STRUCTURE OF THE UPPER CRUST AT THE AXIS SEGMENTATION STAGE OF RIFT EVOLUTION AS REVEALED BY GRAVITY DATA: CASE STUDY OF THE GEDEMSA MAGMATIC SEGMENT, MAIN ETHIOPIAN RIFT

The upper crustal structure of the individual tectono-magmatic segments within the Main Ethiopian Rift is poorly understood. The Gedemsa tectono-magmatic segment (GTMS) within the Central Main Ethiopian Rift is considered to be within an intermediate stage of rift development with extension occurring within the border faults due to far-field stresses and mantle dynamic processes. The interpretation of gravity data from Global Gravity Modelplus2013 has provided constraints on the nature and structure of the upper crust beneath the GTMS. A detailed investigation of the structure of the upper crust based on 2D gravity models, 3D geologic interpretation maps, and residual and upward continued gravity anomaly maps indicated the occurrence of dense, mafic intrusions at depths between 10-20 km and 5-10 km beneath the Tullu Moye-Gedemsa and Boku volcanic complexes, respectively. This magmatic addition to the crust is attributed to be an ongoing dike intrusion which may be part of the cause for current fluid induced seismic activity within the magmatic segment. The gravity analysis indicated that at deeper depths, the GTMS is elongated parallel to the Main Ethiopian Rift axis. The 2D gravity modeling implied that the Quaternary faults within the Wonji Fault Belt are closely associated with the dense magmatic intrusions where the faults could aid in transporting melt from the deeper crustal melt areas to the shallow magma chambers. This transporting deeper mafic melt to shallow melt areas may help to explain the bimodal volcanism observed at the surface. The deeper melt may fractionate into more siliceous magmas within the shallow magma intrusions as observed in the gravity modeling.