Search

In order to decipher the tectono-metamorphic evolution of shallow crustal levels of the active volcanic arc of the Guadeloupe archipelago (Lesser Antilles) we present new geochemical, geochronological, mineralogical and structural investigations of the so-called Basal Complex, the oldest and most eroded volcanic complex of Basse-Terre in Guadeloupe. Based on geochemical and mineralogical criteria we propose an updated geological map of this northern area of Basse-Terre. Using 40Ar–39Ar geochronology we demonstrate first that the eroded “Gros Morne” of Deshaies belong to the Basal Complex, and second that this complex is characterized by 4.3 to 2 Ma old volcanism. Structural analysis reveals a long-lived deformation history with the development through time of N80-N100 schistose zones; N110-N140 and N160-N10 oriented hydrothermal breccias and N140-N150 brittle normal faults. The boundary between the Basal Complex and the southernmost Septentrional Chain corresponds to a series of faults with N 150° and N 50° main directions. Detailed mineralogical and petrological investigations, including thermodynamic modeling, allow the identification of three phases of post-magmatic mineralogical transformations with first a high-temperature stage under Greenschist to sub-Greenschist facies conditions (0.6–2 kbar for 250–300 °C), a re-equilibration under Zeolite facies conditions and finally a sub-surface alteration. The consistency between P–T conditions of metamorphism and the present day measured geothermal gradient demonstrates that the metamorphic pattern is the record of hydrothermal fluids circulation during building and cooling of the Lesser Antilles magmatic arc. The tectono-metamorphic evolution recognized in the Basal Complex enables us to propose a conceptual model for heat and fluid transport within shallow crustal levels of the Guadeloupe active volcanic arc.

Space missions (1,2) and thermal infrared observations (3) have shown that small asteroids (kilometre-sized or smaller) are covered by a layer of centimetre-sized or smaller particles, which constitute the regolith. Regolith generation has traditionally been attributed to the fall back of impact ejecta and by the break-up of boulders by micrometeoroid impact (4,5). Laboratory experiments (6) and impact models (4), however, show that crater ejecta velocities are typically greater than several tens of centimetres per second, which corresponds to the gravitational escape velocity of kilometre-sized asteroids. Therefore, impact debris cannot be the main source of regolith on small asteroids (4). Here we report that thermal fatigue (7-9), a mechanism of rock weathering and fragmentation with no subsequent ejection, is the dominant process governing regolith generation on small asteroids. We find that thermal fragmentation induced by the diurnal temperature variations breaks up rocks larger than a few centimetres more quickly than do micrometeoroid impacts. Because thermal fragmentation is independent of asteroid size, this process can also contribute to regolith production on larger asteroids. Production of fresh regolith originating in thermal fatigue fragmentation maybe an important process for the rejuvenation of the surfaces of near-Earth asteroids, and may explain the observed lack of low-perihelion, carbonaceous, near-Earth asteroids (10)., The collisional and gravitational re-accumulation processes by which small asteroids are formed probably result in the creation of surfaces composed of boulders (11). These boulders are broken up by micrometeoroid [...]

The evolution of life on Earth is marked by catastrophic extinction events, one of which occurred ca. 200 Ma at the transition from the Triassic Period to the Jurassic Period (TrJ boundary), apparently contemporaneous with the eruption of the world's largest known continental igneous province, the Central Atlantic magmatic province. The temporal relationship of the Tr-J boundary and the province's volcanism is clarified by new multidisciplinary (stratigraphic, palynologic, geochronologic, paleomagnetic, geochemical) data that demonstrate that development of the Central Atlantic magmatic province straddled the Tr-J boundary and thus may have had a causal relationship with the climatic crisis and biotic turnover demarcating the boundary. Keywords: Central Atlantic magmatic province, flood basalts, mass extinction, Triassic-Jurassic boundary, Morocco.