Relation Between Regional Lineament Systems and Geological Structures: Implications for Understanding Structural Controls of Geothermal System in the Volcanic Area of Itasy, Madagascar

Abstract

No detailed investigations have been conducted on the specific structural controls of geothermal individual fields in Madagascar. Knowledge of such structures would facilitate exploration models.
In this study, we utilize satellite imagery, field investigations and compilation of existing data, to characterize the age and the role of faults in the geothermal system of the volcanic area of Itasy. The correspondence between the remote sensing-derived aligned features (lineaments) and the geological structures of the area was verified by means of our own geological field surveys.
The lithology of Quaternary section consists of volcanic rocks including Pleistocene trachyte, limburgite, basanite, basalt and ordanchite lavas overlain by sequences of intercalated ash-flow tuffs. The Precambrian basement is mainly composed of migmatitic gneiss and shows high fracture density. This section is fragmented into multiple north to north-northeast-trending fault blocks. Most of the major geothermal sites occur along or near the north to north-northeast-striking faults that roughly parallel the volcanic area. This belt is called to as the Itasy structural zone. Major fractures and faults arranged broadly in north-south structures suggest east-west thinning and extension direction. These faults, distributed across the ancient crystalline basement exhibit a pre-Pleistocene rupture history.
We speculate that the geothermal system may have experienced transtensional strain fields in association with left-lateral slip along a major strike-slip fault zone. The strike-slip basins recognized in this releasing bend may be a negative flower structure or a classical pull-apart basin caused by dilational left-stepover of the master fault.
The structural settings favoring geothermal activity all involve subvertical conduits of highly fractured rock along fault zones oriented approximately perpendicular to the foliation of Precambrian migmatitic gneiss and increase dilation, thereby allowing geothermal fluids to approach the surface.