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



601 



for welding, and perhaps the ignimbrite subprovince was determined not 

 only by abundant water but also by a higher than normal temperature. It 

 corresponds to the postulated projection of the East Pacific Rise under the 

 western United States (Chapter 31). 



TECTONO-IGNEOUS PROVINCES AND DEEP-SEATED EARTHQUAKES 



The South American Andes and adjacent shields and basins are noted 

 for their intermediate depth and deep-seated earthquakes. In charting the 

 foci Renioff ( 1954 ) was led to the conclusion that they lie along an ex- 

 tensive, inclined plane or surface that extends down under the Cordillera 

 and stable region to depths of nearly 700 kilometers. He illustrated the 

 earthquake foci along two sections, one across northern Chile and Argen- 

 tina, and another across Equador to the Guayana shield. For these sec- 

 tions the writer has idealized the crustal geology as shown in Fig. 38.3 in 

 accord with the more detailed cross sections of Fig. 34.5. It will be seen 

 that the volcanic fields as previously pointed out are east of the Nevadan 

 batholithic belt and he principally on the deformed miogeosyncline. They 

 occur somewhat shoreward of the break in slope of the earthquake foci 

 surface. Renioff postulates this surface to be a gigantic reverse fault due 

 to compression in the mantle. It has also been postulated that this great 

 fault is the region of origin of basaltic magma, especially of the alkali 

 olivine variety (Kuno, 1959). Here is a likely place where the partial 

 fusion of the upper mantle shell occurs to supply basalt and heat to the 

 subcrust and crust, and where consequent intrusive and extrusive mag- 

 matic activity takes place. Although andesites are widely recognized in 

 the Andes, Dr. Howel Williams informs the writer that he has a knowl- 

 edge in part and a strong hunch that great volumes of the volcanic piles 

 in South America and Mexico are of the composition called latite or quartz 

 latite in the discussion of the Great Rasin volcanics on previous pages. 

 If the base of the silicic crust is fused partially, then by postulate, more 

 latite than andesite would probably be extruded. 



The deep-seated earthquakes in South America and Mexico are comple- 

 mented by a trench at the continental margin, which is presumed by 

 Benioff and others to be a compressional consequence of reverse move- 



ment along the great fault defined by the earthquakes. Deep-seated earth- 

 quakes have not been recorded in the western United States or western 

 Canada, and no trench exists at the continental margin; yet, the other 

 igneous and tectonic components of the western orogenic belts are present. 

 It seems to the writer that the deep-seated earthquakes have been an 

 integral part of the western Cordillera of Canada and the United States 

 during most of the Tertiary, but that the fault along which they occurred 

 is now inactive. It may have been replaced by the East Pacific Rise and 

 associated expansion of the mantle. 



CRUSTAL TENSION AND MAGMATISM 



Previous References 



The belief that the earth's crust has suffered large amounts of shortening 

 in the orogenic belts has been an orthodox tenet of geologists for many 

 years. Lately several individuals, including de Sitter (1956) and Rucher 

 (1956), have argued for vertical uplift with consequent gravity flow or 

 sliding of the surficial rocks away from the uplift to form the folds and 

 thrusts, and therefore, for minor amounts of, or no horizontal shortening. 

 Others are now contending for expansion of the earth and tension as the 

 primary and dominant force of crustal deformation. 



In Chapters 41 to 43 the concept that the earth is expanding is men- 

 tioned in connection with the possible drifting apart of North and South 

 America. Also in Chapter 31 the Rasin and Range province was explored 

 relative to tension in the crust and expansion of the earth. The Mid- 

 Atlantic rift in Chapter 10 was treated as a tensional structure as a result 

 of earth expansion. It is now absorbing to speculate on magmatism in the 

 framework of crustal tension. 



Evidence of Tension 



Fissure Eruptions and Tension. The most plausible concept of the 

 origin of fissures through which large volumes of magma have passed to 

 the surface is one of tension. Fissure eruptions have always been difficult 

 to explain in the framework of crustal compression. Fissures through which 

 basic magmas in large amounts have flowed from the basaltic subcrust 



