GRAVITY ANOMALIES ALONG THE MAIN ETHIOPIA RIFT

The EAGLE (Ethiopia-Afar Grand Lithospheric Experiment) experiment was a series of seismic, petrologic, gravity and magnetotelluric studies that was conducted during 2001-2003. The main experiment was a controlled source seismic study that consisted of 20 sources that produced a profile across the Main Ethiopian Rift (MER), a profile along the rift’s axis and for scattered seismometers within the magmatic segments surrounding Nazreth. The goals of these experiments were to image and determine the origin of the MER, the role of mantle plumes in its formation, and the crustal/upper mantle structure of the transition zone between the continental and oceanic rifting in the Afar. To support the controlled source experiment along the rift’s axis, 300 gravity stations were collected along two profiles between Awash and Nazreth. Both profiles indicate a steep gravity gradient between Nazreth and Metahara which based on preliminary two-dimensional ray-tracing of the seismic data, is due to a thinning of the crust toward the Afar region. However, a deeper (lower crust/upper mantle) body is needed to fit the gravity data within thinned crust in the Afar. Superimposed on the long wavelength gravity anomaly are a series of high frequency maxima and minima that correspond with the Cenozoic basins and the magmatic segments. Residual gravity anomaly maps using wavelength filtering (125-25 km) shows the abrupt boundaries of these magmatic segments.