Dynamics of the Base Lithosphere

Stanford University, Stanford CA

Investigators

Abstract

Sleep 0000743 Old cratons are a prominent feature of the Earth. The lithosphere is typically 200-250 km deep beneath the older cratonic regions of continents and much thinner beneath ocean basins. Data from rocks entrained in volcanic lavas indicate that this thickness as been stable for over 2 billion years in some locations. The PI seeks to investigate the physics of why the lithosphere is this thick and has remained so over time. A supply of heat from below is needed to offset the heat lost upward by conduction to the surface. He will consider two linked processes that act to maintain this thickness: (1) small-scale free convection that occurs within the thermal boundary layer at the base of the lithosphere, and (2) the transfer of heat by stirring that is caused by horizontal movement of the tectonic plates. Ponding of hot buoyant mantle plume material at the base of the lithosphere is a third process that supplies heat and will be investigated. The stirring process is most efficient if the viscosity of the Earth's interior increases rapidly with depth near the base of the lithosphere. It is also possible that the thick lithosphere beneath continents is stable because it is more buoyant than its surroundings. The PI will investigate how this situation can arise and persist. He will also investigate processes including partial melting which are associated with the lateral flow. The PI proposes to obtain the conditions that self-consistently lead to the present state of cratons and their thickness histories. Potentially observable features including the thickness of the anisotropic layer from shear at the base of plates, changes in the vigor of convection associated with changes in plate speed, and displacement of the thermal boundary layer of cratons by shear form plate motions will be examined.