Magmatically assisted off-rift extension—The case for broadly distributed strain accommodation

Within continental rift settings, extensional strain is initially accommodated along the nascent rift margins, subsequently localizing to zones of focused magmatic intrusion. The migration of strain from rift-border faults to diking places an emphasis on constraining the magmatic plumbing system of zones of focused intrusion to resolve how extension is accommodated in the rift lithosphere. While existing rifting models concentrate on the relationship between extension and focused magmatism within the rift, there is increasing evidence of rift-related magmatic activity outside the rift valley. We examine the Galema range, an area of focused magmatic activity along the eastern margin of the Central Main Ethiopian Rift, which is morphologically similar to areas of focused magmatism within the rift. We find that whole-rock thermodynamic modeling and thermobarometric calculations on mineral-liquid pairs suggest that fractionation (and hence magma stalling depths) within the Galema range is polybaric (~7 and ~3 kbar). These results, when compared to zones of focused intrusion within the rift, indicate an incipient magmatic plumbing system. We contend that diking associated with the Galema range, which predates magmatic belts within the rift, thermomechanically modified the lithosphere along this margin. While the cessation of magmatism within the Galema range may have been precipitated by a change in magma flux, the now thermomechanically modified lithospheric mantle along this margin facilitated the subsequent development of within-rift magmatic chains. The implications of this are that off-rift magmatic activity may play an integral role in facilitating the development of rift architecture.