Collaborative Research on Till Deformation: Linking Microstructural Characteristics to Strain

0136006艾弗森这是爱荷华州州立大学和威斯康星州地质调查局的主要研究人员合作提出的建议。 过去的冰川，包括北方半球的更新世冰盖，有时流动得异常快，造成冰量波动，导致严重的气候变化和景观改变。一个主要的假说将这种快速流动归因于解冻的冰川基底的变形。保存在过去冰盖沉积层中的结构可以提供这种变形的时间综合和空间广泛的记录，并可用于验证这一假设。微观结构是潜在的最有用的变形指标，因为它们系统地演变，甚至在大规模的冰碛单元中也是普遍存在的。虽然变形冰川沉积物的微结构特征已被广泛描述，一直没有系统的努力，这些特征相关的剪切应变幅度。结果是，河床变形的程度，以及它在冰盖运动中的作用，通常不能从地质记录中可靠地推断出来。 需要支持研究冰碛物微观结构特征的演变，作为剪切应变的函数，使用环剪装置使大型沉积物样品变形至高应变。 实验将指导相关的研究，在结构地质学，地球物理学和土壤力学，这表明剪切面取向，粘土颗粒组构，和磁化率（AMS）的各向异性的变化系统与沉积物变形。通过对各种应变进行试验并在每次试验后测量这些特性，将这些显微结构特性与剪切应变相关联。将测试两种不同粘土矿物组分的基底土，并研究结果对初始固结和总正应力的敏感性。将通过光学方法识别剪切平面，并量化Y-剪切发展相对于其他方向剪切的程度（剪切应变大小的指标）。粘土颗粒组构将在密歇根大学用X射线极图测角仪测量，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性研究所进行测量，并用于计算AMS幅度，以及由最大磁化率方向定义的结构强度和方向。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。这一研究也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学中的相关问题，其中颗粒介质的变形及其引起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.