How do viscosity contrasts affect patterns of deformation in multiphase rocks?

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Strain in the lithosphere localizes into tabular shear zones that accommodate large amounts of tectonic strain and separate less strained portions of the lithosphere that form relatively rigid tectonic plates. Therefore, shear zones represent lithospheric-scale examples of strain partitioning that are fundamental to the architecture of the lithosphere and the operation and maintenance of plate tectonics. The idea explored here is that, due to heterogeneities in rocks, some regions will be slightly weaker then neighboring regions. These weaker regions deform more quickly than stronger areas and, as a result, the grain size decreases in the deforming regions by a process called dynamic recrystallization. Therefore, the strength of the initially weaker region decreases further, since strength decreases as grain size decreases. Thus, a positive feedback loop is established in which deformation in a localized region decreases grain size which, in turn, weakens the rock in that localized region which further enhances deformation in that region. To better understand this decrease in strength with increasing amounts of deformation, the research team will deform rocks under well-controlled laboratory conditions to varying amounts of deformation. In particular, rocks composed of at least two different minerals will be studied in order to isolate the effect of one mineral on the behavior of a second mineral. A new method to quantify the internal strain of the mineral grains will be applied to data from optical and electron microscopic analysis of the experimentally-deformed samples. Ultimately, results from this study can be used to address grand challenges in the earth sciences such as what allows lithospheric plates to move past one another along narrow zones of weakness. The research project also has potential to advance desired societal outcomes through increased public scientific literacy and public engagement with STEM through various outreach activities and development of a globally competitive STEM workforce through support and mentoring of an early career post-doctoral fellow.The central question to be addressed by this resesarch project is how different are patterns of lattice rotation axes from different phases in sheared polyphase rocks, and how do these patterns evolve during deformation? A preliminary series of laboratory torsion experiments indicate that the weaker phase reliably tracks the aggregate-scale deformation geometry, whereas the stronger phase does not. To understand this result the research team will conduct rock deformation experiments and microstructural analyses designed to test whether or not contrasts in the viscous strength of the different phases in the same rock results in systematic differences in grain-scale strain partitioning in these phases. The research will test the hypothesis that the weakest phase records the dominant rotational strain pattern. A series of new experiments will be conducted on two-phase mixtures deformed in torsion and in general shear. Torsion experiments will be conducted at a constant equivalent strain rate of 5x10-5 to 5x10-4 s-1 under constant temperature and confining pressure conditions, typically 1423 to 1523 K and 300 MPa, respectively. The torsion experiments will be used to impose a range of finite simple-shear strains on the two-phase aggregates and will be conducted to finite shear strains from 1 to 20 at shear stresses ranging from 30 to 150 MPa. General shear experiments will be conducted to finite shear strains ranging from 1 to 3 at shear stresses ranging from 30 to 100 MPa. Microstructural analyses will be conducted using electron backscatter and electron backscatter diffraction, which will provide detailed maps of all constituent phases in the deformed samples and their crystallographic orientations. The electron backscatter diffraction data will provide the foundational information necessary for quantifying lattice strain in individual grains and across entire sample sections. The internal strain within grains of the different phases will be quantified using new orientation-dispersion methods to calculate lattice-rotation patterns that will be directly compared between phases and also compared to the imposed strain geometry of each experiment. The new experiments will test the effects of viscosity and phase fraction on the development of lattice rotation patterns in three different two-phase systems. Results from analysis of large-strain torsion experiments will explore how lattice rotations in different phases develop with strain and how well rotation axes in the different phases track the deformation geometry.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.