Structure and Evolution
A major cone, flow and vent complex, Sierra Nevada is located in the Salar de Isla area on the frontier between Chile and Argentina, the most remote and inaccessible part of the Central Andes (Figure 39.1;39.2). The whole complex covers an area of about 225 km2 and consists of at least twelve discrete vents visible on the TM image; some are simple craters up to 400 meters across, while others are sources of large andesite lava flows, which extend for up 7 km and have well developed flow ridges and broad marginal levées up to 600 m wide. On the basis of their morphology they are probably of andesitic composition. The oldest part of the complex seems to be at the extreme east end of the Sierras Nevadas de Lagunas Bravas, and consists of two stratocones, one with a broad summit crater ~1 km in diameter.
In character, as well as eruptive style Sierra Nevada is similar to the better known Nevados Ojos del Salado, the Cordons Puntas Negras and Cerro Tipas, all of which are related to regional tectonic features. Although no coherent structure is discernible in the arrangement of the vents of Sierra Nevada, there is a general extension in an east-west direction suggestive of some tectonic control. It is difficult to identify any one of the many andesitic vents as having been more recently active than any other. Two samples examined by Schnurr et al. (2007) had ages of ~1 Ma.
Schnurr et al. (2007) reported 58.1-58.5% SiO2, 206Pb/204Pb = 18.802, 207Pb/204Pb = 15.608, 208Pb/204Pb = 38.734, 87Sr/86Sr = 0.7067, and 143Nd/144Nd = 0.51242 from two whole-rock analyses.
Schnurr, W.B.W., Trumbull, R.B., Clavero, J., Hahne, K., Siebel, W., Gardeweg, M., 2007. Twenty-five million years of felsic magmatism in the southern Central Volcanic Zone of the Andes: geochemistry and magma genesis of ignimbrites from 25-27¡S, 67-72¡W. Journal of Volcanology and Geothermal Research, 166(1):17-46.