Investigating the velocity structure beneath the Southern and Central Atlantic; Implications for evolution of oceanic crust and lithosphere

Presented here is the shear velocity structure of the crust and upper mantle beneath the central and southern Atlantic Ocean from inversion of high resolution group velocity tomography. The path average group velocities from Rayleigh waves were picked using multi filter technique and phase match filtering for 14,000 paths. They were then combined within a tomographic inversion, to obtain the regional variations of velocity structure at a range of short to intermediate periods (14 s - 100 s). These group velocities have depth sensitivities from the surface to approximately 90 km depth, constraining the focus to velocity variations within the crust and mantle lithosphere. Tomographic results highlight short wavelength variations at periods sensitive to shallow depths, implying the possibility for a more complex velocity structure than currently expected for the oceanic region. The results show a clear relationship between increasing group velocities and increasing sea floor age. Group models are then inverted to obtain the shear velocity structure with respect to depth. The shear velocity model highlights slow velocities beneath the ridge, interpreted as the upwelling of asthenosphere between depths between 30 km and 50 km. Models of crustal and lithospheric thickness are extrapolated from the data. These models suggest the evolution of the Atlantic Ocean is more complex than the simple mathematical cooling models. It is suggested that the main control on crustal thickness is tectonic processes associated with the slow spreading rate and not controlled by to the mantle potential temperature. Additionally, results are presented which incorporate 2 azimuthal anisotropy in the tomographic inversions. At the longest periods test show that the recovered anisotropy is an artefact of the inversion process, and cannot be interpreted in terms of mantle flow. At the shortest periods there is a possible relationship between the fast direction and the stress field.