Thermomechanical modeling of the Altiplano-Puna deformation anomaly: Multiparameter insights into magma mush reorganization

Abstract

A 150-km-wide ground deformation anomaly in the Altiplano-Puna volcanic complex (APVC) of the Central Andes, with uplift centered on Uturuncu volcano and peripheral subsidence, alludes to complex subsurface stress changes. In particular, the role of a large, geophysically anomalous and partially molten reservoir (the Altiplano-Puna magma body, APMB), located ~20 km beneath the deforming surface, is still poorly understood. To explain the observed spatiotemporal ground deformation pattern, we integrate geophysical and petrological data and develop a numerical model that accounts for a mechanically heterogeneous and viscoelastic crust. Best-fit models imply subsurface stress changes due to the episodic reorganization of an interconnected vertically extended mid-crustal plumbing system composed of the APMB and a domed bulge and column structure. Measured gravity-height gradient data point toward low-density fluid migration as the dominant process behind these stress changes. We calculate a mean annual flux of ~2 × 10 7 m 3 of water-rich andesitic melt and/or magmatic water from the APMB into the bulge and column structure accompanied by modest pressure changes of <0.006 MPa/yr. Two configurations of the column fit the observations equally well: (1) a magmatic (igneous mush) column that extends to a depth of 6 km below sea level and contains trapped volatiles, or (2) a volatile-bearing hybrid column composed of an igneous mush below a solidified and permeable body that extends to sea level. Volatile loss from the bulge and column structure reverses the deformation, and explains the absence of broad (tens of kilometers) and long-term (>100 yr) residual deformation at Uturuncu. Episodic mush reorganization may be a ubiquitous characteristic of the magmatic evolution of the APVC.

Time-dependent deformation of Uturuncu volcano, Bolivia, constrained by GPS and InSAR measurements and implications for source models

“…16 Pa s are estimated by Gottsmann et al (2017) and <10 14 Pa s in the partially molten APMB by Comeau et al (2016). Considering the uncertainty in both viscosity and appropriate rheological law for the APMB, we think it is worth exploring elastic models to understand their implications and determine if there are other reasons these models could be discarded.…”

“…The quarter-cylinder design permits exploration of 3D subsurface heterogeneity using the seismic tomography of Ward et al (2014), while maintaining reduced computational expense by taking advantage of mirror plane symmetry. Previous finite-element modeling at Uturuncu uti- lized an axis-symmetric formulation to explore 1D subsurface layering (Hickey et al, 2013), and a companion paper in this issue (Gottsmann et al, 2017) uses the model of Ward et al (2014). In our model, a sink reservoir is located at 20 km depth and the source reservoir at 70 km depth along the cylinder axis.…”

Ductility and Compressibility Accommodate High Magma Flux Beneath a Silicic Continental Rift Caldera: Insights From Corbetti Caldera (Ethiopia)

“…A new generation of numerical models aims to constrain the dynamics and evolution of sub‐volcanic reservoirs by considering the thermomechanical feedbacks between magma and crust (Degruyter & Huber, ; Gottsmann et al, ; Karakas & Dufek, ). In a suitable thermal regime, magma influx can be accommodated inelastically without significantly increasing the reservoir pressure, yet still causing surface deformation (Degruyter & Huber, ; Head et al, ; Le Mével et al, ).…”

Thoughts on the criteria to determine the origin of volcanic unrest as magmatic or non-magmatic

“…Because hydrothermal systems are usually less than 10 km deep [39], source depths greater than this likely indicate magmatic unrest. But it is possible that magmatic fluids at depths greater than 10 km are the source of unrest without 'magma on the move' [40] as part of the TCMS model [1].…”