Shear-wave velocity structure of the Chilean subduction zone (39–40°S) based on Rayleigh wave dispersion: evidence of fluid release and melts in the mantle beneath the Villarrica volcano
Martin Thorwart, Yvonne Dzierma, Kathrin Lieser, Heiko Buhs, Wolfgang Rabbel, 2015. "Shear-wave velocity structure of the Chilean subduction zone (39–40°S) based on Rayleigh wave dispersion: evidence of fluid release and melts in the mantle beneath the Villarrica volcano", The Role of Volatiles in the Genesis, Evolution and Eruption of Arc Magmas, G. F. Zellmer, M. Edmonds, S. M. Straub
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A seismic network operated from December 2008 to November 2009 in south-central Chile covering the Chile subduction zone from c. 39°S to 40°S. This segment of the subduction zone includes the highly active Villarrica volcano and the maximum slip area of the 1960 Mw 9.5 earthquake. We applied surface wave dispersion analysis to data from a linear array of broadband stations and to records of four areal sub-arrays. Fifty regional and teleseismic events were used to produce dispersion curves of Rayleigh waves. From the dispersion curves, we determined depth functions of the shear-wave velocity for 4 subregions of the subduction zone: the Coastal Range, the Central Valley, the Volcanic Arc and the Back-arc Region in Argentina. The resulting models reveal the structure of the crust and the depth of the Moho discontinuity. Below the volcanic arc, the shear-wave velocities of the continental mantle are reduced by c. 7% with respect to a background value of 4.3 km s−1. This low-velocity zone coincides with a zone of reduced electrical resistivity that was previously determined from magnetotelluric measurements. The combined occurrences of minima in the S-wave velocity and resistivity can be interpreted as an indicator of partial melts.
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The subduction zone volatile cycle is key to understanding the petrogenesis, transport, storage and eruption of arc magmas. Volatiles control the flux of slab components into the mantle wedge, are responsible for melt generation through lowering the solidi of mantle materials and influence the crystallizing phase assemblages in the overriding crust. Further, the rates and extents of degassing during magma storage and decompression affect magma rheology, ultimately control eruption style and have consequences for the environmental impact of explosive arc volcanism. This book highlights recent progress in constraining the role of volatiles in magmatic processes.
Individual book sections are devoted to tracing volatiles from the subducting slab to the overriding crust, their role in subvolcanic processes and eruption triggering, as well as magmatic-hydrothermal systems and volcanic degassing. For the first time, all aspects of the overarching theme of volatile cycling are covered in detail within a single volume.