We present an application of three-dimensional (3D) simultaneous seismic refraction and reflection tomography for velocity and interface structure. The inversion technique and method for developing the starting model are specifically designed for relatively sparse wide-angle data acquired across strongly-varying structure. The data were recorded in a region of seamount subduction on the Chilean margin and consist of seven receivers and ten intersecting airgun profiles over a 90¿90 km area providing constraint to 25 km depth. The tomographic method and the final model are assessed through a comparison with the large-scale geologic features of the margin and a resolution test. The 3D model shows the Valparaiso forearc basin, the accretionary wedge, the subducting plate, and possibly a subducted seamount. Our results show the potential of relatively sparse 3D wide-angle data. ¿ 1999 American Geophysical Union