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STRUCTURAL GEOLOGY OF NORTH AMERICA 



1. Eruption of dominantly basic (spilitic, keratophyric, basaltic, and 

 andesitic) lavas during the eugeosynclinal phase. 



2. Injection of ultrabasic and basic plutonic intrusions into the eugeo- 

 synclinal sediments and volcanic rocks which are almost constantly being 

 disturbed by episodes of folding. 



3. The climactic folding and dynamic metamorphism of the eugeo- 

 synclinal rocks. In the South American Andean system the folding seems 

 to have been mostly late Paleozoic, preceding the Late Cretaceous batho- 

 liths by a long time. Much eugeosynclinal volcanic rock accumulated 

 between the metamorphism and the batholithic intrusions. In the Siena 

 Nevada a series of orogenic phases stretching at least from the Devonian 

 to the Cretaceous preceded the batholithic intrusions. In Late Jurassic 

 time intense folding and low-grade regional metamorphism climaxed the 

 train of disturbances. Ratholithic intrusions followed immediately in Late 

 Jurassic and again, most voluminously, in Mid-Cretaceous. In the Acadian 

 belt of New Hampshire, specifically the White Mountains, early folding 

 and thrusting resulted in regional low-grade metamorphism, then fol- 

 lowed the main batholithic intrusions with accompanying medium and 

 high-grade metamorphism, and finally a second episode of thrusting. The 

 three stages occurred within late Devonian time. 



4. Emplacement of the great granodioritic and granitic batholiths into 

 the folded and metamorphosed complex. Some batholiths are involved 

 in the folding, but the great bulk of the plutonic rock is post-folding in 

 age. Each great batholith is commonly composed of a number of indi- 

 vidual plutons with each having a slightly different composition. They 

 range from diorite to granite with granodiorite the most abundant. In 

 the Sierra Nevada the sequence of intrusions seems to have occurred over 

 a period of 18 m.y. 



5. An extensive episode of erosion in which the batholithic rocks are 



exposed, with the development commonly of adjacent longitudinal basins 

 of sedimentation. 



6. Renewed volcanism with the building of great lava and pyroclastit 

 fields, chiefly andesitic, on the folded and metamorphosed batholithic belt. 

 These are the Tertiary volcanic fields of the Andes and of the Sierra Madre 

 Occidental of Mexico, and probably the Mississippian (?) Moat vol- 

 canics of the White Mountains. In places latites may be very abundant. 



7. Following shortly the post-batholithic eruptions and probably part of 

 the same renewed igneous activity are new batholithic intrusions which 

 in places reach up to the volcanic accumulations and intrude them. Ex- 

 amples are the White Mountain magma series of the White Mountains 

 the post-volcanic batholiths of the Cascade Range of Washington, and the 

 imposing belt of mid-Tertiary batholiths of western Sonora. This is 

 the second cycle batholithic province of the proposed classification above. 



8. A late volcanic activity occurs in segments of the orogenic belt and 

 results in the building of a majestic row or belt of stratovolcanoes, or an 

 extensive field of basaltic flows and cinder cones. 



In the western United States the broad Paleozoic miogeosyncline and 

 shelf, and the superposed Mesozoic basins and Laramide belts of deforma- 

 tion, are replete with igneous rocks of the trachyte, phonolite, and latite 

 associations. When compared with the Andean, Mexican, and Canadian 

 Cordillera, the wide and complex western United States Cordillera is an 

 exception. Tectonic provinces like the Great Basin, Colorado Plateau, 

 and the Wyoming and Colorado Rocky Mountains belt of orogeny are 

 either not developed to the north and south of the United States or are 

 reflected in a narrower or restricted way. It is the object of the following 

 pages to review the petrographic and igneous provinces of the western 

 Cordillera of South and North America by using the above depicted con- 

 cepts in an attempt better to understand the process of orogeny. 





