TY - JOUR
T1 - Microfabric and microshear evolution in deformed till
AU - Thomason, Jason F.
AU - Iverson, Neal R.
N1 - Funding Information:
We thank the US National Science Foundation (OPP136006) for supporting this research, A. Hansel and T. Hooyer for directing us to till exposures, and A. Carlson and an anonymous reviewer for their reviews of the manuscript.
PY - 2006/5
Y1 - 2006/5
N2 - The bed-deformation model of glacier movement requires that ice masses shear their beds to high strains. For homogeneous basal tills that can result from such deformation, however, there are no sedimentological criteria for estimating strain magnitude. Thus, a large ring-shear device was used to shear two basal tills, and their particle fabrics and microshears were studied at various strains up to 108. Sand-particle fabrics in a longitudinal flow plane became steady at shear strains of 7-39; steady fabrics were moderately strong, with S1 eigenvalues of 0.71-0.74. These values are smaller than those from experiments with pebble-sized particles, perhaps reflecting the greater probability of sand particles colliding with larger grains. Fabrics parallel to flow strengthened progressively, indicating that particles did not reorient with their long axes transverse to flow. R1 and R2 microshears developed and persisted to the highest strains attained. R1 shears became more pervasive with strain and oriented closer to the shear plane. Movement along both sets of shears provides an explanation for the plunge of the steady-state sand fabric, approximately 10° "upglacier." These experiments provide a basis for estimating till-deformation magnitude, but results will require cautious application to the geologic record where deformation processes may be ambiguous.
AB - The bed-deformation model of glacier movement requires that ice masses shear their beds to high strains. For homogeneous basal tills that can result from such deformation, however, there are no sedimentological criteria for estimating strain magnitude. Thus, a large ring-shear device was used to shear two basal tills, and their particle fabrics and microshears were studied at various strains up to 108. Sand-particle fabrics in a longitudinal flow plane became steady at shear strains of 7-39; steady fabrics were moderately strong, with S1 eigenvalues of 0.71-0.74. These values are smaller than those from experiments with pebble-sized particles, perhaps reflecting the greater probability of sand particles colliding with larger grains. Fabrics parallel to flow strengthened progressively, indicating that particles did not reorient with their long axes transverse to flow. R1 and R2 microshears developed and persisted to the highest strains attained. R1 shears became more pervasive with strain and oriented closer to the shear plane. Movement along both sets of shears provides an explanation for the plunge of the steady-state sand fabric, approximately 10° "upglacier." These experiments provide a basis for estimating till-deformation magnitude, but results will require cautious application to the geologic record where deformation processes may be ambiguous.
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U2 - 10.1016/j.quascirev.2005.09.006
DO - 10.1016/j.quascirev.2005.09.006
M3 - Article
AN - SCOPUS:33646359439
SN - 0277-3791
VL - 25
SP - 1027
EP - 1038
JO - Quaternary Science Reviews
JF - Quaternary Science Reviews
IS - 9-10
ER -