Three-dimensional velocity models of the Mount St. Helens magmatic system using the iMUSH active-source data set
Abstract
Building upon previously published 2D results, this presentation will show the first 3D velocity models down to the Moho using the iMUSH (imaging Magma Under St. Helens) active-source seismic data set. Direct P and S wave travel times from 23 borehole shots recorded at approximately 6000 seismograph locations are used to model Vp, Vs, and Vp/Vs over an area extending approximately 75 km from the edifice of Mount St. Helens and down to approximately 15 km depth. At shallow depths, results show several high and low velocity anomalies that correspond well with known geological features. These include the Chehalis Basin northwest of Mount St. Helens, and the Silver Star Mountain, Spirit Lake, and Spud Mountain plutons. Starting at 4 km depth, low velocities and high Vp/Vs values are observed near Mount St. Helens, which may be associated with shallow magmatic fluids. High Vp/Vs values are also observed beneath the Indian Heaven Volcanic Field southeast of Mount St. Helens. At the regional scale, high amplitude north/south trending low and high velocity features extend from the western margin of the resolved models to approximately 30 km west of Mount St. Helens. In general these high and low velocity features also correspond to high and low Vp/Vs anomalies, respectively. These results are in agreement with previous studies that conclude that the accreted terrane Siletzia is composed of multiple igneous bodies interspersed with sedimentary units in this region. Another regional feature of interest is a broad low Vp/Vs area between Mount St. Helens, Mount Adams, and Mount Rainier that spatially correlates with the Southern Washington Cascades Conductor, indicating a non-magmatic origin to this body at shallow and mid-crustal depths. In addition to these shallow results, preliminary 3D velocity models of the entire crust will be presented that utilize both direct and reflected seismic phases from the Moho and other mid-crustal discontinuities. These models will constrain the lateral extents of lower-crustal high and low velocity features observed in previous 2D results. This information will be critical for understanding the distribution of cumulate bodies, magma reservoirs, and accreted terranes in the lower crust, and how these features have affected recent volcanic activity in this region.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2017
- Bibcode:
- 2017AGUFM.V11C0363K
- Keywords:
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- 8410 Geochemical modeling;
- VOLCANOLOGY;
- 8412 Reactions and phase equilibria;
- VOLCANOLOGY;
- 8413 Subduction zone processes;
- VOLCANOLOGY;
- 8439 Physics and chemistry of magma bodies;
- VOLCANOLOGY