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

岩石圈中的应变定位为板状剪切带，板状剪切带容纳了大量的构造应变，并将岩石圈中应变较小的部分分开，形成相对刚性的构造板块。因此，剪切带代表了岩石圈尺度的应变划分例子，这是岩石圈结构和板块构造的运行和维护的基础。这里探讨的想法是，由于岩石的非均质性，一些区域会比邻近区域稍弱。这些较弱的区域比较强的区域变形更快，因此，通过一种称为动态再结晶的过程，变形区域的晶粒尺寸减小。因此，最初较弱区域的强度进一步降低，因为强度随着晶粒尺寸的减小而降低。因此，建立了一个正反馈回路，在这个正反馈回路中，局部区域的变形使晶粒尺寸减小，而晶粒尺寸减小又使该局部区域的岩石变弱，从而进一步增强该局部区域的变形。为了更好地理解随着变形量的增加强度的下降，研究小组将在控制良好的实验室条件下使岩石变形到不同的变形量。特别是，至少由两种不同矿物组成的岩石将被研究，以便分离出一种矿物对另一种矿物行为的影响。一种量化矿物颗粒内部应变的新方法将应用于实验变形样品的光学和电子显微镜分析数据。最终，这项研究的结果可以用来解决地球科学中的重大挑战，比如是什么让岩石圈板块沿着狭窄的薄弱带相互移动。该研究项目还有可能通过各种外展活动提高公众的科学素养和公众对STEM的参与，以及通过对早期职业博士后的支持和指导，培养具有全球竞争力的STEM劳动力，从而促进预期的社会成果。本研究项目要解决的中心问题是，在剪切多相岩石中，不同阶段的晶格旋转轴模式有何不同，以及这些模式在变形过程中如何演变？一系列初步的室内扭转实验表明，较弱的相能够可靠地跟踪集料尺度的变形几何形状，而较强的相则不能。为了理解这一结果，研究小组将进行岩石变形实验和微观结构分析，旨在测试同一岩石中不同相的粘性强度的差异是否会导致这些相的颗粒级应变分配的系统性差异。该研究将验证最弱相位记录主导旋转应变模式的假设。一系列新的实验将进行在扭转和一般剪切变形的两相混合物。在恒定温度和围压条件下，通常分别为1423 ~ 1523 K和300 MPa，在5x10-5 ~ 5x10-4 s-1的恒定等效应变速率下进行扭转实验。扭转试验将用于对两相聚集体施加一系列有限简单剪切应变，并将在30至150 MPa的剪切应力范围内进行1至20的有限剪切应变。一般剪切实验将进行有限剪切应变范围从1到3，剪应力范围从30到100 MPa。显微结构分析将使用电子后向散射和电子后向散射衍射进行，这将提供变形样品中所有组成相及其晶体学取向的详细图。电子背散射衍射数据将为量化单个颗粒和整个样品截面的晶格应变提供必要的基础信息。不同相的晶粒内部应变将使用新的取向-色散方法来量化计算晶格旋转模式，这些模式将直接在相之间进行比较，也将与每次实验的施加应变几何形状进行比较。新的实验将测试粘度和相分数对三种不同的两相体系中晶格旋转模式发展的影响。大应变扭转实验的分析结果将探讨不同相的晶格旋转如何随应变发展，以及不同相的旋转轴如何很好地跟踪变形几何。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

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.