New geodynamic models, based on teleseismic data sets, propose that the westward motion of the North and South American plates is driven by asthenospheric flow coupled to the overlying lithosphere. Implicit in these models is the existence of a zone of partial coupling at the boundary between the lithosphere and asthenosphere. We propose that porphyroclastic peridotite xenoliths from kimberlite are samples of deep-mantle shear zones. These samples are a direct manifestation of the mechanical coupling that arises from the drag applied to the lithosphere by the underlying asthenospheric flow. We postulate that the formation of shear zones at the lithosphere-asthenosphere boundary is a quasisteady-state process attending plate movement beneath all cratons.