Strain accommodation in transitional rifts: extension by magma intrusion and faulting in Ethiopian rift magmatic segments

Active deformation within the northern part of the Main Ethiopian Rift (MER) occurs within approximately 60 kin-long, 20 km-wide 'magmatic segments' that lie within the 80 km-wide rift valley. Geophysical data reveal that the crust beneath the < 1.9 Ma magmatic segments has been heavily intruded; magmatic segments accommodate strain via both magma intrusion and faulting. We undertake field and remote sensing analyses of faults and eruptive centres in the magmatic segments to estimate the relative proportion of strain accommodated by faulting and magma intrusion and the kinematics of Quaternary faults. Up to half the < 10 km-long normal faults within the Boset-Kone and Fantale-Dofen magmatic segments have eruptive centres or extrusive lavas along their length. Comparison of the deformation field of the largest Quaternary fault and an elastic half-space dislocation model indicates a down-dip length of 10 km, coincident with the seismogenic layer thickness and the top of the seismically imaged mafic intrusions. These relations suggest that Quaternary faults are primarily driven by magma intrusion into the mid- to upper crust, which triggers faulting and dyke intrusion into the brittle upper crust. The active volcanoes of Boset, Fantale and Dofen all have elliptical shapes with their long axes in the direction N105, consistent with extension direction derived from earthquake focal mechanisms. Calderas show natural strains ranging from around 0.30 for Boset, 0.55 for Fantale, and 0.94 for Dofen. These values give extension strain rates of the order of 0.3 microstrain per year, comparable to geodetic models. Structural analyses reveal no evidence for transcurrent faults linking right-stepping magmatic segments. Instead, the tips of magmatic segments overlap, thereby accommodating strain transfer. The intimate relationship between faulting and magmatism in the northern MER is strikingly similar to that of slow-spreading mid-ocean ridges, but without the hard linkage zones of transform faults.