“Hotspots” and the opening and closure of oceans

1G.R. Foulger & 2D.L. Anderson

1University of Durham, Science Laboratories, South Rd., Durham, DH1 3LE U.K., g.r.foulger@durham.ac.uk

2California Institute of Technology, Seismological Laboratory MC 252-21, Pasadena, CA 91125 USA, dla@gps.caltech.edu

A model for “hotspot” volcanism based on endgames of plate tectonic processes is maturing. As ocean basins close and continents collide, latest-subducting slab equal in length, at least, to the thickness of the colliding continental crust and lithosphere, is trapped in the suture. This includes overlying sea-floor sediments and back-arc basins. Young oceanic plate is relatively hot and buoyant, and thus does not sink to great depths into the mantle. Some even obducts and underplates. As a result, eclogitised or metasomatized slabs of subducted oceanic upper and lower crust (including both igneous and cumulate) may also be abandoned, after closure, in the upper mantle below the lithosphere of the sutured continents. Sutures are reactivated and exhumed when continents rift and new oceans open. Continental flood basalts and seaward-dipping reflectors are associated with the edges of thick Archean cratons and mobile belts. This “hotspots” volcanism, produces large volumes of melt, some of which has ocean-island basalt (OIB) geochemistry. The transient large melting rate can be explained by the melting of oceanic crust from subducted slabs in the sutures, which is more fusible than mantle peridotite. OIB geochemistry can be explained by large-degree melting of components of old, eclogitised subducted oceanic crust, including slab sediments. Where rifting occurs longitudinally along a suture, the availability of eclogite may increase melt volumes for the first few Myr after opening. Where a new rift crosses a suture transversely, enhanced melt volumes and OIB geochemistry may persist for longer. This raises interesting questions regarding the geometry of rifted continental edges, possible recirculating of continental mantle lithosphere and the depth to which mantle material is transported with overlying continents over periods of hundreds of Myr. Many primary “hotspots”, particularly at the edges of cratons and continents may be readily explained by this model. Abandoned oceanic slabs may be widespread in the upper mantle. Fertility and fusibility variations may explain ``hotspot'' magmatism better than temperature variations.