Altiplano-Puna Magma Body

The Altiplano-Puna Magma Body (APMB), a magma body located within the Altiplano-Puna plateau approximately 10–20 km beneath the Altiplano-Puna Volcanic Complex (APVC)^[1] in the Central Andes. High resolution tomography shows that this magma body has a diameter of ~200 km, a depth of 14–20 km, with a total volume of ~500,000 km³,^[2][3] making it the largest known active magma body on Earth.^[1][4][5] Thickness estimates for the APMB are varied, with some as low as 1 km,^[4][6] others around 10–20 km,^[7] and some extending as far down as the Moho.^[8] The APMB is primarily composed of 7-10 wt% water andesitic melts and the upper portion may contain more dacitic melts^[9][10] with partial melt percentages ranging from 10-40%.^[2] Measurements indicate that the region around the Uturuncu volcano in Bolivia is uplifting at a rate of ~10 mm/year, surrounded by a large region of subsidence.^[5] This movement is likely a result of the APMB interacting with the surrounding rock and causing deformation.^[5][10] Recent research demonstrates that this uplift rate may fluctuate over months or years and that it has decreased over the past decade.^[11] Various techniques, such as seismic, gravity, and electromagnetic measurements have been used to image the low-velocity zone in the mid to upper crust known as the APMB.^[9]

The APMB is likely compositionally zoned with the lower 18–30 km containing andesitic melts and the upper 9–18 km containing dacitic melts.^[9] Estimates for the percentage of andesitic melt vary from 8 vol% on the low end and up to 30 vol% on the high end.^[10] These andesitic melts also have a high water content (~7-10 wt.% water^[10]) indicated by the high electrical conductivity measured in the APMB.^[12] Measurements for the partial melt percentage in the APMB also vary, with seismic imaging indicating that it is anywhere from 10-40% partial melt.^[2] For a magma body with ~20% partial melt, the viscosity is estimated to be <10¹⁶ Pa s.^[13]

The Altiplano-Puna region around the Uturuncu volcano is experiencing a type of deformation termed 'sombrero uplift,' which means a central zone of uplift surrounded by a region of subsidence.^[5] One potential explanation for this sombrero uplift pattern is the formation and growth of a large diapir arising from the APMB.^[5] Lower-density magma than the surrounding rocks is produced during partial melting in the APMB, causing a plume of buoyant magma to rise from the center of the magma body.^[5] This causes material to be removed from the APMB to feed the growing diapir, resulting in a region of subsidence surrounding the uplift zone.^[5]

Data collected between 1992 and 2010 demonstrates that the region is uplifting at ~10 mm/year and subsiding at a slower rate (only a few mm/year).^[5][11] More recent InSAR data, collected between September 2014 and December 2017, shows that the uplift rate over this period has decreased to 3–5 mm/year and may experience short-term velocity reversals.^[11] Additionally, there is evidence that the uplift and subsidence rates have balanced out over the past 16,000 years to create no net deformation.^[9] These aspects of the uplift and subsidence cannot be easily explained by the diapir model, so other possible mechanisms for driving the deformation are being investigated.^[11] One such mechanism that might explain the deformation is the movement of volatiles in a column connected to the APMB.^[10] Movement like this may explain the surface deformation rate that varies on monthly or yearly scales and appears to have resulted in no net deformation over longer periods.^[10][9]

Between 1996 and 1997, several broadband seismic stations were deployed over the Altiplano-Puna Volcanic Complex (APVC) in order to characterize the magmatic structures beneath the surface.^[4] These stations found a low velocity region approximately 10–20 km beneath the surface that was interpreted to be a sill-like magma body associated with the APVC.^[4] Seismic studies and modeling continues to take place in this area, further constraining the extent and characteristics of this magma body.^{[2][14][6][15]}

A 3D density model of the Central Andes was developed based on modeling of Bouguer anomalies and it provided a more detailed view of the region's lithospheric structure and an estimation of the amount of partial melt present in the APMB (~9%).^[16] Continued investigation of Bouguer anomaly data led to the discovery of a column-like, low density structure extending from the top of the APMB with a diameter of approximately 15 km.^[3]

Electromagnetic methods have also been used to investigate structures in the Andes as well as determine characteristics of the APMB. Magnetotelluric stations were deployed across the Central Andes and resolved a highly conductive region beneath the Altiplano-Puna plateau, which appeared to coincide with the low velocity zone associated with the APMB.^[4][17] Further magnetotelluric studies showed that the region has low electrical resistivities of <3 Ωm.^[13] Resistivity values in this range are interpreted to only occur with magma that contains a minimum of 15% andesitic melt.^[13] Additionally, these resistivity values indicate that the melt has a water content up to 10 wt.% H₂O, which makes up approximately 10% of the APMB.^[12]