The Argentine Precordillera as a key to Andean structure.

BALDIS, B. A.; BERESI, M.; BORDONARO, O. AND VACA, A.

EPISODES

International Union of Geological Sciences

Lugar: Ottawa, Canadá; Año: 1984 vol. 7 p. 14 - 19

0705-3797

The Argentine Precordillera as a key to Andean Structure by B. Baldis, M. Beresi, O. Bordonaro and A. Vaca by B. Baldis, M. Beresi, O. Bordonaro and A. Vaca Although the Argentine Precordillera is only only 400 km long is  a key  zone  for studying  of the genesis  of  the Andes. It is  located in the transitional zone between the Pampean craton  and the mobile belt  of  the Pacific edge of South America. This  article summarize  recent studies  on the Precordillera, with emphasis  on  this  Paleozoic evolution and its  significance to the development of the Pacific ocean margin of  southernmost South America. Introduction The 400 km long Precordillera is unique throughout the Andes (Fig. 2) because it contains a complete thick sequence of Early Paleozoic rocks closely linked to the foreland craton and exposed by the tectonic movements which created the Andes. Work done over the last decade has led to an under­standing of the structure of the Precordillera and neigh­bouring areas, as well as to an appreciation of the role these rocks have played in the genesis of the mobile side of South America during the Paleozoic. Much of this work has been carried out within the framework of the International Geological Correlation Programme, specifically IGCP Projects 44 (Lower Paleozoic of South America-completed), 192 (Cambro-Ordovician), 193 (Silu­ro-Devonian) and 202 (Mega-faults). Project 192 is especially important because its objective is to analyse the development of post-Precambrian continental margins. In Argentina, detailed studies have been conducted on the different stratigraphic sequences of the Precordillera, and satellite images have been used to interpret structures. This work has led to new ideas about the genesis and evolution of the Precordillera sedimentary basins and their relation to the lineaments and fracture zones present in the basement. Working conditions in the Precordillera are not easy. It is located in the arid triangle of South America, and its mountain ranges are high and relatively inaccessible. From the standpoint of both climate and landscape, the Precor­dillera region resembles parts of the southwestern United States, including the Death Valley region. Evolution of the Paleozoic Basins The Precordillera contains a thick column of Paleozoic rocks (Fig. 1) actively controlled by a rigid, tectonically stable foundation. This geological province is bounded to the east by the Pampean Ranges, which constituted a stable foreland during the Paleozoic. To the north it ends against the Puna, a high plateau closely linked with the Arequipa massif of Peru. The western border of the Precordillera is marked by the "Cordillera Frontal" and the "Cordillera Principal" belts of Andean structure accreted to the Precorcfffiera. Joroan and others (1983) presented a general view of the deep structure of this part of the Andes, relating the Precordillera to the fiat-subduction segment of the chain. 14 Figure 1: Cambrian limestones of the Zonda Val in the eastern Precordillera. Along the entire western edge of South America mountain chains making up the Andean Cordillera separated from the ancient cratonic landmasses by well-differentiated elements: small chains that lean foothills directly against the eastern flank of the Cordill and depressed areas located to the east of the foothills. small chains are exemplified by the sub-Andean belt extends from Peru to northern Argentina, and the fold-th belt that runs from the southern edge of the Puna to northern Patagonian landmass, entirely in Argentina (Fq The sub-Andean belt forms a transitional system between orogenic Andes area and the broad plains, with a block­structure that separates them from the continental shi outcroping eastwards in the heart of South America. The Precordillera (Fig. 3) is a geological province elong north-south and located between the Andes belt to the and the blocks of the Central Argentine Craton to the (Bracaccini, 1960). It consists of three well-del longitudinal belts: an eastern belt of overthrusts controlle Cambrian and Ordovician limestone; a central zone mad of the Tontal-Tigre-Punilla Paleozoic block; and a we! belt characterized by a sharp mid-Devonian deformation metamorphism together with earlier (Ordovician-Devo basaltic magmatism (Baldis et al., 1982). An analysis 01 structural evolution of the successor basins shows that ~´I´~+~.!l:~ ~ t;.1;>p- P,=~a was basement controllec the following, the paleogeographic setting of the Prec, lera is reviewed at different stages during its evo] (Figs. 4 and 5). Episodes, Vol. 7, No.3, September