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.
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The authors thank A. Adams, L. Ball, B. Bloss, L. Bonner, B. Burton, T. Bye, B. Fry, E. Hart, M. Lee, K. Menoza and M. Wisniewski for their invaluable contributions to the data collection effort. The authors thank the Gifford-Pinchot National Forest, Weyerhaeuser, the Washington DNR, Mount Rainier National Park, Port Blakely Tree Farms, Hancock Forest Resources, Pope Resources, West Fork Timber Company, the White Pass ski area and numerous private landowners for land access without which this work would not have been possible. T. Sisson, C. Finn, O. Bachmann, R. Blakely and J. Glen provided valuable discussion and critical input that helped to shape this manuscript. The authors thank R. Blakely and C. Finn for processing the magnetic field data (available at https://mrdata.usgs.gov/airborne/), R. Evans and P. Wannamaker for making the Café MT data publicly available (available at https://ds.iris.edu/spud/emtf) and D. Ramsey for providing Quaternary vent locations. Seismicity is from the Pacific Northwest Seismic Network. This research used resources provided by the Core Science Analytics, Synthesis, and Libraries (CSASL) Advanced Research Computing (ARC) group at the US Geological Survey (USGS). This work was supported by the USGS Volcano Hazards and Mineral Resources Programs and the US National Science Foundation grant EAR1144353 through the GeoPrisms program. Any use of trade, firm or product names is for descriptive purposes only and does not imply endorsement by the US Government.
The authors declare no competing interests.
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Bedrosian, P.A., Peacock, J.R., Bowles-Martinez, E. et al. Crustal inheritance and a top-down control on arc magmatism at Mount St Helens. Nature Geosci 11, 865–870 (2018). https://doi.org/10.1038/s41561-018-0217-2
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