This project focuses on the generation and segregation of melt at the lithosphere-asthenosphere boundary (LAB), with the aims of (1) characterizing the physical nature of the boundary and (2) constraining the processes that determine its form and evolution. Specifically, the LAB may be substantially modified by processes that are dominantly thermomechanical (e.g., physical delamination of dense parts by drips or shear), or dominantly thermochemical (e.g. rejuvenation by melt impregnation and conversion of depleted material into enriched, partially molten material). This work builds on recent observations suggesting that melt segregation at and migration along the LAB can substantially modify the thermal, chemical, and mechanical properties of the boundary in a "deformation-enhanced thermo-chemical rejuvenation" process. This project explores the geodynamic implications of these ideas by developing a phenomenological description of stress-driven melt segregation based closely on experimental observations, that will be incorporated into fluid-dynamical models of mantle deformation beneath the Colorado Plateau (CP) in the western US. The research builds upon geologic and seismic observations that constrain the distribution and geometry of melt in the CP region, specifically comparing the plateau-margins to the interior. The PIs are interpreting these observations using numerical models of a suite of processes: 1) spatial variations in plate thinning of the CP and surrounding regions, 2) possible convective instability of dense regions of the plate, and 3) motion of the CP lithospheric "keel" through the underlying asthenosphere. These investigations are being coupled with seismic anisotropy observations from surface waves across the USArray to infer mantle deformation scenarios beneath the CP.
2. Non-technical description of broader impact and significance.
One of the fundamental discoveries that the scientific community hopes EarthScope will help achieve is a clearer picture of the "shape" of the North American plate; that is, a measurement of lithosphere thickness and an understanding of the spatial variability in both the thermal and chemical properties of the plate. The base of the lithosphere is an elusive boundary that does not coincide with an easily imaged seismic discontinuity. As data from the USArray component of EarthScope are analyzed to construct seismic models of the upper mantle beneath the tectonically active western US, an important goal for the scientific community is to interpret seismic observations in terms of the structure and dynamics of the plate-mantle interface. A fundamental ambiguity that plagues seismic interpretations is the relative importance of temperature, composition, and fluids (e.g., melt) in controlling the isotropic and anisotropic seismic structure of the upper mantle. This project is developing physics-based models of deformation at the base of the North American plate that can be used to help interpret seismic observations. The project is providing important training for a graduate student in building quantitative models of the Earth and is broadening participation of underrepresented groups in the earth sciences. The combination of fluid dynamics and the experimentally based phenomenology developed here is an innovation that can be generalized to other geodynamic settings. Because the structure and evolution of North America and the CP in particular is of broad public interest, the PIs will produce compelling images from seismic observations, models and interpretive illustrations with pedagogical intent. These images will be utilized in public education and outreach (as well as scientific publications) at museums and national parks in the Southwest, New York and elsewhere.
