The experimental basis for interpreting particle and magnetic fabrics of sheared till
The experimental basis for interpreting particle and magnetic fabrics of sheared till
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解释剪毛颗粒和磁性织物的实验基础
DOI：
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发表时间：
2008
期刊：
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通讯作者：
Jacqueline R. Shumway
中科院分区：
文献类型：
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作者：
N. Iverson;T. Hooyer;J. Thomason;Matt Graesch;Jacqueline R. Shumway
基底冰碛物的颗粒组构可能允许测试冰川流动的河床变形模型，这需要高的河床剪切应变（>100）。然而，现场研究还没有得出剪应变大小和组构发展之间的系统关系。为了分离这种关系，在环剪切装置中将四个基底切片和粘性油灰剪切至高达714的应变。使用细砾石和砂粒的长轴方向以及小型（105·8 cm 3）完整样品的磁化率（AMS）各向异性，在剪切变形区内表征组构。结果表明，冰碛物颗粒以长轴取向朝向剪切平面旋转，其在剪切方向上变得紧密聚集（对于三维数据，0·78 < S1 < 0·94）。这些坚固的稳态组构在7-30的剪切应变下获得，没有证据表明组构随着进一步的应变而变弱，无论所考虑的具体的直到或颗粒尺寸分数如何。这些结果不支持颗粒旋转的Jeffery模型，该模型正确地描述了粘性腻子中的颗粒旋转，但不是在冰碛物中，因为该模型的流体力学假设在冰碛物中被违反。组构发育对剪应变大小的敏感性表明，对于大多数剪应变大小知之甚少的冰碛单元来说，将组构变化归因于其他变量（例如冰碛厚度或含水量）的空间差异本质上是推测性的。将组构特征归因于特定的基底冰碛相是不确定的，因为剪切应变幅度不太可能与冰碛相密切相关。弱的或空间可变的组构，在没有沉积后扰动或单向简单剪切的重大偏差的情况下，表明冰碛物没有被普遍剪切到床变形模型所需的高应变。强流动平行组构是确认模型的必要但不充分的标准。版权所有© 2008约翰威利父子有限公司.
Particle fabrics of basal tills may allow testing of the bed‐deformation model of glacier flow, which requires high bed shear strains (>100). Field studies, however, have not yielded a systematic relationship between shear‐strain magnitude and fabric development. To isolate this relationship four basal tills and viscous putty were sheared in a ring‐shear device to strains as high as 714. Fabric was characterized within a zone of shear deformation using the long‐axis orientations of fine‐gravel and sand particles and the anisotropy of magnetic susceptibility (AMS) of small (∼5·8 cm3) intact samples. Results indicate that till particles rotate toward the plane of shearing with long‐axis orientations that become tightly clustered in the direction of shear (0·78 < S1 < 0·94 for three‐dimensional data). These strong, steady‐state fabrics are attained at shear strains of 7–30, with no evidence of fabric weakening with further strain, regardless of the specific till or particle‐size fraction under consideration. These results do not support the Jeffery model of particle rotation, which correctly describes particle rotation in the viscous putty but not in the tills, owing to fluid‐mechanical assumptions of the model that are violated in till. The sensitivity of fabric development to shear‐strain magnitude indicates that, for most till units where shear‐strain magnitude is poorly known, attributing fabric variations to spatial differences in other variables, such as till thickness or water content, will be inherently speculative. Attributing fabric characteristics to particular basal till facies is uncertain because shear‐strain magnitude is unlikely to be closely correlated to till facies. Weak or spatially variable fabrics, in the absence of post‐depositional disturbance or major deviations from unidirectional simple shear, indicate that till has not been pervasively sheared to the high strains required by the bed‐deformation model. Strong flow‐parallel fabrics are a necessary but insufficient criterion for confirming the model. Copyright © 2008 John Wiley & Sons, Ltd.