Results to date include evidence that tensional faulting or forces 

 exist south of Puerto Rico and in the Anegada Trough area and 

 that oceanic reflectors continue for several kilometers beneath the 

 outer toe of the Antillean structure in the northeastern part of the 

 survey area. 



Leg 4, Venezuela Continental Borderland. TTiis investigation was 

 made along the coast of Venezuela (fig. 28) and included the Is- 

 lands of Aruba. Bonaire, and Curacao, as well as the Bonaire 

 Trough, Curacao Ridge, and Los Roques Trough. The basic ob- 

 jectives were to: (1) determine structural relationships of the vari- 

 ous tectonic features, (2) determine the nature of specific plate 

 boundaries, and (3) outline areas of potential mineral resources on 

 the continental shelf and slope. 



Results to date include: (1) Evidence suggesting that the Islands 

 of Aruba, Bonaire, Curacao represent an area of raised oceanic 

 rocks; (2) findings that in places the basaltic or diabasic layer "B" 

 can be traced southward beneath the deformed deposits of the 

 continental slope; and (3) detection of intensely deformed sedi- 

 mentary rocks under the E-W trending Curacao Ridge. 

 Leg 5, Continental Margin of Liberia. This survey complemented 

 previous work carried out on land by the USGS and the Liberian 

 Geological Survey. The purpose of the survey was to investigate 

 the structural-slratigraphic framework of the continental margin 

 adjacent to Liberia. Of particular interest is the nature of transition 

 of the Precambrian shield area from land to ocean. The tracklines 

 were mostly perpendicular to the coastline and extended about 200 

 km. offshore to water depths in excess of 4 km. (fig. 29). 



Results include: (1) A 100 to 400 gamma magnetic anomaly 

 along the outer edge of the continental shelf has been detected; 

 (2) faulting and slumping, as revealed by seismic reflection profil- 

 ing, when combined with relatively large magnetic anomalies, sug- 

 gest that a major deep-sea fracture zone intersects the West African 

 margin in the area of Cape Palmas; and (3) a relatively thick ac- 

 cumulation of sediments on the continental slope west of 9°W. 

 probably reflects an earlier separation time of the African margin 

 from North America (west of 9°W.) than from South America 

 (east of 9 W.). 



Leg 6, Transatlantic Crossing, West Africa to Virgin Islands. This 

 leg. which was run between Monrovia, Liberia, and Charlotte 

 Amalie, St. Thomas, U.S. Virgin Islands, (fig. 30) included con- 

 tinuous acoustic reflection profiles across three drill sites of the 

 Joint Oceanographic Institutions for Deep Earth Sampling 

 (JOIDES) program and several structural features including the 

 Mid-Atlantic Ridge and the Vema Fracture Zone. Potential re- 

 sults from this cruise will include: (1) Construction of structural 

 models of the Vema Fracture Zone, (2) correlation of oceanic 

 seismic reflection profiles with JOIDES drilling results over a 

 greater extent than previously possible, (3) investigation of the 

 structure of the Sierra Leone Ridge. (4) extension of knowledge 

 of sediment distribution and thickness in the deep ocean, and (5) 

 studies of crustal sound velocity. 



NOAA Surveys and Data Analysis 



The NOAA work in 1971 involved three separate studies. In 

 the northeast Pacific Ocean, a Scientific Exploration and Mapping 

 Program (SEAMAP) survey was made west of Washintgon and 

 Oregon by the NOAA ship OSS Surveyor. In the Atlantic, work 

 was continued on the Trans-Atlantic Geotraverse (TAG) by the 

 OSS Discoverer, and the Caribbean-Atlantic Geotraverse (CAG) 

 was initiated by the OSS Researcher. 



Pacific SEAMAP. Tracklines run by the OSS Surveyor during 

 1971 west of Oregon and Washington were basically oriented east- 

 west, with an 18-km. spacing, supplemented by crosslines (fig. 31). 

 Seismic reflection results were obtained on every sixth E-W line 



and on about every other north-trending crossline. The area cov- 

 ered is very promising from the viewpoint of plate tectonics; also, 

 during the unraveling of the complexities of this area, light will 

 probably be shed on the structure and history of the seismically 

 active and destructive San Andreas fault system to the southeast. 



Included in the survey area are two centers of sea floor spreading 

 (the Gorda Rise and the Juan de Fuca Ridge), seismically active 

 transform faults (including the Blanco Fracture Zone), and inactive 

 fracture zones (such as the Sedna and Surveyor). Results from the 

 seismic reflection data show a buried trench close to the Oregon- 

 Washington coast, adjacent to the string of active volcanoes that 

 runs along the crest of the Cascade mountains from northern 

 California to British Columbia. 



The magnetic anomalies in this area trend mostly north-south 

 and can be correlated very well from trackline to trackline on the 

 37 east-west lines. This correlation will allow the solution of many 

 structural problems, such as apparent offsets of magnetic anomalies 

 across fracture zones. This study includes part of the classical 

 "Mason-Raff" survey area and extends to 144°W. 



Free air gravity anomalies, with a precision of 2 to 3 mgals. 

 r.m.s., were obtained over the survey area. Significant results in- 

 clude: (1) Finding of negative values in the range of minus 50 to 

 minus 80 mgals. along the base of the continental margin in the 

 area of the above-mentioned thick-sediment fill, and (2) finding 

 of a range of 80 mgals. (mostly negative) measured along the 

 Blanco Fracture Zone. 



Additional results include evidence that the Juan de Fuca Ridge 

 is now rising; this rise may result in the eventual emergence of the 

 Bear and Cobb Seamounts as islands. Also, examination of narrow- 

 beam echosounder records reveals the presence of a complex pat- 

 tern of deep-sea channels in the Tufts Abyssal Plain. 



Pacific SEAMAP bathymetry, magnetic, and gravity data col- 

 lected in the North Central Pacific during and after 1961 were 

 reformatted and evaluated by the Marine Sciences Institute (MSI) 



Table 2.— Pacific SEAMAP 1961-70 data reports 



^ Logical Number Index map to guide user from number on map to respective entry 



in data listing. 

 2 Battiymetry. magnetic anomaly, and gravity anomaly maps are being compiled 



by NOS (available separately). 

 ■> Map for area 15248-14 extended to include data in area 15254-14. 

 * Map for area 16648-14 extended to include data in area 16654-14. 

 ^ Longitude 180" is western limit of area. 



19 



