Characterization of Quaternary Extensional Structures: Tulu-Moye Geothermal Prospect, Ethiopia

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1 GRC Transactions, Vol. 39, 2015 Characterization of Quaternary Extensional Structures: Tulu-Moye Geothermal Prospect, Ethiopia Engdawork Admassu 1 and Selamawit Worku 2 1 Geological Survey of Ethiopia, Addis Ababa, Ethiopia 2 United Nations University, Geothermal Training Programme, Orkustofnun, Reykjavik, Iceland engaye@gmail.com soliethio@yahoo.com Keywords Main Ethiopian Rift, Tulu-Moye, geothermal, magmatic segment, faulting, morphology Abstract The area of investigation, Tulu-Moye, is situated in the Main Ethiopian Rift (MER) northwest of Asela close to the eastern margin of the rift. It is a wide zone where tectonic and volcanic activities are concentrated. As a major part of the Koka magmatic segment, The Tulu-Moye area has depicted interesting characteristics of volcanism and geologic structural patterns. The formation and growth of faults in the area could be explained by three progressive phases of faulting. Two fault models are proposed to explain the sequence of faulting, fracturing and lava flow events. The volcanic activities of the area are mostly controlled by the active faults and extension fractures of the Wonji Fault Belt (WFB). A fault morphology survey exhibited the various nature of the fault and fissure morphology that was primarily controlled by strike variations in the geologic units. The fault kinematic data collected from selected localities indicated E-W direction (~ 93 0 ) extension, consistent with previous works conducted in other parts of the MER. A comprehensive relationship between Quaternary faulting and magmatism was realized in the Tulu-Moye geothermal prospect. In such a way that, cone-fault, cone-lava and lava-fault interactions were the most noticeable relationships between Quaternary faulting and magmatism in the area. The fault morphology, fault model, fault-magmatism relationships, suggests a progressive development of an extension fissure to a mature normal fault. Tulu-Moye has both favorable geological and structural features for a prospective geothermal resource, which however deserves detail geothermal investigations towards defining the area s major parameters. 1. Introduction The Ethiopian Rift (ER), part of the East African Rift System (EARS), comprises a series of rift zones extending from the Afar depression in the north to the Turkana rift in the south. The Main Ethiopian Rift (MER) constitutes the northernmost sector connecting the EARS with the Afar Triple Junction and is mainly characterized by active extensional tectonics and volcanism. The MER started to develop during the Miocene (WoldeGabriel et al., 1990; Chernet et al., 1998) and is characterized by well-developed Quaternary faulting mostly related to the Wonji Fault Belt (WFB; Mohr, 1967; Meyer et al., 1975; Boccaletti et al., 1998; Acocella et al., 2003). Despite the overall NE-SW trend of the MER, the WFB is marked by active NNE-SSW trending normal faults and extension fractures with an en-echelon arrangement, and is associated with young volcanic activity. The Ethiopian rift is well explored and many papers have been published about it. Ethiopia started a long-term geothermal exploration in During the last two decades, a great deal of geoscientific surveys, mainly comprising of geological, geochemical and geophysical studies were conducted by the United Nations Development Programme (UNDP) and the Geological Survey of Ethiopia. These reconnaissance surveys consequently led to the identification of about 16 geothermal prospects and many other areas for direct use (Teklemariam and Kebede, 2010). The geothermal prospect areas are currently found at varied stages of exploration and development. Some of the explored prospects include, from south to north, Abaya, Corbetti, Aluto-Langano, Tulu-Moye and Tendaho. Only Aluto-Langano and 225

2 Tendaho have been subjected to exploration drilling to date. The Geological Survey of Ethiopia, apart from planning and executing further exploration works, has responsibly documented previous geothermal exploration studies carried in the country. It is simple to consider that, apart from the favorable geological conditions; geothermal resources in the ER are primarily controlled by rift structures. Even though the inventory of surface hydrothermal manifestation shows a few sites on plateaus, the most significant occurrences are found confined within the rift. Therefore, understanding the way in which rift faults influence subsurface thermal fluid flows plays a key role; Bruhn et al., (1994) also noted the close interaction between fluid circulation and faults. Characterization of faults and their associated magmatism could be considered as the primary stage towards detailed structural investigation of an area. Most of previous geothermal exploration studies carried out in the MER lacked detailed characterization of rift structures, associated magmatism and their control on the thermal manifestations. In this work, therefore, the initiation and growth of faults and their relationship with magmatism in the Tulu-Moye area are tested using an integrated approach that combines field and remote sensing data. 2. Study Area The Tulu-Moye area is located at about 175km south east of Addis Ababa, close to the eastern margin of the MER (Figure 1). The physiography is controlled by young volcanic products and NNE-SSW faults. 3. Methodology The main data sets used in this study include LANDSAT ETM, DEM (Digital Elevation Model), SRTM (Shuttle Radar Topography Mission), Aerial Photographs and other published maps (scale 1:50,000). Image enhancement and interpretation of optical data sets were carried out during pre-field work. The standard image processing techniques provided bychavez et al. (1991), Drury (2001), Gillespie et al.(1987) and Rowan et al. (2003) were tested on the study area. As a result, the standard FCC 473 (in RGB order) and principal component (PC1) images were used to obtain mainly surface lithology and structural information, respectively. Detailed interpretation of stereoscopic pairs of aerial photographs (scale 1:50,000) was conducted for the inventory of Quaternary faults and associated volcanic products. Similarly, the DEM (Figure 1C) was created from the SRTM Figure 1. Location map of the study area A) Inset map of the Ethiopian Rift B); Distribution of Geothermal prospects of Ethiopia indicated by boxes of varied colors (Source: Geological Survey of Ethiopia);C)Digital Elevation Model (DEM) of the Tulu-Moye (yellow box) and Gedemsa prospects. (90m resolution) by fusing the three different illumination directions. The SRTM was mainly used for the extraction of faults and open fissures in the study area. In addition, field work was conducted that aimed to ground truth the surface geology and Quaternary faults which were depicted from the image interpretation. Furthermore, accurate coordinate (x, y, z) surveys both across and along open fissures were done using Kinematic Trimble GPS. 4. Geology, Structure and Geothermal Activity in the Study Area 4.1. Local Geology The MER, like the rest of EARS, experienced a complex geological and tectonic evolution. The regional geology of ER has been extensively described and documented (Mohr 1962, 1970; Kazmin, 1972; Di Paola, 1972; WoldeGabriel et al., 1990). WoldeGabriel et al. (1990) divided the MER into three geographic sectors, and classified the volcanic products of the whole rift into six tectono-magmatic domains. Recent (about 0.3 Ma) volcanic units outcrop along Pleistocene-Recent fault zones; i.e., the Silti-Debrezeit Fault Zones (SDZFZ) and Wonji Fault Belt (WFB) (Di Paola, 1972; Kazmin et al., 1980; WoldeGabriel et al., 1990). Studies (Ebinger, and Casey, 2001) on MER showed the presence of many Quaternary magmatic segments arranged in right stepping en-echelon fashion among which is the Koka magmatic segment to which the Tulu-Moye area belongs. Volcanism in the magmatic segments began about 1.6 Ma. Most lava flows emanated from fissures and central vents (WoldeGabriel et al., 1990; Boccaletti et al., 1999). 226

3 The area is covered by recent volcanic products of the Wonji group (WoldeGabriel et al., 1990) confined to the Koka magmatic segment covering a large area of the Aluto-Gedemsageothermal prospect. The regional geological map of the Lake Ziway-Asela region (Abebe et al., 1998) shows the distribution of various recent volcanic units. These include ignimbrites (east), basalts of Tulu-Moye (center) and pumice falls and lava domes (west) (Figure 2). Basaltic flows, pyroclastic cones, obsidian domes and basalts of Tulu-Moye are found along the axial zone of the WFB. These units are severely affected by intensive fracturing and faulting in the study area. Some of the units are described as follows: A) Ignimbrite and unwelded tuff: This unit is mapped to the southeast and southern end of the area and is characterized by forming fault scarps with maximum thickness exposed along the main border fault near Kulumsa village. According to WoldeGabriel et al. (1990) it has an age of about 1.66Ma, but Holocene ignimbrites were also mapped. The unit consists of poorly welded ignimbrites, and minor lava domes and flow associated with caldera collapses (Figure 3a). B) Basalts: This unit broadly consists of recent rift floor scoracious basalt and relatively older porphyritic basalts of Tulu-Moye. The rift floor basalts of aa type cover the northern Artu area. They outpoured through NE-trending open extension fractures. Petrographically it is composed of abundant plagioclase with some pyroxene and olivine. The porphyritic basalts of Tulu-Moye are abundant in the Artu, Deneba and Danisa areas. They are of pahoehoe type, with groundmass composed mainly of plagioclase laths. These basalts outpoured through recent NE trending open faults (Figure 3b). C) Pyroclastic cones: These cones are found along active graben and horst structures, and showed close association with the rift floor basalts of Tulu-Moye. In some places, the cones have been strongly affected by younger faults and fractures (Figure 3c). D) Obsidians, Trachytes and Lava domes: This unit is a broad category that includes all the varieties of glassy obsidians, lava flows and trachytic domes mapped at the central area, Artu and south of Gedemsa. The trachytic rocks occur either in the form of sheet flows or mushroom-shaped lava domes (Figure 3d) Figure 2. Geological map of Tulu-Moye area overlaid on SRTM as a background. Mainly interpreted from Landsat ETM, aerial photos and field mapping (after Abebe et al., 1998). Figure 3. Photos of (A) welded ignimbrite, (B) basalts of Tulu-Moye, (C) pyroclastic flows, and (D) obsidian and dome-shaped trachytes. 227

4 4.2. Geologic Structures Two main distinct fault systems are recognizable in the MER. That is, a NNE-NE trending late-miocene rift border fault system and a younger N-NNE trending right stepping en echelon Quaternary fault system of the Wonji Fault Belt. According to Meyer et al., (1975) the WFB faulting started developing at the beginning of the early Pleistocene (~1.6 Ma ago). Despite the overall NE- SW trend of the MER, the WFB is characterized by active NNE- SSW trending extension fractures and normal faults, which in many places are associated with fissural and central volcanic activity (Gibson, 1969; Mohr, 1987; Chorowicz et al., 1994; Korme et al., 1997). The normal faults are in most cases arranged in a rightstepping, en-echelon configuration (Mohr, 1968; Boccaletti et al., 1998). Vertical throws are in the order of several meters in the axial zone (Gibson, 1969) to 300 m along the rift margins (WoldeGabriel et al., 1990, Ebinger and Hayward, 1996). The structural map of the study area (Figure 4) was produced mainly from interpretation of enhanced optical data and previous work by Figure 4. Tectonic map of the study area extracted from PC1, DEM, aerial photo and field survey data. Abebe et al., (2007). For convenience, the normal faults in the area are classified based on their measured throws. It was observed that >20m throws were locally seen as border faults for small grabens and half grabens. Whereas, faults with throws <5 m, including open fissures and fractures are densely localized along the axial zone of the rift. This indicates that there are noticeable spatial and geometrical relationships between the local marginal faults, the rift floor minor faults and the extension fractures. In this case, the structural pattern in the Tulu-Moye area is plausibly in agreement with the regional structural framework the MER Geothermal Activity Tulu-Moye has been selected as one of the most promising geothermal prospects in the MER having an estimated 40MWe power generation capacity. Hence, reconnaissance exploration activity has been conducted by the Geological Survey of Ethiopia. Surface geothermal manifestations in the area comprise of hot, altered grounds, steam vents and fumaroles, widely distributed by tracing the N-NNE trending extension fractures. The compiled reports on Tulu-Moye show that the heat source, cap rock, reservoir rock, and favorable structural fabric have been barely identified. This study also attempts to carry out an inventory of steam vent locations (Figure 4). Most of the steam vents were localized along the axial zone characterized by active faults. 5. Results and Discussion 5.1. Fault Morphology Morphology data were collected in the selected Deneba, DalutaTero/Salen and Danisa areas (Figure 5) using Kinematic Trimble GPS usually both across and along the faults. The data was Figure 5. Fault morphology survey shown on the Digital Elevation Models (DEM). (I) Deneba ;( II) Tero/Salen ;( III) Danisa ;( IV) Daluta. 228

5 then converted into 2D cross-section across the faults. The result showed that morphology of faults and fissures depicts variation from place to place in the tested localities. In the Danisa and Salen areas, within the basaltic field, clear flexural faults are observed that have typical fault behavior characterized by steep escarpments (5-25 m) at the center, ending as simple fractures along strike. On the other hand, the morphology in both areas (Figures 6a and 6b) is characterized by monoclonal topography with flat footwalls, and hanging walls tilted away from the fault plane. However, in the northern part of the study area (Shenen and Lugo) the fault morphology changes because of the presence of different rocks, like massive lava flows and pyroclastics. The faults attain maximum height on the massive lava flows and disappear when they reach pyroclastic rocks (Figure 6c) Fault Models Figure 6. Fault/fissure morphology; (A) Deneba area, (B) Daluta area, (C) fault morphology at Shenen. Based on the observed field relationship between faults, extension fractures and associated volcanism; two fault models representing the sequences of events in the study area are proposed. The first one (Figure 7a) would help explain most of the extension fractures and normal faults (throw <10m) with significant openings, which are prevalent in most of the study area (Figure 4). This model suggests that the normal faults are created by the extrusion of the basalts through extension fractures, which is then followed by vertical displacement. The second fault model (Figure 7b) being proposed best explains the faults (throw >20m) devoid of any openings. In this model the formation of sigmoidal extension fractures is followed by small vertical throws that finally develop into mature faults with greater vertical displacements. It is assumed that the original fissures and fractures are filled with either volcanic material erupted through them or with sediments deposited later, which obscure the horizontal displacements. Figure 7. Fault models for the study area. (A) Model to explain faults with small (<10m) throws; (B) Model to explain faults with larger (>20m) throws Fault Kinematics Kinematic data were collected in the Danisa and Salen areas from opposite walls of matching pairs of extension fractures (Figure 8a and 8b). The opening directions and horizontal displacements data collected in the fields were further evaluated using the equation: 229

6 T = n=30 i=1 n=30 Od * HD i=1 where T is total weighted opening direction, Od is approximate opening direction, and HD is horizontal displacement. Based on the about 50 kinematic data (from the Danisa area) and about 85 kinematic data (from the Salen area) that were collected, average extension directions of about 95 and 92 were obtained. Therefore, the kinematic analysis of the two localities suggests that the Quaternary structural pattern of the study area is consistent with the nearly E-W direction of MER s extension. HD Figure 8. (A)Determination of opening direction by matching opposite walls of cooling fractures (after Acocellaet al., 2002); (B) Matching edges from cooling fractures (Salen/ Tero area) Quaternary Faulting and Magmatism The general Quaternary volcanism in the MER is controlled by the en-echelon arrangement of the WFB (Mohr, 1962). Quaternary rift zones in the WFB create areas of active deformation obliquely cutting the rift floor of the MER (Bocaletti et al., 1998; Abebe et al., 2007) with characteristic bimodal volcanism. Quaternary basaltic volcanism in the MER is mainly concentrated along WFB s faults and fractures. Likewise, the volcanism associated with the Koka magmatic segment (Section 4.1) is obviously linked with the WFB. Therefore, strong relationship exists between the volcanic eruptions and associated faults and tension fissures of the Tulu-Moye area. It was noticed that fault related open structures were the most appropriate places for magma outpouring. These open fissures, the so-called tail cracks (Korme et al., 1997), mostly developed at the termination of NNE-striking faults and are marked by the emplacement of score cones, lava domes and tuffs (Figure 3) especially at Shenen and Lugo areas of southern Gedemsa (Figures 9a and 9b). Faulting and Quaternary magmatism of the study area could be expressed in terms of various field relationships like cone-fault, cone- lava flow and fault-basaltic lava flow interactions. Figure 9. (Left) DEM showing tail cracks and the associated volcanic emplacements; (Right) Photo showing fault-cone relations. 230

7 5.5. Formation and Growth of Faults A narrow rift shows an inner younger and active zone of extension characterized by subparallel extension fractures and normal faults, whose vertical displacements increase from the axial zone towards the mature rift margins (Mohr, 1968). According to Gibson (1969), subparallel extension fractures and normal faults formed contemporaneously and independently under the same stress conditions as in the axial part of a rift. Studies on the normal faults on the axial zone of the ER (Acocella et al., 2002) have shown that the openings of the normal faults in the axial zone tend to be proportional to the throw of the faults either along the same fault or different faults. Geometry (length, strike, dip) and kinematics data (dilation, throw, extension direction and hanging-wall tilt angle) were collected for many normal faults and extension fractures particularly from the Shenen and Deneba areas where abundant extension fractures and normal faults were mapped. The results show that three distinct phases of faulting occurred in Tulu-Moye (Figure10a).The first phase started as an extension fracture that progressively developed into a fault with its hanging wall tilted away from the fault plane (phase two) and is characterized by significant opening, but minor throw, which is then followed by a well-developed fault with maximum throw, but without opening (phase three).the relationship between normal faults and the associated extension fractures can be explained by the often branching out of extension fractures from the main faults. This would suggest that normal faults nucleate from simple extension fractures with clear and observable transition zones (Acocella et al., 2002) as shown in (Figure10b). Figure 10. (A) The three faulting phases (I-III; see detail in the text); (B) 3-D fault-morphology showing the transition from a monocline feature to a normal fault. 6. Conclusions The Tulu-Moye geothermal prospect depicts interesting volcanic characteristics and structural patterns. The formation and growth of faults in the area could be explained by three progressive phases of faulting. Two fault models are proposed to explain the sequence of faulting, fracturing and lava flow events. The volcanic activities in the area are mainly controlled by the active faults and extension fractures of the Wonji Fault Belt. A fault survey exhibited the various fault and fissure morphologies, which are primarily controlled by along-strike variations of the geologic units. The fault kinematic data collected from the localities selected in this study indicates a E-W direction of extension that is consistent with the results of earlier investigations conducted in other parts of the Main Ethiopian Rift. A plausible relationship between Quaternary faulting and magmatism was found in the Tulu-Moye geothermal area, noticeably the interrelation between cone-faults, cone-lavas and lava-faults. In most parts of the study area, tail cracks are the most important locations for volcano emplacement. The topographic setting, Quaternary faulting and magmatism in Tulu-Moye suggest that it is a promising geothermal prospect. This study, therefore, recommends detailed geothermal investigation to be conducted mainly towards defining the area s major parameters. 231

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