Reactivation of fault systems by compartmentalized hydrothermal fluids in the southern Andes revealed by magnetotelluric and seismic data
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Pearce, R. K.
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Reactivation of fault systems by compartmentalized hydrothermal fluids in the southern Andes revealed by magnetotelluric and seismic data
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Abstract
In active volcanic arcs such as the Andean volcanic mountain belt, magmatically sourced fluids
are channeled through the brittle crust by faults and fracture networks. In the Andes, volcanoes, geothermal
springs, and major mineral deposits have a spatial and genetic relationship with NNE trending,
margin‐parallel faults and margin‐oblique, NW trending Andean Transverse Faults (ATF). The Tinguiririca
and Planchón‐Peteroa volcanoes in the Andean Southern Volcanic Zone (SVZ) demonstrate this
relationship, as their spatially associated thermal springs show strike alignment to the NNE oriented
El Fierro Thrust Fault System. We constrain the fault system architecture and its interaction with
volcanically sourced hydrothermal fluids using a combined magnetotelluric (MT) and seismic survey that
was deployed for 20 months. High‐conductivity zones are located along the axis of the active volcanic chain,
delineating fluids and/or melt. A distinct WNW trending cluster of seismicity correlates with resistivity
contrasts, considered to be a reactivated ATF. Seismicity occurs below 4 km, suggesting activity is limited to
basement rocks, and the cessation of seismicity at 9 km delineates the local brittle‐ductile transition. As
seismicity is not seen west of the El Fierro fault, we hypothesize that this structure plays a key role in
compartmentalizing magmatically derived hydrothermal fluids to the east, where the fault zone acts as a
barrier to cross‐fault fluid migration and channels fault‐parallel fluid flow to the surface from depth.
Increases in fluid pressure above hydrostatic may facilitate reactivation. This site‐specific case study
provides the first three‐dimensional seismic and MT observations of the mechanics behind the reactivation
of an ATF.
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NERC from the London NERC DTP
NE/M004716/1
RTSG from the London NERC DTP
Comision Nacional de Investigacion Cientifica y Tecnologica (CONICYT)
CONICYT FONDECYT
1141139
Canadian Centennial Scholarship Fund (CCSF)
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Tectonics, 39, e2019TC005997 (2020)
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