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. / Graduation date: 1976
| Identifer | oai:union.ndltd.org:ORGSU/oai:ir.library.oregonstate.edu:1957/29011 |
| Date | 07 August 1975 |
| Creators | Whitsett, Robert Manning |
| Contributors | Couch, Richard W. |
| Source Sets | Oregon State University |
| Language | en_US |
| Detected Language | English |
| Type | Thesis/Dissertation |
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