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.
In this project, we set out to better understand the structure of the continental lithosphere below the Colorado Plateau, as a "natural laboratory" for understanding how the strength and structure of the plate are modified by interactions with the convecting mantle. The Colorado Plateau is enigmatic because it is broadly elevated, surrounded by deforming, volcanic regions (i.e. the Basin and Range to the West and the Rio Grande Rift to the East), and yet is deforming little in its interior. Two major observations suggest that the deep structure of the plate beneath the Plateau is changing rapidly (in geologic time): 1) the fact that volcanism is encroaching towards the center of the Plateau and 2) imaging of the plate structure using seismic waves shows a "strong" (high seismic velocities) root towards the center, but weak (hot, partially molten?) mantle (low seismic velocities), below the margins of the Plateau. There are several possible explanations for the modification of the plate structure, including 1) the deep roots of the plate become unstable and drip off and fall into the deeper convecting mantle, 2) small scale convection occurs at the edge, "eroding" by heating the edges of the base of the plate, or 3) melt from the hotter mantle that is out of thermal and chemical equilibrium with the Plateau root infiltrates the deep plate, and converts it to hotter, weaker material, which we call a process of "corrosion". We set out to test these hypotheses using geodynamical models (computational studies of fluid dynamics scaled to deep earth conditions) and measurement and interpretation of some aspects of seismic data from the EarthScope USArray. In our geodynamical models, we set up a system in which mantle is flowing past a keel that is about 150 km thick, representing the deep structure of the Colorado Plateau. We found an interesting effect, in which melt is essentially sucked into the deforming keel on the upwind side (i.e. the western margin of the CP), due to a fluid mechanical effect (divergence of flow causes a low pressure point in the outer part of the keel). That melt can carry heat and establish the conditions for chemical modification of the keel, which would both act to weaken the material and bring about corrosion. In our measurements of seismic data, we sought to characterize the mechanical nature of the interface between the base of the plate and the convecting mantle, i.e. the lithosphere-asthenosphere boundary (LAB). In detail, we measured seismic attenuation (through the decay of amplitude with frequency) across the western margin of the Plateau. Our aim is to use the geodynamic models to make predictions for various hypotheses and then use the attenuation and seismic velocity measurements to select the more and less plausible scenarios. Towards this aim, we have built a method to interpret the seismic attenuation by calculating the wave propagation for a wide range of attenuation models and plausible plate scenarios. We are in the process of using these models to interpret the measured attenuation. The work is ongoing, but we anticipate that it will lead to higher resolution in the interpretation of USArray data and a better understanding of the range of processes that modify continental lithosphere. Concerning broader impacts, these deep plate processes have direct expressions in the time-space distribution of earthquakes and volcanism, and thus are important for understanding hazards to human population. This project contributed directly to the education of three undergraduate students at UNM and the training of two postdoctoral researchers at LDEO.
