Mapping Geological Structures Using Integrated Geophysical Methods and their Possible Implications, A Case Study of the Ziway-Shala Lakes Basin, Central Main Ethiopian Rift

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Addis Ababa University


The Ziway-Shala Lakes Basin in the Central sector of the Main Ethiopian Rift (38000‟ E - 39030‟ E and 7000‟ N -8030‟ N), has been affected by active Cenozoic tectonic, volcanic and sedimentation processes. These processes are mainly responsible for the current geological and structural setting of the basin. Even though there exists numerous works on the surface and deep structure of the region, the shallow to intermediate depth litho-structural set up of the Ziway-Shala Lakes Basin is least studied. For its implications on hydrology, geothermal and hydrocarbon resources, this PhD research focuses on delineating the shallow to intermediate depth litho-structural features of the basin using gravity, magnetic and seismic noise data with application of various mathematical enhancements and numerical algorithms. These mathematical techniques include derivatives filters, upward continuation, Line module algorithm, Source Parameter Imaging, Power Spectral Analysis, 2D Werner De-convolution, joint 2D forward modeling and 3D structural inversion. The orientations of subsurface lineaments obtained from gravity and magnetic data analysis are dominantly oriented in NNW-SSE to NW-SE and E-W directions. These results appear to differ from the general orientation of surface geologic structures (NNE-SSW to NE-SW) in the Central Main Ethiopian Rift which is the major tectonics of the Cenozoic rift. The shallow to intermediate depth structural trends revealed in this study seem to reflect the influence of the pre-existing Mesozoic Ogaden rift. Gravity and magnetic depth estimation along the rift axis shows the northward thickening of the Hailemichael K. low-velocity volcano-sedimentary layer. Furthermore, about 561 m thick horizon, possibly of Mesozoic sediments, has been identified between the Tertiary ignimbrite layer and the crystalline basement for the first time. The information obtained with regard to water balance of the basin, salinity level of the lakes and the conceptual hydrological flow model adopted appear to reveal that the groundwater flow in the study region is controlled by subsurface structures (lineaments and the mapped interface topographies). Interpretation of the compiled magnetic anomaly maps reveal the existence of a structural path, between 1.5 km and 3 km of depth, along a traverse running from the Aluto-Langano geothermal field to the Silti Debre Zeyte Fault zone. The structural path is thought to serve as a conduit connecting the Aluto-Langano geothermal field and the likely heat source located around the locality of the Silti Debre Zeyte Fault Zone. The study also identified magnetic lineaments most of which are oriented in the direction of pre-existing Mesozoic structures and in the direction of thermally altered structures. Mesozoic sediment delineated under a thick volcanic cover (2.5 km) could be taken as a favorable geologic setting for accumulation of hydrocarbon and CO2 degassing in the region. Generally, the identified geological structures of shallow to intermediate depth are useful in understanding the groundwater dynamics, geothermal heat sources and hydrocarbon accumulations associated with the regions.



Mapping Geological, Structures Using, Integrated Geophysical, Methods and their Possible Implications, Ziway-Shala Lakes Basin, Central Main Ethiopian Rift