Quaternary uplift of the peruvian coast related to the subduction of the Nazca Ridge: 13.5 to 15.6 degrees south latitude

Abstract

Marine terraces between 13.5 and 15.6°S. latitude on the Peruvian coast preserve a history of Quaternary sea-level fluctuations and forearc movement above the subducting Nazca Ridge. Field mapping and amino acid racemization/epimerization, electron spin resonance, radiocarbon, and uranium-series disequilibrium ages from mollusk shells from terrace deposits are used to define a marine terrace chronology. Terraces on the Peruvian coast formed during global high stands of Quaternary sea level, in a similar way to the marine terraces on the west coast of the United States, and Baja California, Mexico. A last interglacial marine terrace (125 ka) is preserved on nearly every terrace flight. Along the coast from northeast to southwest, each terrace varies in elevation because of variations in the rate of coastal uplift. Variations in uplift rate from northeast to southwest in the study area are explained by a simple geometric model in which the variables are the rate of plate convergence, morphology of the Nazca Ridge, and orientation of the ridge relative to the Peruvian coast. The oblique northeastward orientation of the ridge relative to the east-west direction of plate convergence results in a ca. 70 mm/yr southeastward migration of the zone of Nazca Ridge subduction beneath the Peruvian coast. The coastal area above the northern flank and crest of the Nazca Ridge is stable or has undergone net subsidence, while the coast above the ridge crest is rising at about 0.3 m/kyr. Above the southern flank of the Nazca Ridge, the coast is rising at a rate of ca. 0.5 m/kyr, the fastest rate determined from studies of marine terraces on the coasts of Peru and Chile. South of the influence of the Nazca Ridge, uplift rates are ⩽ 0.2 m/kyr. The pattern of faster uplift above the southern flank and slower uplift above the northern flank of the ridge is a predictable consequence of oblique ridge subduction.