The subcontinental lithosphere-asthenosphere boundary

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Our knowledge of the lithosphere, Earth's rigid outer shell, is mainly confined to its upper regions. The lithosphere-asthenosphere boundary (LAB) forms the base of tectonic plates and varies in depth from effectively zero at mid-oceanic ridges to 100's km beneath ancient continental nuclei, where thick lithospheric keels exist. As a mechanical detachment that accommodates relative motion between the plate and underlying, more easily deformed asthenosphere, the LAB constitutes the most extensive class of plate boundary on Earth. Yet, even fundamental physical parameters remain uncertain, particularly beneath continents; these include depth of the LAB, thickness of the downward transition into asthenosphere, abundance and distribution of fluids, as well as the magnitude of basal traction forces that drive (or impede) plate motion and influence intraplate earthquakes. These parameters are expressed in various geophysical observables, including P- and S-wave velocities and seismic anisotropy. This proposed research program will investigate the subcontinental LAB in selected regions using passive seismological methods, including surface-wave tomography, moment-tensor inversion and analysis of S-receiver functions, coupled with numerical simulations of the thermal and rheological evolution of the continental upper mantle. The study areas, located around Hudson Bay, Alberta and Morocco, sample diverse regions that are, respectively, within, near the margin of, and adjacent to ancient lithospheric keels beneath continents. One M.Sc. student, 5 doctoral students, 2 postdoctoral fellows and 5 senior undergraduate students will receive academic training as part of this research program. By improving our knowledge of the base of tectonic plates and internal architecture of continental lithosphere, this work will address fundamental scientific questions about driving forces of plate tectonics and how continents form and interact with underlying mantle convection systems. Practical applications of this knowledge include guiding exploration for diamonds and improving our understanding of factors that control intraplate earthquakes.