IGNEOUS AND TECTONIC PROVINCES IN SOUTH AMERICA 



547 



The broad and somewhat irregular field of Southern Peru, Rolivia, and 

 northern Chile and Argentina is less positively tied to faults even though 

 the modern volcanoes seem to be. The Puna field could perhaps be 

 developed over a Basin and Range type of faulted terrane, judging from 

 the several rows of volcanic vents there. 



Extending from Santiago southward for nearly 1000 miles is a depres- 

 sion that separates the Coast Ranges from the Andean Cordillera. This is 

 called El Valle Central, and is believed to be a complexly faulted graben. 

 The zone of active and dormant stratovolcanoes is almost exactly com- 

 mensurate in length with the depression, and in the central and southern 

 part the volcanoes follow closely the eastern side or are within the 

 graben. At the north end they occur about 60 miles to the east of the 

 graben. 



The fault zones do not bear the same relation everywhere to older 

 tectonic units. In Ecuador the graben occurs between the batholithic 

 belt and the older anticlinorium. In southern Peru the fault zone is mostly 

 within the batholithic belt or along its east side, and in the Puna de Ata- 

 cama it is developed on the nonintruded eugeosynclinal strata. 



The great, tilted, fault blocks that comprise the Pampean Ranges make 

 up a region free of volcanic rocks, and conversely, the volcanic fields of 

 the Argentina foreland are evidently not related to faulting. 



El Valle Central is almost entirely in the batholithic zone, but prefers 

 the eastern side at the north end. 



PARANA BASIN BASALT FIELD 



The Parana basin is one of Paleozoic and Mesozoic age, developed by 

 subsidence of a large, approximately oval-shaped region in the Precam- 

 brian Brazilian shield. The known Paleozoic section consisting of strata 

 representing all periods except the Mississippian is at least 10,500 feet 

 thick, The basin is about 1200 miles long and 400 miles wide ( see map, 

 Fig. 34.1). Desert conditions prevailed in mid- or early Late Triassic time 

 and a windblown sand deposit was spread around irregularly. Then came 

 the eruption of great floods of basalt. Between sheet flows in places more 

 desert sand accumulated. 



In southern Brazil these eruptive rocks are generally at least 400 in thick 

 and are locally as much as 800 m. In Sao Paulo, north of Parana and Rio 

 Grande do Sul, the flows are locally separated by lenticular layers of cross- 

 bedded eolian sandstone, some of which reach a thickness of 40 m. A charac- 

 teristic of these extrusives is the general absence of olivine. Some lava flows 

 are amygdaloidal, and these alternate with flows in which an irregularly de- 

 veloped columnar structure occurs. Pyroclastic rocks seem to be absent; the 

 extrusion was of the calm type of fissure eruption. 



Many feeding dikes and associated sills cut the underlying formations. Al- 

 most all the dikes are vertical. Most of the faults that cut the underlying 

 formations also have steep dips. A number of fault planes, including some 

 along which there was movement of 50 m or more, are filled by dikes of 

 diabase. One of these, cut by the Santa Clara-Urubici highway on the top 

 of the Serra do Panelao, Municipality of Bom Retiro, Santa Catarina, is a fault 

 that vertically displaced the Botucatu sandstone (and apparently the basal 

 part of the overlying eruptives) about 95 m. The fault is occupied by a diabase 

 dike more than 300 m. thick (Avelino, 1956). 



It may be calculated from the above figures and map extension of 

 the field that about 75,000 cubic miles of basalt were extruded in fissure 

 flows. 



It is interesting to note that the Karroo system of sedimentary rocks 

 in South Africa was invaded in Jurassic times by a multitude of diabase 

 dikes and sills which crop out intermittently over an area of 1,500.000 

 square miles or about five times the area of the Parana basalts. The vol- 

 ume has not been figured, but must be as much or more than that of the 

 Parana basin. 



One half of the large island of Tasmania was once covered bv diabase 

 sheets which totaled at least 30,000 cubic miles. The Columbia River field 

 contains about 40,000 cubic miles of basalt, also. All of these dike, sheet, 

 and fissure-flow diabase and basalt fields are of the tholeiitic type. 



Classification of Igneous Provinces 



The Parana basin field is clearly a tholeiitic basalt province, and it is 

 evident that large volumes of primary tholeiitic basalt magma were 

 generated and rose to the surface without differentiation. The origin of 

 such a magma is a controversial question (Turner and Verhoogen, 1951), 

 but it is generally agreed the source was below the silicic crust. The 

 problem will be taken up later. 



