Quartz grain-boundary topology as a stress and strain-rate meter and a new flow law

University Of Maine, Orono ME

Investigators

Abstract

The surface of Earth is constantly subjected to stresses caused by the movement of tectonic plates, and these stresses commonly cause events such as earthquakes that threaten life and infrastructure. To quantify the stresses caused by plate tectonics, geologists examine the shape, size, and other microstructural characteristic of minerals such as quartz. From these characteristics combined with laboratory experiments geologists have developed mathematical equations that relate the applied stress to the rate of rock flow. These equations, known as flow laws, are valuable but the uncertainties remain significant. Here the researchers combine field-based data and modeling with 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 interdisciplinary project supports a postdoctoral associate, as well as the training of undergraduate and 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) Generate a new experimental calibration using quartz to refine the polynomial relations among temperature, stress, flow rate and grain-boundary roughness; 2) Obtain a complete set of quartz grain-boundary roughness data from the Sandhill Corner shear zone in the Norumbega fault system, Maine USA, to test and apply the new experimental calibrations; 3) Develop and refine methods for determining grain-boundary perimeters using optical and electron backscatter diffraction techniques, and provide optical and backscatter diffraction calibrations for the method. 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 strain rates that can be used independently or in concert with flow laws. The chosen approach applies models and concepts that originate largely in the materials and engineering communities, and mostly constitutes novel research in the geosciences. Recognition of the importance of microstructure for macroscopic behavior and the ability to treat it quantitatively is growing in both Earth Sciences and Materials Engineering. Outcomes of this project will provide a framework for future efforts in both fields, and for collaborations between them. 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.