Geophysical measurements over the eastern end of the Tehuantepec Ridge and adjacent continental margins of southern Mexico and northern Guatemala indicate that the ridge is a fracture zone and that it marks the boundary between two different subduction provinces. A positive free-air gravity anomaly whch extends northwestward along the outer continental shelf of Guatemala curves abruptly landward in the Gulf of Tehuantepec. The positive shelf anomaly is on trend with the positive anomaly of the Nicoya Penninsula, Costa Rica, and suggests that rocks genetically related to Cretaceous rocks of the Nicoya Complex extend northwestward along the continential shelf to the Gulf of Tehuantepec. A crustal and subcrustal cross section of the continental margin of Guatemala, constrained by gravity, magnetic, and seismic refraction data, indicates that the rock strata causing the outer shelf gravity high dip land ward, consistent with imbricate thrusting of the oceanic crust beneath and into the continential margin. A model crustal cross section of the continental margin of southern Mexico, north of the Tehuantepec Ridge, shows a markedly different margin structure with a relatively small amount of continental accretion and a continental crustal block extending to within approximately 25 km of the trench axis.