Can rocks palaeomagnetically record palaeo-stress?

The magnetic signals recorded by rocks provide information about the Earth’s evolution. For example, magnetic anomalies on the ocean sea floor were key to theory of plate tectonics, and more recently palaeomagnetic recordings suggest that the Earth’s magnetic field has existed for nearly four billion years; the geomagnetic field protects the Earth making it habitable. However, there is one key process in the Earth Sciences for which we do not understand the magnetic response. That process is stress. This is important, as much of the Earth’s surface is characterised by stress, with stress being integral to dramatic processes like earthquakes, eruptions and impacts, as well as slower processes like mountain building. Yet, we have no comprehensive model for the effect of stress on the magnetic response of minerals. Why have the effects of stress on the magnetic signal of minerals been largely ignored? We know that most rocks have been subjected to stresses from a variety of mechanisms including faulting, burial and in more extreme cases impacts. Historically, however, these stresses were thought to be too small (<1 GPa) to alter or reset existing “stable” magnetic recordings (remanent magnetisations) in all but the most extreme impacts. However, in a recently published paper, we demonstrate this assumption is incorrect. Our numerical models predict that stresses of only ~0.1 GPa are sufficient to affect geologically “stable” magnetic recordings. Whilst 0.1 G Pa is still a high pressure, such stresses are very common in areas such as seismically active fault zones and mountain building. The aim of this proposal is to formulate the first accurate predictive model for stress-induced magnetisation in rocks. To do this, experimental measurements are critical to the development, refinement and extension of our existing numerical model to a wider range of mineralogies and grain morphologies, and to provide detailed experimental validation. We have already undertaken some preliminary experimental measurement on bulk samples – presented in this proposal, which encouragingly support our numerical predictions. However, to build the first accurate, predictive model, we need to experimentally verify and quantify the processes on nanometric scale. To do this we will use state-of-the-art in-situ nanometric magnetic images using the world’s most advanced transmission electron microscopy methods. We will test this new theory on natural rocks through a comprehensive set of bulk measurements. If our numerical prediction is correct and stresses of only ~0.1 GPa can alter magnetic recordings in rocks, this will have significant consequences for many palaeomagnetic studies; however, the real potential breakthrough is in new areas of study and application. One breakthrough area, we are keen to develop, is earthquake hazard quantification. Key to this is knowledge of peak palaeo-stress fields due to previous earthquakes over recent geological history, i.e., ~10 kyr. Currently, palaeo-stress fields are hard to quantify as rocks directly in the fault are heavily altered due to heating, and rocks distal to the fault, which experience limited heating and stresses < 0.2 GPa, display elastic behaviour leaving no detectable microstructural damage. However, that magnetizations are altered by stresses of only ~0.1 GPa, means that rocks have the potential to magnetically record palaeo-stress fields and to retain this information over geological timescales. We plan to use our new knowledge refined in this proposal, to develop methods of quantifying palaeo-stress fields, leading to a step-change in earthquake-hazard risk quantification.