Index
High
temperature friction of diabase
Permeability
of sandstone during extension
Project summaries
High
temperature friction of diabase
It is not atually well known what is the fristion of roks at elevated
temperature, especially if you are interested in something else than granite.
I realized that while working on Venus, which is mostly basaltic, and where
the surface temperature is >400°C. I had trouble just to find a data
point for the value of coefficient of friction, leave alone a study of
the rate- and state-dependence properties which i think are crucial in
tectonic localization (see my projects on localization)
So I got together with Chris Marone (MIT), Greg Hirth (WHOI) and Steve
Karner (MIT, now Texas A&M) and started to test a diabase (coarse basalt)
in a triaxial apparatus and up to 400°C for starters. To our surprised
we encountered anomalously low coefficient of friction (~0.4, while we
expected 0.7). We hope to do more expreiments to understand beter what
happen before going to more Venus-like conditions.
Conference
Abstract: Fall AGU 1999
Permeability
of sandstone during extension
I worked on this project during a 6-month stay in the rock mechanics
lab. of SUNY Stony Brook in 1995, under the direction of Pr. Teng-fong
Wong, and (then-student, now in WHOI) Wenlu Zhu. The project was part of
her thesis. We measured the permeability of a variety of sandstones in
the direction of least compressive stress in conventional triaxial state
of stress. For that doing, we kept the axial stress constant and increased
the confining pressure. We monitored the evolution of permeability as the
rock "collapses" (cataclastic flow) and found some indications of permeability
anisotropy. We coauthored a paper published in the journal "Mechanics of
Materials", and are getting more ready for publication as we "speak".
Zhu, W., Montési, L., and Wong T.-f., Shear-enhanced compaction and permeability reduction: Triaxial extension tests on porous sandstone, Mech. Mat. 25, 199-214., 1997
Conference Abstract: Fall AGU 1995Abstract
Full text (PDF)
Conference
Abstracts
Montési,
L. G. J., Marone, C. J., Hirth, G., and Karner, S. L., Frictional properties
and microstructure of simulated diabase gouge at temperatures up to 400°C,
EOS
Trans. Am. Geophys. Un., 80, Fall Meet. Suppl., F689, 1999
Faulting and sliding on frictional surfaces is the primary mode of deformation in the upper lithosphere. A wide range of deformation conditions are possible in this environment, and these must be investigated to constrain earthquake source processes and the factors that control lithospheric strength. However, the existing database on rock friction is heavily focused on granite and only a handful of experimental studies have been conducted at other than near-surface conditions. We conducted friction experiments on simulated diabase gouge at temperatures up to 400°C to explore the rheological properties of deeper regions of the lithosphere. Friction experiments were conducted in a conventional triaxial apparatus using argon as confining medium. A 1 mm-thick layer of crushed Maryland diabase was placed between rigid forcing blocks made of the same diabase creating a shear zone oriented 30° to the axial direction. Samples were deformed at a confining pressure of 250 MPa and an axial shortening rate of 2 ?m/s. No pore fluid was included. The coefficient of friction decreased from 0.7 at room temperature to 0.4 at 400°C. An analysis of previous data [Stesky et al, Tectonophysics 23, 177-203, 1974; Blandpied et al., JGR 103, 9691-9712, 1998] suggests a similar phenomenon, although not as pronounced as we observed. Microstructural observations of the higher temperature samples indicate that slip localized at the boundary of the gouge layer producing bands with strong lattice preferred orientation. The mechanical properties of samples deformed using diabase blocks roughened with #60 and #100 grit are quite similar. However, when the diabase gouge is sheared between steel forcing blocks, either grooved or roughened with \#60 grit, the coefficient of friction does not decrease with temperature. Velocity steps and slide-hold-slide sequences were conducted during these experiments to characterize the rate- and state-dependence of friction. The system is marginally stable at low temperatures (a-b~0) but a second state variable with longer critical slip distance arises at 400°C, resulting in steady-state velocity-strengthening (a-b1-b2>0). Instabilities can still develop because of the first state variable (a-b1~0) before the second stabilizes the gouge system. Such a transient effect may explain the occurrence of earthquakes at depth approaching brittle-ductile transition near 800°C [Chen and Molnar, JGR 87], in spite of the transition to stead-state velocity strengthening around 400°C.
Zhu, W., Montési, L., and Wong T.-f., Development of permeability during the cataclastic flow of sandstones, EOS Trans. Am. Geophys. Un., 76, Fall Meet. Suppl., 1995
Paper Abstracts
Zhu,
W., Montési, L., and Wong T.-f., Shear-enhanced compaction and
permeability reduction: Triaxial extension tests on porous sandstone, Mech.
Mat. 25, 199-214., 1997
Triaxial extension experiments were conducted to investigate the influence
of radial stress on porosity and permeability (for hydraulic flow along
the axial direction) in three porous sandstones. The effective mean stresses
were sufficiently high that the samples failed by cataclastic flow, with
development of strain hardening and shear-enhanced compaction. Comparison
of the new data with triaxial compression data from a previous study shows
that the critical stress states for the onset of shear-enhanced compaction
sare comparable for the two different loading paths. The initial yield
stress states for each sandstone map out an approximately elliptic envelope
in the stress space. Stress-induced permeability anisotropy was inferred
from synthesis of the triaxial compression and extension data. Before the
onset of shear-enhanced compaction, permeability and porosity reduction
are primarily controlled by the effective mean stress and stress-induced
anisotropy is negligible. With the onset of shear-enhanced comapction and
development of cataclastic flow, coouplingof the deviatoric and hydrostatic
stresses induces considerable permeability and porosity reduction. The
permeability for flow along the direction of the maximum (compressive)
principal stress is greater than that along the minimum principal stress.
Microstructural observations on the shear-enhanced samples show appreciable
increase of grain crushing and pore collapse, which explain the overall
decrease in permeability. The damage from grain crushing is highly anisotropic,
with the stress-induced microcracks preferentially aligned with the maximum
principal stress direction. Because more microcrack conduits are available
to focus the flow in this direction, the permeability is relatively enhanced.
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