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STRUCTURAL GEOLOGY OF NORTH AMERICA 



ened silicic crust; the thickening is due to compression. In the concept of 

 expansion and tension no thickening is possible. In fact, the conditions of 

 rise of the batholithic magmas may be similar to those of the quartz 

 latite magmas of the Basin and Range province where the lower part of 

 the silicic layer is believed to melt without previous thickening. 



The explanation of batholithic belts based on extension fails to provide 

 adequately for dynamic metamorphism and isoclinal folding such as 

 occurs in the Sierra Nevada. Possibly a facility for such metamorphism 

 is present in the slices of rock that settle toward the batholith as exten- 

 sion occurs. See Fig. 38.4. It would appear that an extensive aureole of 

 thermal or hydrothermal alteration would occur around the intruding 

 batholith because of the fracturing. In the Andes of Peru no stage of 

 dynamic metamorphism is reported just prior to batholithic intrusion 

 (H. L. Hosmer, personal comunication), so the problem evidently does not 

 exist there. 



Another phenomenon that occurs and for which an explanation is not 

 readily seen by the writer is the post-batholithic uplift. The belts of 

 batholithic intrusion are elevated and deeply eroded to expose the large 

 intrusive bodies. In the framework of crustal extension what causes the 

 uplift? One may counter that the Sierra Nevada block is a much later 

 affair and not related to the uplift immediately after intrusion. Also it may 

 be observed that parts of the batholithic belt of South America and North 

 America are fairly low-lying today and that the batholiths may have come 

 closer to the surface than illustrated in Fig. 38.4. 



Still another problem is apparent in consideration of the Gulf of Cali- 

 fornia. It was speculated that the eugeosyncline forms by fissure erup- 

 tions as the crust is pulled apart. In Chapters 30 and 31 evidence was 

 presented that Baja California has been pulled apart from the mainland 

 of Mexico during Cenozoic time, so we should expect extensive fissure 

 eruptions there. The volcanism instead is concentrated on the east in the 

 Sierra Madre Occidental with some also on the west in Baja California. 



Only a few volcanic cones exist in the Gulf itself. Possibly no large magma 

 chambers existed where the fractures and separation occurred. 



Basic Conflicts 



The foregoing discussion of the origin of the various magmas is wrought 

 with several conflicting concepts. In certain considerations we conjure up 

 a state of compression in the crust and outer mantle; in others we enter- 

 tain extension of the crust. Where extension, we recognize certain zones 

 of extension complementary to zones of compression, or we imagine 

 world-wide tensional strain. If tension in local zones, then we usually 

 think of convection circulation in the mantle; if world-wide, we say ex- 

 pansion of the entire earth. Some believe a little expansion has occurred, 

 some considerable, but considerable expansion appears impossible ( Cook 

 and Eardley, 1961 ) . Others recognize local or regional vertical uplift due 

 to changes of state in the mantle as the basic tectonic activity, without 

 appreciable overall earth expansion. Secondary, gravity-caused flow move- 

 ments on the flanks create the compressional structures. World-encircling 

 rises underlain by an expanded mantle-crust transition layer seem to be 

 a reality. And finally, there are many geologists who support the concept 

 of drifting and rotating continents without earth expansion. These move- 

 ments are commonly attended by horizontal coupling of varying magni- 

 tudes. Then there is the pointed conflict of granitization versus magmatic 

 intrusion, particularly in regard to the great batholiths of the eugeosyn- 

 cline. The writer finds convincing examples of each and all of the above- 

 mentioned theories, yet none seems adequate to explain the entire 

 panorama of structural and igneous observations. 



It was hoped that the igneous rocks, when their origin was investigated 

 and related to crustal structure, would point out which of the theories 

 are valid, and perhaps the study has accomplished this to some small 

 extent, but there still remains much uncertainty. 



