fault zone fabric and fault weakness cristiano collettini, andre niemeijer, cecilia viti & chris...
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FAULT ZONE FABRIC AND FAULT WEAKNESS
Cristiano Collettini, Andre Niemeijer, Cecilia Viti & Chris MaroneNature 2009
Paradoxe: Medium is “strong”
Laboratory measurements on a wide variety of rock types show that fault friction μ is in the range 0.6–0.8
Several lines of evidence suggest μ = 0.6 is applicable to many faults : failure mainly occurs on
optimally oriented faults No big EQ on mis-oriented
fault “normal” friction Nearly hydrostatic pore
pressure
Byerlee, PAG 1978
But evidences for weak fault
San Andreas fault, CA
Zuccale fault,Isle of Elba
Longitudinal valley fault,Taiwan
Explanation for fault weakness Dynamic weakening mechanisms
Presence of weak minerals
Laboratory measurements of fault friction
Carpenter, Marone and Saffer, GRL 2009
Byerlee, PAG 1978
Serpentinite
Talc
Montmorillonite = smectite
illite
serpentiniteChlorite
Explanation for fault weakness Dynamic weakening mechanisms
Presence of weak minerals High fluid pressure within the fault core Dynamic processes such as :
• Normal stress reduction• Acoustic fluidization ?• Weakening related to high velocity (flash heating, increase of pore
pressure)
Explanation for fault weakness Dynamic weakening mechanisms
Presence of weak minerals but usually not in sufficient abundance High fluid pressure within the fault core Dynamic processes such as :
• Normal stress reduction• Acoustic fluidization ?• Weakening related to high velocity (flash heating, increase of pore pressure)
Explanation for fault weakness Dynamic weakening mechanisms
Presence of weak minerals but usually not in sufficient abundance High fluid pressure within the fault core but required specialized
conditions Dynamic processes such as :
• Normal stress reduction• Acoustic fluidization ?• Weakening related to high velocity (flash heating, increase of pore pressure)
Explanation for fault weakness Dynamic weakening mechanisms
Presence of weak minerals but usually not in sufficient abundance High fluid pressure within the fault core but required specialized
conditions Dynamic processes such as :
• Normal stress reduction• Acoustic fluidization ?• Weakening related to high velocity (flash heating, increase of pore pressure)
but creep and aseismic slip also occur on weak fault
how is frictional slip initiated on mis-oriented fault?
Friction is low in the long term!
Explanation for fault weakness Dynamic weakening mechanisms
Presence of weak minerals but usually not in sufficient abundance High fluid pressure within the fault core but required specialized
conditions Dynamic processes such as :
• Normal stress reduction• Acoustic fluidization ?• Weakening related to high velocity (flash heating, increase of pore pressure)
but creep and aseismic slip also occur on weak fault
how is frictional slip initiated on mis-oriented fault?
Need of other weakening mechanisms Suggestion: brittle, frictional weakening mechanism based
on common fault zone fabrics
Zuccale fault, Isle of Elba
low-angle normal fault (15°)
total shear displacement of 6–8 km
stress field with a vertical maximum compression
The fault is weak
Collettini et al., Nature 2009
Zuccale fault, Isle of Elba
hangingwall and footwall = rocks deformed by brittle cataclastic processes
fault core =highly foliated phyllosilicate-rich horizons, several meters thick, illustrating deformation occurring at less than 8 km depth
Collettini et al., Nature 2009
foliation made of tremolite and phyllosilicate (smectite, talc and minor chlorite)
Experiments
Cohesive foliated fault rocks (L2 & L3) wafers 0.8–1.2 cm thick 5 cm x 5 cm in area oriented with foliation parallel to shear direction
Powders Crushing and sieving intact pieces of fault rock samples used in the solid experiments.
Both kind of samples have been: Sheared in the double direct shear configuration 25 °C Normal stresses from 10 to 150MPa shear slip velocities of 1 to 300 μm/s
Measure of the steady state, residual, frictional shear stress at each normal stress.
Collettini et al., Nature 2009
Results
plots along a line consistent with a brittle failureenvelope
μ = 0.55
μ = 0.31
Measure of the steady state, residual, frictional shear stress at each normal stress.
Collettini et al., Nature 2009
Results
plots along a line consistent with a brittle failureenvelope
powders show friction of about 0.6
foliated rocks have significantly lower values : μ = 0.45 – 0.20
μ = 0.55
μ = 0.31
Collettini et al., Nature 2009
Collettini et al., Nature 2009
Cohesive foliated fault rocks
Powders
Comparison between sliding surface
Sliding surfaces are located along the pre-existing foliation made of tremolite and phyllosilicat
Deformation in powders occurs along zones characterized by grain-size reduction : abundant calcite clasts in a groundmass consisting of tremolite and phyllosilicates
Collettini et al., Nature 2009
Collettini et al., Nature 2009
Conclusion
Although the intact fault rock samples and their powders have identical mineralogical compositions, the foliated samples are much weaker than their powdered analogues.
The frictional strength of the solid wafers is comparable to that of pure talc at similar sliding conditions even with the presence of 65–80% of strong calcite and tremolite minerals.
weakness of the foliated fault rocks is due to the reactivation of pre-existing fine-grained and phyllosilicate-rich surfaces that are absent in the powders
the average value of μ = 0.25 is sufficient to explain: Absence of measurable heat flow along weak faults Frictional reactivation of faults oriented up to 75° from the maximum
compressive stress (SFA) or the low-angle normal faults (Apennines).
Could it explain EQ frictional instability?
Geological investigations have documented the mutual superposition between slip on phyllosilicates and brittle (hydrofractures) or earthquake-related structures (pseudotachylytes)
Continuous strands of phyllosilicates usually bound lenses of stronger lithologies: these lenses could represent sites for stress concentrations and earthquake nucleation near patches of fault creep
Smectite sheared exhibits low friction (μ =0.15 - 0.32) and a transition from velocityweakening at low normal stress to velocitystrengthening at higher normal stress (>40 MPa)
Some crustal faults can behave as weak structures over long timescales (millions of years) and be intermittently seismogenic on shorter timescales.
Talc
Talc is a metamorphic mineral resulting from the metamorphism of magnesian minerals such as serpentine, pyroxene, amphibole, olivine, in the presence of carbon dioxide and water. This is known as talc carbonation or steatization .
Talc is primarily formed via hydration and carbonation of serpentine, via the following reaction:
serpentine + carbon dioxide → talc + magnesite + waterMg3Si2O5(OH)4 + 3CO2 → Mg3Si4O10(OH)2 + 3 MgCO3 + 3 H2O
This is typically associated with high-pressure, low-temperature minerals such as phengite, garnet, glaucophane within the lower blueschist facies.
Fusion temperature : 900 to 1000 °C
Orientation of the stress field, SFA
Provost & Houston, JGR 2001
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