Soft plates and hot spots: Views from Afar

Abstract

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 Te range from 17 ± 2 km to 5 ± 3/−2 km within the 300‐km‐wide Afar Depression. These estimates of Te within the rift are considerably less than values found beneath the uplifted but largely unfaulted plateau to the west (Te ≥ 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.