38 



p. B. KING 



crustal movement. Certain it is that much of the region is still 

 unstable, as attested by many fresh fault scarps along the edges of 

 the mountains and in adjacent alluvial deposits, some of which can 

 be related directly to recorded earthquakes. 



Basin and Range structure is thought to be a mosaic of blocks, 

 which have been variously raised, lowered, or tilted along steeply 

 dipping faults (Fig. 8). High-standing blocks produced the mountain 

 ranges, low-standing blocks the basins; detritus eroded from the 

 higher blocks was trapped in the lower ones, and smoothed their 

 surfaces into gently sloping plains. The faulted sides of the most 

 recently upraised mountain blocks still preserve straight base lines 





10 Miles 



_i 



Approximate scs/e 



Fig. 8. Generalized section showing Basin and Range structure as 

 commonly interpreted, based on Humboldt Range, western Nevada. 

 (After Louderback, 1904.) 1, Deformed bedrock of Paleozoic and 

 Mesozoic age. 2, Lava and tuff, mainly of early and middle Tertiary age. 

 3, Deposits of the intermontane basins, mainly of late Tertiary and 

 Quaternary age. 



and steep escarpments; mountain blocks upraised earlier are more 

 frayed and are embayed by erosion (Davis, 1925). Detrital filling in 

 the basins has generally overlapped the edges of the mountains 

 sufficiently to conceal the faults along their borders, but these faults 

 are exposed at some favorable places. 



Basin and Range structure is a post-orogenic feature that suc- 

 ceeded the strongly compressed structures of the Cordilleran 

 orogenic phase, but opinions differ as to the forces that caused it 

 (Nolan, 1943, pp. 184-186). An early view is somewhat naive — that 

 it was produced by a breakdown and collapse of the region under 

 tension, after relaxation of orogenic compression. But under perva- 

 sive tension the whole region would have subsided from an earlier 

 high-standing position, the ranges less than the basins, whereas the 



