Abstract
This study attempts to delineate subsurface lineaments for the tectonically and volcanically active region of the Ziway-Shala Lakes basin, central Main Ethiopian rift. Most of the previously mapped subsurface structures in the region under consideration focus on delineating crustal structures thicknesses and Moho depths undulations. Moreover, surface structures in the same region were mapped using analysis of Digital Elevation Model image data. On the other hand, there are few studies that have targeted in mapping geologic structures lying at depth levels between the shallower and deeper subsurface. The objective of this research is thus to map the subsurface geologic structures/lineaments to an average depth of 3 km (crystalline basement layer depth) from surface using gravity data. These investigation results are validated by Digital Elevation Model extracted lineaments. Filtering techniques including derivative filters, upward-continuation and line module algorithm of PCI Geomatica are used to extract the gravity and topographic lineaments of the region. Orientation analyses of these subsurface and surface lineaments are made using line direction histogram of the QGIS software. Accordingly, the gravity subsurface lineaments mapped in this study are found to be dominantly oriented in the NNW-SSE to NW-SE and E-W direction on average. These results appear to be contrary to the NNE-SSW to NE-SW trending surface geologic structure mapped on the bases of actual field observation carried out by previous researchers and automatically extracted lineaments based on Digital Elevation Models data considered in this research. The subsurface lineaments mapped using gravity data are believed to govern groundwater dynamics within the basin and the adjacent basins in the area. These structural lineaments which are considered to be masked in the subsurface coincide with the orientation of the Mesozoic Ogaden rift as compared to the overlying surface structures which appear to coincide with the orientation of the Cenozoic Main Ethiopian rift.
Highlights
The Main Ethiopian Rift (MER) encompassing three segments, southern, central and northern MER (Woldegabriel et al, 1990; Bonini et al, 2005) is part of a bigger East African Rift system (EARS) that links the Afar triple junction and the Kenya Rift regions
These includes lineaments extracted based on Line module algorithm (Figure 7(a), tilt derivative techniques (Figure 7(b) and rose diagram plot showing the overall subsurface lineaments orientation constructed based on line direction histogram module of QGIS (Figure 7(c))
The result shows lineaments trending NE-SW (Figure 7(c)) that coincides with the orientation of the quaternary faults of the Main Ethiopian Rift system that comprises the study area
Summary
The Main Ethiopian Rift (MER) encompassing three segments, southern, central and northern MER (Woldegabriel et al, 1990; Bonini et al, 2005) is part of a bigger East African Rift system (EARS) that links the Afar triple junction and the Kenya Rift regions. The mean elevation varies from 1632 masl to 3448 masl (Figure 1) These geology and geologic structures observed in the region are due to active Cenozoic volcanic, tectonic and sedimentation processes (Abebe et al, 2007; Woldegabriel et al, 1990; Le Turdu et al, 1999). These structures are faults, joints and fractures which have surface expression as shown in the geologic map (Figure 2) and the structural map (Figure 3) of the area. These pre-existing structures have been proven to exert a significant control on the accommodation of deformation and on the distribution of strong volcanic activity (Corti et al, 2013; Abebe et al, 2007) in the region
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