Collaborative Research: Determining Mantle Rheology from Field and Microstructural Observations of Naturally-deformed Peridotites

全球地球科学中一个关键且尚未解决的问题是，构成构造板块的不同岩石圈层的相对强度。流变学是用来描述材料行为的术语，它量化了观测到的变形(应变)和施加的力(应力)之间的关系。这笔赠款的主要研究人员将基于现场的定量地质学研究与实验岩石变形研究的数据相结合，以限制自然变形的地幔岩石的流变学。野外工作提供了在地质应变率、空间尺度和成分非均质性水平上的变形分析，而实验室研究提供了单相材料在已知应力和应变率下的受控变形实验数据。岩石圈(顶端)地幔是这项研究的重点，原因有三。首先，地幔岩石的矿物学相对简单(通常只有三种矿物，体积上以矿物橄榄石为主)。其次，对橄榄石进行了大量的实验数据，使实地研究得以校准。第三，由于橄榄石在实验研究中的高强度，许多现有的模型认为岩石圈地幔控制着构造板块的物质行为。本文研究了上地幔岩石的两个出露程度极高的剖面--新西兰的盾山蛇绿岩段(红山段)和美国华盛顿州的孪生姐妹混杂岩段。这两个领域对地幔流变学提供了不同但互补的约束。孪生姐妹橄榄岩是一个理想的野外地点：1)使用被称为岩脉的地质标志来量化地幔剖面中的变形；2)观察地幔变形的空间变化；3)确定第二矿物相(斜方辉石)对天然橄榄岩整体流变学的影响。新西兰盾山蛇绿岩的红山部分允许PI研究地幔变形的另外两个方面：1)自然变形的地幔体内存在的大型结构(例如褶皱)的流变学意义；2)从韧性剪切标志推断的地幔局部化为什么发生在地幔中。根据这些数据集，岩石圈地幔的整体流变性将受到约束。这笔助学金支持两所大学的一名研究生和多名本科生。这笔资金是与新西兰一所大学进行国际合作的基础，允许学生访问这两所大学。

A crucial and unsolved question in global earth science is the relative strength of the different lithospheric layers that make up the tectonic plates. Rheology, the term used to describe material behavior, quantifies the relation between the observed deformation (strain) and the applied forces (stress). The principal investigators of this grant combine quantitative, field-based geologic studies with data from experimental rock deformation studies to constrain the rheology of naturally-deformed mantle rocks. The fieldwork provides analysis of deformation at geologic strain-rates, spatial scales, and levels of compositional heterogeneity, while laboratory studies provide data from controlled deformation experiments at known stresses and strain-rates for monophase materials. The lithospheric (uppermost) mantle is the focus of this study for three reasons. First, mantle rocks have a relatively simple mineralogy (typically only three minerals, volumetrically dominated by the mineral olivine). Second, a large amount of experimental data has been conducted on olivine, allowing calibration of the field studies. Third, because of the high strength of olivine in experimental studies, many current models assume that the lithospheric mantle dominates the material behavior of the tectonic plates. Two extremely well-exposed sections of upper mantle rocks, the Dun Mountain ophiolite body (Red Hills section) in New Zealand and the Twin Sisters dunite in Washington State, USA are investigated in this study. The two field areas provide different, but complimentary, constraints on mantle rheology. The Twin Sisters dunite is an ideal field locality to: 1) Quantify deformation in a mantle section, by using geological markers known as dikes; 2) Observe spatial variations in mantle deformation; and 3) Determine the effect of the 2nd mineral phase (orthopyroxene) on the bulk rheology of natural peridotites. The Red Hills section of the Dun Mountain ophiolite, New Zealand, allows the PIs to investigate two additional aspects of mantle deformation: 1) The rheological implications of large-scale structures (e.g., folds) present in naturally deformed mantle bodies; and 2) Why localization, as inferred from ductilely sheared markers, takes place in the mantle. From these data sets, the bulk rheology of lithospheric mantle will be constrained. The grant supports a graduate student and multiple undergraduate students at two universities. The funding forms the basis for international cooperation with a New Zealand university, allowing students to visit both universities.