Dissecting a Zombie: Joint analysis of density and resistivity models
reveals shallow structure and possible sulfide deposition at Uturuncu
After ca. 250 kyr without a known eruption, in recent decades Uturuncu
volcano in Bolivia has exhibited multiple signs of unrest, making the
classification of this system as “active”, “dormant”, or “extinct”
a complex question. Previous work identified anomalous low resistivity
zones at <10 km depth with ambiguous interpretations. We
investigate subsurface structure at Uturuncu with new gravity data and
analysis, and compare these data with existing geophysical data sets. We
collected new gravity data on the edifice in November 2018 with 1.5 km
spacing, improving the resolution of existing gravity data at Uturuncu.
Gradient analysis and geophysical inversion of these and older gravity
data revealed a 5 km diameter, positive density anomaly beneath the
summit of Uturuncu (1-3 km elevation) and a 20 km diameter arc-shaped
negative density anomaly around the volcano (-3 to 4 km elevation).
These structures often align with resistivity anomalies previously
detected beneath Uturuncu, although the relationship is complex, with
the two models highlighting different components of a common structure.
Based on a joint analysis of the density and resistivity models, we
interpret the positive density anomaly as a zone of sulfide deposition
with connected brines, and the negative density arc as a surrounding
zone of hydrothermal alteration. 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 at a waning volcanic center.