In the northern Gulf of Alaska and Prince William Sound, we have used wide-angle seismic reflection/refraction profiles, earthquake studies, and laboratory measurements of physical properties to determine the geometry of the Prince William and Yakutat terranes, the Aleutian megathrust, and the subducting Pacific plate. In this complex region, the Yakutat terrane is underthrust beneath the Prince William terrane, and both terranes are interpreted to be underlain by the Pacific plate. Wide-angle seismic reflection/refraction profiles recorded along five seismic lines are used to unravel this terrane geometry. Modeled velocities in the upper crust of the Prince William terrane (to 18 km depth) agree closely with laboratory velocity measurements of Orca Group phyllites and quartzofeldspathic graywackes (the chief components of the Prince William terrane) to hydrostatic pressures as high as 600 MPa (66 kbar). A landward dipping reflector at depths of 16--24 km interpreted as the base of the Prince William terrane. This reflector corresponds to the top of the Wadari-Benioff zone seismicity and is interpreted as the megathrust. Immediately beneath the megathrust is a 4-km-thick 6.9-km/s refractor, which we infer to be the source of a prominent magnetic anomaly and which is interpreted by us and previous workers to the gabbro in Eocene age oceanic crust of the underthrust Yakutat terrane. Wide-angle seismic data, magnetic anomaly data, and tectonic reconstructions indicate that the Yakutat terrane has been underthrust beneath the Prince William terrane for at least a few hundred kilometers. Wide-angle seismic data are consistent with a 9¿ to 10¿ landward dip of the subduction Pacific plate beneath the outer shelf and slope, distinctly different from the inferred average 3¿ to 4¿ dip of the overlying 6.9-km/s refractor and the Wadati-Benioff seismic zone beneath the inner shelf. Our preferred interpretation of the geophysical data is that one composite plate, composed of the Pacific plate of a fairly uniform thickness and the Yakutat plate of varying thickness, is subducting beneath southern Alaska. ¿ American Geophysical Union 1994