Segmentation and radial anisotropy of the deep crustal magmatic system beneath the Cascades arc

Abstract

Volcanic arcs consist of many distinct vents that are ultimately fueled by the common process of melting in the subduction zone mantle wedge. Seismic imaging of crustal scale magmatic systems can provide insight into how melt is organized in the deep crust and eventually focused beneath distinct vents as it ascends and evolves. Here we investigate the crustal-scale structure beneath a section of the Cascades arc spanning four major stratovolcanoes: Mt. Hood, Mt. St. Helens, Mt. Adams, and Mt. Rainier, based on ambient noise interferometry measurements from 234 seismographs. Simultaneous inversion of Rayleigh and Love wave dispersion better constrain the isotropic shear velocity (Vs) and identify the unusual occurrence of radially anisotropic structures. Isotropic Vs shows two sub-parallel low-Vs zones at ˜15-30 km depth with one connecting Mt. Rainier to Mt. Adams, and another connecting Mt. St. Helens to Mt. Hood, which are interpreted as deep crustal magma reservoirs containing up to ˜2.5-6% melt, assuming near-equilibrium melt geometry. Negative radial anisotropy is prevalent in this part of the Cascadia margin, but is interrupted by positive radial anisotropy extending vertically beneath Mt. Adams and Mt. Rainier at ˜10-30 km depth and weaker positive anisotropy beneath Mt. St. Helens with a west dipping. The positive anisotropy regions are adjacent to rather than co-located with the isotropic low-Vs anomalies. Ascending melt that stalled and mostly crystallized in sills with possible compositional difference from the country rock may explain the near-average Vs and positive radial anisotropy adjacent to the active deep crustal magma reservoirs. Hosted fileessoar.10512621.1.docx available at https://authorea.com/users/527167/articles/599327segmentation-and-radial-anisotropy-of-the-deep-crustal-magmatic-system-beneath-thecascades-arc Segmentation and radial anisotropy of the deep crustal magmatic system beneath the Cascades arc

Geophysical and Geochemical Constraints on Magma Storage Depths Along the Cascade Arc: Knowns and Unknowns

“…As permanent monitoring networks are densified and older 1-or 3-component seismometers are replaced with modern broadband seismometers, the potential for a new generation of imaging using only permanent installations increases, both through increased station density, and the potential for wavefield-based imaging methods using broadband data. For example, broadband data enables ambient-noise imaging, which provides high-accuracy estimates of shear-wave velocity (V s ) at the upper-crustal depths where magma storage frequently occurs (e.g., Crosbie et al, 2019;Jiang et al, 2023). We also summarize the presence and depth of Deep Long Period earthquakes (DLPs, Nichols et al, 2011), although exactly how these signals relate to the volcanic plumbing system is still enigmatic.…”

“…These RMSE mean that these two Cpx-only barometers can identify magma storage depths within ∼15-18 km of the true value at 1σ confidence. The relatively thick crust in the Cascades (∼40-50 km, Jiang et al, 2023;Kiser et al, 2016;Parsons et al, 1998;Shehata & Mizunaga, 2022) means that Cpx-based barometry can roughly distinguish between storage in the upper, mid and lower crust at best. Another advantage of these two barometers is that they are independent of temperature and H 2 O content, which are difficult to estimate from Cpx compositions alone (Wieser et al, 2023b).…”

“…They speculated that this may indicate fluid or melt present, or high crustal temperatures. Jiang et al (2023) used ambient noise interferometry on a regional seismic network to further investigate the origin of these crustal anomalies. Crucially, their approach used data from the EarthScope array in addition to regional seismic networks, which helped to mitigate edge effects allowing them to expand their reconstruction to cover the area around Mount Hood.…”

“…Magnetotellurics images the conductivity structure of the crust, which can help identify regions of melt and fluid, as well as hot intrusions (e.g., Bedrosian et al, 2018;Bowles-Martinez & Schultz, 2020). Seismic tomography using natural earthquakes or controlled sources (e.g., Kiser et al, 2016;Moran et al, 1999;Ulberg et al, 2020;Zucca & Evans, 1992) and methods that use the ambient seismic noise wavefield (e.g., Flinders & Shen, 2017;Heath et al, 2018;Jiang et al, 2023) are used to probe the elastic velocity structure of the crust. Seismic wave velocities are sensitive to composition and mineralogy, temperature, and the presence of melt or other fluids, while the attenuation of seismic waves is relatively more sensitive to temperature and fluids (Abers & Hacker, 2016;Magee et al, 2018).…”

Geophysical and Geochemical Constraints on Magma Storage Depths along the Cascade Arc: Knowns and Unknowns

“…Magnetotellurics is used to image the conductivity structure of the crust, which can help identify regions of melt and fluid, as well as hot intrusions (e.g., Bedrosian et al, 2018;Bowles-Martinez and Schultz, 2020). Seismic tomography using natural earthquakes or controlled sources (e.g., Kiser et al, 2016;Moran et al, 1999;Ulberg et al, 2020;Zucca and Evans, 1992) and methods that use the ambient seismic noise wavefield (e.g., Flinders and Shen, 2017;Heath et al, 2018;Jiang et al, 2023) have been used to probe the elastic velocity structure of the crust. Seismic wave velocities are sensitive to composition and mineralogy, temperature, and the presence of melt or other fluids, while the attenuation of seismic waves is relatively more sensitive to temperature and fluids (Abers and Hacker, 2016;Magee et al, 2018).…”