Magmatism and crustal accretion in continental rifts

An attempt is made to analyse how magmatism in continental rifts is control led by the geometry of rifting. Distribution of volcanics in some rifts is compatible with the simple shear model of rift evolution. Using data on the Ethiopian and Red Sea Rifts, it is demonstrated that at the early stages o f rifting, magmatic activity concentrates in belts 5-10 km wide coinciding with the break-away and/or hinge zones of the rift. Comparison of two belts exposed at different erosional levels-the Wonji belt (Ethiopian Rift) and the magmatic belt of the Red Sea eastern coastal plain, with addition of da ta on xenoliths in basalts allows the construction of a composite vertical section of the magmatic belt. It appears that processes similar to spreadin g and crustal accretion in oceanic spreading centres may be active in conti nental rifts. However, the geometry of accretion and composition of the acc reting material are different from the ones in oceanic settings. A model sc heme of "magmatic" rift evolution in 5 stages is presented. (1) Formation o f a low angle fault, uplift of the asthenosphere, decompression and partial melting. (2) Migration of melt along the low angle detachment, formation o f sub-crustal reservoir, eruption of plateau basalts on the upper plate. (3 ) Further concentration of extension and magmatic activity in the break-awa y and hinge zones of the rift. At that time, magmatic accretion of the crus t, compensating for extension becomes effective. (4) Blocking of magmatic c onduits by underplating and formation of new ruptures in the axial zone of the rift. (5) Eruption of MORB through the axial ruptures. (C) 2000 Elsevie r Science Limited. All rights reserved.