Mapping of Crustal Thickness Variation in Southern Afar Using Gravity Survey
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Date
2009-06
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Addis Ababa Universty
Abstract
The Afar Depression in north eastern Ethiopia is a scientific treasure trove. It is a site of a triple
junction, where three plates (the Nubian plate, the Arabian plate, and the Somalian plate) are
simultaneously pulling away from each other. The powerful stresses in the crust lead to frequent
earth quakes and volcanism thereby producing deep cracks and fissures in the region. These
tectonic activities and the sedimentation that occurs in the area change the topography, which
affects the crustal thickness.
Analysis of the gravity data, obtained from the Ethiopian institute of geological survey (EIGS),
constrained with seismic refraction results in the study area and the Euler deconvolution results
produced here, was done to study the crustal thickness variation in the southern Afar Depression
including the region between 8.80 to 110N and 39.80 to 42.70E . The Bouguer gravity, which
shows the depression clearly separated from the rest of the study region by relatively higher
values, is dominated by a strong regional gravity while the residual anomaly indicates elongated
higher (positive) gravity values that follows a trend, which over laps with crustal extension
(thinning) revealed by seismic refraction survey conducted in the area. Upward continuation
results also support the idea that deeper effects influence the Bouguer gravity. Higher total
horizontal gradient shows faulted structures. Particularly, this high gradient bounds the
depression in the west and south east, which overlaps with the escarpment faults separating the
depression from the western and south eastern plateaus. Gravity modeling using seismic
refraction model and Euler deconvolution result, produced here, as constraints indicate a
minimum crustal thickness of about 22.7 and 23.5 under profiles BB` and CC` respectively. The
modeling also shows deep seated faults particularly under profile CC.
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Keywords
Thickness Variation