152 



STRUCTURAL GEOLOGY OF NORTH AMERICA 



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*n" — ^~ '/• ■••• : 



* *• •> •• • 



Fig. 10.16. Earthquake epicenters, North Atlantic. Reproduced from Heezen ef a/., 1959. 



place to place. In the crest provinces the 5.1 km/sec layer is commonly 

 exposed. In the flank provinces appreciable thicknesses (to 1 km) of sediment 

 have been measured. 



Under the abyssal floor of the ocean the low velocity sediment layer 

 is underlain by a 6.7-km/sec layer, and this by a 8.1-km/sec layer. The 

 lower is considered the mantle of peridotite and the overlying layer a 



gabbroic or firm basalt layer. Under the Ridge neither of these two are 

 present but instead layers of 5.1-km/sec and 7.3-km/sec. 



Ewing and Ewing (in press) suggest that this intermediate velocity 

 (7.3 km/sec.) is the result of a physical mixture of oceanic crustal rocks and 

 mande rocks. To explain such large-scale mixing they propose that extensive 

 vulcanism and intrusion along the Mid-Adantic Ridge have produced an 



