Multifrequency Inversion of Ps and Sp Receiver Functions: Methodology and Application to USArray Data

We image the thermochemical structure of crust and mantle underneath the North American continent by inverting recordings of P-to-s (Ps) and S-to-p (Sp) converted seismic body waves (receiver functions [RFs]). Through careful data selection and processing, we construct a multifrequency Ps (5-, 8-, and 10 -s) and Sp (10- and 15- s) RF data set from USArray recordings. The inversion is interfaced with petrological phase equilibria computations to build self-consistent radial seismic velocity and density models for RF waveform simulations. Inverted models are combined through back-projection along converted raypaths and interpolation to tomographic images of crust and mantle structure. Through clustering analysis we identify three major tectonic regions based on mantle thermochemical and seismic structure: the tectonically active West (TAW), the central transition region (CTR), and the cratonic-orogenic East (COE). TAW is chemically more fertile with a Mg# ∼ 0.90 (molar Mg# = Mg/(Mg + Fe)) and characterized by an elevated mantle potential temperature of 1490 ± 27°C relative to COE, which is chemically more depleted (Mg# ∼ 0.91) and colder (1419 ± 27°C). CTR is intermediate to TAW and COE. We find significant thermochemically induced topography associated with the base of the lithosphere (±90 km), while the mantle transition zone is mostly influenced by thermally induced topography on the 410-km discontinuity (±15 km). In contrast, the 660-km discontinuity, where variations are only ±5 km, reflects a more complex thermochemical interplay. To place the results in a tectonic context, thermobarometric estimates from basaltic rocks across the western United States are integrated with the seismic inversions to produce a thermal model of the underlying mantle.