Time-Dependent Strengthening Rates in Simulated Fault Gouge and Implications for Fault Zone Processes.
Abstract
The frictional behavior of tectonic faults is ultimately a product of asperity contact processes, fault zone fabric, and granular processes in gouge zones. Contact area evolution likely exhibits significant control on overall fault stability by influencing 1) healing during the interseismic period, 2) rate-dependent friction, and 3) restrengthening at short time scales of dynamic rupture. We report on a series of slide-hold-slide tests conducted on simulated fault gouges, to explore the role of gouge mineralogy on the evolution of contact area and associated fault healing at intermediate time scales. We conducted friction experiments in a double-direct shear configuration, under room conditions using four simulated fault gouges: talc (particles < 125μm), kaolinite (1-4μm), illite shale (< 106μm), and feldspar (andesine) (<106μm). Layers of simulated gouge were sheared at load point velocities of 1, 10, and 100 μm/s under constant normal stress (20 MPa) between rough rigid forcing blocks to a shear strain of ~25, and then subjected to slide-hold-slide tests with holds ranging from 1 to 1000 seconds, with 0.5 mm of slip between each hold. Talc shows little to no healing (Δμ < 0.001) over all hold times and all loading rates, whereas kaolinite (Δμ = 0.0025 - 0.025), andesine (Δμ = 0.002 - 0.021) and illite shale (Δμµ = 0 - 0.011) all exhibit healing. Illite shale shows increasing healing in tests that were run at increasing load point velocity during shear before and after the holds. All gouges exhibit log-linear healing rates over all velocities and hold times, with the exception of feldspar, which deviates from this relationship at hold times > 100s. Layer compaction during holds increases with hold time and with the load point velocity. Compaction ranges from < 1 μm at 1 μm/s to 3.5 μm at 100 μm/s. Talc compacts the least and feldspar compacts the most. High compaction at high velocities is attributable to gouge dilation during shear. Low compaction is associated with the weak mineral talc, suggesting that most of the porosity reduction in weak minerals occurs under initial loading and that large porosity changes are unlikely. The low strength of talc likely facilitates saturation of contact area even at low effective stress, as evidenced by extremely low rates of healing and compaction. Because velocity-weakening frictional behavior (a necessary condition for unstable slip) is predicated on the disruption of time-dependent strengthening processes, the presence of talc or similarly weak minerals would promote stable sliding in a fault zone. Ongoing work will consider water saturated conditions and the possible effects of compaction and porosity reduction on permeability and pore pressure evolution.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2008
- Bibcode:
- 2008AGUFM.T13A1905C
- Keywords:
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- 5475 Tectonics (8149);
- 8034 Rheology and friction of fault zones (8163);
- 8118 Dynamics and mechanics of faulting (8004)