Seismic structure of the young oceanic Lithosphere and the Lithosphere-Asthenosphere Boundary in the equatorial Atlantic Ocean

Before a jury composing of: Harm Van Avendonk (University of Texas, USA) - Rapporteur Nicholas Harmon (University of Southampton, UK) - Rapporteur Wayne Crawford (IPGP, France) - Examinateur Audrey Galve (GEOAZUR, France) - Examinatrice Alexandrine Gesret (Mines Paris - PSL, France) - Examinatrice Valenti Sallares (Institut de Ciències del Mar, Spain) - Examinateur Louise Watremez (Université de Lille, France) - Examinatrice Satish C. Singh (IPGP, France) - Directeur de thèse The defense will take place on Tuesday, 19th July at 3:00 pm in the IPGP Cuvier amphitheater. You can also follow it remotely with this Zoom link: https://u-paris.zoom.us/j/87315187971?pwd=UStSWWpVeDhjWmM2anhTZVlxYTR0dz09 Meeting ID : 873 1518 7971 Passcode : 011215 ----- Abstract: The idea of a rigid and elastically deforming lithosphere floating over a weaker and visco-plastically deforming asthenosphere is central to the theory of plate tectonics. Yet, the formation and evolution of the Lithosphere-Asthenosphere Boundary (LAB) and the role it plays in coupling/decoupling the mantle convection to the driving forces of plate motions remains poorly understood. Many seismic studies and drilling programs have sampled the upper oceanic crust well, whereas the lower crust, Moho boundary and upper mantle and their evolution with age at slow spreading ridges have not been well studied. In this thesis, I present the results of P-wave and S-wave velocity structure between 0-27 Ma obtained using traveltime tomography along two E-W trending profiles in the equatorial Atlantic ocean. The LITHOS profile on the African plate is 600 km long with 34 OBS/OBH and the ILAB-SPARC profile on the South American plate is 800 km long and both profiles cross the slow-spreading Mid-Atlantic ridge between 1°S and 3°S. I have also imaged the Lithosphere-Asthenosphere Boundary between 11-21 Ma using Kirchhoff pre-stack depth migration of wide-angle reflections at an offset range of 150-400 km and present a synthesis of observations of the LAB in the equatorial Atlantic with possible models to explain the dynamics of the LAB in this region. Along both the profiles of LITHOS and ILAB-SPARC, the upper/lower crustal and upper mantle velocities increase rapidly up to 10 Ma and change gradually beyond this age. At older ages, seismic velocity anomalies in the crust can be attributed to the presence of inactive faults and/or chemical heterogeneity. Average crustal thickness is 5.8 km along both the profiles with the variations in crustal thickness attributed to the heterogeneity of the accretionary processes at slow spreading ridges and also by crustal thinning due to faulting and tectonic extension. Elevated temperatures (>1200°C) beneath the ridge axis requires the presence of partial melt (~1-2%) in the lower crust. The Vp/Vs ratio reaches a value of ~2.1 in the axial valley further indicating the presence of fluids/melt as the S-wave velocity is more affected in the presence of fluids and increased porosity than the P-wave velocity. The LAB is imaged along the ILAB-SPARC profile at 38±3 km at 11 Ma and monotonically deepens with age up to 70±3 km at 18 Ma. The contrasting observation with respect to previous studies is that after 18 Ma, the LAB starts to become shallower towards 20 Ma and reaches a depth of 57±3 km at 21 Ma. This trend in the LAB reflector correlates well with the variations in basement topography. This depth variation of the LAB can be explained by two models, one concerning regional geology and the other with small scale convection. By considering the bathymetry and evolution of the crustal accretion at the western end of Chain transform fault, it is more probable that the LAB is controlled by the regional geology which is formed at the ridge axis at zero age. The structure of the lithosphere at the ridge axis influenced by magmatic/amagmatic accretion due to the thermal anomalies in the upper mantle is preserved though the lithosphere thickens with age. The LAB along SPARC profile does not follow a particular isotherm of plate cooling model, but instead, a range of isotherms between 1250°C and 1350°C cross the LAB along the profile. An estimated ~6-8% sharp velocity drop is required to produce the observed reflections from the LAB which could be due to the presence of frozen or partially molten sills at the LAB with the melt fraction being at least 0.9% - 1.3%. The presence of melt could decrease viscosity in the asthenosphere by a few orders of magnitude and this could enable the highly viscous and rigid lithosphere to glide over the easily deformable and less viscous asthenosphere facilitating the translational plate motions on the surface of the Earth.