Guiana Basin. 



Underway data will include (along the entire track) bathy- 

 mctric, gravity, magnetic, and seismic reflection measure- 

 ments. Station work, at the rate of approximately one station 

 per day, will consist of measurements of bottom currents and 

 heat flow, as well as bottom photography, nephelometry, and 

 piston coring. Seismic refraction work and dredging stations 

 will be added when appropriate. 



During May through July 1972, the RV Robert D. Con- 

 rad of Lamont-Doherty Geological Observatory made recon- 

 naissance geophysical studies on the continental margin of 

 Brazil (fig. 28). Brazilian scientists from Petrobras and the 

 National Department of Mineral Production participated in 

 planning the cruise and assisted in gathering and reducing the 

 data. 



Although the scope of the 1972 field work was primarily 

 of a reconnaissance nature to determine whether more detailed 

 studies will be needed, significant accomplishments were re- 

 corded. 



The North Brazilian Ridge was found to be primarily a 

 discontinuous feature extending from 34°W to 45.5°W except 

 for a section between 34°W and 38°W. The ridge contains 

 gaps which progressively widen west of 38°W. The acoustic 

 basement is consistently 1 to 2 kilometers deeper on the land- 

 ward side of the ridge than on the seaward side. 



The Romanche Fracture Zone can be traced into the 

 North Brazilian Ridge 1°48'S, and the Chain Fracture Zone 

 extends into the Fernando de Noronha and Atol das Rocas 

 chain of seamounts. 



Diapiric structures, attributed to salt, were observed on 

 the continental margin of southern Brazil. The zone of offshore 

 diapirs is roughly triangular, with the Sao Paulo plateau as the 

 base. The northern apex of the zone is not closely defined and 

 appears to be located at the junction of the Brazilian coast and 

 the Columbia Seamount chain. Sonobuoy refraction and reflec- 

 tion information and normal reflection data were used to outline 

 the diapir zone. 



Mid-Atlantic Ridge Study 



A better understanding of the geological processes opera- 

 ting along midocean ridges is the basis for study of the 

 Mid-Atlantic Ridge. Despite numerous individual studies of 

 this feature, little is known and nothing on a scale smaller than 

 approximately 20 to 50 kilometers. 



Prior to undertaking a major study of the Mid-Atlantic 

 Ridge, IDOE supported a National Academy of Sciences 

 workshop to examine major questions concerning the processes 

 operating along the crest. Scientists from Canada, France, Ice- 

 land, Netherlands, United Kingdom, and United States met at 

 Princeton, N.J., in January 1972. The workshop results sum- 

 marize present knowledge and outline a comprehensive pro- 

 gram for a concerted attack on the major problems. 



Scientific exploration of the ocean floor will enter a new 

 phase with the Mid-Atlantic Ridge Study. Present knowledge, 

 based on studies from surface ships, is analogous to geophysi- 

 cal surveys of the land made from altitudes of 4,500 meters. Sub- 

 mersible craft will enable scientists to make firsthand observa- 

 tions of the active zones, collect samples, and emplace instru- 

 ments with precision. French and United States submersibles 



and the British vehicle, Gloria (side-scan radar technique), 

 will be used in the diving area southwest of the Azores (fig. 29). 



Preliminary surveys are now being done to select the most 

 geologically active points in the rift valley of the ridge. The 

 techniques include detailed bathymetry, heat flow, sampling, 

 seismic reflection, and seismicity. In addition to England, 

 France, and the United States, Canada, Iceland, and Portugal 

 plan to participate. The first submersible dives by the French 

 vessel, Archimede, took place in July 1973. A joint United 

 States-French program is scheduled for summer 1974. 



Other studies of processes operating along the Mid-Atlan- 

 tic Ridge include work in Iceland, the Azores, and along the 

 equatorial zone in the vicinity of the Romanche Fracture Zone. 



J. F. Hermance of Brown University, a geophysicist spe- 

 cializing in the field of magnetotelluric measurements, is inves- 

 tigating the deep tectonic processes that generate gcothermal 

 activity at the surface in Iceland. This study is being carried 

 out in cooperation with the National Energy Authority of 

 Iceland. The mechanism by which heat generated by deep- 

 seated processes, such as those associated with sea floor 

 spreading as related to hydrothcrmal activity in the crust, is 

 obscure at this time. 



N. D. Opdyke of Lamont-Doherty Geological Observa- 

 tory, in cooperation with scientists at Dalhousie University, 

 Nova Scotia, plans a 1 ,500-mctcr borehole on the island of San 



_l I M M II II U I M I 



76° 70° V_60 



— SOUTH AMERICA 



iiii|lliiMiii|llli|llll|lliiilMI|iillliiil|lili33o|. 



50° 40° 30° 20° 10° W 0' 



20° -z 



10°N- 



0°- 



10°S- 



20°- 



30° 



40° 



50° — 



~lllll IIIiImIIIIIIiIiIIIIIIIiIiIII Illllll[llllllllllllllll|56° _ 



Figure 28. — Lamont-Doherty Geological 

 Observatory's RV CONRAD tracklines during 

 1972. 



29 



