Main Ethiopian Rift

The Main Ethiopian rift (MER) forms one arm of the complex Afar triple junction zone. Rifting initiated in the southern and central Main Ethiopian rift between 18 Ma and 15 Ma, but the northern MER only developed after ~11 Ma (WoldeGabriel et al., 1988; Wolfenden et al., 2004). Between 12 and 10 Ma, the southern Red Sea margin propagated southward as the MER propagated NE, effectively linking the southern Red Sea and Ethiopian rifts, and forming a triple junction for the first time (Wolfenden et al., 2004).

The MER formed within Precambrian metamorphic basement of the Pan-African shield (Kazmin et al., 1978). Within the study area, however, Precambrian basement is exposed in only one locality at the base of the footwall to the Guraghe border fault. Over 1 km of Mesozoic to early Tertiary marine passive margin sequences overlie basement, and are in turn covered by 1-2 km of Oligocene to early Miocene basalt-ignimbrite sequences (Abebe et al., 2007). The Mid-Miocene to recent infill within the MER comprises interbedded basalt and ignimbrite flows, with isolated pockets of lacustrine and volcaniclastic strata (e.g., WoldeGabriel et al., 1988; Wolfenden et al., 2004).                                          

The NE-trending northern Main Ethiopian rift is a series of linked half grabens bounded by steep NE-striking Miocene (11 - 5.3 Ma) border faults (WoldeGabriel et al., 1988; Wolfenden et al., 2004). Structural patterns suggest N105E-directed extension (Wolfenden et al., 2004). Extensional strain migrated from border faults to smaller offset ~N10oE-striking faults and aligned eruptive centers in the central rift valley (Wolfenden et al., 2004). These Quaternary (<1.8Ma) faults and volcanic centres define a number of ~20 km-wide, ~60 km-long, right-stepping en echelon “magmatic segments” (Ebinger and Casey, 2001). Quaternary faults within magmatic segments show predominantly normal slip and ~50% of faults have eruptive centres or extrusive lavas along their length. GPS measurements show that approximately 80 % of present day extension across the MER is localized within these magmatic segments (Bilham et al., 1999), and InSAR analysis shows that several of the volcanic centers are deforming.

Thick grey lines are border faults with the annotated dates showing the age of the onset of rifting. Thin grey lines are Pliocene age faults and thin black lines are Quaternary - Recent faults along the rift axis. Red shapes show major Quaternary to Recent volcanic centers.

Simplified structural map of the Main Ethiopian rift (from Keir et al. 2011).

References

Keir, Derek, Pagli, Carolina, Bastow, Ian D. and Ayele, Atalay (2011), The magma-assisted removal of Arabia in Afar: Evidence from dike injection in the Ethiopian rift captured using InSAR and seismicity, Tectonics, TC2008, doi: 10.1029/2010TC002785.

Abebe, B., V. Acocella, T. Korme, and D. Ayalew (2007), Quaternary faulting and volcanism in the Main Ethiopian Rift, J. Afr. Earth Sci., 48 (2-3), 115-124. 

Biggs, J., I. Bastow, D. Keir, and E. Lewi (2011), Pulses of deformation reveal frequently recurring shallow magmatic activity beneath the Main Ethiopian Rift, Geochem. Geophys. Geosyst., doi:10.1029/2011GC003662. 

Bilham, R., R. Bendick, K. Larson, P. Mohr, J. Braun, S. Tesfaye, and L. Asfaw (1999), Secular and tidal strain across the Main Ethiopian Rift, Geophys. Res. Lett., 26, 2789–2792. 

Ebinger, C., and M. Casey (2001), Continental breakup in magmatic provinces: an Ethiopian example, Geology, 29, 527 - 530. 

Kazmin, V., A. Shifferaw, and T. Balcha (1978), The Ethiopian basement: stratigraphy and possible manner of evolution., International Journal of Earth Sciences (Geologische Rundschau), 67, 531–546. 

WoldeGabriel, G. (1988), Volcanotectonic history of the central sector of the main Ethiopian rift; a geochronological, geochemical and petrological approach., Ph.D. thesis, Case Western Reserve University. 

Wolfenden, E., C. Ebinger, G. Yirgu, A. Deino, and D. Ayalew (2004), Evolution of the northern main Ethiopian rift: birth of a triple junction, Earth Planet. Sci. Lett., 224, 213–228.