FAULT ZONE FABRIC AND FAULT WEAKNESS Cristiano Collettini, Andre Niemeijer, Cecilia Viti & Chris Marone Nature 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, CAZuccale 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 serpentinite Chlorite

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 μ m/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 failure envelope 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 failure envelope  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

 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

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 = ) 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.serpentinepyroxeneamphiboleolivine  Talc is primarily formed via hydration and carbonation of serpentine, via the following reaction: serpentine + carbon dioxide → talc + magnesite + waterserpentinecarbon dioxidemagnesitewater Mg 3 Si 2 O 5 (OH) 4 + 3CO 2 → Mg 3 Si 4 O 10 (OH) MgCO H 2 O  This is typically associated with high-pressure, low-temperature minerals such as phengite, garnet, glaucophane within the lower blueschist facies.phengitegarnetglaucophaneblueschist facies  Fusion temperature : 900 to 1000 °C

Orientation of the stress field, SFA Provost & Houston, JGR 2001