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

Nature Geosciencevolume 11pages865870 (2018) | Download Citation


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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  1. 1.

    Hildreth, W. Quaternary Magmatism in the Cascades—Geologic Perspectives, U.S. Geological Survey Professional Paper 1744 (USGS, 2007).

  2. 2.

    Blair, J. L., McCrory, P. A., Oppenheimer, D. H. & Waldhauser, F. A Geo-referenced 3D Model of the Juan de Fuca Slab and Associated Seismicity (US Geological Survey Data Series 633, USGS, 2013).

  3. 3.

    Snavely, P. D. Jr in Geology and Resource Potential of the Continental Margin of Western North America and Adjacent Ocean Basins—Beaufort Sea to Baja California (eds. Scholl, D. W., Grantz, A. & Vedder, J. G.) Ch. 14, 305–335 (Earth Science Series, Circum-Pacific Council for Energy and Mineral Resources, Houston, TX, 1987).

  4. 4.

    Wells, R. et al. Geologic history of Siletzia, a large igneous province in the Oregon and Washington Coast Range: correlation to the geomagnetic polarity time scale and implications for a long-lived Yellowstone hotspot. Geosphere 10, 692–719 (2014).

  5. 5.

    Wells, R. E. & McCaffrey, R. Steady rotation of the Cascade arc. Geology 41, 1027–1030 (2013).

  6. 6.

    Evarts, R. C., Ashley, R. P., & Smith, J. G. Geology of the Mount St. Helens Area: record of discontinuous volcanic and plutonic activity in the Cascade arc of Southern Washington. J. Geophys. Res. 92, 10155–10169 (1987).

  7. 7.

    Stanley, W. D., Finn, C., & Plesha, J. L. Tectonics and conductivity structure in the Southern Washington Cascades. J. Geophys. Res. 92, 10179–10193 (1987).

  8. 8.

    Hill, G. J. et al. Distribution of melt beneath Mount St Helens and Mount Adams inferred from magnetotelluric data. Nat. Geosci. 2, 785–789 (2009).

  9. 9.

    Law, L. K., Auld, D. R. & Booker, J. R. A geomagnetic variation anomaly coincident with the Cascade volcanic belt. J. Geophys. Res. 85, 5297–5302 (1980).

  10. 10.

    Egbert, G. D. & Booker, J. R. Imaging crustal structure in Southwestern Washington with small magnetometer arrays. J. Geophys. Res. 98, 15967–15985 (1993).

  11. 11.

    Pallister, J. S. et al. in A Volcano Rekindled: The Renewed Eruption of Mount St. Helens 2004–2006 (eds Sherrod, D. R., Scott, W. E. & Stauffer, P. H.) Ch. 30 (USGS, Reston, VA, 2008).

  12. 12.

    McGary, R. S., Evans, R. L., Wannamaker, P. E., Elsenbeck, J. & Rondenay, S. Pathway from subducting slab to surface for melt and fluids beneath Mount Rainier. Nature 511, 338–340 (2014).

  13. 13.

    Schultz, A. et al. USArray TA Magnetotelluric Transfer Functions (2006–2018);

  14. 14.

    Finn, C. Geophysical constraints on convergent margin structure. J. Geophys. Res. 95, 19533–19546 (1990).

  15. 15.

    Wells, R. E., Weaver, C. S. & Blakely, R. J. Forearc migration in Cascadia and its neo tectonic significance. Geology 26, 759–762 (1998).

  16. 16.

    Parsons, T. et al. A new view into the Cascadia subduction zone and volcanic arc: implications for earthquake hazards along the Washington margin. Geology 26, 199–202 (1998).

  17. 17.

    Stanley, D., Villasenor, A. & Benz, H. Subduction Zone and Crustal Dynamics of Western Washington: A Tectonic Model for Earthquake Hazards (US Geological Survey Open-File Report 99-311, USGS, 1995).

  18. 18.

    Williams, D. L. & Finn, C. Evidence for a shallow pluton beneath the Goat Rocks Wilderness, Washington, from gravity and magnetic data. J. Geophys. Res. 92, 4867–4880 (1987).

  19. 19.

    Iveson, A. A., Webster, J. D., Rowe, M. C. & Neill, O. K. Magmatic–hydrothermal fluids and volatile metals in the Spirit Lake pluton and Margaret Cu–Mo porphyry system, SW Washington, USA. Contrib. Mineral. Petrol. 171, 20 (2016).

  20. 20.

    Bedrosian, P. A. & Feucht, D. W. Structure and tectonics of the northwestern United States from EarthScope USArray magnetotelluric data. Earth Planet. Sci. Lett. 402, 275–289 (2014).

  21. 21.

    Blundy, J., Mavrogenes, J., Tattitch, B., Sparks, S. & Gilmer, A. Generation of porphyry copper deposits by gas–brine reaction in volcanic arcs. Nat. Geosci. 8, 235–240 (2015).

  22. 22.

    Patro, P. K. & Egbert, G. D. Regional conductivity structure of Cascadia: preliminary results from 3D inversion of USArray transportable array magnetotelluric data. Geophys. Res. Lett. 35, L20311 (2008).

  23. 23.

    Worzewski, T., Jegen, M., Kopp, H., Brasse, H. & Castillo, W. T. Magnetotelluric image of the fluid cycle in the Costa Rican subduction zone. Nat. Geosci. 4, 108–111 (2010).

  24. 24.

    Wannamaker, P. E. et al. Segmentation of plate coupling, fate of subduction fluids, and modes of arc magmatism in Cascadia, inferred from magnetotelluric resistivity. Geochem. Geophys. Geosyst. 15, 4320–4253 (2014).

  25. 25.

    Blatter, D. L., Sisson, T. W. & Hankins, W. B. Voluminous arc dacites as amphibole reaction-boundary liquids. Contrib. Mineral Petrol. 172, 27 (2017).

  26. 26.

    Pommier, A. & Le-Trong, E. ‘SIGMELTS’: a web portal for electrical conductivity calculations in geosciences. Comp. Geosci 37, 1450–1459 (2011).

  27. 27.

    Claiborne, L. L., Miller, C. F., Flanagan, D. M., Clynne, M. A. & Wooden, J. L. Zircon reveals protracted magma storage and recycling beneath Mount St. Helens. Geology 38, 1011–1014 (2010).

  28. 28.

    Cashman, K. V., Sparks, R. S. J. & Blundy, R. J. Vertically extensive and unstable magmatic systems: a unified view of igneous processes. Science 355, 1280 (2017).

  29. 29.

    Flinders, A. F. & Shen, Y. Seismic evidence for a possible deep crustal hot zone beneath Southwest Washington. Sci. Rep. 7, 7400 (2017).

  30. 30.

    Nichols, M. L., Malone, S. D., Moran, S. C., Thelen, W. A. & Vidale, J. E. Deep long-period earthquakes beneath Washington and Oregon volcanoes. J. Volcanol. Geotherm. Res. 200, 116–128 (2011).

  31. 31.

    Kiser, E. et al. Magma reservoirs from the upper crust to the Moho inferred from high-resolution Vp and Vs models beneath Mount St. Helens, Washington State, USA. Geology 44, 411–416 (2016).

  32. 32.

    Sisson, T. W., Salters, J. V. M. & Larson, P. B. Petrogenesis of Mount Rainier andesite: magma flux and geologic controls on the contrasting differentiation styles at stratovolcanoes of the southern Washington Cascades. Geol. Soc. Am. Bull. 126, 122–144 (2014).

  33. 33.

    Hughes, G. R. & Mahood, G. A. Silicic calderas in arc settings: characteristics, distribution, and tectonic controls. Geol. Soc. Am. Bull. 123, 1577–1595 (2011).

  34. 34.

    Robertson, E. A. M., Biggs, J., Cashman, K. V., Floyd, M. A. & Vye-Brown, C. in Magmatic Rifting and Active Volcanism (eds. Wright, T. J., Ayele, A., Ferguson, D. J., Kidane, T. & Vye-Brown, C.) 43–67 (Special Publication 420, Geological Society, 2015).

  35. 35.

    Weaver, C. S., Grant, W. C. & Shemeta, J. E. Local crustal extension at Mount St. Helens, Washington. J. Geophys. Res. 92, 10170–10178 (1987).

  36. 36.

    Egbert, G. D. & Kelbert, A. Computational recipes for electromagnetic inverse problems. Geophys. J. Int. 189, 251–267 (2012).

  37. 37.

    Kelbert, A., Meqbel, N., Egbert, G. D. & Tandon, K. ModEM: a modular system for inversion of electromagnetic geophysical data. Comput. Geosci. 66, 40–53 (2014).

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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, R. Evans and P. Wannamaker for making the Café MT data publicly available (available at 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.

Author information


  1. United States Geological Survey, Denver, CO, USA

    • Paul A. Bedrosian
  2. United States Geological Survey, Menlo Park, CA, USA

    • Jared R. Peacock
  3. Oregon State University, Corvallis, OR, USA

    • Esteban Bowles-Martinez
    •  & Adam Schultz
  4. University of Canterbury, Gateway Antarctica, Christchurch, New Zealand

    • Graham J. Hill


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The iMUSH MT experiment was conceived by P.A.B. and A.S. A.S. and P.A.B. coordinated and led the data collection effort, with data collection primarily carried out by E.B.M. and J.R.P. Time-series processing of the data was performed by P.A.B., J.P. and G.J.H. P.A.B., J.R.P. and E.B.M. carried out the inversion and model development. The interpretation and development of the conceptual model was led by P.A.B. All authors contributed to understanding the results and editing the manuscript.

Competing interests

The authors declare no competing interests.

Corresponding author

Correspondence to Paul A. Bedrosian.

Supplementary information

  1. Supplementary Figures

    Supplementary Figs 1–10

  2. Supplementary Video

    A video showing features of the resistivity model

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