Topography and structure of the Peru-Chile trench

Abstract

During the Scripps Institution's IGY Expedition Downwind of 1957–1958 the research vessels Horizon and Spencer F. Baird spent 41 ship-days conducting geological-geophysical studies in two portions of the Peru-Chile Trench, concentrating their efforts off northern Chile and central Peru. Data from 5400 miles of echo-sounding traverses and eight seismic-refraction stations (three off Peru, five off Chile) are here reported. The peru-Chile Trench, lying off southern Ecuador to central Chile, is interrupted off southern Peru by the northeast-trending Nasca Ridge. North of Nasca Ridge the trench reaches a maximum depth of nearly 6500 m. Characteristically, the trench floor is flat; this flatness is attributed principally to land-derived sediment, introduced by the intermittenly-flowing rivers of the coastal zone, passing down the trench flank. South of Nasca Ridge, off the Atacama Desert, relatively little sediment reaches the narrow trench bottom. Here the maximum depth is slightly more than 8000 m. South of 27° 30′S the trench shoals in a series of nearly flat-floored basins. River-transported solid phases here form a major portion of the trench-floor sediment. Seismic refraction studies of two sections across the trench off Chile and Peru are very similar in that both show that crustal thickening begins westward of the trench and reaches a thickness of about 11 km beneath the trench axis. The Mohorovičić Discontinuity, which is found at a depth of about 17 km below sea level at the trench axis, plunges steeply to the east with a slope not inconsistent with the depths of about 65 km beneath the Andes, estimated from the observations of Tuve and Tatel.