A relatively large density contrast between the Cenozoic and pre-Cenozoic rock that forms the continental margin of southern Peru causes gravity anomalies which outline the topography of the pre-Tertiary rock of the continental shelf of southern Peru. A coastal gravity high of +20 to +50 mgal extends from Mollendo to Lima and is associated with andesites and basalts of Mesozoic age and Precambrian gneisses and granodiorites that crop out along the coast. The gravity anomalies indicate that this coastal structural high extends nearly 100 km out to sea from the Paracas Peninsula southwest of Pisco. Two prominent gravity lows on the margin are those associated with the Pisco Basin on the shelf at lat 13°25'S and the Mollendo Basin on the slope at about lat 17° 10'S. Sediment thickness in the Pisco Basin indicated by the gravity anomalies is approximately 2.2 km, and the thickness of the sediments in the Mollendo Basin is approximately 4 km. A series of closed gravity lows occurs on the outer continental shelf and upper slope seaward of the coastal structural high. The lows outline relatively small depositional basins that have sediment thicknesses of approximately 1 km. Between lat 13° and 13°30'S, a marked change occurs in the character of the gravity field of the shelf and slope. The amplitude of the anomaly associated with the coastal structural high decreases abruptly, and a linear negative anomaly of less than-70 mgal, which extends northwestward, indicates a sedimentary basin on the upper continental slope.The northeastern end of the Nazca Ridge is isostatically compensated by a relatively thick crust whose layers have a slightly lower density than similar layers of the adjacent oceanic crust. Most of the increase in crustal thickness occurs in the basal crustal layer. Approximately 330 km west of southern Peru, the depth to the Mohorovicic discontinuity increases from approximately 10.5 km on the northeast side of the Nazca Ridge to 18 km under the center of the ridge, then rises again to 9.7 km southwest of the ridge. Measurements show that the free-air gravity anomaly along the trench axis has a maximum value 200 km northwest of the point of minimum bathymetric depth of the trench. This requires the rock of a mass column off the northeast end of the Nazca Ridge to be less dense than the 569 on July 15, 2015 memoirs.gsapubs.org Downloaded from 570 COUCH AND WHITSETT rock of a mass column under the axis of the trench for at least 200 km northwest of the Nazca Ridge, and is consistent with subduction of the Nazca Ridge beneath the continental margin of southern Peru.

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