Structures of the continental margin of Peru and Chile
Published:January 01, 1981
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Richard Couch, Robert Whitsett, Bruce Huehn, Luis Briceno-Guarupe, 1981. "Structures of the continental margin of Peru and Chile", Nazca Plate: Crustal Formation and Andean Convergence, La Verne D. Kulm, Jack Dymond, E. Julius Dasch, Donald M. Hussong, Roxanne Roderick
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Model crustal cross sections contrained by gravity and seismic refraction data indicate that pre-Cenozoic continental rocks extend to within 50 km of the trench off Mollendo and 20 km off Pisco in southern Peru and to within 30 km of the trench off northern and central Chile. The oceanic plate dips beneath the continental margin at approximately 5.1° west of Pisco, 3.8° west of Mollendo, 3.8° off northern Chile, and 4.1° off central Chile. The dip increases in the vicinity of the coast and coastal mountains in all sections. The oceanic crust decreases in thickness near the outer gravity high seaward of the trench. Magnetic anomaly 12 is identified beneath the continental slope of central Chile, and anomaly 24 is beneath the margin of northern Chile.
The presence of the marine magnetic anomalies beneath the margin, the shallow dip of the oceanic plate, and the continuity of the model structures suggest that the subduction process is relatively smooth in central and northern Chile and off Mollendo south of the Nazca Ridge. However, the computed model of the continental margin near Pisco north of the ridge indicates structural complications in the lower continental crust and/or uppermost mantle between the coast and the west flank of the Andes. The complications are likely associated with ongoing crustal deformation in the vicinity of the zone of contact between the subducting and overriding plates.
Free-air gravity anomalies and bathymetry suggest that the continental margin of Chile is segmented at intervals of approximately 280 km. However, only small structural differences are noted along the margin. Based on sediment thickness on the Nazca plate, plate-closure rates, and the interpretation of continental-margin low-density rocks as post-Paleozoic sediments, calculations indicate that the sediment now on the margin is less than 30% of the sediments transported to the margin during post-Paleozoic time. The decrease in volume of the oceanic sediments caused by compaction and dewatering (indicated by the difference in densities of the sediments) is at least partially offset by the addition of terrigenous sediments. These observations indicate that not all of the oceanic and terrigenous sediments have been accreted to the margin and that consumption of the material must occur, at least on occasion, during the subduction of the Nazca plate.