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Geological Society, London, Special Publications; 1982; v. 10; p. 171-189;
DOI: 10.1144/GSL.SP.1982.010.01.11
© 1982 Geological Society of London

South America

Forearc and other basins, continental margin of northern and southern Peru and adjacent Ecuador and Chile

R. Moberly, G. L. Shepherd & W. T. Coulbourn

Hawaii Institute of Geophysics, University of Hawaii, Honolulu, Hawaii 96822, U.S.A.
Cities Service East Asia, Inc., 606 Cathay Bldg., Mount Sophia, Singapore 9
Geologic Research Division, Scripps Institution of Oceanography, La Jolla, California 92093, U.S.A.

The continental margin between the Peru-Chile Trench and the west coast of South America is the type example of the Andean margin, where one of the two converging blocks of lithosphere is oceanic and the other is continental. Along Peru and Chile, basement at the coast and under the landward side of forearc basins is dominantly metamorphic and plutonic. Apparently, the edge of continental crust has been stoped away during subduction. The seaward side of basins may be structurally high dams of sediments accreted and deformed by subduction, or they may be horsts of continental basement. Subsurface mapping in the on-shore and off-shore oilfields of the forearc basin of north-western Peru has shown a mosaic of faulted blocks of tensional origin, overlain by low-angle gravity glides. Details of this faulting, however, cannot be seen on single- or multi-channel seismic records of forearc basins, even where they cross the fields. Off northern and southern Peru, forearc basins are separated from one another along strike by culminations of the basement; across strike there may be more than one forearc basin. The pattern of culminations and depressions may be inherited from structures of the oceanic crust and trench. Deeper seismic reflectors of the basins generally dip landward and converge seaward, and show progressive migration shoreward of the centres of deposition. Sediment movement into the basins and down the slopes is by turbidity currents and slumps. Subduction is an efficient process, removing about four-fifths of the trench sediments.

The Progreso Basin under the Gulf of Guayaquil owes its origin to a transform fault, the Dolores-Guayaquil megashear, oblique to the trench. It is a pull-apart basin or rhombochasm opened in the wake of the northward movement of a small plate. A forearc basin at Talara and the adjacent Progreso Basin have substantial hydrocarbon accumulations.