Title of the talk: Subgrain-size piezometry as a tool for measuring stress in polymineralic rocks
Subgrains are defined as intragranular surfaces across which the crystal structure is misoriented. At conditions where dislocation creep is active, subgrain-size is inversely proportional to the Von Mises stress experienced by a rock. As such, subgrains offer a useful tool to estimate the past stresses supported by exhumed natural rocks — a technique referred to as piezometry. Recently, we calibrated a new subgrain-size piezometer using electron backscatter diffraction, a now common technique for microstructural characterisation. However, samples used in the calibration were from monomineralic rocks that had reached a steady-state stress. In contrast, natural rocks are often comprised of multiple minerals and have complex stress histories. If subgrain-size piezometry is to be applied to natural, polymineralic rocks, it is first necessary to explore three questions:
1. Does our monomineralic-subgrain-size piezometer apply to polymineralic samples?
2. How do rapid changes in stress modify the subgrain size?
3. How do grain-scale stresses measured within individual minerals in an aggregate related to the bulk stress experienced by the sample?
We tested the application of the subgrain-size piezometer to polymineralic rocks, and the effect of complex stress histories, using a Deformation-DIA apparatus at synchrotron X-ray beamline 6-BM-B at the Advance Phone Source, at the Argonne National Laboratory, Illinois. Samples that are deformed at stresses within the original calibration range of the subgrain-size piezometer exhibit good agreement between in-situ and ex-situ stress measurements in olivine, both in monomineralic and polymineralic rocks. However, fine grain sizes limit the application of subgrain-size piezometry at high stresses, influencing which portion of the stress history is recorded. To explore stress partitioning, we also deformed olivine-orthopyroxene mixtures of various phase proportions in a gas-medium apparatus. We found that the stress within individual phases did not match theoretical models for calculating the bulk stress in aggregates. Finally, we explore the stress recorded in the subgrain size for three natural shear zones: the Great Slave Lake shear zone (Northwest Territories Canada), the Atlantis Bank oceanic core complex (ODP Hole 735B), and the Karakoram fault (Himalayas). To conclude, subgrain-size piezometry offers an important tool for exploring how stress partitions in natural systems, however care must be taken before drawing conclusions about the bulk strength of rocks.
Co-authors: Lars N. Hansen, David Wallis, Kathryn Kumamoto, Andrew J. Cross, Veronique Le Roux, Brendan Dyck, Namitha Kumar, & Diede Hein.
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