Using Seismic Discontinuities to Image Melt and Dynamics in the Sub-Continental Upper Mantle

Abstract:

Continents are assembled from multiple Proterozoic and Archean terranes to form stable cratonic platforms with associated deformation typically localized to margins and/or rift zones. Successive episodes of subsequent extension, compression, magmatism, accretion, and rifting have left the sub-continental upper mantle with a complex signature of thermal and chemical heterogeneity. One key interest is the history of melt production, migration, and storage in sub-continental upper mantle as it provides a window into past and present dynamical processes, including the differentiation and formation of continental structure.

Here we examine seismic discontinuities within the mantle that arise from a wide range of mechanisms, including changes in mineralogy, major element composition, melt content, volatile abundance, anisotropy, or a combination of the above. Using a dataset of broadband seismograms of underside reflected S-waves arriving as precursors to the seismic phase SS, we determine the depth and impedance contrast of discontinuities in the depth range of 80-410 km. Our observations are compared to predictions for the seismic moduli from a mineral physics database using the software MuMaP (Multiphase Material Properties). MuMaP modeling allows us to vary the average regional temperature, mantle composition and account for the effects of melt (if present).

In our initial study of the western North American plate, we detect the presence of the 410 km discontinuity, a discontinuity at 300 km depth (X), and a G discontinuity at 60-80 km depth. The X is indicative of the coesite to stishovite phase transition in the upper mantle and suggests substantial mixing of subducted basalt with the mantle. The presence of the G may indicate partial melt in the asthenosphere, melt frozen into the lithosphere, and/or anisotropic fabrics preserved beneath the continent. These hypotheses are evaluated against MuMap predictions for melt content and anisotropic structure in the upper mantle.