Géosciences Montpellier

• Planète et Univers/Sciences de la Terre/Tectonique
  
• Sciences de l'environnement/Milieux et Changements globaux
  
• Sciences de l'environnement/Biodiversité et Ecologie
  
• Planète et Univers/Sciences de la Terre
  
• Planète et Univers/Sciences de la Terre/Paléontologie
  
• Planète et Univers/Sciences de la Terre/Volcanologie
  
• Planète et Univers
  
• Planète et Univers/Sciences de la Terre/Stratigraphie
  
• Planète et Univers/Sciences de la Terre/Géochimie
  
• Sciences du Vivant/Biodiversité/Systématique, phylogénie et taxonomie
  
• Planète et Univers/Sciences de la Terre/Géomorphologie
  
• Planète et Univers/Sciences de la Terre/Géophysique
  
• Physique/Physique/Géophysique
  
• Planète et Univers/Interfaces continentales, environnement
  
• Sciences de l'Homme et Société/Etudes de l'environnement
  
• Sciences de l'environnement
  

The sedimentary model of coastal deposits in eastern Rhodes over the last 2 Ma is refined and improved in accuracy. New field investigations and U/Th dating of Spondylus bivalve shells, combined with micropalaeontological and sedimentological data, allow the recognition of four synthems separated by major erosional surfaces. We present here evidence for two of these erosional surfaces. This new model allows the identification and quantification of the vertical movements recorded by the studied exposures. The history of these vertical motions is characterized by two periods of uplift and two periods of subsidence. Such an evolution is unique at the regional scale in the eastern Hellenic forearc. We interpret these results as reflecting the individualization of Rhodes as a single tectonic block during increasing trench bending. This trench bending is accommodated by an increase in the curvature of the forearc during the last 2 Ma.

Fossil remains of extinct terrestrial sloths have been discovered in numerous localities throughout the Americas, but knowledge of these animals remains poor in the tropical latitudes in comparison with the austral ones. Even where Pliocene mylodontine sloths are known from North and South America, well-preserved craniodental remains are extremely rare, hindering reliable assessment of their taxonomic assignment and phylogenetic affinities. Here, new craniodental remains of Simomylodon uccasamamensis, from the latest Miocene–Pliocene of the Bolivian Altiplano, are described and compared with those of other Neogene Mylodontinae from South and North America. The resulting morphological observations, combined with morphometric analyses, permit reliable differentiation among these moderate-sized Miocene–Pliocene mylodontids. Simomylodon uccasamamensis appears to be the smallest Pliocene mylodontine, and it is closely related phylogenetically to the late Miocene species Pleurolestodon acutidens. Simomylodon uccasamamensis is also an endemic taxon of the Andean highlands during the Pliocene, with a continuous chronological range extending throughout the Montehermosan, Chapdamalalan and (early) Marplatan South American Land Mammal Ages. This terrestrial sloth may have found its ideal ecological conditions in the Bolivian Altiplano, during a span of time falling between the important South American Late Miocene–Pliocene faunal turnover and the Great American Biotic Interchange around the Pliocene–Pleistocene transition.

The upper plate deformation pattern reflects the mechanical behavior of subduction zones. Here we focus on the consequences of the entrance of a buoyant bank into the Caribbean subduction zone during the Eocene by studying the oldest exposed rocks belonging to the Lesser Antilles volcanic arc. Using a novel geochronological data set, we show that the volcanic arc activity on the island of St. Barthelemy spanned over the mid‐Eocene to early Miocene with a westward migration of the tectono‐volcanic activity, which is comparable to what has already been observed on other volcanic islands in the Lesser Antilles. The kinematics analysis allows us to identify a switch in the stress field from pure to radial extension at the Oligo‐Miocene hinge with a subhorizontal σ3 that has a mean trend of N20°. A three‐step restoration of the regional deformation indicates that this switch from pure parallel‐to‐the‐trench extension to radial extension may reflect a strain partitioning initiation affecting the upper Caribbean Plate in response to trench bending that followed the entrance of the Bahamas Bank into the subduction zone. We show that the northern end of the Lesser Antilles arc shows a tectono‐volcanic evolution which is similar to the southern one. The north‐south dichotomy in the perpendicular‐to‐the‐trench extension, 15% in the north versus 30% in the south, may reflect different slab ends that are highly curved to the north (restraining the extension in the upper plate) versus a tear to the south (allowing a larger amount of extension within the upper plate).

The Altiplano formed a broad Cenozoic inverted sedimentary basin (> 200 km wide) with up to 8 kmof fluvio-lacustrine filling in various sub-basins. This work is focused on the Corque basin which recordedevolution of the central Altiplano and Neotropical terrestrial mammalian assemblages during the lateNeogene. Indeed, this area is inferred to have experienced a rise from ∼1,500 to 4,000 m, between ∼10and 6 Ma. Paleoclimates rapidly changed from warm climate at 10 Ma, with a mean annual temperature(MAT) ˜20◦C, to the current highland climate (MAT ˜8-9◦C). These series yielded both rich land mammalassemblages essential for late Neogene biochronology and interbedded volcanic tuffs ranging a ∼10–2.8Ma interval, marked by drastic faunal changes by the Miocene-Pliocene transition. A multidisciplinary studyof late Neogene deposits from the Corque basin near the village of Pomata allows for describing thesedimentary evolution, refining chronostratigraphy through Ar/Ar datings of interbedded volcanic tuffs,and implementing the corresponding fossil record. The studied section encompasses the Totora, Pomata,and Mauri Formations, with 15 interbedded volcanic tuffs suitable for dating. The Totora Fm is mainlycomposed of continental fine-grained sediments yielding desiccation features indicating temporaryemersions and some palynomorphs in finest sediments. Paleoenvironments correspond to ephemeralbraided streams and lakes. In the channels, eastward paleocurrent directions indicate that sediments camefrom the Cordillera Occidental. The Pomata Fm begins few meters above an index volcanic tuff that cropsout over the whole basin and consists of mud or sand-supported mass flow deposits yielding numerousterrestrial mammal fossils. The most conspicuous taxa are the mesotheriid notoungulate Plesiotypotheriumachirense and new echimyid and caviid rodents. The upper surface of the Totora Fm, eroded, correspondsto an angular unconformity underlying the Mauri Fm. The latter starts with an index volcanic tuff (Toba 76;5.34 +/- 0.003 Ma) and corresponds to four thinning upward fluvial sequences, each one starting withcross-bedded conglomerates/coarse-grained sandstones, passing upwards into fine-grained argillaceoussandstones. In these southward-flowing deposits, we recently found a new nothrotheriid ground sloth,Aymaratherium jeani. The erosion of previous units and the change in transport direction may be related toa major phase of relief growth.

In Western Mediterranean, the Rif belt in Morocco is part of the Gibraltar Arc built during the Tertiary in the framework of Eurasia-Africa convergence. The structural and metamorphic evolution of the internal units of this belt as well as their timing, crucial to constrain the geodynamic evolution of the Alboran Sea, is still largely debated.Our study on the Ceuta Peninsula (Northern Rif) provides new structural, petrological and geochronological data (U-Th-Pb, Ar-Ar), which allow to precise the tectono-metamorphic evolution of the Lower Sebtides metamorphic units with:(1) a syn-metamorphic thrusting event developed under granulite facies conditions (7–10 kbar and 780–820 °C). A major thrust zone, the Ceuta Shear Zone, drove the emplacement of metapelites and peridotitic lenses from the Ceuta Upper Unit over the orthogneisses of the Monte Hacho Lower Unit. This compressional event ended during the Upper Oligocene.(2) an extensional event developed at the boundary between amphibolite and greenschist facies conditions (400–550 °C and 1–3 kbar). During this event, the Ceuta Shear Zone has been reactivated as a normal fault. Normal ductile shear zones contributed to the final exhumation of the metamorphic units during the Early Miocene.We propose that the compressional event is related to the formation of an orogenic wedge located in the upper plate, in a backward position, of the subduction zone driving the geodynamic evolution of the Alboran domain. In this context, the episode of lithospheric thinning could be related to the opening of the Alboran basin in a back-arc position.Furthermore, unlike the previous models proposed for the Rif belt, the tectonic coupling between mantle peridotites and crustal metamorphic rocks occurred in Ceuta Peninsula at a depth of 20–30 km under high temperature conditions, before the extensional event, and thus cannot be related to the back-arc extension.