Magnetotelluric images of magma distribution beneath Volcán Uturuncu, Bolivia: Implications for magma dynamics

The Altiplano-Puna volcanic complex in the central Andes records a history of major caldera-forming eruptions over the past 10 m.y. Geophysical and geodetic data indicate that magma is still present, and perhaps mobile, in the crust. Broadband magnetotelluric data were used to generate two-dimensional and three-dimensional electrical resistivity models of the Altiplano-Puna magma body (APMB) with a focus on the zone of inflation around Volcán Uturuncu in southern Bolivia. Low electrical resistivities (<3 Ωm) at a depth of ∼15 km below sea level are interpreted as being due to the presence of andesitic melts of the APMB and require a melt fraction >20%. The upper surface of the APMB is shallowest beneath Uturuncu and the geometry is consistent with geodynamic models that require the upward movement of a melt layer at this location. The shallower resistivity structure is characterized by discrete electrically conductive bodies, oriented east-west near sea level (depth of 5 km), which are interpreted as a combination of partial melt and fluids.