494 



STRUCTURAL GEOLOGY OF NORTH AMERICA 



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form is due largely to middle and late Tertiary deformation. Literature 

 on the structure of the desert ranges of the western part of the Plateau 

 Central of Mexico is almost nonexistent. 



All the great trenches of central Utah, eastern Idaho, western Wyo- 

 ming and Montana, and British Columbia are probably fault valleys and 

 of middle and late Tertiary age. They extend as a narrow belt from the 

 High Plateaus of northern Arizona and central Utah through the Wasatch 

 Mountains in Utah and northward along the boundary of Idaho and 

 Wyoming to the Teton Range and Jackson Hole in northwestern Wyo- 

 ming, thence northwestward as a wider belt through western Montana 

 to the great trenches of British Columbia. See the map, Fig. 31.1 for 

 boundaries. Much also remains here to be worked out; but sufficient is 

 known, it is believed, to compose these great valleys into a structural 

 system and to treat them collectively as such. 



With few exceptions, the middle and late Tertiary high-angle faults of 

 the Great Basin and the folds and thrusts of southern California are 

 superposed on earlier Nevadan or Laramide structures. 



BASIN AND RANGE SYSTEM 



Evidence of Faulting 



Four types of evidence have been used to prove that the individual 

 ranges in the Great Basin are bordered by block faults: physiographic! 

 evidence, stratigraphic evidence, exposure of a fault plane, and presence| 

 of recent fault scarps along die range fronts. As the boundary betweenl 

 mountain and valley blocks is commonly concealed by the alluvium ac- 

 cumulating in one or more closed basins, die second and third types:; 

 of evidence are rarely found; for most places physiographic evidence has 

 been called upon to determine the existence of a fault block. 



Fig. 31.1 Tectonic map of the western Cordillera in late Miocene, Pliocene, and early Pleistocene T> 

 time. The sediments along the Pacific are late Miocene and Pliocene in age. They are horizontally 

 dashed. The obliquely ruled area denotes the Basin and Range and Sonoran-Chihuahua systems, 

 the faulting of which took place chiefly in Pliocene and early Pleistocene time, although in «i 

 places it started earlier and lasted longer, even to the present. The cross-ruled belt is thei i 

 system of great trenches. Miocene and Pliocene basin deposits are common in all three fault" 

 systems. 



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