An oceanic crustal section has been derived for the Nazca plate and across the Peru-Chile Trench at lat 12°S. Although based on unreversed seismic refraction data obtained by the Airgun-Sonobuoy-Precision-Echo-Recorder (ASPER) technique, 12 crustal determinations along the profile provide sufficient data density to permit good correlation of velocity structure between the stations. A thin, high-velocity oceanic crust characterizes the region. A major apparent offset in the crustal layers is interpreted as a low-angle thrust fault dipping east at an average of 6° from the ocean floor 300 km west of the trench axis. This feature, together with other indications of thrust faulting and crustal foreshortening, is taken as evidence that the upper lithosphere, at least to the depth of the Moho, is undergoing compression prior to subduction in the trench.