The lithosphere-asthenosphere boundary: integrated modeling of scattered wave observations and mantle dynamics

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The physical and chemical factors that distinguish the Earth's lithosphere from its asthenosphere are not uniquely known, and neither is their variation between different tectonic environments. However, high resolution imaging of seismic velocity gradients at the lithosphere-asthenosphere boundary (LAB) has the potential to constrain the combinations of temperature, composition (depletion and volatile content), and melt fraction that could create the LAB. For example, P-to-S scattered waves (Ps) observed at broadband stations in eastern North America have revealed LAB velocity gradients (a 3 to 11% shear-wave velocity drop over less than 11 km) that are too strong and sharp to be explained by temperature alone, suggesting compositional effects (a more depleted, dry lithosphere over a less depleted, hydrated asthenosphere) or perhaps the presence of partial melt in the asthenosphere.The goal of this project is to better determine the sharpness of the transition from the lithosphere to the asthenosphere, the combination of factors dominate this transition, and the variance of their relative influence between different tectonic settings. The approach is to integrate constraints on velocity gradients at the LAB from scattered wave observations with models of mantle flow and melting. Specifically, the PI's are: using Ps and Sp phases to constrain LAB shear velocity gradients in a variety of continental and oceanic settings; numerically modeling mantle flow and melting to calculate ranges of temperature, major element depletion, volatile enrichment, and partial melting near the LAB; determining which conditions are consistent with the observed seismic velocity gradients, using existing relationships between seismic velocity and mantle composition and temperature.An over-arching hypothesis to be tested is that exceptionally steep seismic velocity gradients beneath the LAB may be caused by a small degree of partial melt formed through adiabatic decompression melting, and trapped by a less permeable lower lithosphere. Less steep gradients may be explained by temperature or volatile content alone. The project will initially focus on three cases: eastern North America, southern Africa, and ocean islands of varying age. These regions were chosen because they contain the distributions of long-term broadband stations necessary for imaging LAB gradients, and because they permit the evaluation of specific interactions between mantle flow, composition, temperature, and melting. In eastern North America and southern Africa, the idea to be tested is that mantle upwelling driven by rapid changes in lithospheric thickness generates a small degree of partial melt through adiabatic decompression. In the case of ocean islands, the project will examine whether LAB gradients requiring melt occur near young ocean islands, and LAB gradients consistent with dehydration and depletion of the lithosphere, or with temperature alone, are more likely beneath the oldest ocean islands.This work is benefiting from the participation of a graduate student and undergraduate researchers and is in turn contributing to their education.