498 

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STRUCTURAL GEOLOGY OF NORTH AMERICA 



Ruby Mtns. 



Lamoille Valley 

 Th Qa 



5,000 



dip 60 to 70 degrees basinward. Displacements on the east boundary- 

 faults have been at least twice as great ( 5500 to 6000 feet ) as on the west 

 boundary faults (2000 feet). The northern termination of the mountains 

 is due to intersection of the east and west boundary faults. The structure 

 of the pre-Miocene rocks is discordant with the trend and shape of the 

 range. 



Five periods of basin-range faulting have been established: (1) late 

 middle or early late Miocene, displacement small (open to question); 

 ( 2) late Miocene, during deposition of the Humboldt formation, displace- 

 ment larger; (3) latest Miocene to Pliocene, younger than the Humboldt 

 formation and older than the Pliocene (?) lava, amount of displacement 

 unknown; (4) Pliocene to Pleistocene, later than the Pliocene (?) lava 

 and extending to middle or late Pleistocene, the period of last major uplift 

 of the range, displacement large; (5) late Pleistocene to Recent, later than 

 the earliest Wisconsin, displacement small. 



The history of faulting in the Sierra Nevada is fairly completely known. 

 Toward the end of the Eocene, volcanoes were intermittently active, and 

 they emitted rhyolite lava and mud that filled the existing valleys. This 

 volcanic activity, interspersed in an erosion cycle, continued well into 

 the Oligocene; at the same time die Sierras were gaining elevation by 

 vertical arching. The country lying to the east was warped and flexed; 

 low ranges came into existence, and between them were formed wide 

 basins in which the water collected in shallow lakes. 



According to Matthes (1930), the disturbances died out at last and 



were followed by a long interval of relative quiet, during which most 

 of the rhyolite and much other rock waste was stripped from die Sierra 

 region and deposited on its western border and in the basins to the east 

 of it. Then, presumably in the second half of the Miocene epoch, volcanic 

 activity and earth movements began anew on a large scale. This time, 

 the eruptions yielded mostly andesitic lava of brown, reddish, and grayish 

 colors. Down the valleys this material flowed, sheet upon sheet, obliterat- 

 ing the stream beds and compelling the waters to seek new paths. In the 

 north half of the range, the outpourings were especially frequent and 

 voluminous; they piled up to thicknesses of a thousand feet or more, 

 overwhelming all the features of the country save the higher peaks and 

 crests. In the soudiern parts of the range, the volcanic flows were less ex- 

 tensive and less thick; they filled only the bottoms of certain valleys, and 

 caused no notable displacements in the drainage system. Only the drain- 

 age basin of the Merced River, in the central part of the range, remained 

 free from volcanic outpourings. 



The crustal movements of this epoch increased the height of the Sierra 

 region by several thousand feet and gave it the aspect of a mountain 

 range, or rather a belt of mountains, that dominated all the country round 

 about. Mount Lyell probably attained an altitude of about 7000 feet 

 Strong faulting took place along some parts of the eastern border, an 

 the great depression in which Lake Tahoe is situated was formed by 

 subsidence; the lava which dams the lake itself was not poured out, ap 

 parently, until after the depression was formed. The ranges and valleys 





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