les roches ignées du massif d’El Aouana font partie de la chaîne magmatique miocène des Maghrébides qui s’étend du nord de la Tunisie jusqu’au Maroc. Ces roches sont composées de faciès volcaniques (andésites et dacites) et subvolcaniques (microdiorites et microgranodiorites), qui ont été mis en place au sein des flyschs crétacés, oligo-miocènes et sédiments miocènes post-nappes. Les andésites sont composées de phénocristaux de plagioclase, d’amphibole et de pyroxène dans une mésostase microlithique. Les dacites sont plus riches en plagioclase avec de rare minéraux ferromagnésiens souvent altérés. Les microdiorites et microgranodiorites sont généralement à hornblende verte, plagioclase, pyroxène et rare biotite dans une mésostase microcristalline. Les résultats géochimiques montrent que ces roches sont calco-alcalines avec une affinité type-I et elles présentent un enrichissement en éléments à grand rayon ionique (LILE) et éléments de terres rares légers (LREE) par rap­port aux éléments à champs électrostatique élevé (HFSE) et aux éléments de terres rares lourds (HREE). Les anomalies négatives en Nb, P et Ti témoignent d’un magma de zones de subduction. Les observations de terrains, les données pétrographiques et géochimiques montrent que les granitoïdes miocènes d’El Aouana ont été mis en place dans un environnement post-collisionnel. Ces roches sont similaires aux granitoïdes métalumineux, post-collisionnels de l’Algérie du nord, considérés comme dérivés d’une source mantellique métasomatisée suite au processus de ‘slab break-off’ sous la marge nord-africaine.

The Mesozoic Margin of the Maghrebian Tethys in the Rif Belt (Morocco): Evidence for Polyphase Rifting and Related Magmatic Activity

AbstractThe Rif belt (northern Morocco) is a mountain chain located at the junction between the Mediterranean and Central Atlantic Domains. Although the Rif belt underwent important Cenozoic (i.e., Alpine) shortening, remnants of the Mesozoic North African rifted margin are preserved in its external zones. This contribution aims to characterize the Mesozoic architecture and polyphase rifting history of this rifted margin. We present detailed field evidence and geochronological data from two palaeogeographic zones (Mesorif and Intrarif) preserving remnants of the former North African distal margin. The Mesorif conserves lithostratigraphic associations characterized by mafic intrusive rocks overlain by dismembered and discontinuous blocks of Lower Jurassic carbonates covered by Middle to Upper Jurassic sediments. U‐Pb zircon dating of four samples from this gabbroic complex shows ages close to the Triassic‐Jurassic boundary (195–200 Ma). The gabbros were emplaced within the continental crust at the end of the first Triassic rift event and exhumed shortly after during a second Middle Jurassic rift event, which presents exceptional rift‐related structures. The most distal part of the margin is exposed in the Intrarif. In this unit, the Beni‐Malek serpentinized peridotites exhibit ophicalcites with uppermost Jurassic limestones resting conformably on top, suggesting that exhumation of the mantle occurred at the distal part of the North African margin at this time. When integrated, these new evidences enable us to discuss the evolution of the western part of the North African rifted margin and its relations with the Moroccan Atlantic margin and Tethys system.

Interpretation of gravity data using 3D inversion and 2D continuous wavelet transform in Hedil deformed structures, northern Tunisia

The Hedil region in northern Tunisia shows several geological structures associated with intense tectonic deformations that induced genesis of overthrusts and tectonic imbrications with rising of Triassic evaporites. This region constitutes a transition zone, between the Atlassic and Tellian domains, and exhibits multi-scale tectonic deformations related to the basins structure and the regional geodynamic of the Northern African margin. Gravity data processing and continuous wavelet transform, coupled with field data, lead to underline the subsurface geometric configuration and to propose an evolutionary structural framework for the studied area.The horizontal gradient magnitude (HGM) and its maxima highlight several lineaments trending NE-, E−, NW and N however the NE-SW direction is predominantly expressed. Depths of the main buried structures were estimated using the Euler deconvolution method. The highest values that exceed 3 km seem to underlines the structures associated to NE-SW, NW-SE and rarely E-W inherited features. The NE-SW oriented structures are mainly represented by overthrusts and tectonic imbrication zones that were occurred along inherited faults in the southern part of the area, as a result of the Atlassic compressional tectonics of Late Miocene and Quaternary ages. However the other lineaments correspond mainly to strike-slip and reverse. The application of 3D inversion method allows identification and characterization of geological structures in subsurface. The most deformed structures are marked by positive anomalies and are sometimes underlined by intrusion of Triassic evaporite rocks. However, the collapsed structures are underlined by negative anomalies testifying a material of low density. The extracted inversion sections highlight the deep rooted geometry of the Triassic bodies, locally associated with overturn of pierced strata.The continuous wavelet transform method was used to get an idea about the deep structures of the region, to estimate the boundary between the sedimentary filling and the Precambrian basement, and determining the depth and dipping of some identified contacts. Some uplifted structures and Triassic evaporite intrusions especially that of the Bazina are marked by risings of pre-Triassic deep causative sources. These zones of significant anomalies in the Atlassic domain are probably areas of intense deformation related to the ancient Hercynian tectonics that affected the North African margin. The identified structures should be related to the regional tectonic inversion, related to the African and European plates subduction.

The lower crust of the Northern broken edge of Gondwana: Evidence for sediment subduction and syn-Variscan anorogenic imprint from zircon U-Pb-Hf in granulite xenoliths

The continental basement in the Eastern Mediterranean area represents the northern edge of Gondwana, which has been the site of repeated crustal accretion and has subsequently been modified by consecutive rifting events. We investigated the geologic and thermal history of the North Gondwana lower crust by examining the U-Pb-Hf isotope systematics in zircons within 6 mafic granulite xenoliths from Pliocene lava cone in North Israel. The lava cone protrudes through the platform cover that seals the late Neoproterozoic junction between the Arabian-Nubian basement to the South and the Cadomian basement exposed in the Taurides to the North. The mafic granulite xenoliths are composed of plagioclase + orthopyroxene + clinopyroxene ± garnet ± spinel ± secondary amphibole. U-Pb zircon ages from the granulites vary among the different samples with distinct zircon age populations at 400–1200 Ma, 170–350 Ma, and 3.6–4.2 Ma, attesting the lower crust preserves a prolonged thermal and igneous history. While 400–550 Ma U-Pb ages are interpreted to be the result of Pb loss, the wide scatter of zircon grains aged between 550 and 1200 Ma, alongside their diverse εHf(t) values (−25–+10), is an extraordinary evidence for the accretion of Neoproterozoic sediments into the North Gondwana arc root lower crust. The U-Pb-Hf signature of these zircons resembles Cadomian sediments of the Tauride block to the north, indicating southward (present coordinates) subduction under North Gondwana and possible accretion of fore-arc sediments to the lower crust through relamination in the latest Neoproterozoic. One xenolith contained metamorphic-shaped zircons aged 170–350 Ma with positive εHf values and Hf-TDM of 0.85 Ga interpreted to reflect Paleozoic recycling of the Neoproterozoic juvenile Arabian basement, which we consider to form a major component of the lower crust in the region. An overwhelming cluster of Carboniferous zircons concentrating at 305 Ma with exclusively negative εHf values around −6, was retrieved from three xenoliths. Some of these zircons portrayed igneous textures and shape. While Carboniferous igneous activity is the hallmark of Western Europe's Variscan orogeny, the latter did not affect the southern rifted edge of Neo-Tethys where our xenoliths were retrieved. The Paleozoic age-Hf composition in our xenoliths is therefore interpreted to result from syn-Variscan recycling of Neoproterozoic sedimentary remains in the lower crust, and some degree of melting in a non-orogenic environment. Rather than with horizontal plate motions and orogeny, the Carboniferous zircon ages in the xenoliths appear to coalesce with significant vertical movements that created continental scale unconformities and a broad basin and swell architecture known to develop over the entire North Gondwana margin at that time. The Carboniferous aged zircons in northern Israel lower crustal xenoliths are therefore a unique gauge of the thermal perturbation that accompanied the large-scale mantle dynamics below the then passive North African margin of Gondwana, while Variscan orogenic accretion occurred on the Eurasian margin. These lower crustal granulite xenoliths therefore contain important information with respect to the nature of the lower crust under Israel, with implications on the geodynamic setting during the Cadomian and Variscan cycles.

Towards subduction inception along the inverted North African margin of Algeria? Insights from thermo-mechanical models

While ocean subduction at continental margins is a prominent process of plate tectonics, understanding how and where it begins is still being debated, especially because examples of emerging ocean–continent subduction in the world are rare. Northern Algeria is currently undergoing a slow compression deformation due to the ongoing African–Eurasian convergence. Active compressional seismic activity recorded both on land and at sea indicates that the margin might be transitioning from a passive stage to an active one.In order to test this hypothesis, we perform thermo-mechanical models of margin inversion. Varying thermal and rheological parameters as well as the geometry of the margin boundary, we find that tectonic inversion of a young passive margin localizes at the margin toe only if the latter is strongly heated (i.e., with an abrupt thermal gradient between oceanic and continental lithospheres); otherwise, deformation propagates into the weak, hot oceanic lithosphere. The presence of a thinned continental crust at the ocean–continent transition favors either subduction of the oceanic lithosphere when the transition zone plunges towards the continent, or an indentation of the lower continental crust by the oceanic lithosphere when the transition zone is vertical. If the oceanic lithosphere is directly in contact with the continental margin, subduction-like deformation occurs during the early stages of the model but rapidly gives place to intra-oceanic buckling and faulting.Comparing the results of the simulation to the active tectonic structures of the Algerian margin, we conclude that both processes (emerging subduction or indentation) are possible and that the presence of a thermal anomaly beneath the thinned continental margin is probable, in relation with slab rupture at depth or with other thermal weakening processes.

Contribution of integrated geophysical data in the structural characterization of the El Hamma graben zone (Southern Tunisia): Example of a pull-apart basin along the South Atlasic Fault Corridor

The El Hamma-Gabes Region in southeastern Tunisia belongs to the crustal shear zone of the South Atlasic Fault Corridor, which corresponds to the Meso-Cenozoic North African margin. So far, most geological works on this region are on the stratigraphic order. However, there are few tectonic and structural studies carried on the style and mechanism of structural deformation. The present work analyzes the subsurface structure of the El Hamma grabens area based on gravity and seismic data, calibrated by stratigraphic and tectonic elements. The El Hamma-Gabes Zone is characterized by a horst and graben system associated with N130˚-150° trending parallel extensive/transtensive faults. Two main grabens are recognized: the El Hamma graben in the west and the Chenchou graben in the east. They are separated by the Jebel Hallouga horst. These structures relate to two N100˚-120° trending principal shear corridors (PSC) arranged in right echelon, being the Oum Ali-Fejej Shear corridor towards the northwest and the Matmata-Medenine Shear corridor towards the southeast.Stratigraphic and structural data collected from several outcrops were correlated with the tectonic events and paleostress reconstruction. New data shows that the PSCs marked a dextral transtensive kinematic framework during the N80° directed Upper Cretaceous extension. The extensional movements are the N150˚-180° Paleogene extension and associated transpression during the N130° Neogene shortening. The dextral sliding generated and maintained the opening of the El Hamma-Gabes graben system, creating a “pull-apart” basin that opened in an extensive ramp between the two PSCs. The overall extension of this ramp is estimated to 15% of the initial width.

The Tell-Rif orogenic system (Morocco, Algeria, Tunisia) and the structural heritage of the southern Tethys margin

The Tell-Rif (Tell in Algeria and Tunisia; Rif in Morocco) is the orogenic system fringing to the south the West Mediterranean basins. This system comprises three major tectonic-palaeogeographic zones from north to south: (1) the internal zones (AlKaPeCa for Alboran, Kabylies, Peloritan, Calabria) originating from the former northern European margin of the Maghrebian Tethys, (2) the “Flyschs zone” regarded as the former cover of the oceanic domain and (3) the external zones, forming the former southern Maghrebian Tethys margin more or less inverted. The Tell-Rif is interpreted as the direct result of the progressive closure of the Maghrebian Tethys until the collision between AlKaPeCa and Africa and, subsequently, the propagation of the deformation within Africa. This gives a consistent explanation for the offshore Neogene geodynamics and most authors share this simple scenario. Nevertheless, the current geodynamic models do not completely integrate the Tell-Rif geology. Based on the analysis of surface and sub-surface data, we propose a reappraisal of its present-day geometry in terms of geodynamic evolution. We highlight its non-cylindrical nature resulting from both the Mesozoic inheritance and the conditions of the tectonic inversion. During the Early Jurassic, we emphasize the development of NE-SW basins preceding the establishment of an E-W transform corridor connecting the Central Atlantic Ocean with the Ligurian Tethys. The Maghrebian Tethys developed just after, as the result of the Late Jurassic-Early Cretaceous left-lateral spreading between Africa and Iberia. By the Late Cretaceous, the occurrence of several tectonic events is related to the progressive convergence convergence between the two continents. A major pre-Oligocene (pre-35 Ma) compressional event is recorded in the Tell-Rif system. The existence of HP-LT metamorphic rocks associated with fragments of mantle in the External Metamorphic Massifs of the Eastern Rif and Western Tell shows that, at that time, the western part of the North-African margin was involved in a subduction below a deep basin belonging to the Maghrebian Tethys. At the same time, the closure of the West Ligurian Tethys through east-verging subduction led to a shift of the subduction, which jumped to the other side of AlKaPeCa involving both East Ligurian and Maghrebian Tethys. Slab rollback led to the development of the Oligo-Miocene back-arc basins of the West-Mediterranean, reworking the previous West Ligurian Tethys suture. The docking of AlKaPeCa against Africa occurred during the Late Burdigalian (17 Ma). Subsequently, the slab tearing triggered westward and eastward lateral movements that are responsible for the formation of the Gibraltar and Tyrrhenian Arcs respectively. The exhumation of the External Metamorphic Massifs occurred through tectonic underplating during the westward translation of the Alboran Domain. It resulted in the formation of both foredeep and wedge-top basins younger and younger westward. The lack of these elements in the eastern part of the systems signs a different evolution dominated by frontal accretion. In the discussion, we precisely address the origin of the non-cylindrical behavior of the orogenic system and question the mechanisms explaining at large scale the phases of coupling/uncoupling between the major plates.

Lower Jurassic brachiopods from the Ibero-Levantine Sector (Iberian Range): Faunal turnovers and critical bioevents

Brachiopod fauna from the peri-Iberian platform system is well-known in the Late Pliensbachian-Toarcian interval. However, the diversity dynamics and the stratigraphical distribution of this group in the Ibero-Levantine Sector of the Iberian Range require more accurate analysis due to the patchy distribution of the outcrops. The abovementioned timespan involved a period of changes in long-term environmental conditions which led to a severe extinction and critical turnovers on the brachiopod fauna (ETMEE). The Ibero-Levantine sector has notable palaeobiogeographical significance as it represents the south-easternmost outcrops of the Iberian Range connecting with the Betic Domain positioned in the South-Iberian Palaeomargin. In this region, the brachiopod fauna has been arranged into six assemblages, showing a discontinuous stratigraphic distribution since its diversity dynamics is markedly conditioned by the extinction phases linked to the ETMEE: Ass. 1, recorded in the Spinatum-lower Tenuicostatum chronozones; Ass. 2, mainly recorded in the Tenuicostatum chronozone; Ass. 3, typifying the lower Serpentinum chronozone; Ass. 4, documented in the Serpentinum-lowermost Bifrons chronozones; Ass. 5, recorded in the Bifrons-Variabilis chronozones, and Ass. 6, distinctive of the Pseudoradiosa-Aalensis chronozones. The faunal succession shows strong affinities with the Euro-Boreal province, what rules out the transitional or even Mediterranean palaeogeographical affinity previously attributed to the southernmost part of this area. Amidst the main brachiopod-based bioevents must be considered the predominance of the genus Lobothyris in the pre-ETMEE assemblages, the conspicuous record of Liospiriferina? undulata, distinctive taxon of the north-African margin and, especially, the first record in the Iberian Peninsula of thecideid marker beds prior to the ETMEE. After the opportunistic strategy conducted by Soaresirhynchia bouchardi the distribution of the post-ETMEE brachiopod assemblages is markedly influenced by the depositional sequences, entailing premature turnovers and involving the earlier record of homeomorphic morphotypes of younger terebratulides and rhynchonellides.

Permo-Carboniferous and early Miocene geological evolution of the internal zones of the Maghrebides – New insights on the western Mediterranean evolution

Geochemical and geochronological analyses were performed on metamorphic rocks from the footwall of an oceanic unit thrusted onto the North African margin. The footwall is mainly constituted by metamorphosed sediments and interlayered mafic rocks with scarce ultramafic lenses. Major and trace elements geochemistry of the sediments is consistent with a passive margin setting (16.9<K2O/Na2O<20.9; 18.2<Zr/Th<24.6). U–Pb analyses of detrital zircons indicate that deposition of the sedimentary sequence is younger than 307±7Ma (2σ) and concur with a provenance of the detritus from the Eastern Meseta and High Atlas of Morocco. The age of a crosscutting orthogneiss (281±4Ma) further constrains deposition to the upper Carboniferous/lower Permian times (307–281Ma), in good agreement with the 295±2Ma age of an interbedded amphibolite. Geochemistry of the mafic and ultramafic rocks displays subduction related features (LILE and LREE enrichments, HFSE depletion). Pb, Sr, Nd and Hf isotopes further indicate a depleted mantle reservoir modified by a continental crust-derived component brought into the mantle during an ancient subduction event. The range of isotopic compositions and geochemical signatures displayed by the Carboniferous mafic rocks suggest they inherited their geochemical characteristics during melting of a heterogeneous lithospheric mantle containing variable metasomatic additions. Geochronological results indicate that the HT metamorphic evolution of this upper Paleozoic sequence resulted from two distinct events, each followed by fast cooling. The first one, dated at 20.85±0.34Ma (2σ), followed HP metamorphism and thrusting of the Kef Lakhal oceanic complex onto the Northern margin of Africa. The second event occurred at 17.65±0.46Ma (2σ) and is coeval with exhumation and anatexis of the lower crustal units of the neighbouring Edough dome. Fast cooling of the footwall rocks, through the closure temperatures of muscovite and biotite, occurred synchronously at 16.94±0.10Ma (2σ) and 17.02±0.19Ma (2σ) respectively. This study documents a further example of the complex history recorded by rocks of the Peri-Mediterranean area that contains an imbrication of geological units, witnesses of a protracted history related to Permo-Carboniferous and Miocene events.

Toarcian ammonitico rosso facies from the South Iberian Paleomargin (Betic Cordillera, southern Spain): paleoenvironmental reconstruction

The Toarcian ammonitico rosso facies were widespread in the Mediterranean Tethys (between 15 and 30°N latitude) in the North Gondwana Paleomargin (Apulian promontory and North African Margin) and southern Iberian Paleomargin (Betic Cordillera). These facies were associated with epi-oceanic slopes of a sedimentary swell-trough system related to the extensional phase of continental rifting. In the Median Subbetic (southern Iberian Paleomargin), ammonitico rosso facies show a progressive change through the Toarcian on the hemipelagic swells after the fragmentation of a carbonate platform. During the latest Pliensbachian to the Bifrons Zone (middle Toarcian), sedimentation was dominated by epi-oceanic limestone and marl with a high influence of neighboring shallow-water environments represented by common turbidite–tempestite beds (with foraminifera and ooids). Microfossils and trace fossils provide no evidence of oxygen-restricted conditions. In the Gradata Zone (middle Toarcian), the ammonitico rosso facies appear (red nodular limestone and marly-limestone rich in the trace fossils Phycodes, Planolites, Thalassinoides, and Chondrites). Progressively more pelagic conditions and a restricted influence of emergent lands and carbonate platforms are reflected by the reduced input of turbidite–tempestite beds and increase of ammonitellas and radiolaria. A sea-level fall affected the hemipelagic swell during the middle–late Toarcian and favored sediment-winnowing by currents, with subsequent nodulation. The combined action of burrowing, compaction, and dissolution controlled nodulation, which ranges from diffuse nodules to sharp-edged nodules. The sedimentation rate conditioned the time available for nodule growth, the migration of the Ca2+ and HCO3 − precipitation horizon, and the degree of nodulation (from horizons with diffuse-edged nodules to semi-continuous to continuous layers formed by the coalescence of sharp-edged nodules).