Pierre BOYMOND

Advisor: Feuillet Nathalie (IPGP)

Co-advisor: Isabelle Thinon (BRGM), Luc Sholtes (ENSG, Univ. Lorraine)

Host lab/ Team: IPGP- Marine Geoscience – UMR7154

Funding: ANR COYOTES

Summary: On May 2018, the Island of Mayotte (Comoros, Indian Ocean) was struck by several Magnitude 5+ earthquakes as a result of a magmatic intrusion across the whole lithosphere (Lemoine et al., 2020, Cesca et al., 2020, Feuillet et al., 2019). This magmatic activity gave birth to a large deep-sea volcanic edifice (Feuillet et al., 2019). Such deep magmatic sources and diking in the brittle mantle were never documented by geophysical data before. High-resolution marine data were acquired offshore Mayotte Island during several marine cruises in 2019 and 2020 ((Mayobs cruises, Feuillet et al., 2019). Those data revealed that the new volcanic edifice of Mayotte was built at the tip of a NW-SE trending volcanic ridge (Mayotte ridge). This ridge is composed by dozens of volcanic edifices as a result of multiple magmatic intrusions along a NW-SE extensional tectonic structure (Feuillet et al., 2019). This structure opens at the scale of the whole lithosphere to transfer the strain between the East african rift offshore branches and the Madagascar graben, and serves as pathways for the magma straddling at the lithosphere asthenosphere boundary. The objective of this thesis is to better constrain the processes that led to the building of the Mayotte ridge and many other volcano-tectonics structures at the scale of the whole Comoros archipelago by combining marine geophysical data analysis and numerical modeling based on the discrete element method (Jiao et al., 2018). In addition to the exploitation of the MAYOBS cruise data set, the student will participate to the acquisition and processing of high-resolution marine data that will be acquired during the SISMAORE 2020 cruise (scheduled on the RV Marion Dufresne by the end of 2020). On the basis of the whole dataset of bathymetry, backscatter, HR seismic and sub-bottom profiles, the student will identify, map and characterize all the volcanic and tectonic structures at different spatial scales. By combining the information she/he will obtain at the subsurface to other information that will be gathered by the other work-packages of the COYOTES project*, the student will be able to built discrete element models. These models will bring constraints on the origin of such structures by investigating the mechanical coupling between volcanism and tectonics at plate scale. We aim to understand better the complex interactions between magmatism and tectonics in complex faulted media of different rheologies, to reproduce magma pathways, the conditions leading to failure of deep reservoirs and vertical or lateral transfer of the magma between the base of the lithosphere and the seafloor. Competencies required: Mechanics, physics, computing, modeling. Strong background in geophysics and geology

Subjet of the thesis

Anais RUSQUET

Supervision: Laurent Michon, professor (50%); Vincent Famin, associate professor HDR (50%)

Collaborations: Xavier Quidelleur (GEOPS, Paris Saclay), François Nauret (LMV, Clermont-Ferrand)

Host lab/ Team: Laboratoire Géosciences Réunion (LGSR), university of Réunion Island/IPGP

Funding: ANR COYOTES, INSU Tellus MAYVOLTE, INTERREG HATARI.

Summary The Comoros archipelago is the subject of a longstanding controversy about the origin of its volcanism, alternatively interpreted as a hotspot or as due to lithospheric deformation. This controversy is part of a broader debate about the nature of hotspots in general, and whether they are deeply rooted at the base of the lower mantle, or more shallowly in the asthenosphere. On the one hand, the hotspot interpretation of the Comoros is based on the He-Sr-Nd-Os-Pb istopic signature of some lavas in the archipelago, and also on the erosion of the islands that suggests a progression of volcanism toward the west. On the other hand, the presence of a regional seismicity, connecting the Comoros archipelago with the East-African Rift System to the west and with the Madagascar grabens to the east, suggests that its volcanism might be related to lithospheric deformation. Is the volcanism of the Comoros related to a hotspot, to an intraplate deformation zone ? Does it represent the northern boundary between the Lwandle and Somalia plates ? Is the Comorian volcanism related in time and origin with the East-African Rift System and/or with the volcanism of northern Madagascar ? A better geological knowledge of the Comorian region is necessary to answer any of those questions. The volcanic and seismic crisis that Mayotte undergoes since may 2018 reactivates the debate about the origin of the Comoros archipelago. The PhD thesis will study three aspects of the Comorian volcanism and its geodynamic framework. (1) The tectonics of the archipelago, through a survey of deformation structures observed in the field or by remote sensing, and their relationship with submarine structures identified by ongoing and future offshore campaigns. The objective of this first task is to determine if the deformation observed in Mayotte can be found in Anjouan, Grande Comore and northern Madagascar, and if all these structures are consistent with a regional kinematics. (2) The timing of volcanism relative to tectonics in the archipelago, with the use of K-Ar geochronology. The goal of this second task is to determine if volcanic building follows a synchronous pattern or an age progression along the archipelago, and/or if the timing of volcanism can be related to that of the East-African Rift System or of northern Madagascar. (3) The source of magmatism, through a geochemical study of the Sr-Nd-Pb isotopic evolution of volcanism through time, and its comparison with the magmatism of the East-African Rift System and northern Madagascar. The idea is to highlight isotopic similarities and differences among these magmatic provinces, in order to better understand the origin of volcanism. The PhD thesis is funded by the french national program ANR COYOTES involving several french laboratories. A strong collaboration is expected among teams as other PhD students will work on the Comoros geodynamics with complementary methods.

Subjet of the thesis

Keywords: Comoros archipelago, hotspot volcanism, lithospheric deformation, regional tectonics, plate boundary, Lwandle plate, Somalia plate, East-African Rift System, geochronology, isotopic geochemistry

Charles MASQUELET

Supervision: Sylvie LEROY (ISTeP) ; Daniel SAUTER (EOST)

Co-supervision: Nicolas CHAMOT-ROOKE (ENS); Isabelle THINON (BRGM)

Host lab/ Team: ISTeP - Sorbonne University

Funding: ANR COYOTES

Summary:

Subjet of the thesis (french version)

Keywords: North Mozambique channel, Ocean-Continent transition, inheritance, deformation, regional tectonics, multichannel reflection seismic (processing and interpretation), magnetism, gravity.