The East-Mayotte new volcano in the Comoros Archipelago: structure and timing of magmatic phases inferred from seismic reflection data Le nouveau volcan de l'Est-Mayotte dans l'archipel des Comores : structure et chronologie des phases magmatiques déduites des données d'imagerie sismique

Volcanism and tectonics unveiled in the Comoros Archipelago between Africa and Madagascar Volcanisme et tectonique découverts le long de l'archipel desComores entre l'Afrique et Madagascar

Heat flow in the Northern Mozambique Channel is poorly constrained, with only a few old measurements indicating relatively low values of 55-62 mW/m 2 . During the SISMAORE cruise to the Northern Mozambique Channel, we obtained new heat flow measurements at four sites, using sediment corers equipped with thermal probes. Three of the sites yield values of 42-47 mW/m 2 , confirming low regional heat flow in this area. Our values are consistent with a Jurassic oceanic lithosphere around Mayotte, although the presence of very thin continental crust or continental fragments could also explain the observed heat flow. Our values do not support a regional thermal anomaly and so do not favor a hotspot model for Mayotte. However, at a fourth site located 30 km east of the submarine volcano that appeared in 2018 east of Mayotte, we measured a very high heat flow value of 235 mW/m 2 , which we relate to the circulation of hot fluids linked to recent magmatic activity.Résumé. Le flux de chaleur dans le nord du canal du Mozambique, basé sur des mesures anciennes et peu nombreuses, est faible (55-62 mW/m 2 ). Durant la campagne à la mer SISMAORE, nous avons obtenu quatre nouvelles mesures de flux de chaleur. Nos mesures donnent des valeurs de 47, 45 et 42 mW/m 2 qui confirment un flux de chaleur régional faible. Ces valeurs sont cohérentes avec une lithosphère océanique d'âge Jurassique autour de Mayotte, mais elles peuvent aussi être expliquées par une croûte continentale amincie, ou des enclaves de croûte continentale dans une croûte océanique. Nos faibles valeurs de flux ne sont pas en faveur d'une anomalie thermique régionale ni du modèle point chaud pour expliquer le volcanisme de Mayotte. Cependant, nous avons une mesure de flux,

The source of massive magma production at volcanic rifted margins remains strongly disputed since the first observations of thick lava piles in the 1980s. However, volumes of extruded and intruded melt products within rifted continental crust are still not accurately resolved using geophysical methods. Here we investigate the magma budget alongside the South Atlantic margins, at the onset of seafloor spreading, using high-quality seismic reflection profiles to accurately estimate the oceanic crustal thickness. We show that, along ~ 75% of the length of the Early-Cretaceous initial spreading centre, the crustal thickness is similar to regular oceanic thickness with an age > 100 Ma away from hot spots. Thus, most of the southernmost Atlantic Ocean opened without anomalously hot mantle, high magma supply being restricted to the Walvis Ridge area. We suggest that alternative explanations other than a hotter mantle should be favoured to explain the thick magmatic layer of seaward dipping reflectors landward of the initial mid-oceanic ridge.

<p>The birth of a new volcano offshore the eastern coast of Mayotte, one of the oldest volcanic island in the Comoros archipelago rose questions about the origin of the volcanism in this area. The volcano-tectonic context of this region is poorly known mainly because high-resolution marine data was missing. Here we present new marine geophysical data (bathymetry, backscatter and seismic reflexion data) acquired between December 23 2020 and February 11 2021 during the SISMAORE cruise (Thinon et al., 2021) in the framework of the French ANR COYOTES project. The high-resolution multibeam bathymetric and backscatter data reveal the existence of two submarine volcanic provinces we named N’Droundé and Mwezi (Thinon et al., 2022). In these provinces, we identified faults scarps, volcanic structures, lava flows and flat-top sedimentary domes on the seafloor. Those volcanic and tectonic features are very well preserved in the morphology and very reflective in the backscatter attesting that they are recent and probably active. Several seismic reflection profiles crosscut those structures. They reveal that the sedimentary layers are cut by faults and intruded by sills and dykes. We showed that the recent deformations of the seafloor such as flat-topped domes and grabens are promoted by those intrusions. The recent deformation of the sediments accommodating the magmatic intrusions are used as indirect markers to establish a relative chronology of magmatic activity in the two volcanic provinces. We showed that the magmatism is older in the N’Droundé volcanic province, near Grande Comore than in Mwezi’s, North-East of Anjouan. We also showed from the analysis of sills and dykes in the sedimentary cover that the magmatism intruded during two non-concurring episodes.</p><p>Those volcano-tectonic features align in a mean NW-SE direction and may have likely emplaced in a NE-SW extensional stress field. At a smaller spatial scale, some diking-induced graben form swarms of different directions implying local perturbation of the regional stress field by volcanic intrusions.</p><p>Overall, those observations are crucial to improve our knowledge of the geodynamics in the area and to constrain boundary conditions for future numerical modeling of deformation at lithospheric scale.</p>