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Teleseismic receiver-functions and Rayleigh-wave dispersion curves are jointly inverted for quantifying S-wave velocity profiles beneath the active volcanic zone off Mayotte. We show that the lithosphere in the east-northeast quadrant is composed of four main layers, interpreted as the volcanic edifice, the crust with underplating, the lithospheric mantle, and the asthenosphere, the latter two presenting a main low-velocity zone. The depths of the old (10-11 km) and new Moho (28-31 km) coincide with the two magma reservoirs evidenced by recent seismological and petrological methods. We propose that the main magma reservoir composed of mush with an increasing amount of liquid extends down to 54 km depth. This magma storage develops from a rheological contrast between the ductile lower and brittle upper lithospheric mantle and accounts for most of the volcanic eruption-related seismicity. Finally, the abnormally small thickness of the lithosphericmantle (33 km) is likely a result of a thermal thinning since the onset of Cenozoic magmatism. [ABSTRACT FROM AUTHOR]

The Comoros archipelago has attracted renewed attention since 2018 due to the submarine volcano growing east of the island of Mayotte and the associated ongoing seismic crisis. However, the origin of Comorian magmatism remains controversial, as it is either interpreted as related to a hotspot trail, to a fracture zone, or to a plate boundary. Lying in the central part of the archipelago, Anjouan is a key island to better understand the relationship between volcanism and geodynamics. Together with a careful selection of published whole-rock K–Ar ages, our new set of 13 groundmass K–Ar ages on lava flows and one radiocarbon age on a charcoal from a strombolian deposit, allow us to reassess the volcano-tectonic evolution of Anjouan Island. New groundmass K–Ar ages lie within the last 1 Ma, i.e. from 899 ± 14 to 11 ± 1 ka. They suggest that most of the subaerial volcanism in Anjouan is much younger than previously inferred, and occurred as pulses at 900–750 ka, perhaps 530 ka, 230–290 ka, and since 60 ka, with erosional periods in between. Among our new data, one 14C age of 7513–7089 yrs calBCE (9.3 ± 0.2 ka) and five K–Ar ages younger than 60 ka show that recent volcanism occurred in Anjouan. Moreover, the concentration of eruptive vents along a N150° alignment, parallel to the maximum horizontal stress, suggests a strong link between regional tectonics and volcanism. Considering the presence of active volcanoes on both the western and eastern extremities of the Comoros archipelago, our discovery of Holocene activity on Anjouan provides strong arguments against a chronological progression of volcanism along the archipelago, and therefore contradicts the hotspot hypothesis for the origin of volcanism. Finally, this study provides a robust geochronological timeframe of the different volcanic stages of Anjouan. It demonstrates that Anjouan is an active island and suggests that volcanism and tectonics can both resume at any time. • New groundmass K–Ar ages in Anjouan are between 899 ± 14 and 11 ± 1 ka. • Volcanism in Anjouan occurred in successive pulses. • A14C age of 7513–7089 yrs calBCE has been obtained for strombolian deposits. • Anjouan is an active island where volcanism and tectonics can both resume. • Holocene volcanism is present all over the Comorian archipelago. [ABSTRACT FROM AUTHOR]

The origin of the Comoros archipelago in the Mozambique channel is the subject of a longstanding controversy. This volcanic chain has been successively interpreted as a hotspot track, as built on an intraplate fracture zone or a passive margin, or as the northern limit of a diffuse deformation zone in the Nubia-Somalia plate system, none of these interpretations being entirely satisfactory. It is also possible that the volcanism of the Comoros is a branch of the East African Rift System, and delineates the still poorly constrained northern boundary between the Lwandle and Somalia plates. To better understand the origin of the Comoros archipelago, we combined a formal stress inversion of earthquake focal mechanisms and deformation structures (faults and dykes) observed on three islands (Mayotte, Anjouan, and Mohéli) with a morphologic study of the repartition of onshore and offshore volcanic vents in the area. Earthquake focal mechanisms and field-observed deformation structures both yield a generalized strike-slip regime with a maximum horizontal stress oriented NW-SE. In tandem, the study of topographic and bathymetric features shows that volcanic vents follow a preferential repartition along two trends: N105 ± 10° lineaments up to 100 km-long, and N145 ± 10°, less than 60 km-long lineaments organized in an en échelon fashion. Earthquakes, deformation structures, and volcanic vents all consistently designate the Comoros archipelago as an E -W elongated, 150 km-wide zone of right-lateral tear in the lithosphere. Because the clockwise rotation of Somalia relative to Lwandle requires the existence of such a right-lateral shear zone, we interpret the Comoros archipelago as the northern boundary between the two plates. This plate boundary is still in an immature state, which is why it remained undetected until now. • Strike-slip stress field occurs in the Comoros archipelago with a NW-SE shortening. • Volcanic cones align as crustal-scale Riedel shear and en échelon extension fractures. • Tectonics, volcanism, and seismicity indicate right-lateral incipient strain localization along the Comoros archipelago. • Dextral motion is consistent with the clockwise rotation of Somalia relative to Lwandle. • The Comoros are built on the northern boundary between the Lwandle and Somalia plates. [ABSTRACT FROM AUTHOR]

The volcanism of the Comoros archipelago (from west to east: Grande Comore, Mohéli, Anjouan, and Mayotte islands) has been under renewed scientific scrutiny since the eruption of a new submarine volcanic edifice (Fani Maoré) which begun in May 2018 east of Mayotte. West of Mayotte, Mohéli island has received little attention from the geoscience community, despite its largely uneroded volcanic landscapes suggesting a recent activity that has not been dated. Here we address this gap in knowledge by applying K Ar geochronology on groundmass and major-trace element analyses to subaerial and submarine rocks, in an attempt to reconstruct the volcano-tectonic and geochemical evolution of Mohéli. Our results show that Mohéli's volcano-tectonic evolution proceeded in two constructional stages, including a primary edification along a N070°E axis from before ca. 3.8 Ma to ca. 3 Ma (Stage 1), and a second construction along a N110°E ridge shaping the present-day island since ca. 2 Ma (Stage 2). The two stages were separated by an increased subsidence (≥0.2 mm/yr) that drowned a large part of the N070°E primary edifice. The two identified volcano-tectonic stages are reflected also in the geochemical evolution of emitted magmas, which were moderately silica-undersaturated in Stage 1 and moderately to highly silica-undersaturated in Stage 2. Silica undersaturation increases with time up to the olivine melilitite field, together with enrichments in Ca, P, and incompatible trace elements, suggesting the increasing contribution of a metasomatized mantle in melts. The coeval changes of construction orientations and magmas compositions of Mohéli suggest a modification in the tectonics of the Comoros related to magmatism at about 2 Ma. The most recent volcanic morphology investigated is an olivine nephelinite lava flow on the north coast of the island dated at 8 ± 2 ka. This finding of Holocene volcanism on Mohéli implies that the entire Comoros archipelago should be considered as an active zone in any volcanic hazard assessment. The melilite-bearing rocks of Mohéli share geochemical similarities with those of Mayotte and of the Cenozoic volcanic provinces of Madagascar, further pointing to similar melting sources and magmatic processes over the Mozambique channel. • New robust K Ar ages constraints on the volcano-tectonics of Mohéli Island. • Two construction stages at ≥3.8–3 Ma and 2–0.008 Ma, separated by a main subsidence. • Volcano-tectonic shift, from N070°E in Stage 1 to N110°E in Stage 2. • Moderate silica-undersaturation in Stage 1 magmas, high undersaturation in Stage 2. • Holocene volcanism in Mohéli implies that the entire Comoros archipelago is active. [ABSTRACT FROM AUTHOR]