SHAPE AND STRUCTURE OF OCEAN BASINS 17 



enclosed by island arcs. The arcs possibly have originated through 

 metamorphism and uplift of sediments, which were earlier collected 

 in trenches and later incorporated into magma to form andesitic 

 volcanic rocks. It has been found, by seismic refraction surveys, 

 that the crust inside the Caribbean Sea is not of a single type, but 

 that it varies widely between a typical oceanic type in the deeper 

 areas to a pseudo-continental type in the enclosing ridge, as well 

 as in many subsidiary ridges within (Officer et al., 1959; Ewing 

 etal., 1960). 



Continental Margins 



The continental margins are a region which has been difficult to 

 explore by means of seismic refraction measurements, owing to 

 sharp topographic changes, plus a rapid transition in thickness 

 (and usually in composition) of many of the layers concerned. 

 Gravity measurements (Worzel and Shurbet, 1955) have shown 

 that the transition is a sharp one in the regions studied. This 

 conclusion is confirmed by numerous recent studies on the behavior 

 of "crustal" Love and Rayleigh waves at continental margins. 

 Refraction effects have been noted over the relevant period range, 

 and strong attenuation occurs, for waves crossing the continental 

 margins, in some parts of the period range. This is probably 

 attributable to a strong, sharp contrast in the impedances of the 

 wave guides responsible for the wave transmission (Oliver et al., 

 1955). 



Heat Flow 



The flow of heat through the crust has been found to be about 

 1 X 10~*^ cal cm~- sec ~^ for typical continental and oceanic areas, 

 but is significantly higher for all areas sampled on the Mid-Oceanic 

 Ridge system. Bullard et al. (1956), von Herzen (1959), Maxwell 

 and von Herzen (1959), and E. C. Bullard and A. Day (personal 

 communication) have reported significantly higher flux for the 

 ridge both in the x^tlantic and the Pacific. This important result 

 is confirmed by measurements (not yet published) made on Vema 

 in the Atlantic, Pacific, and Indian oceans. 



