Speculations on the causes of crustal rifting and subduction,with applications to the atlantic margin of Brazil

It is suggested that ideal regions for continental separation involve regions of crust dominated by granulite and amphibolite facies rocks. Such rocks are strong and refractory. The formation of thick crust of this type requires, at some stage, that the crust be tectonically thickened to about twice the equilibrium thickness which is itself controlled by melting. It is suggested that subduction is initiated by the basalt-eclogite transition in the upper oceanic lithosphere. A large sediment load may induce this reation and lead to subsidence or subduction. If the rate of sediment supply is small, subduction will be transient, or may not occur at all and sinking slow. Such motions could account for sporadic igneous activity by partial melting of slowly sinking ocean-floor materials.