Date of Award
Doctor of Philosophy
G. Randy Keller
The combined effects of magmatism and stretching due to asthenosphere upwelling modifies the crustal structure of the Earth as seen in the Ethiopian rift and adjacent areas. The Ethiopian rift provides unique opportunities to understand the nature of rifted crust and the intensity of its modification by magmatic processes. I used geological and geophysical data to conduct an integrated study in and around the Ethiopian rift including the northern Kenyan rift and the northern part of the Kenyan dome. New gravity, controlled source seismic, and teleseismic data from the EAGLE (Ethiopia-Afar Geoscientific Lithospheric Experiment) were used as additional constraints in my analysis of the crustal structure of Ethiopian rift and adjacent plateaus.
Application of a residual gravity anomaly filtering technique using upward continuation revealed various crustal features within the Ethiopian rift and the flanking plateau regions. Short wavelength high amplitude positive anomalies coincide with the local volcanic complexes and calderas. In addition low gravity anomalies are associated with areas of thicker sediments within the rift valley.
Axial and cross rift gravity profiles were modeled in 2.5 dimensions constrained with seismic refraction and geologic data. The axial model connects the Kenyan dome through Turkana rift and Main Ethiopian rift (MER) up to the Afar triple junction and provides a new integrated picture of lithospheric structure along the rift for over 1000 km. This model indicates a thin crust (26 km) underlying the Afar region. The crust gradually thickens towards the MER where it is about 35-40 km thick. Towards the south the crust thins and is only 22 km thick when it reaches the Turkana area. The southern section of the axial model indicates that the crust is about 35 km thick beneath the central Kenyan rift. All these thickness values are in agreement with the EAGLE and Kenya Rift International Seismic Project (KRISP) and earlier refrresults and ties these results together to form a complete picture of the axial structure of the rift. The cross profiles, which are interlocked with the axial rift profile, indicate that thick (~45km) crust is present beneath a broad region of the western plateau. The EAGLE seismic results indicate that the part of the western plateau adjacent to the rift is thickened via underplating. The Bale Mountain region on the eastern rift flank has relatively thick (~40 km) crust, which is in agreement with receiver function results. In general, asthenospheric upwelling affects a wide zone near Afar and the southern Ethiopian rift, whereas the area of upwelling is narrower around the MER.
The Abbay or Blue Nile basin was another target of my study. Integrated geophysical (seismic, remote sensing, and gravity) and geological data suggest that the sedimentary section of Abbay basin extends well to the east of the known extent of its sedimentary fill. Gravity modeling results suggest approximately 3 km of sub-volcanic sedimentary strata exist over a wide area.
I also undertook an integrated analysis of the Ogaden basin that lies east of the rift valley and is associated with the break-up of Gondwanaland by Karroo rifting. Seismic reflection data were processed and interpreted and combined with gravity and magnetic data to study the evolution of the basin and its geometry. The existence of a tri-radial rift that connects to the Abbay basin is suggested by the isostatic residual gravity anomaly map produced in this study. This result provides new evidence for the relationship of the Ogaden and Abbay basins via a northwest-southeast trending Permo-Triassic rift system.
The northeastern part of the Ogaden basin shows distinct gravity anomalies trending in a northeast-southwest direction that appear to be due to a series of grabens and horsts. 3D Euler deconvolution of gravity data and modeling results suggest a sedimentary thickness of about 5 km sedimentary strata in some of the grabens.
Integrated gravity models in the southwest part of the Ogaden basin indicate a sediment thickness of 8 km. Interpretation of seismic reflection data indicates potential stratigraphic and structural traps for hydrocarbons in the Ogaden basin. Older strata such as the Karroo strata appear to pinch out towards the uplifted basement to the northwest. Fault structures are associated with the basement. Channels that appear as distinct features on 2D reflection seismic data may be developed in various places with hanging wall incision. Attribute analysis and interpretation suggest possible hydrocarbon bearing zones or at least porous formations and continuity of reflection horizons.
In summary, this Dissertation presents a new integrated analysis of Permian and younger rifting in Ethiopia and northern Kenya. The basins related to Permo-Triassic were analyzed and connections across Ethiopia from the Ogaden basin to the western plateau are suggested. The seismic results of the EAGLE and KRISP project were tied together and extended beyond the rift valley regions and revealed large variations in structure along the rift system.
Received from ProQuest
Tadesse, Ketsela, "Integrated Geophysical Data Processing and Interpretation of Crustal Structure in Ethiopia with Emphasis on the Ogaden Basin and Adjacent Areas" (2007). Open Access Theses & Dissertations. 365.