Structure of oceanic lithosphere and asthenosphere from forward modeling of absolute velocities

Understanding the structure and properties of the oceanic lithosphere and asthenosphere system is key to understanding the dynamics of plate tectonics. Recently a number of ocean bottom seismic deployments in the Pacific have allowed the construction of high-resolution models of the seismic velocities of the oceanic upper mantle at a range of ages. In particular, the NoMelt experiment located about 900 km SE of Hawaii was designed to image the lithosphere-asthenosphere system in an area with no seamounts or transform faults indicating asthenospheric disturbances. To explore whether the seismically observed velocities can be explained without the presence of melt at the lithosphere-asthenosphere boundary we forward model velocities including anelasticity. To determine the elastic moduli we calculate phase diagrams for MORB source compositions using Perple_X. A particular aspect is the effect of Cr on the stability fields of spinel and garnet. Densities and anharmonic shear moduli of the bulk rock extracted from Perple_X are used as input in Burgers model for calculations of moduli and attenuation as a function of frequency. Comparison with NoMelt velocities shows that the top of the low velocity zone is well matched, and the minimum seismically observed velocities are consistent with anelastic velocities at a grain size of 1 mm. Melt is therefore not needed to account for the observed velocities, consistent with relatively low observed attenuation. The spinel to garnet transition calculated with Perple_X results in discontinuous variations in velocities, but the variations are too small to produce detectable receiver functions. Suitable velocity models for the Atlantic will also be explored.