This award is funded under the American Recovery and Reinvestment Act of 2009 (Public Law 111-5).
Nontechnical explanation: The uppermost 200 kilometers of geologically stable regions behaves nearly rigidly over geological times. The temperature in this region, called the lithosphere, gradually increases with depth. The rock near the base of the lithosphere deforms slowly as a very viscous fluid. Cold material sinks into the underlying mantle and hot material rises from below. Thermal conduction through the rigid lithosphere carries heat to the surface. Scientists constrain the temperature change by studying rocks that were carried quickly from great depths to the surface in diamond pipes. The proposed work involves the physics of heat and mass transfer at the base of the lithosphere. This process occurs everywhere on the Earth. Over geological time, it determines whether continents are above or below sea level and strongly influences the depositional history of hydrocarbon-bearing sediments in basins where subsidence occurs. The investigation will include related topics on the long-term habitability of planets, including the axes of mid-oceanic ridges that spread at rates below 2 kilometers per million years. The heat and mass transfer computer code developed for the project has proved useful for understanding the thermodynamic basis of widely used empirical relationships that represent time-dependent friction in geological situations including earthquakes as well as industrial applications.
involves dynamic effects that occur at the base of the lithosphere or equivalently at the base of tectonic plates. The lack of tectonics in cratonal continental crust (older than ~2.5 billion years) is a salient feature of the Earth. Data from xenoliths erupted from diamond pipes provide hard constraints on physical models. Cratonal lithosphere approached its present thickness, about 200 kilometers, soon after it formed before 2.5 billion years ago and has persisted since then. The second key observation from xenoliths is that the rheologically active boundary layer (the zone of active flow) at the base of the lithosphere has a temperature range of less than 200 K. The investigation will focus on observable processes. Thermal convection occurs within the basal transition region (the rheological boundary layer) between the lithosphere and the underlying nearly adiabatic mantle. The overlying nearly rigid cold lithosphere provides a stagnant lid. The lithosphere cools and subsides when the basal convective heat flow (plus lithosphere radioactive heat generation) is less than the heat flow through the surface. This situation prevails in young ocean lithosphere, which subsides as it ages. Convective heat flow comes into quasi-equilibrium with surface heat flow below stable continental regions. Cratons with crust typically over 2.5 billion years old have chemically buoyant highly viscous lithosphere that provides a chemical lid to convection. Lithosphere that differs only thermally from deeper mantle provides a stagnant lid beneath younger platform crust. At present, the Earth is in a state where the lithosphere beneath platforms is almost as thick as that beneath cratons. That is, the chemical-lid convection beneath cratons is transitional to stagnant-lid convection. (1) The proposal concentrates on the scaling and 3-D numerical physics of the transition including 3-D effects from the disruption of convection by changes in plate motion. (2) Mantle plumes of hot buoyant material ascend from great depths and impinge on the base of the lithosphere. The buoyant material ponds beneath the lithosphere and flows laterally toward thin lithosphere like oil beneath pack ice. The plume material convectively exchanges heat with the lithosphere, thinning it. Scaling relationships and 3-D numerical models indicate that situations that favor vigorous thinning also favor vigorous lateral flow. The duration at a given place of thinning and lateral flow is thus brief, except above a slowly moving plume-tail orifice. The proposal concentrates on 3-D modeling of real features. (3) The proposal begins physical investigation of ultraslow ridge axes with kinematic thermal models.
