Between 1°N and 3°S the South American trench system is obliquely subducting oceanic crust of the Nazca plate at about 90 mm/year. In the past 3 × 10⁶ years, entry of the shallow, thick Carnegie Ridge into the subduction zone has shoaled the trench off Ecuador to a minimum axial depth of 2,920 m, and has modified its structure and sedimentation. Seismic reflection profiles and bottom samples show that the seaward side of the trench is part of the eastern Carnegie Ridge. It is composed of lower Miocene igneous rock with a thick though variably dissected blanket of calcareous and siliceous pelagic sediments, and has been deformed into an outer rise, a smoothly convex upper oceanic slope, and a lower slope 10 km wide broken by large (200 to 300 m of set) normal step faults. Although there are local turbidity-flow and debris-flow deposits in the trench axis, there is no thick, extensive turbidite lens because of limited terrigenous supply, an uneven structurally controlled long profile, and the presence of a fast thermohaline current which sweeps loose sediment out of the northern part of the trench.

Most of the landward side of the trench is subaerially exposed, except for trench-slope basins in the Gulf of Guayaquil and the steep inner wall, which has pelagic sediments scraped off the down-going oceanic plate. A terminology derived from other arc-trench systems is applied to the structures of Ecuador, where there are oil and gas reserves in trench-slope, fore-arc, and back-arc basins. The well-mapped lithologies and patterns of faulting in these basins and intervening ridges may help interpretation of less accessible homologs elsewhere. The history of this cordilleran subduction system can be interpreted only by combining the evidence from marine geophysics and continental geology.