Dissecting a Zombie: Joint Analysis of Density and Resistivity Models Reveals Shallow Structure and Possible Sulfide Deposition at Uturuncu Volcano, Bolivia

Abstract

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.

Transcrustal Compressible Fluid Flow Explains the Altiplano‐Puna Gravity and Deformation Anomalies

“…Smaller dimensions of the ABC ensemble than considered here would require larger density and pressure changes, with a larger ensemble requiring smaller changes. Given that we have used optimal model constraints we conclude that the results of the study fall in the range of geophysical solutions of a set of mutually consistent subsurface models of the target area (e.g., Comeau et al, 2016;del Potro et al, 2013;Hudson et al, 2022;Kukarina et al, 2017;MacQueen et al, 2021;McFarlin et al, 2017).…”

“…The high‐density body is also associated with a positive gravitational tilt angle and interpreted to represent an intrusive complex and/or a zone of disseminated sulfides by MacQueen et al. (2021). Alternatively, the gravity variations may mirror water level perturbations in a river bed which extends NNE‐SSW along D1 (FOOF is located at the E shoulder of D1).…”

“…(i.e., z = −3 km) by MacQueen et al. (2021). Benchmarks JIMY, LDZP, MTLC, and SEXP are located in a ring‐like structure of high negative tilt angles and representative of the column‐like low‐density anomaly beneath Uturuncu (del Potro et al., 2013), whereas all other benchmarks, bar BLND, are located in structures with high positive tilt angles.…”

“…At FOOF, anomalous behavior may be associated with water table variations and or/localized stress addand density perturbations along an SW-NE elongated topographic ridge. The ridge is composed of older eroded volcanics at the surface and underlain by >3 km thick material of higher than background density material (labeled anomaly D1 by MacQueen et al, 2021) and shown in Figure 3c for reference). The high-density body is also associated with a positive gravitational tilt angle and interpreted to represent an intrusive complex and/ or a zone of disseminated sulfides by MacQueen et al (2021).…”

“…(c) Gravimetric tilt angle derived from the local Bouguer anomaly using data presented in MacQueen et al. (2021) where angles are given in degrees. Small yellow circles mark static gravimetric measurement points.…”