We use guided waves traveling updip along the surface of the Nazca slab to image subducted oceanic crust at the Chile-Peru subduction zone. Observed P onsets of intermediate depth events near 21¿S in northern Chile reveal waveguide behavior: with growing focal depth, low-frequency energy (<2 Hz) becomes more and more dominant, and higher frequencies arrive delayed, sometimes resembling two distinct phases. To explain the observations, we employ two-dimensional finite difference (FD) simulations of complete P-SV wave propagation along an updip profile of the subduction zone. The FD calculations shed some light on several basic issues regarding crustal waveguides. The development of guided waves dependent on event focal depth is simulated. Further, we show that the observed guided wave energy must decouple from the waveguide near 100 km depth to reach the deployed stations and that the decoupling process is related to variations in subduction angle. Simulations also yield constraints on source locations relative to the low-velocity structure. Finally, the frequency content of P onsets is used to constrain the thickness of the waveguide. The results indicate that a structure of <4.5 km average width and 7% low-velocity remains seismically slow compared to the surrounding mantle down to a depth of at least 160 km. The layer is interpreted as the unaltered lower part of the subducted oceanic crust, suggesting that complete eclogite transformation in the Chile-Peru subduction zone is unlikely to take place until beyond the volcanic front.