Regional electrical structure of the Andean subduction zone in central Chile (35°–36°S) using magnetotellurics

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.

Fluid and Melt Pathways in the Central Chilean Subduction Zone Near the 2010 Maule Earthquake (35–36°S) as Inferred From Magnetotelluric Data

“…There are seven primary conductive features present in both models in Figure and several significant resistive features, all of which were investigated using sensitivity tests. At the surface, a conductor (S1) coincides with the volcaniclastic sedimentary fill of the Central Valley (Reyes‐Wagner et al, ). Feature C1 is a conductor (<10 Ωm) located at shallow depth (<15 km) beneath the trench.…”

“…This was done using the tensor decomposition methods of McNeice and Jones (2001) as well as an analysis of the phase tensors (Caldwell et al, 2004). Multisite, multifrequency tensor decomposition shows a regional strike of N15°E ± 19°for periods >1 s, which is within error of the N5°E regional strike obtained using only broadband MT (Figure 2; Reyes-Wagner et al, 2017). Mean strike and one standard deviation uncertainty was estimated using directional statistics (Mardia, 1972) where one standard deviation in radians equals…”

“…In 2016, broadband MT data (0.001‐ to 1000‐Hz frequency response) were collected along a profile that extended from the Central Valley to the Argentine border perpendicular to the trench (Reyes‐Wagner et al, ; Figure ). This profile included some additional MT data collected between 2009 and 2016 (Cordell et al, ; Hickson et al, ).…”

Field observations and numerical models of a Pleistocene-Holocene feeder dyke swarm associated with a fissure complex to the east of the Tatara-San Pedro-Pellado complex, Southern Volcanic Zone, Chile

“…This constitutes one of the widest segments of the SVZ, up to 60 km wide in the main arc and over 200 km wide in the back arc region (Stern, 2004). Magnetotelluric surveys in this region show conductive anomalies in the upper crust of the volcanic arc, roughly consistent in location with the TSPP, LMVF and Mariposa Geothermal System (MGS) (Reyes Wagner et al, 2017;Cordell et al, 2018;Hickson et al, 2011).…”

“…The crustal properties are kept constant throughout all of the model simulations. The size of the magmatic hydrothermal reservoir is constrained by the shape of conductive bodies obtained from a magnetotelluric profile (Reyes Wagner et al, 2017), while the orienta tion of dykes was measured directly in the field. The lengths of the dykes in the models are deliberately one order of magnitude larger than measured in the field for purposes of mesh resolution.…”