Seismic anisotropic signatures of complex crustal magmatic pathways beneath the Mount St. Helens region
Abstract
Mount St. Helens (MSH) is the most active volcano within the Cascades Arc and it is located unusually close to the cold forearc. Recent local seismic tomography and resistivity imaging suggest shallow magmatic reservoirs at ~4-12 km depth beneath MSH. Yet, the deep origin of the magma beneath MSH remains hotly debated due to the location atop the cold front of the mantle wedge. Regional geophysical studies suggest a deep crustal magmatic mush zone ~20 km east of the volcano fed by hotter mantle closer to the axis of the Cascadia arc. However, limited resolution obscures whether this mush zone provides the deep source for MSH, and if so, how it connects to the shallow reservoir. Seismic anisotropy can illuminate structural fabric or layering with fine length scales difficult to resolve in most seismic tomography. In this study, we construct a radially anisotropic shear-velocity (Vs) model covering the broad arc volcanic systems in the region, including MSH, Mount Adams and Mount Rainer, from surface down to ~50 km to explore the crustal magmatic pathways for the Washington Cascade volcanoes.
Our new model shows concentrated velocity reductions with moderate-weak positive anisotropy at 8-15 km depth beneath all three volcanoes, whose locations are generally consistent with previously identified shallow magmatic reservoirs. We interpret them as sill complexes where ascending melts stall and spread laterally. The shallow reservoirs appear to be mostly isolated, however, the anisotropic anomalies beneath Mount Adams and Mount Rainer merge at >15 km depth and form an elongated arc-parallel region of reduced velocities. In contrast, the positive anisotropy beneath MSH deteriorates in amplitude and likely switches to a negative sign at deeper depths. This region of negative anisotropy is adjacent to the estimated hypocentre of deep-long-period events that may be related to vertical fluid migration. Cumulatively, our model provides seismic signatures for complex magmatic pathways linking deep crustal hot zones to distinct volcanoes of the Washington Cascades.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2020
- Bibcode:
- 2020AGUFMT001.0007J
- Keywords:
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- 7209 Earthquake dynamics;
- SEISMOLOGY;
- 7280 Volcano seismology;
- SEISMOLOGY;
- 8105 Continental margins: divergent;
- TECTONOPHYSICS;
- 8413 Subduction zone processes;
- VOLCANOLOGY