Géosciences Montpellier

• Planète et Univers/Sciences de la Terre/Tectonique
  
• Planète et Univers/Sciences de la Terre/Volcanologie
  
• Planète et Univers
  
• Planète et Univers/Sciences de la Terre
  
• Planète et Univers/Sciences de la Terre/Stratigraphie
  
• Planète et Univers/Sciences de la Terre/Paléontologie
  
• Planète et Univers/Sciences de la Terre/Géochimie
  
• Sciences de l'environnement/Milieux et Changements globaux
  
• 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 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.

In this study, we investigate the Lesser Antilles forearc basin, focusing on the late Pliocene to Pleistocene sedimentary archives in order to track the occurrence of extreme events triggered by enhanced subduction-related tectono-volcanic activity. We identify late Piacenzian deposits covering a major regional erosional surface, displaying sedimentary dykes and large marine boulders embedded in a mixed continental–marine matrix, characteristic of tsunamites. We interpret this episode of platform emersion and the successive cataclysmic deposits as resulting from enhanced tectonic activity at the interface of the subduction zone, synchronous with the initiation of the Lesser Antilles volcanic arc. We then discuss the implications in terms of the mechanical behaviour of the Lesser Antilles subduction zone.

Imaging deep active volcanic areas remains a challenge in our understanding of their activity and evolution, especially in subduction zones. Study of magnetic anomalies is appropriate to access such dynamics in depth. The magnetic anomaly pattern of the Lesser Antilles Arc (LAA) subduction is studied through Curie Point Depth (CPD), interpreted as the depth of the 580 °C isotherm, and developed to better assess the deep thermal structure of the arc. The depth of the estimated CPD exhibits a complex topography. Keeping in mind the overall uncertainty associated with this method, a main doming is evidenced below the Guadeloupe archipelago. Its apex is shifted towards the ancient arc, suggesting a very hot state of the fore-arc/arc domain. To better understand the LAA thermal state, we perform 2D thermo-mechanical simulations of the subduction zone. Recalling that magnetite is a serpentinization by-product, we simulate water transfer triggered by slab dehydration to test the assumption of fore-arc serpentinization suggested by the positive magnetic anomaly in the vicinity of the Guadeloupe archipelago. In this area, the subduction-induced arc lithosphere hydration and related weakening trigger a fast heating of the upper plate by basal convective removal. This process of fast arc lithosphere thinning may apply where simultaneously the volcanic arc is split in two and normal convergence is high enough. As serpentinization strongly decreases P-wave velocity, we propose a new interpretation of a published seismic profile below Guadeloupe. The seismic layer previously interpreted as the arc lower crust may rather be a layer of serpentinized mantle, as supported by spatial correlations between gravimetric and magnetic anomalies. Consequently, at the scale of Guadeloupe Island, the fore-arc Moho would be shallower than initially assumed, with a dome shape more consistent with both the extensive deformation active since the Oligocene in the inner fore-arc and the CPD doming.