HR: 10:35h
AN: C52A-02 INVITED    [Abstracts]
TI: Testing the bed-deformation hypothesis using magnetic fabrics of tills sheared experimentally and subglacially
AU: * Iverson, N R
EM: niverson@iastate.edu
AF: Iowa State University, Department of Geological and Atmospheric Sciences, Ames, IA 50011, United States
AU: Thomason, J F
EM: thomason@isgs.uiuc.edu
AF: Illinois State Geological Survey, 615 E. Peabody Drive, Champaign, IL 61820, United States
AU: Shumway, J R
EM: shumjr@iastate.edu
AF: Iowa State University, Department of Geological and Atmospheric Sciences, Ames, IA 50011, United States
AU: Hooyer, T S
EM: tshooyer@wisc.edu
AF: Wisconsin Geological Survey, 3817 Mineral Point Rd., Madison, WI 53705, United States
AU: Lagroix, F
EM: lagroix@ipgp.jussieu.fr
AF: Institut de Physique du Globe de Paris, 4, place Jussieu, Paris, 75252, France
AU: Graesch, M
EM: mgraesch@mchsi.com
AF: Iowa State University, Department of Geological and Atmospheric Sciences, Ames, IA 50011, United States
AB: The bed-deformation hypothesis asserts that glaciers can move and transport sediments primarily by shearing their soft beds. Numerous models of both glacier flow and development of subglacial landforms (e.g., till plains, drumlins, flutes, megalineations, tunnel valleys, Rogen moraines, and boulder pavements) incorporate this process. It requires that basal sediments be sheared to very high strains (> 100) and over depth ranges sufficient to account for most basal motion and sediment transport. Thus, the key issue in testing this hypothesis is not whether basal sediment has been sheared but how much and over what thickness of the bed. We have studied basal tills of southern lobes of the Laurentide ice sheet in ring-shear experiments to determine effects of strain magnitude and direction on anisotropy of magnetic susceptibility (AMS). These experiments indicate that fabrics defined by directions of the maximum principal susceptibility (k1) strengthen progressively with shear strain but at exponentially decreasing rates, becoming steady at strains of 10-20. Steady k1 fabrics are strong (S1 eigenvalue = 0.83-0.94), with k1 orientations clustered in the direction of shear and plunging gently "up-glacier." Ancillary experiments indicate that anisotropy results from alignment of silt-sized and smaller magnetite grains present in most tills. AMS fabrics integrate effects of many such particles and involve little subjectivity, so AMS fabrics are more accurate strain indicators than traditional particle fabrics. AMS fabrics measured along profiles through till units of the Superior (Douglas till, Wisconsin) and Lake Michigan lobes (Batestown till, Illinois) indicate that deformation to strains greater than ~ 10 can be ruled out in many parts of the bed. Elsewhere, fabrics are sufficiently strong to indicate that such strains were equaled or exceeded. However, spatial variability of fabric strength and direction indicates that only minor fractions of the thicknesses of these till units sheared at one time and that deformation was heterogeneous over the bed area. These field data are the first to benefit from laboratory fabric-strain calibrations and provide little support for deep, pervasive deformation of the bed to high strains.
DE: 0720 Glaciers
DE: 0738 Ice (1863)
DE: 0776 Glaciology (1621, 1827, 1863)
DE: 1824 Geomorphology: general (1625)
DE: 1862 Sediment transport (4558)
SC: Cryosphere [C]
MN: 2007 Fall Meeting