is that the crest of the East Pacific Rise is not a smooth, broad 

 arch without a well-defined topographic axis as has been 

 thought in the past. Rather, the topography is very rugged with 

 local relief of several hundred meters. The axis is denoted by a 

 well-defined topographic block, 25 to 35 kilometers wide, 

 standing about 200 meters above the surrounding region. 

 Areas adjacent to the axial block display topographic steps 

 descending from the axis, somewhat similar to the topography 

 present on some portions of the Mid-Atlantic Ridge. 



Other preliminary results of this study include the discov- 

 ery of unexpectedly large magnetic anomalies encountered at 

 latitudes 6'S near the magnetic equator, thereby allowing defi- 

 nition of the more recent magnetic events. Spreading half-rates 

 for the area calculated from these initial data are far higher 

 than earlier estimates. 



Geological sampling further suggests that metalliferous 

 sediments rich in transitional elements (Co, Cu, Fe, Mn, Ni) 

 are abundant on the East Pacific Rise, with measured accumu- 

 lations of up to 17 meters. The Bauer Deep, an elongated 

 sediment trap on the eastern flank of the East Pacific Rise, 

 contains the highest concentration of these transitional me- 

 tals so far measured in open-ocean metalliferous sediments. 



The continental shelf, slope, and Peru-Chile Trench be- 

 tween latitudes 7°S and 17°S were surveyed in a reconnais- 

 sance fashion by OSU's RV Yaquina using seismic reflection 

 profiles. HIG's RV Kana Keoki also conducted separate and 

 joint work in this area. 



Acoustic basement was traced from the Nazca Plate 

 into the Peru Trench and beneath the lower continental slope 

 for a distance of about 10 kilometers. In some areas, a broad 

 zone of deformation occurs over a distance of 10 kilometers in 

 the trench-margin complex. Uplifted turbidite basins separated 

 by a long linear volcanic ridge characterize the trench axis. 



Deformed calcareous sediments from the Nazca Ridge 

 were discovered in the Peru Trench-Margin complex suggest- 

 ing that Nazca Plate sediments have been scraped off onto 

 the lower continental slope landward of the trench axis. 



Field investigations are now continuing with both the RV 

 Kana Keoki and the NOAA RV Oceanographer participat- 

 ing in detailed studies. 



Plate Tectonics and Metallogenesis Bibliography 



Burnett, W. C, H. H. Veeh, and A. Soutar. Geochemistry and 

 Radiometric Ages of Sea-Floor Phosphorites from the 

 Continental Margin of Peru and Chile, Geol. Soc. Ainer. 

 Abst. 5(1): 18-19, February 1973. 



Dorman, L. M., R. K. Lattimore, and G. Peter. NOAA-IDOE 

 Equatorial Atlantic Gravity Investigations, EOS, Trans., 

 Amer. Geophysi. Union 54(4): 326, April 1973. 



Corliss, J. B. Volcanism and Ore-Formation Related to the 

 Subduction of Oceanic Crust: The "Geostill" Concept, 

 Geol. Soc. Amer. Abstr. 5(1): 26, February 1973. 



Dymond, J., G. R. Heath, J. B. Corliss, C. W. Field, E. J. 

 Dasch, and H. H. Veeh. Elemental and Isotopic Geo- 

 chemistry of Metalliferous Sediments on the East Pacific 

 Rise, Geol. Soc. Amer. Abstr. 5(1): 36, February 1973. 



Fein, C. D. Chemical Composition and Submarine Weathering 

 of Some Oceanic Tholeiites from the East Pacific Rise, 

 Geol. Soc. Amer. Abstr. 5(1): 41, February 1973. 



Hussong, D. M., P. Edwards, S. H. Johnson, and J. F. Camp- 

 bell. Crustal Structure of the Eastern Nazca Plate and 

 Peruvian Continental Shelf from Seismic Refraction, 

 Geol. Soc. Amer. Abstr. 5(1): 61, February 1973. 



Hussong, D. M., S. H. Johnson, G. P. Woollard, and J. F. 

 Campbell. Crustal Structure of the Nazca Plate; 1972 

 Preliminary Report. £05, Trans. Amer. Geophys. Union 

 53(4): 413, April 1972. 



Kulm, L. D., D. M. Hussong, R. A. Prince, R. W. Couch, and 

 K. S. Scheidegger. Deformation in the Peru Trench, Geol. 

 Soc. Amer. Abstr. 4(7): 570, October 1972. 



Kulm, L. D., R. A. Prince, K. F. Scheidegger, R. W. Couch, 

 A. Masias, and D. M. Hussong. Shallow Structure of the 

 Peru Continental Margin and Trench 7° -12° S, Geol. 

 Soc. Amer. Abstr. 5(1): 68, February 1973. 



Lattimore, R. K., O. E. DeWald, and G. Peter. NOAA-IDOE 

 Equatorial Mid-Atlantic Ridge Investigations, EOS 

 Trans. Amer. Geophys. Union 54(4): 326, April 1973. 



Northrop, J. Seaquakes on the Nazca-Cocos Plate Boundary. 

 Geol. Soc. Amer. Abstr. 5(1): 87, February 1973. 



Peter, G., O. E. DeWald, G. Merrill, and L. M. Dorman. 

 NOAA-IDOE Equatorial Atlantic Studies 1971, EOS. 

 Trans. Amer. Geophys. Union 54(4): 326, April 1973. 



Rea, D. K., and L. W. Kroenke. The East Pacific Rise Crest at 

 6° and 11° S., Geol. Soc. Amer. Abstr. 5(1): 54, Febru- 

 ary 1973. 



Rose, J. C, and R. W. Couch. Gravity Measurements over the 

 Nazca Plate and Its Boundaries, Geol. Soc. Amer. Abstr. 

 5(1): 57, February 1973. 





-"^-^^^^s:^ 



^rA '^-^'^^'^J^^-^- 





Lamont Doherty Geological Observatory scien- 

 tists photographed a large accumulation of ferro- 

 manganese nodules on the seafloor 5,320 m deep 

 in the Central South Pacific. 



33 



