Deformation Distribution and Type in the Main Ethiopian Rift

Abstract The Main Ethiopian Rift (MER) is magmatically segmented and seismically active. The magmatic segments are arranged en echelon (e.g. Ebinger, C.J., Casey, M., 2001. Continental breakup in magmatic provinces: An Ethiopian example. Geology Society of America Bulletin, vol. 12, pp. 527–530). Furthermore they are about 50 km long and up to 20 km wide, orientated about N10°–20°E, and are therefore oblique to the main direction of Mid-Miocene border faults (N35°E). In this paper we propose that these segments not only represent magmatic but also tectonic segmentation. We use the association of the high spectral resolution of Landsat data (principal component analysis and classifications) and the very high spatial resolution of digitized aerial photographs to characterize and quantify the deformation along the rift. Additionally, several digital elevation models of metric resolution have been produced at different locations along the segments in order to quantify and describe the faulting. We define 4 tecto-magmatic segments (Gedemsa, Bosetti, Kone and Fantale), 40–70 km, 10–15 wide, arranged en echelon and N10°–20°E orientated. Each tecto-magmatic segment (TMS) is separated by 2–18 km wide domains exhibiting minor deformation. Furthermore, the TMS overlap in a N–S direction and show similar symmetrical deformation patterns. Within an individual segment deformation varies from its centre, where it is essentially magmatic, to its tips, where brittle deformation is predominant. The largest volumes of recent basalt are erupted in the centres and nearby. In contrast, recent magma injections are absent in the tip domains. It seems probable that dikes connect laterally to, and are fed by, the magmatic centres. Dikes are sub-parallel to the segment axis and therefore orientated normal to the recent extension direction. Within the TMS, aligned cones, atypical fault geometries, en echelon fault segments, and atypical displacement–length ratios all suggest that faulting is dike-induced. Along axis propagation of the TMS generate interactions between younger (Quaternary) intra-rift faults and older (Mid-Miocene) border faults. Thus, at the deformed tips, cross-cutting and long curved faults are produced. The fault interactions in the MER confirm that the border faults have been inactive during the recent stage of rifting. We discuss the implications of oblique rifting with moderate obliquity ( α  = 27°, angle between the extension direction and the trend perpendicular to the rift) at slow extension rates on active rifting processes associated with intermediate continental crust. The tecto-magmatic segmentation mirrors crustal segmentation beneath the rift axis, as indicated from 3-D-tomography (Daly, E., Keir, D., Ebinger, C., Stuart, G., Ayele, A., Waltham, D., 2007. Crustal structure of the northern Main Ethiopian Rift from a tomographic inversion of local earthquakes. Geophysical Journal International, in press; Keranen, K., Klemperer, S.L., Gloaguen, R., EAGLE Working Group, 2004. Three-dimensional seismic imaging of a protoridge axis in the Main Ethiopian rift. Geology 32 (11), 949–952). These long and narrow zones (40–70 km long and 10–15 km wide), where deformation is generated principally by diking and is associated with magmatic centres, have similarities with along-axis segmentation at mid ocean ridges. Consequently, deformation distribution and type in the MER can be considered the outcome of a transition between continental and active oceanic rifting.

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