PRELIMINARY GRAVITY ANALYSIS OF THE CRUSTAL STRUCTURE WITHIN THE TENDAHO GRABEN AND MANDA HARARO RIFT IN THE CENTRAL AFAR, ETHIOPIA

The Afar region of Ethiopia lies at the triple junction where northern section of the East African Rift System, the Red Sea rift and Aden rift meet. Afar represents the transition of continental crust to the south to oceanic crust to the north. The Red Sea and Aden rifts seem to be propagating in SE- and NW-trending directions, respectively but the two rifts have actually bifurcated and stepped onto Afar through a series of active rifts and volcanic systems known as propagators. One of the Red Sea propagators, the Manda Hararo rift in the central Afar is currently active and ends near the capital of Afar, Aseyta. The Manda Hararo rift is within the largest basin in the Afar, Tendaho Graben which formed approximately 1.8 Ma ago, is approximately up to 60 km wide, is filled with lacustrine sediments and basalt flows, and bounded by basaltic and rhyolitic flows. Despite the numerous geological studies of the Manda Hararo rift and the Tendaho graben, there have been few studies of the subsurface crustal structures. To aid in determining the region's crustal structure, a detailed gravity, magnetic and broadband seismic study has been undertaken. Detailed gravity data were collected using differential GPS methods along all available roads at intervals between 0.5 and 1 km to supplement the available widely-spaced gravity data. Preliminary analysis of the gravity data indicates that Tendaho graben produces a low amplitude gravity minimum on both sides of the Manda Hararo rift which is characterized by a regional gravity maximum. This gravity minimum has a higher amplitude toward the south indicating a greater thickness of the lacustrine sediments. The gravity anomaly due to the Tendaho graben dies out toward surface expression of the southern end of Red Sea propagator indicating that the upper crustal manifestation of the propagator ends in the same region. Future work will be to construct two and three-dimensional models and to collect additional data to further define the crustal structure of the region.