HR: 0800h
AN: T41B-0589    [Abstracts]
TI: Onset of Strain Localization in Sheared Granular Materials
AU: * Rathbun, A P
EM: arathbun@geosc.psu.edu
AF: Pennsylvania State University, 522 Deike Building, University Park, PA 16802, United States
AU: Marone, C
EM: cjm@geosc.psu.edu
AF: Pennsylvania State University, 522 Deike Building, University Park, PA 16802, United States
AB: Strain localization plays a key role in determining the frictional stability of brittle shear zones, which in turn influences the rheology and seismic/aseismic behavior of fault zones and deforming glacial till. Recent studies show that seismic, stick-slip motion occurs in dilatant till layers. We report on detailed laboratory experiments to measure the onset of shear localization of Caesar till sampled from the Scioto Lobe of the Laurentide Ice Sheet, collected in central Ohio, USA. Experiments were conducted in a servo-controlled, double-direct shear apparatus with saturated samples at normal stresses of 0.5, 1, and 5 MPa. The nominal frictional contact area is 100 cm² and remains constant during shear. The layer thickness was either 0.5, or 1 cm prior to shear. Till was sheared under constant velocity or constant shear stress (creep) boundary conditions. Constant velocity experiments were conducted with a series of velocity steps from 10 μm/s to 30 μm/s. Constant shear stress experiments were employed to study frictional creep and the localization of strain. Creep was induced after an initial shear strain ranging from 0 to 5 to investigate the role of shear fabric. In creep experiments, shear stress increments began at ~ 2/3rds of the shear strength and continued until tertiary creep occurred. Stress steps were 5% of the shear strength and we determined the resulting strain rate and layer dilation. Microstructures are analyzed in transmitted light microscopy and under SEM. Both velocity stepping and creep experiments show a transition from distributed deformation to localized deformation at low strains. Velocity steps show that Caesar till is a velocity strengthening material, with the critical slip distance decreasing with strain up to 0.7 then remaining constant. Creep experiments show that dilatancy after a stress step decreases exponentially from ~10 μm at shear strain of zero to ~2 μm at a shear strain of 1. Beyond strains of 1 no variation in dilatancy is observed. Decreasing initial layer thickness decreases dilation by the same factor at low strains, but has no effect at strains of 1 or greater. These results imply that shear becomes more localized over a finite displacement in a velocity strengthening material. Beyond the onset of localization, variations in strain and driving velocity do not change the characteristics of strain localization. Localized deformation in till implies shallow deformation, which does not regulate fast glacial slip.
DE: 3902 Creep and deformation
DE: 5112 Microstructure
DE: 5120 Plasticity, diffusion, and creep
DE: 8004 Dynamics and mechanics of faulting (8118)
DE: 8034 Rheology and friction of fault zones (8163)
SC: Tectonophysics [T]
MN: 2007 Fall Meeting