In this work, we focus on the influence of an interface on liquid rise through an immersed granular bed. Based on laboratory experiments, we consider the migration of water injected at constant flow rate at the bottom center of a Hele-Shaw cell filled with two layers of grains immersed in water. The bottom layer is made of coarse grains, large enough to ensure liquid percolation without grain motion. The top layer consists of a bidisperse medium of fine grains and dusts about four times smaller in diameter, which can penetrate the interface between coarse and fine grains. When the liquid invades the cell, above a critical flow rate, the dusts are washed out of the coarse grains, a process called elutriation. The flow pattern self-organizes, generating fluidization chimneys at the interface between coarse and fine grains with regular spatial distribution. A model based on pressure-drop estimations predicts the pattern wavelength, which depends on the dust size and the number of coarse grains in the cell gap.
Due to climate change, tropical islands are being increasingly exposed to coastal hazards. Under energetic incident swells, infragravity (IG) waves can have a key contribution to extreme water levels and flooding, but their dynamics at barrier reef remains little studied. In this context, this study analyses IG wave generation mechanisms and transformations across a barrier reef to the southwest of Mayotte (Indian Ocean), combining a new comprehensive field dataset with phase resolving wave modelling. This analysis reveals that IG waves are mostly breakpoint-forced and suffer a substantial dissipation by bottom friction, particularly at low tide. Numerical experiments with reduced bottom friction representing a degraded coral reef suggest that IG waves would grow by about 20
Samples from the Cap de Creus shear zone network were investigated to constrain the timing of the basement's polycyclic evolution by combining structural and microstructural analyses, electron imaging chemical investigations, and laser ablation mass spectrometry in situ RbSr and Th-U-Pb geochronology. Two generations of muscovite (Ms I and Ms. II) yield in situ RbSr isochron ages of 290–270 Ma and 60–50 Ma, consistent with UPb ages of c. 290 Ma in monazite cores and 50–30 Ma rims. Microstructure study shows that Ms. II grew during dynamic recovery of quartz ribbon precursors (Qz I). Fe-enrichment in Ms. II, localized in mica fish tails and C′ shear planes, along with Fe-rich veins marking Qz II grain boundaries, evidence that the RbSr system in muscovite recorded an episode of dynamic fabric recovery of quartz in presence of fluids. We thus demonstrate that rocks fabrics were annealed during late Variscan HT-LP metamorphism and deformation in the Early Permian, followed by ductile-plastic reactivation of the shear zones during the Eocene. This reactivation was facilitated by external fluid influx, drained by Ms. II neocrystallization and Qz II boundary migration. The Cap de Creus shear zones, usually interpreted as Variscan, also record a major Eocene reactivation during the formation of the Pyrenean orogenic prism through underthrusting of basement slices. These zones acted as early accommodation structures for crustal shortening. This study highlights the critical role of fluid-induced rheological softening in ductile reactivation of polycyclic basements and provides a context-dependent framework for interpreting the behavior of the RbSr geochronometer in muscovite during deformation.
Mantle processes control plate tectonics and exert an influence on biogeochemical cycles. However, the proportion of mantle sampled in-situ is minimal, as it is buried beneath igneous crust and sediments. Here we report the lithological characteristics of two mantle sections from an embryonic ocean drilled by the International Ocean Discovery Program (IODP) in the Tyrrhenian Sea. Contrary to the mantle drilled at Mid Ocean Ridges (MORs) and hyperextended passive margins, our findings reveal exceptionally heterogeneous and fertile mantle lithologies, ranging from fertile lherzolites to depleted harzburgites and dunites, interlayered with pyroxenites. Plagioclase- and clinopyroxene-rich layers, hydrous potassic magmatic veins, and mafic intrusions indicate substantial mantle refertilization and delayed inception of magmatic crust. We propose that magma-poor rifts do not require a chemically depleted mantle, too refractory to melt. Deep lithospheric processes such as mantle refertilization and prolonged lithospheric thinning delayed melt focusing and the formation of a steady-state spreading center. Integrated Ocean Discovery Program Expedition 402 drilled two in-situ long mantle sections in the Tyrrhenian Sea, revealing that retention of large quantities of melts at mantle levels delayed the birth of an ocean basin.
Tectonically emplaced peridotites and mantle xenoliths present complementary aspects of the evolution of the Variscan lithospheric mantle. The former have diverse origins and document complex histories of melt-rock reactions, exhumation along various pressure–temperature–time (P–T–t) paths, and emplacement into the crust, unravelling plate boundary evolution during Variscan subduction and collision. Mantle xenoliths exhumed by Cenozoic volcanism reveal ancient partial melting and mostly post-Variscan metasomatism episodes. Yet, their coarse-grained textures potentially record Variscan deformation. Dominantly belt-parallel fast seismic directions of the in situ Variscan lithospheric mantle may record flow normal to the convergence direction, but parallel to the boundaries of the Baltica and Avalonia blocks in central Europe, and to the main strike-slip faults and late extension in the Massif Central and Iberia.
