Thire are three regional joint sets in the central Ebro, basin. The southern part is. dominated by a primary N-S striking joint set and a secondary E-W set. In the nortnern part, the primary joint set strikes NW-SE Detailed outcrop observations where the three sets do coexist suggest that the E-W set is the result of stress perturbation originated by the previous N-S set over the extensionat NE-SW stress field responsible for the development of the NW-SE joints in the northern area