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STRUCTURAL GEOLOGY OF NORTH AMERICA 



Fig. 32.1. Pacific submarine relief provinces off North America. After Menard, 1955. 



The higher elements are comparable to the short mountain ranges of 

 the adjacent land. The submarine relief is also comparable in magnitude, 

 but not in the intricacy of detail. The San Bernardino Range rises about 

 9000 feet above the adjacent basins, and the San Juan seamount rises 

 about 10,000 feet above the adjacent ocean floor. Santa Cruz Island rises 

 almost 9000 feet above the floor, and Catalina Island about 6000 feet. 



Some of the relief features have flat tops. The most extensive are banks 

 under about 3000 feet of water. Another group of flat-topped seamounts 

 ranges in depth from 1200 to 3480 feet. 



Continental Shelf 



Shelf. North of Point Conception, the basin and range type of 

 topography on the sea floor composes itself into a continental shelf gen- 

 erally not over 500 feet deep. Off central and northern California, the 

 shelf is about 25 miles wide, and off Oregon and Washington, somewhat 

 less. The borderland of southern California, after deepening southward, 

 shoals again and abuts against the 80-mile-wide shelf of Sebastian Viz- 

 caino Bay of central Baja California. From Sebastian Vizcaino Bay south- 

 ward, a distinct shelf and straight shelf slope extend all the way to the 

 southern tip of the peninsula. See map, Figs. 32.1 and 32.5. 



The shelf zone continues fairly regularly along the coast of British 

 Columbia and southeastern Alaska to a point off Yakutat Bay, where it 

 turns southwestward along the Aleutian Islands and borders the Aleutian 

 trench. It is a submerged surface of great glacial valleys off British Colum- 

 bia and southeastern Alaska (see Fig. 17.18). Along the Aleutians, it is 

 over 100 miles wide in places, and generally less than 500 feet deep. 



Longitudinal depressions just off shore in the shelf of southeastern 

 Alaska (off Yakutat Bay and Cross Sound) are interpreted to be due to 

 faulting incident to the Pleistocene uplift of the adjacent ranges (Hol- 

 tedahl, 1958). 



Shelf Slope. From Yakutat Bay, Alaska, to Baja California, the shelf 

 and basin and range borderland are terminated oceanward by a slope of 

 great proportions. The decline where greatest extends from the brink at 

 500 feet to the base at 10,000 feet. In places it is sufficiently steep to be 

 comparable with the Sierra Nevada scarp, and hence considered b; 





