ELSEVIER Tectonophysics 313 (1999) 363–382 www.elsevier.com/locate/tecto Crustal structure of the Main Ethiopian Rift from gravity data: 3-dimensional modeling R. Mahatsente a , G. Jentzsch b,Ł , T. Jahr b a Formerly at Institut fu ¨r Geophysik, Technische Universita ¨t Clausthal, Arnold-Sommerfeld Straße 1, D-38678 Clausthal-Zellerfeld, Germany b Institut fu ¨r Geowissenschaften, Friedrich-Schiller Universita ¨t, Burgweg 11, D-07749 Jena, Germany Received 4 September 1998; accepted 9 July 1999 Abstract A three-dimensional interpretation of the newly compiled Bouguer anomaly map of the Main Ethiopian Rift is presented. A high-resolution 3-D model constrained with the seismic results reveals a possible crustal thickness and density distribution beneath the graben. The Bouguer anomalies along the axial portion of the rift floor, as deduced from the results of the regional and residual separation, are mainly caused by the deep-seated structures. The inferred zone of intrusion, which is the main subject of the present study, coincides with the maximum gravity anomaly of the rift floor. The intrusion is displaced at several sectors along the east–west direction, and the two major displacements coincide with the locations of the major rift offsets on the surface. Because of the asthenospheric uplift, the crust under the Main Ethiopian Rift is slightly thinned. The zone of crustal thinning (31 km) coincides with the location of the intrusion beneath the rift floor, and the maximum of which is attained in the northern and central sectors of the graben. The trend of the crustal thinning zone, which is from south to north, is the same as the one obtained in the Afar depression. The southeastern and western plateaus, on the other hand, show by far the largest crustal thickness in the region (38–51 km). In contrast to the Afar depression, where the crust is partly oceanized, the thickness and density of the crust suggest that the Main Ethiopian Rift is underlain by a purely continental crust. The deep and relatively large nature of the intrusion leads to the conclusion that a large-scale asthenospheric upwelling might be responsible for the thinning of the crust and subsequent rifting of the graben.  1999 Elsevier Science B.V. All rights reserved. Keywords: Main Ethiopian Rift; intrusion; Wonji Fault Belt; Bouguer anomaly; 3-dimensional gravity modeling 1. Introduction The unique geological setting of the Ethiopian rift system including the Main Ethiopian Rift and the Afar depression, where the inception of plate bound- aries within the continental rift is clearly observed, Ł Corresponding author. Fax: C49 3641 630280; E-mail: jentzsch@geo.uni-jena.de makes it the main focus of interest for geoscien- tific researchers. The three major rifts, namely the Ethiopian rift system, the Red Sea rift, and the Gulf of Aden rift converge in the northern part of the Afar depression (Fig. 1 and inset map of Fig. 2). The nature of the crust underlying the northern segment of the East African rift system (the Afar depression) has been controversial among researchers for the past three decades. Consequently, the crustal struc- 0040-1951/99/$ – see front matter  1999 Elsevier Science B.V. All rights reserved. PII:S0040-1951(99)00213-9