The Evolution and Seismic Expression of a Lithospheric-scale Magmatic System: The Puna Plateau

Abstract:

Recent seismic images of the crust and uppermost mantle of the Puna Plateau reveal an active lithospheric-scale magmatic plumbing system. The main driving mechanism responsible for volcanism in the Puna Plateau is thought to be the southward migration of flat-slab subduction, subsequent re-steepening of the slab, and associated lithospheric delamination. Multiple large slow velocity bodies (magma reservoirs) are observed in the mid-crust and coincide with large negative gravity anomalies and voluminous ignimbrite volcanism. Comparisons between these slow velocity bodies show variations from south to north that consist of: (1) an increase in the volumes of low-velocity zones associated with magmatic reservoirs, (2) a progressive decrease in the lowest velocities observed in the core of any reservoir, and (3) a systematic shallowing in depth to the center of the reservoirs. The southward migration of the mechanism responsible for the magmatic flare-up allows us to interpret these along-strike variations as different temporal stages in the magmatic evolution of a large silicic system in the thermally warming and tectonically widening thickened crust of the Puna Plateau.

Plutonic-to-volcanic (P:V) ratios of >30:1 are obtained throughout the Puna Plateau when the volumes of these magma reservoirs are compared to that of the ignimbrites. However, these ratios only consider the evolved silicic component of this magmatic system, and shear-wave velocities near the crust-mantle transition beneath these bodies are also much slower than expected for mantle lithosphere. These velocities are consistent with the presence of melt in a matrix of ultramafic composition (i.e., “MASH” zone) which likely feeds the large mid-crustal magma reservoirs, in agreement with geochemical evidence for crustal assimilation and fractional crystallization at similar depths.

If these interpretations are correct, the Puna Plateau presents a unique opportunity to investigate the evolution and characteristics of a large silicic system. In presenting these results in the context of existing geochemical and petrologic data, we seek a to understand what the most useful direction/pressing questions are in studying these systems, and how seismic methods can contribute to this from a multidisciplinary point-of-view moving forward .