Dissecting a Zombie: Shallow Volcanic Structure Revealed by Multiple
Geophysical Data Sets at Uturuncu Volcano, Bolivia
Abstract
Uturuncu volcano in southern Bolivia is something of a “zombie”
volcano – presumed dead, but showing signs of life. The volcano has not
erupted in 250 kyr, but is exhibiting unrest in the form of ground
deformation, seismicity, and active fumaroles. Elucidating the
subsurface structure of the volcano is key for interpreting this recent
unrest. Magnetotelluric measurements revealed alternating high and low
resistivity anomalies at depths <10 km beneath the volcano,
with a low-resistivity anomaly directly beneath Uturuncu. A key question
is, what is the nature of this anomaly? To what extent is it partial
melt, a hydrothermal brine reservoir, or a mature ore body? Knowing the
density of this anomaly could distinguish between these scenarios, but
existing density models of the area lack sufficient resolution. To
address this issue, we collected additional gravity measurements on the
Uturuncu edifice with 1.5 km spacing in November 2018. Gradient analysis
and geophysical inversion of these data revealed several features: a 5
km diameter, high density anomaly beneath the summit of Uturuncu (1 – 3
km elev.), a 20 km diameter ring-shaped negative density anomaly around
the volcano (-3 – 4 km elev.), a NNE trending, positive density anomaly
northwest of the volcano (0 – 4 km elev.), and a NW trending, negative
density anomaly to the southeast. These structures often (but not
always) align with resistivity anomalies, features in new seismic
tomography models, and relocated earthquake hypocenters. Based on a
joint analysis of these data, we interpret the positive density anomaly
as a crystallizing dacite pluton, and the negative density ring anomaly
as a zone of hydrothermal alteration. Earthquakes around the edges of
the crystallizing pluton may represent escaping fluids as the magma
cools. The high density anomaly to the northwest likely represents a
solidified pluton, and the low density anomaly to the southeast may
represent a fractured fault zone. We posit that the alternating zones of
high and low resistivity anomalies represent zones of low and high
fluid/brine content, respectively. Based on this analysis we suggest
that the unrest at Uturuncu is unlikely to be pre-eruptive. This study
shows the value of joint analysis of multiple types of geophysical data
in evaluating volcanic subsurface structure.