Figure 18. — Survey tracks for African Atlantic 

 Margin Study. 



NOAA Environmental Data Service's National Geophysical 

 and Solar-Terrestial Data Center cover 24,165 nautical miles, 

 including bathymetric, magnetic, gravity, seismic, and 3.5-kHz 

 echo sounder data. Profiles across the continental margin and 

 adjacent deep-sea floor from Congo Canyon to Cape Francis, 

 South Africa, completed in 1972, are described by Elazer Uchupi 

 and K. O. Emery, in "Reflection. Magnetic, and Gravity Profiles. 

 The Eastern Atlantic Continental Margin and Adjacent Deep- 

 sea Floor. T. Cape Francis (South Africa) to Congo Canyon 

 (Republic of Zaire)," Woods Hole Oceanogr. Inst. Tech. Rep., 

 Ref. No. WHOI-72-95, 1972 (one page of text and nine plates). 

 Marine geophysical data received from this activity last year 

 by NOAA Environmental Data Service's National Geophysical 

 and Solar-Terrestrial Data Center cover 26,300 nautical miles, 

 including bathymetric, magnetic, gravity, seismic, 3.5-kHz echo 

 sounder, and sonobuoy data. 



Southwest Atlantic Margin 



The margins of the South Atlantic are ideally suited to 

 the testing of models of pre-drift reconstruction, because of 

 the "exactness" of the geometrical fit of South America and 

 Africa (Bullard et al. 1965) and the presence of numerous 

 structures trending normal to the coastlines, which may have 

 counterparts at conjugate positions off opposing coastlines 



of the once-joined continents (Ludwig et al. 1968; Leyden et al. 

 1971). Information gained from the study of these margins 

 should help to form working hypotheses for other, perhaps 

 more complicated, margins. The present IDOE study of the 

 continental margin of Argentina and Brazil and the com- 

 plementary program of study of the African margin by Woods 

 Hole Oceanographic Institution are expected to provide clues 

 to basic scientific questions regarding the mechanism of rifting 

 of a continental land mass and the effects of rifting on the 

 margins. Among these questions are: 



1. Where is the true edge of the continent and what is 

 the nature of the transition zone between continental and 

 oceanic crust? 



2. When and how did the South Atlantic Ocean open, and 

 what effects were produced on the rifted continents? 



3. What remnants of structures of the once-joined con- 

 tinents can be found on the present-day margins? Offshore 

 sedimentary basins containing evaporites (salt) of Early Cre- 

 taceous age trend normal to the coastline of West Africa. 

 Piercement structures in sediments that may be salt diapirs 

 have been found off Brazil. Are these features related? To 

 what extent is the structural framework of one continent a 

 mirror image of the other? 



4. What is the origin of oceanic fracture zones and how 

 does the pattern of fracture zones determine the geologic 

 setting of the continental margins? Francheteau and Le Pichon 

 (1971) and Le Pichon and Hayes (1971) suggest that the struc- 

 tural framework of the margin off Argentina-Brazil and West 

 Africa has been controlled by marginal ridges that are the 

 prolongations of fractures imposed onto the continental margin 

 during separation. Such marginal ridges, in the early stages of 

 rifting, may have acted as barriers to oceanic circulation and 

 thus create an environment for the precipitation of salt in small 

 basins. 



5. In a precise fit (or reconstruction) of South America 

 and Africa, where are the paleo-positions of the Falkland 

 Islands and the Falkland Plateau? 



6. What is the relationship of the Falkland Plateau, Falk- 

 land (Malvinas) basin, and the North Scotia ridge? Does the 

 Falkland Plateau -Malvinas basin system constitute a modern 

 mio-eugeosynclinal couple? Is the North Scotia ridge a frag- 

 mented and dispersed section of the Andean Cordilleran system? 



The approach to these problems has been to conduct 

 an integrated program of geophysical measurements, primarily 

 vertical reflection, magnetics, and gravity, with additional sup- 

 port from bathymetry and station work in deep water. Instru- 

 ments for station work include a rock dredge, piston corer, 

 bottom camera, nephelometer, and thermograd. Seismic refrac- 

 tion work using expendable radio sonobuoys is included where 

 velocity information is required and (or) where the section is 

 too thick or absorptive for penetration by vertical reflection 

 profiling technique. 



The work accomplished to November 1973 includes two 

 periods of marine field work of about 6 months each (early 

 1972 and early 1973) and additional field work in the southern 

 end of the Andean Cordillera and on the island of South 

 Georgia (early 1973). 



During the past two austral summers U.S. and Argentine 

 scientists, working aboard Lamont-Doherty Geological Observa- 

 tory's research vessel Robert D. Conrad, have made marine 

 geological and geophysical measurements of the Argentine 



22 



