Gravity anomalies uncovered by a survey in the Central Andes provide evidence for magma transport by diapir, not dyke
The Altiplano-Puna Magma Body (APMB), located in the Central Andes, is the largest known continental, midcrustal zone of partial melt on Earth. Fed by mantle-derived hydrous magmas, this volume of molten rock lies directly beneath the Altiplano-Puna Volcanic Complex (APVC), the biggest ignimbrite province generated in the last 20 million years. At the height its of activity, volcanoes in this area spewed out >12,000 km3 of APMB-sourced lava in episodic supereruptions.
Previous geophysical surveys have detected the presence of 15-30 % interconnected melt in the APMB by virtue of low seismic velocities and very high electrical conductivity. Del Potro and co-workers investigated how this molten rock travels from its storage zones in the mid-crust (>20 km depth) to the surface by conducting a high resolution gravity survey over the entire area above the APMB. By assuming a homogeneous crustal composition, anomalies in the overlying gravitational field can be attributed directly to difference in density when compared with the surrounding rock. Calculations demonstrate that the measured density contrast is consistent with the crust containing, on average, ~ 25 % liquid by volume.
3D imaging of inversions of the gravity anomalies reveal bulbous, vertically elongated structures of 15 km width rooted at the top of the APMB. The distinctive morphology of the gravity anomalies in conflict with magma transport by dyking, instead supporting a theory of magma transport by diapiric ascent. Evolved melts, buoyant due to high SiO2 and H2O contents, accumulate at the top of the APMB and ascend en masse, undergoing chemical and physical differentiation and stratification.
It is notable that the APVC is showing current signs of unrest, as evidenced by large, decade-long, deep-sourced ground deformation at Uturuncu volcano in Bolivi, making Del Potro et al.’s study extremely timely.
del Potro, R, Diez, M, Blundy, JD, Gottsmann, JH & Camacho, AG (2013) ‘Diapiric ascent of silicic magma beneath the Bolivian Altiplano’ Geophysical Research Letters, vol 40, no. 10, pp. 2044-2048. http://dx.doi.org/10.1002/grl.50493
The vertical transport of large volumes of silicic magma, which drives volcanic eruptions and the long-term compositional evolution of the continental crust, is a highly debated problem. In recent years, dyking has been favored as the main ascent mechanism, but the structural connection between a distributed configuration of melt-filled pores in the source region and shallow magma reservoirs remains unsolved. In the Central Andes, inversion of a new high-resolution Bouguer anomaly data over the Altiplano-Puna Magma Body (APMB) reveals ~15 km wide, vertically elongated, low-density, 3D structures rooted at the top of the APMB at 20 km depth. We integrate our gravity inversion with the available geophysical, geological, and petrological observations, and in agreement with petrological/mechanical considerations propose that, in this region of the Andes, partially molten granitic bodies ascend diapirically through the hot ductile mid-upper crust.