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GEOREF RECORD

Clast-fabric development in a shearing granular material; implications for subglacial till and fault gouge

Thomas S. Hooyer and Neal R. Iverson
Clast-fabric development in a shearing granular material; implications for subglacial till and fault gouge
Geological Society of America Bulletin (May 2000) 112 (5): 683-692

Abstract

Elongate clasts in subglacial till and in fault gouge align during shearing, but the relation between clast-fabric strength and cumulative shear strain for such materials is effectively unknown. This relation was explored in experiments with a large ring-shear device in which a till and a viscous putty that contained isolated clasts were sheared to high strains. As expected, rotation of clasts in the putty is closely approximated by the theory of G. B. Jeffery, who derived the orbits of rigid ellipsoids in a slowly shearing fluid. Clast rotation in the till, however, is strikingly different. Rather than orbiting through the shear plane as predicted by Jeffery, most clasts rotate into the shear plane and remain there, resulting in strong fabrics regardless of the aspect ratios and initial orientations of clasts. This divergent behavior is likely due to slip of the till matrix along the surfaces of clasts, which is a natural expectation in a granular material but violates the no-slip condition of Jeffery's model. These results do not support the widespread belief that subglacial till deformation results in weak clast fabrics. Thus, many tills with weak fabrics thought to have been sheared subglacially to high strains, like many basal tills of the Laurentide Ice Sheet, may have been sheared only slightly with little effect on either ice-sheet dynamics or sediment transport. In addition, these results indicate that in simple shear the rotation of clasts in till and in fault gouge is best analyzed with the model of A. March, who treated inclusions as passive markers.


ISSN: 0016-7606
EISSN: 1943-2674
Coden: BUGMAF
Serial Title: Geological Society of America Bulletin
Serial Volume: 112
Serial Issue: 5
Title: Clast-fabric development in a shearing granular material; implications for subglacial till and fault gouge
Affiliation: Iowa State University, Department of Geological and Atmospheric Sciences, Ames, IA, United States
Pages: 683-692
Published: 200005
Text Language: English
Publisher: Geological Society of America (GSA), Boulder, CO, United States
References: 72
Accession Number: 2000-036253
Categories: Structural geology
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 3 tables
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2019, American Geosciences Institute. Reference includes data supplied by the Geological Society of America, Boulder, CO, United States
Update Code: 200012
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