Few constraints on the timing, amount and distribution of lithospheric extension associated with flood-basalt magmatism were available from the southern Main Ethiopian rift system, where the base of the Cenozoic volcanic succession is exposed by faulting. New structural observations, together with K–Ar and 40 Ar/ 39 Ar geochronology data from a transect of the Chamo basin–Amaro horst–Galana basin, show that basins are bounded by faults with steep dips at the surface, and the stratal dips of Eocene–Recent volcanic and sedimentary units are generally less than 20°. Little or no extension accompanied the extrusion of a 0.5 to 1 km thick sequence of transitional tholeiitic flood basalts between 45 and 35 Ma. Stratigraphical correlations with basins to the north and southwest suggest that felsic eruption(s) at c. 37 Ma blanketed much of the southern Ethiopian plateau region with a felsic tuff unit. A second, less widespread, episode of alkali basalt and trachyte volcanism occurred between 18 and 11 Ma, and Recent alkali basalt volcanism occurs within the Chamo basin. The attitude, distribution, and diversity of Neo–gene–Recent volcanic and sedimentary strata within the Chamo and Galana basins indicate that crustal extension, basin subsidence, and rift flank uplift began during or after the second flood-basalt phase. Based on cross-sectional reconstruction to the top of the Oligocene tuff, we estimate a minimum of 12 km crustal extension (β ≈ 1.12), and infer that maximum extension across the southern Ethiopian rift is less than 25 km. Extension is primarily accommodated by slip along the border faults bounding the asymmetric basins, with small amounts of extension occurring within the hanging walls. Crude estimates of original basalt layer thickness prior to erosion in the Amaro region suggest that roughly comparable volumes of basaltic material erupted during the two episodes of flood-basalt magmatism (45–35 Ma and 18–11 Ma). The small amounts of lithospheric extension and the large volumes of magma estimated in this study of the southern Main Ethiopian rift suggest a very hot plume and/or efficient thinning of the mantle lithosphere from below by mantle plume processes during the two discrete episodes of flood-basalt volcanism.

The uplifted Ethiopian plateau region encompasses amagmatic rift basins and basins with nascent seafloor spreading. Rift segments in the Main Ethiopian rift and southern Afar show a distinct structural segmentation, with a south to north reduction in the length, width, and spacing of fault zones; rift segments in northern Afar, where extension exceeds 100%, show a magmatic segmentation. The objectives of remote sensing, gravity, and modeling studies of the Ethiopian plateau area are (1) to summarize morphological patterns along the length of the rift system and (2) to relate variations in effective elastic thickness of the lithosphere within the Ethiopian plateau region to tectonics. Inverse models of new and existing Bouguer gravity data from the rifted regions of the uplifted plateau constrained by seismic data are used to relate the variations in the geometry of the along-axis structural and magmatic segmentation to variations in flexural rigidity. We use the wavelength dependence of the coherence between gravity and topography to estimate the flexural rigidity, or, equivalently, the effective elastic plate thickness (Te), of the lithosphere. Estimates of T e range from 17 + 2 km to 5 + 3/-2 km within the 300-km-wide Afar Depression. These estimates of T e within the rift are considerably less than values found beneath the uplifted but largely unfaulted plateau to the west (T e > 56 km). These results show that the transition from a "continental segmentation" to "oceanic segmentation" corresponds to a decrease in rift basin segment length and the separation of faults and magmatic centers, an increase in magmatic construction, and a marked decrease in effective elastic thickness. We suggest that the length scales of extensional segments prior to the onset of seafloor spreading are controlled primarily by plate strength, at least in rift areas affected by mantle plumes. Introduction Continental rifts and slow spreading oceanic ridges show a number of first-order morphological similarities, despite their distinct differences in crustal/lithospheric thickness, rheology, and magma supply. Both exhibit fault-bounded depressions flanked by escarpments that may rise over 2 km above the valley floor. The faulted rift valleys and flanks show an along-axis structural segmentation, with discrete structural, morphological, and magmatic segments 30-100 km in length in both settings [e.g., Sempere et al., 1993]. Continental segments are separated along their length by oblique-slip faults or magmatic ridges that crosscut the rift valley (accommodation zones), and are analogous kinematically to non-transform offsets of oceanic segments [e.g., Bosworth, 1992; Grindlay and Fox, 1993]. The similarities in structural and magmatic styles, and the similar length scales of the extensional segments suggest that the continental and oceanic lithospheres are responding in similar ways to rifting processes, or that the oceanic segmentation initiates prior to the onset of seafloor spreading. The relationship between continental or oceanic ...

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