Collaborative Research on Till Deformation: Linking Microstructural Characteristics to Strain

0136006Iverson这是爱荷华州立大学和威斯康星州地质调查局首席研究员的合作提案。 过去的冰川，包括北半球的更新世冰盖，有时流动得异常快，导致冰块波动，导致严重的气候变化和景观改变。一个主要的假设将这种快速流动归因于融化的冰川基质的变形。过去冰盖沉积床中保存的结构可以提供这种变形的时间整合和空间广泛记录，并可用于检验这一假设。微观结构可能是最有用的变形指标，因为它们系统地演化并且无处不在，即使在巨大的耕作单元中也是如此。尽管变形冰川沉积物的微观结构特征已被广泛描述，但尚未有系统地努力将这些特征与剪切应变大小相关联。结果是，地床变形的程度及其在冰盖运动中的作用通常无法从地质记录中可靠地推断出来。 正在请求支持，以使用环剪装置来研究蒂尔微观结构特征随剪切应变的变化，该装置使大型沉积物样本变形至高应变。 这些实验将以结构地质学、地球物理学和土壤力学的相关研究为指导，这些研究表明剪切面方向、粘土颗粒结构和磁化率（AMS）各向异性随着沉积物变形而系统地变化。通过对各种应变进行实验并在每次测试后测量这些特征，将这些微观结构特征与剪切应变相关联。将测试两种具有不同粘土矿物比例的底土，并研究结果对初始固结和总正应力的敏感性。剪切平面将被光学识别，Y 剪切相对于其他方向剪切的发展程度（剪切应变大小的指标）将被量化。粘土颗粒织物将使用密歇根大学的 X 射线极图测角仪进行测量，并通过计算特征值和马奇应变进行量化。 AMS 将在明尼苏达大学岩石磁学研究所进行测量，并用于计算 AMS 幅度以及由最大磁化率方向定义的织物的强度和方向。这些实验将有助于改进过去冰盖的模型以及对冰川沉积物和地貌的解释。这项研究还可能有助于解决构造地质学、地球物理学、石油地质学和岩土工程中的相关问题，其中颗粒介质的变形和随之而来的各向异性往往是核心问题。

0136006IversonThis is a collaborative proposal by Principal Investigators at Iowa State University and the Wisconsin Geological Survey. Past glaciers, including the Pleistocene ice sheets of the Northern Hemisphere, sometimes flowed unusually fast, causing ice-mass fluctuations that resulted in severe climate change and landscape modification. A leading hypothesis attributes this rapid flow to deformation of the thawed glacier substrate. Structures preserved in the sediment beds of past ice sheets can provide a time-integrated and spatially extensive record of such deformation and can be used to test this hypothesis. Microstructures are potentially the most useful indicators of deformation, because they evolve systematically and are ubiquitous, even in massive till units. Although microstructural characteristics of deformed glacier sediments have been described extensively, there have been no methodical efforts to correlate these characteristics to shear-strain magnitude. The result is that the degree of bed deformation, and hence its role in ice-sheet motion, usually cannot be inferred reliably from the geologic record. Support is being requested to study the evolution of till microstructural characteristics as a function of shear strain with a ring-shear device that deforms a large sediment specimen to high strains. The experiments will be guided by related studies in structural geology, geophysics, and soil mechanics, which indicate that shear-plane orientations, clay-particle fabric, and anisotropy of magnetic susceptibility (AMS) change systematically with sediment deformation. These microstructural characteristics will be correlated with shear strain by conducting experiments to various strains and measuring these characteristics after each test. Two basal tills with different clay-mineral fractions will be tested and the sensitivity of the results to initial consolidation and total normal stress will be studied. Shear planes will be identified optically and the extent of Y-shear development relative to shears at other orientations, an indicator of shear-strain magnitude, will be quantified. Clay-particle fabrics will be measured with an X-ray, pole-figure goniometer at the University of Michigan and quantified by computing both eigenvalues and March strains. AMS will be measured at the Institute for Rock Magnetism at the University of Minnesota and used to compute AMS magnitudes, as well as strengths and directions of fabrics defined by orientations of maximum susceptibility. These experiments will help improve models of past ice sheets and interpretations of glacigenic sediments and landforms. This research may also help solve related problems in structural geology, geophysics, petroleum geology and geotechnical engineering, in which deformation of granular media and consequent anisotropy are often central issues.