Untangling the Roles of Viscous, Elastic, and Plastic Deformation in Slab Bending

Sinking of a tectonic plate beneath another plate into the mantle is called subduction. It is a fundamental process that controls the evolution of Earth and holds a key to understanding important processes, such as earthquakes, volcanism, and mantle convection. However, how subduction initiates and how the tectonic plate changes its shape during subduction initiation (SI) are still unclear. The key driving forces are the negative buoyancy of the subducting plate and external forces that push the tectonic plates laterally. The key resisting forces include bending resistance of the subducting plate, frictional resistance between the subducting and overriding plates, and viscous resistance that is exerted by the surrounding mantle on the subducting plate. The details of SI and how the subducting plate bends depend strongly on the balance between these forces and also on the strength of the subducting slab, which has been defined by some combination of viscosity, elasticity, and plasticity of Earth materials. Further, these material prosperities are often assumed to be uniform in numerical simulations of SI despite their dependence on pressure, temperature, stress, and composition. Which of the three mechanical properties (i.e., viscosity, elasticity, and plasticity) controls the bending of the subducting plate depends on their relative values. Using numerical models, this study aims to better understand how the bending behavior of the subducting plate changes with the three mechanical properties. The project provides support for an early-career female faculty member and a postdoctoral scholar.

A suite of generic 2-D and 3-D dynamic subduction models with a visco-plastic and a visco-elasto-plastic rheology will be developed to quantify the effect of elasticity on down-dip and lateral bending of the subducting slab. These models will incorporate a composite viscous rheology that accounts for both dislocation creep and diffusion creep, and models with the visco-elasto-plastic rheology incorporates pressure- and temperature-dependent shear modulus, using the thermodynamic code Perple_X. The models are used to address two key questions: (1) Can the negative buoyancy of the oceanic lithosphere be sufficient to overcome the elastic bending resistance and to initiate subduction, and (2) Is elasticity a rheological component that resists or facilitates down-dip and lateral bending/unbending? Systematic tests on the effect of subduction parameters on the slab evolution will also be performed to address these questions. Further, 2-D and 3-D models will be developed for selected real subduction systems, including Marianas, Tonga, and Puysegur, and the results will be compared with the tectonic history and the current slab geometry. The numerical models to be developed will provide not only the geometrical evolution of the slab but also the temperature and mantle flow fields of subduction systems over a range of parameters, and the information will be useful for the interpretations of geophysical and geochemical observations. The results will be summarized in scientific videos to be distributed through the PI's research website and YouTube as well as in conference presentations and peer-reviewed journal articles.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.