Sjoberg, Lars E (Profossor)Asfaw, Laike M (PhD)Alemu, Abera2019-11-042023-11-092019-11-042023-11-091992-01-05http://10.90.10.223:4000/handle/123456789/19774All Existing Data (1500 taken by the the author and 800 reprocessed older data )in the Main Ethiopia Rift and the adjacent plateaus (between latitudes 4045’ N and 12000’ N)were reduce to Bougure and free-air values The resulting values are compiled in to 5 mGal and 10 mGal contour interval Bouger anomaly and free-air anomal maps respectively. The accuracy of the Bouger anomaly at each gravity station is estimated to Type equation here. ±2.9 mGal. The Bouguer anomaly map reveals at :P the Western and Eastern Plateaus are associated with gravity minima(e.g ≤265mGal near Asasa on the Eastern plateau and near fiche on the Waster plateaus),and those gravity minima are located in area od ,aximum elevation. Within the middle of the rift floor,a trend of gravity maximum(median highs) whose locations generally coincide with th location of various Quaternary volcanic centers runs parallel to the main escarpments. Between 7 30'N and 8 45'N,trend of elongated gravity maxima corresponding to marginal grabbers appear to run along appear to run along the whole length of thr east side of he the rift floor. Along the rift axis gravity values general increase north and south latitude of Awasa (7 N).This increase is frome-195 mGal in the Awasa district to -65 mGal in the Gewane area when going northward and to -135 mGal in the konso area south of Lake Chamo when going southward along the rift axis. This trend of gravity maxima coincides with the locations of maximum crustal thinning along the youngest structural deformation of the wonji Fault Belt Six Profiles were extracted from the Bouger anomaly map through the major plate tectonic system(the western platean ,the Eastern platea the Main Ethiopia Rift and southern Afar)of interest in the area. Two-dimational gravity models;i.e infinite in strike extent, were computed along these profile. The objective was to obtain modale whose gravity effect possessed the same qualitative feature and approximately the same anomaly shapes and magnitudes as the observed profiles. The densities used to compute the models were constrained by the results of the deep seismic refraction sounding profiles of the region and the adjacent area the results of these calculation are that: the crust in the plateaus is underlinain by a normal mantle. It is 35 to 41 km thick beneath both the Western and the Eastern plateaus and occurs at the locations of gravity minima in the region surveyed. Within the main Ethiopian Rift, the curst thins from about 30km in the south to23 km in the north. The observe things is some 7 to 8km and is cased by an upward progression of low density mantle material (anomalous mantl) that intrudes itself into the lower curst. The Quantity of the anomalous mantle material decreases frame north to south beneath the rift It is about 22 km thick and 150 km wide in the north, whilst in it central and southern parts, it is 15 to 20 km think over a 100 km wide zone To determine the degree to which the crust attained isostic balance, the following methods were employed. (i)Computation of mass distributions along the seismically controlled2-D gravity models for 20,30,40,50 and and 60 km depth levels; (ii)computation of an isostatic Moho-depth map according to the Airy-Heiskanen model of compensation. The result of these calculation are that: with reference to the mass distributions, the curves show that the crust is isostically balanced at the 30 km depth level, north of latitude 90N.South of this latitude, isostatic equilibrium is first reached between the 40 and 60 km depth levels. Isostic equilibrium north of latitude 90N is achieved though the contribution of the anomalous upper mantle lying at shallow depths. The normal mantl. As regards the isostatic Moho-depth map, there is a significant correlation between the Bougure anomaly and the calculated Moha-depth maps. The steep gravity gradient which marks the transition from the plateau area to the rift is also reflected in the Moho-depth map. As in the the2-D crustal density models, the Moho-depth map reveals that the crust beneath the rift thins from south to north. The magnitude of the axial positive gravity anomalies observed on the compiled map increase in magnitude north and south of latitude 70N. The compiled gravity maps, isotatic Moho-depth map and Geoid undulation map are found to be sensitive indicators of the tectonic and geologic structures of the region . A two dimensional correlation analysis made between the different parameters considered in this study (Bouguer anomaly, free-air anomaly, free-air anomaly, elevation and Moho-depth) shows that there are significant correlations between some of them.enGravity FieldCrustal StructureMain Ethiopia RiftThesis