Understanding Sub-Lithospheric Small-Scale Convection by Linking Models of Grain Size Evolution, Mantle Convection, and Seismic Tomography

The interaction between aging oceanic plates and their underlying mantle is a crucial component of the plate tectonic cycle. Sub-lithospheric small-scale convection (SSC) explains why plates appear not to thicken after a certain age. Here, we link grain-scale processes, dynamic models of asthenospheric flow, and seismic observations to gain new insights into the mechanisms of SSC. We present high-resolution 3D geodynamic models of oceanic plate evolution with an Earth-like rheology including coupled diffusion/dislocation creep and their interplay with evolving olivine grain size. Our models quantify how rheology affects the morphology and temporal stability of SSC, and we directly relate these quantities to geophysical observations from the Pacific OBS Research into Convecting Asthenosphere (ORCA) experiment. We convert variations in temperature, pressure, grain size, water content and stable melt fraction to seismic velocity and attenuation, seeking to match the wavelength and pattern of observed longitudinal convective rolls, the young SSC onset age, the large seismic velocity heterogeneity, low absolute seismic velocities, and high seismic attenuation. This requires low ( Pa s) asthenospheric viscosity, the contribution of both diffusion and dislocation creep to deformation, and the presence of volatiles and melt. Although SSC occurs at plate ages $\ll$60 Ma in our best-fit model, the plate thermal structure approximately matches global observations of heat flux and bathymetry, indicating an important role of vigorous SSC in Earth's plate dynamics. However, reconciling all seismological observations is challenging, and additional mechanisms are required to explain the strong velocity heterogeneities suggested by body wave tomography.