522 



STRUCTURAL GEOLOGY OF NORTH AMERICA 



Pacific Ocean Trench Bench 



Aleutian Ridge 



Bering Sea 



Fig. 32.7. Speculative and diagrammatic cross section of western end of the Aleutian Ridge 

 and Aleutian trench. Reproduced from Gates and Gibson, 1956. 



in large measure appears to be the great delta of the Yukon and Kuskok- 

 wim rivers. See Chapter 39. 



PACIFIC FLOOR OFF MEXICO AND CENTRAL AMERICA 



Middle America Trench 



The Middle America trench is continuous at depths greater than 14,400 

 feet for 1260 miles, except for two submarine volcanoes which lie in the 

 trench. (See Figs. 32.8 to 32.10). Northwest of Acapulco the trench is 

 generally U-shaped in cross section, with a steeper shoreward flank and a 

 flat bottom suggesting sedimentary fill. Off Guatemala for a distance of 

 380 miles it is over 18,000 feet deep with a maximum sounding of 21,000 

 feet. Thence southeastward it shoals gradually to merge into the sea floor 

 off Costa Rica. The southeast segment is also asymmetrical in cross section, 

 but V-shaped with irregular bottom, in contrast to the flat bottom north- 

 west of Acapulco. 



Along the trench as explored to date, a series of breaks in slope or 

 terraces suggests a downwarped or downfaulted shelf below the more 

 normal shallow shelf. Faulting across the shelf may have been important 

 south of the Isthmus of Tehuantepec ( Fisher and Shor, 1959 ) . 



Fig. 32.8. Middle American trench and related features. Compiled from Fisher 1961, and Shori 

 and Fisher, 1961. Rows of dots are submarine canyons. 



