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  1. Abstract

    Novel fluid medium pressure cells were used to deform antigorite under constant stress creep conditions at low temperature, low strain rate (10−9 − 10−41/s), and high pressure (1 GPa) in a Griggs‐type apparatus. Antigorite cores were deformed at constant temperatures between 75°C and 550°C, by applying 8–12 stress‐strain steps per temperature. The microstructures of deformed samples share features documented in previous work (e.g., shear microcracks), and highlight the importance of basal shear and kinks to antigorite plasticity. Rheological data were fit with a low temperature plasticity law, consistent with a deformation mechanism involving large lattice resistance. When applied at geologic stresses and strain rates, the extrapolated viscosity agrees well with predictions based on subduction zone thermal models.

     
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  2. Abstract

    Deformation experiments were conducted to investigate the influence of apparatus stiffness on frictional stability during dehydration of antigorite. These experiments also provided a novel way to constrain relationships between fault slip rate and reaction weakening. Experiments were conducted at a confining pressure of 1 GPa using a Griggs apparatus with a modified loading column. During experiments, the temperature was ramped from 400°C to 700°C over 600 seconds, while deformation was imposed with varying load‐point displacement rates. No stick‐slip behavior was observed during the experiments, despite an order of magnitude lower apparatus stiffness, suggesting there are inherent differences in the frictional stability between low pressure (where stick‐slip is observed during dehydration) and high pressure experiments. Fault slip at high pressure may be stabilized by higher dehydration temperatures and higher shear stresses prior to the onset of dehydration reactions. In comparison to reference experiments conducted with an unmodified stiffness, the weakening rate during dehydration increases by a factor of two. In addition, the time (and therefore imposed temperature) at which the maximum weakening rate is observed decreases linearly with increasing maximum weakening rate. Microstructures in the deformed samples illustrate much greater reaction extents (locally >50%) relative to the reference experiments (<1%), with decreasing reaction extent away from the primary slip surfaces. These observations indicate that the dehydration reaction is enhanced by additional shear dissipation resulting from a tenfold increase in displacement per increment of weakening. The shear‐enhanced reaction mechanism could potentially generate runaway slip in subduction zone settings, leading to intermediate‐depth earthquakes.

     
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  3. Abstract

    Large earthquakes and slow slip events typically nucleate along plate boundaries near the depth limit of the seismogenic zone, which is also recognized as the brittle‐plastic transition zone (BPT). HighVp/Vsratios are commonly observed at the BPT in subduction zones, indicating the presence of aqueous fluid in pore spaces. We conducted experiments to investigate the rheology of quartz with different fluid fractions at deformation conditions that cross the BPT. The strengths of quartz aggregates with fluid‐filled porosities of 5–25 vol% are significantly lower than predicted by wet quartzite flow laws, and decrease with increasing fluid fraction. Recovered samples deformed in the ductile regime exhibit S‐C´ mylonitic structures characterized by elongate grains, shear localization and fluid segregation. Variations in strength are explained by a combination of a constitutive law for dislocation creep that includes the geometric effects of fluid fraction, a friction law that includes the effect of fluid fraction through its role on the real area of contact, and an empirical function to describe the smooth brittle‐plastic transition. Our results indicate that the presence of fluid‐filled porosity promotes significant weakening in shear zones, and that variations in fluid fraction (together with temperature) can explain transitions in the spectrum of slip behaviors observed along plate boundaries.

     
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