Collaborative Research: The Role of Rock Composition and Microstructural Evolution on Strain Localization and the Effective Viscosity of the Crust

控制大陆地壳力学行为的知识是理解广泛地质过程的基础。例如，地壳物质在深部的长期流动控制着地壳如何由于与海平面上升和下降、冰川前进和后退以及造山和侵蚀有关的加载或卸载而变形。大地震前后地球表面的变形也受地壳岩石力学行为的控制。长期以来，科学家们一直利用大陆地壳组成矿物的力学性质来估计地壳应如何反应，但令人惊讶的是，对这些矿物（岩石）的聚集体如何反应知之甚少。来自布朗大学和伍兹霍尔海洋研究所的一个研究小组与挪威和新西兰的科学家合作，旨在更好地了解地壳岩石在高温高压下如何在受到外部应力时流动。他们将在实验室中使地壳材料变形，并进行计算机建模，以提高对短期（地震）和长期（山区）负荷下地壳材料流动的理解。该研究项目还通过培养具有全球竞争力的多元化STEM人才队伍，培养研究生和本科生实验室实验和数值模拟方面的培训，进一步推进预期的社会成果。该项目将获得新的实验和显微结构数据，并对地壳多相岩石的变形进行建模研究，以研究大陆地壳的流变学特性，重点在于组成和应变局部化的影响。实验和显微结构观测的重点是石英+石榴石，石英+白云母，石英+钠长石系统，以提高地壳流变学的理解和粒度敏感的蠕变剪切带的形成和流变学的作用。研究小组发现，将流变混合模型（结合单相流动定律）与稳定矿物组合的计算相结合，是研究岩石成分对地壳粘度作用的一种有前途的方法。这种模型与大地测量观测之间的一致性是令人鼓舞的，然而，这种方法有几个局限性，这项研究将解决：（1）石榴石流动定律预测在地壳条件下广泛变化的粘度，严重阻碍了地震属性与流变学相关的可能性;（2）云母集合体和云母单晶的现有流动定律也预测了地壳条件下的广泛不同的强度，主要是由于云母含量和应变速率的影响有关的不确定性;（3）剪切区的形成过程，这是被忽略的混合模型，似乎产生微观结构，其中混合层的晶粒尺寸由齐纳钉扎设置;（4）最近的实验工作表明，石英集合体中存在着对粒度敏感的蠕变和晶界滑动。实验将在700？1100摄氏度和应变速率从3e-7/s到1 e-4/s，围压从0.8到2.0 GPa。为了补充实验数据的解释，研究人员将对多相岩石中的粒度演化和剪切带发展进行数值模拟。模型将研究剪切带的各向同性，均匀系统和2-D剪切带的发展在非均质系统中的应力的局部变化可以影响强相和弱相的粒度演变。

Knowledge of the controls on the mechanical behavior of the continental crust is a fundamental underpinning for understanding a wide range of geological processes. For example, the long-term flow of crustal materials at depth controls how the crust deforms due to loading or unloading associated with sea level rise and fall, glacial advance and retreat, and mountain building and erosion. Deformation of the Earth's surface before and after large earthquakes is also controlled by the mechanical behavior of crustal rocks. Scientists have long used knowledge of the mechanical properties of the continental crust's constituent minerals to estimate how the crust should respond but, surprisingly, little is known about how aggregates of these minerals (rocks) respond. A research team from Brown University and Woods Hole Oceanographic Institution, in collaboration with scientists from Norway and New Zealand, aims to develop a better understanding of how crustal rocks flow under high temperature and pressure when subjected to external stresses. They will deform crustal materials in the laboratory and carry out computer modeling to improve understanding of the flow of crustal materials under both short-term (earthquakes) and long-term (mountain belts) loads. The research project additionally advances desired societal outcomes through the development of a diverse, globally competitive STEM workforce by training graduate and undergraduate training in laboratory experiments and numerical modeling.This project will acquire new experimental and microstructural data and conduct modeling studies of deformation in crustal multi-phase rocks to investigate the rheological properties of the continental crust, with emphasis on the effects of composition and strain localization. The experiments and microstructural observations focus on quartz+garnet, quartz+muscovite, and quartz+albite systems in order to improve understanding of crustal rheology and the role of grain size sensitive creep in the formation and rheology of shear zones. The research team finds that combining rheological mixing models (incorporating single-phase flow laws) with calculations of stable mineral assemblages is a promising way to investigate the role of rock composition on crustal viscosity. Agreement between such models and geodetic observations is encouraging, however, there are several limitations to this approach that this research will address: (1) garnet flow laws predict widely varying viscosities at crustal conditions, severely hampering the potential for relating seismic properties to rheology; (2) existing flow laws for mica aggregates and mica single crystals also predict widely different strengths at crustal conditions, primarily due to uncertainties related to the influence of mica content and strain rate; (3) shear zone formation processes, which are neglected in the mixing models, appear to produce microstructures in which the grain size of the mixed layers is set by Zener pinning; and (4) recent experimental work is suggestive of grain size sensitive creep and grain boundary sliding in quartz aggregates. Experiments will be conducted using Griggs apparatus at 700?1100 degrees C and strain rates from 3e-7/s to 1e-4/s at confining pressures from 0.8 to 2.0 GPa. To compliment the interpretation of the experimental data, the researchers will conduct numerical simulations of grain size evolution and shear zone development in polyphase rocks. Models will investigate shear zone evolution in isotropic, homogeneous systems and 2-D shear zone development in heterogeneous systems where local variations in stress can influence grain-size evolution in both the strong and weak phases.