Gravity measurements and their structural implications for the continental margin of southern Peru Public Deposited

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  • A free-air gravity anomaly map of the continental margin of Peru between 12° and 18° S. Lat. shows a -110 to -220 mgl anomaly associated with the Peru-Chile Trench, a -60 mgl anomaly over the Pisco Basin on the continental shelf, and -120 mgl anomaly over the Mollendo (or Arequipa) Basin on the upper continental slope. Anomalies observed over the continental slope and shelf consist of slope and basin anomalies superposed on a very large, broad regional anomaly. The approximately zero mgl anomaly observed in the region of the Nazca Ridge indicates the ridge is isostatically compensated. A structural model constrained by the observed gravity anomalies and seismic refraction data indicates that compensation is due to a crust approximately 8 km thicker and about 0. 04 g/cm³ less dense than the oceanic crust on either side of the Nazca Ridge. Gravity anomalies are consistent with mass distributions expected at the Peru-Chi1e Trench as a consequence of subduction of the Nazca Ridge and the Nazca Plate. Crustal and subcrustal cross sections constrained by free-air gravity anomalies, seismic refraction data, and geologic information indicate approximately 2 km of crustal thinning seaward of the trench on the southeast side of the Nazca Ridge but no crustal thinning on the northwest side of the ridge. Crustal thickness increases from approximately 10 km near the trench to about 25 to 30 km under the southwestern flank of the Andes and to approximately 70 km under the Andes. The crust is inferred to be 33 km thick under the Amazon Basin. A cross section north of the Nazca Ridge suggests a rupture of the crust at depth under the coast mountains, and earthquake hypo centers projected onto this cross section indicate a relatively shallow, nearly horizontal Benioff zone under the Andes and the Amazon Basin. A cross section south of the Nazca Ridge does not show these features, hence a different subduction process on each side of the Nazca Ridge is indicated. Free-air gravity anomalies indicate a structural high extending northwest from 17° S. Lat, along the coast, the Paracas Peninsula and nearly 100 km offshore along the edge of the continental shelf. Computations based on gravity data suggest the Pisco Basin immediately east of this structural high contains approximately 2. 2 km of sediment. A similar computation for the Mollendo Basin yields a sediment thickness of approximately 1.4 km. Gravity anomaly patterns are consistent with uplift beneath the continental shelf edge and upper slope and suggest a continental margin composed of compacted, dewatered sediments of both continental and oceanic origin.
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