The surface of the earth is in constant motion via a process known as plate tectonics. The surface of the earth is divided into about a dozen tectonic plates whereby one plate moves (at a few inches per year) with respect to other plates along great faults and fault zones. Although these motions seem small, they are responsible for earthquakes, including great earthquakes, mountain building, and the formation of sedimentary basins. Consequently, plate tectonics is simultaneously responsible for both some of the most significant geological hazards as well as the formation of the largest hydrocarbon reservoirs. With this project, we aim to better understand the forces that drive and resist plate motions and thereby obtain a deeper understanding of the forces that drive natural hazards and natural resources. Within this project, by linking the most advanced computational tools to existing observations, we will determine the forces and physical properties associated with plate tectonics. The most important impact will be the training of two Ph.D. students in geophysics and a postdoctoral scholar. They will gain enormous experience with sophisticated numerical methods, supercomputing technologies, and the linkage of data with numerical models. These are essential skills in science and the new knowledge economy. The proposed research will impact several large observational programs by developing hypotheses that could be tested with deep sea drilling under the auspices of IODP and geological sampling and seismic imaging of the Aleutian Island chain under the GeoPRISM project.
We propose to test hypotheses for changes in the plate tectonic system with a new generation of geodynamic models and bring new observational constraints to bear on the dynamics of plate motions. We will focus on changes in plate speed & direction and formation of new subduction zones because the observations will allow us to constrain uncertain mantle and lithospheric properties. Specifically, plate motions are likely strongly resisted by bending of the slab in a subduction zone, a force that depends on lithospheric viscosity, but this viscosity remains poorly known and we propose to place tighter bounds on it and other uncertain properties by matching observations in the time domain -- in much the same way that a combination of time-dependent and present day observations of post-glacial rebound are used to constrain the background mantle viscosity. First, we will use generic, fully dynamic 3D models to self-consistently explore the balance between changes in plate motions and changes to subduction and plumes. Second, we will use realistic models to better establish the link between dynamics and detailed geological and geochemical observations of specific examples of subduction initiation. Third, we will build detailed structural models of the mantle and lithosphere in the geological past using an approach that assimilates plate tectonic reconstructions into models of mantle convection along with inverse methods that allow estimation of initial conditions, density anomalies, and viscosity structures. Through these three approaches, we aim to more fully establish the dynamic link between changing plate motions and how they translate into and result from changes in subduction, with specific linkages to geophysical and geological observations. The models will be high resolution (e.g. locally 1 km or less where needed) and fully incorporate the extreme variations in viscosity between the slab and mantle, hinge zone and interface between subducting and over-riding plates.
