| Title | Abrourahman Omar Haga, Said Kaireh Youssouf, Jacques Varet |
|---|---|
| Authors | The Manda-Inakir Geothermal Prospect Area, Djibouti Republic |
| Year | 2012 |
| Conference | ARGeo |
| Keywords | hydrothermal, fumaroles, volcanic, tectonic, fault, Djibouti, Manda-Inakir, Moussa Ali |
| Abstract | The northern half of Djibouti Republic is made of volcanic formations of late Miocene age, with the extensive and deeply faulted Mabla Rhyolites (12 to 8 My) partly covered (western side) by the Dalha Basaltic Plateau (7 to 4 My) gently dipping towards NW. This geologic block is rather stable and appears to be a part of the Arabic Plate since oceanic spreading stopped along the Bab-El-Mandeb straight of the Red Sea and developed inside Afar for the last 3 to 4 My (Marinelli& Varet, 1973). No real high enthalpy geothermal potential is therefore expected there. On the western flank of this block however, the Dalha Plateau sinks under the more recent basalts of the stratoïd series, itself deeply faulted. Near to the triple state boundary with Ethiopia and Eritrea (Moussa Ali central volcano, of quaternary age), an active axial range called Manda-Inakir was identified (La Volpe et al. 1974). Since then, the huge N-S fault zone linking Manda-Inakir and Asal Rift segments was interpreted as the surface expression of a transform fault (Tapponier and Varet, 1974). Hence an active plate boundary crosses through the westernmost part of this region, with related geothermal heat source. The Manda-Inakir axial range extends ENE-WSW across the boundary with Ethiopia, and displays similar volcano-tectonic behavior, as the Asal and other axial ranges. However, a shift is noticed, the most recent volcanic activity having migrated from Inakir - an elongated and rifted shield displaying the whole petrologic sequence from basalts to trachytes - to the north, in the Manda range, where historical basaltic eruptions did occur. Open fissures affecting the most recent basaltic flows and nearby sediments in the rift direction show that the extensional phenomena is still active at present. This implies that there are the benefit of both a magmatic heat source and important fracture permeability. Although the climate in the area is particularly dry, any meteoritic water concentrates in these recent volcanic lowlands and nearby sedimentary plains, and quickly infiltrates deep in the basalts due to the open faults. Part of the large hydrographic basin of the Dalha Plateau also feeds these reservoirs. A field study undertaken by the authors identify several sites where gas vents, hot-springs and fumaroles occur, mainly located in the faulted formations surrounding the plains of Andaba and Awda’a near the villages of Dorra and further south towards Malaho and Balho. They are mostly located in sites where the NW-SE open faults cross with the N-S trending transverse faults. Suitable sites for geothermal development are hence certainly present in this district. However, geochemical and geophysical investigations should be undertaken to enhance the location of sites for geothermal exploration wells. Although no geothermal gradient drilling has been undertaken, it is important to note that several sites drilled for water production in the area and its surroundings display high temperature gradients (up to 60°C/100m). At present, the local consumption of electricity is very low. The nearby villages rely on small diesel engines. There is no doubt that such a local energy source would boost the demand, notably for groundwater pumping. In addition, future needs will increase due to the development of transport. An important highway is under construction and will be followed by a railway line linking Makalé, the capital of Tigré regional state in the Federal republic of Ethiopa, to the sea port of Tadjoura in the Republic of Djibouti. The modular character of power production development on geothermal sites should ,provide - once the resource have been proved by exploration drilling – a solution to these successive needs (from a few MW initially to a few ten or more if needed). |