An Assessment of the Tectonic Control in Defining the Geothermal System(s) of the Southern Chilean Andes

Geothermal manifestations between Villarrica and Chihuio (39°15'-40°15'S, 71°40'-72°10'W), in the southern Chilean Andes, have been studied to assess the tectonic control in defining the geothermal systems of the area. These surface manifestations are in close spatial relationship with either the stratovolcanoes or the Liquiñe-Ofqui Fault Zone (LOFZ, Cembrano et al., 1996). Volcanism and regional tectonics control the two vital components of the geothermal systems, viz., heat source and permeability. Two distinct domains of the geothermal systems, viz., structural (or non-volcanic) and volcanic have been identified, based on the chemical signatures of the thermal discharges and structural analysis of the lineaments. These two domains are distinct in their ways of heating up of meteoric water. The geothermal system(s) of the volcanic domain are closely associated with the volcanic centers, spatially as well as geochemically. In the case of the geothermal system(s) of volcanic domain, the heating of meteoric water is through absorption of heat and condensation of steam and gases by meteoric water during lateral circulation. These discharges do not exhibit the typical signatures of steam heated waters, which are subdued by near surface processes. The relation between the geothermal systems and fault and fracture density (FFD) is quite evident from the lineaments analysis. FFD correlates very well with the surface geothermal manifestations, as well as with their recharge areas. An increase in the (secondary) permeability in the uppermost 200-300 m in the areas of relatively high FFD values, necessary for lateral flow, is consistent with the lithology, structure and stratigraphy of the area. Although the lineaments scatter in a wide range, the absence of lineaments between N60°E and N100°E is noticeable, and is consistent with displacement and stress data of LOFZ (Lavenu and Cembrano, 1999; Cembrano et al., 2007; Lara and Cembrano, 2009). This indicates that such lineaments, which represent fractures and faults, are the result of recent deformation, causing secondary permeability that facilitates the subsurface flow particularly in NW-SE and N-S directions. Differential rates of exhumation and frictional heating (Parada et al., 2000; Thomson, 2002; Adriasola et al., 2006; Glodny et al., 2008) are the other two controlling factors for the geothermal systems of the area. (Project Funding: PBCT-PDA07) References: Adriasola, A.C., Thomson, S.N., Brix, M.R., Hervé, F., Stóckhert, B., 2006. Int. J. Earth Sci. 95: 501-528. Cembrano, J., Hervé, F., Lavenu, A., 1996. Tectonophysics 259: 55-66. Cembrano, J., Lara, L., 2009. Tectonophysics 471: 96-113. Cembrano, J., Lavenu, A., Yañez, G. (coordinators), Riquelme, R., Garcia, M., González, G., Herail, G., 2007. In: T. Moreno, W. Gibbons (Eds), The Geology of Chile, The Geological Society, London, 147-178. Cembrano, J., Schermer, E., Lavenu, A., Sanhueza, A., 2000. Tectonophysics 319: 129-149. Glodny, J., Gräfe, K., Echtler, H., Rosenau, M., 2008. Int. J. Earth Sci. 97: 1271-1291 Lavenu, A., Cembrano, J., 1999. J. Struct. Geol. 21: 1669-1691. Parada, M.A., Lahsen, A., Palacios, C., 2000. Geological Society of America Special Papers 2000, 91: 169- 179. Thomson, S.N., 2002. Geol. Soc. Am. Bull. 114: 1159-1173.