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Neotectonics of the East African Rift System: new interpretations from conjunctive analysis of field and remotely sensed datasets in the Lake Magadi area, Kenya

Sequar, Ghebretinsae Woldu (2009) Neotectonics of the East African Rift System: new interpretations from conjunctive analysis of field and remotely sensed datasets in the Lake Magadi area, Kenya.

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Abstract:Proper characterization of tectonic extension of a rift helps to understand its seismcity, geothermal activity, volcanicity and general geodynamics. For the Kenya Rift, two different models of extension directions (E-W to ESE-WNW and NW-SE) have been proposed. However, there is a knowledge gap in the understanding of neotectonics of the southern Kenya Rift (Lake Magadi area) since previous works in the area were based on limited field sites and they lacked subsurface investigations. Hence, conjunctive analysis of field and remotely sensed datasets of the Lake Magadi area is essential. Field data from several locations in a larger area are required in order to obtain representative information about the conditions of extensional stress in the Lake Magadi area. Remotely sensed datasets, which include space-borne multispectral imagery and geophysical (resistivity plus magnetic) data sets are likely to provide useful surface and subsurface geological information, respectively and would enable characterization of the neotectonic extension in the Lake Magadi area. In this research, ASTER datasets, SRTM DEM, airborne magnetic data, ground magnetic data, resistivity data, structural field data, existing geological maps and location of earthquake epicenters were used. In order to extract optimum structural and lithologic information, the remotely sensed datasets were enhanced using different enhancement methods. Lithologic information extracted from the ASTER images, from field and from the airborne magnetic data was integrated in order to update geological map of the study area. Structural information was extracted from the enhanced ASTER datasets, SRTM DEM and airborne magnetic data. Combined results of these datasets revealed four fault sets in the area; normal N-S fault, dextral NW-SE fault, strike slip ENE-WSW fault and sinistral NE-SW fault. All fault sets have been observed in the field where their relative age relationship showed that the N-S faults are the oldest and the reactivated NE-SW faults are youngest. The existence of four set of faults having different styles and different relative ages suggests that there has been a geodynamic change in tectonics of the rift. These faults were created under an E-W extension direction which is explained by Anderson’s fault model. Preliminary paleostress reconstruction of the field data also showed that the neotectonic extension direction of the study area is E-W. The new finding of sinistral NE-SW trending fault in the study area was created under E-W extension direction, which counterbalanced the dextral NW-SE. This finding solves the puzzle that some researchers were associating the dextral NW-SE movement with a NW-SE extension direction. Besides, the clustering of the earthquake swarms in the northern part of the Lake Magadi area is associated with the intersection of the N-S, NW-SE and NE-SW to ENE-WSW trending structures. In the earthquake clustering area, a surface rupture has been observed by Seht et al.(2001). Ground magnetic and resistivity data characterized the subsurface nature of the rupture and revealed other hidden rupture in the site. The 2D resistivity imaging of the ruptures showed that both ruptures persist to a depth of greater than 66m signifying that the ruptures were intense. Besides, field observation showed that the surface rupture aligns along N-S to NNE-SSW signifying that the neotectonic extension direction is E-W. The conjunctive analysis of field and remotely sensed datasets were useful in updating geological map of the study area, characterizing surface and subsurface structures and in deducing neotectonic extension of the Lake Magadi area and in explaining the clustering of earthquake in the area.
Item Type:Essay (Master)
Faculty:ITC: Faculty of Geo-information Science and Earth Observation
Programme:Geoinformation Science and Earth Observation MSc (75014)
Link to this item:https://purl.utwente.nl/essays/91635
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