HR: 1340h
AN: T23A-0556    [Abstracts]
TI: Some Mechanical Implications of the Development and Evolution of Y shears in Simulated Granite Gouge
AU: * Hadizadeh, J
EM: hadizadeh@louisville.edu
AF: University of Louisville, Department of Geography & Geosciences, Louisville, KY 40292 United States
AU: Goldsby, D L
AF: Brown University, Department of Geological Sciences, Providence, RI 02912 United States
AU: Konkachbaev, A I
AF: University of Louisville, Department of Computer Engineering & Computer Science, Louisville, KY 40292 United States
AU: Yazdanpanah, M
AF: University of Louisville, Department of Electrical & Computer Engineering, Louisville, KY 40292 United States
AU: Tullis, T E
AF: Brown University, Department of Geological Sciences, Providence, RI 02912 United States
AU: Beeler, N
AF: US Geological Survey, 345 Middlefield Rd., Menlo Park, CA 94025-3591 United States
AB: Frictional sliding experiments were performed in a rotary shear machine at 25 MPa normal stress on 2-mm thick layers of simulated Westerly granite gouge(particle size$\leq$85$\mu$m)with the objective of studying microstructural aspects of displacement dependence of the frictional behavior of the gouge. Sliding velocity was regularly stepped between 1 and 10$\mu$m/s in all the experiments. Successful trials were terminated for examination of the samples after: (1) 60mm of sliding involving a persistent reduction in friction, (2) 144mm of sliding involving that reduction followed by a persistent increase in friction, and (3) 386mm of sliding involving several varied amplitude fluctuations in friction. The sheared gouge sections were imaged at 0.25K-64K times magnification in SEM/BSE mode. In experiment (1) a highly comminuted zone was separated from marginal gouge by a sharp particle size gradient. A single 5-10$\mu$m-wide linear Y slip surface appeared to have nucleated within the highly comminuted zone. The gouge also included a field of large survivor particles interlaced by numerous R slip surfaces. In contrast, a complex system of microstructures including a full set of Riedel shear surfaces and pervasive comminution had occurred in experiment (3). The most notable microstructure, absent in both (1) and (2), was an intermittent central zone of faulted, thrusted, and rotated sliver-shaped particles. The slivers appeared to be riding in a less competent surrounding gouge. Detailed microscopy revealed that in every case the slivers (5-25$\mu$m thick) have an internal layering defined by a mass of particles graded in size from 20nm on one side to about 300nm on the other side. At the highest magnification the sliver material had an image-estimated porosity of 10-15$%$, and consisted of a mixture of rounded and sub-rounded quartz and feldspar particles. The correlation of microstructural and mechanical data in experiments (1) and (2) is generally consistent with previously proposed models (Beeler et al. 1996) that associate Y slip surfaces with significant shear localization and reduction in friction followed by hardening of the localized layers and consequent delocalization. The data suggests that fluctuations in friction observed in experiment (3)may be related to occurrences of hardening and subsequent brecciation of the gouge during slip on multiple Y shear surfaces as well as to the discontinuous nature of the sliver breccias along the gouge zone. The internal layering of the brecciated slivers and their well-developed particle size grading indicates that once created Y slip surfaces may continue to localize shear strain until particles that line the slip surface are reduced to a critical average size or perhaps achieve a critical packing density. The mechanism for hardening of gouge and disruption into the observed slivers, although appears to be size-related, is not well understood. If subsequent studies show that it does involve an increase in packing density due to wider PSD, then the hardening could result from an increase in the contact area per volume or the contact area along potential shear surfaces.
DE: 8010 Fractures and faults
DE: 8100 TECTONOPHYSICS
DE: 5100 PHYSICAL PROPERTIES OF ROCKS
DE: 7209 Earthquake dynamics and mechanics
DE: 7215 Earthquake parameters
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting