MIDDLE AND LATE CENOZOIC SYSTEMS OF THE CENTRAL CORDILLERA 



509 



GRADIENT, ACROSS VALLEY 



•170 



-180 



--I90 <r 



o 



■200 ffl 



-4000 



MANTLE 



MANTLE 



j. 31.20. Gravity profile and section across Fairview Valley. Also alternate interpretations of 

 ;ulting of crust under extending forces. From Thompson, 1959. 



will be considered tensional features as well as the graben blocks. 



If the crust has been extended some 30 miles between the Sierra Nevada 

 and Wasatch Mountains, then our understanding of the penetration of 

 magma into and through it comes into better focus. In Chapter 33 it is 

 suggested that the large volumes of quartz monzonite magma originated 

 in the base of the silicic (granitic) layer of the crust at depths of 10 to 20 

 kilometers, and we can see that the tensional fractures illustrated by 

 Thompson in Fig. 31.20 would penetrate such magma chambers and 

 conduct the magma upward. From this point of view both the block fault- 

 ing and magmatism are the result of the tensional tectonism, and only 

 in the local examples of pluglike basin subsidence should we conclude 

 that the evacuation of a magma chamber is the direct cause of the 

 faulting. 



We are led to speculate that fractures have penetrated to the basaltic 

 subcrust in Oregon and Washington to conduct the olivine and tholeiitic 

 magmas to the surface. 



EXPLORING TENSIONAL TECTONISM IN WESTERN NORTH AMERICA 



The theory of expansion of the Basin and Range province in late Ceno- 

 zoic time in the magnitude of 30 miles piques one's curiosity to consider 

 the entire framework of movements in western North America. The strike- 

 slip movement along the San Andreas system and the postulated extension 

 of the Basin and Range province with its components of horizontal move- 

 ment should be related. Figure 31.21 has been prepared to show the 

 directions of fault traces and the horizontal movement on the San Andreas. 

 Only a few of the faults of the Great Basin are shown such as to indicate 

 the direction of tensional forces that must be entertained. 



Figure 31.22 is a diagrammatic map which resolves in bold strokes 

 the distention cracks and horizontal movements of the crust previously 

 postulated. The expansion fractures of the Basin and Range province are 

 distributed across the entire basin, but for purposes of illustration are 

 concentrated along the eastern and western margins. The width of the 

 lines represents the approximate amount of postulated expansion. The 

 main pulling away appears to have been in a west-northwesterly direction 



