Karen Fischer Professor Earth, Environmental, and Planetary Sciences Brown University “ The variable N ature of the Li thosphere - A sthenosphere T ransition ” 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 fr om 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 lithospher e - asthenosphere transition, combined with surface wave tomography, provide key constraints. At the base of thick, stable cratonic lithosphere, the typical absence of clear Sp conversions is consistent with a shear velocity decrease over >100 km and a litho sphere - asthenosphere boundary (LAB) that reflects an increase in temperature alone. In contrast, strong Sp conversions from the LAB in tectonically active continental regions indicate a vertically - localized negative velocity gradient (<20 - 30 km) that requi res other factors, such as ponded partial melt, embedded in a broader thermal gradient. Within the cratonic mantle, the most widespread feature is a sub - horizontal negative velocity gradient, typically at depths of 70 - 90 km, that can be explained by volat ile - rich products of now - solid partial melt; inward - dipping mid - lithospheric discontinuities likely represent the remains of past plate collisions.