5 SO 



STRUCTURAL GEOLOGY OF NORTH AMERICA 



NEVAOAN BATHOLITHIC 

 BELT 



GEOSYNCLINE INVOLVEO IN SHELF AREA INVOLVED IN 

 POST-BATHOLITHIC OROGENY POST-BATHOLITHIC OROGENY 



Fig. 36.6. Relations of tectonic provinces of western United States to igneous. To identify 

 igneous provinces compare with Fig. 36.3. 



The sill in the upper Belt series of Glacier National Park is a dark diorite 

 but in places is pink due to orthoclase feldspar and in other places green 

 from the presence of mica. Stratigraphically several hundred feet above 

 the sill is the Purcell lava unit 50 to 275 feet thick. It is made up of several 

 submarine flows with pillow structure. Hundreds of feet higher forming* 

 the top of the exposed Belt strata are several other flows. It is not known 

 whether the flow rocks are spilites and keratophyres (Dyson, 1949). 



Basic sills of diabase ( ? ) occur in the Ray-Superior-Globe area of the 

 Mountain Region of south-central Arizona, and are the only other ones 

 known to the writer in the Laramide systems similar to those in the Belt 

 series of western Montana. 



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RELATION OF TECTONIC TO IGNEOUS PROVINCES 



The major tectonic and igneous provinces of the western United States 

 are related to each other on the map of Fig. 36.6. The Nevadan batholithic 

 belt has about the same distribution as the previously existing eugeosyn- 

 cline. The Mexican geosyncline has considerable volcanic material on the 

 north and west in the Cretaceous sediments, but the Nevadan belt of 

 metamorphism and batholithic intrusions developed to the west in Baja 

 California. The later second cycle batholiths intruded the western flank 

 of the geosyncline, however, in great volume. 



The batholithic belt was the site of later basaltic and andesitic eruptions 

 in two regions : ( 1 ) a narrow zone extending from northern California to 

 southern British Columbia, and (2) in central British Columbia along the 

 east-central part of the broad batholithic belt with the vast Coast Range 

 batholith entirely on the west. As in the Sierra Madre Occidental no row 

 of late Cenozoic stratovolcanoes occurs in the broad fields of central 

 British Columbia. The Cascade basalt-andesite field with its row of strato- 

 volcanoes correlates in north-south extent with the eastern bulge of the 

 batholithic belt in Oregon, Washington, Idaho, and southern British 

 Columbia. It does not continue southward where the batholithic belt 

 narrows in central and southern California. A genetic relation to the bulge 

 is implied. 



The igneous rocks east of the batholithic belt in the miogeosyncline of 

 Nevada and western Utah are mostly of the monzonite-latite clan with 



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