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STRUCTURAL GEOLOGY OF NORTH AMERICA 



tains, Uinta Mountains, and San Rafael Swell as results. These elongate 

 uplifts have lengths of 75 to 150 kilometers and widths of 40 to 75 kilo- 

 meters. Giant-sized laccolithic intrusions in the subcrust of similar hori- 

 zontal dimensions could have originally arched and upfaulted die struc- 

 tures, whose structural relief would thereafter have been augmented by 

 sediment transfer to adjacent basins and gravitational adjustments. A 

 giant-sized laccolith would need to be perhaps only 1 kilometer thick to 

 result in a final structural relief at the surface of perhaps 3 or 4 kilo- 

 meters. See Fig. 36.4. 



Still other considerations of the theory of primary basalt magma gener- 

 ation in the outer mantle remain. They are in the fields of gravity and 

 seismicity. Dr. Kenneth L. Cook's reactions to the gravity problems are 

 as follows. If the outer shell of the mantle should expand and elevate the 

 crust, say of the Colorado Plateau, 2 kilometers, isostatic anomalies in 

 the order of 10 to 20 milligrams would probably occur, but effects of local 

 surficial density variations might mask the overall isostatic anomaly pic- 

 ture to the extent that it would be unrecognizable. The problem is fairly 

 complex. At least the concept of partial melting and expension of the 

 outer mantle under regions as large as the Colorado Plateau does not run 

 afoul of any gravity observations or interpretations that he could see 

 off hand. 



Dr. Joseph W. Berg's reactions to the seismic problems are as follows. 

 Melting of 5 percent of a certain column of the mantle in a disperse sys- 

 tem of some kind would lower earth wave velocities, but not any more 

 than the observed range of velocities interpreted from seismic records in 

 the upper mantle or lower crust. As far as he could see, the concept of 

 partial melting of parts of the upper mantle 50 to 200 miles across is not 

 contrary to any seismologic analysis. 



From the above considerations it is concluded that under the Laramide 

 systems of the alkalic igneous province olivine basalt was generated by 

 partial fusion of the upper mantle and rose to the subcrust where it was 

 intruded, probably in giant sill bodies; only minor amounts escaped up 

 through the silicic crust to the surface. Large bodies of the molten basalt 

 lay directly under the silicic crust, affected some melting and assimila- 

 tion, and by various routes of fractional crystallization, mixing, and sieving, 



the contaminated primary magmas bore in small amounts the unusual 

 alkalic and calc-alkalic suites of the Colorado Plateau, Wyoming, and 

 Montana. 



Magmas of the Nevadan Systems 



The conclusion has been reached on prevous pages that the batholithic 

 masses of the Nevadan belt represent such an enormous bulk of quartz- 

 monzonitic and granodioritic material that it is impossible to conceive of 

 their derivation from a basaltic parent by fractional crystallization, and, 

 providing they were once mobile, we are forced to conclude that they rep- 

 resent a primary acid magma. Further, the primary magma originated by 

 the melting of a part of the silicic crust in a master belt of orogeny along 

 the continental margin. The conventional concept involves a thickened 

 crust whose roots melt. The thickness of the silicic layer under the Sierra 

 Nevada is now about 20 kilometers and about 25 kilometers in north- 

 central Utah, but possibly before melting and isostatic adjustments, the 

 crust there was much thicker. See Fig. 38.1. The basaltic subcrust seems 

 about as thick as the silicic crust under the Sierra Nevada, but if upward 

 adjustment has occurred after orogeny then the silicic crust would have 

 been thinned by erosion, as well as viscous flow, and this consideration 

 points to a previous much thicker silicic crust. 



The theory of origin of primary basalt, therefore, contrasts sharply 

 with that of primary granodiorite; the first by partial fusion of the upper 

 mantle shell and upward migration to the subcrust and crust, and the 

 second by fusion of large masses of the lower part of the thickened silicic 

 crust in belts of master orogeny. 



The above discussion is in the manner of those who believe that the 

 great batholiths were emplaced by mobile magma, but there are many 

 authorities who believe that the batholithic rock formed in place by a 

 transformation of previously existing rock. Strong evidence is presented 

 to support this point of view, namely that of granitization. For a review of 

 the evidence see Gilluly (1948). One's point of view changes radically 

 in considering crustal layering, roots, and intrusion space problems when 

 convinced that granitization is the process at hand. It will be commented 

 on later under the headings of andesite magmas and quartz latite magmas. 



