Le Pichon, X., and D. E. Hayes. Marginal Offsets, Fracture 

 Zones and the Early Opening of the South Atlantic, /. 

 Geophys. Res. 76(26): 6283-6293, 1971. 



Leyden, R., M. Ewing, and E. S. W. Simpson. Geophysical 

 Reconnaissance on African Shelf; 1. Cape Town to East 

 London, Anier. Assoc. Petrol. Geol. Bull. 55: 651-657, 

 1971. 



Leyden, R., G. Bryan, and M. Ewing. Geophysical Recon- 

 naissance on African Shelf; 2. Margin Sediments From the 

 Gulf of Guinea to the Walvis Ridge, Amer. Assoc. Petrol. 

 Geol. Bull. 56: 682-693, 1972. 



Ludwig, W. J., J. L Ewing, and M. Ewing. Structure of the 

 Argentine Continental Margin, Amer. Assoc. Petrol. Geol. 

 Bull. 52(12): 2337-2368, 1968. 



Miura, K., and J. C. Barbosa, Geologic Plataforma Continental 

 do Maranhao, Piaui, Ceara e Rio Grande do Norte, Trans. 

 26th Brazilian Geol,Congr.. in press, 1973. 



Talwani, M., and O. Eldholm. Boundary Between Continental 

 and Oceanic Crust at the Margin of Rifted Continents, 

 Nature 291: 325-330, 1973. 



Plate Tectonics and Metallogenesis 



Geological processes operating along mid-oceanic ridges 

 and active trenches may be responsible for the generation of 

 heavy metal ore deposits. Metalliferous sediments and hydro- 

 thermal rocks in the crust have been dredged up from the bot- 

 tom near the active spreading centers. Heat-flow measurements 

 show anomalously high values in these zones suggesting that 

 the metal-rich crust and overlying sediments emanate from the 

 rift and move toward the active trenches. Preliminary isotope 

 studies suggest that ore bodies in igneous rocks above subduction 

 zones result from partial melting of the subducted crust — the 

 melting of which in conjunction with upward transport can be 

 considered the second stage of a two-stage geochemical enrich- 

 ment process. Since such ore bodies are known only on land, 

 the study of metallogenesis on the margin edges will be a partial 

 contribution to the understanding of a much larger problem. 



Nazca Plate 



The Nazca Plate Project was initiated in May of 1971 to 

 gain more definitive knowledge of the oceanic rift-plate-trench 

 tectonic cycle, and to ascertain the degree to which plate tec- 

 tonic theory influences metallogenesis and can be used as a guide 

 for economic mineral exploration and earthquake prediction. 

 This continuing investigation, utilizing marine geophysical ex- 

 ploration techniques, includes both the study of the origin of 

 metals on the seafloor and the eventual development of continen- 

 tal ore deposits as a consequence of crustal subduction at the 

 continental margin. 



The Nazca lithospheric plate (fig. 19) is a well-defined, 

 self-contained tectonic entity which spans both a sufficient time 

 interval and a large enough areal extent to encompass most of 

 the elements of the plate tectonics model. Nevertheless, the 

 region is small enough to study as a unit in a reasonable time 

 period, and so is ideally suited to this type of comprehensive 

 investigation. The diverging edge at the East Pacific Rise has 

 been identified as a potentially important locality of minerali- 

 zation of the crust and overlying sediment, whereas the con- 

 verging edge at the South American continental border exempli- 

 fies all the major effects of continental convergence, from vol- 

 canism through shallow- to deep-focus earthquakes to extensive 



_ A 



110° 



90° 



<r 







I 

 I 



' GALAPAGOS rIfT 



V 





BAUER 



D 



DEEP 



SALA-Y- GOMEZ 

 RIDGE 



'^o;V'"- 



> 



r 

 > 



> 

 o 

 o 



m 



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Figure 19. — Nazca Plate. 



deposits of base metals. The time frame for this cooperative 

 international investigation is that originally proposed — 5 years 

 of field investigations, terminating in 1976, followed by 3 addi- 

 tional years of laboratory work and interpretation and integra- 

 tion of data, terminating in 1979 with completion of the final 

 synthesis. 



During the past year the Hawaii Institute of Geophysics 

 (HIG), University of Hawaii, and Pacific Oceanographic Lab- 

 oratory (POL)* of the National Oceanic and Atmospheric Ad- 

 ministration, cooperated in the field effort to collect data on the 

 Nazca Plate. They were assisted by scientists from elsewhere in 

 the United States, Colombia, Ecuador, Peru, and Chile. The 

 effort concentrated on investigating the dynamic processes at 

 the plate boundaries. HIG and POL completed simultaneous 

 geological studies of selected regions of the Nazca Plate, and 

 cooperated in joint (two-ship) seismic refraction experiments 

 designed to reveal both shallow and deep crustal structure of 

 these regions. The NOAA ship Oceanographer was equipped 

 with a seismic refraction laboratory. 



The RV Kana Keoki (HIG) departed Honolulu on No- 

 vember 8, 1972 for cruise 72-11-08 and began the IDOE por- 

 tion of the cruise upon departure from Tahiti on January 21, 

 1973. Except for a short diversion to participate in the Narino 

 project (an NSF-funded cooperative seismic refraction investi- 

 gation of the structure of the Andes Mountains having direct 

 bearing on Nazca Plate problems), the Kana Keoki worked 

 exclusively on the Nazca Plate Project until returning to Tahiti 

 on May 30, 1973. 



The Oceanographer (POL) departed Seattle on February 



Pacific Marine Environmental Laboratory (PMEL) 



24 



