Transient surface deformation over thousand-year timescales at Uturuncu volcano, Bolivia

Abstract:

The Altiplano-Puna Volcanic Complex (APVC) in southern Bolivia has repeatedly sourced large-volume ignimbrite eruptions since 11 Ma. Though eruption rates have waned since 3 Ma, ­it is uncertain whether eruptions of a similar scale will occur in the future. Uturuncu volcano, a long-dormant dacite stratovolcano located within the APVC in southern Bolivia, currently exhibits sustained surface deformation over a broad, 70 km diameter area, with a peak uplift rate of ~1-2 cm/yr. This deformation corresponds to a magmatic source within the Altiplano-Puna Magma Body (APMB), a zone of partial melt within the mid-crust of the APVC that is thought to have sourced its large-volume ignimbrites. Given the short timescale of current geodetic measurements, however, it is unclear how this modern unrest relates to the evolution of Uturuncu’s underlying magmatic source. As geomorphic processes operate on timescales similar to those of magma chamber growth, we use the topography of lakes and rivers surrounding Uturuncu as strain markers to constrain its surface deformation history since the late Pleistocene. We investigate Uturuncu’s geomorphology through both field surveys and analysis of a high-resolution (0.5 m) digital elevation model (DEM). We focused our field efforts on two lakes proximal to Uturuncu: Laguna Mama Khumu and Laguna Loromayu. OSL data reveal shorelines of Tauca age (~15 ka), suggesting a regional basin response to climate. Differential GPS surveys of Mama Khumu’s shorelines show no evidence of deformation along the volcano’s flanks. Assuming a constant inflation rate, we estimate a maximum onset of Uturuncu’s present deformation phase at ~1 kyr before present. DEM-derived shoreline elevations match well with dGPS surveys, and DEM-derived river longitudinal profiles similarly show a lack of sustained surface deformation since the Tauca highstand. Together these observations imply a relatively stable topography at Uturuncu over the late Pleistocene, and thus the recent unrest here likely reflects a transient pulse of inflation rather than a snapshot of long-term tumescence. These results provide a valuable constraint for modelers seeking to link surface deformation to magmatic processes at depth, and more generally highlight the time-transient nature of large silicic magma chamber systems.