250 



HEEZEN AND AIENAKD 



[chap. 12 



arriving after the echoes from both sides of the trench (Fisher, 1954). In some 

 areas ^^■here detailed siirve3's have been made of the continental margin, it has 

 been discovered that much of the fine-scale relief is subjjarallel to the adjacent 

 margin. This seems to suggest that the tectonic relief throughout the continental 

 margin province is controlled by a single structural system, which Ues pre- 

 dominantly parallel to the continental slope. 



SCALE IN NAUTICAL MILES 



400 



Fig. 16. Profiles illustrating the marginal trench and outer ridge in the northern Caribbean. 

 (After Heezen et al., 1959.) 



h. Island arcs 



Lying on the northwestern margin of the Pacific Ocean are a series of great 

 island arcs — the Aleutians, the Kuriles, the Japanese and the Marianas. The 

 outer convex side of these island arcs is considered to be the seaward limit of 

 the Asian continent by some petrologists because of the occurrence of andesite. 

 However, consideration of other criteria, such as the depth of water or crustal 

 thickness, makes it clear that most of the great island arcs lie within the Pacific 

 Basin rather than at the margin of the continent. The Phili])pine Sea is roughly 



