Abstract
Magnetotelluric studies were carried out in 1993 and 2008 inside the caldera of Copahue Volcanic Complex, located in South America, at the border between Chile and Argentina (37°45ʹ S; 71°10.2ʹ W). The main effusive centre of this complex is the Copahue Active Volcano, which constitutes an important geothermal zone. The study of the crust and the investigation of possible magma chambers were the objectives of this survey. Six magnetotelluric soundings were interpreted taking in mind the geologic and tectonic background. Two 2D bimodal modelling along two profiles approximately perpendicular to geological strike were performed. In addition, two magnetovariational studies were made, using two magnetic variometers. Induction (Wiese) vectors were thus estimated for three MT sites. Among the results, the lithosphere in the region is suggested to be 60—66 km thickness, with upper and lower crusts of 8 km and 10 km thickness respectively. A magma chamber is suggested at lower crust with top at 3—8 km depth, with 1 Ωm of resistivity, thus indicating partial-melting or melted-rocks. Data suggest this chamber could be genetically associated with a deep fault system into the caldera. Heat flows of 130—278 mW/m 2 were estimated at surface, above magma chambers, using empirical formulas linking depths of thermal conductive layers, in the crust and upper mantle, with heat flows values measured at surface. The estimated heat flows, thus obtained, are rather consistent with heat flows measured in wells drilled into the caldera. Taking into account that magnetotelluric soundings, at present, are not usual in volcanic studies, the present work may give valuable information about this active volcano, (which at present is in yellow alert), mainly because there are people living at about 15 km from the volcano or less in Argentina and Chile. In addition this study is important from geothermal point of view, for the possibility to obtain energy without air contamination.
Highlights
Introduction and geological outlineIn the South American Continent, southward of 33°S, there are, at least, 60 historically and potentially active volcanoes [Stern, 2004], mainly located along the Andean Range
Taking into account all results, we can synthesize the conclusions as follow: 1) subsidence of basement inside the Copahue Caldera, relevant to surrounding region, is about 700—1400 m; 2) asthenosphere is suggested with top to about 60—66 km depth, indicating a lithospheric thinness
A magma chamber seems to be seated at the lower crust with top between 4 and 8 km depth
Summary
Introduction and geological outlineIn the South American Continent, southward of 33°S, there are, at least, 60 historically and potentially active volcanoes [Stern, 2004], mainly located along the Andean Range. The Copahue Volcanic Complex (CVC), located in Patagonia at the Chile—Argentina border (about 37°52ʹS, 71°09ʹW), has four effusive centres whose activities began from Pliocene [Pesce, 1989]. The main of these is the Copahue Volcano (37°51ʹ S, 71°10,2ʹ W) (Fig. 1), seated at the Chile-Argentina border, which constitutes one of the main geothermal areas in Argentina. The role of faults and lineaments, present in the zone, in the genesis of magma chambers was investigated To perform this survey, deep magnetotelluric (MT) soundings were carried out in the region in 1993 and 2008. A preliminary publication was already done, using field data from the 1993 campaign [Mamani et al, 2000]
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