600 



STRUCTURAL GEOLOGY OF NORTH AMERICA 



considered. This has been done in Chapter 31 and Fig. 31.26. 



If the boundary layers are fairly flat, as suspected from previous geo- 

 logical analysis, then widespread adjustment of the deeper crystalline 

 basement must be postulated incident to the folding and thrusting of the 

 Paleozoic and Mesozoic basin sediments above. The sections of Fig. 38.1 

 have been prepared to show the folding and faulting where seismic in- 

 formation on crustal layers is most available. Section R extends from the 

 blast site at the south end of the Promontory Range westward to Elko, 

 Nevada. Graduate theses at the University of Utah furnish stratigraphic 

 and structural information on Promontory Range (Richard Olsen), New- 

 foundland Range (R. E. Paddock), Silver Islet Range (Fred Schaeffer 

 and W. L. Anderson), and Pilot Range (Donald Rlue). Sharp's (1942) 

 mapping of the Ruby Range and Dott's ( 1955 ) work at Elko and eastward 

 permit the drawing of a fairly satisfactory, if somewhat generalized and 

 simplified, cross section. The 9-kilometer surface, if of uniform depth 

 across the entire section, is just about tangent to the troughs of maximum 

 downfolding of the Paleozoic and Proterozoic sedimentary sequences. A 

 more detailed and larger-scaled section is shown in Fig. 38.2 which takes 

 a course northeasterly across the Oquirrh Mountains, across the Jordan 

 (Salt Lake) Valley to the Cottonwood dome of the Wasatch Mountains, 

 and then northward to the major exposure of the Archean crystalline 

 (Farmington) complex. If the transition surface between the silicic and 

 the basaltic crust is as illustrated in restored sections R and C, then con- 

 siderable flowage must have taken place in the base of the silicic crust 

 during and after folding of the sediments. 



Origin of Latite Magma. The chief reason for postulating the primary 

 nature of the latite assemblage of the Great Rasin is great volume with 

 only minor amounts of rocks of other composition. A few basalt flows 

 have been noted as part of the latite assemblage but most basalt occur- 

 rences are later, and were extruded in Pliocene-Pleistocene time. Andesite, 

 dacite, and rhyolite generally occur along with the latite, but in relatively 

 small amounts. A plausible theory for the origin of the magma must there- 

 fore account for variations as indicated by the above observations, and 

 even, on occasion, to explain the transit of basalt to the surface. 



Two possibilities occur to the- writer: (1) the base of the silicic crust 



melts in part or in bulk to form the primary magma, or ( 2 ) basaltic magma 

 is intruded in megasills at the base of the silicic crust at temperatures 

 sufficiently above the melting temperature of the silicic crustal rock to 

 melt an appreciable layer of it or to melt it partially in decreasing amounts 

 upward from the basalt sills; some mixing of the basalt with the melted 

 silicic crust might occur. The second theory supplies heat for the phenom- 

 enon and basalt, on occasion, as required. An expanding column of the 

 mantle to produce a surface uplift of about 2 kilometers is needed every- 

 where in the Rocky Mountains, Colorado Plateau, and Great Rasin region 

 (see Chapter 31), and a primary basalt magma is needed under most of 

 it, so it seems logical to start with the premise of rising basalt from the 

 mantle where, it has been concluded, differences in density exist. The 

 basalt would furnish a good part of the heat needed to raise the tempera- 

 ture of the base of the silicic crust to melting. The idea of partial melting 

 of the base of the silicic crust, especially those parts thrust slightly down- 

 ward during the Laramide orogeny, is attractive because, thereby, a 

 magma of monzonitic composition might be formed rather than one of 

 granodioritic composition as in the case of bulk melting of great roots. 

 Partial melting will not only facilitate viscous flow to level out the base of 

 the silicic crust (Fig. 38.2) but also will produce the great volumes of 

 various gneisses and schists called migmatites whose features characterize 

 them as transitional to igneous. A granitic or monzonitic magma would 

 have been squeezed out, and represent the first minerals to melt, hence 

 more acidic, and more basic varieties would represent the melting of a 

 larger percentage of a basal portion of the silicic crust nearby. The basalt 

 immediately beneath may be tapped by a fissure conduit from time to 

 time and add its conspicuously dark and perhaps unexpected presence to 

 the surface assemblage. 



Mixing of a small amount of the silicic magma with basalt would pro- 

 duce an andesite, or the basalt could fractionate to an andesite. Very 

 little andesite is needed in this province. 



The latitic magma of the ignimbrite subprovince contained sufficient 

 water such that effervescence of water vapor at a temperature high enough 

 for welding occurred. The extrusion temperatures of the tuff-breccias in 

 the Pine Valley Mountains is interpreted to be lower than that necessary 



