Karen Fischer (Brown University) Fri, October 27, 10:30am - 11:30am
Host: Thorsten Becker
Abstract: The paradigm of plate tectonics is fundamental to our understanding of the Earth, yet the question of what makes the lithosphere plate-like remains unanswered. As Earth's outer thermal boundary layer, the lithosphere derives its high viscosity largely from its cold temperatures, relative to the warmer asthenosphere. However, the roles of partial melt and volatiles in further reducing asthenospheric viscosity are still debated. Seismic wave conversions generated by a velocity gradient within the lithosphere-asthenosphere transition, combined with surface wave tomography, provide key constraints on these questions.
Data from the EarthScope USArray has enabled imaging of the lithosphere-asthenosphere boundary (LAB) and discontinuities internal to the lithosphere across the contiguous U.S. Strong S-to-P (Sp) conversions from the base of the thin lithosphere in the tectonically active western U.S. indicate a vertically-localized negative velocity gradient (<40 km) that is consistent with ponded partial melt, embedded in a broader thermal gradient. In contrast, beneath the thick, stable and ancient cratonic lithosphere in the center of the continent, the typical absence of clear Sp conversions is consistent with a gradual shear velocity decrease over >80 km; patches with a sharper gradient are spatially intermittent. The very gradual cratonic lithosphere-asthenosphere boundary can be explained by temperature without the effects of higher volatile content or partial melt in the asthenosphere. Middle-aged lithosphere in the eastern U.S., largely undisturbed since Mesozoic rifting, manifests an LAB velocity gradient of intermediate sharpness, except for a few zones that correlate with low mantle velocities and the possible presence of partial melt. Across active plate boundaries, variations in the depth and amplitude of the LAB velocity gradient across rifts and strike-slip fault systems indicate localized deformation in the deep mantle lithosphere. Within the cratonic mantle lithosphere, the most widespread feature is a sub-horizontal negative velocity gradient, typically at depths of 70-90 km, that can be explained by volatile-rich products of now-solid partial melt that once ponded beneath the carbonated peridotite solidus.