NATURE OF THE CRUST BENEATH MAGMATICALLY ACTIVE CONTINENTAL RIFTS

A terse review of the geology and geophysics of continental rift valleys associated with large-scale volcanism, points to the underlying crust being composed essentially of new igneous rock. The process of transformation from old cratonic to new igneous crust, still only sketchily known, is considered to involve two phenomena: (1) Intrusion of mafic sills into the extending, thinning and diked cratonic crust, is accompanied at a deeper level by accretionary 'underplating'. (2) A steepening geothermal gradient facilitates anatexis of sill gabbro/amphibolite, a process aided by fresh batches of mafic magma passing up through the crust. Silicic magma produced from the partial melting process may mix with that fractionated from mafic magma in sill-like chambers in the middle crust. Through these processes, the cratonic component of the rift crust may ultimately become wholly subordinate to the new igneous component well before the onset of crustal separation.