5:15 PM - 7:15 PM
[SSS10-P22] Shear zone development in the deep part of subduction zone recorded in the Kerama Islands, Okinawa
Keywords:Subduction zone, Shimanto Belt, Shear zone, Ductile deformation
Slow earthquakes, characterized by slow slips compared to regular earthquakes, have been observed along the plate boundaries in subduction zones from seismological and geodetic studies. Deep slow earthquakes, which occur at greater depths than the sources of megathrust earthquakes, are thought to occur in ductile shear zones where temperatures exceed 350°C (Behr and Burgmann, 2021; Kirkpatrick et al., 2021). However, identifying deformation features of geological outcrops specifically attributable to slow earthquakes remains a significant challenge due to the overprinting by multiple deformations, metamorphic processes, and uplifting processes. Therefore, this research aims to find deformation processes occurring in the deeper parts of subduction zones by analyzing deformed rocks formed under pressure-temperature conditions corresponding to deep slow earthquakes with minimal retrograde deformation and metamorphism.
In this study, we focus on understanding shear zone formation and its deformation mechanisms in the Kerama Islands, Okinawa Prefecture. The research area, the Kerama Islands, are located west of Okinawa Island. They belong to the Ryukyu Arc (Takami et al., 1999) and contain high-grade metamorphic rocks (epidote-amphibolite metamorphic facies with maximum temperature of approximately 530°C) in the Shimanto Belt (Yamamoto, 2021JpGU). Especially in Aka Island, it is reported that shear zones developed near the boundary between greenstone and sandstone exhibit highly deformed features like mylonite (Yamamoto, 2021JpGU).
We adopted several methods with different scales to investigate the types of deformed rocks and their deformation characteristics in this region, with a particular focus on the distribution and deformation mechanisms. A field survey was conducted in October 2024 to observe the overall picture of the shear zone in Aka Island. Chemical composition mapping by using micro-XRF and thin-section observation were performed to investigate microstructures of highly deformed samples.
As a result, the deformation structures and compositions of the shear zone on Aka Island were characterized across scales ranging from micrometers to approximately 100 m. The shear zone is about 160 meters wide, but the degree of deformation within the rocks is heterogeneous. Highly deformed sections (mylonites) are localized within the shear zone and the layers with varying degrees of deformation are adjacent to each other with distinct boundaries. At both the outcrop and sample scales, clasts of quartz and feldspar, and white and black bands exhibit distinct structures. Based on microscopic observations, single-crystal quartz clasts showed undulatory extinction, suggesting subgrain boundary migration caused by dynamic recrystallization. Feldspar clasts contained inclusions with no regular arrangement, indicating that the clasts grew during the early stages of the deformation. Lens-shaped, elongated polycrystalline quartz clasts and fine-grained white bands composed mainly of quartz and feldspar exhibited grain boundary morphologies indicative of dynamic recrystallization through subgrain rotation (SGR).
Based on these observations and the inferred deformation mechanisms, the deformation processes of the shear zone were classified into six stages (Stages 0 to 5). This progressive sequence highlights the transition from initial lithification to advanced deformation, governed by crystal growth of quartz and feldspar and competing dislocation creep. Shear localization into fine-grained bands is possibly attributed to the dominance of grain boundary sliding, which could be facilitated by fine-grained quartz and feldspar formed through grain size reduction during dynamic recrystallization.
In this study, we focus on understanding shear zone formation and its deformation mechanisms in the Kerama Islands, Okinawa Prefecture. The research area, the Kerama Islands, are located west of Okinawa Island. They belong to the Ryukyu Arc (Takami et al., 1999) and contain high-grade metamorphic rocks (epidote-amphibolite metamorphic facies with maximum temperature of approximately 530°C) in the Shimanto Belt (Yamamoto, 2021JpGU). Especially in Aka Island, it is reported that shear zones developed near the boundary between greenstone and sandstone exhibit highly deformed features like mylonite (Yamamoto, 2021JpGU).
We adopted several methods with different scales to investigate the types of deformed rocks and their deformation characteristics in this region, with a particular focus on the distribution and deformation mechanisms. A field survey was conducted in October 2024 to observe the overall picture of the shear zone in Aka Island. Chemical composition mapping by using micro-XRF and thin-section observation were performed to investigate microstructures of highly deformed samples.
As a result, the deformation structures and compositions of the shear zone on Aka Island were characterized across scales ranging from micrometers to approximately 100 m. The shear zone is about 160 meters wide, but the degree of deformation within the rocks is heterogeneous. Highly deformed sections (mylonites) are localized within the shear zone and the layers with varying degrees of deformation are adjacent to each other with distinct boundaries. At both the outcrop and sample scales, clasts of quartz and feldspar, and white and black bands exhibit distinct structures. Based on microscopic observations, single-crystal quartz clasts showed undulatory extinction, suggesting subgrain boundary migration caused by dynamic recrystallization. Feldspar clasts contained inclusions with no regular arrangement, indicating that the clasts grew during the early stages of the deformation. Lens-shaped, elongated polycrystalline quartz clasts and fine-grained white bands composed mainly of quartz and feldspar exhibited grain boundary morphologies indicative of dynamic recrystallization through subgrain rotation (SGR).
Based on these observations and the inferred deformation mechanisms, the deformation processes of the shear zone were classified into six stages (Stages 0 to 5). This progressive sequence highlights the transition from initial lithification to advanced deformation, governed by crystal growth of quartz and feldspar and competing dislocation creep. Shear localization into fine-grained bands is possibly attributed to the dominance of grain boundary sliding, which could be facilitated by fine-grained quartz and feldspar formed through grain size reduction during dynamic recrystallization.