Collaborative Research: Effects of Structural and Compositional Heterogeneity on Upper Mantle Deformation and Rheology

岩石圈地幔的变形行为对构造板块的强度起着至关重要的作用。岩石变形实验研究是我们理解地幔变形和流变学的基础，而野外对暴露的地幔切片的研究需要在更大的空间尺度(10厘米)上研究自然变形的地幔的流变学和变形。最近的野外研究表明，复杂的变形模式挑战了经典的地幔变形均质流变学模型。一种多尺度(厘米到公里)的、导致岩石圈地幔局部化和变形叠加的过程的综合方法，允许在更大的空间尺度上评估地幔流变性。盾山蛇绿岩带包含多个完整的岩石圈地幔块体，包括红山、红山和墩山，揭示了自然变形的岩石圈地幔变形的时空模式，阐明了地幔变形、局部化和流变学的基本过程。本研究涉及的关于岩石圈地幔变形的第一级构造问题包括：1)地幔不均匀的规模和程度；2)什么过程导致不同尺度的地幔不均匀；3)不均匀组构是时间和/或空间局部变形的结果？以及4)不均匀变形对组构和结构保存以及大小的影响？这些问题的答案是关于地幔变形非均质性的规模和程度的原始数据，包括结构分析(组构分析、成分和结构域表征、局部化或叠置现象的识别、应变分析、有效粘度的估计)、显微结构和结构分析(具有非均质组构的岩石中的结构和组构之间的关系)，以及通过分析内在(例如，组成、熔体分数、粒度)和外部(例如，压力、温度、应力、氧逸度)岩石特征。地幔的最上部(岩石圈)被认为是形成地球的构造板块的最强部分--S表面。因此，岩石圈地幔的变形行为对正在进行的和古老的造山过程施加了一级控制。部分岩石圈地幔在造山事件中被抬升和剥离，现在可以在地球表面找到S。这些地点提供了在自然界中发生变形的尺度上研究地幔过程的机会。这项研究的主要问题是，地幔中不同矿物的确切比例和取向的变化在多大程度上控制了变形行为。这项工作只能在世界上少数几个有足够曝光率的地方进行；拟议的工作将在新西兰进行。这些数据对于综合岩石变形实验和地球物理观测的结果，以创建真实和预测的地幔顶部变形模型至关重要。该项目是德克萨斯农工大学和威斯康星大学的研究人员之间的多学科合作努力。除了该项目的研究目标外，它还培养了研究生和本科生的科学培训。该项目还涉及与新西兰奥塔哥大学的一名教职员工的国际合作。

The deformational behavior of the lithospheric mantle exerts a critical effect on the strength of the tectonic plates. Whereas experimental rock deformation studies form the basis of our understanding of mantle deformation and rheology, field-based studies of exposed mantle sections are needed to investigate the rheology and deformation of naturally deformed mantle at larger spatial scales ( 10 cm). Recent field studies have illustrated complex patterns of deformation that challenge a classical homogeneous rheological model for mantle deformation. A multi-scale (cm to km), integrated approach of the processes that result in localization and deformational overprinting in lithospheric mantle, allows an assessment of mantle rheology at larger spatial scales. The Dun Mountain ophiolite belt contains multiple intact blocks of lithospheric mantle - including the Red Hills, Red Mountain, and Dun Mountain - that reveal spatial and temporal patterns of deformation in naturally deformed lithospheric mantle and elucidate fundamental processes of mantle deformation, localization, and rheology.First-order tectonic questions about deformation in the lithospheric mantle addressed by this research include: 1) What is the scale and extent of mantle heterogeneity?; 2) What processes result in mantle heterogeneity at different scales?; 3) Do heterogeneous fabrics result from temporally and/or spatially localized deformation?; and 4) What is the effect of heterogeneous deformation on fabric and texture preservation and magnitude? These questions are answered with primary data on the scale and extent of mantle deformational heterogeneity including structural analyses (fabric analysis, compositional and structural domain characterization, identification of localizing or overprinting phenomena, strain analysis, estimates of effective viscosity), microstructural and textural analyses (relationship between texture and fabric in rocks with heterogeneous fabrics), and analyses of deformation processes by analyzing both intrinsic (e.g., composition, melt fraction, grain size) and extrinsic (e.g., pressure, temperature, stress, oxygen fugacity) rock characteristics.The uppermost (lithospheric) part of the mantle is thought to be the strongest part of the tectonic plates that form the Earth?s surface. As such, the deformational behavior of the lithospheric mantle exerts a first-order control on ongoing and ancient mountain building processes. Portions of the lithospheric mantle are uplifted and exhumed during mountain building events and can now be found on the Earth?s surface. These localities offer the opportunity to study mantle processes at the scales at which deformation occurs in nature. The major question in this research is the extent to which variations in the exact proportion and orientation of different minerals in the mantle control the deformational behavior. This work can only be done in a few places in the world with sufficient exposure; the proposed work will take place in New Zealand. These data are critical in synthesizing results from rock deformation experiments and geophysical observations, to create realistic and predictive models of uppermost mantle deformation.This project is a multidisciplinary collaborative effort between researchers at Texas A&M University and the University of Wisconsin. In addition to the research goals of the project, it is fostering scientific training of graduate and undergraduate students. The project also involves an international collaboration with a faculty member from the University of Otago, New Zealand.