Plate tectonics remains the over-arching context for solid earth geophysics and describes the motion of the solid Earth's surface by the relative movement of ~14 rigid plates that interact along weak boundaries. These boundaries provide the locus for most of the planet's earthquakes and volcanoes and while their location and relative motions are well described by plate tectonics, their dynamics and properties remain poorly understood. A key feature of plate boundaries, however, is that many of them are magmatic with active volcanism and it may be that the interaction of molten rock (magma) with its solid host can lead to the necessary structures and weakness preserving plate boundaries. Laboratory experiments by the PI and others, demonstrate that deformation of partially molten rocks can lead to spontaneous localization into melt-rich networks that weaken the rock and provide efficient paths for heat and melt transport. Thus understanding the dynamics of partially molten systems is critical for understanding the behavior and evolution of plate boundaries. The aim of this project is to advance our theoretical understanding of the process, in order to extrapolate results from experiments to a wide range of conditions in the Earth.
There exists a wealth of data from high-pressure and temperature experiments on partially molten rocks deformed in torsion, from which a great deal of information about the mechanisms active in stress-driven segregation may be inferred. The emphasis of this project is to develop better theoretical and computational models of these experiments. We are using two methods in parallel to model the process and compare model to experiment. The first is to develop numerical models of the partial differential equations derived from two-phase flow or magma dynamics theory, solved in torsional deformation geometry. A spinoff of this work will be the development and release of an advanced computational system for general multi-physics problems. The effects to be tested will include various constitutive models for matrix deformation as well as various effects of surface energy and damage. The second methodology is to develop effective macroscopic constitutive models within a non-equilibrium thermodynamic framework, using a formalism that is well-established in metallurgy but is just beginning to be applied to earth science. This method describes the structural characteristics of the material with "internal state variables", and tracks the stored and dissipated energy associated with those. The result will be thermodynamically consistent constitutive equations that can be solved in geodynamic models that explore the large-scale effects of stress-driven segregation occurring at length scales much smaller than can ever be resolved in a geodynamic model. This effective constitutive model will also include melt transport properties so that the potential consequences of coupling between deformation and fluid flow can be explored in subduction zones, ridges, rifts and other planetary settings.
