258 



HKRZEN AND MENARD 



[chap. 12 



It is 1-5 miles wide and has 10-100 fm relief below an abyssal plain. Except 

 where it passes through a narrow gorge in a submarine mountain range, it is 

 steep-walled, Hat-iloored, and generally flanked by levees. A very similar 

 feature, the Equatorial Atlantic Mid-Ocean Canyon, has been subject to a 

 rather detailed siu'vey which has confirmed the continuity of the canyon for 

 several hinidred miles. Although incomplete, the study suggests that the mid- 

 ocean canyon disappears somewhere on the continental rise and does not 

 connect with a submarine canyon. Some question exists, therefore, as to 

 A\hether what appear to be identical morphological forms are actually the 

 same. All known deep-sea channels extend without interruption from the 

 continental slope portions of submarine canyons to deep-sea fans or cones to 

 abyssal plains, and they are clearly the channels followed by turbidity currents 



M!rv-Oci-_\N OxNvoN No, 2 



ff-w-iimpHiiiiiiPiiimpiil 



*»» ijrj::3l 





Mid-Ocean Canyons in" HA-rniiiAs AcissAL Pi-ajn, West or Vem* Gap 



Fig. 21. P.D.R. records of Mid-Ocean Canyon No. 2 and mid-ocean canyons at Vema Gap. 

 Position of Vema Gap and Mid-Ocean Canyon No. 2 as indicated in Fig. 3. Depth 

 in fathoms. (After Heezen et al., 1959.) 



originating at the heads of the canyons. Each mid-ocean canyon discovered in 

 the North Atlantic, on the other hand, leads to an abyssal gap in a manner 

 suggesting a genetic relationship. It is not known whether these mid-ocean 

 canyons connect to submarine canyons. 



Mid-ocean canyons or deep-sea channels (if they are not the same thing) 

 have been found by random crossings on several abyssal plains and it would 

 appear that they are widespread (Fig. 21). All recognized mid-ocean canyons in 

 the xA.tlantic parallel the adjacent continental margin, suggesting a tectonic 

 control. Although all have a continuous downward slope, they cut across the 

 regional slope of the abyssal plain. This latter characteristic is prominently 

 displayed in the northeastern Pacific where deep-sea channels hook to the left 

 across the contours of fans. 



f. Oceanic rises 



Heezen, Tharp and Ewing (1959) define oceanic rises as " aseismic areas 

 slightly elevated above the abyssal floor which do not form parts of the con- 

 tinental margin or the mid-oceanic ridge" (Fig. 4). Menard (1960) and others 

 use a broader, less restricted definition. 



