Imaging the Laguna del Maule Volcanic Field, Central Chile Using Magnetotellurics: Evidence For Upper-to-mid Crustal Melt Regions Laterally-offset From Surface Vents and Holocene Lava Flows

Monday, 8 January 2018
Salon Maule (Hotel Quinamavida)
Darcy Raymond Cordell, University of Alberta, Edmonton, AB, Canada, Martyn Jonathan Unsworth, University of Alberta, Department of Physics, Edmonton, AB, Canada and Daniel Diaz, University of Chile, Santiago, Chile
Abstract:
Broadband magnetotelluric (MT) data were collected at the Laguna del Maule volcanic field (LdMVF), located in central Chile (36° S, 70.5° W), which has been experiencing unprecedented upward ground deformation since 2007. These data were inverted to create a three-dimensional electrical resistivity model of the LdMVF and surrounding area. Four conductive features were identified:
  • A near-surface conductor (C1; 0.5 Ωm) located 100 m beneath the lake. This feature is interpreted as a clay cap related to a shallow hydrothermal system.
  • At 1 km depth directly beneath C1, a small conductor (C2; <10 Ωm) is imaged but is not required by the MT data due to the attenuation of low frequency signals by C1 and should not be over-interpreted. Attenuation from C1 does not affect the imaging of C3 and C4.
  • At 4 km depth, a strong con­ductor (C3; 0.3 Ωm) is located beneath recent vents in the northwest LdMVF and dips to the north. Feature C3 is interpreted as a hydrous (>6 wt% H2O), dacitic-to-rhyolitic partial melt (>35% melt).
  • C4 (1 Ωm) is located north of the lake at 9 km depth. C4 is interpreted as an andesitic-to-dacitic partial melt (<40% melt) responsible for the largest eruptions in the LdMVF.

A large, extensive conductor beneath LdMVF is not imaged with MT and modelling tests suggest that such a feature is not required by the MT data. Studies of the properties of partial melt suggest that any hypothesized large crystal mush volume beneath LdMVF must be anhydrous (<2 wt% H2O) and low melt fraction (<25%) in order to go undetected. Additional model tests show that a 30 km3, shallow, eruptible volume with high melt fraction (>50%), beneath the inflation center is not supported by the MT data. However, a smaller (e.g. 10 km3) anomaly cannot be resolved with the current MT array.

The presence of large conductors to the north is significant in studying the magma dynamics of the LdMVF as it suggests that magma and hydrothermal fluids may have moved laterally more than 10 km as they rise from depth (C4) to create small, ephemeral volumes of eruptible melt closer to the surface (C3). This lateral movement is likely facilitated by existing weaknesses or faults. This has important implications for long-term volcanic hazards at LdMVF as well as other volcanoes as it raises the possibility that underlying magma bodies may not be directly beneath recent surface vents.

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