Integrated analysis of geochemical and geophysical data from Alalobeda geothermal field, Northern Afar Region

This paper illustrates the first results of the geological, geochemical and geophysical investigations in the Alalobeda geothermal field located in the Tendaho Graben, Northern Afar Region (Ethiopia). The tectonic pattern of the Alalobeda field is characterized by the superposition of three regional rift zones: the Red Sea (NW-SE), the Main Ethiopian Rift (NNE-SSW) and, to a lesser extent, the Gulf of Aden (ENE-WSW). Gravity anomalies highlight the graben bedrock and the main fault systems which in turn well correspond to magnetic anomalies recorded by the basalts of the Afar Stratoid Series. Geochemical analyses of water samples collected from hot springs indicate reservoir temperatures of 200-220°C. Such temperatures are substantially consistent with those (on average 185-225°C) inferred from fumaroles gases. The geothermal field is characterized by a quite continuous seismic activity of low-energy, consistently with the regional pattern, mostly exhibiting shallow earthquakes with magnitude <4.0. The seismicity cutoff-depth, which can represent a rheological, temperature-dependent boundary, occurs at 3.5-5 km depth. A possible increase of temperature gradient in correspondence of the major surface manifestations of the geothermal system could be indicated by the decrease of focal depths. The results of a magnetotelluric survey give a picture of the possible extension of the cap-rock and reservoir formations. The surveyed area unexpectedly shows extremely low resistivity values for the basaltic rocks forming the bedrock, due to intense argillification phenomena. Below the high-conductivity cap-rock, the more resistive basement can act as the reservoir formation, although its resistivity is somewhat higher than that commonly recorded in other geothermal fields.