Volcano-tectonic interactions within two recently discovered submarine volcanic fields: Implication for geodynamics in the Comoros

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.

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.

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.