Structural evidence of the evolution of fore-arc basins off South America

Abstract

The continental margin of southern Peru and northern Chile was surveyed during the 1973 and 1974 expeditions of the research vessel Kana Keoki. Seismic reflection profiles reveal three large basins at about 1000 m depth between Mollendo, Peru (17°00′ S) and Iquique, Chile (20°00′ S). Only small basins and discontinuous terraces are seen on profiles crossing the Iquique-to-Antofagasta, Chile (23°30′ S) segment of the continental margin.The structural cross-sections of the basins resemble those of arc-trench gaps. The undeformed uppermost reflectors probably represent turbidites, as evidenced by displaced shallow-water benthic foraminifera and coarse sands in cores. Deeper reflectors are generally inclined land-ward, with dips and deformation increasing in the lower reflectors down to about 1.5 seconds penetration. Seaward convergence of these reflectors indicates a progressive shoreward migration of the axis of maximum sedimentation. If the deeper beds are also turbidites, this axis marks the axis of the sediment trap on the continental slope.The structure is consistent with subduction of an oceanic plate and obduction of a portion of its sediment cover. The imbricate stacking of obducted material is lifting an anticlinal ridge visible in most traverses across the trench side of the basin. The growing ridge is deforming the older sediment trapped in the upperslope basin and shifting the locus of deposition shoreward. The irregular distribution of the basins apparently is a product of culminations and depressions of the surfaces of imbricate fault planes. These undulations may result from the transference of the irregular structure of the oceanic plate to the face of the continental block.