Collaborative Research on Till Deformation: Linking Microstructural Characteristics to Strain

[136006 . iverson]这是爱荷华州立大学和威斯康星地质调查局的主要研究人员合作提出的一项建议。过去的冰川，包括北半球更新世冰原，有时流动得异常快，造成冰块波动，导致严重的气候变化和景观改变。一种主流假设将这种快速流动归因于融化的冰川基底的变形。保存在过去冰盖沉积物床中的结构可以提供这种变形的时间整合和空间广泛的记录，可以用来检验这一假设。微观结构可能是最有用的变形指标，因为它们是系统地进化的，无处不在，即使是在大规模的单位中。虽然变形冰川沉积物的微观结构特征已被广泛描述，但没有系统的努力将这些特征与剪切应变大小联系起来。结果是，通常无法从地质记录中可靠地推断出床层变形的程度，以及它在冰盖运动中的作用。利用环剪装置将大型沉积物试样变形为高应变，研究其微观结构特征随剪切应变的演化。实验将以构造地质学、地球物理学和土力学的相关研究为指导，表明剪切面取向、粘土颗粒结构和磁化率各向异性（AMS）随沉积物变形而系统变化。通过对各种应变进行实验，并在每次试验后测量这些特征，将这些微观结构特征与剪切应变相关联。将测试两个具有不同粘土矿物组分的基墩，并研究结果对初始固结和总法向应力的敏感性。剪切面将被光学识别，y型剪切相对于其他方向剪切的发展程度（剪切应变大小的一个指标）将被量化。粘土颗粒织物将用密歇根大学的x射线极形角计进行测量，并通过计算特征值和March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的震级，以及由最大磁化率方向定义的结构的强度和方向。这些实验将有助于改进过去冰盖的模型，以及对冰期沉积物和地貌的解释。该研究还有助于解决构造地质学、地球物理、石油地质学和岩土工程等相关问题，这些问题中颗粒介质的变形及其各向异性往往是中心问题。

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.