Magnetotelluric Investigation of the Laguna del Maule Volcanic Field, Central Chile

• Author / Creator

  Cordell, Darcy R

• This thesis uses the magnetotelluric (MT) method to image the electrical resistivity structure of the Laguna del Maule Volcanic Field (LdMVF) and the regional subduction zone structure of central Chile (36°S) in the vicinity of the 2010 Maule earthquake. The LdMVF surrounds the eponymous Laguna del Maule (LdM) and the area has been experiencing unprecedented and ongoing upward ground deformation greater than 20 cm/year since 2007. This, along with extensive rhyolite eruptions in the last 25 ka, suggests the presence of a restless magmatic system at depth. Broadband MT data were collected at the LdMVF and the data were processed, analyzed and inverted to produce the first three-dimensional image of the electrical resistivity structure of the LdMVF. The shallow subsurface included small low resistivity anomalies directly beneath the lake at 100-300 m depth and at 1 km depth, both interpreted as hydrothermal fluids and alteration. The most significant low resistivity anomaly in the model is located in the mid-crust (10 km depth) and is laterally-offset northwards from vents, lava flows, and the center of deformation. A steeply-dipping conductor is imaged on the western side of the LdMVF in the upper crust (3 km depth) and connects to the mid-crustal conductor. Both the mid- and upper-crustal conductive anomalies are interpreted as zones of partial melt and hydrothermal fluids which suggests that deep source melts may migrate both laterally and vertically as they approach the surface. Detailed sensitivity analyses were performed to elucidate discrepancies between the MT resistivity model and published seismic velocity and density models. These simulations suggest that the MT data are better fit with isolated, steeply-dipping conductors along the mapped Troncoso fault, rather than a single large conductor in the upper crust. This suggests that any large (e.g. 450 km3) homogeneous mush zone in the upper crust beneath the LdMVF contains relatively little interconnected melt. The MT is imaging a structure-driven magmatic plumbing system which contains batches of eruptible magma and hydrothermal fluids in the upper crust. A relatively small (e.g. 10 km3) ephemeral magma reservoir in the shallow crust beneath the inflation center could go undetected but larger volumes (e.g. 30 km3) with high-melt fraction would have a detectable signature in the MT data. The southeastern LdMVF—on the footwall side of the Troncoso fault—contains no significant geophysical anomalies despite being volcanically active which further suggests that eruptible volumes of magma must be small and ephemeral. The lack of a large electrical resistivity anomaly directly beneath the LdMVF provides an important constraint on the magma plumbing system.

  To better understand the regional context in which the LdMVF is situated, broadband and long-period MT data were collected along a 380-km profile from the Pacific Ocean to western Argentina. These data were used to image the deeper subduction zone structure and better understand the role that fluids play in earthquake rupture and magma genesis. The data measured a regional geoelectric strike of N15°W ± 19° with a notable westward shift at sites on the volcanic arc. The data also suggested some three-dimensional geoelectric structure and possible anisotropic features but two-dimensional isotropic inverse modelling was employed as an approximation. The preferred inversion model included several conductors along the plate interface related to fluid release from compaction and metamorphic reactions in the forearc, and higher pressure-temperature metamorphic reactions and flux melting in the backarc. A resistor on the plate interface near the Moho is interpreted as a strong, dry asperity which may affect the co-seismic slip behavior of large megathrust earthquakes at this latitude. This resistor is correlated with the previously identified Cobquecura high velocity anomaly from seismic tomography. Beneath the volcanic arc, two conductors in the upper crust (25 km depth) underlies both volcanoes and suggests a connected network of melt in a thermally-mature lower crust.

• Subjects / Keywords

  • Volcanology
  • Magnetotellurics
  • Chile
  • Geophysics

• Graduation date

  Spring 2020

• Type of Item

  Thesis

• Degree

  Doctor of Philosophy

• DOI

  https://doi.org/10.7939/r3-x17z-hv08

• License

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