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A 3-D, Technicolor Zombie: Joint Analysis of Multidisciplinary Geophysical and Geochemical Data at Uturuncu Volcano, Bolivia Reveals Active Hydrothermal System and Possible Sulfide Deposition
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  • Patricia MacQueen,
  • Thomas Hudson,
  • Ying Liu,
  • Elizabeth Eiden,
  • Karissa Rosenberger,
  • Scott Henderson,
  • Matthew Comeau,
  • Joachim Gottsmann,
  • Matthew Pritchard,
  • Michael Kendall,
  • Martyn Unsworth,
  • Tobias Fischer,
  • Jonathan Blundy
Patricia MacQueen
Cornell University

Corresponding Author:pgm65@cornell.edu

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Thomas Hudson
University of Oxford
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Ying Liu
USTC University of Science and Technology of China
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Elizabeth Eiden
Cornell University
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Karissa Rosenberger
University of New Mexico
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Scott Henderson
University of Washington Seattle Campus
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Matthew Comeau
University of Münster
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Joachim Gottsmann
University of Bristol
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Matthew Pritchard
Cornell University
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Michael Kendall
University of Oxford
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Martyn Unsworth
University of Alberta
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Tobias Fischer
University of New Mexico
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Jonathan Blundy
University of Oxford
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Abstract

Uturuncu volcano in southern Bolivia is a member of a distinctive class of volcanoes – systems that show unrest despite not having erupted in the Holocene. Uturuncu has not erupted in 250 kyr, but has been deforming (uplift with a moat of subsidence) for several decades, along with seismic swarms and active, sulfur-encrusted fumaroles. Our work builds on previous geophysical imaging at Uturuncu by jointly analyzing multidisciplinary datasets, focusing on imaging the shallow (<15 km depth below surface) structure of the system with geophysical and geochemical data. Whereas previous research pointed to andesite melt at depths >15 km depth, results were ambiguous as to what proportions of melts vs. brines are present at shallower depths. Identifying fluids (melt, brine, etc.) and structures at shallow depths is key for evaluating the hazard potential of the volcano and understanding the source of the unrest. We present new results from gravimetry, seismology (hypocenter relocation, seismic velocity and attenuation tomography), gas geochemistry, and InSAR observations. The results point to an extensive and active hydrothermal system extending ~20 km laterally and ~10 km vertically from Uturuncu, with possible connections at depth to the deeper magmatic system. A combined view of the new density, seismic velocity and attenuation models, and the existing resistivity model is crucial for revealing key features of the hydrothermal system: a vapour-rich conduit beneath Uturuncu (low resistivity/high attenuation column extending from 1.5 to 12.5 km depth), an extensive alteration zone surrounding Uturuncu (complex zone of annular shaped anomalies surrounding Uturuncu from 1.5 to 12.5 km depth), and a possible zone of sulfide deposition just below the western flank of Uturuncu at 1.5 km depth (high density/low resistivity/high attenuation). High fluxes of diffuse CO2 degassing at sub-magmatic temperatures and a small area directly above a low resistivity anomaly subsiding from 2014 to 2017 show that the hydrothermal system is currently active. Analyzed jointly, this multidisciplinary data set suggests that current activity within the shallow structure at Uturuncu is dominated by hydrothermal, rather than magmatic processes.