524 



STRUCTURAL GEOLOGY OF NORTH AMERICA 





500 400 



DISTANCE IN KILOMETERS 

 300 200 



100 









t SHORELINE 







5 











- -£^~ 2 4- 

 ,^ 32 



-— — -~T~ < ^^' 4.7 

 ^"~-\57 



^<X0WER CRUSTAL LAYER^ 

 ^\^ 69 



SEDIMENTS 

 BASEMENT 





^57 

 67 



"-S7 - 

 66 





10 

 IS 



. 81 



VERTICAL EXAGGERATION 5X 



. 



10 

 15 



61 



20 



61 



6.2 



MANTLE 



- 



20 



Fig. 32.11. Crustal layers across Middle America trench after Shor and Fisher, 1961. Numbers 

 represent wave velocities in kilometers per second. 



Age of Trench 



The Gulf of Tehuantepec marks a major change in trench configuration 

 and possibly in age. Northwest of Tehuantepec the flat trench bottom 

 suggests a greater age than the deep V-shaped profile southeast of the 

 Gulf. Thicker crustal layers and a bordering volcanically active coast 

 also mark the younger division. The zone of ridge-and-trough topography, 

 the Tehuantepec Ridge, trending southwest from the point of change 

 may be another evidence of the division of the trench into older and 

 younger parts. 



FRACTURE ZONES 



Four great bands of linear relief features, named fracture zones (H. 

 W. Menard, 1955), have been discovered in the northeastern Pacific 

 basin. They are the Mendocino, Murray, Clarion, and Clipperton, and 

 are shown on the map of Fig. 32.12. A lesser zone, the Pioneer Ridge, is 

 labeled on Fig. 32.16. It had not been surveyed well at the time the map 

 of 32.2 was constructed. 



The zones range from 1400 to 3300 miles long and average 60 miles 

 wide. The Mendocino and Murray stretch across the Pacific floor to the 

 Hawaiian Ridge. They follow great circle courses and are approximately 

 parallel. Topographic relief within the fracture zones is characterized by 

 large seamounts, deep narrow troughs, asymmetrical ridges, and escarp- 



ments. Two escarpments are about 1 mile high and more than 1000 miles 

 long. See Fig. 32.2. 



The Clipperton fracture zone is more varied and irregular than those 

 to the north (Menard and Fisher, 1958). The western half consists of 

 narrow ridges and low seamounts, but the eastern is dominated by an 

 enormous ridge, about 60 miles wide, 330 miles long, and 8000 to 10,000 

 feet high. A trough about 10 miles wide and a mile deeper than the 

 surrounding region borders the ridge. See Fig. 32.13. 



The over-all easterly trend of the ridge is complicated by a southeasterly 

 cross trend indicated by the alignment of volcanoes, by orientation of minor 

 ridges on the south side of the main ridge, and by the marked change in trend 

 of the main ridge at its eastern end. Clipperton Island, the only feature in 

 the whole Clipperton fracture zone that reaches the sea surface, is one volcano 



• PLEISTOCENE OR 

 RECENT VOLCANOES 

 (INCLUDING ISLANDS) 



-y-GUYOTS, FORMER 

 VOLCANIC ISLANDS 



X SEAMOUNTS, SUBMARINE 

 VOLCANOES NOT KNOWN 

 TO BE GUYOTS 



Fig. 32.12. Fracture zones and seamounts of northeastern Pacific. Reproduced from H. W. 

 Menard, 1955b. Also volcanoes of adjacent coastland. 



