38. 



SPATIAL RELATIONS OF 

 MAJOR TECTONO-IGNEOUS 

 ELEMENTS AND 

 THE ORIGIN OF MAGMAS 



PREVIOUS OROGENY IN EUGEOSYNCLINE 



In all parts of the eugeosyncline of South and North America, evidence 

 of mid- or late Paleozoic orogeny is at hand, and especially in the Sierra 

 Nevada of California and the Coast Ranges of southeastern Alaska we 

 note a succession of orogenies of both Paleozoic and Mesozoic age. The 

 isoclinal folding and development of slaty cleavage in the Jurassic Mari- 

 posa formation previous to the intrusion of the batholiths in the Sierra 

 Nevada has clouded the effects of an orogeny of late Paleozoic age there, 

 but the importance of the older orogeny is emphasized by a study of the 

 eugeosyncline in South America where little note is made of the Late 

 Jurassic or Early Cretaceous orogeny and commonly only the older 

 Paleozoic one is recognized. 



The Paleozoic orogeny in South America affected a belt from the 

 present coast to the eastern mountain ranges in places. It shows today 

 as the metamorphic basement of the Coast Ranges and as the meta- 

 morphic rocks in the anticlinoria 150 to 300 miles inland from the coast. 

 The anticlinoria are in the eastern part of the Mesozoic eugeosyncline 

 and in the western part of the miogeosyncline, and have formed during or 

 later than the batholithic orogeny. The great width of the belt of dynamic 

 metamorphism indicates orogeny of superior intensity long before the 

 invasion of the great batholiths. The batholiths generally were emplaced 

 in the oceanward margin of the metamorphic belt. 



RELATION OF BATHOLITHIC BELT TO EUGEOSYNCLINE 



The spatial relations of the major tectono-igneous elements of the west- 

 ern Cordillera of South and North America will now be summarized. 

 The batholithic belt coincides almost exactly with the previous eugeosyn- 

 cline. In places eastern segments of the eugeosyncline have not been in- 

 vaded by the great batholiths, and in one known place, the batholiths 

 have invaded the entire width of the eugeosyncline and also part of the 

 miogeosyncline. This is in Idaho, western Montana, and southeastern 

 British Columbia. 



RELATION OF POST-BATHOLITHIC COMPRESSIONAL 

 OROGENY TO GEOSYNCLINE AND SHELF 



After the main batholithic intrusions, mainly in the eugeosyncline, 

 strong folding and thrusting occurred in the miogeosyncline. In South 

 America Cretaceous and Tertiary sediments have accumulated on the 

 former transition region from miogeosyncline to shelf, and thus the rela- 

 tion to folding is obscured. Aside from the Pampean Ranges it appears 

 that no conspicuous orogeny has occurred in the shelf. The width of 

 the belt of folding ranges from 50 to 300 miles. 



Post-batholithic folding in Mexico is extensive. Laramide folding from 



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