Browsing by Author "French, Melodie E."
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Item Fracture-induced pore fluid pressure weakening and dehydration of serpentinite(Elsevier, 2019) French, Melodie E.; Hirth, Greg; Okazaki, KeishiWe investigate the strength, deformation processes, and pore fluid weakening during localized shear of antigorite serpentine. Recent work has shown that some phyllosilicates, including antigorite, undergo a reverse transition from ductile to localized deformation at the pressure-temperature conditions of deep slow slip and tremor in subduction zones. Here, we investigate the processes that lead to and occur during localized deformation. Because high pore fluid pressure is hypothesized to control the location and style of fault slip at these conditions, we investigate the role of pore fluids on these deformation processes. We present the results of undrained general shear experiments on antigorite-rich serpentinite deformed to varying strains at 500°C, 1 GPa pressure, and with 0 to 2 wt.% added pore water. At all fluid conditions, the serpentinite exhibits strain hardening during distributed deformation and subsequent strain weakening associated with the formation of a prominent shear fracture zone. The magnitude of strain weakening correlates with increasing pore water content. We evaluate two end-member scenarios for how the effective stress influences strength during localized deformation and find that either an increase in fluid pressure or increase in the parameter α in the effective stress law can explain the weakening. At all fluid conditions, we also find evidence for localized dehydration of antigorite within the fracture zones, at pressures and temperatures where antigorite is considered stable. Although the extent of the reaction did not measurably affect fault strength in our experiments, at the time scales of in-situ deformation in the Earth, reaction weakening and associated pore fluid pressurization may occur.Item Localized Slip and Associated Fluidized Structures Record Seismic Slip in Clay‐Rich Fault Gouge(Wiley, 2018) French, Melodie E.; Chester, Judith S.Fault rocks can weaken dramatically with increasing slip rate, which results in localization of slip and earthquakes. Exhumed fault zones and fault rocks deformed at seismic rates in the laboratory both show that deformation can become extremely localized to zones less than or equal to millimeters thick. However, localization can occur during aseismic slip, so evidence of localization cannot necessarily be interpreted as having occurred coseismically. Dynamic weakening that occurs during earthquakes is the result of processes that are unique to seismic slip rates, and previous results from carbonates show that these processes produce unique microstructures. We evaluate whether coseismic deformation at low normal stress produces unique structures within the localized slip zones and adjacent gouge that develop in clay‐rich gouge from the Central Deforming Zone of the San Andreas fault. We measured the thickness and orientations of localized slip zones and their internal lamina, particle orientations, and particle size distributions of gouge sheared from 0.35 to 1.3 m/s velocity, up to 25‐m displacement, and 1‐MPa normal stress, under water‐wet and room‐dry conditions. We find that the thicknesses of localized slip zones and their internal laminae are consistent with numerical formulations for thermal pressurization in wet gouge and both thermal decomposition and cataclastic deformation in dry gouge. Localized zones form coincident with a zone of fluidized gouge that accommodates at most 10% of the shear strain. We conclude that the combined occurrence of foliated localized shear zones with a zone of fluidized gouge may provide a record of seismicity in clay‐rich gouges.Item Measurements of Wave-Induced Attenuation in Saturated Metapelite and the Band-Limitation of Low-Frequency Earthquakes(Wiley, 2023) Fliedner, Céline; French, Melodie E.The most common explanation for the depletion of high frequency waves that defines low-frequency earthquakes (LFEs) and very low-frequency earthquakes (VLFEs) is that fault rupture and slip are slower than typical earthquakes. However, it is difficult to rule out the possibility that the high frequency waves are produced during slip, but attenuated near the LFE source. One reason this hypothesis has been poorly tested is that there are no measurements of attenuation on the relevant rocks. We present the results of forced oscillation experiments that measure the frequency-dependent attenuation of a chlorite-rich metapelitic schist, a lithology found along the subduction plate boundary where LFEs and VLFEs have been documented. Experiments were run on dry and water-saturated schist at effective pressures of 2–10 MPa and at frequencies of 2 × 10−5–30 Hz. We find that pore fluids and low effective pressure result in the attenuation of high frequencies. The frequency-dependent attenuation is consistent with the concomitant operation of two wave-induced fluid flow mechanisms, squirt flow, and patchy saturation. When the effects of these mechanisms are extrapolated to geologic conditions using rock physics models, our results predict that attenuation is capable of completely diminishing the frequencies depleted in LFEs and VLFEs. Therefore, LFEs and VLFEs may not necessarily record slow fault slip, but possibly the presence of high fluid pressure.Item Slab dehydration in warm subduction zones at depths of episodic slip and tremor(Elsevier, 2020) Condit, Cailey B.; Guevara, Victor E.; Delph, Jonathan R.; French, Melodie E.Non-volcanic tremor (NVT) and episodic slow slip events (SSEs) have been observed below the seismogenic zone of relatively warm subduction zones for the past 20 years. Geophysical and geologic observations show that this portion of the subduction interface is fluid-rich, and many models for these slip behaviors necessitate high pore fluid pressures. However, whether these fluids are sourced from local dehydration reactions in particular lithologies, or require up-dip transport from greater depths is not known. We present thermodynamic models of the petrologic evolution of four lithologies typical of the plate interface (average MORB, seafloor altered MORB, hydrated depleted MORB mantle, and metapelite) along predicted plate boundary pressure–temperature (P-T) paths at several warm subduction segments where NVT and SSEs are observed at depths between 25-65 km. The models suggest that 1-2 wt% H2O is released at the depths of NVT/SSEs in Jalisco-Colima, Guerrero, Cascadia, and Shikoku due to punctuated dehydration reactions within MORB, primarily through chlorite and/or lawsonite breakdown. These reactions produce sufficient in-situ fluid across a narrow P-T range to cause high pore fluid pressures at NVT/SSE depths. Dehydration of hydrated peridotite is minimal at these depths for most margins, and metapelite releases H2O (<1.5 wt%) gradually over a wide depth range compared to MORB. We posit that punctuated dehydration of oceanic crust provides the dominant source of fluids at the base of the seismogenic zone in these warm subduction zones, and up-dip migration of fluids from deeper in the subduction zone is not required.Item Slip partitioning along an idealized subduction plate boundary at deep slow slip conditions(Elsevier, 2019) French, Melodie E.; Condit, Cailey B.Below the base of many subduction seismogenic zones, the plate interface periodically slips at rates 1 to 2 orders of magnitude faster than tectonic plate velocities. A number of competing hypotheses exist to explain the mechanisms for these slow slip events (SSEs), but they remain incompletely tested because we do not know how deformation is partitioned across the lithologically complex plate boundary interface. We use the deepest exposure of the Arosa zone, a ∼520 m-thick exhumed subduction interface, as a case study to evaluate the partitioning of strain between lithologic units throughout the SSE cycle. We review and synthesize published constitutive relations for the five lithologic units present to express shear stress as a function of deformation rate. We use these results to predict (1) the shear stress across the plate boundary as a function of slip velocity and (2) the partitioning of deformation among the different lithologic units for SSE and aseismic creep velocities. We conduct this analysis for pore fluid pressures from hydrostatic to near-lithostatic. Our results show that, at pore fluid pressure close to hydrostatic, aseismic creep and SSE velocities occur by viscous deformation of calcareous and quartzose units. However, once the pore fluid pressure increases above 80% of lithostatic, plate boundary slip migrates from the calcareous and quartzose rocks during aseismic creep to frictional deformation of talc schist during slow slip. This result is insensitive to differences in the thicknesses of metasedimentary units that may be present along subduction plate boundaries and, therefore, may apply to subduction plate boundaries in general.