| Abstract |
The Aluto Volcanic Complex (AVC) is situated in the central sector of the Main Ethiopian Rift (MER), close to its eastern margin in respect to the main axis. It is also located at the junction between two important hydrological basins, whose central portions are presently occupied by Lake Ziway in the north and by the three sister lakes of Langano, Abiyata and Shala in the south. The local geological setup of the AVC is quite complex. The oldest rocks are dated at about 150 Ka, while some of the post/syn caldera rocks are dated at about 55 Ka (ELC, 1986). The caldera collapse (8.5 x 5 km) elongated in E-W direction, might have taken place between 50 to 80 Ka ago. After the caldera collapse, volcanism continued from many small centers forming cones of pyroclastics and lava flows mainly following the caldera rim and the NE and the NNE regional fault systems. Although at present no volcanic center is active in the AVC, the complex itself is considered to be dormant, but not extinct. Biggs et al., 2011, using satellite based interferometric synthetic aperture radar observations, demonstrated the presence of uplifting and sinking of the surfaces of Aluto and other central volcanoes in the MER between 1993 and 2010.This indicates the presence of a hot magma chamber, which is continuously replenished, wherefore pressure build-up and collapse could be manifested on the ground surface. One of the youngest flow was dated at 2 Ka (ELC, 1986). Areal distribution of these very young products, together with the associated hydrothermal manifestations and the structural pattern, are important indicators of the favorable geological conditions of these sectors from the point of view of geothermal energy exploration. Other than along the Jawe Fault Zone (JFZ), most of the hydrothermal manifestations are concentrated in the eastern and western sides of the E-W elongated caldera rims, presumably due to the displacement of magma pockets in these sectors. Comparing the two sides of the caldera, it can be observed that the AVC products in the eastern part (Bobessa Zone) has an average thickness of 340 m and in the west (Adonsha Zone) about 600 m. Moreover, the lacustrine sediments and the remaining older volcanic successions are much thicker in the west, as a result of the rift-ward inclination of the layers and the presence of faults down-steping towards the rift axis. Due to the larger thickness of materials in the western rim of the caldera, hydrothermal manifestations and younger lava flows may appear in lower quantities as compared to the eastern rim. Nevertheless, a higher geothermal potential is expected to be in the western sector of the AVC, due to larger hydraulic recharge and thicker lacustrine sediments that may act as cap rock of the geothermal system. |