MIDDLE AND LATE CENOZOIC SYSTEMS OF THE CENTRAL CORDILLERA 



497 



Fig. 31.5. Block diagram showing nature of crustal deformation by block faulting in the Ruby- 

 East Humboldt Range, Nevada. The view is to the southwest. The block is about 50 miles long 

 (S) and 30 miles wide. (W). The diagram is approximately to scale with the maximum throw 

 of the faults about 6000 feet (Sharp, 1939). The faults acquired their present displacement 

 by four stages of movement from upper Miocene to the Pleistocene. 



Studies by Hudson ( 1955 ) indicate that the uplift of the Sierra Nevada 

 is not due to simple tilting of a rigid block. An important zone of faulting 

 about midway between summit and the western edge of the range 

 divides the range into two blocks of deformation, and Hudson, from 

 gradient calculations, thinks there may be five separate bocks of adjust- 

 ment. 



Most of the blocks throughout the Great Basin are rotated or tilted. 

 [Study Osmond's representation in Fig. 31.3, which includes thirteen up- 

 tilted blocks and probably a number of down-dropped additional blocks 

 ijin a distance of 200 miles. In the eastern part of the Great Basin Mackin, 

 Working with the ignimbrite sheets, shows a series of blocks all rotated 

 n the same direction. See Fig. 31.4. Some blocks, however, are horstlike, 

 ;uch as the Ruby Mountains and East Humboldt Mountains (Fig. 31.5). 

 Mapping in the Wasatch Range (Eardley, 1939, 1944) indicates that 

 i master fault, the Wasatch fault, 115 miles long with displacement of 

 1000 to 6000 feet, forms the eastern limit of the faulted part of the Great 

 3asin. In places its displacement is distributed along step faults with 

 jhe west side down. It is a dip-slip normal fault and dips 50 to 70 

 jegrees west. A quasi en echelon pattern of smaller normal faults 

 preads across the thick sediments of the Pennsylvanian Oquirrh basin 



of west central Utah. The Basin and Range faults are not aligned with 

 the Precambrian or Laramide structures. Neither have the crystalline 

 rocks of the northern Utah highland, the intrusions of the Cottonwood 

 uplift, nor the late Precambrian basins influenced perceptibly the course 

 or the throw of the faults. The widths of the fault blocks range from 4 to 

 30 miles, but a fairly uniform width of 18 to 24 miles is found in the four 

 major blocks of the area, the Wasatch, Oquirrh, Stansbury, and Cedar 

 mountain blocks. A relief of 3000 feet or more is believed to have existed 

 at the inception of faulting. 



Age of Block Faulting 



Ferguson (1926) and Ferguson and Cathcart (1924), in addition to 

 presenting physiographic evidence that the block faulting occurred at 

 different times, found that similar faults, though without present topo- 

 graphic expression, both preceded and followed the deposition of sedi- 

 ments belonging to the Esmeralda formation (late Miocene and early 

 Pliocene). The conclusion that these earlier faults were of the same 

 character as the later block faults is based on the fact that the Esmeralda, 

 adjacent to the pre-Esmeralda faults, is composed of material similar to 

 that now being deposited in the fans along range-front scarps, and 

 further that at least some of the topographically expressed faults have fol- 

 lowed the lines of these early faults. Westgate and Knopf (1932) have 

 also found evidence in the Boulder Dam region for block faulting that 

 preceded, accompanied, and followed the deposition of his Muddy Creek 

 formation, of questionable Pliocene age. Gianella (1936), similarly, has 

 distinguished two major epochs of movement at the Comstock Lode. 



A typical range in the north central part of the Basin and Range 

 province for which the geology has been worked out is the Ruby-East 

 Humboldt. According to Sharp (1939) the range consists of pre-Miocene 

 igneous, metamorphic, and sedimentary rocks of complex structure. The 

 adjoining basins contain deformed beds of the upper Miocene Humboldt 

 formation. The boundary structure of the mountain block is well exposed 

 because of dissection by the through-flowing Humboldt River. See Fig. 

 31.6. 



This range is a westward-tilted horst, bounded by normal faults which 



