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Crustal inheritance and a top-down control on arc magmatism at Mount St Helens

September 3, 2018

In a subduction zone, the volcanic arc marks the location where magma, generated via flux melting in the mantle wedge, migrates through the crust and erupts. While the location of deep magma broadly defines the arc position, here we argue that crustal structures, identified in geophysical data from the Washington Cascades magmatic arc, are equally important in controlling magma ascent and defining the spatial distribution and compositional variability of erupted material. As imaged by a three-dimensional resistivity model, a broad lower-crustal mush zone containing 3–10% interconnected melt underlies this segment of the arc, interpreted to episodically feed upper-crustal magmatic systems and drive eruptions. Mount St Helens is fed by melt channelled around a mid-Tertiary batholith also imaged in the resistivity model and supported by potential–field data. Regionally, volcanism and seismicity are almost exclusive of the batholith, while at Mount St Helens, along its margin, the ascent of viscous felsic melt is enabled by deep-seated metasedimentary rocks. Both the anomalous forearc location and composition of St Helens magmas are products of this zone of localized extension along the batholith margin. This work is a compelling example of inherited structural control on local stress state and magmatism.

Citation Information

Publication Year 2018
Title Crustal inheritance and a top-down control on arc magmatism at Mount St Helens
DOI 10.1038/s41561-018-0217-2
Authors Paul A. Bedrosian, Jared R. Peacock, Esteban Bowles-Martinez, Adam Schultz, Graham Hill
Publication Type Article
Publication Subtype Journal Article
Series Title Nature Geoscience
Index ID 70209687
Record Source USGS Publications Warehouse
USGS Organization Geology, Geophysics, and Geochemistry Science Center