Pearce, Rebecca K.; (2020) Andean Hydrothermal and Structural System Dynamics: Insights from 3D Magnetotelluric Inverse Modelling. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

In an active volcanic arc, magmatically sourced fluids are channelled through the brittle crust by structural features inherent in the lithological setting. This interaction is observed in the Andean volcanic mountain chain, where volcanoes, geothermal springs and major mineral deposits are spatially coherent with first-order NNE oriented thrust fault systems, and convergent-margin oblique WNW striking Andean Transverse Faults (ATF). The volcanic and hydrothermal activity at Tinguiririca and Planchón-Peteroa volcanoes demonstrate this relationship, as both volcanic complexes and their spatially associated thermal springs show strike alignment to the outcropping NNE oriented El Fierro thrust fault system. This study aims to constrain the 3D architecture of this fault system in the proximity of the volcanoes and its interaction with volcanically sourced hydrothermal fluids from a combined magnetotelluric (MT) and seismic field study. Data from a 24 station broadband magnetotelluric survey were interpreted using 3D inversion. Over 700 seismic hypocentres from a 12 station coeval seismic survey are also presented in support of the final 3D conductivity model. The combined results show a correlation of conductivity anomalies with seismic clusters in the top 10 km of the crust, including a distinct seismogenic WNW oriented feature that occurs at an abrupt electrical conductivity contrast, which is most apparent at a 6 km depth. It is concluded that this discrete feature is an Andean Transverse Fault (ATF), and that the conductors are signatures of either geothermal fluid reservoirs or fluid saturated lithologies at depth. The associated fluids are channelled parallel to the margin-oblique ATF plane and cause fault reactivation due to increased pore fluid pressure acting on the fault plane. Seismicity induced by this mechanism is limited to the east of the El Fierro fault system, as fluids are compartmentalized along the footwall due to the low permeability fault core that prevent cross-fault fluid migration. This study thus contributes novel insight into how WNW oriented AFT systems interact with local volcanic, structural and hydrothermal systems.

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