Oceanic isostasy and intraplate stresses

Intraplate deformation is, by definition, unexplained by plate tectonics. Because intraplate strain rates are relatively small, dominant intraplate stresses driving observed deformation can derive from a number of different, non-mutually exclusive, sources. Driving processes include, but are not necessarily limited to, gravitational potential energy variations, glacial isostatic adjustment, and tractions at the base of the lithosphere from flow in the underlying asthenosphere. Tractions at the base of the lithosphere have long been suggested to contribute to plate motions as well as intraplate stresses. Any stationary asthenospheric flow field will contribute to plate driving or resistance, depending on whether asthenosphere is leading or lagging the overlying plate. Stationary flows that are also spatially variable will induce tractions imparting differential stress on the overlying lithosphere. One overlooked driver of asthenosphere flow at the asthenosphere-lithosphere boundary that could have implications for understanding intraplate stresses in the oceans is oceanic isostasy. In a manner similar to an icecap on a continent, the addition of ocean mass on top of subsiding lithosphere drives a small degree of flow in the asthenosphere to accommodate the excess mass accumulated on top. Typically, oceanic lithosphere is understood to cool and subside away from mid ocean ridges to a Pratt-like isostasy condition. However, the presence of seawater added on top of subsiding lithosphere necessitates an additional isostatic response that cannot be achieved through densification of lithosphere alone. The basic mathematics behind the isostasy-driven asthenospheric flow demonstrates that the flow is systematically from beneath younger seafloor towards older seafloor. The asthenosphere flux is variable, but systematic across the plate. The flow rate peaks beneath seafloor of about one-quarter the plate age and decreases to zero at the plate extremities. The maximum flow rate is a few tenths of a percent of the plate rate and superimposed on the regional asthenospheric flow field. The modification to the regional flow induces a small, but systematically positive drag (mantle push) component on the overlying lithosphere. Potentially importantly, the form of the drag predisposes young lithosphere to extensional stress and older lithosphere to compressional stress. While difficult to tie any particular intraplate observation to this mechanism, a statistical evaluation of intraplate stress indicators is consistent with the predicted pattern.