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 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 ...

Spatial variations in Te in the southern Appalachians, eastern United States

“…Initial concepts of isostasy proposed that the Earth was in hydrostatic equilibrium at depth, requiring topography to be compensated either by lateral variations in crustal thickness [Airy, 1855] [Forsyth, 1985;Ebinger and Hayward, 1996], that is, for a plate with both surface and subsurface loading that has a finite, nonzero rigidity, coherence will approach 1 at wavelengths greater than the characteristic flexural wavelength and will approach 0 at wavelengths shorter than it, where loads can be supported by stresses within the plate. The transition from coherent to incoherent topography and gravity will occur in the same wavelength range as the transition from compensated to uncompensated topography [Forsyth, 1985] As we are using Lowry and Smith's ME codes, the shallowest first-order density-velocity discontinuity is chosen to serve as the subsurface load depth, as the downward continuation operation implicit in the load deconvolution is most stable at shallow depths [Blakely, 1995].…”

The protracted development of the continent–ocean transition in Afar

“…Constraining this phase of plate stretching to recent times has the implication that larger volumes of melt can be produced than if thinning had been achieved more gradually over the ∼30 Myr since breakup began: slow stretching allows cooling to the surface by conduction without the production of appreciable melt volumes 7 . Observations of reduced effective elastic plate thickness in northernmost Afar to ∼5 km compared with ∼9 km farther south 25 , along with the spatial coincidence of voluminous young volcanism and thinned plate (Fig. 2), strongly indicate that plate weakening by heating owing to protracted and localized magma intrusion has reduced the tectonic force required for plate stretching.…”

The August 2002 earthquake sequence in north Afar: Insights into the neotectonics of the Danakil microplate

“…tensor inversion are 5-7 km, which is the approximate depth of the brittle-ductile transition zone in the Main Ethiopian rift (Ebinger and Hayward, 1996;Keranen et al, 2004;Ayele et al, 2006). Fig.…”