Structural Controls Analysis and Its Correlation with Geothermal Occurrence at Barrier Volcanic Complex (BVC), Turkana, Kenya ()
Structural Controls
1. Introduction
Barrier Volcanic Complex (BVC) is a geothermal prospect situated at Turkana, along the East African Rift System (EARS) specifically the Kenyan rift. It is 20 km in length and 15 km wide, and forms a natural dam across the inner trough separating Lake Turkana from Suguta Valley. The prospect covers an area of 380 km2. Geographically, it is located at coordinates 2.3192˚N, 36.5879˚E (Figure 1).
BVC is a composite structure that is made up of four separate volcanic centers. These centers include Barrier, Kalolenyang, Kakorinya, and Likau (Figure 2). They are made up of different kinds of lava, namely; basanite, basalt, hawaiite, mugearite, benmorite, trachyte, and phonolite, among others. Kakorinya is the youngest of the volcanic centers. Dindi (2020) reported that the Kenyan rift is an active system with volcanic activities taking place hence surface geothermal manifestations like hot springs, geysers, fumaroles, altered grounds, occurrence at the rift zone [1] . This is evident in BVC through occurrence of
![]()
Figure 1. Map of the Kenya showing the location of Barrier Volcanic complex in the rift.
![]()
Figure 2. Elevation map of barrier volcanic complex as a study area with volcanic centers.
fumaroles and the upper western and southern flanks of the caldera having hot hydrothermally altered grounds. A number of sites on the rim of the caldera have silica veins and sinters, both of which are indicators of the presence of active hot springs in the past.
Synoptic view in remote sensing enables enhanced visualization and better understanding of the relation between the whole terrain of an area and its features [2] . Several studies have demonstrated the effectiveness of remote sensing techniques for identifying the structural controls. Hamimi et al., 2020 applied remote sensing technique in mapping geological structures at Atalla Shear Zone and Environs, Central Eastern Desert, Egypt where band rationing was performed for Landsat images for the true and false color composites which revealed the NE to NNE orientation of the structures in the study area [3] .
Skakni et al. (2022) used an integrated Geographic Information System (GIS), Remote Sensing and resistivity approach to conduct a structural mapping of initially inaccessible zones in the North-Western Rift Belt in Morocco [4] . The study combined Principal Component Analysis, directional filtering, and Optimal Index Factor to generate images for mapping the lithology of the zone.
Due to active tectonic setting of a prospect such as BVC, remote sensing approach is essential in order to detect morphological changes by identifying the significant difference between the multi-temporal Landsat images [5] and the elevation changes using Shuttle Radar Topography Mission (SRTM). Field mapping of lineaments is expensive and time consuming, hence use of remote sensing has been widely applied [5] in lineaments delineation.
Geothermal Development Company (GDC) of Kenya did geoscientific studies (geophysics, geology and geochemistry) in 2011 to ascertain the potential of geothermal resource in the study area. They found that the area has a potential of about 700 MWe for electrical generation. Since the potential of the study area was estimated by GDC, there was need to study the structural controls that act as conduits to recharging the geothermal system in the area. This therefore, prompted the application of remote sensing technique in this research to map and analyze the structural controls at Barrier Volcanic complex (BVC). This is because understanding fracture distribution is crucial in siting of exploratory wells, reservoir management, as well as ensuring sustainable geothermal resource production. Presence of water is important to the future exploitation of the geothermal resource in the area to avoid drilling of tight wells. All these needs necessitated the application of remote sensing satellite imagery and Shuttle Radar Topography Mission (SRTM) in mapping of structural controls in the study area since it has been applied in many geothermal fields for exploration and exploitation expansive purposes.
2. Geological Setting
2.1. Tectonic and Geological Setting
BVC is located in the Turkana rift zone which is within the Kenyan Rift system. The Kenya Rift is comprised of both the Turkana Rift Zone and the Kenya Rift Zone [6] . It generally follows the boundary between the Proterozoic Mozambique Belt and the Archean Tanzania Craton [7] . The northern, central, and southern segment of the Kenya Rift have half graben geometries due to earlier faulting and pronounced subsidence along the western boundary faults [6] . For most of the structural evolution of the Kenya Rift, faulting has been controlled by an E-W oriented position of minimum stress [8] .
Volcanic activity in Turkana rift, which was followed by activity moving southwards, was the initial step in the formation of the Kenya rift during the early Miocene (14 - 23 Ma BP). Faulting that occurred in conjunction with rifting took place in many stages, beginning with faulting on the western side, which was accompanied by basaltic and phonolitic volcanism on the crust of the uplift [9] . Major faults extended along the western side, generating a half graben that was bordered on the eastern side by a monoclinic flexure. At the same time, the development of massive basaltic-trachytic shield volcanoes was taking place. On the eastern side, major faults formed, which resulted in the transformation of the half graben into a complete graben and was followed by basalt-trachyte volcanism.
![]()
Figure 3. Simplified geological map of the barrier volcanic complex.
Around 5 million years ago (Miocene epoch), the graben structure began to take shape. Subsequently, fissure eruptions in the axis of the rift led to the creation of flood lavas between 2 and 1 million years ago (Quaternary period). Within the axis of the rift, volcanic activity has been growing increasingly intense over the past two million years. Large Quaternary shield volcanoes, notably the Barrier Volcanic Complex, began to form along the Kenya rift axis during this time period. The majority of these volcanoes are geothermal potential areas.
2.2. Stratigraphy
Volcanism in BVC commenced with the eruption of strongly undersaturated flood basalts from a series of fissures along the eastern margin of the trough. Trachyte volcanism commenced with the eruption of lavas and pyroclastic flows from Likaiu East and Kalolenyang volcanoes in the period extending from at least 1.3 Ma to 0.7 Ma [10] . These eruptions build up steep sided edifices, the summit of which later collapsed to form craters. Small volumes of basalts were erupted at a late stage. Rejuvenation of rift margin structures faulted and tilted this center before the next phase of activity.
Likau West (0.5 - 0.2 Ma) is poorly exposed. The summit area is covered by small trachyte and phonolite lava domes and associated pyroclastic deposits. Volcanism on the BVC culminated with the formation of Kakorinya volcano (>0.2 Ma to Recent). The construction of the lava shield was accompanied by the eruption of extensive pyroclastic flow deposit and air-fall tuff on the western flanks of the volcano. Collapse of the summit area of Kakorinya took place in two stages. Two outer ring faults formed immediately after the Upper trachytes were erupted and were associated with limited basaltic activity on the summit area (Figure 3). These fractures are post-dated by trachyte lava domes and widespread air-fall tuffs, the eruption of which triggered the formation of the caldera at 92 ka [10] , as shown in Table 1.
Table 1 below shows the generalized stratigraphy of the Barrier Volcanic Complex.
![]()
Table 1. Stratigraphy of Barrier Volcanic Complex (BVC) as constructed by [10] .
3. Materials and Methods
3.1. Shuttle Radar Topography Mission (SRTM) Data
The digital elevation SRTM data was downloaded from Earth Explorer USGS website. The period of the data was 01-01-2000 to 31-12-2020, a range of 20 years. The image with less atmospheric noise was selected and downloaded for processing and analysis using the WGS-84 datum, and 37N UTM zone map projection. The study area was clipped out using the BVC shapefile. The clipped image was double-blurred to smooth it for clear geological structural identity.
PCI Geomatica was used for generating major geological structures in the area. The digital elevation model of BVC was imported into the software, run it, and the results was generated in terms of a shapefile of major geological structures i.e. faults in the area. ArcGIS was used for drawing the lineaments and generating a structural density map. The lineaments generated from ArcGIS were exported into Rockworks 17 in form of file for generation of a rose diagram.
The geological structural analysis was done in three steps namely; delineation of lineaments, determination of lineaments density and finally determination of major trends of lineaments. The results of the analyses gave a clear map on the trend of structures that influence the geothermal system in the study area. The steps used are further discussed in the proceeding text.
3.1.1. Delineation of Lineaments
Lineaments are surface manifestations of structurally controlled features, such as joints, straight course of streams and vegetation alignments [11] . Hill-shading technique was used in delineation of structures. This technique recognizes the use of Digital elevation model (DEM) to generate lineaments by producing a map with different shades of gray [12] . Hill shading map shows the terrain representation and geomorphological tilting [13] . The technique has been applied by map users in many fields including Geographic Information Systems (GIS), cartography, and 3D terrain visualization.
The aim was to map all the possible lineaments that can be recognized on the SRTM Satellite Imagery in the study area by utilizing four illumination directions of 45˚, 90˚, 150˚, and 315˚, a constant elevation of 45˚ and exaggeration of 10. ArcGIS was used in applying this concept.
3.1.2. Determination of Lineaments Density
This stage provides the concentration of lineaments delineated from the previous step. The frequency of the lineaments per unit area (pixel) that will result in a lineament density map was determined at this stage. In each pixel grid, the grid sections were measured in length. The calculation’s outcome is then represented by a middle point in the middle of each grid. The output of the representation was contours that display the significant amount of obtained contour based on the concentration of lineaments.
3.1.3. Determination of Major Trends of Lineaments
Major trends were determined by use of lineament densities. All delineation step results are collated, and Rose diagrams were created to determine the main direction of the lineaments produced. Major trends show the regional flow of fluids and the occurrence of geothermal resource. The results can be supported by the sites/locations of surface manifestations within the study area.
3.2. Ground Truthing
The GPS was used for collecting coordinates of the structures mapped on-site. The Branton Compass was used for recording the strikes and dips, see Table 2. The data on the location of hot springs and fumaroles was obtained from Ol Suswa Energy Limited which has been licensed to explore the geothermal resource in BVC. The raw data on strikes and dips was input into the Georose Software and the results were the generation of a Rose diagram. Figure 13 shows the distribution and location of the mapped structures.
4. Results and Discussion
4.1. Lineaments Delineation
The total number of lineaments generated in the study area were 260; whereby at an azimuth of 45˚ a total of 60 lineaments, at 90˚ was 95 lineaments, at 150˚ was 61, and at 315˚ was 44 lineaments were generated. The lineaments generated for different Azimuths are shown in Figures 4-7. The results were compiled into one map as represented in Figure 8.
![]()
Figure 4. Azimuth of 45˚, 45˚ elevation, exaggeration of 10.
![]()
Figure 5. Azimuth of 90˚, 45˚ elevation, exaggeration of 10.
![]()
Figure 6. Azimuth of 150˚, 45˚ elevation, exaggeration of 10.
![]()
Figure 7. Azimuth of 315˚, 45˚ elevation, exaggeration of 10.
![]()
Figure 8. Lineaments map of Barrier Volcanic Complex (BVC).
The hot springs in the southern part of the area occurring in the East-West direction are evident with results generated by PCI Geomatica but not evident with ArcGIS drawn lineaments. It is also evident that the geothermal manifestations i.e., hot springs and fumaroles (Figure 9) in the area occur along the delineated geological structures. The E-W occurring structures indicated by hot springs can be as a result of folding.
![]()
Figure 9. Major geological structures in the area, generated from PCI Geomatica.
4.2. Lineaments Density
For analysis and construction of lineaments density map the lineaments shown in Figure 8 were used. From the color ramp of Figure 10, the highest density areas are represented in red while low density areas are represented in green, and lowest/no lineament areas are in white color. From the results, there is localised high concentration of lineaments in the four volcanic centers which shows high potential for occurrence of geothermal resource. However, there could be an impermeable layer that separates Kalolenyang to the western part of the area with the other three volcanic centers hence having a separate geothermal system with distinct recharge.
4.3. Determination of Major Trends
The results from the two previous steps were used to determine the major trends in the study area. The delineated lineaments were compiled and processed to generate a Rose diagram that shows the dominant orientation of the lineaments as shown in Figure 11.
![]()
Figure 11. Rose diagram generated from delineated lineaments.
From Figure 11, the geological structures at BVC are trending in the NNE-SSW, and N-S. These trends of geological structures might have been as a result of faulting towards the east as reported by (10). The North-South trending is associated with the North trending normal faulting at the rift margin that led to the formation of the inner trough of BVC as well basalts of Logipi.
4.4. Ground Truthing
To ensure the accuracy of the data, ground truthing was necessary for the calibration and validation of remote sensing models. This increased the precision and reliability of the analysis and result in more informed decision-making processes by cross-referencing data from remote sensing with information from the ground (See Figure 12). Ground truthing was done by measuring the strikes and dips at BVC in order to confirm the major trends (See Table 2). The raw data on strikes and dips was input into the Georose Software and the results were the generation of a Rose diagram shown in Figure 13.
This involved collection of on-site data to enhance the accuracy of data obtained remotely and was done by visiting the study area and taking measurements of strikes and dips, coordinates of the mapped structures, and general ground topography (Table 3). 12 accessible structures in the area were mapped and the result was a generation of a rose diagram shown in Figure 13. The trend of structures is in the NE-SW direction which supports the structural trends in Figure 11. This validated the reliability and legitimacy of information extracted from satellite imagery.
![]()
Figure 12. Map showing the mapped structures at BVC during ground truthing.
![]()
Figure 13. Rose diagram generated from ground truthing data.
![]()
Table 2. Location data for mapped structures at BVC.
![]()
Table 3. Coordinates of fumaroles and hot springs at BVC.
5. Conclusion
The study shows good distribution of lineaments with major trends occurring in the NNE-SSW, and N-S, hence presumed to have been influenced by the east faulting in the area. This is supported by the high density of lineaments in the same direction. Geothermal surface manifestations in the area, fumaroles and hot springs, occur in the NE-SW & E-W direction along the mapped lineaments/faults, which can be used as an effective observatory method for exploring and locating possible drilling locations in future exploitation of geothermal resource in the area.