Regional electrical structure of the Andean subduction zone in central Chile (35\xb0\u201336\xb0S) using magnetotellurics

Valentina Reyes-Wagner,Daniel Díaz,Darcy Cordell,Martyn Unsworth

doi:10.1186/s40623-017-0726-z

Abstract

A profile of broadband magnetotelluric stations was acquired between 2009 and 2016 at 35°–36°S in the Southern Volcanic Zone of the Chilean Andes to image the subduction zone and its relation with the volcanic arc at this latitude. This transect extends from the Coastal Cordillera across the Central Valley and the volcanic arc of the Principal Cordillera to the Argentine border. Two active volcanic complexes are found along this profile: Tatara-San Pedro is located on the modern volcanic front, and the Laguna del Maule volcanic field is found approximately 30 km to the east. The latter exhibits considerable signs of unrest, such as uplift rates of up to 25 cm/year, and has produced a high concentration of silicic eruptions in the last 25 ky. The data covered the period range from 0.001 to 1000 s. Robust processing techniques were used, including remote reference, and dimensionality was investigated by estimation of geoelectric strike, skew and analysis of the induction arrows. The data were modeled using a 2D inversion algorithm to produce a resistivity model which was consistent with surface geology and seismicity. The final resistivity model shows a generally resistive fore-arc structure, coincident with the tectonic environment, and a wide conductive region from the volcanic front to the east. This suggests a broad region of magmatism throughout the arc, related to three distinct magma bodies, associated with the Tatara-San Pedro and Laguna del Maule volcanic complexes and the Mariposa Geothermal System.

Highlights

The Southern Volcanic Zone (SVZ) of the Andes extends between 33° and 46°S and has been formed as the Nazca Plate subducts obliquely beneath the South American Plate in a direction of approximately N78°E (Somoza 1998)
In the Central Volcanic Zone, which extends from 14° to 27°S, deep conductive features are found below the Altiplano and Puna plateaus, approximately 100 km east of the modern volcanic arc (Brasse and Eydam 2008; Diaz et al 2012; Comeau et al 2016)
The goal of this paper is to present a regional transect which relates the regional structure of the subduction zone to the shallow magma bodies beneath this area of the Andes

Summary

Introduction

The Southern Volcanic Zone (SVZ) of the Andes extends between 33° and 46°S and has been formed as the Nazca Plate subducts obliquely beneath the South American Plate in a direction of approximately N78°E (Somoza 1998). As in other subduction zones, fluids released from the slab facilitate partial melting in the upper mantle by reducing the melting point of the rocks (Grove et al 2012). These mafic melts rise due to buoyancy and differentiate on their way to the surface where they are erupted (Hildreth and Moorbath 1988). A number of previous MT studies have imaged subduction zones worldwide, including the Central and Southern Andes (Brasse and Soyer 2001; Brasse and Eydam 2008; Brasse et al 2009; Diaz et al 2012; Comeau et al 2016). In the Central Volcanic Zone, which extends from 14° to 27°S, deep conductive features are found below the Altiplano and Puna plateaus, approximately 100 km east of the modern volcanic arc (Brasse and Eydam 2008; Diaz et al 2012; Comeau et al 2016)

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