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| Titel |
Frictional properties of Zuccale Fault rocks from room temperature to in-situ conditions: results from high strain rotary shear experiments |
| VerfasserIn |
A. Niemeijer, C. Collettini, S. A. F. Smith, C. J. Spiers |
| Konferenz |
EGU General Assembly 2012
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 14 (2012) |
| Datensatznummer |
250065784
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| Zusammenfassung |
| The Zuccale fault is a regionally-important, low-angle normal fault, exposed on the Isle of
Elba in Central Italy, that accommodated a total shear displacement of 6-8 km.The
fault zone structure and fault rocks formed at less than 8 km crustal depth. The
present-day fault structure is the final product of several deformation processes
superposed during the fault history. Here, we focus on a series of highly foliated and
phyllosilicate-rich fault rocks that represent the basal horizon of the detachment. Previous
experimental work on foliated, intact samples, sheared in their in-situ microstructural
(foliated) condition, demonstrated a markedly lower friction coefficient compared to
homogeneously mixed powdered samples of the same material. In this study, we
report results from a series of rotary shear experiments performed on 1 mm thick
powdered gouges made from several fault rock types obtained from the Zuccale Fault.
The tests were done under conditions ranging from room temperature to in-situ
conditions (i.e. at temperatures up to 300 ºC, applied normal stresses up to 200
MPa and fluid-saturated.) The ratio of fluid pressure to effective normal stress was
held constant at either λ=0.4 or λ=0.8 to simulate an over-pressurized fault. The
samples were sheared at a constant sliding velocity of 10 μm/s for at least 5 mm, after
which a velocity-stepping sequence from 1 to 300 μm/s was started to determine
the velocity dependence of friction. This can be represented by the rate-and-state
parameter (a-b), which was determined by an inversion of the data to the rate-and-state
equations.
Friction of the various fault rocks is between 0.3 and 0.7, similar to values obtained in a
previous study, and decreases with increasing phyllosilicate content. Friction decreases
mildly with temperature whereas normal stress and fluid pressure do not affect friction
values systematically. All samples exhibited velocity-strengthening, inherently stable
behavior under room temperature conditions and (a-b) increases with increasing
sliding velocity. In contrast, velocity-weakening, accompanied by stick-slips, is
observed for the strongest samples at 300 ºC and sliding velocities below 10 μm/s. An
increase in fluid pressure under these conditions leads to a further reduction in
(a-b) for all samples, so that they exhibit unstable, stick-slip behavior at low sliding
velocity.
The results suggest that phyllosilicate-bearing fault rocks can deform by stable, aseismic
creep at low resolved shear stress and low shear rate conditions. An increase in fluid pressure
or loading of stronger portions could lead to a runaway instability. However, this instability
could be limited in size and velocity due to the observed strengthening at higher sliding
velocities. This has important implications for potential rupture dimensions in this
geometrically complex fault zone. |
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