| Title | Exploring for Geothermal Sites in Northern and Central Afar (Ethiopia) |
|---|---|
| Authors | Jacques Varet, Tadiwos Chernet, Girma Woldetinsae and Knutur Arnason |
| Year | 2012 |
| Conference | ARGeo |
| Keywords | exploration, Ethiopia, Afar, volcanic ranges, rift, fracture, faults, Dallol, Boina, Manda Harraro |
| Abstract | While geothermal exploration and development has occurred until now in Ethiopia along the Rift Valley and in Tendaho (central Afar), the northern part of the Afar Triangle was relatively neglected. The aim of this paper is to show that sites of interest for electric power production certainly exist in this area and would be worth analysing for prefeasibility and feasibility studies leading to industrial development. Conditions for the development of high enthalpy geothermal fields were shown to be met in North-Eastern Afar when the superposition of 3 geological features was encountered (J.Varet, 2011). These include: - The presence of active spreading segments, with important and recent volcanic activity developed in the “rift-in-rift” structures of the Erta-Ale, Alayta and Manda Harraro axial ranges, allowing for the development of significant and rather shallow heat sources; - The development of transverse faulting crossing through the dominantly NNW-SSE normal and open faults, which frequently correspond to offsets in the scarp of the Nubian plateau as well as between axial ranges, which allow for the development of fracture permeability in geothermal reservoirs; and - The feeding of the geothermal reservoirs from eventually wide basins developed along the intensively eroded escarpment of the Nubian Plateau, with frequent lateral grabens allowing for the infiltration of meteoritic water from the wet highlands into Mesozoic sedimentary (Jurassic limestone and cretaceous sandstone) as well as Tertiary and Quaternary detrital formations. Due to the evaporation of a former southern branch of the Red Sea (200 to 25 ka ago), the northern part of the depression is floored with thick (at least 1000m) salt deposits - this allowing for the development of Sylvite deposits in addition to Halite. Numerous hot-springs, explosion craters and fumaroles are reported at Dallol (Chernet 2012, Frazson and Helgadóttir, 2012). Therefore, the ongoing geothermal exploration focuses on the detection of geothermal reservoir fed by less saline waters in the Mesozoic and Tertiary formations at accessible depth underneath the salt formation. Further south, another geothermal site may have developed around Lake Afrera, where the Erta-Ale Range ends while the spreading is transferred to the two axial ranges of Alayta to the SW and Tat-Ali to the SE. Important hot-springs are interpreted as the products of the heating of superficial groundwater crossing through the Alayta Range by deeper geothermal fluids, the location of which remains to be identified by further exploration. South of Alayta, another offset is also in correspondence with the geometry of the basement scarp, where the Mand-Harraro Range develops along 120km. In the area of transition between these two axial ranges, the Boina – Dabbahu recent silicic center developed with a fractionation sequence up to pantellerites showing the presence a shallow and active magma chamber. (Barberi and Varet, 1975). This is also the site of intense hydrothermal manifestations, with numerous hot-springs, fumaroles and silica deposits, notably along the sedimentary plains bordering the volcanic zones to the West. Mand-Harraro, the most active axial range in Afar, displaying recent basaltic emissions, and surrounded by numerous fumaroles, hot grounds and hot-springs, was subject of an intense volcano-tectonic event with the opening in 2005 along its NNW axis of a fault 120km long and up to a few meters wide, followed by several basaltic magma injections from the mantle, 10km deep, until 2011. To the south of the range, rather intense faulting, transverse to the regional NNW direction expressing a high permeability favourable to the development of a geothermal reservoir fed by the important Awash and Mille river basins. The apex of this system is located immediately south of the lava fields, whereas the Tendaho Geothermal Prospect was developed in the graben filled by recent unconsolidated sediments of the Awash River. |