of more than about 100 m are too great for 

 present economic exploitation of oil and gas, 

 they may justify testing by the drill within a 

 decade. Successful exploitation of the deep- 

 water features can greatly modify the econ- 

 omy of the adjacent countries and broaden the 

 petroleum supply for the rest of the world."' 

 On one or more of the various legs, twenty- 

 one scientists, technicians and students from 

 Argentina, Brazil, the RepubHc of the Congo, 

 the United Kingdom, France, Portugal, the Re- 

 public of South Africa and Spain participated. 

 Preparatory to the cruise about 150 African 

 and other interested scientists received a 

 bathymetric atlas and preliminary reports on 

 geomagnetics, gravity and sediments. The 

 profiles and charts of geophysical data from 

 the 1972 cruise were printed and distributed 

 in January 1973. 



South American Atlantic Assessment 



A similar study, being carried out in the 

 southwest Atlantic extends from the Scotia 

 Ridge to the Caribbean. This cooperative pro- 

 gram involves scientists from the Lamont- 

 Doherty Geological Observatory, the Woods 

 Hole Oceanographic Institution, Brazil, Argen- 

 tina and France. A significant portion of the 

 work is being done from a Brazilian research 

 vessel, and scientists from both Brazil and 

 Argentina have received additional training 

 aboard ship and in residence at Lamont and 

 Woods Hole. The study includes the entire 

 western margin of the South Atlantic as a unit. 

 The thickness and areal extent of the entire 

 sedimentary section is being measured, its 

 internal structure is being analyzed, and, 

 where geological formations subcrop along the 

 ocean bottom, samples are taken in order to 

 work out a stratigraphic section. In areas of 

 great sedimentary thickness, such as the Ama- 

 zon cone, a two-ship refraction technique is 

 used in order to record the depth and attitude 

 of the acoustic basement. The prominent frac- 

 ture zones are mapped in order to answer 

 questions involving their relationship to mar- 

 gins off Africa. For example, the diapir-like 

 structures of the Sao Paulo Plateau appear to 

 be the counterpart of the diapir field off 

 Angola. The Rio Grande Rise, off Brazil — the 

 target of an investigation by French scien- 



' K. O. Emery, "Eastern Atlantic Continental Margin. 

 Some Results of the 1972 Cruise of the R. V. Atlantis 

 II." Science. Vol. 178, Oct. 20, 1972, p. 300. 



tists — may have played the same role in con- 

 trolling the salt deposition as the Walvis Ridge 

 off Africa. Indeed, in the early stages of rift- 

 ing they may have been parts of the same 

 structural feature. Further definition of the 

 several sedimentary basins off Argentina and 

 detailed study of the Malvinas (Falkland) 

 Plateau and Scotia Ridge — the active segment 

 of the southwest Atlantic continental mar- 

 gin — will add to our understanding of how 

 South America and Africa separated. 



Paleogeographic Maps — Concentrating on 

 the continental margins on both sides of the 

 South Atlantic simultaneously has broad ap- 

 peal. The exactness of the geometrical fit of 

 the continents offers a textbook example of 

 the way they may have been united (Figure 17) 

 and suggests that the process of spreading 

 must have been relatively straightforward 

 here. The concepts which evolve from a study 

 of these margins will form useful working 

 hypotheses for other, perhaps more compli- 

 cated, margins. 



A significant product of this cooperative in- 

 vestigation will be a series of paleogeographic 

 maps, one for each successive stage of the 

 "opening" of the South Atlantic Ocean. De- 

 spite the acceptance of the general principle 

 that present day geologic conditions are a key 

 to the past, it is difficult to account for either 

 the existence of 500-million-barrel oil fields or 

 the enormously thick salt deposits in the 

 geologic column. A reconstruction of geologic 

 conditions during past geologic ages may pro- 

 vide an understanding of the causes of these 

 phenomena. 



New continental margin studies may be un- 

 dertaken when international groups of scien- 

 tists recommend major areas or problems 

 which can best be investigated on a large- 

 scale, cooperative basis. 



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 

 hydrothermal rocks in the crust have been 

 dredged up from the ocean bottom near the 

 active spreading centers. Heat-flow measure- 

 ments show anomalously high values in these 



30 



