260 



HEEZEN AND MENARD 



[chap. 12 



Tn the North Atlantic, the Bermuda Rise is the best known example 

 (Figs. 3 and 22). Tn contrast to the mid-oceanic ridges, oceanic rises are 

 non-seismic ; their relief is more subdued and they are asymmetrical in cross- 

 section. The western and central parts of the Bermuda Rise are characterized 

 by gentle, rolling relief. The average depth gradually decreases to^^•ard the 

 east. In the eastern third, the rise is cut by a series of 500-1000-fm-high scarps 

 which drop the sea floor to the level of the abyssal plain on the east. The series 

 of eastward-facing scarps suggest block faulting. Situated approximately in 

 the center of the Bermuda Rise is the volcanic pedestal of Bermuda. A small 

 archipelagic apron surrounds the pedestal. The turbidity-current origin of the 

 smooth surface of the apron is supported by cores containing shallow-water 

 carbonate clastic sediments in depths of 2300 fm. 



g. Fracture zones 



Fracture zones are long, thin bands that are conspicuously more mountainous 

 than the sea floor in general and ordinarily separate regions with diff"erent 

 depths. The first wholly submarine fracture zones were discovered only a few 



120' 



100* 



80° 



— MAJOR LINE«ION 

 •~~ MINOR LINEATION 

 -• LINE OF SEAMOUNTS 



lec 



Mcr 



Fig. 23. Major and minor lineations in the northeastern Pacific. (After Menard, 19.59.) 



years ago in the northeastern Pacific basin. Now scores are known in the Pacific 

 and Atlantic. They form a pattern of east-west parallel lines, relatively evenly 

 spaced down the whole eastern Pacific (Fig. 23). Three follow great circles and 

 all are remarkably straight. Each is about 100 km wide and more than 2000 km 

 long. Within the zones are asymmetrical ridges and narrow troughs parallel to 



