The fractal dimension of grain boundaries in deformed rocks: A novel method for estimating differential stress and strain rate

The surface of Earth, where humans live and build infrastructure, is constantly subjected to stresses caused by the movement of tectonic plates. These stresses commonly cause events such as earthquakes that threaten life and infrastructure. To quantify the stresses on Earth, geologists examine the shape, size, and mineral characteristics of rocks. Observations of these rocks and laboratory experiments allow geologists to develop mathematical equations that relate the applied stress to the rate of rock flow. In this project, the researchers use laboratory experiments to develop a new method for extracting stress and flow-rate information from rocks that can be directly compared to results obtained from applying flow laws. They quantify how the roughness of mineral grain boundaries directly records stress and flow rate. They develop new protocols for measuring roughness using electron beam methods. The project supports a postdoctoral associate, as well as the training of graduate students at the University of Maine. The developed codes and analytical protocols are made openly available through public portals. These outcomes can be applied beyond Earth Sciences in Material Sciences and Engineering, notably to engineer the strength of metals, ceramics, and advanced composite materials. The objectives of this project are to (1) collect 0.2 μm step-size electron backscatter diffraction (EBSD) data from representative areas in 12 experimental samples deformed in axial shortening, and (2) collect 0.2 μm EBSD data on 7 experimental samples deformed in general shear. These new data will allow the team to (a) deliver robust calibrations for axial shortening experimental samples, (b) evaluate the effects of water on the stress and flow-rate determinations, and (c) evaluate the effects of deformation geometry (axial shortening versus general shear) on the stress and flow-rate determinations. A third objective is to collect EBSD data on 15 naturally deformed samples to test natural application of the experimentally derived calibrations. Addressing the above objectives provides an opportunity to quantify the relations between deformation conditions and resulting rock microstructures, and to develop a new method for estimating stresses and flow rates that can be used independently or in concert with traditional flow laws. 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.