Fabien Paquet

The outcrops of the Flamborough Head promontory in Yorkshire, UK, including sea cliffs and inland quarries, provide an exceptional view of the Upper Cretaceous sedimentation. These exposures reveal chalks uplifted during the Tertiary inversion. A detailed sedimentological study of the Turonian to Campanian sections, incorporating both macroscopic and microscopic observations, reveals well-preserved sedimentary features consistent with depositional processes in a deep marine environment. The study identified three main types of facies: contourites, turbidites, and hemipelagites. Contourites, influenced by bottom currents, are composed of fine-grained chalk and exhibit thinly laminated beds with planar and undulating laminations, as well as current ripples. These beds, which can reach a total thickness of several dozen meters, exhibit internal organization with topographic compensation. That suggests deposition within larger contourite drifts – as observed in nearby offshore seismic profiles. Turbidites are characterized by massive beds with sharp bases fining upward, laminated deposits, and minimal bioturbation. Hemipelagites, in contrast, are intensely bioturbated mudstones with sparse fauna and show no evidence of current reworking. These deposits in Flamborough Head provides a valuable analogue for the North Sea deposits, offering insights into the dynamics of chalk deposition along the paleo-slope influenced by bottom currents. The identification of these contouritic facies in chalk across the Flamborough Head promontory highlights periods of increased bottom current activity, with hemipelagite-dominated intervals in between. The outcrop presents a unique combination of stylolitization from burial diagenesis and differential weathering, which highlight its sedimentary features. This provides insights into sedimentary processes that are usually hardly observable in chalk from cores. Although contourites geometries are widely reported through seismic data, facies observations are rare. This study not only provides a valuable analogue for North Sea Chalk deposits but also enhances our understanding of the sedimentology and current dynamics within the Chalk bassin.

The 4.9 Mw earthquake of 11 November 2019 at Le Teil (France) occurred at a very shallow depth (about 1 km), inducing the surface rupture of La Rouvière fault. The question was raised shortly after about the potential impact of a nearby surface quarry. Thanks to satellite differential interferometry, here, we revealed the existence of a secondary surface rupture of the quasi-parallel Bayne Rocherenard fault. A newly processed seismic cross-section allowed us to shape the three-dimensional geometry of the local three-fault system. Assuming that the earthquake was triggered by the impact of meteoric water recharge, our numerical simulations show that the hydraulic pressure gradient at depth was at a maximum during the period of 2010–2019, just before the seismic event. The estimated overpressure at the intersection of the two faults, which is the most probable place of the hypocenter, was close to 1 MPa. This hydraulic effect is about two and a half times larger than the cumulative effect of mechanical stress release due to the mass removal from the surface quarry over the two past centuries. This work suggests a rapid hydraulic triggering mechanism on a network of faults at a shallow depth after a heavy rainfall episode.

Since May 2018 till the end of 2021, Mayotte island has been the locus of a major submarine volcanic eruption characterized by the offshore emission of more than 6.5 km 3 of basanitic magma. The eruption occurred along a WNW–ESE trending submarine ridge on the east flank of the island where, in addition, several seemingly recent phonolitic bodies were also identified close to the island. To define realistic scenarios of magma ascent and potentially predict the style of an upcoming event, it is crucial to have a precise understanding on the plumbing system operating below volcanoes. The putative relationships between basanites emitted by the new volcano and these recent phonolites have been experimentally explored by performing crystallization experiments on a representative basanite over a large range of pressures (up to 400 MPa). The results show that the crystallization of basanite at crustal levels ( ≤ 12–15 km) yields a phonolitic residual liquid containing up to 3–4 wt% H 2 O after ≥ 65 wt% of an assemblage of olivine + plagioclase + amphibole + clinopyroxene + biotite + magnetite + ilmenite + apatite. The final iron content of the residual phonolitic liquids is strongly controlled by the depth/pressure of fractionation. Fe-rich phonolites from the submarine ridge are produced at 6–8 km depth, while a shallower differentiation ( ≤ 4–5 km) results in the production of liquids with trachyte–benmoreite affinities. If the fractionation process occurs at depths higher than 8 km, the resulting phonolitic melts are progressively enriched in SiO 2 – Al 2 O 3 but depleted in FeO*, ie unlike those erupted. We therefore conclude that phonolitic magma production and storage at Mayotte is a rather shallow process.

Unlike subaerial volcanic activity, deep submarine eruptions are difficult to detect, observe and monitor. The objective of this paper is to describe a large and complex volcanic region, named the Horseshoe area, recently discovered at ∼ 1500 m below sea level on the eastern upper submarine slope of Mayotte Island. The area is crucial because, since 2018, it has experienced an exceptionally deep seismic activity associated with the ongoing submarine eruption that formed a new volcanic edifice, Fani Maoré, about 40 km to the east. We present the results of a multiscale study, based on high-resolution bathymetry and in-situ seafloor observations carried out with autonomous underwater vehicles (AUVs) and deep-towed camera systems. In-situ imagery provides ground-truth for the geological interpretation of seafloor textures mapped with the bathymetry. The combination of both datasets allows us to discuss the nature of the volcanic structures and to propose a relative chronology of previous eruptive events in the Horseshoe area. Based on our analyses, we propose the following chronology: (a) the emplacement of a large explosive volcanic cone, the Horseshoe edifice, (b) the later collapse of this edifice that resulted in the formation of an elongated, 2 km wide horseshoe-shaped depression, crosscutting older hummocky lava flows, (c) the development of an E–W eruptive fissure associated with numerous explosive craters, east of the Horseshoe edifice, and (d) late volcanism emanating from the rim of the horseshoe-shaped depression that fed elongated thin lava flows both towards and away from the depression. While all volcanic features mapped at the Horseshoe area were emplaced prior to the 2018 eruption, our study shows that this region has still been volcanically active in the recent past. Our results thus document a complex geological history at small spatial scales involved in the construction of major submarine edifices, and that are controlled by volcano-tectonic processes at larger scales.

Geophysical and geological data from the North Mozambique Channel acquired during the 2020–2021 SISMAORE oceanographic cruise reveal a corridor of recent volcanic and tectonic features 200 km wide and 600 km long within and north of Comoros Archipelago. Here we identify and describe two major submarine tectono-volcanic fields: the N’Droundé province oriented N160°E north of Grande-Comore Island, and the Mwezi province oriented N130°E north of Anjouan and Mayotte Islands. The presence of popping basaltic rocks sampled in the Mwezi province suggests post-Pleistocene volcanic activity. The geometry and distribution of recent structures observed on the seafloor are consistent with a current regional dextral transtensional context. Their orientations change progressively from west to east ( ∼ N160°E, ∼ N130°E, ∼ EW). The volcanism in the western part appears to be influenced by the pre-existing structural fabric of the Mesozoic crust. The 200 km-wide and 600 km-long tectono-volcanic corridor underlines the incipient Somalia–Lwandle dextral lithospheric plate boundary between the East-African Rift System and Madagascar.

A multichannel seismic reflection profile acquired during the SISMAORE cruise (2021) provides the first in-depth image of the submarine volcanic edifice, named Fani Maore, that formed 50 km east of Mayotte Island (Comoros Archipelago) in 2018–2019. This new edifice sits on a ∼ 140 m thick sedimentary layer, which is above a major, volcanic layer up to ∼ 1 km thick and extends over 120 km along the profile. This volcanic unit is made of several distinct seismic facies that indicate successive volcanic phases. We interpret this volcanic layer as witnessing the main phase of construction of the Mayotte Island volcanic edifice. A ∼ 2.2–2.5 km thick sedimentary unit is present between this volcanic layer and the top of the crust. A complex magmatic feeder system is observed within this unit, composed of saucer-shape sills and seal bypass systems. The deepest tip of this volcanic layer lies below the top-Oligocene seismic horizon, indicating that the volcanism of Mayotte Island likely began around 26.5 Ma, earlier than previously assumed.

In 2019, a new underwater volcano was discovered at 3500 m below sea level (b.s.l.), 50 km east of Mayotte Island in the northern part of the Mozambique Channel. In January 2021, the submarine eruption was still going on and the volcanic activity, along with the intense seismicity that accompanies this crisis, was monitored by the recently created REVOSIMA (MAyotte VOlcano and Seismic Monitoring) network. In this framework, four hydrophones were moored in the SOFAR channel in October 2020. Surrounding the volcano, they monitor sounds generated by the volcanic activity and the lava flows. The first year of hydroacoustic data evidenced many earthquakes, underwater landslides, large marine mammal calls, along with anthropogenic noise. Of particular interest are impulsive signals that we relate to steam bursts during lava flow emplacement. A preliminary analysis of these impulsive signals (ten days in a year, and only one day in full detail) reveals that lava emplacement was active when our monitoring started, but faded out during the first year of the experiment. A systematic and robust detection of these specific signals would hence contribute to monitor active submarine eruptions in the absence of seafloor deep-tow imaging or swath-bathymetry surveys of the active area.