Arc Crustal Structure around Mount Rainier Constrained by Receiver Functions and Seismic Noise

Volcanic arcs along subduction zones are thought to be loci for continental growth. Nevertheless, the amount of material transferred from the mantle to crust and the associated magmatic plumbing are poorly understood. While partial melting of mantle peridotite produces basaltic melt, the average composition of continental crust is andesitic. Several models of magma production, migration and differentiation have been proposed to explain the average crust composition in volcanic arcs. The formation of mafic cumulate and restite during fractional crystallization and partial melting has potential to alter the structure of the crust-mantle interface (Moho). The computed composition and distribution of crust and mantle rocks based on these different models convert into distinctive vertical velocity profiles, which seismic imaging methods can unravel . With a view to put more constraints on magmatic processes in volcanic arc, we analyze the shear wave velocity (Vs) distribution in the crust and uppermost mantle below Mount Rainier, WA, in the Cascadia arc. We resolve the depth of the main velocity contrasts based on converted phases, for which detection in the P coda is facilitated by source normalization or receiver function (RF) analysis. To alleviate the trade-off between depth and velocity intrinsic to RF analysis, we jointly invert RF with frequency-dependent surface wave velocities. We analyze earthquake surface waves to constrain long period dispersion curves (20-100 s). For shorter period (5-20s), we use seismic noise cross-correlograms and Aki's spectral formulation, which allows longer periods for given path. We use a transdimensional Bayesian scheme to explore the model space (shear velocity in each layer, number of interfaces and their respective depths). This approach tends to minimize the number of layers required to fit the observations given their noise level. We apply this tool to a set of broad-band stations from permanent and EarthScope temporary stations, all within 35 km of Mt Rainier. The receiver functions significantly differ from one station to another, indicating short wavelength lateral contrast in the lithospheric structure. Below arc stations offset from Mount Rainier, preliminary models show a rather clear Moho transition around 40km, separating lower crust with 3.6-3.9 km/s shear velocity, from a ~ 20 km thick mantle lid with Vs ~ 4.2 km/s. In contrast, at station PANH located 9 km east of Mount Rainier, the exact location of the Moho is not clear. Shear velocity ranges from 3.3 to 3.9 km/s from the surface down to 55 km depth, with the exception of a fast layer imaged between 25 and 32 km depth with Vs ~ 4.2 km/s. It seems likely that partial melt in the mantle, combined with high-velocity underplated or differentiated lower crust, are acting in various ways to create a complicated structure around the Moho.