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 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.

The morphology and shallow structure of the Peru continental margin has been mapped using bathymetric and seismic reflection profiles from lat 6°S to 16°S. Other geophysical and geologic data are used to constrain interpretations of the margin’s deeper structure and to relate the offshore to the onshore Andean geology. Two prominent structural ridges, subparallel to onshore Andean trends, control the distribution of the offshore Cenozoic sedimentary basins. The Coastal Cordillera, which surfaces north of lat 6°S and south of lat 14°S, can be traced onto the offshore as an Outer Shelf High (OSH); it is evidently cored with Precambrian and Paleozoic metasediments and crystalline rocks. A series of shelf basins is situated between the Coast Range/OSH and the Andean Cordillera: from north to south, these are the Sechura, Salaverry, and East Pisco Basins. A second set of upper-slope basins flanks the Coast Range/OSH to the southwest, limited seaward by an Upper-Slope Ridge (USR) of deformed sediment: from north to south, these are the Trujillo, Lima, and West Pisco Basins. The Yaquina Basin lies within divergent arms of the USR. The shelf and upper-slope basins are set on continental massif. An anastomosing network of elongate ridges and ponded sediments is the surficial expression of the subduction complex, which apparently begins just seaward of the USR. The effect of the late Paleocene/Eocene Andean orogeny has been extrapolated offshore as a distinct interface of seismic velocity in the Salaverry Basin. Though Cenozoic marine sedimentation in the shelf basins did not begin until after this event, sedimentation in the upper-slope Trujillo Basin may have been more continuous through the early Tertiary. In the Trujillo Basin, the bulk of the nearly 4 km thick sedimentary section is of Paleogene age, while in the adjoining upper-slope Lima Basin to the southeast, the bulk of the nearly 2 km thick sedimentary section is of late Miocene or younger age. Apparently, post-Oligocene tectonism caused uplift, deformation, and a gross reduction of sedimentation in the Trujillo Basin; this event is evidenced by boundaries of differential structural deformation in seismic reflection profiles. In middle to late Miocene time, while orogenic activity affected the inland Andean Cordillera, the upper-slope Lima Basin subsided and began its depositional record. Unconformites in shelf basins apparently reflect the inland tectonism at this time. The boundary between the Lima and Trujillo Basins, and between the contrasting styles of upperslope tectonic movement, is near lat 9.5°S, coincident with the present day intersection of the Mendaña Fracture Zone with the continental margin. A final phase of upper-slope deformation closed the Pliocene. Like earlier tectonic activity, the major break in structural style of this epoch occurs near lat 9.5° S: compressional faulting and folding characterize the younger sediments of the Trujillo Basin, while the Lima Basin appears as a broad, open syncline, distrubed only in its southernmost occurrence.

Abstract Along the coast of Peru, the shelf between the Ancles mountains and the 100-fm isobath is narrow and reaches a width of 100 km in few places. Between lats. 6 and 14°S, nearly all the shelf lies offshore. This shelf was the site of marine deposition throughout the Tertiary. Basement highs and irregularities define six subbasins. Beginning in the north, the mainly offshore Progreso basin extends into Peru from the Gulf of Guayaquil. It is filled with upper Tertiary shales, sandstones, and conglomerates with a maximum thickness of 6,000 m. The small, abandoned Zorritos field is located in this basin, and some oil and gas have been discovered recently. The basin has an estimated potential of 350 million bbl. The Talara basin, which has produced over 800 million bbl of oil, is both onshore and offshore. The strata are Upper Cretaceous and lower Tertiary silicate clastic rocks with a maximum thickness of 8,000 m; nearly all production comes from Eocene deltaic, fluvial, and turbidite sandstones. Intense block faulting, gravity sliding, and submarine slumping complicate development operations. The onshore part does not have significant potential for future discoveries, but the offshore is estimated to have a potential of 1 billion bbl. The Sechura basin is situated between the Andes and a discontinuous coastal chain of low mountains which separates it from the Talara basin. It is mostly onshore but extends offshore to the south. Up to 3,000 m of marine, brackish, and nonmarine deposits, including diatomite and phosphatic and tuffaceous elements, fills the basin. The most widespread sedimentary rocks are upper Tertiary, but lower Tertiary and Cretaceous rocks are also present. In the 1950s, 28 wildcat wells were drilled and moderate reserves of gas were discovered. Total potential for the basin is estimated to be 100 million bbl. The Salaverry basin, the largest of the coastal basins, has a length of 500 km and a width up to 100 km. Entirely offshore, it extends southward to about 100 km north of Lima. It contains up to 3,000 m of Tertiary marine shales, siltstones, and calcareous rocks with sandstones at the base. Two wells have been drilled in this basin. The estimated potential for the basin is 500 million bbl. The Pisco basin begins about 100 km south of Lima in the offshore, but splits into onshore and offshore parts southward. Up to 2,000 m of lower and upper Tertiary rocks are present. The lower Tertiary is composed of conglomerates, sandstones, and calcareous shale. The upper Tertiary consists of strata similar to those of the upper Tertiary in the Sechura basin. One well has been drilled in the basin. The estimated potential for the basin is 100 million bbl. The Moquegua basin is a narrow onshore basin between the Andes and the coastal chain of mountains. Marine strata are found only in the northern part and are of insufficient volume to have significant potential. The total potential of the coastal basins is estimated to be about 2 billion bbl.