| Abstract |
The Menengai geothermal prospect is located in the middle of the East African Rift System (Kenyan Rift Valley). This prospect covers an area of about 90 km2 in correspondence of the ¡§triple junction¡¨ of such rift, at the intersection of the ENE-WSW trending Kavirondo Rift with the Gregory Rift. The reconstruction of the structural setting of the Menengai area was based on the existing literature, integrated with the interpretation of Aerial Photos and Satellite Images (Landsat ETM+7 and Aster). Available information indicated that the structural setting of the area is dominated by the presence of tensional-type N-S systems related to the regional stress. The main tectono-volcanic axes (TVA) identified in the area correspond to the Molo and Solai systems. As refers to the classification and recognition of volcanic forms, mention was made of the likely presence of another calderic structure, in addition to the Menengai one. This caldera, named ¡§Olbanita caldera¡¨, located NW of Menengai and visible only along its western rim, has an estimated diameter of about 12 km. Moreover, evidence was found that the so-called ¡§Menengai shield volcano¡¨ is actually constituted by two separate edifices, one of them encompassing the whole Menengai caldera and the other corresponding to the N-S oriented Ol Rongai chain. The main findings of the linear features orientation and density analysis, illustrated by means of maps and azimuth distribution diagrams, are the following: „h The structure at regional level is controlled by a family of tensional-type N-S faults extending along the Rift and constituting the main TVA recognized in the area, i.e. the Molo and Solai systems allegedly forming a complex graben-horst system. However, other fault systems become predominant within the Menengai caldera, in particular the NNW-SSE ones, which are highlighted by the occurrence of well aligned eruption centres and fumarolic activity. Another important system corresponds to the E-W direction expressed by a long-linear extending between MW-01 and MW-03 exploration wells and by other parallel faults, which seem to partly control the areal extent of the geothermal reservoir. „h Two structures of the NNW-SSE system (trending N.7W and N.30W) appear to be of major importance, as indicated by their pronounced surface expression, their control over the volcanic emission centres and the association with high density linear features. These structures, also clearly expressed by the distribution of the micro-seismic events, define a block 4 to 6 km wide, extending throughout the caldera, which is deemed to correspond to the sector where the magmatic chamber is shallowest and the thermal anomaly most pronounced. |