Magmatism and crustal accretion in continental rifts

An attempt is made to analyse how magmatism in continental rifts is controlled 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 of 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 data on xenoliths in basalts allows the construction of a composite vertical section of the magmatic belt. It appears that processes similar to spreading and crustal accretion in oceanic spreading centres may be active in continental rifts. However, the geometry of accretion and composition of the accreting material are different from the ones in oceanic settings. A model scheme of "magmatic" rift evolution in 5 stages is presented. (1) Formation of a low angle fault, uplift of the asthenosphere, decompression and partial melting. (2) Migration of melt along the low angle detachment, formation of sub-crustal reservoir, eruption of plateau basalts on the upper plate. (3) Further concentration of extension and magmatic activity in the break-away and hinge zones of the rift. At that time, magmatic accretion of the crust, compensating for extension becomes effective. (4) Blocking of magmatic conduits by underplating and formation of new ruptures in the axial zone of the rift. (5) Eruption of MORB through the axial ruptures. (C) 2000 Elsevier Science Limited. All rights reserved.