546 



STRUCTURAL GEOLOGY OF NORTH AMERICA 



the northerly trends of Chile and Argentina to the northwesterly trends 

 of Bolivia and Peru. The southern division is notably an assemblage of 

 individual smaller fields. 



Although the eugeosynclinal, batholithic, and fold belts are continuous 

 from Tierra del Fuego to Colombia, the Cenozoic volcanic fields are not. 



Spatial Relations to Older Belts 



In a broad way the volcanic fields lie between the Cordillera Oriental 

 and the Cordillera Occidental, and in part fill a graben whose sinking 

 between the two linear relief elements was about contemporaneous with 

 the eruptions. The volcanic rocks, however, spread over both adjacent 

 Cordillera extensively in places, and in northernmost Chile extend west- 

 ward across the batholithic belt to the coast. In the Puna de Atacama 

 region of northern Chile opposite Antofagasta, the extrusions are entirely 

 east of the batholithic belt and mostly on the eugeosynclinal strata free 

 of batholithic intrusions. They spread eastward, also, to the deformed 

 miogeosynclinal and shelf sediments. Possibly 800 active and inactive 

 cones exist in this large field and seem to be arranged in several rows. A 

 few scattered fields are on the Precambrian and Paleozoic anticlinorium 

 to the north. 



In Ecuador and southwestern Colombia the main volcanic field fits 

 rather snugly in a graben between the batholithic belt on the west and 

 the anticlinorium of older metamorphosed rock on the east. Some of the 

 great stratovolcanoes have vents through the cordilleran rocks on either 

 side, however, and have built considerable volumes of ejecta on these 

 foundations beyond the faults that bound the graben. 



The southern division of volcanic fields is generally east of the batho- 

 lithic belt on the batholithic-free Mesozoic volcanic rocks, and as men- 

 tioned, a number of isolated fields lie on the miogeosyncline and shelf 

 areas of the foreland. This division is singular in that the row of great 

 stratovolcanoes is mostly in the batholithic belt and not a part of the vol- 

 canic fields. At the north end of the division, the row cuts acutely into the 

 eugeosyncline, and several vents are offset sufficiently far east so that they 

 are in the miogeosyncline (see map, Fig. 34.1). South of Santiago numer- 

 ous volcanoes have been active in recent years. The great isolated vol- 



canoes or groups of volcanoes are spaced at about 30- to 40-kilometer 

 intervals in this part of the zone. 



Composition 



The flows associated with active volcanoes in Chile are mostly basalts, 

 ranging from hypersthene basalt in the oldest flows to olivine basalt in the 

 more recent, with the exception of Calbuco, which still erupts hypersthene 

 basalt (Cristi, 1956, p. 213). 



The volcanic fields of the foreland in Argentina are nearly all basalt 

 flows of Pliocene-Quaternary or Quaternary age. The Eocene and Oligo- 

 cene volcanic rocks in the cordilleran region are andesites and dacites, 

 and overlying Miocene flows are basalts. 



The great Puna field consists of augite and hypersthene andesite with 

 the latest flows of olivine basalt. Rhyolite is also reported. 



The Tertiary and Quaternary volcanic rocks of southern Peru range 

 in composition from basalt to rhyolite, with andesite, trachyandesite, and 

 trachyte very abundant. Cutting the extrusives are numerous small stocks 

 of diorite, monzonite, quartz monzonite, syenite, and dacite porphyry. 



The volcanic rocks of Ecuador and southeastern Colombia are domi- 

 nantly andesitic pyroclastic and flow rocks. 



It may be concluded, therefore, that andesites are the most abundant 

 of the Cenozoic volcanic rocks which appear within the Cordillera, with 

 olivine and hypersthene basalts probably next in abundance and also 

 usually latest in the eruptive sequence. In southern Peru the trachyte 

 volcanics and the monzonite and syenite stocks are unusual because of 

 their high alkalic content. 



Relation to Graben Faulting 



Both Gerth ( 1955 ) and Cristi ( 1956 ) emphasize the relation of fault- 

 ing to volcanism, or more generally stated, to "recent tectonic depres- 

 sions." In the entire Andes only where a well-developed longitudinal 

 valley exists do volcanoes occur. This concept relates specifically to the 

 rows of active and dormant stratovolcanoes. In the Ecuador division, 

 however, the entire field is fairly closely tied to the graben faulting which 

 here has been interpreted as of compressional nature. 



