The continental margin of southern Mexico north of the Tehuantepec Ridge and south of the Rivera Fracture Zone is a narrow crustal transition zone. The margin lacks a well-developed forearc basin and the active volcanic arc is subparallel to the trench and not clearly related in a normal fashion to the Benioff Zone. A geological survey of a small part of the margin 100 km south of Acapulco reveals significant variations in structure over short distances. Multi-channel seismic reflection data, sonobuoy refraction data and dredging indicate that the shelf and upper slope are underlain by continental crust. Geometry derived from the reflection data suggests that the continental crust is less than 10 km thick at its seaward termination. Migrated reflection data reveal structures on the lower slope and within trench axis turbidite fill which are interpreted as folds and thrust faults. Structures and landward dipping reflections suggest that the base of the slope is an accreting zone with trench turbidite fill being uplifted and emplaced. Both reflection and refraction data suggest the trench inner wall contains both slope deposits and uplifted trench deposits. There is no evidence of significant amounts of oceanic crust within the slope. Volume calculations reveal that some of the sedimentary rocks are being consumed along with the oceanic crust.