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Present address: Geological Research Division, A-015, Scripps Institution of Oceanography, University of California, San Diego, La Jolla, California 92093.
Hawaii Institute of Geophysics Contribution 1105

The morphology and shallow structure of the region bracketed by lat 18°S and 23° S off the South American coast shows that the Nazca plate landward of an outer swell and seaward of the Peru-Chile Trench is broken along an anastomosing network of fault scarps. In contrast, ridges that are discontinuous and variable in strike characterize the continental margin. These structural highs form culminations and depressions along strike and have no magnetic signature. The ridges are uplifted relative to depressions along their landward flanks, and seismic reflectors from basin-filling sediment document a history of rotational movement. Free-air gravity values suggest that the loci of maximum deposition subdivide the three forearc basins of the Arica Bight into subbasins.

Correlation between onshore and offshore tectonic units is tenuous, but the onshore geology suggests that part of the continental crust between the coast and the trench may have been removed. Interpretations of the shallow structure between the coast and the trench, based on single-channel seismic reflection profiles, suggest that the continental margin may either be underlain by granitic fault blocks, or composed of a wedge of deformed and dewatered sediment. The evidence at hand describing the South American subduction zone does not allow an unequivocal choice between the alternatives of subcrustal tectonic erosion of the continental edge or imbricate accretion of a wedge of sediment. Gravity may be the all-important force controlling the form of the hemipelagic cover. Both accretion and tectonic erosion may be acting to some degree with varying quantities of accreted sediment pasted against the disrupted face of the South American plate.

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