Crustal structure of the Main Ethiopian Rift from gravity data: 3-dimensional modeling

A three-dimensional interpretation of the newly compiled Bouguer anomaly ma p of the Main Ethiopian Rift is presented. A high-resolution 3-D model cons trained with the seismic results reveals a possible crustal thickness and d ensity distribution beneath the graben. The Bouguer anomalies along the axi al portion of the rift floor, as deduced from the results of the regional a nd residual separation, are mainly caused by the deep-seated structures. Th e inferred zone of intrusion, which is the main subject of the present stud y, coincides with the maximum gravity anomaly of the rift floor. The intrus ion is displaced at several sectors along the east-west direction, and the two major displacements coincide with the locations of the major rift offse ts on the surface. Because of the asthenospheric uplift, the crust under th e Main Ethiopian Rift is slightly thinned. The zone of crustal thinning (le ss than or equal to 31 km) coincides,with the location of the intrusion ben eath the rift floor, and the maximum of which is attained in the northern a nd central sectors of the graben. The trend of the crustal thinning zone, w hich is from south to north, is the same as the one obtained in the Afar de pression. 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 an d 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 in trusion 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. (C) 1999 Elsevier Science B.V. All rights reserved.