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
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.