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