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的地质过程的基本基础。例如，地壳物质在深处的长期流动控制着地壳如何因与海平面上升和下降、冰川进退以及山体建造和侵蚀有关的加载或卸载而变形。大地震前后地表的变形也受地壳岩石力学行为的控制。长期以来，科学家们一直使用大陆地壳组成矿物的机械性质知识来估计地壳应该如何反应，但令人惊讶的是，人们对这些矿物(岩石)的集合体如何反应知之甚少。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和压力下受到外部压力时是如何流动的。他们将在实验室中使地壳物质变形，并进行计算机模拟，以改善对短期(地震)和长期(山脉地带)载荷下地壳物质流动的了解。此外，该研究项目还通过培训研究生和本科生在实验室实验和数值建模方面的培训，通过发展一支多样化的、具有全球竞争力的STEM劳动力队伍，促进预期的社会成果。该项目将获得新的实验和微观结构数据，并进行地壳多相岩石变形的模拟研究，以调查大陆地壳的流变性，重点是成分和应变局部化的影响。实验和显微构造观察主要集中在石英+石榴石、石英+白云母、石英+钠长石体系，以加深对地壳流变学和粒度敏感蠕变在剪切带形成和流变学中的作用的认识。研究小组发现，将流变混合模型(结合单相流定律)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的作用的一种很有前途的方法。这些模型与大地观测之间的一致性令人鼓舞，然而，这项研究将解决这种方法的几个局限性：(1)石榴石流动定律预测地壳条件下的粘度变化很大，这严重阻碍了将地震性质与流变学联系起来的可能性；(2)现有的云母聚集体和云母单晶流动定律也预测了地壳条件下云母聚集体和云母单晶的不同强度，主要是由于与云母含量和应变率的影响有关的不确定性；(3)剪切带形成过程，在混合模型中被忽略，似乎产生了混合层的颗粒尺寸由齐纳钉扎确定的微结构；(4)最近的实验工作表明，石英集料中存在尺寸敏感的蠕变和晶界滑移。实验将使用Griggs装置，温度为700~1100℃，应变速率为3E-7/S至1E-4/S，围压为0.8~2.0 Gpa。为了补充对实验数据的解释，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。模型将研究各向同性均质体系中的剪切带演化和非均质体系中的二维剪切带发展，其中应力的局部变化可以影响强相和弱相中的粒度演变。

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.