The southern edge of the Larzac carbonate plateau in Occitanie (France) is subject to various landslide processes from rock falls, toppling, and roto-translational slides to rock spreading. To constrain the strength of the carbonate rock mass involved, field and laboratory approaches have been employed. The first approach involves field investigations using the Rock Mass Rating method and the Geological Strength Index, as well as estimations of uniaxial compressive strength conducted with the Schmidt hammer. The second approach consists of various laboratory tests under uniaxial compressive and triaxial stress. A comparison of mechanical parameters obtained from the field and laboratory approaches for characterising the rock mass (cohesion, friction angle and Young’s modulus) illustrates that laboratory measurements provide much higher (70%) cohesion values than field measurements. The friction angles derived from the field investigations are also weaker (20%). Such differences are considered the result of a scale effect, which is less evident for the Young’s modulus. The field approach appears to be more representative of the in situ rock mass strength as it characterises the rock mass by considering both the intact rock and discontinuities.
Biodiversity hotspots often coincide with regions along subduction zones where tectonic activity continuously make and break geographic connections promoting biological diversification and speciation. A puzzling biodiversity hotspot is the northern Caribbean islands that contain endemic terrestrial and freshwater biotas mainly evolved from South American colonizers that dispersed during the Cenozoic. However, tectonic reconstructions have always assumed a mostly inactive and coherent eastern Caribbean plate, such that migration routes must either have been overwater, or through an inner-plate land bridge. Nevertheless, recent studies revealed that the northeastern Caribbean region underwent tectonically induced uplift, subsidence and large-scale block rotations questioning the assumed plate coherency. Here we present a plate reconstruction including these novel constraints and reveals how tectonic and volcanic activity along the Lesser Antilles subduction zone have established a transient land corridor connecting South America and the Greater Antilles from ~45 to 25 Ma ago offering a new avenue to explain Caribbean biotic interchanges and diversification.
The Comoros archipelago located in the western Indian Ocean has been a topic of debate for ca. 50 years regarding its origin. Various mechanisms have been proposed to explain its formation, ranging from the impact of a mantle plume to the development of a plate boundary between the Somalia and Lwandle plates. Determining the timing of the volcanic activity is crucial to understanding the archipelago's origin. Despite recent geochronological studies, the age of the initial volcanic eruptions on the islands remains uncertain due to the difficulty of accessing the earliest lavas. This study uses high- quality seismic reflection profiles and regional stratigraphy to identify the first volcanic series that marked the onset of volcanism on each island of the Comoros Archipelago. Our findings reveal that localized volcanic activity began ca. 32 Ma at Z & eacute;l & eacute;e and Geyser banks and Mayotte edifice, the eastern portions of the archipelago, much earlier than previously believed. Volcanism spread across the Comoros archipelago ca. 9-8 Ma, from the northern part of the Mayotte edifice to Moh & eacute;li Island in the west. Ca. 4 Ma, volcanic activity occurred on Anjouan Island and the Jumelles seamounts, followed by Grande Comore Island ca. 2 Ma. This progression of ages from east to west indicates a chronological sequence over time. The timing of volcanic activity in the Comoros archipelago is similar to the magmatism evolution documented at Madagascar and along the East African Rift System (EARS). Magmatic activity began in the late Oligocene, followed by quiescence during the middle Miocene, and resumed in the late Miocene, coinciding with widespread deformation along the EARS, including its offshore branches and Madagascar. Our study shows that the regional tectonic control of volcanic activity in the Comoros archipelago began during the early Miocene, thereby suggesting that the Comoros archipelago developed as an offshore branch of the EARS south of the Somalian plate at that time.
The Mesozoic was punctuated by several Oceanic Anoxic Events (OAE) characterised by widespread black shale deposition and global carbon cycle perturbations. Among them, the OAE2 spanning the Cenomanian-Turonian boundary (~ 94 Ma), has been identified as one of the most severe OAE of the Mesozoic. The OAE2 is characterised by a global > 2‰ positive carbon isotope excursion (CIE) recorded in both inorganic and organic sedimentary material. This CIE is interpreted as a massive burial of 13C-depeleted organic carbon driven by a global decrease in seafloor oxygenation. Previous models suggested that volcanism related to Large Igneous Provinces (LIPs) increased atmospheric CO2 concentrations, thereby increasing, through enhanced continental weathering, oceanic nutrient and primary production in superficial waters, ultimately leading to higher oxygen consumption at depth. However, the role of enhanced weathering and accelerated continental fluxes on marine primary productivity and organic carbon burial during OAE2 remains highly debated, largely due to the difficulty of reconstructing highly resolved changes in terrigenous and organic fluxes from sedimentary records. In this study, we use new extra-terrestrial 3He (3HeET) measurements from hemipelagic marine samples from Pont d’Issole (Vocontian Basin) and Cuba (Western Interior Basin) sections to reconstruct carbonate, organic carbon and terrigenous sedimentation rates and fluxes across the OAE2.Our results from Pont d’Issole reveal that over 70 % of the 3He in the analysed samples is extra-terrestrial in origin. Assuming a constant flux of 3HeET-bearing Interplanetary Dust Particles, we used 3HeET concentrationsto reconstruct relative changes in sedimentation rates at an unprecedented high resolution (every 15 cm/ 5 ka). Our preliminary results indicate constant terrigenous fluxes (ca 1.6 g/cm²/ka) across different carbonate-rich and carbonate-poor lithologies prior to and across the OAE2 onset. On the contrary, carbonate fluxes plummet (from 26 to 1 g/cm²/ka) prior the CIE. This decrease occurs exactly at the level recording a marked shift towards more radiogenic osmium isotope values, pointing to a possible volcanically triggered collapse of carbonate productivity. Sediments from the Plenus Cold Event (colder and more oxygenated period within the OAE) interval and Cenomanian-Turonian boundary show more modest but significant changes in both terrigenous and carbonate fluxes (from 0.4 to 3 g/cm²/ka and from 0.8 to 11 g/cm²/ka, respectively). Our data also show that organic matter accumulation occurred mostly as short orbitally paced pulses across the entire OAE2 interval with varying responses to changes in terrigenous fluxes. Ongoing 3HeET analyses from Cuba will provide comparative data from a different setting that will be pivotal to determine whether the reconstructed fluxes have a local or geographically widespread significance. The implication of 3HeET data from both locations will be discussed in the context of the debated role of increased detrital input on marine primary productivity and organic carbon burial during the OAE2.
