Browsing by Author "French, Melodie"
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Item Data for the article: Pore fluid pressures and strength contrasts maintain frontal fault activity, northern Hikurangi(Rice University, 2020-07-29) French, Melodie; This work was funded by NSF EAR 1759127 to Melodie French and USSSP-IODP Subaward 66B(GG009393) 309 of NSF Award OCE1450528 to Julia Morgan.; Earth, Environmental, and Planetary SciencesThis data set archives the experimental rock deformation data reported in "Pore fluid pressures and strength contrasts maintain frontal fault activity, northern Hikurangi". In this paper, we present the results of five deformation experiments conducted on samples recovered during the Intergrated Ocean Drilling Program Expedition 375.Item Diagenetic controls on fault zone structure along the Sestola-Vidiciatico unit, as an analogue to the shallow subduction megathrust(2023-12-01) Mckenzie, Emory Isaac; French, Melodie; Morgan, Juli; Lee, Cin-TyAlong the subduction megathrust, the up-dip limit of the seismogenic zone is thought to occur at 100 to 150 °C and the shallower region hosts diverse modes of fault slip. The transition to the seismogenic zone is thought to occur due to evolving material properties that promote seismicity; however, which properties are responsible and how they evolve is poorly understood. The Sestola-Vidiciatico Unit (SVU) has been interpreted as an analog of the sedimentary component of active subduction megathrusts, near the up-dip transition to seismicity. The SVU accommodated Miocene convergence between the subducting Adriatic plate and the overriding accretionary prisms of the European plate. Deformation was dominantly accommodated within a 500 m thick shear zone along a well-exposed basal decollement and inaccessible roof decollement. Here we evaluate the role that diagenesis of sedimentary rocks plays in controlling fault zone architecture and strain partitioning. We use HAWK pyrolysis to determine new temperature constraints for deformation at multiple outcrops along the basal decollement and show that deformation occurred at temperatures between ~180 to 190 °C at our four field sites. We characterize the macroscopic fault architecture through field measurements and rock composition and microfabrics of the basal decollement using scanning electron microscopy and electron probe microanalysis. Diagenetic reactions such as silicification, chloritization, and pyritization occur in samples from each outcrop. We show that fault architecture is variable even at similar deformation conditions. We quantify the abundance and occurrence of diagenetic reactions and discuss their correlations with variations in fault architecture and deformation textures.Item Fluid-Mediated Slip of Shallow Subduction Thrust Faults(2023-12-01) Belzer, Ben; French, MelodieThe shallow segment of subduction plate boundary faults (<10 km) hosts diverse slip modes including low-frequency earthquakes and slow slip. These slip behaviors are thought to be controlled by the mechanical properties of rocks and sediment present along the subduction interface and their thermal and hydrologic environment. The roles of fluids have particularly gained a lot of attention over the past 20 years as numerous geophysical studies have shown a correlation between slow earthquakes and evidence of high pore fluid pressure along subduction thrusts. However, the mechanics of slip in shallow, fluid-rich subduction shear zones are not well constrained. We present three studies addressing different roles that fluids play in affecting the strength and slip behavior of shallow subduction thrust faults. Using deformation experiments, we first quantify the frictional constitutive behavior of chlorite, which is a ubiquitous and important hydrous mineral in subduction zones. Rate-stepping shear experiments were performed under shallow hydrothermal conditions with varying temperature, pore fluid pressure, and slip rates from 10-9 to 10-5 m/s, representing slow slip speeds and the faster velocities previously used to study chlorite deformation. Our results show that chlorite strengthens with increasing temperature and transitions from stable to unstable frictional behavior with decreasing slip rate, indicating that slow slip can nucleate in chlorite-rich fault zones. Based on microstructural observations and micromechanical analyses, we interpret this transition is controlled by a competition between rate-strengthening mineral deformation at grain contacts, which promotes stable sliding, and the time-dependent strength of water films between grains, which promotes rate-weakening (unstable) behavior. Along shallow subduction faults, pore fluid pressure can become elevated due to disequilibrium compaction of sediments or fluid release due to dehydration reactions like the smectite to illite transition. In our second study, we investigate whether the source of fluid overpressure places a role on the mechanics of shallow subduction fault slip. We conducted hydrothermal shear experiments on chlorite gouge and natural cataclasite from the Rodeo Cove thrust (RCT) in California along stress paths that simulate disequilibrium compaction and dehydration reactions. Our results show that the effects of pore fluid pressurization on fault strength can generally be described using the critical state soil mechanics (CSSM) framework. The effects of path are more pronounced and persist to greater displacements in chlorite fault rock than in cataclasite, which we attribute to differences in microstructure. Because of the effects on microstructure, effective stress path also imparts a much more significant control on the stability of chlorite-rich faults, which is not predicted by CSSM. Our second study thus provides constraints on how both the source of pore fluids and fault rock composition should be considered in models of shallow subduction faulting. In our third study, we characterize the effects that fluid-mediated reactions have on the rheology of oceanic crust in a shallow subduction thrust environment (8-10 km). To do so, we present a geochemical and microstructural study of metabasaltic fault rock from the Rodeo Cove thrust zone. At the RCT, deformation is distributed along a dense network of reddish and greenish foliated cataclasites, which surround blocks of altered basalt that contain abundant calcite veins and cement. Our study indicates that faulting and cataclasis of the altered basalt followed by seawater-mediated K-metasomatism promoted extensive mineralization of celadonite within the RCT, which influenced its overall strength and deformation style. Transitions from spilitization to K-metasomatism of oceanic crust may therefore play an important role in the mechanics of shallow subduction fault slip, especially along active sediment-poor margins.Item Localized slip and associated fluidized structures record seismic slip in clay-rich fault gouge [Replication Data](Rice University, 2018-09-21) French, Melodie; GeophysicsItem Mechanical Strength of Water-Saturated Solnhofen Limestone at Elevated Temperatures(Rice University, 2021-10-07) French, Melodie; Zhu, Wenlu; Xiao, Xiaohui; Evans, Brian; Prior, David; National Science Foundation; U.S. Geological Survey; Earth, Environmental, and Planetary SciencesThe data is a record of the stress and strain during experimental deformation of Solnhofen limestone across its brittle (localized deformation) to ductile (distributed deformation) transition. We conducted conventional triaxial compression tests on both water-saturated and nominally dry cores of Solnhofen at temperatures up to 200 Celsius and effective confining pressures up to 350 MPa to evaluate the roles of pore water and temperature on the deformation mechanisms of low-porosity limestone at conditions of the upper crust.Item Microstructural and environmental controls on the elastic wave properties of phyllosilicate-rich rocks(2023-02-14) Fliedner, Celine; French, MelodieSubduction zones generate the most earthquakes, and the most destructive earthquakes often nucleate at the plate boundary near the base of the seismogenic zone, located at a depth of 30-40 km. This active region correlates with evidence of near lithostatic pore pressure from geophysical imaging methods, indicating conditions that significantly impact seismic activity and slip mode. Additionally, the pressure-temperature conditions at 30-40km correspond to rocks undergoing greenschist facies metamorphism, although little is known about their rock properties under high fluid pressure. As a result, it remains unclear how the rock properties of greenschists under high fluid pressure control seismic waves, possibly leading to misinterpretation in geophysical imaging. This thesis aims to collect data on a greenschist metapelite, the Orocopia schist, to better understand how the microstructure and pore fluids control the rock properties and seismic waves. We measured velocities, elastic moduli, and attenuation of the Orocopia schist in the laboratory to determine the effect of pore fluids. Then, we extracted the critical microstructure, like pore shape, porosity, and permeability, with models that helped us extrapolate to the geologic scale. Chapter 2 presents ultrasonic wave measurements to explore the effect of mineralogy, anisotropy, and pore network on velocities. Although mineralogy and mineral anisotropy influence the wave velocities, thin elongated pores aligned within the foliation are necessary to explain the anomalous velocities in subduction zones. The forced oscillations technique is introduced in Chapters 3 and 4 to explore wave-induced fluid flow at a small scale, which causes frequency-dependent attenuation and elastic moduli. Models in Chapter 3 reveal that the microstructure is fundamental for generating fluid-flow mechanisms and a single property control dispersion and attenuation in saturated rocks. The attenuation results in Chapter 3 are extrapolated to the scale of subduction zones in Chapter 4 to understand the effect on earthquakes. We find that high fluid pressure causes enough attenuation to deplete high frequencies, consistent with amplitude spectra of low frequencies of earthquakes. This thesis demonstrates that metapelites under high fluid pressure can represent regions of low velocities and high attenuation in subduction zones but also significantly impact seismic waves.Item Ultrasonic measurements on Orocopia schist(Rice University, 2021-07-27) Fliedner, Celine; French, Melodie; Earth, Environmental, and Planetary SciencesThe purpose of the study is to calculate the speed of waves in a rock from the time of arrival. Four datasets are representing experimental measurements at 4 different experimental conditions. The data provided are waves measured in time, with amplitude and energy of the wave. The heading is the experimental setup.