Investigating the Effect of Lithosphere Thickness and Viscosity on Mantle Dynamics Throughout the Supercontinent Cycle

Abstract

The relationship between the lithosphere and the mantle during the supercontinent cycle is complex and poorly constrained. The processes which drive dispersal are often simplified to two end members: slab pull and plume push. We aim to explore how lithosphere thickness and viscosity during supercontinent assembly may affect the interaction of deep mantle structures throughout the supercontinent cycle. We consider supercontinental lithosphere structure as one of many potential processes which may affect the evolution of upwellings and downwellings and therefore systematically vary the properties of continental and cratonic lithosphere, respectively within our 3D spherical simulations. The viscosity and thickness of the lithosphere alters the dip and trajectory of downwelling material beneath the supercontinent as it assembles. Focusing on Pangea, we observe that plumes evolve and are swept beneath the center of the supercontinent by circum-continental subduction. The proximity of these upwelling and downwelling structures beneath the supercontinent interior varies with lithosphere thickness and viscosity. Where slabs impinge on the top of an evolving plume head (when continental and cratonic lithosphere are thick and viscous in our simulations), the cold slabs can reduce the magnitude of an evolving plume. Conversely, when the continental lithosphere is thin and weak in our simulations, slab dips shallow in the upper mantle and descend adjacent to the evolving plume, sweeping it laterally near the core-mantle boundary. These contrasting evolutions alter the magnitude of the thermal anomaly and the degree to which the plume can thin the lithosphere prior to breakup.