Graduate Thesis Or Dissertation

 

Morphology, shallow structure, and evolution of the Peruvian continental margin, 6⁰ to 18⁰ S Public Deposited

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  • Detailed bathymetric survey data were collected along the Peruvian continental margin and were compiled by the author and other investigators to construct a new bathymetric map for the area between 6° to 18°S latitude. Based on this map and individual bathymetric profiles, the continental shelf topography is essentially flat. Four different physiographic provinces (A-D) are defined on the continental slope. Each province apparently reflects the structural and tectonic settings in a given area. The trench is separated into three provinces on the basis of regional depth differences. Three sedimentary basins, Sechura, Salaverry, and Pisco, are recognized on the Peruvian continental shelf between 6° to 15°S. The landward migration on the axis of deposition within these basins is interpreted to be the sedimentary response to tectonism (uplift) taking place along the outer edge of the continental margin. Prominent sedimentary basins also occur on the upper continental slope; these basins have been named with respect to their geographic position as the Lima and Arequipa Basins. Landward and seaward migration of the axis of maximum deposition in these basins is interpreted to be the result of variable rates of uplift along the outer margin. Uplift along the seaward edge of these basins suggests that the accretion of trench and oceanic plate deposits is taking place along the lower and middle continental slope. Seismic reflection profiles, extrapolation of regional structural trends onshore to offshore along the Peruvian margin, and positive free-air gravity anomalies off southern Peru (Whitsett, 1975), show that an outer continental shelf high is present off northern and central Peru. This high is believed to be composed of Paleozoic rocks with a possible Precambrian core. The high is linked with the Amotape Mountains in northern Peru and the Coastal Ranges in southern Peru. It is an important element in the development of the Peruvian continental margin. Using all of the data available, a four stage model is proposed for the Mesozoic-Cenozoic evolution of the Peruvian continental margin. Stage I describes conditions prior to the formation of the subduction zone during Triassic time. Plate collision is postulated at the beginning of Stage II with the formation of a Benioff Zone about 180 m.y. ago (Triassic-Jurassic boundary). Accretion of trench and oceanic plate deposits occurs as a result of the initiation of under-thrusting of the South America Block by the Nazca Plate. Stage III describes the continuous seaward growth of the continental slope during middle to late Cretaceous time. During late middle (Turonian) and middle late Cretaceous (Santonian) time diastrophism in southern Peru restricted the marine conditions to central and northern Peru. During late Cenozoic time (Stage IV) the Peruvian margin attained its present configuration through continuous seaward growth of the continental slope and buildup of the sedimentary sequences found in the Sechura, Salaverry, and Pisco Basins, in central and northern Peru. Late Cenozoic volcanism in southern Peru is apparently associated with the large amount of sediments that reached the southern Peru Trench since late Cretaceous time. The Nazca Ridge apparently approached the Peruvian continental margin during Cenozoic time, and locally changed the morphology of the continental slope. The proposed model shows that the Peruvian continental margin is growing seaward and that continental erosion of the crystalline continental block is not necessary along the Peruvian continental margin. The model explains and justifies the presence of the outer continental shelf high off northern and central Peru.
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