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The recent identification of unrest at multiple volcanoes that have not erupted in over 10 kyr presents an intriguing scientific problem. How can we distinguish between unrest signaling impending eruption after kyr of repose and non-magmatic unrest at a waning volcanic system? 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, ±15 μGal precision, and ±5 cm positioning precision, improving the resolution of existing gravity data at Uturuncu. This high quality data set permitted both gradient analysis and full 3-D geophysical inversion, revealing a 5 km diameter, positive density anomaly beneath the summit of Uturuncu (1.5–3.5 km depth) and a 20 km diameter arc-shaped negative density anomaly around the volcano (0.5–7.5 depth). 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.
Bibliographical noteFunding Information:
JG and MP conceived the project underpinning the research and secured funding (NERC grant NE/S008845/1 and NSF grant 1757495). PM wrote the initial draft, analyzed the data (inversion, derivative analysis, joint analysis), and secured additional funding (FINESST award number 80NSSC19K1339). JG collected the gravity and GNSS data together with NY, FT, ET, and RT, post-processed the data, and performed the gravity data reductions. All authors contributed to the discussion and interpretation of the findings. The writing of the final draft was led by PM with contributions from JG and MP.
PM was supported by NASA FINESST award number 80NSSC19K1339. MP and PM were both supported by NSF grant number 1757495. JG and NY were supported by NERC grant number NE/S008845/1. FT, RT, and ET were jointly supported by NSF grant 1757495 and NERC grant NE/ S008845/1.
© Copyright © 2021 MacQueen, Gottsmann, Pritchard, Young, Ticona J, Ticona and Tintaya.
- Central Andes
- hydrothermal system
- volcanic structure
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Investigating the relationship between pluton growth and volcanism at an active intrusions in the central Andes
1/10/09 → 1/01/14