The stru c tu ra l character o f the Andean Precordille ra represents a hig h-le vel o f fold and thrust b e lt fo rm e d during the alp in e orogeny. The studied area is characterised by a set o f eastwards Im bricated overthrusts and folded rocks which are In movement, as evidenced by a number of earthquakes in recent times. The main rivers flow approximately perpendicular to the main structural lines and show several terrace levels with different morphologies. The generation ot the terraces has often been attributed to the main climatic changes. In our opinion this development Is directly related to the repeated base level changes controlled by neotectonics