Leader: Sylvie Leroy (ISTEP- Sorbonne U.)

Participants & coll.: S. Leroy , D. Sauter (EOST), N. Chamot-Rooke (ENS Paris), M. Delescluse (ENS Paris), Louise Watremez (LOG), I. Thinon (BRGM), S. Jorry (Ifremer), S. Zaragosi (EPOC), F. Paquet (BRGM), A. Peyrefitte (BRGM), A. Canva (BRGM), C. Masquelet (IsTep)

This WP is organized in three tasks:

• Task 3.1. Depth structuration and inheritance (Leader: D. Sauter)
  

The objectives are to study the regional crustal structuration and the role of inheritance in order to propose a long-term geodynamic evolution of the North Mozambique Channel and the Comoros archipelago. These aspects will be addressed in a PhD thesis (PhD3) using:

• New high-resolution long offset multichannel reflection seismic profiles from the SISMAORE cruise (6000 m long streamer with 960 channels and 18 airguns (~3600 inch3)). These profiles will be calibrated in age with a DSDP242 well.
• Gravimetric and magnetic anomalies maps (processed in T3.3).

These new seismic data will allow to observe the Moho (oceanic or transitional) if present and thus to assess the nature of the crust in the southern part of the Somali basin and in the Comoros basin. Interpreting these new geophysical data will provide information on the nature of the Comoros Basin, the southern Somali Basin architecture, the relationships between the volcanic archipelago and the basins, and identification of the transition between Continental and Oceanic domain offshore NW Madagascar. All the seismic data will be processed, interpreted and integrated by a PhD student with the existing data set in order to end up, together with T3.3, with a new model for the nature and structure of the Comoros basin. Underway gravimetric and magnetic data collected during the cruise will also be used to make forward models in support of the interpretation of the seismic data that will further constrain the nature and structure of the crust in the southern part of the basin.

• Task 3.2. Recent tectono-sedimentary evolution (Leader: F. Paquet)
  

In addition to the understanding of the regional deep structuration, we propose to evaluate its imprint on the basin evolution with a focus on the Plio-Pleistocene period. The whole data set will be complemented by a regional grid of both multichannel high-resolution seismic (SISRAP; SISMAORE dataset) and the very high resolution seismic (sub bottom profiler – SBP; MD136, MAYOBS and SISMAORE datasets) that will image the sedimentary cover geometry and deformation of the Comoros Basin, in order to detail its tectono-sedimentary evolution until present. The recent dynamics of the basin will be specifically addressed through SBP interpretation as well as descriptions and analyses of several Calypso or Kullenberg sediment piston-cores (sedimentology, spectrocolorimetry, P-wave velocity, Gamma Ray, magnetic susceptibility, both X-Ray radiography and fluorescence Spectrometry (XRF), Grain size, thin sections (20), radiocarbon dating (foraminifera tests), nannofossils, 210Pb on volcanic material, trace-elements). Sediment cores will provide key informations to determine (i) depositional environment and events (hemipelagic, turbidites, volcanoclastic, mixed systems…), (ii) age and short-timescale (Holocene-Pleistocene) sedimentation rates along the slope of Mayotte down to the adjacent basin, and (iii) recent volcanic events that may be recorded in the sedimentary pile as volcanic glass or ash layers. This latter aspect will eventually integrate the study of the recent volcanic activity carried out in WP2. Correlation between cores and very-high resolution seismic profiles will give an overview of sedimentary dynamics and of the “Pleistocene” evolution of the area where the current seismicity crisis occurs. Analyses will be carried out in priority over sediment cores from the SISMAORE survey. In case of poor recovery rates in SISMAORE cores or if considered valuable and meaningful, analyses could also be performed on previous cores from MD104/MOZAPHARE (1996) and MD136/VIGO (Michel & Waelbroeck, 2004).

• Task 3.3. Geophysical characteristics of the crust: Magnetism and gravimetric modelling (Leader: A. Peyrefitte)
  

We propose to process, adjust and compile the gravimetric and magnetic data from SISMAORE, MAYOBS and older surveys to create the regional Bouguer and Reduced to pole anomaly maps. In addition to being made available to other WPs, these maps will be the subject of a dedicated study, implementing advanced processing (different kind of operators, depth sources analyses, 2D geophysical modeling…) that will constrain the in-depth localization of the sources of density and magnetization contrasts, as well as give insight on the overall crustal structure, thickness and nature. Around the seismic swarm, potential field data will also be used in order to extend the 3D geometries of structures imaged in the deep seismic profiles. This data will complement other estimates of location, structure and volume of the deep sources responsible of Mayotte magmatic crisis. These works will be carried out by a post-doc (one year funded by the COYOTES project, possible 6 months additional BRGM funding may be asked in case of need), supervised and assisted by two geophysical experts.