from 23 December 2020 to 11 February 2021

Citation: THINON I., LEROY S., LEMOINE A. & SISMAORE team (2020) SISMAORE cruise, RV Pourquoi pas ?, https://doi.org/10.17600/18001331

Despite a seismic context classified as moderately active, a major seismic crisis has been affecting the Mayotte Island since May 2018. About twenty earthquakes of magnitude greater than 5.0 were recorded, as well as volcanic tremors detected worldwide. This large-scale phenomenon is associated with a significant deformation of the island (both subsidence and horizontal displacement) and with an exceptional active underwater volcanism off eastern Mayotte discovered during the “crisis monitoring” works (REVOSIMA). Mayotte is considered to be the oldest island of the Comoros archipelago, volcanism ranging from Miocene to Holocene. The archipelago, of volcanic origin, is located between the Cretaceous Somali Basin with its oceanic crust and the Mesozoic Comoros Basin, whose crustal nature is still under discussion. The cause of the Comoros volcanism itself remains controversial, between hot spot and regional thermal anomaly associated with a tectonic component. Limited geological knowledge of the maritime surroundings of the Comoros region has not made it possible to determine the recent geodynamic evolution of this region, leaving a set of unanswered questions: on which basin architecture is the new active underwater volcanic structure being built, and more generally how the Comoros archipelago was established? Where is the Ocean-Continent Transition located in the northern part of the Mozambique Channel? What is the cause of this large-scale seismo-volcanic crisis? Is it an isolated magmatic phenomenon and/or related to a tectonic event (activation or reactivation of faults)? What is the role of structural inheritance? How does this current crisis fit into regional geodynamics? Is the Comoros archipelago associated with a nascent plate boundary and/or a diffuse intraplate deformation zone? How does this current deformation fit into the Somalia/Africa regional geodynamic context?

The SISMAORE campaign aims to fill this regional knowledge gap, an essential condition for understanding the current exceptional seismo-volcanic activity and placing it in a more global context. The campaign is based on the latest onshore and offshore observations (TELLUS INSU/ CNRS MAYOTTE projects, REVOSIMA bulletins). Complementary to these “crisis monitoring” programme, all concentrated in East Mayotte, SISMAORE is of a more regional nature and targets more particularly the sedimentary basins around the volcanic islands of the archipelago, where the current and ancient deformation is recorded. SISMAORE will provide essential data for the work funded under the COYOTES project, which has just been accepted by the ANR (2020-2024), including three PhDs, one post-doc, sept Master students and forty participants. To answer the questions asked, we propose a regional-scale acquisition of geophysical and geological data in the Comoros archipelago, with a particular focus on the eastern domain and the Comoros basin. The acquisition of new geophysical data will make it possible to image from the seabed to the base of the crust, using three complementary types of seismic with different penetration and resolution. Subottom profiler (SBP) and 48-channel reflection seismic (SISRAP) profiles will detail the architecture of the sedimentary cover and will identify and characterize tectonic and/or volcanic structures, some of which may be related to recent and/or historical earthquakes. These data will complement onshore observations of recent tectonic deformations observed on the different islands (Mayotte, Anjouan). Regional 960-channel reflection seismic profiles (MCS) will image the entire sedimentary cover but also the basement hopefully down to Moho depth. Three Ocean Bottom seismometer (OBS) were deployed to obtain some refraction data. Combined with gravity and magnetic surveys, these data will also provide information on the crustal structure of the Comoros and Somalia basins, the transition between these two basins and the basin/volcanic archipelago relationships in the Comoros. Geological data (coring, dredging) will provide lithology and ages of structures and surficial deposits, and will be used to quantify vertical movements and understand recent sedimentary flux. Cores will be used to measure heat flow in an attempt to provide information on the thermal structure of the lithosphere. The SISMAORE data will provide the regional knowledge useful for the MAYOBS actions and possibly for the Geo-flamme and MyVista Refraction Ride campains.

In progress