Fifty years into the plate-tectonic era, Earth scientists struggle to constrain two fundamental components of the tectonic model: (1) what is the nature and scale of the mantle convection processes that drive the plate motion?; and (2) what processes control the localized weakening of plate boundaries that enables plates to move? New autonomous ocean-bottom seismic (OBS) instrumentation deployed in the deep ocean basins provide an exceptional new opportunity to investigate these processes, as the simple geological history of oceanic crust and lithosphere provide a relatively clear imaging of the underlying mantle. Using data from the innovative NoMelt experiment from the central Pacific basin, we demonstrate that the strongest deformation in the oceanic mantle is associated with seafloor spreading at the mid-ocean ridge. Flow deeper in the asthenosphere is driven by large-scale buoyancy and/or pressure forces that are decoupled from the motion of the overlying plate. The strength and orientation of the deformation agree remarkably well with structural geology estimates from peridotite samples, enabling us to probe the microscopic mechanisms of deformation. We also infer that weakening in the asthenosphere beneath the center of the Pacific plate is controlled by water or other volatiles and not silicate partial melt. These local, high-resolution geophysical observations provide fundamental new constraints on the nature of oceanic plates and their relationship to the convecting mantle.