Northern Margin Of Africa Research Articles

The Cenomanian–Turonian Boundary Event (CTBE) as recorded in the northern margin of Africa: palaeoceanography of the Oceanic Anoxic Event (OAE-2), North-Central Tunisia

ABSTRACT Samples spanning the Cenomanian–Turonian transition at Guern Halfaya Section of North-Central Tunisia were analysed for elemental and isotopic chemostratigraphic characterization of the OAE-2 global anoxic event. Comparison with other published sections of the same age was achieved. These geochemical analyses, which cover the local Bahloul Formation, add new insights into the palaeoceanographic evolution of the southern Tethyan margin during the Cenomanian–Turonian transition. The use of discrete redox-sensitive and primary productivity-sensitive trace element data coupled to isotopic and biostratigraphic events is shown as leading proxies for the determination of prevailing warming palaeoceanographic conditions, in terms of productivity and climate, during the OAE-2 in the northern margin of Africa. Such event is believed to have generated one among the most significant source rocks of the region.

Age of UHP metamorphism in the Western Mediterranean: Insight from rutile and minute zircon inclusions in a diamond-bearing garnet megacryst (Edough Massif, NE Algeria)

Diamond-bearing UHP metamorphic rocks witness for subduction of lithospheric slabs into the mantle and their return to shallow levels. In this study we present U–Pb and trace elements analyses of zircon and rutile inclusions from a diamond-bearing garnet megacryst collected in a mélange unit exposed on the northern margin of Africa (Edough Massif, NE Algeria). Large rutile crystals (up to 300 μm in size) analyzed in situ provide a U–Pb age of 32.4±3.3 Ma interpreted as dating the prograde to peak subduction stage of the mafic protolith. Trace element analyses of minute zircons (≤30 μm) indicate that they formed in equilibrium with the garnet megacryst at a temperature of 740–810 °C, most likely during HP retrograde metamorphism. U–Pb analyses provide a significantly younger age of 20.7±2.3 Ma attributed to exhumation of the UHP units. This study allows bracketing the age of UHP metamorphism in the Western Mediterranean Orogen to the Oligocene/early Miocene, thus unambiguously relating UHP metamorphism to the Alpine history. Exhumation of these UHP units is coeval with the counterclockwise rotation of the Corsica–Sardinia block and most likely resulted from subduction rollback that was driven by slab pull.

Neo-Tethys geodynamics and mantle convection: from extension to compression in Africa and a conceptual model for obduction

Since the Mesozoic, Africa has been under extension with shorter periods of compression associated with obduction of ophiolites on its northern margin. Less frequent than “normal” subduction, obduction is a first order process that remains enigmatic. The closure of the Neo-Tethys Ocean, by the Upper Cretaceous, is characterized by a major obduction event, from the Mediterranean region to the Himalayas, best represented around the Arabian Plate, from Cyprus to Oman. These ophiolites were all emplaced in a short time window in the Late Cretaceous, from ∼100 to 75 Ma, on the northern margin of Africa, in a context of compression over large parts of Africa and Europe, across the convergence zone. The scale of this process requires an explanation at the scale of several thousands of kilometres along strike, thus probably involving a large part of the convecting mantle. We suggest that alternating extension and compression in Africa could be explained by switching convection regimes. The extensional situation would correspond to steady-state whole-mantle convection, Africa being carried northward by a large-scale conveyor belt, while compression and obduction would occur when the African slab penetrates the upper–lower mantle transition zone and the African plate accelerates due to increasing plume activity, until full penetration of the Tethys slab in the lower mantle across the 660 km transition zone during a 25 Myr long period. The long-term geological archives on which such scenarios are founded can provide independent time constraints for testing numerical models of mantle convection and slab–plume interactions.

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.

Siliceous and organic-rich sedimentation during the Cenomanian–Turonian Oceanic Anoxic Event (OAE2) on the northern margin of Africa: an evidence from the Bargou area, Tunisia

The late Cenomanian–early Turonian deposits in Tunisia recording the OAE2 event are commonly attributed to the Bahloul Formation. These deposits are composed of dark clayey limestone and organic-rich black shales with abundant planktic foraminifera. At the Bargou area, Oued Kharroub section, these organic-rich deposits include siliceous beds with abundant radiolarians recalling the “Livello Bonarelli”. A total of 25 radiolarian species are identified with a maximum of 18 species per sample. Moreover, the species skeleton preservation is usually poor to moderate in the most studied samples [estimated preservation index (PI) values ranging between 4 and 6]. Two successive assemblages are distinguished. The oldest one (RI) is dominated by nassellarians, which includes mainly Stichomitra, Pseudodictyomitra and Rhopalosyringium genera. It is included within the upper part of foraminiferal Rotalipora cushmani Zone and lower part of Whiteinella archaeocretacea zone. In contrast, the youngest assemblage (RII), which belongs to the upper part of the W. archaeocretacea zone and the earliest Turonian Watinoceras ammonite assemblage, is dominated by spumellarians and includes mainly Archaeocenosphaera, Cavaspongia, Pseudoeucyrtis and Pyramispongia genera. In addition, major and trace elements are analysed to test whether the geochemical record was synchronous to the biotic event or not. Thus, selected crossplots, Al2O3 vs. SiO2 and Al2O3 vs. TiO2, are used to evaluate the detrital input, and V/Cr vs. V/(V + Ni) and U/Th vs. V/(V + Ni) are used to evaluate the bottom redox conditions and the primary productivity within the Bahloul Formation. A strong Si/Al increase is marked in the lower part of the siliceous and organic-rich Bahloul Formation that could be explained by a local increase in upwelling-related biogenic SiO2 (silica-secreting organisms). Relative low abundance of terrigenous-related Ti/Al and K/Al ratios and enrichment of some productivity proxies such as Ba (organic matter related trace elements) suggests that the Bahloul was of relatively elevated primary productivity and minimal detrital input. Enrichments in redox-sensitive trace metals U and V in the Bahloul Formation deposits and redox indices, such as V/(V + Ni), U/Th and V/Cr, indicate that oxygen-restricted conditions prevailed during the late Cenomanian–earliest Turonian times and correlate well with relative abundances of some radiolarian and foraminiferal paleo-environmental indicators. In addition, indicators of detrital flux variations are used. Al2O3 shows a slight positive correlation with TiO2 and a less distinct correlation with SiO2.

The Adriatic lithosphere is a promontory of the African Plate: Evidence of a continuous mantle lid in the Ionian Sea from efficient Sn propagation

Evidence of a continuous lithosphere between the northern margin of Africa and Italy is given from the analysis of regional waveforms recorded by the Italian Seismic Network. Frequency and amplitude of shear waves Sn propagating over the Ionian Sea and the western part of the Levantine Sea are examined in this study. An important observation is that Sn waves are efficiently transmitted without exception at epicentral distances ranging from 3° to 22.5°. The very efficient propagation of uppermost mantle shear waves implies that mantle lithosphere is continuous in this part of the Mediterranean region. This in turn implies that the lithosphere underlying the Adriatic Sea is attached to the African lithosphere and can be considered a promontory of the major African plate. The regional shear phase Lg, that typically propagates within the continental crust, is not transmitted in this area. This observation is consistent with the presence of oceanic crust in the eastern Mediterranean, as hypothesized by previous studies.

East-west extension and Holocene normal-fault scarps in the Hellenic arc

Examination of surface fault traces with Spot images and in the field corroborates the inference that the active tectonics of southern Peloponnesus and Crete are dominated by approximately north-south normal faulting and approximately east-west extension. The heights of Holocene normal-fault scarps yield first-order regional estimates of fault slip rates between 0.1 and 2-3 mm/yr. Most of the surface scarps probably ruptured during past earthquakes, such as that which destroyed Sparta in 464 B.C. On the Sparta fault the Holocene average slip rate and the recurrence time of large earthquakes may be ∼1 mm/yr and 3000 yr, respectively. The regional pattern of Quaternary faulting suggests that the east-west extension near the Hellenic subduction zone is fast (about 5%-10%/m.y.). The change from north-south to east-west extension in the late Pliocene (∼2-4 Ma) implies that the Aegean is starting to collide with the northern margin of Africa.

The Western Mediterranean Region as an upper-mantle transition zone between two lithospheric plates

The upper-mantle structure of the Western Mediterranean Region, i.e. the Western Mediterranean basins with the surrounding land areas, has been investigated by means of surface-wave analysis. Phase velocities of teleseismic Rayleigh waves have been obtained from the records of a specially designed long-period seismic station network. The dispersion data have been regionalized employing a computerized procedure and have then been inverted by means of the so-called “hedgehog” method. The upper mantle shows pronounced lateral heterogeneities which are correlated with the tectonic activity of the region. Some deep anomalous features are revealed such as the “soft” continent-like structure of the lithosphere under the Alboran Sea, the absence of lithospheric roots below the mountain range of the High Atlas, the continental platform-like upper mantle along the northern margin of Africa. While the upper mantle under the Provençal and the Algerian basins shows oceanic or back-arc basin-like structure, the Balearic Promontory and particularly the Valencia Trough are shown to possess upper mantle structure with characteristics close to those of a continental rift system. We therefore suggest that the boundary—at the level of the upper mantle—between the African and the European plates, should rather be viewed as a broad transition zone involving the upper mantle of the entire Western Mediterranean region across which the plate interaction takes place.