Collaborative Research on Till Deformation: Linking Microstructural Characteristics to Strain

这是爱荷华州立大学和威斯康星州地质调查局的首席研究人员共同提出的建议。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。一种主要的假说将这种快速流动归因于融化的冰川基质的变形。保存在过去冰盖沉积床中的结构可以提供这种变形的时间综合和空间上的广泛记录，并可以用来检验这一假设。微结构可能是最有用的变形指标，因为它们是系统地演化的，而且无处不在，即使在大规模耕作单位中也是如此。尽管变形冰川沉积物的微结构特征已被广泛描述，但还没有系统的努力将这些特征与剪切应变大小联系起来。其结果是，通常不能从地质记录中可靠地推断出海床变形的程度，从而也就是它在冰盖运动中的作用。需要用环形剪切装置研究作为剪切应变函数的TIL微观结构特征的演变，该装置可将大的沉积物样品变形为高应变。这些实验将在构造地质学、地球物理学和土力学的相关研究的指导下进行，这些研究表明，剪切面取向、粘土颗粒组构和磁化率各向异性(AMS)随沉积物变形而发生系统变化。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进行关联。对两种不同粘土矿物含量的底耕进行了试验，并研究了试验结果对初始固结和总法向应力的敏感性。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将被量化，这是剪切应变大小的指标。粘土颗粒织物将在密歇根大学用X射线极图测角仪进行测量，并通过计算本征值和马奇应变进行量化。AMS将在明尼苏达大学岩石磁性研究所进行测量，并用于计算AMS的大小，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。这项研究还有助于解决构造地质、地球物理、石油地质和岩土工程中的相关问题，其中颗粒介质的变形和由此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.