Structural Model of the Itasy Geothermal Prospect in Central Madagascar: Preliminary Review

Abstract

The Itasy geothermal prospect, situated at about 110 km west of Antananarivo capital, exhibits a striking density and variety of volcanic structures on the western side of Lake Itasy. The volcanic edifices all lie directly on the Precambrian basement mainly composed of migmatite and gneiss. The majority of the volcanic vents lie roughly within a rectangle of 45 km north-south (N-S) and 15 km east-west (E-W) extension suggesting a graben structure in the basement. Deformation has been analyzed from tension fractures  which are perpendicular to the direction of extension  and from field structural analysis. The Precambrian basement shows high fracture density and is fragmented into multiple north (N) to north-northeast (NNE)-trending fault blocks. Most of the major geothermal sites occur along or near the N to NNE-striking faults that roughly parallel the volcanic area. Kinematic data indicate essentially dip-slip normal displacement on the NNE-striking faults (principal displacement zone). Stress data indicate that the principal stress axe (σ1 = N25W) is orthogonal to the regional east-northeast (ENE) thinning direction. Thus, on a regional scale, the tectonic regime responsible for the distribution of volcanic vents and geothermal sites is extension. Left-lateral shear along NNE-striking fault zones within the Itasy structural zone may accentuate ENE-directed regional extension. This mechanism, combined with a greater density of faults and fractures induced by the transfer of strain between the en echelon overlapping normal faults, may promote deep circulation of geothermal fluids along the N-striking fault zones. We speculate that the geological structures of the Itasy prospect are mainly composed of strike-slip faults and related basins. Two hypotheses are possible to explain the origin of these strike-slip basins: the first one concern a classical pull-apart basin or rhomb-shaped graben and the second one may be a negative flower structure. Few low temperature thermal springs are indicative for persisting geothermal resources. The geothermal system is volcano-tectonic type. It has been termed magmatic geothermal and is associated with recent faulting in areas of thinned and extending crust.