Chapter 24 Geometry of the coastline and morphology of the convergent continental margin of Ecuador

Abstract The pacific border of the South American plate presents a more or less symmetrical sinuosity, with a central concave curvature (the Arica Angle located between two side rays along Chile and Peru) and ending in convex arcs (the Patagonian and Talara arcs, respectively). The width of the continental and coastal margins varies significantly according to the geometry of the border. The continental margin of Ecuador corresponds to the northern part of the Talara Arc. Three different segments showing different coastal geomorphology and continental platform characteristics are identified from north to south: the northern segment (Mataje River–Galera Point) shows a wide continental shelf and slope, the upper subducted slab of the subduction plane presents a low dip; the central segment (Galera Point–Santa Elena) stands in front of the Carnegie Ridge, and presents a moderate uplift in the Manta Peninsula, in front of the Carnegie Ridge, and the upper subduction plane is subhorizontal; the southern segment includes the side and inner coasts of the Gulf of Guayaquil, below the gulf the subduction plane shows a low dip. A comparison with published 3D numerical modelling of curved subduction suggests that the geometry of the continental boundary has a significant effect on uplift or subsidence along the continental margin. Also, the subduction of asperities in the trench, such as the Carnegie Ridge, may change the coastal motion from subsiding to uplifting, as is observed in the Esmeraldas area. There is no clear evidence of a shelf developed during the Last Glacial Maximum (LGM) sea-level lowstand, probably due to the vertical motion – uplift or subsidence – observed all along the coastal margin.