Calibration of a new model for mantle viscosity: the role of grain boundaries from bicrystal experiments

The solid rocks within Earth's interior can flow, analogous to ice in a glacier, given sufficient time and temperature. This flow, or viscous deformation, has a strong influence on a variety of processes over short and long time scales. Over long time scales, the viscous deformation of rocks controls the motion of Earth's tectonic plates. Over short timescales, the viscous deformation of rocks controls the rate at which stresses buildup on overlying, earthquake-generating faults. However, there are major gaps in our understanding of how these rocks deform, which results in significant uncertainties in modeling these large-scale processes on Earth.

One of the largest sources of uncertainty is in understanding how grain boundaries, that is the regions between crystals, deform at extreme conditions. This lack of understanding has major implications for predicting processes in Earth. For instance, if grain boundaries are weak relative to the interiors of crystals, then the rates at which stresses build up on large, earthquake-generating faults may increase tenfold. To address this shortcoming, we will carry out experiments at extreme conditions in which we slide two crystals past each other. In some cases, we will add water to the boundary to test if water increases how fast the crystals slide. The data from many experiments will be used to create an equation that describes how fast the crystals slide under a wide range of conditions. To investigate how individual grain boundaries influence the properties of a rock made up of many crystals, these equations will be incorporated into numerical simulations that predict the behavior of an aggregate of crystals. These simulations will be used to understand the importance of grain boundaries in a variety of important large-scale geologic processes.

The tools generated in this study will have several important economic and societal impacts. Hazards, including earthquakes and sea level rise pose serious risks to life and economic stability in many regions. One of the best way to mitigate these risks is by implementing policy based on robust assessment of risk and predictive models to minimize their impact. However, generating predictive models of these risks requires a robust understanding of the flow behaviour of rocks in Earth's mantle over a wide range of timescales, which is currently incomplete. The constitutive equations we aim to develop in this work will greatly aid in generating more accurate predictive models in the context of several geodynamic hazards.

Earthquakes, which are regions of frictional sliding along lithospheric fault zones, are concentrated near the boundaries between tectonic plates. The rates at which stresses build up on the most major of these faults between earthquakes are largely controlled by the properties of the underlying, plastically deforming mantle rocks. However, current models of deformation of mantle rocks are based on an incomplete understanding of the physical mechanisms responsible for deformation. This time-dependent loading of faults is rarely built into models of seismic risk, but when it is incorporated, only the long-term plate motion rates are used. However, rates of loading directly after an earthquake are likely significantly faster than long-term rates due to deformation along grain boundaries. Better modeling of time-dependent loading of faults is one of the key focuses of current seismic risk analysis (e.g, uniform California earthquake rupture forecast, see Ref. 39). Our constitutive equations, based on the physical processes governing flow, will provide an integral new tool for incorporating time-dependent flow into the next generation of earthquake risk models.

The hazards associated with sea level rise are also a global concern. Urban planners require accurate models of sea level rise to safely plan and minimize the influence of this phenomenon on residents of coastal regions. However, sea-level changes are especially difficult to predict after the melting of large ice sheets because of glacial-isostatic adjustment. As glaciers melt, their weight is removed and the rocks below may be uplifted in response. However, other nearby portions of the lithosphere may undergo downward movement as part of the receding of a flexural bulge. A relevant example is the current uplift of northern Scotland and simultaneous subsidence of southern England, both in response to the retreat of the Fennoscandian ice sheet. Importantly, this complex viscoelastic response during glacial isostatic adjustment is intimately dependent on the flow of mantle rocks, and likely the deformation along grain-boundaries. Therefore, our improved model of mantle flow will provide a more accurate characterization of the viscoelastic properties of Earth for incorporation into new modelling efforts of the influence of glacial-isostatic adjustment on local variations in sea-level rise.

In addition to geological applications, the proposed work will also benefit the Engineering and Materials Science communities. There are undoubted benefits in bringing together scientists from different disciplines both engaged in research aimed at understanding mechanical processes in materials - naturally occurring or synthetic. Better knowledge of high temperature, high pressure deformation methods and production of high quality ceramic bicrystals may bring significant advantages to Materials Scientist and Engineers interested in tuning grain-boundary properties to enhance material behavior for special applications. Furthermore, understanding high-temperature processes at grain boundaries in rocks may give insight into grain-boundary sliding in superplastic forming of engineering ceramics, which plays a significant role in manufacturing of a wide range of products.