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



591 



It may also be concluded that trenches within and east of the batho- 

 lithic belt are nearly everywhere present along the entire Cordillera of 

 North and South America, and that in places volcanism seems fairly well 

 localized to the trench or immediately adjacent to it. Extensive volcanism 

 occurs in Mexico, however, on either side of and at a considerable 

 distance from the depressed zone. 



RELATION OF ANTICLINORIA TO OTHER ELEMENTS 



Anticlinoria of Precambrian or metamorphic Paleozoic rock occur 

 parallel to and on the inside of the batholithic belt. These generally ele- 

 vated areas are encompassed in the belt of post-batholithic folding and in 



; places are separated from the batholithic zone by the fault-depressed 

 zone. The anticlinoria range in width from 50 to 150 miles. They are 



i present in the more typical South American and Canadian Cordillera but 



i not present in the atypical United States Cordillera. They lie generally 

 east of the major volcanic fields, although some volcanics occur on them 



! and even east of them. 



| ORIGIN OF MAGMAS 



Physical Considerations 



Crustal Structure. The crust forming the continental masses according 

 to seismic information (Tatel and Tuve, 1955), has a general thickness of 

 28 to 35 kilometers, but interpretations as low as 20 kilometers in coastal 

 California and as great as 65 kilometers under the Sierra Nevada and 72 

 kilometers under the eastern Great Rasin are given. An abrupt change in 

 seismic velocities at the base marks the Mohorovicic discontinuity which 

 is believed to be world-wide. 



The upper layer of the crust which has low velocity is called the granitic 

 crust, silicic crust, or sial, and the lower, the basaltic crust, gabbroic crust, 

 sima, or subcrust. Tatel and Tuve concluded that the two are probably 

 transitional, but others have postulated distinct layers locally of inter- 

 mediate velocity and of different relative thicknesses. 



The silicic layer consists of the fighter rock-forming silicates and is high 

 in Si and Al, and the basaltic layer, as the name implies, consists of the 



darker and heavier silicates and is lower in Si and Al and higher in Fe 

 and Mg. 



The floor of the oceans, beneath a thin veneer of lava flows and un- 

 consolidated sediments, consists of a basaltic laver 5 to 10 kilometers 

 thick, which overlies the mantle. Extensive volcanic accumulations, aLso 

 composed mostly of basalt, rest on the basaltic crust in many places. 



The outer part of the mantle down to a depth of several hundred 

 kilometers is crystalline. It consists mainly of dense silicates of Mg and 

 Fe, prominent among which is olivine, and is often referred to as peri- 

 dotitic (Turner and Verhoogen, 1951), but many be eclogite, a high- 

 density phase of gabbro (Kennedy, 1960). 



Geothermol Gradient and Melting Points. Measurements in mines and 

 wells indicate that the earth temperature increases with depth at a rate 

 of approximately 30 °C per kilometer. Gradients as low as 7°C per kilo- 

 meter and as high as 50°C per kilometer are known but are exceptional. 

 According to Turner and Verhoogen ( 1951 ) the temperature at the base 

 of the crust, say at 40 kilometers, is 500 to 600°C, at 100 kilometers 800 

 to 900°C, and at 2900 kilometers 1500°C. 



Magmas erupted from volcanoes have been found to have temperatures 

 as high as 1000 to 1200° C, and the melting temperature at the surface of 

 basalt of about 1000° C is in this general high-temperature range. Rut such 

 a temperature is not normal to the rocks at the 40-kilometer depth. Con- 

 sequently, according to Turner and Verhoogen, either the magma origi- 

 nates by fractional melting of deep-seated earth material of peridotitic 

 or eclogitic composition, or it is the result of melting of shallower rocks 

 in place under temperatures temporarily raised far above the average 

 temperature normally prevailing at that depth. 



Temporary and local increase in temperature within the crust or outer 

 shell of the mantle might be developed in three ways : ( 1 ) by the blanket- 

 ing effect of a thick sediment-filled basin; (2) local radiogenic heat; and 

 (3) frictional heat due to diastrophism. Turner and Verhoogen conclude 

 that the blanketing effect of sediments 10 kilometers thick would result in 

 an increase in temperature of less than 200 or 300° C in the crustal rocks 

 beneath. Regarding radiogenic heat in the outer mantle shell, they be- 

 lieve that this could result in cyclical convective overturn, and that the 

 temperature of the crust immediately above would be raised appreciably 



