Some of the best-studied examples of tectonically active, narrow intracontinental rifts occur in East Africa (Fig. 7.2). Southwest of the Afar triple junction, the Nubian and Somalian plates are moving apart at a rate of approximately 6-7 mm a-1 (Fernandes et al., 2004). This divergent plate motion results in exten-sional deformation that is localized into a series of discrete rift segments of variable age, including the Western Rift, the Eastern Rift, the Main Ethiopian Rift, and the Afar Depression. These segments display characteristics that are common to rifts that form in relatively strong, cool continental lithosphere. Key features include:
1 Asymmetric rift basins flanked by normal faults. Continental rifts are associated with the formation of sedimentary basins that are bounded by normal faults. Most tectonically active rift basins show an asymmetric half graben morphology where the majority of the strain is accommodated along border faults that bound the deep side of the basins (Fig. 7.2b-e). The polarity of these half grabens may change along the strike of the rift axis, resulting in a segmentation of the rift valley (Fig. 7.3a). In plan view, the border faults typically are the longest faults within each individual basin. Slip on these faults combined with flexural isostatic compensation of the lithosphere (Section 7.6.4) leads to uplift of the rift flanks, creating a characteristic asymmetric topographic profile. The lower relief side of the basin may be faulted and exhibits a monocline that dips toward the basin center. Deposition during slip on the bounding normal faults produces sedimentary and volcanic units that thicken towards the fault plane (Fig. 7.3b). The age of these syn-rift units, as well as units that pre-date rifting, provide control on the timing of normal faulting and volcanism. In plan view, displacements decrease toward the tips of border faults where they interact with other faults bounding adjacent basins. Within these transfer zones faults may accommodate differential horizontal (including strike-slip) and vertical displacements between adjacent basins.
2 Shallow seismicity and regional tensional stresses. Beneath the axis of most continental rifts earthquakes generally are confined to the uppermost 12-15 km of the crust, defining a seismogenic layer that is thin relative to other regions of the continents (Section 2.12). Away from the rift axis, earthquakes may occur to depths of 30 km or more. These patterns imply that rifting and thinning locally weaken the crust and affect its mechanical behavior (Section 7.6).
In Ethiopia, the record of seismicity from 1960 to 2005 (Fig. 7.4a) shows that the majority of large earthquakes occur between the Afar Depression and the Red Sea. Analyses of seismic moment release for this period shows that more than 50% of extension across the Main Ethiopian Rift is accommodated aseismically (Hofstetter & Beth, 2003). The earthquakes show combinations of normal, oblique and strike-slip motions. North of the Afar Depression, the horizontal component of most axes of minimum compressive stress strike to the north and northeast at high angles to the trend of the rift segments. Keir et al. (2006) used nearly 2000 earthquakes to determine seismicity patterns within the northern Ethiopian Rift and its flanks
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