HR: 0800h
AN: NG31A-0860    [Abstracts]
TI: A Microstructural Study of Small-Displacement Faults in Aztec Sandstone
AU: * Hagg, K
AF: University of Louisville, Department of Geography & Geosciences, Louisville, KY 40292 United States
AU: Yazdanpanah, M
AF: University of Louisville, Department of Electrical & Computer Engineering, Louisville, KY 40292 United States
AU: Harfenist, S
AF: University of Louisville, Department of Electrical & Computer Engineering, Louisville, KY 40292 United States
AU: Otto, M
AF: University of Louisville, Department of Geography & Geosciences, Louisville, KY 40292 United States
AU: Hadizadeh, J
EM: hadizadeh@louisville.edu
AF: University of Louisville, Department of Geography & Geosciences, Louisville, KY 40292 United States
AB: Previous studies indicate that brittle shear localization microstructures develop during the initial increments of shear displacement. We studied gouge samples from faults in Aztec quartz sandstone (Valley of Fire State park, Nevada) with shear displacements in the order of 10-1500mm. Undeformed Aztec sandstone near the sampling site had well-rounded grains with average size of 550$\mu$m and typical porosity of 23$%$. The total gouge zone thickness, measured parallel with slickenside lineation/perpendicular to shear plane ranged from 1-47mm. Average shear strain $\gamma$ in the samples, calculated as the ratio of shear displacement (true displacement) to average zone thickness measured over 1m length of fault, ranged from 14 to 100. Sections of the gouge were imaged under SEM in backscattered mode at 250, 500, and 1000 times magnifications. Individual particle outlines on images from each sample were manually traced on digital overlays, which were reproduced as binary images and used in microstructural measurements. Particle shapes were measured by a new shape descriptor algorithm POL (particle outline linearity), which returned POL values in N straight sides per particle (Ns/P). In the undeformed Aztec Ns/P$\sim$0. We combined data from all magnifications to reduce measurement errors due to large particle size variability of the gouge. The gouge microstructures became more complex with increasing shear strain (rather than shear displacement). Y-shears were absent at $\gamma$14, but began to appear in the gouge at $\gamma$26. However, incipient damage zones at the lowest shear strain contained clear sets of R shears. Multiple Y shear surfaces and damage zones containing R-shears are observed at $\gamma$31. Microstructures suggestive of multiple R and Y shear surfaces were observed at $\gamma$97 and $\gamma$100. In all cases, Y shear surfaces appeared as surrounded by zones of high comminution that varied in thickness as much as an order of magnitude along the sample length. Median size of particles in the core region determined on PSD curves was reduced from 9$\mu$m ($\gamma$14) to 3 $\mu$m ($\gamma$97). In contrast, particle size reduction in the damage zone was minimal compared to the average undeformed particle size. POL values for the core region dropped from 1.5s/P at $\gamma$14 to 0.6s/P at $\gamma$97. The fractal dimension values (slope of the log-log cumulative PSD) for the core region changed from 2 to 2.3 with increasing $\gamma$. The particle shape profiles across the gouge zone reach maximum in the damage zone region then approach the undeformed Ns/P values for the core region. The form of the profiles remains more or less unchanged for all samples in terms of the maximum value for the danage zone. The particle shape and size evolution with increased $\gamma$ indicates that most comminution occurs in the core region, leaving the damage zone mostly inactive at $\gamma$$>$20. Our results suggest that the damage zone/core structure in Aztec faults developed at $\gamma$30-$\gamma$50. Reported results for experimental faults with quartz gouge sheared at 25-75MPa pressures suggest that the Aztec faults were deformed under lower normal stresses.
DE: 7260 Theory and modeling
DE: 7200 SEISMOLOGY
DE: 3210 Modeling
DE: 3220 Nonlinear dynamics
SC: Nonlinear Geophysics [NG]
MN: 2004 AGU Fall Meeting