.506 



STRUCTURAL GEOLOGY OF NORTH AMERICA 



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material, and the cause of subsidence of the pluglike block to be due to 

 the relief of pressure from below by the movement away of magma in a 

 supporting chamber. The magma is presumed to have found escape at 

 the surface, but only part of the extrusives accumulated in the subsiding 

 basin. The magmatism and pluglike faulting are believed to be related to 

 the general tectonic framework of deforming forces of the Basin and 

 Range provinces. The nature of the relation will be considered in follow- 

 ing paragraphs. 



Region West of Wasatch Range 



Cook and Berg ( 1957 and 1961 ) report on an extensive gravity survey 

 in Salt Lake and Utah counties where they made 1100 observations over 

 an area of 5000 square miles. Steep gravity gradients reveal buried faults 

 unrecognized by surface geologic surfaces, and although the downfaulted 

 valley block between the Wasatch and the ranges on the west was known 

 to contain over 2000 feet of unconsolidated or semiconsolidated sediments 

 a deep inner trough was discerned which with a number of irregularities 

 extends north-south for over 100 miles. "Several large fragments . . . have 

 apparently dropped deeper than the other fragments, as if slipping into a 

 great crevasse." 



Fallon-Austin Earthquake Area 



A major earthquake occurred in the Dixie Valley-Fairview Valley area of 

 west-central Nevada in 1954, and fresh scarps were formed. Their pattern 

 is shown in Fig. 31.19. The faulting is most advantageous to study because 

 a first order triangulation net and a first order line of levels had been 

 established across the area before the movements. The stations were re- 

 occupied and the amount of vertical and horizontal movement accurately 

 determined. A vertical displacement of 7 feet occurred in Dixie Valley 

 and also 7 feet where the fault is in bedrock east of Fairview Peak. The 

 arrows of Fig. 31.19 indicate the horizontal extension that occurred and 

 which averages about 5 feet in magnitude in a northwesterly direction. 



Fig. 31.15. Earthquake epicenters of the Rocky Mountain region showing coincidence of zone 

 of concentrated seismic activity and the belt of trenches. Taken from map compiled by G. P. j 

 Woolard from U.S.C. & G.S. reports. 



