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



603 



to the surface cannot be explained as surficial features of the folding of 

 sedimentary sequences. 



Basin and Range Province and Tension. In Chapter 31 it was postu- 

 lated that the Rasin and Range province has been distended about 30 

 miles since Miocene time, and the suggestion made that this process could 

 provide for the intrusion from great depth of much magma. The earth- 

 quake foci have been interpreted to mean that the great faults extend 

 to depths of 20-40 kilometers. 



Eugeosyncline and the Tension Hypothesis 



With an expanding earth and a crust cracking apart in places we may 

 devise a scheme of magmatism for the eugeosyncline. See Fig. 38.4A. The 

 crystalline crust of the continents seems to end abruptly at the ocean basins, 

 and in the realm of an expanding earth the continent-ocean boundary may 

 generally be a zone of weakness where extension will be focused. If so, 

 then here will be a likely site for the rise of magma from the upper 

 mantle. The continental margins are commonly sites of deep-seated seis- 

 micity as well as unusual thermal activity in the mantle. Fissure eruptions 

 in the eugeosyncline have been postulated. 



Provision must be made for the evolution of the andesites, and if they 

 arise by fractional crystallization from basalt, then there must exist large 

 magma chambers in the subcrust where the process takes place. It would 

 appear that basaltic eruptions should be dominant in the early stages of 

 the eugeosyncline with andesites more abundant later; also undifferenti- 

 ated basalt could be conducted to the surface from time to time as new 

 fissures break through to great depths. It is not known if observations in 

 the eugeosynclines support the supposition that andesites become more 

 abundant in the later stages. 



Batholithic Belts and the Tension Hypothesis 



Speculating further, as the eugeosyncline develops the crust is depressed 

 under it, and the depression is mostly the result of removal of support 

 by the ejection of magma through fissures to the surface. See Fig. 38.4R 

 and C. Eventually, the silicic upper crust or the base of the eugeosyncline 

 comes into the domain of melting, and it is at this stage, with continued 



PACIFIC OCEAN 



DISTENSION AND 

 FISSURE ERUPTIONS 



MELTING OF LOWER PART 

 OF BASALTIC CRUST 



EUGEOSYNCLINE SUBSIDES 

 ZONE OF MELTING MOVES UP 



ZONE OF DISTENSION 

 FRACTURES AND 

 BATHOLITHIC INTRUSION 



Fig. 38.4. Speculations on crustal structure at the continental margin and the relation of 

 magmatism to the eugeosyncline and batholithic belt, if the earth should be expanding and 

 the crust distended. 



expansion and tension that the growth of the batholiths of intermediate 

 and acidic composition begins. The fluidity of the basaltic melts provides 

 for rapid flow to the surface, but the greater viscosity of the more silicic 

 magmas makes for slower, more irregular intrusions, with attendant varied 

 intrusive relations. The space problem is largely accounted for, however, 

 by irregular Assuring and pulling apart of the crust (Fig. 38.4C). 



Several adjacent fissures may develop, and each is invaded by the silicic 

 magmas, thus perhaps accounting for the great septa of metamorphosed 

 country rock noted in some of the batholithic belts. The batholithic belts 

 in some places are narrow and linear and seem to fit nicely the tension 

 hypothesis, but others are more irregular with the batholiths in clumps, 

 and therefore do not accord with the hypothesis very well. 



Problems 



The conventional explanation for the origin of large volumes of magma 

 of intermediate composition is the melting of the lower part of a thick- 



