346 



STRUCTURAL GEOLOGY OF NORTH AMERICA 



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Fig. 22.20. Diagrammatic cross section of southwestern Utah restored to the time before eruptive activity 

 started. After Mackin, 1960. Section 20, Fig. 22.11. 



The complex structure is believed by Cohenour to have evolved as 

 follows: (a) an early folding (monocline) predates the thrusting and 

 may be of the Cedar Hills orogeny, (b) Overturning and thrusting east- 

 ward — the Sheeprock thrust formed as a wide flat sheet, now dissected 

 so that several windows appear through it. (c) Southwesterly thrusting 

 in which the Pole Canyon and Lion Hill thrusts formed, (d) Intrusion 

 of the West Tintic monzonite pluton probably in Eocene time, (e) 

 After extensive erosion basin and range faulting started and the Sheep- 

 rock granite was intruded which has been dated as Miocene. Erosion, 

 volcanism, and renewed block faulting ensued. 



In a notable study of central-northeast Nevada and adjacent parts of 

 Utah Misch (1960) sees eastward thrusting of the decollement type in 

 nearly every range. He regards it as large scale. As a reasonable work- 

 ing hypothesis, the individually exposed decollements are considered 

 to have moved on a regionally large plane. The main thrusting was mid- 

 Mesozoic and predated the Laramide movements of central Utah. 



SOUTHERN NEVADA 



A well-known group of major thrusts occurs in southern Nevada, near 

 the eastern margin of the Basin and Range province. Examine the Tec- 

 tonic Map of the United States. The group comprises eight or more thrusts 

 with variable, but commonly low, westerly dips. The absence of basement 



rocks in the overriding block, together with the observed changes in dip, 

 suggests that the thrusts pass downward into a nearly horizontal sole 

 (Hewett, 1931; Longwell, 1928). Hewett (1931) regards the thrusts 

 in the Goodsprings area as being successively younger westward, and 

 cites evidence indicative of an erosion interval between them. The belt of 

 thrusting has been traced 100 miles north-south in this region. 



Longwell has reported several times on the geology of southernmost 

 Nevada, particularly on the Muddy Mountains, and his latest diagnosis 

 of the complex structure there is as follows. Figures 22.26 and 22.27 

 should be referred to. 



Instead of a single large thrust in the Muddy Mountain area, Nevada, as 

 reported from earlier field study, the writer distinguishes two superposed thrusts 

 which may represent distinct orogenic episodes separated by a considerable 

 time interval. Both thrusts "root" to the west. The structurally lower thrust (for 

 which the name Muddy Mountain thrust is retained) is the more extensive; as 

 reported previously, it has brought Paleozoic carbonate formations over Jurassic 

 sandstone, with the heave-component of slip at least 15 miles. The higher 

 thrust (here called the Glendale thrust) has heave-displacement of at least 5 

 miles. Together with associated smaller thrusts, it involves formations of early 

 Upper Cretaceous age, as well as thick piedmont deposits that may be consid- 

 erably younger. "Orogenic deposits" several thousand feet thick were laid down 

 in front of the Glendale thrust as it advanced. 



Conglomerate at the base of the Upper Cretaceous section, containing boul- 

 ders and cobbles derived from resistant units in older systems as low as the 

 Permian, indicates earlier strong deformation not far west of the Muddy Moun- 

 tain area. This earlier orogenic episode may have included development of the 



