512 



STRUCTURAL GEOLOGY OF NORTH AMERICA 



80* 70* 60* 50' 



■60* ISO* 140" 130* 120* 110* 100* 90* 60* 70' 60* 50' 



Fig. 31.24. East Pacific Rise and pattern of heat flow. Reproduced from Menard, 1960. 



Interpretations 



The East Pacific Rise of the ocean floor has been considered by Menard 

 (1960) to extend to the Gulf of California and hence under the western 

 part of the continent of North America appearing in the Pacific again off 

 Oregon, Washington, and British Columbia. See Fig. 31.24. 



The puzzling slope between California and Hawaii is the west flank of the 

 rise. . . . Where the crest and east flank of the rise intersect Mexico are found 

 the plateau of Mexico, the Colorado Plateau, and the Basin Ranges comprising 

 a topographic bulge of the continent comparable in scale to the bulge of the 

 sea floor. 



Cook (1961) follows Menard in projecting the East Pacific Rise under 

 the continent, and assigns the broad uplift to the development of the 

 7.4-7.7-kilometer-per-second velocity layer under it. In fact, he believes 

 from still incomplete data that the oceanic rises of the Pacific, Atlantic, 

 and Indian oceans with their accompanying rift systems and volcanism 

 are due to the uplift of the crust as the 7.5 layer develops. He calls it the 

 mantle-crust mix layer, and regards it as a change from eclogite to basalt 

 with attendant expansion. 



The views of Menard and Cook related to the western United States 

 lead to many thoughts which will only be summarized here. First, the 

 Late Cenozoic uplift should be considered. Approximate uplift contours 

 are shown in Fig. 31.25. They are admittedly approximate, and in the 

 Great Basin represent an average of the uplift of the Tertiary deposits in 

 the valley blocks and the uplift of the mountain blocks. From the picture 

 presented the Snake River downwarp and associated Columbia basalt 

 region may represent a transverse break in the continuity of the 7.5- 

 km/sec layer from south to north. The Colorado Plateau has been uplifted 

 more than the Great Basin, and it has generally been considered that the 

 Great Basin is one of collapse or subsidence in relation to the Plateau, 

 although in relation to sea level, both have been uplifted. It will be very 

 interesting to see what the relative heat-flow measurements will indicate 

 as to the central part of the rise over the 7.5 layer. None has been made 

 yet. Cook seems to infer that the zone of Great Trenches and accompany- 

 ing seismicity is the central rift zone of the rise. Fig. 31.15. 



In Chapter 36, the igneous rocks of the western United States are re- 





