“Ductile to brittle” transition in thermally stable antigorite gouge at mantle pressures

Proctor, B.; Hirth, Greg

doi:10.1002/2015jb012710

Cited by 40 publications

(83 citation statements)

References 56 publications

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“…Frictional sliding at the grain scale in confined antigorite is possibly suggested by the presence of cracking along the cleavage planes in samples deformed under moderate confinement ( R = 0.46) at temperatures and pressures within the antigorite stability field [ Auzende et al ., , Figure 4]. Also possibly suggestive of frictional sliding in antigorite are microfracture networks evident within the sheared hot‐pressed antigorite samples of Proctor and Hirth [] that also were deformed under moderate confinement ( R = 0.34 and 0.4) at temperatures and pressures within the antigorite stability field. We note, however, that many of the faults observed in Proctor and Hirth [] occurred at the sample edge along the solid confining media, and thus, it is unclear to what extent edge effects may have affected these results.…”

Section: Discussionmentioning

confidence: 99%

“…Also possibly suggestive of frictional sliding in antigorite are microfracture networks evident within the sheared hot‐pressed antigorite samples of Proctor and Hirth [] that also were deformed under moderate confinement ( R = 0.34 and 0.4) at temperatures and pressures within the antigorite stability field. We note, however, that many of the faults observed in Proctor and Hirth [] occurred at the sample edge along the solid confining media, and thus, it is unclear to what extent edge effects may have affected these results. However, that frictional sliding occurs in antigorite samples under moderate confinement is consistent with the measured range of stable antigorite friction coefficients which are observed to be as low as 0.4 [ Murrell and Ismail , ; Proctor et al ., ; Raleigh and Paterson , ; Reinen et al ., ].…”

Section: Discussionmentioning

confidence: 99%

“…For example, in antigorite, the high‐temperature polymorph of serpentine common in subduction zones [ Reynard , ] and often cited as a potential source of intermediate‐depth seismicity [ Auzende et al ., ; Chernak and Hirth , ; Garth and Rietbrock , ; Green , ; Hacker et al ., ; Hilairet et al ., ; Jung et al ., ; Murrell and Ismail , ; Peacock , ; Raleigh and Paterson , ; Yamasaki and Seno , ], high‐confinement brittle‐like faults are notably narrow, are often extensively recrystallized along the shear zone (indicated by grains with a strong crystallographic preferred orientation (CPO) along the in‐plane extension of the fault), and again are relatively free from microcracks. Fault orientation, however, is more variable than observed in ice, although high angle (>40°) faults are observed in many samples [ Chernak and Hirth , ; Proctor and Hirth , ]. Failure is characterized by a partial stress drop and macroscale stable slip along the fault [ Chernak and Hirth , ; Proctor and Hirth , ].…”

Section: Character Of High‐confinement Brittle‐like Failurementioning

confidence: 99%

“…Fault orientation, however, is more variable than observed in ice, although high angle (>40°) faults are observed in many samples [ Chernak and Hirth , ; Proctor and Hirth , ]. Failure is characterized by a partial stress drop and macroscale stable slip along the fault [ Chernak and Hirth , ; Proctor and Hirth , ]. Narrow, high‐angle faults lacking a concentration of microcracks near the main fault also have been observed in granite [ Shimada , ; Shimada and Cho , ; Tullis and Yund , ] and in olivine [ Schubnel et al ., ] deformed under high confinement, with terminal failure again characterized by partial stress drops.…”

Section: Character Of High‐confinement Brittle‐like Failurementioning

confidence: 99%

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Strength‐limiting mechanisms in high‐confinement brittle‐like failure: Adiabatic transformational faulting

Renshaw

Schulson

2017

JGR Solid Earth

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Brittle‐like failure under high confinement impacts many geophysical phenomena, including earthquakes at depths beyond a few tens of kilometers. We reanalyze recent observations of high‐confinement brittle‐like failure of nominally thermodynamically stable ice and antigorite and show that the brittle‐like failure only occurs when the strain rate within an incipient adiabatic instability accelerates to the point that the attendant increase in localized temperature approaches a phase transition temperature, initiating transformational faulting. We test a closed‐form model for this process based on independently measureable parameters by comparing model predictions to observations of brittle‐like failure of ice and antigorite under high confinement. Model results do not constrain the mechanism of transformational faulting. However, the similarity of brittle‐like failure characteristics across a variety of geologic materials suggests a similarity of process. We propose that localized heating due to an adiabatic instability initiates dynamic recrystallization near the phase transformational temperature. Transformational superplasticity during recrystallization concentrates plastic strain ahead of the transformed region, causing dynamic recrystallization there and propagating slip in an autocatalytic manner. The newly recrystallized grains rapidly harden as strain accumulates, limiting the stress drop. As the recrystallized grains harden and the stress increases, localized heating reinitiates the adiabatic instability in the still warm region, concentrating plastic strain due to thermal softening and initiating another wave of dynamic recrystallization and partial stress drop. This process, termed adiabatic transformational faulting, could explain how deep earthquakes can propagate beyond the metastable wedge since the propagation of slip is due to dynamic recrystallization rather than to the primary phase transformation.

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Section: Discussionmentioning

confidence: 99%

Section: Discussionmentioning

confidence: 99%

Section: Character Of High‐confinement Brittle‐like Failurementioning

confidence: 99%

Section: Character Of High‐confinement Brittle‐like Failurementioning

confidence: 99%

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Strength‐limiting mechanisms in high‐confinement brittle‐like failure: Adiabatic transformational faulting

Renshaw

Schulson

2017

JGR Solid Earth

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“…Furthermore, laboratory friction experiments indicate that serpentine faults are characterized by a low healing rate and a large slip-weakening distance that promotes conditional stability, consistent with the slip mechanism of slow earthquakes 42 . The dominant flow mechanism of antigorite-rich serpentinite shows a transition from semi-brittle (localized) flow by strain localization to ductile (distributed) flow by intracrystalline deformation with increasing temperature from 300 °C to 500 °C at confining pressures from 1 to 2 GPa 43,44 . Serpentinite exhibits nonlinear plastic flow with an effective viscosity much lower than that of the major mantle-forming minerals 38 allowing it to localize deformation.…”

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confidence: 99%

Slow-slip events in semi-brittle serpentinite fault zones

Goswami¹,

Barbot²

2018

Preprint

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Slow-slip events are earthquake-like events only with much lower slip rates. While peak coseismic velocities can reach tens of meters per second, slow-slip is on the order of 10 −7±2 m/s and may last for days to weeks. Under the rate-and-state model of fault friction, slow-slip is produced only when the asperity size is commensurate with the critical nucleation size, a function of frictional properties. However, it is unlikely that all subduction zones embody the same frictional properties. In addition to friction, plastic flow of antigorite-rich serpentinite may significantly influence the dynamics of fault slip near the mantle wedge corner. Here, we show that the range of frictional parameters that generate slow slip is widened in the presence of a serpentinized layer along the subduction plate interface. We observe increased stability and damping of fast ruptures in a semi-brittle fault zone governed by both brittle and viscoelastic constitutive response. The rate of viscous serpentinite flow, governed by dislocation creep, is enhanced by high ambient temperatures. When effective viscosity is taken to be dynamic, long-term slow slip events spontaneously emerge. Integration of rheology, thermal effects, and other microphysical processes with rate-and-state friction may yield further insight into the phenomenology of slow slip.The dynamics of fault slip at subduction zones can assume a wide range of behaviors. A continuum of slip modes that includes fast earthquakes, slow-slip events, and stable creep determines seismic hazards 1,2 . Along the megathrust, earthquakes, very low frequency earthquakes, and tsunami earthquakes are typically found updip of the continental Mohorovicic (Moho) discontinuity. In contrast, slow-slip events are often found close to or below the continental Moho depth at many subduction zones around the circum-Pacific seismic belt 3 , even though some happen at shallow depth, notably in New Zealand 4-6 and Costa Rica 7,8 . Slow-slip events at subduction zones worldwide often embody similar, characteristic features [9][10][11][12][13] . In particular, we observe small stress drops in the range 10-200 kPa, short recurrence times of a few months to a few years, and sometimes more regular periodicity than for earthquakes 14 . Some characteristics of slow-slip events can be explained within the context of rate-and-state friction 15,16 with conditionally stable behavior [17][18][19][20][21] . Runaway frictional instabilities require a critical size for nucleation that depends on the frictional parameters and the effective confining pressure. For sufficiently small velocity-weakening asperities, only creep spontaneously occurs. Slow-slip events emerge spontaneously when the asperity size is commensurate to critical nucleation size 22 . As slow-slip events are now found close to universally at subduction zones [23][24][25][26][27][28] , it is unrealistic to expect the same combination of parameters across widely distant regions (Fig. 1A). Several authors developed and investigated other friction l...

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Dehydration‐induced instabilities at intermediate depths in subduction zones

2017

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We formulate a model for coupled deformation and dehydration of antigorite, based on a porosity‐dependent yield criterion and including shear‐enhanced compaction. A pore pressure and compaction instability can develop when the net volume change associated with the reaction is negative, i.e., at intermediate depth in subduction zones. The instability criterion is derived in terms of the dependence of the yield criterion on porosity: if that dependence is strong, instabilities are more likely to occur. We also find that the instability is associated with strain localization, over characteristic length scales determined by the hydraulic diffusivity, the elasto‐plastic parameters of the rock, and the reaction rate. Typical lower bounds for the localization length are of the order of 10 to 100 for antigorite dehydration and deformation at 3 GPa. The fluid pressure and deformation instability is expected to induce stress buildup in the surrounding rocks forming the subducted slab, which provides a mechanism for the nucleation and propagation of intermediate‐depth earthquakes.

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“Ductile to brittle” transition in thermally stable antigorite gouge at mantle pressures

Cited by 40 publications

References 56 publications

Strength‐limiting mechanisms in high‐confinement brittle‐like failure: Adiabatic transformational faulting

Strength‐limiting mechanisms in high‐confinement brittle‐like failure: Adiabatic transformational faulting

Slow-slip events in semi-brittle serpentinite fault zones

Dehydration‐induced instabilities at intermediate depths in subduction zones

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