Application of a plate model to reproduce surface deformations observed at Uturuncu volcano, Bolivia
Abstract
Between 1992 and the mid 2010s, strong uplift has been observed centred on Uturuncu volcano in Bolivia. More recently it was shown that a smaller-amplitude moat of subsidence surrounds the uplift. We propose that this "sombrero" pattern can be the signature of elastic-plate deformation due to a buoyant fluid stored beneath it. We take the base of the "plate" to be the brittle ductile transition which, in this region, is shallow when compared with the crustal thickness. First, we show experimentally that this type of storage can produce the observed deformation pattern. Secondly, we adapt a plate deformation model and use it to invert the surface deformation published by Henderson and Pritchard (2017). A total of 1681 inversions was performed to span the wide range of physical-parameter values. We found that our model can find a source that reproduces the vertical deformation well for a large range of parameters. The viable sources mostly correspond to a thin plate (≈5 km) or a magma with high buoyancy (possibly suggesting the presence of a volatile phase in the storage zone). Interestingly, the petrologically-determined magma storage depth before the last eruption of Uturuncu (≈4±2 km) is approximately the same as the plate thickness deduced. The radial displacement is less correctly reproduced than the vertical. The, at most, 15.6 m magma layer needed to reproduce the total observed uplift since 1992 is smaller than the typical thickness for seismic detection, which can explain why it was not detected. Storage of buoyant silicic magma at a shallow brittle-ductile transition in hot continental crust may be applicable to other large silicic systems.
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