Four different mechanisms (frictional heating, magmatic intrusion, free convection, and forced convection) have been tested as possible causes to explain the localized and relatively high geothermal anomalies found along masters faults of graben structures. Forced convection is the only mechanism found to be generally able to generate the anomalies. The Vall¿s-Pened¿s basin (NE Spain) is used to illustrate the results of the different mechanisms. In general, it is concluded that forced convection will generate geothermal anomalies when several conditions are met: water circulation has to be deep and slow in its descent; deep reaching faults must exist to allow a rapid ascension of water; and some sort of sealing along the ascending conduit has to prevent mixing with cold water. The hydraulic model of the Vall¿s-Pened¿s graben rules out a possible circulation through the sediments filling the basin, and therefore a circulation model through the adjacent horst is proposed. This model is supported by isotopic data. Such a model is not restricted to horst and graben structures but can be used to explain anomalies found in compressive geological settings. ¿ American Geophysical Union 1990