Understanding heat transfer along extensional faults: The case of the Ambilobe and Ambanja geothermal systems of Madagascar

•Forced convective heat transfer along extensional faults is numerically assessed.•Simulations are characteristic of petrothermal to transitional system.•Favorable faults dip is when facing the direction of the groundwater flow.•Favorable fault location is near a discharge zone (i.e. a river).•Fault characteristics and heat flow influence the depth of reservoir temperature. Understanding the role of faults where forced convective heat transfer is the dominant mechanism giving rise to hot springs is critical in geothermal exploration in extensional environments. This study uses two-dimensional models of coupled fluid flow and heat transfer along cross-sections perpendicular to faults and the regional topography to identify favorable fault conditions for geothermal system development in northern Madagascar. Structural data collected at surface were used to define fault scenarios and simulate the ascension of hot fluids to reproduce hot spring temperatures in the Ambilobe normal fault zone area and the Ambanja graben structure. Fault dips facing topography‐driven groundwater flow was shown to be favorable, and hot spring temperatures could be reproduced when the fault permeability was > 10−14 m2. Faults located in a discharge zone near a river were the most favorable for fluid ascension, regardless of their dip. Constraining the model with a basal heat flow between 90 and 148 mWm−2 at a depth of 10 km allowed the reservoir temperature to reach 150–200 °C at depths of 2 km or shallower along favorable faults.