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

粘度对比如何影响多相岩石的变形模式？

基本信息

批准号：
1755805
负责人：
David Kohlstedt
金额：
$ 32.41万
依托单位：
University of Minnesota-Twin Cities
依托单位国家：
美国
项目类别：
Continuing Grant
财政年份：
2018
资助国家：
美国
起止时间：
2018-07-15 至 2022-06-30
项目状态：
已结题

来源：
https://www.nsf.gov/awardsearch/showAward?AWD_ID=1755805&HistoricalAwards=false
关键词：
How do viscosity contrasts affect

项目摘要

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.

岩石圈中的应变集中在板状剪切带中，这些剪切带容纳了大量的构造应变，并将形成相对坚硬的构造板块的岩石圈应变较小的部分分开。因此，剪切带代表了岩石圈规模的应变分配的例子，这对岩石圈的结构以及板块构造的运行和维护是基本的。这里探讨的想法是，由于岩石的非均质性，一些地区将比邻近地区略弱。这些较弱的区域比较强的区域变形得更快，因此，通过一种称为动态再结晶的过程，变形区域中的晶粒度减小。因此，最初较弱区域的强度进一步降低，因为强度随着晶粒度的减小而降低。因此，建立了一个正反馈回路，在该回路中，局部区域的变形减小了颗粒尺寸，进而削弱了该局部区域的岩石，从而进一步增强了该区域的变形。为了更好地理解随着变形量的增加，岩石强度的下降，研究小组将在控制良好的实验室条件下将岩石变形为不同的变形量。特别是，将对至少由两种不同矿物组成的岩石进行研究，以分离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被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，比如是什么允许岩石圈板块沿着狭窄的薄弱地带相互移动。该研究项目还有可能通过提高公众的科学素养和公众对STEM的参与，通过各种外联活动和通过支持和指导早期职业博士后研究员发展一支具有全球竞争力的STEM劳动力队伍来促进预期的社会结果。该研究项目要解决的中心问题是，在剪切的多相岩石中，不同阶段的晶格旋转轴的图案有多大不同，以及这些图案在变形过程中如何演变？一系列初步的实验室扭转实验表明，较弱的相可靠地跟踪骨料尺度的变形几何，而较强的相不能。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中颗粒尺度应变分配的系统差异。这项研究将检验这样一种假设，即最弱的阶段记录了主要的旋转应变模式。一系列新的实验将在扭转变形和一般剪切变形的两相混合物上进行。扭转实验将在等值应变率为5x10-5到5x10-4 S-1的恒定温度和恒定围压条件下进行，通常分别为1423K到1523K和300兆帕。扭转实验将被用来在两相集合体上施加一系列有限的简单剪切应变，并将在剪应力从30到150兆帕的范围内进行从1到20的有限剪切应变。一般剪切实验将在30至100兆帕的剪应力范围内对1至3的有限剪应变进行。将使用电子背散射和电子背散射衍射进行微观结构分析，这将提供变形样品中所有组成相及其晶体取向的详细地图。电子背向散射衍射数据将为量化单个颗粒和整个样品截面中的晶格应变提供必要的基本信息。不同相的内部应变将使用新的取向色散方法来量化，以计算晶格旋转图案，这些图案将被直接在相之间进行比较，并与每个实验的施加的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两相体系中晶格旋转图案发展的影响。大应变扭转实验的分析结果将探索不同阶段的晶格旋转如何随应变发展，以及不同阶段的旋转轴如何很好地跟踪变形几何。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。