392 



HEEZEN AND EWING 

 40° 20° 0» 



[chap. 16 



A 



I 



E 



HI 



Continental margin 



Continentol shelf 

 Marginal plateau 



Continental slope 

 and marginal 

 escarpment 

 Marginal trencti 



Continental rise 

 Outer ridge 



a 



Ocean- basin floor 



Abyssal floor 



Abyssal plain \,£P \ 



Abyssal tiills pT^ 



Oceanic rise i ^ = = 1 



E53 



C Mid- ocean ridge 



Crest 



Rift valley ond ^^ 

 rift mountains '-^^ 

 Flanks fcr^ 



Fracture zone 



Fig. 4. A generalized province map of the South Atlantic showing the threefold division 

 of the ocean into continental margin, ocean-basin floor and mid-oceanic ridge. In 

 the equatorial Atlantic a series of left slip faults or fracture zones has offset the 

 mid-oceanic ridge westward for over 2000 miles. Although fracture zones have been 

 known since 1939 in the Pacific (Menard, 1955), this is the first time that they have 

 been mapped in the Atlantic. (After Heezen and Tharp, 1961.) 



epicenter belt actually coincides with the rift valley. The data available would, 

 however, allow the conclusion that the rift mountains also lie within the seismic 

 belt. 



All epicenters of the mid-oceanic belt are of shallow-focus. This distin- 

 guishes the mid-oceanic l)elt from the circum-Pacitic epicenter belt. The 



