590 



STRUCTURAL GEOLOGY OF NORTH AMERICA 



volcanoes for the most part stand as isolated piles having been fed by 

 conduits through the batholithic complex and deformed eugeosyncline. 



The stratovolcanoes of the southern Cascades of Oregon and Washing- 

 ton have been built on an older andesite complex but in the northern 

 Cascades of northern Washington and southwestern British Columbia 

 they stand as isolated cones fed by conduits through the batholithic com- 

 plex. 



The stratovolcanoes of southern Mexico are built on an extensive older 

 volcanic field and are part also of extensive fields evidently as young as 

 the volcanoes themselves. The belt of stratovolcanoes seems to lie on 

 the inner margin of the metamorphic belt and also partly on the de- 

 formed adjacent geosyncline. See Chapter 43. 



POST-BATHOLITHIC VOLCANICS TO TRENCHES 



Trenches are of two kinds, the submarine deeps marginal to the con- 

 tinents and the fault-depressed zones generally within the batholithic 

 complex or separating it from the anticlinoria. The volcanics are of the 

 stratovolcanic and basalt-andesitic field types. 



The fault-depressed trenches are the sites of both basalt-andesite com- 

 plexes and stratvolcanoes. The stratovolcanoes occur in both the de- 

 pressed blocks and on the adjacent upraised blocks. In Chile south of 

 Santiago the cones are chiefly on the east side of the depressed zone. In 

 northern Chile and southern Peru the volcanic deposits may have filled 

 depressed blocks, with extensively faulted regions both east and west of 

 the volcanic accumulations. In northern Peru, Ecuador, and southern 

 Columbia, the volcanic deposits have filled a long graben-like depression, 

 and stratovolcanoes are present within the depression and on its marginal 

 uplifted blocks, particularly on its eastern block. 



The fault-depressed blocks of the South American Cordillera are gen- 

 erally described as compressional structures, bounded on one side or both 

 by uplifted overthrust blocks. The southern Mexico stratovolcanic 

 province is probably bounded on the south by a block-faulted region, but 

 little is known of the structures there. The great Sierra Madre Occidental 

 field is broken and bounded on the west by a thrust-faulted zone and 

 then by the major depressed zone of the Gulf of California. The de- 



pressed zone here is postulated to be due to the drift of Baja California 

 away from the continent and to the northwest, in connection with the 

 San Andreas fault movements of California. A young volcanic field exists 

 on the west or outer side of the depressed zone. 



In western Canada the andesite complex is west of the depressed zone, 

 here the Rocky Mountain Trench, which separates the Nevadan complex 

 and geanticline from the deformed miogeosyncline. 



In the United States the relations are very complicated, and compari- 

 sons with the South American and Canadian can only be imagined. 

 The broad Basin and Range province would be the fault-depressed belt, 

 which in Nevada and Utah is superposed on the eastern side of the eugeo- 

 syncline and across the entire miogeosyncline. It is replete with volcanics 

 but not of the basalt-andesite complex, but rather of the monozonite- 

 latite clan. A zone of particularly conspicuous trenches (graben and 

 horst blocks) make up the eastern side of the Basin and Range province 

 and these extend northward through Idaho and western Montana into 

 British Columbia. Relatively minor volcanic activity is noted in the zone 

 of trenches from the High Plateaus field of south-central Utah to the 

 Finlay River field of northern British Columbia. 



A spatial coincidence of the stratovolcanoes of South America to the 

 offshore, submarine trenches is immediately conspicuous, but in detail we 

 may note first, the Chilean row south of Santiago extends southward 

 beyond the limits of the submarine trench and second, the trench is con- 

 tinuous but the stratovolcanoes occur in three separate rows or seg- 

 ments. 



The Central American Trench lies opposite the stratovolcanoes of south- 

 ern Mexico and also the active and dormant volcanoes of Central America, 

 but the trench, as an ocean-floor phenomenon, does not continue north- 

 ward where the major volcanic complex of Mexico occurs. The sub- 

 marine trench coincides well with recent volcanic activity but not with 

 the older activity. 



The Cascade andesite complex and row of stratovolcanoes is not com- 

 plemented by a submarine trench. It may, therefore, be concluded that 

 a submarine trench is not a necessary accompaniment of a row of adjacent 

 stratovolcanoes; one may exist without the other, but their coincidence 

 spatially is more likely than not. 



