The western Principal Cordillera consists of thick volcanic successions accumulated in an extensional basin and deformed in Early Neogene times during tectonic inversion of the basin. In the study region, key-layers are lacking and, for this reason, geometrical reconstructions are difficult. The main structure is a N-S-oriented syncline bounded by faults. Reconstructions using the fault-propagation fold and trishear models indicate that this structure is controlled by faults, a western (WF, cut-off angle ~40-60º) and an eastern fault (EF, cut-off angle ~20º). WF, the most important one, is interpreted as an inverted normal fault associated with basin inversion, whereas EF is interpreted as a short-cut related to WF. Shortening is 2-3 km (~30%).