Investigating superswells and sea level changes caused by superplumes via normal mode relaxation theory

The superplume hypothesis, particularly for one in the mid-Cretaceous, has stimulated the search for evidence for or against such an event and the associated superswell. An approach based on normal mode relaxation theory makes it possible to simulate the surface deformation caused by the rising plume using a spherical, self-gravitating and stratified viscoelastic Earth model. The results for the superswell are consistent with those of previous analyses using convection and experimental modeling. The self-consistent coupling of elastic and viscous properties based on Maxwell rheology reveals that elasticity plays an important role contributing up to 30% to surface swelling when the plume head crosses the upper mantle. We show that the deformation at great distances from the superswell is relatively small, of the order of meters at most, and opposite in sign with respect to the large uplift over the plume, of the order of kilometers. The global subsidence over a wide region of the planet surrounding the superswell counteracts its effects on sea level changes and triggers a "eustatic'' signal of a few meters at most, a negligible magnitude with respect to 250 m characteristic for the mid-Cretaceous. Previous arguments against the mid-Cretaceous superplume hypothesis, which assume that the large amount of water displaced by the superswell causes a "eustatic'' sea level rise of about 200 m thus competing with the displacement due to oceanic crust production, are no longer tenable since such estimates did not account for the global behavior of the planet under internal loads.