514 



STRUCTURAL GEOLOGY OF NORTH AMERICA 



to the base of the silicic crystalline basement layer. This is necessary to 

 feed basalt to the surface in ways listed in the geologic requirements 

 previously mentioned. The basalt layer is visualized as growing in thick- 

 ness as the molten basalt from below rises and is added to it. In case of 

 tensional fractures in the crust which reach downward through the base- 

 ment, the basalt reservoir is tapped, and fissure flows result. When eclogite 

 changes to solid basalt through polymorphic phase transitions, much heat 

 is consumed in the process, and unless considerably more is generated in 

 the mantle or the basalt so formed, none converts to liquid basalt. 



The heat of the liquid basalt which has risen to the base of the silicic 

 crustal layer mobilizes, if not melts, a considerable amount of it; and it is 

 this silicic magma which is postulated to have erupted at the surface to 

 form the voluminous silicic flows of the Great Rasin and the alkalic 

 igneous rocks of the shelf province ( Chapter 33 ) . 



Menard postulates a convection current rising under the East Pacific 

 Rise and flowing westward under the crust. The drag of this current 



creates tensional block fault features in the central zone of uplift, it trans- 

 lates the adjacent crust westward, and in the region of downward plunge 

 of the current, compressional structures are formed. He has difficulty, 

 however, fitting the San Andreas fault into the convection current 

 hypothesis. 



Reference to Figs. 31.21, 31.22, 31.25, and 32.15 should convince one 

 that the cause of late Cenozoic tectonism must be complex, and more is 

 involved than westward movement of the convection cell. In addition to 

 the San Andreas fault with large strike-slip movement to the northwest, 

 there is the Snake River fault which appears to separate the western 

 Cordillera into two distant segments. A drift of the crust to the northwest 

 with extension to the west-northwest is fairly clearly indicated. Resides 

 variations of convection circulation and expansion of the mantle to ac- 

 complish these movements of the crust, there is need to consider the (as 

 yet intangible) forces presumed to cause polar migration, drift, and rota- 

 tion of the continents. The pattern suggests such forces to the writer. 



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