SOFT PLATES AND HOT-SPOTS - VIEWS FROM AFAR

The uplifted Ethiopian plateau region encompasses amagmatic rift basin s and basins with nascent seafloor spreading. Rift segments in the Mai n Ethiopian rift and southern Afar show a distinct structural segmenta tion, with a south to north reduction in the length, width, and spacin g of fault zones; rift segments in northern Afar, where extension exce eds 100%, show a magmatic segmentation. The objectives of remote sensi ng, gravity, and modeling studies of the Ethiopian plateau area are (1 ) to summarize morphological patterns along the length of the rift sys tem and (2) to relate variations in effective elastic thickness of the lithosphere within the Ethiopian plateau region to tectonics. Inverse models of new existing Bouguer gravity data from the rifted regions o f the uplifted plateau constrained by seismic data are used to relate the variations in the geometry of the along-axis structural and magmat ic segmentation to variations in flexural rigidity. We use the wavelen gth dependence of the coherence between gravity and topography to esti mate the flexural rigidity, or, equivalently, the effective elastic pl ate thickness (T-e), of the lithosphere. Estimates of T-e range from 1 7 +/- 2 km to 5 +/- 3/-2 km within the 3QO-km-wide Afar Depression. Th ese 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 greater than or equal to 56 km). These results show that the tran sition from a ''continental segmentation'' to ''oceanic segmentation'' corresponds to a decrease in rift basin segment length and the separa tion of faults and magmatic centers, an increase in magmatic construct ion, and a marked decrease in effective elastic thickness. We suggest that the length scales of extensional segments prior to the onset of s eafloor spreading are controlled primarily by plate strength, at least in rift areas affected by mantle plumes.