SPATIAL RELATIONS OF MAJOR TECTONO-IGNEOUS ELEMENTS AND THE ORIGIN OF MAGMAS 



589 



the Nevadan batholiths to the east front of the Sierra Madre Oriental 

 forms a belt up to 450 miles wide. Its relation to Paleozoic sedimentary 

 basins is largely unknown, but it embraces the Cretaceous eugeosyncline 

 and miogeosyncline. 



In the United States folding and thrusting extend through the mio- 

 geosyncline. See Fig. 36.6. Basins and asymmetrical anticlinal uplifts 

 expressive of significant vertical components of force are common in the 

 shelf. The maximum width of the belt of folding in the Paleozoic mio- 

 geosyncline is about 300 miles, and the front of the belt of deformation 

 in the shelf is another 400 miles farther east at its most easterly point. 

 As in South America rather thick Mesozoic sedimentary sequences have 

 accumulated along the Paleozoic miogeosyncline and shelf transition zone 

 and also in places over the shelf. The shelf in the United States was also 

 the site of building of the Ancestral Rockies in late Paleozoic time. 



The belt of Laramide folding in British Columbia and Alberta is intense 

 and about 100 miles wide. It is confined to the Paleozoic miogeosyncline 

 and the western part of the Mesozoic basins over the miogeosyncline and 

 shelf. Farther north, the folding is less intense but has a maximum width 

 of about 300 miles. It spreads here mostly over a Paleozoic cratonic 

 ■basin and an overlying Cretaceous basin, and the force component is 



vertical. 



I 

 ■ 



RELATION OF POST-BATHOLITHIC VOLCANICS TO BATHOLITHIC BELT 



In Chile south of Antofagasta and Argentina the post-batholithic vol- 

 canics are chiefly on the deformed eugeosyncline, miogeosyncline, and 



shelf east of the batholithic belt. The maximum width of the general 



belt of extrusive rocks is 300 miles. In northern Chile and southern 

 Teru the belt of volcanic deposits is partly on the batholithic belt, and 



spans across the narrow eugeosyncline to the deformed miogeosyncline. 



Only a small amount of extrusive rock is found as far east as the anti- 



jclinorium. The main belt of volcanism is about 150 miles wide. 



I 1 The belt of volcanic deposits in Ecuador and Columbia spreads across 



the boundary of the batholithic belt and the anticlinorium. This volcanic 

 .•field is narrow in comparison with the others, and does not exceed 50 



miles unless some of the stratovolcanoes are considered part of it, in 

 which case the belt reaches 80 miles in width. 



There are two major volcanic fields in Mexico. The larger of the two, 

 the Sierra Madre Occidental, rests on the deformed Cretaceous eugeo- 

 syncline and in places over the miogeosyncline, a considerable distance 

 east of the Nevadan batholithic belt. The smaller field, and the more- 

 recent, rests on the batholithic complex in the southern part of Baja 

 California. The great Sierra Madre Occidental field is 200 to 300 miles 

 wide. The smaller field in Baja California is 30 to 60 miles wide. 



The volcanic fields in western Canada lie almost entirely within the 

 batholithic belt. The largest accumulations, in British Columbia, are 150 

 miles in width. 



The volcanic fields of the western United States are broad and varied. 

 Where the batholithic belt is narrow in southern and central California, 

 volcanic deposits are few, but all through the broad miogeosyncline 150 

 to 300 miles wide, they are extensive. They also occur in considerable 

 quantity in scattered fields in the shelf to the east which in part has been 

 moderately deformed in post-batholithic time. Volcanic eruptions in 

 Colorado are 500 miles east of the miogeosyncline, and the Black Hills 

 igneous rocks are 350 miles east of the miogeosyncline. 



In northwestern United States, where the Nevadan belt is very wide, 

 the great basalt fields occur. The tholeiitic (Columbia River) basalt field 

 is entirely on the batholithic belt and is nearly 300 miles wide. The olivine 

 vent basalt field is mostly on the batholithic complex but extends east- 

 ward over the miogeosyncline to the shelf. These two large basalt fields 

 are exceptional to all other fields in the western Cordillera of South 

 and North America, and seem related to the great batholithic bulge at the 

 intersection of two Nevadan orogenic arcs. 



RELATION OF POST-BATHOLITHIC VOLCANIC FIELDS 

 TO STRATOVOLCANOES 



The three rows of stratovolcanoes of the South American Cordillera 

 are closely related to the orogenic andesite complexes. The southern row. 

 south of Santiago, however, is not accompanied by voluminous fields; the 



