508 



STRUCTURAL GEOLOGY OF NORTH AMERICA 



Fig. 31.18. 

 1960. 



Generalized geologic section across Mono basin. Reproduced from Pakiser er a/., 



about 2 miles has occurred since the inception of faulting, which he 

 assumes here was in the Miocene. If the basin is bounded by normal faults 

 considerable distention of the crust must have occurred over the course 

 of movement. If the basin is bounded by faults dipping 60 degrees ( lower 

 diagram, Fig. 31.20), the extension normal to the strike amounts to about 

 a mile on each side of the basin or a total extension of 2 miles. If the faults 

 dip 70 degrees, the extension amounts to about VA miles. 



The location of the focal depths probably reveals the depth to which 

 faulting extended. Two earthquakes occurred 4 minutes apart in time and 

 35 miles apart in distance. The southern Fairview Peak focal depth was 

 determined by Romney ( 1957) to be 15 kilometers below the surface, and 

 the northern Dixie Valley one to be 40 kilometers. Also a close correspond- 

 ence of dip and direction of motion at the surface was found to obtain 

 at the 15-kilometer focus. These points lead Romney to believe that the 

 fault fracture extended to a depth greater than 15 kilometers. The even 

 greater depth of the northern focus supports the conclusion that the entire 

 crust to the Moho discontinuity is possibly affected. Two possible fault 

 structures are shown in Fig. 31.20, with the one on the right coming 

 closest to fitting the facts (Thompson, 1959). 



The amount and rate of distention of the entire Basin and Range prov- 

 ince are estimated by Thompson as follows: 



The data indicate that the region of Dixie and Fairview Valleys has been 

 distended in a nearly east-west direction about a mile and a half. If we assume 



that each of the principal basins between the Sierra Nevada and the Wasatch 

 Mountains has been deformed this much on the average, the total distention 

 amounts to 30 miles or 5 pet. And if the deformation took place in the last 

 15 million years, as suggested by the geologic history (deformation of Miocene- 

 Pliocene and younger rocks ) , the rate is 2 mi/million years or only 1 ft/century. 

 The rate of extension indicated by several fault movements within historic times 

 appears to be at least 1 ft/century. The faults lie in a north-south belt about 

 250 mi long. For at least this distance the data are consistent with an extension 

 of 1 ft or more in the last hundred years. Prehistoric Quaternary faults are also 

 numerous; they strongly suggest that the historic rate of deformation is not 

 abnormally high. 



Tilted blocks, which are characteristic of large parts of the Great Basin, 

 may or may not be the result of extension of the crust. If they are an ex- 

 pression of tension then the general level of the surface is depressed and 

 the crust thinned. Since the Great Basin appears from other geo- 

 logical considerations to be a depressed region, the tilted blocks 



Fig. 31.19. Horizontal movements in the 

 Fairview and Dixie valleys earthquake. After 

 Thompson, 1959. 



SCALE OF MAP 

 IN MILES 



SCALE OF VECTORS 

 IN FEET 





