Searching for the Deep Roots of Arc Volcanoes: Results From IMUSH Seismic Imaging in the Washington Cascades

Date: 4/25/2019

Time: 04:15 PM

Room: Cascade II

Many arc volcanoes erupt mantle-sourced basalts and high-temperature lower-crustal magmas, yet seismic images of this deep plumbing are almost non-existent. Are deep partial-melt bodies too small for seismology to see, or are typical seismic arrays too limited to detect them? We address these questions with a 2014-16, 70-station broadband array around Mount St. Helens (MSH), termed iMUSH (imaging Magma Under St. Helens). iMUSH provides some of the highest resolution 3D images of arc crust and underlying upper mantle anywhere. Local earthquake signals only sample the upper crust, requiring teleseismic and ambient wavefield methods. Receiver functions image 35-40 km of upper-plate crust and a subducting plate 65-68 km directly beneath MSH. This leaves very little mantle to melt, yet basalts are present. As elsewhere in the Cascades, the upper-plate Moho vanishes in the forearc (west of MSH), an observation usually interpreted as serpentinized mantle. Ambient-noise tomography shows low wavespeeds here consistent with hydrated mantle, requiring temperatures low enough for serpentines to be stable, as does low forearc heat flow. Seismic body-wave attenuation also confirms large temperature contrasts between the forearc and back arc. However, much of the reduction in Moho strength is a consequence of upper-plate geology: very high wavespeeds are observed in the lower crust west of MSH, likely the mafic Siletz terrane, while typical crustal wavespeeds are seen to the east. This lithological variation controls the wavespeeds, with little evidence for melt except far east of MSH. Overall, these observations show little sign of the deep magma plumbing system beneath MSH, and they imply that the lower crust and uppermost mantle are too cold directly beneath the edifice to generate the observed melts. To reconcile these observations with the volcanism at MSH requires significant lateral melt transport within the crust. However, these pathways are not directly visible with seismic imaging techniques currently available, motivating the need for other approaches.

Presenting Author: Geoffrey A. Abers

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