Collaborative Research: Deformation Thermometry and Water Weakening of Quartz Tectonites - Case Studies from the Himalaya and the Caledonides of NW Scotland

合作研究：石英构造岩的变形测温和水弱化——喜马拉雅山和苏格兰西北部喀里多尼亚山脉的案例研究

• 批准号: 1220138
• 负责人: Andreas Kronenberg
• 金额: $ 15.59万
• 依托单位: Texas A&M University
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2012
• 资助国家: 美国
• 起止时间: 2012-08-15 至 2016-07-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1220138&HistoricalAwards=false
• 关键词: Collaborative; Research; Deformation; Thermometry; Water

Quantitative determination of the ambient temperatures at which rocks deform under different tectonic settings in the earth's crust is of critical importance for constraining thermo-mechanical modeling of the earth's tectonic plates, and their mechanical and thermal evolution. Ambient deformation temperatures control the physical processes that govern deformation and the rates at which deformation occurs. Pressure-Temperature-time (PTt) histories of rocks can be obtained from the compositions of minerals and mineral assemblages, through reaction equilibrium relations, but these determinations bear on the PT conditions of the mineral reactions, not necessarily those of deformation. Development of thermometers that directly constrain conditions of deformation is important because temperatures indicated by deformation features and those indicated by equilibrium mineral assemblages may differ, recording different intervals of a rock's PTt history. In quartz-rich continental crust, two types of deformation thermometers (based on quartz recrystallization processes and crystallographic fabrics) have been used as analytical tools over the last two decades in a wide range of tectonic studies, assuming that temperature is the primary controlling factor in recrystallization regime and fabric development. However, recrystallization regimes and fabric development may also be influenced by trace levels of water, and the effects of water on deformation mechanisms and fabric development may be of comparable importance to temperature. We propose to test, refine, and validate the deformation thermometers for quartz-bearing rocks and to examine those conditions under which water contents or variations in deformation rate need to be evaluated. We have chosen Himalayan and NW Scotland field areas as case studies because one of us has already collected extensive suites of appropriate oriented quartz-rich tectonites from these areas under prior NSF funding. These samples have been deformed under a wide range of tectonic settings and deformation thermometry, together with more restricted compositionally based thermometry, have already been completed on many of these samples. Deformation temperatures indicated by recrystallization regime and crystallographic fabrics in individual samples will be independently compared/tested by titanium-in-quartz thermometry, while the potential influence of water on recrystallization and crystallographic fabrics will be investigated by infrared spectroscopy and transmission electron microscopy. Integration of theoretical concepts and analytical techniques developed in Geosciences and Materials Science over the last half century has led to major advances in our understanding of both the mechanisms by which rocks deform/flow and the influence of environmental factors such as temperature on flow in the earth's crust. However the most commonly applied analytical techniques for determining the temperatures at which rocks now exposed at the earth's surface have been deformed are known to also be sensitive to fluctuations in chemically-induced weakening that may have occurred during deformation in the mineral grains making up the rock. Thus, deformation temperatures calculated using these thermometers may be in error, and thermomechanical numerical models developed to simulate flow in the crust using such temperature data may give unrealistic results. This project is designed to test the validity of these thermometers using a recently developed thermometer that takes such chemical processes in to account. Rock samples collected under previous NSF funding from ancient mountain belts in the Himalaya and Scotland have been chosen as case studies, and the potential role of chemical weakening in these samples will also be evaluated using complementary analytical techniques. These studies have important applications to plate tectonics and the results may change our understanding of the mechanical and thermal character of plates. Plate thickness and strength were originally considered to be due solely to temperature and the geothermal gradient in the earth. Our study addresses this concept and tests whether the internal strength of plates may also be influenced by water content, in addition to temperature. The project is a collaborative effort between researchers at Virginia Tech, Texas A&M University and Rensselaer Polytechnic Institute. In addition to the scientific goals of the project, this research is contributing to the training of Ph.D. students at Virginia Tech and Texas A&M, and providing support for an early career post-doctoral researcher at RPI. Undergraduate students at all of the participating institutions will be involved in the research.

定量确定地壳不同构造环境下岩石变形的环境温度对于约束地球构造板块的热机械模型及其机械和热演化至关重要。 环境变形温度控制着控制变形的物理过程以及变形发生的速率。 岩石的压力-温度-时间（PTt）历史可以通过反应平衡关系从矿物和矿物组合的组成中获得，但这些确定取决于矿物反应的 PT 条件，而不一定是变形的 PT 条件。 开发直接限制变形条件的温度计非常重要，因为变形特征指示的温度和平衡矿物组合指示的温度可能不同，记录岩石 PTt 历史的不同间隔。在富含石英的大陆地壳中，两种类型的形变温度计（基于石英重结晶过程和晶体结构）在过去二十年中被用作广泛的构造研究中的分析工具，假设温度是再结晶状态和结构发展的主要控制因素。 然而，再结晶机制和织物发展也可能受到微量水的影响，并且水对变形机制和织物发展的影响可能与温度具有相当的重要性。我们建议测试、改进和验证含石英岩石的变形温度计，并检查需要评估含水量或变形率变化的条件。我们选择喜马拉雅山和苏格兰西北部地区作为案例研究，因为我们中的一个人已经在国家科学基金会之前的资助下从这些地区收集了大量适当定向的富含石英的构造岩。 这些样品在广泛的构造环境下发生了变形，并且许多样品已经完成了变形测温以及更受限制的基于成分的测温。 将通过石英中钛测温法独立比较/测试各个样品中再结晶状态和晶体结构所指示的变形温度，同时将通过红外光谱和透射电子显微镜研究水对再结晶和晶体结构的潜在影响。过去半个世纪地球科学和材料科学中发展的理论概念和分析技术的整合，使我们对岩石变形/流动机制以及温度等环境因素对地壳流动的影响的理解取得了重大进展。 然而，众所周知，用于确定现在暴露在地球表面的岩石变形温度的最常用分析技术也对化学引起的弱化的波动敏感，这种弱化可能在构成岩石的矿物颗粒变形期间发生。 因此，使用这些温度计计算的变形温度可能是错误的，并且使用此类温度数据来模拟地壳中的流动而开发的热机械数值模型可能会给出不切实际的结果。 该项目旨在使用最近开发的温度计来测试这些温度计的有效性，该温度计考虑了此类化学过程。 在美国国家科学基金会之前的资助下，从喜马拉雅山和苏格兰的古代山区收集的岩石样本已被选为案例研究，这些样本中化学弱化的潜在作用也将使用补充分析技术进行评估。 这些研究对板块构造有重要的应用，其结果可能会改变我们对板块机械和热特性的理解。 板的厚度和强度最初被认为仅取决于地球中的温度和地温梯度。 我们的研究解决了这个概念，并测试了除了温度之外，板材的内部强度是否也可能受到含水量的影响。该项目是弗吉尼亚理工大学、德克萨斯农工大学和伦斯勒理工学院的研究人员共同努力的结果。除了该项目的科学目标外，这项研究还有助于博士的培训。弗吉尼亚理工大学和德克萨斯农工大学的学生，并为 RPI 的早期职业博士后研究员提供支持。所有参与机构的本科生都将参与这项研究。