# Volcanism and tectonics unveiled in the Comoros Archipelago between Africa and Madagascar

Isabelle Thinon, Anne Lemoine, Sylvie Leroy, Fabien Paquet, Carole Berthod, Sébastien Zaragosi, Vincent Famin, Nathalie Feuillet, Pierre Boymond, Charles Masquelet, et al.

► To cite this version:

Isabelle Thinon, Anne Lemoine, Sylvie Leroy, Fabien Paquet, Carole Berthod, et al. Volcanism and tectonics unveiled in the Comoros Archipelago between Africa and Madagascar. Comptes Rendus. Géoscience, 2022, TheMayotte seismo-volcanic crisis of 2018-2021 in the Comoros archipelago (Mozambique channel) / La crise sismo-volcanique de 2018-2021 deMayotte dans l'archipel des Comores (Canal duMozambique), 354 (S2), pp.1-28. 10.5802/crgeos.159. hal-03857244

HAL Id: hal-03857244

https://brgm.hal.science/hal-03857244

Submitted on 16 Jan 2023

HAL is a multi-disciplinary open access
archive for the deposit and dissemination of sci-
entific research documents, whether they are pub-
lished or not. The documents may come from
teaching and research institutions in France or
abroad, or from public or private research centers.

L'archive ouverte pluridisciplinaire HAL, est
destinée au dépôt et à la diffusion de documents
scientifiques de niveau recherche, publiés ou non,
émanant des établissements d'enseignement et de
recherche français ou étrangers, des laboratoires
publics ou privés.

<figure><img src="image_1.png" /></figure>

Distributed under a Creative Commons Attribution 4.0 International License
# Comptes Rendus
## Géoscience
### Sciences de la Planète

Isabelle Thinon *et al.*

**Volcanism and tectonics unveiled in the Comoros Archipelago between Africa and Madagascar**

Published online: 19 December 2022

https://doi.org/10.5802/crgeos.159

**Part of Special Issue:** The Mayotte seismo-volcanic crisis of 2018-2021 in the Comoros archipelago (Mozambique channel)

**Guest editors:** Jérôme Van der Woerd (Institut Terre Environnement de Strasbourg, UMR 7063 CNRS / Université de Strasbourg, 67084 Strasbourg, France), Vincent Famin (Laboratoire Géosciences Réunion, Université de La Réunion - IPGP, 97744 Saint-Denis, France) and Eric Humler (Professeur Université de Nantes, Laboratoire de Planétologie et Géosciences, UMR 6112, Faculté des Sciences et Techniques, Nantes Université, 44322 Nante, France)

<figure><img src="image_2.png" /></figure>

This article is licensed under the

CREATIVE COMMONS ATTRIBUTION 4.0 INTERNATIONAL LICENSE.

http://creativecommons.org/licenses/by/4.0/

<figure><img src="image_3.png" /></figure>

Les Comptes Rendus. Géoscience — Sciences de la Planète sont membres du
Centre Mersenne pour l'édition scientifique ouverte

www.centre-mersenne.org

e-ISSN : 1778-7025
J Laboratoire de Géologie, Ecole Normale Supérieure (ENS), PSL Univ., CNRS UMR 8538; Laboratoire de Géologie, France

k BGR Bundesanstalt für Geowissenschaften und Rohstoffe, Germany

l Université de Lille, CNRS, IRD, Université Littoral Côte d'Opale, UMR 8187 - LOG - Laboratoire d'Océanologie et de Géosciences, Lille, France

m Université Côte d'Azur, CNRS, Observatoire de la Côte d'Azur, IRD, Géoazur, 250 rue Albert Einstein, Sophia Antipolis 06560 Valbonne, France

n Faculté des Sciences et Techniques - Université des Comores, Union des Comores, Comoros

o CNDRS (centre national de documentation et des recherches scientifiques) Union des Comores, Comoros

*Current address:* BRGM - DGR/GBS, 3 avenue Claude Guillemin, BP 36009, 45060 Orléans Cedex 2, France (I. Thinon)

E-mails: i.thinon@brgm.fr (I. Thinon), a.lemoine@brgm.fr (A. Lemoine), sylvie.leroy@sorbonne-universite.fr (S. Leroy), f.paquet@brgm.fr (F. Paquet), carole.berthod@uca.fr (C. Berthod), sebastien.zaragosi@u-bordeaux.fr (S. Zaragosi), vincent.famin@univ-reunion.fr (V. Famin), feuillet@ipgp.fr (N. Feuillet), boymond@ipgp.fr (P. Boymond), charles.masquelet@sorbonne-universite.fr (C. Masquelet), n.mercury@brgm.fr (N. Mercury), rusquet@ipgp.fr (A. Rusquet), Carla.Scalabrin@ifremer.fr (C. Scalabrin), jerome.vanderwoerd@unistra.fr (J. Van der Woerd), j.bernard@brgm.fr (J. Bernard), julie.julie-bignon@etu.u-bordeaux.fr (J. Bignon), valerie.clouard@get.omp.eu (V. Clouard), Cecile.Doubre@unistra.fr (C. Doubre), jacques@ipgp.fr (E. Jacques), stephan.jorry@ifremer.fr (S. Jorry), frederique.rolandone@upmc.fr (F. Rolandone), rooke@geologie.ens.fr (N. Chamot-Rooke), delescluse@geologie.ens.fr (M. Delescluse), Dieter.Franke@bgr.de (D. Franke), louise.watremez@univ-lille.fr (L. Watremez), p.bachelery@opgc.fr (P. Bachèlery), laurent.michon@univ-reunion.fr (L. Michon), Daniel.Sauter@unistra.fr (D. Sauter), stephane.bujan@u-bordeaux.fr (S. Bujan), Albane.CANVA@geoazur.unice.fr (A. Canva), emilie.dassie@u-bordeaux.fr (E. Dassie), vincent.roche@sorbonne-universite.fr (V. Roche), nasaidali@gmail.com (S. Ali), alawia.abdoulhamid@gmail.com (A. H. Sitti Allaouia), deplus@ipgp.fr (C. Deplus), s.rad@brgm.fr (S. Rad), l.sadeski@brgm.fr (L. Sadeski)

**Abstract.** 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.

**Résumé.** Des données géophysiques et géologiques ont été acquises lors de la campagne océanographique SISMAORE (2020-2021). Deux grands champs tectono-volcaniques sous-marins ont été découverts tout le long et principalement au nord de l'archipel des Comores : la province N'Droundé orientée N160°E au nord de Grande-Comore, et la province Mwezi orientée N130°E au nord d'Anjouan-Mayotte où des roches basaltiques de type popping-rocks suggèrent une activité volcanique possiblement actuelle à pléistocène. La géométrie et la distribution des structures récentes sont cohérentes avec un contexte régional actuel transtensif dextre. Leurs orientations évoluent d'Ouest en Est (~N160°E, ~N130°E, ~EW), suggérant pour la partie occidentale, une mise en place du volcanisme influencée par la structuration crustale préexistante. Le corridor tectono-volcanique de
200 km de large et de 600 km de long dessine une limite de plaque lithosphérique Somalie-Lwandle immature en décrochante dextre entre le système du rift est-africain et Madagascar.

**Keywords.** Volcanic province, Active tectonics, Incipient plate boundary, Bathymetry, Backscatter, Northern Mozambique Channel, Comoros Archipelago.

**Mots-clés.** Province volcanique, Tectonique active, Frontière de plaque naissante, Bathymétrie, Rétrodiffusion, Canal du Mozambique Nord, Archipel des Comores.

Published online: 19 December 2022

# 1. Introduction

The Comoros Archipelago, in the Mozambique Channel between east Africa and northern Madagascar (Figure 1), is located along the poorly constrained plate boundary between the Lwandle and Somalia lithospheric plates [Famin et al., 2020, Kusky et al., 2010, Stamps et al., 2018, 2021]. This boundary has probably evolved jointly with the southward propagation of the East African Rift System (EARS) [e.g., Franke et al., 2015, Mougenot et al., 1986].

Seismicity in the Mozambique Channel is of moderate intensity and is concentrated along two large-scale structures: Davie Ridge and the Comoros Archipelago [Bertil and Regnoult, 1998, Bertil et al., 2021, Lemoine et al., 2020; Figure 1]. Focal mechanisms mainly indicate regional E–W extension along Davie Ridge, considered to be a N–S trending offshore arm of the EARS [Courgeon et al., 2018, Déprez et al., 2013, Deville et al., 2018]. West of the Comoros Archipelago, sparse focal mechanisms along approximately N–S trending normal faults are also compatible with E–W extension and may be related to the EARS [e.g., Klimke and Franke, 2016]. In contrast, seismicity in the Comoros Archipelago indicates ESE–WNW distribution west of Mayotte and ENE–WSW distribution east of Mayotte [Bertil et al., 2021]. Elsewhere in the Archipelago, sparse focal mechanisms suggest mainly strike-slip faulting with a normal component, including the seismo-volcanic sequence associated with the 2018–2021 East Mayotte eruption [Cesca et al., 2020, Feuillet et al., 2021, Lemoine et al., 2020, Lavayssière et al., 2022, Mercury et al., 2022]. Holocene tectonic and volcanic activity is also evident on the islands of Anjouan and Mayotte [Famin et al., 2020, Quidelleur et al., 2022] and in the abyssal plain north of the Archipelago [Tzevahirtzian et al., 2021]. Recent studies [e.g., Bertil et al., 2021, Famin et al., 2020, Feuillet et al., 2021, Kusky et al., 2010, Lemoine et al., 2020, Michon et al., 2016, Stamps et al., 2018, 2021] have suggested that the Comoros Archipelago is lo-

cated along a diffuse or incipient plate boundary between the Lwandle and Somalia plates, although the sense of displacement is poorly constrained, being consistent with transtension, pure strike-slip, or transpression.

Early work has focused on the Comoro Islands, and few papers have treated the offshore parts of the Archipelago (Figure 2). Using bathymetric data, Audru et al. [2006] and Tzevahirtzian et al. [2021] provide overviews of the volcanic structures and slope instabilities on the flanks of the islands. Since 2019, Mayotte Volcanological And Seismological Monitoring Network [REVOSIMA (Mayotte Volcanological And Seismological Monitoring Network), 2022] has closely monitored the ongoing seismo-volcanic events and reported detailed information, such as seafloor morphology and seismic and volcanic activity on the eastern slope of Mayotte, where ~6.55 km³ of magma erupted in January 2021), as well as the lithology of volcanic structures [e.g., Berthod et al., 2021a,b, Deplus et al., 2019, Feuillet et al., 2021, Foix et al., 2021, Jacques et al., 2019, Lavayssière et al., 2022, REVOSIMA newsletter, 2021, Saurel et al., 2022]. Yet, the age and nature of the volcanic Comoros Archipelago and surrounding areas are constrained only by analysis of onshore samples [Class et al., 2005, Michon et al., 2016, Pelleter et al., 2014, Quidelleur et al., 2022] and rare marine rock samples, mostly from east of Mayotte [Berthod et al., 2021a,b, Feuillet et al., 2021].

To fill this data gap, the geophysical and geological SISMAORE oceanographic cruise, linked to the COY-OTES project (COMoros & maYotte: vOlcanism, Tectonics and Seismicity), took place off the Comoros Archipelago from December 2020 to February 2021 [Thinon et al., 2020b]. This cruise acquired high-resolution multibeam bathymetry and backscatter data, shallow and deep seismic reflection profiles and refraction data, heat flow, magnetic and gravity data, and rock and sediment samples. In this paper, we focus on the main characteristics of the recent volcanic and tectonic structures at a regional scale, to bet-
<figure><img src="image_4.png" /></figure>

**Figure 1.** (a) Regional geological context of the study area. Volcanism in the northern Mozambique Channel from Michon et al. [2022] and Roche and Ringenbach [2022]. Earthquake locations from the database of the International Seismological Centre [2021]; plate boundaries from Michon et al. [2022]; fracture zones from Davis et al. [2016]; boundary of oceanic crust and exhumed mantle (COB) from Roche and Ringenbach [2022]; major faults of the East African Rift System (heavy black lines) from Michon et al. [2022]; bathymetry from GEBCO, 2014 [Weatherall et al., 2015]. A, Ambilobe basin; DR, Davie Ridge; K, Kerimbas basin; M, Majumba basin; N, Nacada basin. (b) Focal mechanisms compilation [Bertil et al., 2021, and references therein] and present-day relative plate motions with respect to Nubia plate [Stamps et al., 2021].

ter map the distribution of deformation around the Archipelago. Better knowledge of active structures and their regional geodynamic context will improve the identification and assessment of regional volcanic, seismic, tsunamigenic, and landslide hazards.

In this paper, we propose a geodynamic configuration that takes into account the pre-volcanism history of the northern Mozambique Channel and current regional kinematics.
<figure><img src="image_5.png" /></figure>

**Figure 2.** Map of major geographic features of the study area and data acquired during the 2020–2021 SISMAORE cruise. SBP, sub-bottom profiler lines; thin lines, SISRAP, 48-channel seismic reflection; Multichannel seismic (MCS) data, 960-channel seismic reflection Masquelet et al. [2022]; OBS, ocean bottom seismometer. Multibeam echosounder system (MBES) and water column acoustic data were obtained along all black lines Thinon et al. [2020b]. Volcanic islands: GC, Grande-Comore; Mo, Mohéli; A, Anjouan; Ma, Mayotte. Major seamounts and chains: V, Vailheu; J, Jumelles; Z, Zélée Bank; G, Geyser Bank; L, Leven Bank; C, Cordelière Bank. White stars: K, Karthala volcano; FMV, Fani Maore volcano; FaC, Fer à Cheval volcanic complex [Feuillet et al., 2021]. Detailed bathymetry from the compilation of Tzevahirtzian et al. [2021] (surveys listed in references and in Counts et al. [2018]) superimposed on GEBCO regional low-resolution bathymetry [Weatherall et al., 2015]. White outlines correspond to other figures in this paper.

**2. Regional settings**

**2.1. Volcanism of the Comoros Archipelago**

The Comoros Archipelago includes the four main islands, Grande-Comore, Mohéli, Anjouan and Mayotte, as well as submerged features including the Jumelles seamounts, Vailheu seamount and the Zélée–Geyser banks [e.g., Tzevahirtzian et al., 2021; Figure 2]. Seismic stratigraphy [Leroux et al., 2020] indicates that the main volcanic phase of Mayotte (late Paleogene to Neogene) is much younger than

Zélée–Geyser banks (Cretaceous–Paleogene transition), which in turn are younger than the Glorieuses seamounts (Late Cretaceous). Mayotte, oldest of the four main islands, has a low elevation and a well-developed insular shelf, as does Mohéli [Tzevahirtzian et al., 2021]. The onset of Mayotte's volcanism was estimated at about 20 Ma by Michon et al. [2016] and between 26 and 27 Ma by Masquelet et al. [2022]. For Anjouan and Grande-Comore, high-elevation volcanic islands with young relief, volcanism could have started at 7 to 10 Ma [Michon et al., 2016].
Subaerial Mayotte is made up of three morphologically and structurally distinct units corresponding to three eruptive phases, which date from 10.6 to 1.5 Ma in the south part, from 7.1 to 1.0 Ma in the northwest part and from 2.4 Ma to 0.15 Ma in the northeast part and the nearby islet Petite-Terre [Debeuf, 2004, Nehlig et al., 2013]. Volcanic ash layers in dated lagoon sediments suggest that the most recent volcanic and explosive activity on land is younger than 7 ka [Zinke et al., 2003, 2005, and references therein]. On the northern slope off Mayotte, several small volcanic edifices are part of a longer line of volcanoes trending N140°E from Anjouan to Petite-Terre [Audru et al., 2006]. On the seafloor east of Mayotte, the MAYOBS monitoring cruises [Rinnert et al., 2019, REVOSIMA newsletter, 2021] during the 2018–2021 Mayotte seismo-volcanic crisis have documented the N110°E trending Eastern Volcanic Chain of Mayotte (EVCM; Figure 2), a line of numerous volcanic structures with a new active volcanic edifice 800 m high and 5 km in diameter at its tip [Berthod et al., 2021a,b, Cesca et al., 2020, Feuillet et al., 2021, Lemoine et al., 2020]. This active submarine volcano is called Fani Maor'e volcano (name submitted to the UNESCO's International Marine Chart Commission, Figure 2).

Before the 2018–2021 Mayotte eruption, the only volcano with known historic activity (since the early 18th century) is Karthala volcano on Grande-Comore [Bachèlery et al., 1995, 2016b]. Late Holocene activity (from ¹⁴C ages of 1300 ± 65 yr BP and 740 ± 130 yr BP) is known for La Grille volcano on Grande-Comore [Bachèlery and Coudray, 1993, Bachèlery et al., 2016b], and early Holocene activity is known for Anjouan [¹⁴C ages of 8335 ± 50 yr BP; e.g., Quidelleur et al., 2022].

The origin of the volcanism of the Comoros Archipelago is debated [Michon et al., 2016]. A mantle plume model, proposed mainly based on large-scale geomorphological arguments and scant early K–Ar dates, is inconsistent with the inferred distal transtension between the Somalia and Lwandle plates [Calais et al., 2006, Stamps et al., 2018] and by geochronological data showing no clear age progression through the islands [Debeuf, 2004, Quidelleur et al., 2022, Tzevahirtzian et al., 2021]. One alternative explanation is that the volcanism represents an offshore extension of the EARS to the east of Davie Ridge, possibly along a diffuse or incipient Somalia–Lwandle plate boundary [e.g., Deville et al., 2018

and references therein; Famin et al., 2020, Feuillet et al., 2021, Lemoine et al., 2020, Michon et al., 2022, Stamps et al., 2018].

## 2.2. Distribution of recent and old deformation

Deformation onshore on Mayotte took place predominantly on thrust and strike-slip faults consistent with the N135°E trend in earthquake focal mechanisms [Famin et al., 2020]. Offshore structures trending N130°E include the Jumelles volcanic chain and some alignments of volcanic cones, mounds and faults [e.g., Tzevahirtzian et al., 2021]. Moreover, Lemoine et al. [2020] and Feuillet et al. [2021] have suggested that the deep dyke feeding the current Mayotte eruption is oriented NW–SE, consistent with the context of transtension.

Toward Davie Ridge on the west side of Mozambique Channel, Neogene strike-slip and normal faults imply a transtensional component associated with volcanism [Franke et al., 2015, Roche and Ringenbach, 2022]. Other than there and in associated basins such as the Kerimbas and Nacala basins [Courgeon et al., 2018, Mahanjane, 2014, Mougenot et al., 1986, Roche and Ringenbach, 2022, Vormann and Jokat, 2021], no Quaternary deformation has been described in the Comoros basin or in the Maungunga basin in the NW Madagascar margin (Figure 1).

## 2.3. Regional geological history—inheritance

Recent studies suggest that rifting of Gondwana began in late early to early Middle Jurassic time, ~170 Ma to ~185 Ma [Davis et al., 2016, Eagles and König, 2008, Gaina et al., 2015, Leinweber and Jokat, 2012, Mueller and Jokat, 2019, Senkans et al., 2019]. However, precise dating is difficult given the poorly developed seafloor spreading anomalies from this time. North of the Comoros Archipelago, kinematic reconstructions [e.g., Davis et al., 2016] and limited gravity and magnetic data suggest that the Somali basin consists of oceanic crust with an extinct EW trending oceanic spreading axis and transform faults trending N–S to NW–SE [see Figures 4 and 8 in Davis et al., 2016 and Figures 5 and 9 in Phethean et al., 2016] (Figure 1). Seafloor spreading is thought to have begun during the Jurassic Magnetic Quiet Zone (~166 Ma) and ceased at ~120 Ma [Davis et al., 2016, Segoufin and Patriat, 1980]. In
Mozambique and Tanzania to the west, the breakup
unconformity is of early Middle Jurassic (Aalenian)
age [Fossum et al., 2021, Roche et al., 2021, Senkans
et al., 2019]. The age and the nature of the crust be-
neath the Comoro Islands and the Comoros Basin
are still debated. On the one hand, an oceanic crust
is supported by isostatic data, magnetic and grav-
ity anomalies, P-wave velocities, and seismic reflec-
tion data [Coffin and Rabinowitz, 1987, Coffin et al.,
1986, Klimke et al., 2016, Phethean et al., 2016, Roche
and Ringenbach, 2022, Talwani, 1962, Vormann and
Jokat, 2021, Vormann et al., 2020]. On the other hand,
the presence of quartzite inclusions in volcanic rocks
and a massif of quartzite on Anjouan [Flower, 1972,
Flower and Strong, 1969, Montaggioni and Nougier,
1981, Quidelleur et al., 2022, Wright and McCurry,
1970] and low *V*<sub>p</sub>/*V*<sub>s</sub> ratios [Dofal et al., 2021] suggest
that the Archipelago is built on continental crust or
at least upon a succession of continental sediments
[Dofal et al., 2021, Roach et al., 2017]. In Figure 1, we
choose to represent the crust as oceanic [Roche and
Ringenbach, 2022].

Whatever the nature of the crust, gravity data ap-
pear to indicate some relict structural segmentation
of both continental and oceanic domains beneath
the Comoros Archipelago. Both transforms and frac-
ture zones [Davis et al., 2016, Phethean et al., 2016]
could be pre-existing structures in the Comoros and
Somali Basins (Figure 1) that have influenced later
structures.

**3. Dataset from SISMAORE cruise and methods**

The SISMAORE cruise (December 2020 to February
2021) of R/V (Research Vessel) *Pourquoi Pas?* col-
lected 80,000 km² of multibeam bathymetry and
backscatter data around the Comoros Archipelago
with the vessel-mounted MultiBeam Echosounder
System (MBES RESON 7150 at 12 or 24 kHz) that
complemented previous surveys around Mayotte
[Figure 2; Thinon et al., 2020b; for details, see Sup-
plementary Information]. In addition, the ship ac-
quired 10,000 km of sub-bottom profiler (SBP) data
and 6730 km of 48-channel seismic reflection profiles
(for details, see Supplementary Information), imag-
ing the subsurface down to 0.1 s two-way travel time
(TWT) with very high resolution and down to ~3 s
TWT below the seafloor, respectively. Five dredging

operations (Figures 2 and 3) collected mainly vol-
canic rocks on the flanks of Zélée Bank (sample set
SMR1), the Jumelles volcanic chain (SMR2), Mo-
héli (SMR3), the Chistwani volcanic chain (SMR4)
and a seamount in the abyssal plain north of May-
otte (SMR5) [Thinon et al., 2020b]. These rocks are
macroscopically similar to those described on land in
the Comoro Islands [Bachlery and Hémond, 2016,
Debeuf, 2004, Pelleter et al., 2014, and references
therein] and offshore of Mayotte [Berthod et al.,
2021a,b]. The rocks in the SMR5 dredge samples
are CO2-rich fresh basaltic rocks called popping
rocks (Supplementary Figure S1) that contain arkose
and quartzite xenoliths, occasionally with melted
shapes, and rare olivine xenocrysts [Thinon et al.,
2020b].

**4. Observations and interpretation**

On the basis of the new regional MBES bathymetric
and backscatter reflectivity data (Figure 3), we pre-
pared a new regional geomorphological map around
the Comoros Archipelago (Figure 4). In addition to
the islands and seamounts previously mentioned,
it portrays two newly mapped large volcanic fields
with tectonic structures (the N'Droundé and Mwezi
provinces, and the Domoni, Chistwani and Safari
submarine volcanic chains that extend between
the islands [first described by Tzevahirtzian et al.,
2021].

The depth of the abyssal plain ranges between
3000 m and 3800 m (Figure 3a). North of the Co-
moros Archipelago, the smooth abyssal plain of the
Somali basin is interrupted by two large fields of
heterogeneous bathymetric features (Figure 3a) and
strong reflectivity contrasts (Figure 3b). The first of
these, north of Anjouan and Mayotte, is here named
the Mwezi Province ("Moon" in Comorian; Figures 4
and 5), and the second, north of Grande-Comore,
is here named the N'Droundé Province (from the
town of N'Droundé on Grande-Comore; Figures 4
and 6). In the eastern part of the Somali basin are
two major valleys trending N–S to NNE–SSW, one
on the seabed between the Glorieuses Islands and
Leven Bank at 3800 m water depth and one north
of Jumelles Seamount and Zélée–Geyser banks at
3600 m (Figures 3a and 4).
<figure><img src="image_6.png" /><figcaption>**Figure 3.** Maps showing (a) bathymetry and (b) acoustic backscatter imagery from the SISMAORE cruise combined with previous bathymetric data. Also shown are locations of newly described features: Vailheu, Domoni, Chistwani and Safari volcanic chains and N'Droundé and Mwezi provinces. Yellow and red circles represent earthquakes from Bertil et al. [2021]. All other symbols as in Figure 2.</figcaption></figure>

The abyssal plain south of the Archipelago, in the Comoros Basin, has a homogeneous and flat morphology at 3500 m depth (Figure 3a) without strong reflectivity contrasts (Figure 3b). Small reflectivity variations of the flat seabed southeast of Mayotte

(Figures 3b and 4) appear to be associated with the distribution of surficial sediment.

East of Mayotte, the EVCM (Figures 3 and 4) comprises elongated ridges, several volcanic cones up to 500–900 m high including the submarine volcano
<figure><img src="image_7.png" /></figure>

**Figure 4.** Geomorphological map of the study area showing major structural, volcanic and sedimentary features identified on the SISMAORE bathymetry and backscatter maps (Figure 3) plus features on the insular slopes from earlier work [Feuillet et al., 2021, Paquet et al., 2019, Tzevahirtzian et al., 2021]. The two grey dashed lines mark the boundaries of the Comoros tectono-volcanic corridor described in this paper.

formed during the 2018–2021 eruption, lava flows, and a horseshoe-shaped volcanic complex with current activity [MAYOBS cruises, REVOSIMA newsletter, 2021, Feuillet et al., 2021, Rinnert et al., 2019]. Two great dome-shaped forced folds, faults, and possible submarine mass transport deposits described by Paquet et al. [2019] are also imaged (Figure 4).

On the insular slopes, the map depicts the major mass-wasting deposits identified by Tzevahirtzian et al. [2021] and Audru et al. [2006] in new detail. On the abyssal plain, seismic profiles (Figure 7) show that the thickness of the entire sedimentary cover can exceed 2.5 s TWT, or ~3.1 km based on an average 2500 m·s⁻¹ velocity in the sediments [Masquelet et al., 2022].

## 4.1. *North of the Comoros Archipelago*

### 4.1.1. *The Mwezi tectono-volcanic province*

The Mwezi Province covers an area 100 by 60 km (~6000 km²) in the flat abyssal plain of the Somali basin (~3400 m depth) and represents a westward extension of the N130°E trending Jumelles volcanic chain (Figure 5). It contains abundant bathymetric features (Figures 5a and d) corresponding to patches with moderate to strong reflectivity (Figures 5b and e). These relief structures include a large number of elongated and steep-sided ridges and conical features (Figure 5d). The ridges are typically 1–4 km in length and reach heights of several hundred metres above the surrounding seafloor. The highest seamount in this area (C1 on Figures 5d
<figure><img src="image_8.png" /></figure>

**Figure 5.** (a) Bathymetric map and (b) backscatter map of the Somali basin north of Anjouan and Mayotte, showing seismicity and an incised valley (Va). Outlined regions are shown in (d) and (e), respectively. (c) Interpretive map showing the Mwezi province, Jumelles volcanic chain, Zélée-Geyser Bank and other structural, volcanic, and sedimentary features (for legend see Figure 4). (d) Detail map of seafloor morphology and (e) reflectivity in the central Mwezi Province. Red circle indicates the 24 July 2018 earthquake [3.62 mwu, 15 km depth; Bertil et al., 2021]. C, conical feature; Cc, seamount with summit crater; Cd, seamount with summit dome; D, dome-shaped forced fold; De, depression; F, fault; Lf, lava flow; R, ridge. Supplementary Figure S2 presents oblique views of this area.
<figure><img src="image_9.png" /></figure>

**Figure 6.** (a) Bathymetric, (b) backscatter and (c) interpretive maps of the N'Droundé Province. Three sub-parallel volcanic chains (numbered circles) are marked with thick dashed lines in (c). Note that volcanic chain 1 changes orientation from NNW-SSE to NW-SE as it approaches Grande-Comore (GC), unlike chains 2 and 3. (d) Details of bathymetry and (e) backscatter maps. See Figure 5 for explanation of symbols.
<figure><img src="image_10.png" /><figcaption>**Figure 7.** Selected 48-channel seismic reflection and SBP profiles and their interpretations, showing the subsurface architecture of different volcanic structures visible on the seafloor. Locations shown in Figure 2. (a, b) Orthogonal profiles in the Mwezi Province (MAOR21R002 and MAOR21R075, respectively). (c) NNE-SSW profile in the N'Droundé Province (MAOR21R070) and (d) WNW-ESE profile in the Somali basin north of Zélée-Geyser Bank (MAOR21R001). Gr, graben; V, volcanic edifice; v.e., vertical exaggeration; other abbreviations as in Figure 5.</figcaption></figure>

and e) stands ~600 m above a basal depth of 3355 m. The conical features may contain craters (Cc) or domes (Cd) at their summits, and they typically reach diameters of a few kilometres (up to 4 km) and

heights up to 600 m. The reflectivity of these features suggests a hard seabed consisting of rocky outcrops or bedrock lightly covered with sediment [Le Gouidec et al., 2003]. Dredge sample SMR5 from one
of these seamounts contained popping rocks (Sup-
plementary Figure S1). We interpreted these features
as volcanic edifices [Wessel et al., 2010]. They have
the same morphology and geophysical characteris-
tics of volcanic edifices to the EMVC, where pop-
ping rocks have been sampled from the summit area
and fresh lava flows of Fani Maor'e volcano [Berthod
et al., 2021a,b, Feuillet et al., 2021], and features of
similar bathymetry and reflectivity on the slopes of
the Comoro Islands [Audru et al., 2006, Feuillet et al.,
2021, Tzevahirtzian et al., 2021].

The backscatter map contains patches of strong reflectivity, some reaching more than 15 km in extent (Lf on Figure 5e), that have little bathymetric relief (Figure 5d). Generally associated with complex roughness and various shapes (commonly lobes), only one patch has a linear shape (Lf1 on Figure 5e, oriented N160°E). We interpret these patches as lava flows, cropping out on the seafloor or lightly covered by very fine sediment. The shapes and sizes of these lava flows suggest low-viscosity lavas like those observed and sampled during the 2018 eruption east of Mayotte [Berthod et al., 2021a,b, Feuillet et al., 2021].

The bathymetric map displays many large and flat
circular domes with smooth seabed that reach ele-
vations of 100 m and diameters of several kilometres
(D on Figure 5d). They have no reflectivity signature
(Figure 5e). Seismic profiles across these domes show
an uplifted and faulted sedimentary cover overlying
an abrupt, high-amplitude terminating reflector, of-
ten displaying a saucer-shaped profile (Figure 7a,b).
These deep reflectors are consistent with shallow
magmatic intrusions (sills or laccoliths), which would
induce domal uplift and faulting of the overlying
sedimentary cover [Kumar et al., 2022, Medialdea
et al., 2017, Montanari et al., 2017, Omosanya et al.,
2017, and references therein]. We interpret these
domes as forced folds by analogy to the description
of Paquet et al. [2019] and other sources [Monta-
nari et al., 2017, and references therein]. The ana-
logue modelling of Montanari et al. [2017] shows that
the growth of dome-shaped forced folds produces
tensional and compressional deformation (normal
and reverse faults) in the sedimentary cover. Some of
the dome-shaped forced folds in the Mwezi province
are accompanied by faults with sub-vertical offsets
reaching 10 m (Figure 7a2–a4, b). The disruption of
seismic signals under and over the sills may result

from the presence of overlying lava flows (acous-
tic masking) or fluid migration pathways (chimneys)
[Masquelet et al., 2022, and references therein].

Most of the volcanic structures, ridges and
seamounts are distributed along ~N130°E trend
(Figure 5c). In addition, steep escarpments identi-
fied on the seafloor are mainly oriented ~N130°E
in the centre of the Mwezi province and mainly ori-
ented ~N-S on the northern edge. They are as long
as 10 km and have vertical offsets of up to 10 m (see
F₁, F₂, F₃ on Figures 5d and 7a3). Some of these es-
carpments consist of discontinuous, slightly shifted
segments (Supplementary Figure S2). Some connect
seamounts or domes, and some cut across domes
producing vertical offsets of 10–20 m (see F₁ cut-
ting domes D₃ and D₄ on Figure 5d; Supplementary
Figure S2). We interpret these escarpments as faults
with sub-vertical components.

4.1.2. *The N'Droundé tectono-volcanic province*

The N'Droundé Province covers an area 40 by
100 km (~4000 km²) at the western end of the Co-
moros Archipelago, north of Grande-Comore, and
consists of three subparallel NNW-SSE striking sub-
marine topographic features (Figure 6). The south-
ernmost of these is 60 km long and ~15 km wide, with
a summit reaching a depth of 1230 m above a base at
3000 m (feature 1 in Figure 6). Near its southeastern
tip, its orientation changes smoothly from N160°E to
N130°E. The other two are parallel lines of relief fea-
tures, oriented N160°E, about 130 km long and aver-
aging 5 km in width at 3100 to 3400 m depth (fea-
tures 2 and 3 in Figure 6). They have a wide variety
of shapes, including narrow ridges and conical edi-
fices some of which have craters or breached craters
at their summits (Figure 6d). These are between 1
and 5 km in diameter and a few kilometres long
and reach heights of several hundred metres above
the surrounding seafloor. The highest seamount rises
735 m to a depth of 2472 m (C1 on Figure 6a). We
interpret these seamounts as volcanic edifices. Some
dome-shaped forced folds appear in seismic profiles
in the N'Droundé Province (D on Figure 7c), but
few affect the seafloor and have elevations less than
100 m. Most of them are covered by a thin layer of a
chaotic or non-reflective unit (max. 0.1 s TWT; V and
S on Figure 7c). Significant deposition of sediments
from the Grande-Comore may result in a smoother
bathymetry here than in the sediment-starved Mwezi
Province. Alternatively, the N'Droundé seamounts are older than those in the Mwezi Province and have a thicker sedimentary cover. Along with the seamounts are N160°E-trending sub-vertical escarpments a few metres high, forming relays that cut the seafloor (F on Figure 6d).

## 4.2. Other evidence of recent volcanism in the Comoros Archipelago

### 4.2.1. Submarine volcanic chains

The Safari volcanic chain between the islands of Anjouan and Mayotte has several volcanic edifices consisting of conical seamounts, ridges, eruptive fissures and lava flows (Figures 3, 4 and 8). The conical seamounts are aligned in a 130°E trend and reach heights of up to 400 m and diameters of 1 to 1.5 km (Figure 8, Supplementary Figure S3). Large high-reflectivity patches near them are interpreted as lava flows, and low-reflectivity patches are interpreted as lava flows covered by a thin layer of sediment (Figure 8b,e). The ridges, which trend NW-SE, are up to several kilometres long, hundreds of metres wide, and 200 m high (R on Figure 8d) and may contain depressions on their summits as deep as about 20 m (Ef1 on Figure 8d-f). One of these ridges transitions into a crack 2 km long with walls up to 20 m high and a high-reflectivity signature (Ef2 on Figure 8e,f). Similar features on Axial Seamount (Juan de Fuca Ridge) have been interpreted as eruptive fissures [Chadwick Jr et al., 2019, Clague et al., 2011].

Other major submarine volcanic chains, such as the Domoni and Chistwani chains (Figures 4 and 9, Supplementary Figure S4) contain volcanic edifices including conical seamounts, ridges and lava flows. The dredged rocks from the SE flank of the Chistwani chain (SMR4) are basaltic [Thinon et al., 2020a]. This widespread array of volcanic structures is evidence of several phases of volcanism that have formed submarine volcanic chains throughout the Comoros Archipelago, both between the islands (Domoni, Chistwani, and Safari chains) and beyond them (Vailheu chain and the EMVC).

### 4.2.2. Other volcanic evidence

Grande-Comore, Mohéli, and Anjouan have rugged insular slopes covered with mass-wasting deposits (detrital cones) and outcropping blocks [Tzevahirtzian et al., 2021; Figure 4]. In the volcanoclastic

deposits on the south slope of Grande-Comore, some small conical edifices and various patches with high reflectivity have been identified (Figure 10a).

Unlike the other Comoro Islands, the slopes of Mayotte (except the eastern slope) have a relatively smooth bathymetry and a mottled appearance [Audru et al., 2006, Tzevahirtzian et al., 2021; Figures 4 and 11a]. Major mass-wasting deposits are absent, and only a superficial submarine landslide deposit no higher than 20 m is apparent at the foot of the western flank (Figure 11a,b). In this area, the strongest reflectivity features mainly correspond to canyons and a few small isolated seamounts not covered by sediments (Figure 11c,d). Seismic profiles indicate that the smooth and mottled-bathymetry features correspond to sediment deposits (Figure 11e,f). The sedimentary unit covering the acoustic basement off Mayotte can reach thicknesses of ~0.5 s TWT (roughly 500 m in using 2000 m·s⁻¹ velocity) and often covers volcanic edifices. On the western slope of Mayotte, three nearly conical seamounts exhibit a N160°E alignment (Figure 11a,c,d). By creating a barrier for sediment transport on the western slope, these seamounts potentially control the shape of a submarine canyon. A larger number of conical seamounts on the northern slope of Mayotte lie in a N130°E trending corridor between the Safari volcanic chain and Fani Maor'e volcano, in agreement with previous observations [Audru et al., 2006, Famin et al., 2020, Tzevahirtzian et al., 2021].

On the flanks of Zélée Bank, dredging sample SMR1 consists of volcanic rocks [Figure 3; Thinon et al., 2020a]. In the abyssal plain north of Zélée-Geyser banks, 0.2 s TWT high dome-shaped forced folds are observed in the seismic profiles (i.e. ~200 m with 2000 m·s⁻¹ velocity; Figure 7d). However, they have no surface expression owing to the thick sediment cover (0.15 to 0.4 s TWT), which is thicker than the cover on the dome-shaped forced folds in the Mwezi (quasi-nonexistent) and N'Droundé Provinces (max. 0.1 s TWT). Dome-shaped forced folds and sills exist also east of Mayotte near Fani Maor'e volcano [Figure 4; Masquelet et al., 2022, Pauquet et al., 2019, Rolandone et al., 2022].

To the east of the Comoros Archipelago, we identified evidence of volcanism near the Madagascar margin. A small conical seamount and an 85 km² high-reflectivity patch with lobe-shaped and roughened facies were identified at the foot of the western slope
<figure><img src="image_11.png" /></figure>

**Figure 8.** (a) Morphology of the Safari volcanic chain between Anjouan and Mayotte islands from SISMAORE MBES bathymetry data (greyscale) and previous surveys (colour, from Tzevahirtzian et al., 2021). (b) Backscatter map. (c) Interpretive map showing conical volcanic edifices (C), ridges (R), lava flows (Lf) and eruptive fissures (Ef). (d) Detail of (a) and (e) detail of (b). (e) Cross-sections across the ridges (R) and crack fissures (Ef2) observed on the seafloor. Supplementary Figure S3 presents oblique views of this area.
<figure><img src="image_12.png" /></figure>

**Figure 9.** (a) Oblique view showing the morphology of the Chistwani volcanic chain between Mohéli and Anjouan. Oriented N 55°E, the chain is narrow (~20 km wide), asymmetric (steeper SE flank), and rises ~500 m (up to 2830 m depth) above the surrounding abyssal plain (3400 m depth). (b) Interpretive map showing volcanic edifices (purple) and lava flows (pink) at the top of the volcanic chain. Black dashed lines indicate en echelon lineaments that may represent a dextral motion.

of Cordelière Bank (Figure 10b). Their morphology and reflectivity suggest that they are recent volcanic structures, in agreement with the evidence of volcanism suggested by dredge sampling in this area [Daniel et al., 1972].

### 4.3. Distribution of volcanic and tectonic structures in the Archipelago

The great N'Droundé and Mwezi volcanic provinces in the Somali abyssal plain, together with the four Comoro Islands and the major seafloor volcanic chains around them, constitute a volcanic corridor 600 km long and 200 km wide (Figure 4). Mapping based on the SISMAORE cruise data shows on the seafloor the presence of up to 2200 recent volcanic edifices and lava flows, covering an area of ~5300 km² (~2500 km² of cones and ridges and ~2800 km² of lava flows),

plus dome-shaped forced folds on the seafloor with an area larger than 972 km² throughout the Comoros Archipelago.

At the western end of the Comoros Archipelago, volcanic features are distributed mainly along a N160°E direction (N'Droundé, Grande-Comore, Domoni). To the east of Mohéli and Anjouan, the volcanism is distributed mainly along a N130°E direction (Mwezi, Jumelles, Safari, EMVC). The topography of the Mwezi Province and the two northernmost volcanic chains in the N'Droundé Province is less significant than that of the highest volcanic chains (Jumelles, Domoni, Chistwani, EMVC). The Mwezi and N'Droundé volcanic fields thus may be nascent volcanic chains, younger than the major and polyphase chains elsewhere in the corridor.
<figure><img src="image_13.png" /><figcaption>**Figure 10.** Bathymetric (left) and backscatter (right) maps of (a) the southern slope of Grande-Comore Island and (b) the lower western slope of Cordelière Bank. Also shown in (b) is detailed bathymetry of a conical volcanic edifice (C) with a potential lava flow (Lf). Lf0, lava flow with relatively low reflectivity due to sediment cover; other symbols as in Figure 5.</figcaption></figure>

The faults identified in the abyssal plain of the Somali basin and east of Mayotte display two main orientations. Faults trending N130°E are mainly in the Mwezi province, east of Mayotte, and in the SAFARI volcanic chain, and faults trending N160°E are mainly in the N'Droundé province, in the Domoni volcanic chain and on the western upper slope of Mayotte (Figure 4).

North-south to NNE-SSW trend- ing faults exist in the northern part of the Mwezi Province. These sets of faults and volcanic structures, both too recent to be covered by sediments, appear to be coeval as they crosscut each other in many cases. Some of the faults cut the surface of the dome-shaped forced folds or connect two major structures (e.g., fault Lf1 in Figure 5e, Supplementary Figure S2). They can form graben systems (e.g., Figures 5d, 7a2
<figure><img src="image_14.png" /></figure>

**Figure 11.** (a) Shaded bathymetric map of the western slope of Mayotte with location of MARO21R105 seismic line (black line). Small cluster of volcanic edifices (C) forms a NW-SE trend and outlines the western rim of a major canyon. (b) Detail of (a) showing the mass-wasting deposit—at the foot of the slope (3544–3521 m depth). (c) Detail of (a) showing the N160°E alignment of volcanic cluster. (d) Reflectivity map of the area in (c). (e) Interpreted and (f) migrated 48-channel seismic profile MARO21R105, oriented EW (location in (a)).
and 7a4) or lines of offset segments (e.g., Figure 6c,d). Most of the faults show sub-vertical offsets reaching 20 m, often with a normal component (Figure 7, Supplementary Figure S2). Strike-slip motion could exist, but is not easy to identify in the seismic profiles.

The eastern flank of the Chistwani volcanic chain contains three lines of volcanic edifices and ridges with ENE-WSW (N60°E to N90°E) orientations (Figure 9). Some east-facing scarps are present. These lineaments are arranged en echelon, as if they reflected the influence of a dextral motion.

**5. Discussion**

Our interpretations of bathymetry and backscatter maps and seismic profiles revealed a widespread regional distribution of recent volcanic and tectonic deformation that allows us to propose a geodynamic context for the Comoros Archipelago and address questions about the role of the structural inheritance (Figure 12).

**5.1. Tectono-volcanism events in the Comoros Archipelago**

The volcanic and tectonic structures in the abyssal plain rest on a sedimentary sequence ~3 km thick (Figures 7 and 11), in agreement with previous authors [Coffin et al., 1986, Leroux et al., 2020, Masquelet et al., 2022].

In the sediment-starved Mwezi Province (Figures 4 and 5), the near-absence of sediments on lava flows, volcanic edifices, forced folds, and faults suggests that volcano-tectonic activity, though undated, is very recent. The high-reflectivity of these seafloor features is similar to that of the lava flows from the 2018-2021 Fani Maor'e volcano eruption [Deplus et al., 2019, Feuillet et al., 2021]. Moreover, the presence of popping rocks in dredge sample SMR5, in the Mwezi province (Supplementary Figure S1), implies that the residence time of these seafloor basalts has been too short to permit complete degassing. The evidence is consistent with very young volcanic activity in the Mwezi Province. We propose that fresh outcrops and seafloor features represent similarly recent volcanic and tectonic activity throughout the Comoros Archipelago, including in the N'Droundé Province, on the slope of Grand-Comore, in the Safari

volcanic chain, at the foot of the Jumelles volcanic chain and on the Cordelière Bank (as shown in purple in Figures 4 and 12). However, no current volcanic eruptions or fluid activity were apparent in the water column acoustic data from the SISMAORE cruise except east of Mayotte [REVOSIMA newsletter, 2021, Thinon et al., 2020b].

There are more lava flows and dome-shaped forced folds visible on the seafloor in the Mwezi Province than in the N'Droundé Province. This difference could be due to magmatic events of differing intensities or ages; however, in the N'Droundé Province, many dome-shaped forced folds and lava flows are covered by as much as 100 m (0.1 s TWT in using 2000 m·s⁻¹ velocity) of sedimentary deposits or other volcanic products (Figure 7c). This province receives an abundant sediment supply from wave erosion and coastal hydrodynamic processes at Grande-Comore, whereas the Mwezi Province is more distant from sediment sources.

North of the Zélée-Geyser banks, a significant sedimentary cover onlaps the dome-shaped forced folds (0.15–0.4 s TWT or 100–400 m in using 2000 m·s⁻¹ velocity, Figure 7d), hides the surface deformation induced by magmatic intrusions. Even if sedimentation rates differ between the two areas, these forced folds appear to be older than those in the Mwezi province, which disrupt the seafloor. This difference is consistent with the long-term magmatic history of the north Mozambique Channel [Michon et al., 2016], and with the emplacement of the Zélée-Geyser banks, which are older than the Comoro Islands to the west [Leroux et al., 2020]. We suggest that the Mwezi and N'Droundé provinces consist mainly of monogenetic volcanic cones and lava flows whereas submarine volcanic chains (Jumelles, Chistwani and Domoni) and Zélée-Geyser banks are products of a complex evolution and are akin to multiphase constructional structures such as the EVCM [Feuillet et al., 2021, Masquelet et al., 2022] or islands such as Mayotte [e.g., Nehlig et al., 2013]. Based on our comparisons of topographic features and backscatter signatures, we propose that most of the volcanic and tectonic structures in the Mwezi and N'Droundé provinces are younger than those of volcanic chains and Zélée-Geyser banks.

The evidence of widespread recent volcanic activity on submarine volcanic chains and islands (purple shades on Figure 4) suggests the occurrence of
<figure><img src="image_15.png" /></figure>

**Figure 12.** (a) Regional geological context of the Comoros Archipelago, including the distribution of recent volcanism and tectonics identified from this study and the proposed diffuse and nascent Somalia–Lwandle plate boundary. See Figure 1 for legend. (b) Geodynamic interpretation of recent tectonic and volcanic structures and the fossil Mesozoic crustal fabric of the study area. Shown are major faults and lineaments from this study as well as tensile fractures from this study (Tf4) and from Famin et al. [2020] and Feuillet et al. [2021] (Tf1, Tf2, Tf3). Locations, focal mechanisms and dates of earthquakes (up to Mw 5) are from Bertil et al. [2021] and references therein and pre-existing crustal fabric, derived from magnetic data, is from Davis et al. [2016]. (c) Regional tectonic setting between the EARS and Madagascar. K, Kerimbas basin; A, Ambilobe basin. Grey dashed lines mark the diffuse, immature Somalia–Lwandle plate boundary.
multiple volcanic events throughout the Comoros Archipelago. The volcanic structures identified on Cordelière Bank (conical edifices and lava flows; Figure 10b) appear to be the products of recent eruptive events from their morphology and strong reflectivity, more recent than those of the Glorieuses Islands [Late Cretaceous according to Leroux et al., 2020]. A pattern of multiple phases separated by periods of quiescence would be similar to that of Mayotte, where volcanic activity occurred between 10.6 and 1.9 Ma and then resumed during the Pleistocene [Bachlery et al., 2016a, Michon et al., 2016, Pelletier et al., 2014].

These faults and volcanic structures appear to be coeval as they crosscut each other in many instances yet are both too recent to be covered by sediments. Some of the faults form and grow in response to local stress fields caused by magmatic intrusions and eruptions [see Sections 4.1 and 4.2; Figure 7; Montanari et al., 2017]. However, the major fault trends (N130°E and N160°E to N-S) and the alignment of seamounts in the Mwezi and the N'Droundé provinces are consistent with the regional stress field. During the ongoing eruption off Mayotte, magma appears to ascend through NW-SE trending lithospheric structure, consistent with the regional stress field [Berthod et al., 2021b, Feuillet et al., 2021, Lemoine et al., 2020].

## 5.2. Spatial distribution of deformation and current kinematic context

From the distribution of recent volcanic and tectonic structures, we identified geologically recent diking events throughout the Comoros Archipelago (Figure 12a), including the Mwezi Province (N130°E), N'Droundé Province (N160°E), Safari volcanic chain (N130°E) and Domoni volcanic chain (N160°E). Figure 12a shows groups of tensile fractures (Tf1-Tf4) corresponding to the main tectono-volcanic provinces and volcanic chains that were identified from the SISMAORE data and previous studies [Famin et al., 2020, Feuillet et al., 2021], as well as major faults and lineaments. These observations suggest the presence of major en echelon tensile fractures and secondary Riedel shears (Figure 12) in the Comoro Islands, Jumelles volcanic chain, Zélée-Geyser banks, and north of Anjouan. Feuillet et al. [2021]

also interpreted the EVM as a tensile fracture. Features with roughly orthogonal (ENE-WSW) azimuths in the Comoros Archipelago, such as those in the Chistwani volcanic chain (Figure 9), may have originated in diking events with right-lateral strike-slip motions.

The shift in the orientation of principal tensile fractures from N160°E in the west to N130°E to E-W in the east suggests a segmentation of the Archipelago (Figure 12). At the western end, the azimuths of the major volcanic structures tend to be parallel to the offshore branches of the EARS, such as the N-S trending Kerimbas graben along the Tanzanian/Mozambican coast [Franke et al., 2015, McGregor, 2015, Mougenot et al., 1986; Figures 1 and 12]. Taken as a whole, seismicity in the Comoros Archipelago outlines a rather narrow deformation zone that changes direction around Mayotte (Figure 12b). On the east, recent seismicity is distributed within an ENE-WSW (N80°E) corridor from west of Jumelles to Madagascar [especially from 2018; Bertil et al., 2021], and on the west it is concentrated in an NW-SE direction to the west of Mayotte. The N80°E trend of the eastern segment is parallel to the ENE-WSW Ambilobe sedimentary basin of northern Madagascar, of Cenozoic age [Piqué et al., 1999, Roig et al., 2012]. The observed tectono-volcanic structures on Zélée-Geyser banks are oriented roughly E-W. The same change in the orientation of tectono-volcanic structures corresponds to a change in the pattern of earthquakes, which are relatively diffuse to the west, where the structures trend from NNW-SSE to NW-SE, and less diffuse to the east, where they form a corridor trending N80°E and the tectono-volcanic structures trend approximately E-W. The few available focal mechanisms support a transtensional stress regime (Figure 12b). The current stress regime is consistent with a NE-SW direction of least stress (σ3) and a NW-SE direction of greatest stress (σ1), which implies an overall right-lateral deformation pattern that produces en echelon tensile fractures parallel to σ1 and perpendicular to σ3 in alignment with the principal volcanic chains. The transtensional stress field promotes dikes, as attested by the recent Mayotte eruptions.

The new data presented in this paper show that the recent volcanic and tectonic structures on the seafloor form a corridor some 200 km wide and 600 km long that includes the southern abyssal plain
of the Somali Basin and the Comoro Islands from west of Grande-Comore to Madagascar (Figures 4 and 12). Volcanism and faulting are absent on the seafloor in the Comoros basin south of the Comoro Islands. Note that this tectono-volcanic corridor does not closely match the “broad zone of deformation” proposed by Stamps et al. [2021] from GNSS modelling, which extends 1000 km from the Comoros Archipelago, including the northern half of the Lwandle plate, and the Comoros basin. Indeed, the regional GNSS network is not optimally configured [Bousquet et al., 2020] to clearly constrain the regional kinematics. The tectono-volcanic corridor shown in Figure 12 more closely corresponds to the Somalia–Lwandle plate boundary previously proposed by previous studies [e.g., Calais et al., 2006, Famin et al., 2020, Stamps et al., 2021].

In this tectono-volcanic corridor, no major fault systems affect the seafloor like those observed, for example, along the NE–SW Atlantic plate boundary in the Azores region [Sanchez et al., 2019]. The faults in the corridor are small (less than 10 km long with apparent vertical throws no greater than 20 m) and discontinuous (Figures 4 and 12). The absence of large (M6+) earthquakes and the fact that the largest reported events are associated with volcanic events, such as the 1918 eruptions of Karthala in Grande-Comore [Bachèlery et al., 1995, 2016b] and the 2018–2021 offshore Mayotte eruption, are further evidence of the lack of major faults in the Comoros Archipelago. If a dextral transform boundary between the Somalia and Lwandle plates is present in the Comoros Archipelago, the low intensity of deformations suggests that it is very immature or incipient.

### 5.3. Relationships between plate boundary forces, the role of inheritance and magmatism

The study area is the locus of interactions between deformation at plate boundaries and intense magmatism.

We propose that the young tectonic and volcanic structures identified in this study mark the location of a diffuse, immature Somalia–Lwandle plate boundary between the EARS and Madagascar (dashed lines in Figures 4 and 12). A recent to current regional regime of dextral transtension explains

well the location and orientation of volcanic and tectonic structures like the Mwezi province and the part of the EMVC associated with the active Fani Maor'e volcano. Particularly to the west of Mayotte, these features are well aligned with the pre-existing crustal fabric of the Somalia basin, interpreted from magnetic and gravity data as Mesozoic transform fault zones by Davis et al. [2016] and Phethean et al. [2016] (Figure 12a). We propose that these pre-existing oceanic fracture zones play a role in the distribution of the N–S to N160°E younger seafloor features by acting as weak zones, prone to reactivation in the current regional stress field (dextral shear with NE–SW tension axis or transtension), as Sauter et al. [2018] have established in the northwestern Somali basin. The volcanic complexes of the Comoro Islands appear to be located at intersections between tensile fractures guided by pre-existing fracture zones and NE–SW trending volcanic chains lying nearly parallel to the lithospheric fabric evident from magnetic anomalies in the southern Somali basin. These observations are consistent with a reactivated fossil fabric that guides magma ascent and controls the sites of volcanism on the seafloor.

However, the relationship between young seafloor features and earlier tectonic fabric is less clear east of longitude 44.5°E, from Anjouan to Madagascar. The N160°E trend of free-air gravity anomalies is subdued in this region, and the N130°E and E–W orientations of tectono-volcanic features in the Mwezi Province, the Jumelles and Safari volcanic chains, the EVCM, and the Zélée–Geyser banks are clearly oblique to the N–S to N160°E orientation of the fossil fabric. It may be that the N130°E trending structures have formed in response to the current geodynamic context of an immature plate boundary, independent of the inherited lithospheric structure. However, studies of other regions with heterogeneous lithosphere and active regional and local geodynamic contexts, especially in the Wharton basin in the Indian Ocean [Delescluse and Chamot-Rooke, 2007, Deplus et al., 1998, Hananto et al., 2018, Stevens et al., 2020, and references therein] have reported that the orientations of new and reactivated faults, and of deep and surface structures, may be different. They also suggest that shallow deformations can be influenced by both pre-existing structures and local or regional stress fields. A strong influence of a mantle melting anomaly or mantle convection can-
not be excluded [Tsekhmistrenko et al., 2021]. Thus, clarifying the relationship between recent seafloor deformations, fossil fabrics of the oceanic crust, and the presence or absence of oceanic crust under the Comoros Archipelago, requires further work that is being conducted in the framework of the ANR COYOTES project [e.g., Boymond et al., 2022, Masquelet et al., 2022, Rolandone et al., 2022, Thinon et al., 2020a].

**6. Conclusions**

Our SISMAORE cruise has shed light on a hitherto poorly known area around the volcanic Comoros Archipelago. Bathymetric and backscatter data, seismic profiles and dredge samples have revealed up to 2200 volcanic edifices and lava flows on the seafloor of the Comoros Archipelago, in the abyssal plain, on the submarine volcanic chains and on the slopes of the Comoro Islands. Other newly identified features include dome-shaped forced folds linked to sill intrusions in the thick sedimentary cover of the abyssal plain. The majority of these structures are located two great volcanic and tectonic fields in the abyssal plain: the N'Doundé Province, north of Grande-Comore and the Mwezi Province, north of Anjouan and Mayotte. Dredge samples of fresh rich-gas basaltic rocks in the Mwezi Province suggest that volcanic activity, although still undated, is very recent. The Comoro Islands, the major submarine volcanic chains and the N'Doundé and Mwezi provinces are interpreted as tensile fractures associated with diking events, consistent with the current regional tectonic setting of right-lateral transtension. Although the distribution of present-day to Pleistocene tectonic and volcanic structures in the western part of the Comoros Archipelago appears fairly congruent with the pre-existing crustal fabric, in the eastern part the role of inheritance is less clear. The distribution of recent deformation appears mainly compatible with the current kinematic context, with possible influence from the pre-existing crustal fabric and heterogeneities in the lithosphere. A strong influence of a mantle melting anomaly or mantle convection cannot be excluded.

The recent volcanic and tectonic structures, as well as the regional seismicity, are distributed within a corridor at least 200 km wide, including the Comoro Islands and the abyssal plain to their north, that

extends 600 km between the EARS and Madagascar. The distribution of volcanic and tectonic structures, and by inference the orientation of major tensile fractures, shifts in segments from a roughly N-S orientation in the west, to N160°E, to N130°E to E-W in the east. The Comoro Islands tend to lie at the junctions between tensile fractures (N160°E and N130°E) and ENE-WSW lineaments. This regional distribution of the recent structures is consistent with seismicity patterns in the Comoros Archipelago.

We interpret the Comoros tectono-volcanic corridor as a marker of the Somalia-Lwandle plate boundary. Although the corridor is prone to episodes of intense volcanism, the low intensity and style of deformation tends to confirm the very immature state of this dextral strike-slip plate boundary.

**Conflicts of interest**

Authors have no conflict of interest to declare.

**Acknowledgments**

The SISMAORE cruise was mainly funded by the Flotte Océanographique Française and the French geological survey (BRGM). We thank Captains P. Moimeaux and G. Ferrand of R/V *Pourquoi Pas?* and its crews and technicians especially and GENAVIR team for the bathymetry processing and the seismic deployment. Thanks to the BRGM regional department of Mayotte, Ifremer, REVOSIMA and the Délégation interministérielle aux risques majeurs Outremer for their assistance during the COVID health crisis. We also thank the Rectorate of Mayotte and two of its teachers (J. Jacquot, P. Deparis) for their participation, the Mission Sismaoré blog (https://sismaore.ac-mayotte.fr/) and the primary and high school students for the richness of exchanges. Thanks to the laboratory of BRGM for gas analysis and to D. Bertil for discussions on regional seismicity.

This paper is a contribution of the COYOTES (COmores & maYotte: vOlcanisme, TEctonique et Sismicité) and SISMAORE teams. This paper benefited from the previous works of the REVOSIMA community. The reviewers are greatly acknowledged for their comments and recommendations, which helped us to significantly improve an early draft of the manuscript. We also thank M. A. Alden for language editing. The processing and the detailed
analysis of these geophysical and geological data are carried out mainly in the framework of the COYOTES project (ANR-19-CE31-0018; https://anr.fr/Projet-ANR-19-CE31-0018) funded by the French Agence Nationale de Recherche and the BRGM.

## Supplementary data

Supporting information for this article is available on the journal's website under https://doi.org/10.5802/crgeos.159 or from the author.

## References

Audru, J.-C., Guennoc, P., Thinon, I., and Abellard, O. (2006). *Bathymay : La structure sous-marine de Mayotte révélée par l'imagerie multifaisceaux.* C. R. Geosci., 338, 1240-1249.

Bachlery, P., Ali, D. B., Desgrolard, F., Toutain, J. P., and Coudray, J. (1995). *L'éruption phréatique du Karthala (Grande-Comore) en Juillet 1991.* C. R. Acad. Sci. Sér. 2. Sciences de la terre et des planètes, 320(8), 691-698.

Bachlery, P. and Coudray, J. (1993). *Carte volcanotectonique (1/50000e) de la Grande-Comore et notice explicative.* The French Embassy in Moroni, Comores, and The University of La Réunion, St. Denis de La Réunion.

Bachlery, P. and Hémond, C. (2016). *Geochemical and petrological aspects of Karthala volcano.* In *Active Volcanoes of the Southwest Indian Ocean*, pages 367-384. Springer, Berlin, Heidelberg.

Bachlery, P., Lenat, J. F., Di Muro, A., and Michon, L. (2016a). *Active Volcanoes of the Southwest Indian Ocean: Piton de la Fournaise and Karthala*. Springer, Berlin.

Bachlery, P., Morin, J., Villeneuve, N., Soulé, H., Nas-sor, H., and Ali, A. R. (2016b). *Structure and erup-tive history of Karthala volcano.* In *Active Volca-noes of the Southwest Indian Ocean*, pages 345-366. Springer, Berlin, Heidelberg.

Berthod, C., Médard, E., Bachlery, P., Gurioli, L., Di Muro, A., Peltier, A., et al. (2021a). *The 2018-ongoing Mayotte submarine eruption: Magma mi-gration imaged by petrological monitoring.* *Earth Planet. Sci. Lett.*, 571, article no. 117085.

Berthod, C., Médard, E., Di Muro, A., Hassen Ali, T., Gurioli, L., Chauvel, C., Komorowski, J.-C., et al. (2021b). *Mantle xenolith-bearing phonolites and basanites feed the active volcanic ridge of Mayotte (Comoros archipelago, SW Indian Ocean).* *Contrib. Mineral. Petrol.*, 176(10), 1-24.

Bertil, D., Mercury, N., Doubre, C., Lemoine, A., and Van der Woerd, J. (2021). *The unexpected Mayotte, 2018-2020 seismic sequence: a reappraisal of the regional seismicity of the Comoros.* C. R. Géosci., 353(S1), 1-25.

Bertil, D. and Regnoult, J. M. (1998). *Seismotectonics of Madagascar.* *Tectonophysics*, 294, 57-74.

Bousquet, O., Lees, E., Durand, J., Peltier, A., Duret, A., Mekies, D., et al. (2020). *Densification of the Ground-Based GNSS Observation Network in the Southwest Indian Ocean: Current Status, Perspec-tives, and Examples of Applications in Meteorol-ogy and Geodesy.* *Front. Earth Sci.*, 8, article no. 566105.

Boymond, P., Feuillet, N., Thinon, I., Scholtès, L., Leroy, S., Rusquet, A., Masquelet, C., and Jacques, E. (2022). *Volcano-tectonic interactions within two recently discovered submarine volcanic fields: im-plication for geodynamics in the Comoros.* In *EGU General Assembly 2022, Vienne, Talk*.

Calais, E., Ebinger, C., Hartnady, C., and Nocquet, J. M. (2006). *Kinematics of the east African rift from GPS and earthquake slip vector data.* *Geol. Soc., Lond. Spec. Publ.*, 259(1), 9-22.

Cesca, S., Letort, J., Razafindrakoto, H. N., Heimann, S., Rivalta, E., Isken, M. P., et al. (2020). *Drainage of a deep magma reservoir near Mayotte inferred from seismicity and deformation.* *Nat. Geosci.*, 13(1), 87-93.

Chadwick Jr, W. W., Rubin, K. H., Merle, S. G., Bob-bitt, A. M., Kwasnitschka, T., and Embley, R. W. (2019). *Recent eruptions between, 2012 and, 2018 discovered at west mata submarine volcano (NE Lau Basin, SW Pacific) and characterised by new ship, AUV, and ROV data.* *Front. Mar. Sci.*, 6, article no. 495.

Clague, D. A., Paduan, J. B., Caress, D. W., Thomas, H., Chadwick Jr, W. W., and Merle, S. G. (2011). *Volcanic morphology of west mata volcano, NE Lau Basin, based on high-resolution bathymetry and depth changes.* *Geochem. Geophys. Geosystems*, 12(11), 1-21.

Class, C., Goldstein, S. L., Stute, M., Kurz, M. D., and Schlosser, P. (2005). *Grand Comore Island: a well-constrained "low 3He/4He" mantle plume.* *Earth Planet. Sci. Lett.*, 233(3-4), 391-409.
Coffin, M. F. and Rabinowitz, P. D. (1987). Reconstruction of Madagascar and Africa: evidence from the Davie Fracture Zone and Western Somali Basin. *J. Geophys. Res.*, 92(B9), 9385–9406.

Coffin, M. F., Rabinowitz, P. D., and Houtz, R. E. (1986). Crustal structure in the western Somali Basin. *Geophys. J. Int.*, 86(2), 331–369.

Counts, J. W., Jorry, S. J., Leroux, E., Miramontes, E., and Jouet, G. (2018). Sedimentation adjacent to atolls and volcano-cored carbonate platforms in the Mozambique Channel (SW Indian Ocean). *Mar. Geol.*, 404, 41–59.

Courgeon, S., Bachélery, P., Jouet, G., Jorry, S. J., Bou, E., Boudagher-Fadel, M. K., Révillon, S., Camoin, G., and Poli, E. (2018). The offshore east African rift system: neo insights from the Sakalaves seamounts (Davie Ridge, SW Indian Ocean). *Terra Nova*, 30(5), 380–388.

Daniel, J., Dupont, J., and Jouannic, C. (1972). Relations Madagascar-Archipel Des Comores (Nord-Est Du Canal De Mozambique): Sur La Nature Volcanique Du Banc Du Leven. *C. R. Acad. Sci. Ser. D: Sci. Naturelles*, 274(12), 1784–1787.

Davis, J. K., Lawver, L. A., Norton, I. O., and Gahagan, L. M. (2016). Neo Somalie basin magnetic anomalies and a plate model for the early Indian Ocean. *Gondwana Res.*, 34, 16–28.

Debeuf, D. (2004). *Etude de l'évolution volcano-structurale et magmatique de Mayotte (Archipel des Comores, Océan Indien): approches structurale, petrographique, geochimique et geochronologique*. PhD thesis, Université De La Réunion. Unpublished.

Delescluse, M. and Chamot-Rooke, N. (2007). Instantaneous deformation and kinematics of the India-Australia Plate. *Geophys. J. Int.*, 168(2), 818–842.

Deplus, C., Diament, M., Hébert, H., Bertrand, G., Dominguez, S., Dubois, J., et al. (1998). Direct evidence of active deformation in the eastern Indian oceanic plate. *Geology*, 26(2), 131–134.

Deplus, C., Feuillet, N., Bachelery, P., Fouquet, Y., Jorry, S., et al. (2019). Early development and growth of a deep seafloor volcano: preliminary results from the MAYOBS Cruises. In *AGU Fall Meeting Abstracts*, volume 2019, pages V43I–0227.

Déprez, A., Doubre, C., Masson, F., and Ulrich, P. (2013). Seismic and aseismic deformation along the East African Rift System from a reanalysis of the GPS velocity field of Africa. *Geophys. J. Int.*, 193(3), 1353–1369.

Deville, E., Marsset, T., Courgeon, S., Jatiault, R., Ponte, J. P., Thereau, E., Jouet, G., Jorry, S. J., and Droz, L. (2018). Active fault system across the oceanic lithosphere of the Mozambique channel: implications for the Nubia–Somalia southern plate boundary. *Earth Planet. Sci. Lett.*, 502, 210–220.

Dofal, A., Fontaine, F. R., Michon, L., Barruol, G., and Tkalčić, H. (2021). Nature of the crust beneath the islands of the Mozambique channel: constraints from receiver functions. *J. African Earth Sci.*, 184, article no. 104379.

Eagles, G. and König, M. (2008). A model of plate kinematics in Gondwana breakup. *Geophys. J. Int.*, 173(2), 703–717.

Famin, V., Michon, L., and Bourhane, A. (2020). The Comoros archipelago: a right lateral transform boundary between the Somalia and Lwandle plates. *Tectonophysics*, 789, article no. 228539.

Feuillet, N., Jorry, S., Crawford, W., Deplus, C., Thimon, I., Jacques, E., Saurel, J.-M., et al. (2021). Birth of a large volcano offshore Mayotte through lithosphere-scale rifting. *Nat. Geosci.*, 14(10), 787–795.

Flower, M. F. J. (1972). Petrology of volcanic rocks from Anjouan, Comores Archipelago. *Bull. Volcanol.*, 36(1), 238–250.

Flower, M. F. J. and Strong, D. F. (1969). The significance of sandstone inclusions in lavas of the Comores Archipelago. *Earth Planet. Sci. Lett.*, 7(1), 47–50.

Foix, O., Aiken, C., Saurel, J. M., Feuillet, N., Jorry, S. J., Rinnert, E., and Thinon, I. (2021). Offshore Mayotte volcanic plumbing revealed by local passive tomography. *J. Volcanol. Geotherm. Res.*, 420, article no. 107395.

Fossum, K., Dypvik, H., Haid, M. H., Hudson, W. E., and van den Brink, M. (2021). Late Jurassic and early Cretaceous sedimentation in the Mandawa Basin, coastal Tanzania. *J. African Earth Sci.*, 174, article no. 104013.

Franke, D., Jokat, W., Ladage, S., Stollhofen, H., Klimke, J., Lutz, R., and Schreckenberger, B. (2015). The offshore east African rift system: structural framework at the toe of a juvenile rift. *Tectonics*, 34(10), 2086–2104.

Gaina, C., Van Hinsbergen, D. J. J., and Spakman, W. (2015). Tectonic interactions between India and Arabia since the Jurassic reconstructed from ma-
rine geophysics, ophiolite geology, and seismic tomography. *Tectonics*, 34, 875–906.

Hananto, N., Boudarine, A., Carton, H., Singh, S. C., Avianto, P., Dyment, J., et al. (2018). Evidence of pervasive trans-tensional deformation in the northwestern Wharton Basin in the 2012 earthquakes rupture area. *Earth Planet. Sci. Lett.*, 502, 174–186.

International Seismological Centre (2021). International Seismograph Station Registry (IR). https://doi.org/10.31905/EL3FQQ40.

Jacques, E., Feuillet, N., Aiken, C., Lemoine, A., Crawford, W. C., Deplus, C., et al. (2019). The 2018–2019 Mayotte seismic crisis: evidence of an upper Mantle rifting event? In *AGU Fall Meeting Abstracts*, volume 2019, pages V431–0221.

Klimke, J. and Franke, D. (2016). Gondwana breakup: no evidence for a Davie fracture zone offshore northern Mozambique, Tanzania and Kenya. *Terra Nova*, 28(4), 233–244.

Klimke, J., Franke, D., Gaedicke, C., Schreckenberger, B., Schnabel, M., Stollhofen, H., and Chaheire, M. (2016). How to identify oceanic crust-evidence for a complex break-up in the Mozambique channel, off east Africa. *Tectonophysics*, 693, 436–452.

Kumar, P. C., Niyazi, Y., Eruteya, O. E., Moscariello, A., Warne, M., Ierodiazionou, D., and Sain, K. (2022). Anatomy of intrusion related forced fold in the offshore Otway Basin, SE Australia. *Mar. Pet. Geol.*, 141, article no. 105719.

Kusky, T., Toraman, E., Raharimahefa, T., and Rasoazanamparany, C. (2010). Active tectonics of the Alaotra–Ankay Graben system, Madagascar: possible extension of Somalian–African diffusive plate boundary? *Gondwana Res.*, 18, 274–294.

Lavayssière, A., Crawford, W. C., Saurel, J. M., Satriano, C., Feuillet, N., Jacques, E., and Komorowski, J. C. (2022). A new 1D-velocity model and absolute locations image the Mayotte seismo-volcanic region. *J. Volcanol. Geotherm. Res.*, 421, article no. 107440.

Le Gonidec, Y., Lamarche, G., and Wright, I. C. (2003). Inhomogeneous substrate analysis using EM300 backscatter imagery. *Mar. Geophys. Res.*, 24(3–4), 311–327.

Leinweber, V. T. and Jokat, W. (2012). The Jurassic history of the Africa–Antarctica corridor—new constraints from magnetic data on the conjugate continental margins. *Tectonophysics*, 530, 87–101.

Lemoine, A., Briole, P., Bertil, D., Roullé, A., Foumelis, M., Thinon, I., et al. (2020). The 2018–2019 seismo-volcanic crisis east of Mayotte, Comoros islands: seismicity and ground deformation markers of an exceptional submarine eruption. *Geophys. J. Int.*, 223(1), 22–44.

Leroux, E., Counts, J. W., Jorry, S. J., Jouet, G., Révillon, S., et al. (2020). Evolution of the Glorieuses seamount in the SW Indian Ocean and surrounding deep Somali Basin since the Cretaceous. *Mar. Geol.*, 427, article no. 106202.

Mahanjane, E. S. (2014). The Davie fracture zone and adjacent basins in the offshore Mozambique margin—a neo insight for the hydrocarbon potential. *Mar. Pet. Geol.*, 57, 561–571.

Masquelet, C., Leroy, S., Delescluse, M., Chamot-Rooke, N., Thinon, I., Lemoine, A., Franke, D., Watremez, L., Werner, P., and Sauter, D. (2022). The east-Mayotte new volcano in the Comoros archipelago: structure and timing of magmatic phases inferred from seismic reflection data. *C. R. Geosci.*, 354. Online first.

McGregor, D. (2015). History of the development of the east African rift system: a series of interpreted maps through time. *J. African Earth Sci.*, 101, 232–252.

Medialdea, T., Somoza, L., González, F. J., Vázquez, J. T., de Ignacio, C., Sumino, H., et al. (2017). Evidence of a modern deep-water magmatic hydrothermal system in the Canary Basin (eastern central Atlantic Ocean). *Geochem. Geophys. Geosystems*, 18(8), 3138–3164.

Mercury, N., Doubre, C., Lemoine, A., Van Der Woerd, J., Bertil, D., Hoste-Colomer, R., and Battaglia, J. (2022). Onset of a submarine eruption east of Mayotte, Comoros archipelago: the first ten months seismicity of the seismo-volcanic sequence (2018–2019). *C. R. Geosci.*, 354(S2). Forthcoming.

Michon, L. et al. (2016). The volcanism of the Comoros archipelago integrated at a regional scale. In Bachèlery, P. et al., editors, *Active Volcanoes of the Southwest Indian Ocean*, pages 333–344. Springer, Berlin, Heidelberg.

Michon, L., Famin, V., and Quidelleur, X. (2022). Evolution of the east African rift system from trap-scale to plate-scale rifting. *Earth-Sci. Rev.*, 231, article no. 104089.

Montaggioni, L. F. and Nougier, J. (1981). Les enclaves de roches détritiques dans les Volcans d'Anjouan
(Archipel des Comores), Origine et interprétation dans le cadre de l'évolution du Canal de Mozambique. *Bull. Soc. Géol. Fr.*, 7(6), 595-601.

Montanari, D., Bonini, M., Corti, G., Agostini, A., and Del Ventisette, C. (2017). Forced folding above shallow magma intrusions: insights on supercritical fluid flow from analogue modelling. *J. Volcanol. Geotherm. Res.*, 345, 67-80.

Mougenot, D., Recq, M., Virlogeux, P., and Lepvrier, C. (1986). Seaward extension of the east African rift. *Nature*, 321(6070), 599-603.

Mueller, C. O. and Jokat, W. (2019). The initial Gondwana break-up: a synthesis based on new potential field data of the Africa–Antarctica corridor. *Tectonophysics*, 750, 301–328.

Nehlig, P., Lacquement, F., Bernard, J., Audru, J.-C., Caroff, M., et al. (2013). *Notice Explicative, Carte Géologique France (1/30 000), Feuille Mayotte (1179)*. BRGM, Orléans. Carte géologique par Lacquement F, Nehlig P, Bernard J., 2013.

Omosanya, K., Johansen, S., Eruteya, O. E., and Waldmann, N. (2017). Forced folding and complex overburden deformation associated with magmatic intrusion in the Vøring Basin, offshore Norway. *Tectonophysics*, 706–707, 14–34. ISSN 0040-1951.

Paquet, F., Jorry, S., Deplus, C., Le Friant, A., Bernard, J., et al. (2019). The Mayotte seismo-volcanic crisis: characterising a reactivated volcanic ridge from the upper slope to the abyssal plain using multi-beam bathymetry and backscatter data. In *AGU Fall Meeting Abstracts*, volume 2019, pages V431-0219.

Pelletier, A. A., Caroff, M., Cordier, C., Bachèlery, P., Nehlig, P., Debeuf, D., and Arnaud, N. (2014). Melilite-bearing lavas in Mayotte (France): an insight into the mantle source below the Comores. *Lithos*, 208, 281–297.

Phethean, J. J., Kalnins, L. M., van Hunen, J., Biffi, P. G., Davies, R. J., and McCaffrey, K. J. (2016). Madagascar's escape from Africa: a high-resolution plate reconstruction for the western Sahali Basin and implications for supercontinent dispersal. *Geochem. Geophys. Geosystems*, 17(12), 5036–5055.

Piqué, A., Laville, E., Chotin, P., Chorowicz, J., Rakotondraompiana, S., and Thouin, C. (1999). L'extension à Madagascar du Néogène à l'Actuel: arguments structuraux et géophysiques. *J. African

*Earth Sci.*, 28(4), 975-983.

Quidelleur, X., Michon, L., Famin, V., Geffray, M. C., Danišík, M., Gardiner, N., Rusquet, A., and Zakaria, M. G. (2022). Holocene volcanic activity in Anjouan Island (Comoros archipelago) revealed by new Cassignol-Gillot groundmass K-Ar and ¹⁴C ages. *Quart. Geochronol.*, 67, article no. 101236.

REVOSIMA (Mayotte Volcanological And Seismological Monitoring Network) (2019-2022). Institut de Physique du Globe de Paris (IPGP), Bureau de Recherches Géologiques et Minières (BRGM), Institut Français de Recherche pour l'Exploitation de la MER (IFREMER), Centre National de la Recherche Scientifique (CNRS). Data collection of the Mayotte volcanological and seismological monitoring network (V.1.0). Institut de Physique du Globe de Paris (IPGP).

REVOSIMA newsletter (2021). Bulletin REVOSIMA n°25 (janvier 2021) de l'activité sismo-volcanique à Mayotte. IPGP, Université de Paris, OVPF, BRGM, Ifremer, CNRS, January, 6th, 2021. http://www.ipgp.fr/sites/default/files/ipgp_1er_bulletin_info_sismo_volcanique_mayotte-cor.pdf.

Rinnert, E., Feuillet, N., Fouquet, Y., Jorry, S. J., Thion, I., and Lebas, E. (2019). MD228/MAYOBS.

Roach, P., Milsom, J., Toland, C., Matchette-Downes, C., Budden, C., Riaroh, D., and Houmadi, N. (2017). Neo evidence supports presence of continental crust beneath the Comoros. In *Pesgb/Hgs Africa Conference, August, 2017*.

Roche, V., Leroy, S., Guillocheau, F., Revillon, S., Ruffet, G., Watremez, L., et al. (2021). The Limpopo Magma-rich transform margin, south Mozambique—2: implications for the Gondwana breakup. *Tectonics*, 40(12), article no. e2021TC006914.

Roche, V. and Ringenbach, J.-C. (2022). The Davie fracture zone: a recorder of continents drifts and kinematic changes. *Tectonophysics*, 823, article no. 229188.

Roig, J.-Y., Tucker, R. D., Peters, S. G., Delor, C., and Théveniaut, H. (2012). Carte géologique de la république de Madagascar à 1 / 1 000 000. Ministère des Mines, PGRM, Antananarivo, République de Madagascar, 1 color sheet.

Rolandone, F., Poort, J., Masquelet, C., Leroy, S., Thion, I., Lemoine, A., and Paquet, F. (2022). Heat flow measurements in the northern Mozambique channel. *C. R. Geosci.*, 354. Online first.
Sanchez, G., Merle, R., Hinschberger, F., Thinon, I., and Girardeau, J. (2019). Post-spreading deformation and associated magmatism along the Iberia–Morocco Atlantic margins: insight from submarine volcanoes of the Tore-Madeira Rise. *Mar. Geol.*, 407, 76–93.

Saurel, J. M., Jacques, E., Aiken, C., Lemoine, A., Retailleau, L., Lavayssière, A., et al. (2022). Mayotte seismic crisis: building knowledge in near real-time by combining land and ocean-bottom seismometers, first results. *Geophys. J. Int.*, 228(2), 1281–1293.

Sauter, D., Ringenbach, J. C., Cannat, M., Maurin, T., Manatschal, G., and Mcdermott, K. G. (2018). Intraplate deformation of oceanic crust in the west Somali basin: insights from long-offset reflection seismic data. *Tectonics*, 37, 588–603.

Segoufin, J. and Patriat, P. (1980). Existence d'anomalies Mésozoïques dans le Bassin de Somalie. Implications Pour Les Relations Afrique–Antarctique–Madagascar. *C. R. Acad. Sci. Sér. Iia: Sciences De La Terre Et Des Planètes*, 291b, 85–88.

Senkans, A., Leroy, S., d'Acremont, E., Castilla, R., and Despinois, F. (2019). Polyphase rifting and break-up of the central Mozambique margin. *Mar. Pet. Geol.*, 100, 412–433.

Stamps, D. S., Kreemer, C., Fernandes, R., Rajaonarison, T. A., and Rambolamanana, G. (2021). Redefining east African rift system kinematics. *Geology*, 49(2), 150–155.

Stamps, D. S., Saria, E., and Kreemer, C. (2018). Geodetic strain rate model for the east African rift system. *Sci. Rep.*, 8(1), article no. 732.

Stevens, D. E., McNeill, L. C., Henstock, T. J., Deslecluse, M., Chamot-Rooke, N., and Bull, J. M. (2020). A complete structural model and kinematic history for distributed deformation in the Wharton basin. *Earth Planet. Sci. Lett.*, 538, article no. 116218.

Talwani, M. (1962). Gravity measurements on HMS Acheron in south Atlantic and Indian Oceans. *Geol. Soc. Am. Bull.*, 73(9), 1171–1182.

Thinon, I., Lemoine, A., Feuillet, N., Michon, L., Leroy, S., et al. (2020a). ANR COYOTES project (Comoros, maYotte: vOlcanism, TEctonics and Seis-

micity). In *AGU Fall Meeting Abstracts 2020-Online*, volume 2020, pages V040–0003.

Thinon, I., Leroy, S., and Lemoine, A. (2020b). SIS-MAORE cruise, RV Pourquoi Pas?

Tsekhmistrenko, M., Sigloch, K., Hosseini, K., and Barruol, G. (2021). A tree of Indo–African mantle plumes imaged by seismic tomography. *Nat. Geosci.*, 14(8), 612–619.

Tzevahirtzian, A., Zaragosi, S., Bachèlery, P., Biscara, L., and Marchès, E. (2021). Submarine morphology of the Comoros volcanic archipelago. *Mar. Geol.*, 432, article no. 106383.

Vormann, M., Franke, D., and Jokat, W. (2020). The crustal structure of the southern Davie Ridge offshore northern Mozambique—a wide-angle seismic and potential field study. *Tectonophysics*, 778, article no. 228370.

Vormann, M. and Jokat, W. (2021). The crustal structure of the Kerimbas basin across the offshore branch of the east African Rift System. *Geophys. J. Int.*, 226(3), 2073–2102.

Weatherall, P., Marks, K. M., Jakobsson, M., Schmitt, T., Tani, S., et al. (2015). A new digital bathymetric model of the world's oceans. *Earth Space Sci.*, 2(8), 331–345.

Wessel, P., Sandwell, D. T., and Kim, S. S. (2010). The global seamount census. *Oceanography*, 23(1), 24–33.

Wright, J. B. and McCurry, P. (1970). The significance of sandstone inclusions in lavas of the Comores Archipalago, M.F.J. Flower and D.F. Strong: (Earth Planet. Sci. Lett. 7 (1969) 47–50). *Earth Planet. Sci. Lett.*, 8, 267–268.

Zinke, J., Reijmer, J. J., Taviani, M., Dullo, W. C., and Thomassin, B. (2005). Facies and faunal assemblage changes in response to the Holocene transgression in the Lagoon of Mayotte (Comoro Archipelago, SW Indian Ocean). *Facies*, 50(3–4), 391–408.

Zinke, J., Reijmer, J. J. G., Thomassin, B. A., Dullo, W. C., Grootes, P. M., and Erlenkeuser, H. (2003). Postglacial flooding history of Mayotte lagoon (Comoro archipelago, southwest Indian Ocean). *Mar. Geol.*, 194(3–4), 181–196.