Structure of the Crust Beneath the Gedemsa Tectono-Magmatic Segment and Its Environs Perceived from Geologically Constrained Geophysical Studies: an Implication for Geothermal Heat Source Assessment, Central Main Ethiopian Rift, Ethiopia
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Date
2020-08-28
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Addis Ababa University
Abstract
The Main Ethiopian Rift is in a stage of tectono-magmatic segmentation where the mechanism of rifting is primarily enhanced by a magmatic addition to the crust. However, the extent, nature and structure of the crust beneath each magmatic segment and occurrence of the associated geothermal resources remain unclear. Geologically constrained gravity and magnetic studies have been conducted in the Gedemsa tectono-magmatic segment and its environs.
Interpretation of combined potential field data in this study have provided important contributions regarding the geodynamic setting, crustal structure and geothermal resources of the study area. 2D gravity modelling of the entire crust across and along the Gedemsa tectono-magmatic segment reveals three density layers (upper volcanics/sediments, upper crust with intrusion, and lower crust) overlying the upper mantle with underplating beneath the Ethiopian plateau. The along axis tectono-magmatic segmentation and the bilateral off rift volcanic zones revealed by gravity data show the distribution of deformation beyond the rift axis and the complex geodynamic setting of the central to northern Main Ethiopian Rift transition zone.
Gravity modelling of the upper crust show the occurrence of intrusions with an estimated depth range of 10-20 km and 5-10 km beneath the Tulumoye-Gedemsa and Boku Volcanic complexes respectively. This magmatic addition to the crust is attributed to be an ongoing dyke intrusion which induces surface fracturing/faulting. The gravity data reveals that at deeper depths the Gedemsa tectono-magmatic segment (25 km wide and 60 km long) is elongated parallel to the Main Ethiopian Rift axis and changes its orientation oblique to the rift axis at the near surface.
A detailed investigation of the structure of the shallow crust based on 2D gravity models, 3D geologic interpretation and gravity/magnetic maps offer the presence of promiceable thermal heat sources (as shallow magma chambers at the Boku, Gedemsa and Tulumoye volcano). Quaternary faults/fractures of the Wonji Fault Belt play a triple fold role regarding the geothermal system of the area: 1) Transporting melt from the deep intrusion to shallow magma chambers. 2) Acting as pathways for the surface/groundwater migrating to the shallow magma chambers. 3) Transporting thermal fluid to the surface to be detected as thermal manifestation.
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Keywords
Tectono-Magmatic Segment, Gravity/Magnetic Anomaly, Crustal Structure, 2d Gravity Model, Geothermal Heat Source