530 



STRUCTURAL GEOLOGY OF NORTH AMERICA 



and trends to the northwest (E. L. Hamilton, 1957). The islands are 

 believed to have formed during the Tertiary with volcanic activity pro- 

 gressing southeastward. Present volcanic activity is confined to the island 

 of Hawaii, which may have had its inception as late as the Pliocene 

 (Stearns and Macdonald, 1946). 



Submarine contouring has indicated a sag, the Hawaiian deep, adjacent 

 to the ridge, which in its deepest part is about 3600 feet below an outer 

 gentle arch. See Fig. 32.18. The bottom of the deep is above the level 

 of the ocean floor beyond the arch. 



The peripheral deep and arch are believed by Hamilton to be due to 

 the loading of the earth's crust by the volcanic piles, and to consequent 

 downbowing and lateral bulging. 



MID-PACIFIC MOUNTAINS 



A submerged relief feature known as the Mid-Pacific Mountains, ex- 

 tends southwesterly from Hawaii. There a series of flat-topped volcanic 

 peaks, called guyots, are submerged 4200 to 5400 feet. The study of 

 dredged samples from the flat tops yielding Upper Cretaceous, Paleocene, 

 and Eocene foraminifera indicate that in Cretaceous time the guyots were 

 a chain of basaltic islands, wave-decapitated with coral-rudistid reefs 

 lodged on and among the erosional debris. Submergence followed to the 

 depths indicated (E. L. Hamilton, 1956). The recognition of broad sub- 

 sidence of the ocean floor in the magnitude of one mile is very significant 

 in understanding the processes of mountain building. 



CIRCUM-PACIFIC TECTONICS 



In Chapter 29 the San Andreas fault and associated structures were 

 depicted, and there the theories of Hill and Dibblee and of Benioff on 

 the mechanics of formation were outlined. It is recognized that the 

 major movement on the San Andreas fault has been right strike-slip move- 

 ment. Hill and Dibblee (1953) have suggested a horizontal displacement 

 of 560 kilometers. 



Incident to the study of aftershock sequences Benioff ( 1957 ) recognized 



-2". 



Fig. 32.19. Circum-Pacific tectonics. Reproduced from Benioff, 1957. 



that the extent of faulting for earthquakes where the fault is not visible 

 could be determined. Since the direction of slip can also be determined, 

 a study of Circum-Pacific earthquakes leads to the presumed discovery 

 that around the entire margin the slip is dextral as indicated in Fig. 

 32.18. Only for Antarctica are observations wanting. 



Critical of Benioff's hypothesis of counterclockwise rotation of the 

 Pacific basin crust, Chingchang (1958) points out that the section be- 



