Plate reconstructions, aseismic ridges, and low-angle subduction beneath the Andes

Abstract

Low-angle—subduction segments beneath Peru and Chile are believed to represent interaction of the subduction zone with the aseismic Nazca and Juan Fernandez Ridges. On the basis of symmetric sea-floor–spreading models and plate-hotspot reconstructions, predicted continuations of the relatively buoyant ridges correspond well with the limits of the low-angle–subduction segments. Further, the history of interaction of the aseismic ridges with the subduction zone, as predicted by plate reconstructions, is not inconsistent with available information on volcanic episodes of the Andes, if the contemporary correlation of volcanic gaps and low-angle subduction is also applicable to the rest of the late Cenozoic. The occurrence of deep seismicity beneath the eastern flanks of the Andes is also believed to reflect the effects of low-angle subduction. The deep-seismic segments are adjacent to segments which are presently, or were recently, experiencing low-angle subduction. Thus, the deep seismicity is interpreted as representative of remnants of “normal” oceanic lithosphere subducted before interaction of aseismic ridges with the subduction zone. Other geologic phenomena are associated with low-angle subduction beneath South America, including landward shift in loci of tectonism, surficial subsidence, and Laramide-style deformation. The aseismic-ridge—buoyancy hypothesis appears to be a valuable predictive tool for the interpretation of such geologic effects.