. Collected reprints / Atlantic Oceanographic and Meteorological Laboratories [and] Pacific Oceanographic Laboratories. Oceanography 5250 Dash et al.: Geophysical Investigation of the Cape Verde Archipelago I5°N 14° N. 26°W 25°W 24°W 23° W 22°W Fig. 1. The Cape Verde archipelago and its location with respect to the African continent. A, B, and C are the refraction profiles showing the recording stations on the islands of Sao Vicente, Sao Tiago, and Sal. The Sal-Maio fracture zone is indicated. Contours are at 100 fm () intervals. the recently recognized trends on th
. Collected reprints / Atlantic Oceanographic and Meteorological Laboratories [and] Pacific Oceanographic Laboratories. Oceanography 5250 Dash et al.: Geophysical Investigation of the Cape Verde Archipelago I5°N 14° N. 26°W 25°W 24°W 23° W 22°W Fig. 1. The Cape Verde archipelago and its location with respect to the African continent. A, B, and C are the refraction profiles showing the recording stations on the islands of Sao Vicente, Sao Tiago, and Sal. The Sal-Maio fracture zone is indicated. Contours are at 100 fm () intervals. the recently recognized trends on the island of Maio [Serralheiro, 1970]. It appears that this Cretaceous sequence on Maio represents a section of marine limestones and shales invaded by highly undersaturated alkaline intrusive and extrusive rocks along a NNW-SSE zone of weakness in the North Atlantic sea floor. Carbonatites are present on Maio, Fogo, Brava, Sao Vicente, and possibly Sal [Assuncao et al., 1968]. On Brava the syenite-carbonatite series forms a ring complex intruded into a series of palagonitic basaltic pillow lavas of submarine origin and probable Cretaceous age [Machado et alâ , 1967]. Essexite-syenite-carbonatite associations commonly occur as a compound central plug enclosed in a ring complex, a structural pattern suggesting emplacement of the carbonate as well as the alkaline basic rocks by intrusion. This is the prevailing pattern in some of the east African rift valleys [Turner and Verhoogen, I960]. Such intrusions are thought to be characteristic of a prerifting up-doming phase in continental areas [LeBas , 1971] Area of Investigation and Data Collection The first phase of the geophysical investi- gation carried out in 1969 consisted of refraction seismic work supplemented by magnetic profiling. Three seismic stations were established on the islands of Sal, Sao Tiago, and Sao Vicente, forming a triangle with sides of over 200 km (Figure 1) . Shots ranging from 50 to 350 lb (2
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