We obtained a 2D crustal model beneath central Chile and western Argentina using apparent Pn phase velocities recorded along an EW trending transect at 30¿S. This model is characterized by a thick (65 km) crustal root beneath the High Cordillera, a 60 km thick crust beneath the Precordillera, and a gradual shallowing of Moho depths beneath the Sierras Pampeanas from 55 km in the west to 40 km in the east. Beneath the Sierras Pampeanas, where very little crustal shortening is observed, the thick crust must be explained by means other than simple tectonic shortening. Isostatically balanced crustal thickness is consistently shallower than our model shows, suggesting the existence of dynamic forces possibly linked to the flat slab.