Géosciences Montpellier

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Intriguing latest Eocene land-faunal dispersals between South America and the Greater Antilles (northern Caribbean) has inspired the hypothesis of the GAARlandia (Greater Antil-les Aves Ridge) land bridge. This landbridge, however, should have crossed the Caribbean oceanic plate, and the geological evolution of its rise and demise, or its geodynamic forcing, remain unknown. Here we present the results of a land-sea survey from the northeast Caribbean plate, combined with chronostratigraphic data, revealing a regional episode of mid to late Eocene, trench-normal, E-W shortening and crustal thickening by *25%. This shortening led to a regional late Eocene-early Oligocene hiatus in the sedimentary record revealing the location of an emerged land (the Greater Antilles-Northern Lesser Antilles, or GrANoLA, landmass), consistent with the GAARlandia hypothesis. Subsequent submergence is explained by combined trench-parallel extension and thermal relaxation following a shift of arc magmatism, expressed by a regional early Miocene transgression. We tentatively link the NE Caribbean intra-plate shortening to a well-known absolute and relative North Ameri-can and Caribbean plate motion change, which may provide focus for the search of the remaining connection between 'GrANoLA' land and South America, through the Aves Ridge or Lesser Antilles island arc. Our study highlights the how regional geodynamic evolution may have driven paleogeographic change that is still reflected in current biology.

The small (≈500 km2) Azrou-Timahdite basaltic area (Middle Atlas Magmatic Province, Northern Morocco) lies on a Lias (200-180 Ma) karst plateau of medium altitude (between 1,750 and 1,950 m). This area includes 23 small-sized Early Pleistocene (2.5–0.8 Ma) scoria cones and 30 Middle Pleistocene (around 0.6–0.5 Ma) hydrovolcanic edifices. The Pleistocene was a period of strong climatic instability in North Africa: three peaks of aridity would have occurred near 2.8 Ma, 1.7 Ma and 1.0 Ma (DeMenocal, 2004), but the period between 500 and 650 ka would belong to a predominantly wet stage (Jimenez et al., 2010). The scoria cones of the Azrou-Timahdite area would therefore formed when the climate was dry and the hydrovolcanic edifices when it was wet. The hydrovolcanic edifices of the Azrou-Timahdite area are peculiarly interesting because they display an exceptional diversity in terms of morphologies and ejecta deposits which can be only explained by interaction of magma with variable amounts of water, sometimes even during a single eruptive event. The karst regions like the Middle Atlas Plateau are very sensitive to fluctuations in rainfall and during a wet climatic episode the deep karst is permanently saturated with water while the epikarst has degrees of saturation that vary with the topography. Our study suggests that the diversity of hydrovolcanic dynamics in the Azrou-Timahdite area could be linked not only to the degree of saturation of the epikarst but also to the thickness of the transmission zone separating the deep water-saturated karst and the epikarst which also depends on the topographic position of the eruptive sites.

Saint Barthélemy is the only island of the northern Lesser Antilles where it is possible to investigate in the detail the chronostratigraphy of the mixed carbonate-volcaniclastic deposits of the Paleogene Caribbean volcanic arc. Based on field mapping and new biostratigraphical and sedimentological data, we study the limestone units interbedded in the Paleogene volcaniclastic deposits. We find that four main carbonate units occur instead of previously believed six ones. Based on the ages given by the foraminifera assemblages, and taking into account the recently published 40Ar/39Ar ages of magmatic rocks, the Lower Limestone unit dates Lutetian, the Intermediate Limestone unit late (?) Bartonian-late Priabonian, the Upper Limestone late Priabonian and the Top Limestone unit early Miocene. The Paleogene carbonate beds were deposited on gently dipping submarine volcaniclastic deposits issued from emergent volcanoes, in muddy, unrimmed inner to mid-ramp setting dominated by bottom communities. A major subaerial unconformity is evidenced during the Oligocene, most probably corresponding to uplift affecting Saint Barthélemy. Our work offers a revised lithostratigraphic succession of Saint Barthélemy as a first key-point for further studies of the Paleogene Caribbean arc deposits which were dismembered following the entrance of the Bahamas Bank in the Lesser Antilles subduction zone.

Dans le Rif interne (domaine d’Alboran), le massif de Beni Bousera expose un empilement de péridotites et d’unités métamorphiques crustales. Ce massif est recoupé par des filons granitiques et affecté par plusieurs zones de cisaillement ductiles normales. Des analyses structurales, pétrologiques et des datations 40Ar–39Ar réalisées sur ces deux types d’objets montrent que, (1) les filons granitiques se sont mis en place dans des zones de failles et des zones de cisaillement normales, orientées en majorité entre N70° à N140° résultant d’une extension NNE-SSW, (2) la faille d’Aaraben dans sa partie NE est caractérisée par des zones de cisaillement ductiles normales, orientées N70° à N150° résultant d’une extension N-S et (3) l’âge de cet évènement extensif est compris entre 22 et 20 Ma. Les données paléomagnétiques disponibles, permettent de restaurer l’orientation initiale de cette extension, qui était approximativement E-O contemporaine de l’ouverture du bassin d’Alboran en position d’arrière-arc, au cours du Miocène inférieur. Au début de l’extension, les péridotites reposaient sur une croûte continentale partiellement fondue, et exhumées pendant cet évènement par la zone de cisaillement normale d’Aaraben. Le domaine d’Alboran a ensuite été affecté par plusieurs événements compressifs.