Two-dimensional crustal models of northwest South American margins presented in this thesis are based on geologic, seismic, and potential field data.

The Romeral fault system separates dense high-velocity oceanic crust to the west from lighter lower velocity continental crust to the east. Accreted and obducted oceanic materials lie west of the Romeral suture zone. Pronounced gravity and seismic gradients exist across much of the fault system.

The geophysical and geological data in southwestern Colombia could be explained by obduction of an oceanic ridge 70 km to the east over continental crust during Cretaceous to Paleogene time. During the Tertiary, subduction moved 200 km westward to the Colombian trench while thin-skinned folding and thrusting moved 70 km eastward uplifting Paleozoic and Precambrian metamorphics of the Eastern Andes. The Benioff zone is clearly defined. The crust under the Altiplano was thickened to 50 km by magmatic differentiation, overthrusting, and flexural loading. Positive gravity anomalies along the coastline are explained by ophiolitic obduction rather than by phase changes in the descending slab. The Guaitara (Romeral?) fault dips approximately 12^o to the west into the lower crust under the Western Andes.

In northern Colombia, because convergence is much slower and sedimentation is more rapid, up to 10 km of Tertiary fluvial, lacustrine, turbidities and carbonate sediments, and small isolated volumes of obducted cherts and volcanic rocks have been preserved in the North Colombian fold belt. Subduction features are masked by recent sediments which are also involved in deformation, folding, and faulting.

The devastating Popayan earthquake of March 31, 1983, and the microseismicity along a northern segment of the Romeral fault system demonstrated that the Romeral suture zone is still active. Composite focal mechanism determinations for the northern segment of the fault were compatible with left-lateral movement on a north-south fault plane and a NW-SE principal stress direction.

Deformation in the North Panama fold belt is associated with underthrusting of the Isthmus by the Caribbean plate.