Reactivation of caldera structures in active extensional settings: implication for geothermal exploration

Abstract

Collapsed calderas are prominent volcano-tectonic features occurring in active tectonic settings and bear intrinsic risks associated with their explosiveness. Nonetheless, they also represent key targets for geothermal fluid exploration, their structures being often the preferential pathway for geothermal fluids migration. In active tectonic settings such as continental rifts, caldera faults may be reactivated, enhancing therefore their permeability. However, specific structures may be subject to clamping, consequently reducing their secondary porosity. Discriminating if and how caldera structures may respond to tectonic stresses, represents therefore a critical question to address when calderas become the locus of potential geothermal exploration. We performed an experimental series of analogue models of caldera collapse exploring whether caldera structures may reactivate under extensional tectonic conditions. This analysis is important for evaluating which caldera fault segments may be regarded as the best potential target for fluid interception. Our experimental series shows that regional extension and fault dip can explain the reactivation of specific caldera fault segments. In particular, outer normal ring faults do reactivate under extensional conditions only in the sectors trending orthogonally to the direction of extension. Conversely, inner outward-dipping reverse faults do not reactivate, likely because of their lower dip angle, whichever their trend might be. This implies that inward-dipping normal faults trending orthogonal to direction of extension likely increase their permeability, thus becoming a favourable locus for geothermal fluid migration and therefore a preferable target for exploration. Conversely, our models show that sectors of inward-dipping normal caldera faults trending parallel to the direction of extension may experience clamping, and so reducing their secondary permeability. Therefore, our setup, with due approximations and limitations, represents a useful predictive tool for identifying potential target structures for geothermal exploration at caldera sites. The model setup can also provide insights into similar caldera systems developing in other geological settings (e.g., compressional).