The Record of Mass Transfer in Ancient Accretionary Prisms and Its Role in Fault Behavior
Open Access
- Author:
- Chen, Tsai Wei
- Graduate Program:
- Geosciences
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- July 12, 2023
- Committee Members:
- Donald Fisher, Program Head/Chair
Chris Marone, Major Field Member
Andrew Smye, Major Field Member
Derek Elsworth, Outside Unit & Field Member
Donald Fisher, Chair & Dissertation Advisor - Keywords:
- mélange
seismogenic zone
scaly fabric
mass transfer
ancient accretionary complex - Abstract:
- A comprehensive understanding of the factors that govern subduction zone earthquakes is crucial for assessing their spatial distributions, magnitudes, and recurrence intervals, thus offering substantial possibilities for forecasting behavior. Dissolution-precipitation creep, hereinafter referred to as mass transfer, has been identified as a potential deformation mechanism that could modulate megathrust slip behavior by exerting composite effects on the mineralogy, mechanics, and hydrology within subduction fault zones. In this dissertation, I integrate micro-structural, geochemical, and geomechanical analyses to investigate various aspects of this deformation mechanism. These studies focus on characterizing the microstructures including scaly fabrics and mineral veins, which act as sources and sinks of local mineral redistribution, in exhumed tectonic mélanges of the Cretaceous Shimanto belt in Japan and Kodiak accretionary prism in Alaska. These mélange units preserve a variety of paleotemperature records that correspond to the range of the seismogenic zone, thereby providing insights into the evolution of subduction fault materials resulting from mass transfer along the depth of the plate boundary. I first derived records of temperature from fourteen mineral veins in five mélange units of the Cretaceous Shimanto belt and one unit in the Kodiak accretionary prism using quartz-calcite oxygen isotope thermometry and carbonate clumped isotope thermometry. The C-isotopic composition of calcite in the values shows δ13CPDB values ranging from -17.2 to -6.8‰, which is consistent with a mixed carbon source from organic matter and sedimentary carbonates. The O-isotopic composition of calcite and quartz yield δ18OSMOW values ranging from +11.1 to +17.2‰ and +14.9‰ to +21.7‰, respectively, which suggests the host-rock buffered vein-forming fluid sources. Notably, the differing δ18OSMOW values observed in veins of the Cretaceous Shimanto belt and Kodiak accretionary prism are likely indicative of their distinct paleogeographic locations. Additionally, temperature records obtained from the veins generally fall in the range of 100 to 250°C, which likely reflects the predominant availability of fluid sources at the corresponding depths along the plate interface. The observation suggests the prevalence of mass transfer in the temperature range of 100-250°C and its stronger control on shallow seismicity of sediment-dominated subduction zones. The cyclic process involving interseismic cementation resulting from mass transfer and the subsequent coseismic rupture of the seal may contribute to frequent and smaller earthquakes, as supported by numerical modeling and observations from several modern convergent margins. I further investigate the temperature-dependency of mass transfer by constructing sets of element maps for selected areas on polished thin sections made from nine mélange samples with an electron microprobe. My results show an amplified depletion of fluid-mobile elements (e.g., Si) and enrichment of fluid-immobile elements (e.g., Ti) along scaly fabrics of mélanges with higher paleotemperature records. The strain magnitudes related to mass transfer are quantified using the chemical mass-balance approach. I find a difference of 67% in average volume loss along scaly fabrics of the Lower Mugi and Makimine mélanges, which preserve paleotemperature records near the updip and downdip isotherms bounding the seismogenic zone, respectively. I apply image analysis to determine the proportion of scaly fabrics in each sample and integrate the strain over the areas of interests. There is no temperature-dependency observed in the spatially-integrate strain, indicating the need for larger scales of characterization or the differences in time durations during which the mélange units remain underthrust. I further compare the spatially-integrate volume loss with the volume gain, represented by the proportions of mineral veins within the areas of interests. A larger amount of volume gain is observed compared to the amount of volume loss in the mélanges with higher temperature records, which suggests the existence of external sources for vein materials in addition to the locally redistributed materials originating from scaly fabrics. Finally, I perform two types of friction experiments, velocity-step experiments and slide-hold-slide experiments, to explore the effects of mass transfer on the frictional velocity-dependence and healing behavior of fault materials. Velocity-step experiments using powered samples of the Lower Mugi and Makimine mélanges reveal that the former exhibit velocity-weakening behavior under low normal stress, while the latter shows velocity-strengthening behavior under high normal stress. The higher content of framework silicates in the Makimine sample resulting from mass transfer suggest this mechanism has the potential to alter the frictional properties of plate boundary fault materials along the depth. Furthermore, I observe an acceleration and amplification in healing during slide-hold-slide experiments conducted under hydrothermal conditions, using the Lower Mugi mélange sample. This is likely attributed to mass transfer during the experiments, as evidenced by microstructures found in the post-experimental gouges. My laboratory studies reveal the potential of mass transfer to modulate the frictional properties and healing behavior of subduction zone fault materials, ultimately influencing the slip behavior of subduction megathrusts.