Degree 2 structure with a supercontinent.
This is another end-member scenario -- after a supercontinent is formed
in the downwelling hemisphere. Cirucm-supercontinent subduction quickly causes a
return flow beneath the supercontinent which induces a second upwelling
system that is antipodal to the original upwelling system. Noticing that the
African superplume is below where Pangea was, these results suggest that the
African and Pacific superplumes which are antipodal are not by accident.