Presentation Information
[T5-O-6]Dynamic fluid-rock interactions during the seismic cycle from microstructural observations: Minami-Awa fault of the Mugi mélange in the Cretaceous Shimanto Belt
*Takahiro HOSOKAWA1, Yoshitaka HASHIMOTO1, Rüdiger KILIAN2, Michael STIPP2 (1. Kochi University, 2. Martin Luther University Halle-Wittenberg)
Keywords:
Shimanto Belt,microstructures,fluid-rock interactions,ultracataclasite,cathodoluminescence
Fluids play a key role in deformation and mass transfer along subduction plate interfaces (e.g., Fisher et al., 2021). Deformation and healing through quartz cementation significantly influence fluid pressure and fault strength. The study of fluid-rock interactions in ancient subduction zones is therefore important. Previous studies have employed scanning electron microscopy -cathodoluminescence (SEM-CL) to investigate such interactions (e.g., Knipe and Lloyd, 1994). CL imaging enables the detection of microstructural features and trace element variations (e.g., Ti, Al) that are not visible through conventional methods and have been widely used to reconstruct complex histories of quartz vein formation (e.g., Götze, 2012). In this study, we reveal fluid-rock interactions that occurred during the seismic cycle by inspecting microstructures of ultracataclasite from the Minami-Awa fault, the northern boundary fault of the Mugi mélange in the Cretaceous Shimanto Belt.
The Mugi mélange is composed of tectonic mélanges, mainly with sandstone blocks and shale matrices, and small amounts of basalt, chert, tuff, and red shale. The Minami-Awa faults separates the predominantly coherent sandstone of the Hiwasa Formation from the Mugi merange below, which is interpreted to represent a roof thrust of a subduction plate boundary. The fault zone is composed of an approximately 1–2 m thick layer of foliated cataclasite and thin (up to several mm) layers of ultracataclasite. Pseudotachylyte has also been reported within the cataclasite, indicating past seismic slip events (Kitamura et al., 2005).
We analyzed fault rocks containing a 0.1–0.2 mm thick ultracataclasite layer using optical microscopy, scanning electron microscopy and cathodoluminescence (SEM-CL). The ultracataclasite is composed of fine-grained, poorly sorted clasts. Quartz grains within and near ultracataclasites exhibit CL-pattern indicative of microcracks, hydraulic fracturing, extension cracks, and growth zoning. Domains appearing as single quartz grains in BSE and polarized light images are aggregates of medium to high CL intensity quartz, surrounded by a matrix of low CL intensity quartz. Cement within microcracks also shows low CL intensity, suggesting that grain-size reduction and microcracking due to brittle deformation is accompanied by quartz precipitation.In contrast, cement within hydraulic fracturing and extension cracks displays high CL intensity, compatible with incorporation of higher trace element concentrations caused by rapid growth (e.g., Landtwing and Pettke, 2005). These features can be explained by fluid overpressure exceeding lithostatic stress, consistent with seismic slip. The subsequent rapid depressurization or fluid advection allows rapid mineral precipitation.
Our observations suggest that the ultracataclasite records a cycle of brittle fracturing and healing, reflecting dynamic fluid-rock interactions during the seismic cycle. These processes may influence fault strength recovery and, consequently, earthquake recurrence intervals.
References
Fisher et al., 2021, Geosphere, 17(6), 1686–1703.
Knipe and Lloyd, 1994, Pure and Applied Geophysics, 143(1), 229-254.
Götze, 2012, Microscopy and microanalysis, 18(6), 1270-1284.
Kitamura et al., 2005, Tectonics, 24(5).
Landtwing and Pettke, 2005, American Mineralogist, 90(1), 122-131.
The Mugi mélange is composed of tectonic mélanges, mainly with sandstone blocks and shale matrices, and small amounts of basalt, chert, tuff, and red shale. The Minami-Awa faults separates the predominantly coherent sandstone of the Hiwasa Formation from the Mugi merange below, which is interpreted to represent a roof thrust of a subduction plate boundary. The fault zone is composed of an approximately 1–2 m thick layer of foliated cataclasite and thin (up to several mm) layers of ultracataclasite. Pseudotachylyte has also been reported within the cataclasite, indicating past seismic slip events (Kitamura et al., 2005).
We analyzed fault rocks containing a 0.1–0.2 mm thick ultracataclasite layer using optical microscopy, scanning electron microscopy and cathodoluminescence (SEM-CL). The ultracataclasite is composed of fine-grained, poorly sorted clasts. Quartz grains within and near ultracataclasites exhibit CL-pattern indicative of microcracks, hydraulic fracturing, extension cracks, and growth zoning. Domains appearing as single quartz grains in BSE and polarized light images are aggregates of medium to high CL intensity quartz, surrounded by a matrix of low CL intensity quartz. Cement within microcracks also shows low CL intensity, suggesting that grain-size reduction and microcracking due to brittle deformation is accompanied by quartz precipitation.In contrast, cement within hydraulic fracturing and extension cracks displays high CL intensity, compatible with incorporation of higher trace element concentrations caused by rapid growth (e.g., Landtwing and Pettke, 2005). These features can be explained by fluid overpressure exceeding lithostatic stress, consistent with seismic slip. The subsequent rapid depressurization or fluid advection allows rapid mineral precipitation.
Our observations suggest that the ultracataclasite records a cycle of brittle fracturing and healing, reflecting dynamic fluid-rock interactions during the seismic cycle. These processes may influence fault strength recovery and, consequently, earthquake recurrence intervals.
References
Fisher et al., 2021, Geosphere, 17(6), 1686–1703.
Knipe and Lloyd, 1994, Pure and Applied Geophysics, 143(1), 229-254.
Götze, 2012, Microscopy and microanalysis, 18(6), 1270-1284.
Kitamura et al., 2005, Tectonics, 24(5).
Landtwing and Pettke, 2005, American Mineralogist, 90(1), 122-131.