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[SMP28-18] Deformation environment and strain localization of small shear zones in the Ryoke granites, Teshima Island, SW Japan
Keywords:small-scale shear zone, strain localization, quartz CPO, disGBS, Ryoke Granite
The Ryoke granite on Teshima Island in the Seto Inland Sea (SW Japan) contains numerous small-scale shear zones. As they have not experienced large-scale deformation or complex histories, they are considered suitable for fundamental research of shear zones (Hara et al., 1973; Yamagishi et al., 1996; Michibayashi et al., 1999; Michibayashi & Murakami, 2007; Ono et al., 2010; Uhmb & Michibayashi, 2022). Nagara & Hara (1993) reported Y-max c-axis fabric (hereafter CPO) in dynamically recrystallized quartz aggregates at the center of a small-scale shear zone, attributed to increasing strain. However, as some shear zones contain quartz veins and exhibit distinct characteristics, further analysis remains necessary. This study aims to clarify their development by conducting (1) geometric descriptions and (2) microstructural analyses of dynamically recrystallized quartz aggregates using polarizing microscope and EBSD.
The older Ryoke granite in southern Teshima Island has undergone regional ductile shear deformation and exhibits weak mylonitization. Its foliation trends ENE–WSW with a steep dip and shows a sinistral shear sense. Most small-scale shear zones develop along the southern coastline, trending WNW–ESE with steep dips. Lineation plunges 10–20° toward the ESE, indicating dextral shear. Their orientation resembles dextral shear zones along Late Cretaceous granite dikes in Awaji Island (Kano & Takagi, 1996), suggesting formation under a similar stress field. The shear zones range from 1–100 cm in thickness, while their total lengths remain unknown due to outcrop constraints. The foliation of surrounding granite exhibits locally symmetric sigmoidal drag. Some shear zones contain quartz veins, whereas others without veins often have a dark-colored center (a dark gray, high-strain zone <30 cm thick). In shear zones with quartz veins, multiple vein generations exist from pre- to late-stage deformation. Most veins occur in the shear zone center and are intensely deformed, suggesting that fractures formed before mylonitization, allowing fluid infiltration and quartz precipitation. This suggests the deformation of brittle-ductile transition regime. Additionally, low-angle quartz veins branching from the shear zone center show weaker deformation, suggesting formation during mylonitization. Furthermore, quartz veins adjacent to the dark-colored center exhibit minimal deformation, indicating late-stage formation.
In the small-scale shear zones, CPO in the shear zone center exhibits Y-max, while surrounding quartz displays type I cross-girdle and single-girdle patterns. If these CPO patterns reflect stress-dependent deformation, then the shear zone center suggests lower stress conditions. However, the mean grain size of dynamically recrystallized quartz decreases toward the center, implying increasing differential stress (e.g., Stipp & Tullis, 2003; Cross et al., 2017). This apparent contradiction, as noted by Nagara & Hara (1993), is likely due to increasing shear strain (Heilbronner & Tullis, 2006; Muto et al., 2011; Kilian & Heilbronner, 2017). Shear strain (γ) can be determined using cotβ = cotβ' + γ, where β' is the angle between the C-plane and S-plane, and β is the angle between the foliation of the sinistral older Ryoke granite and the C-plane (Ramsey, 1980). With β' nearly 0°, shear strain was high.
Additionally, Tokle et al. (2019) demonstrated that during the transition from basal<a> to prism<a> slip, dislocation-accommodated grain boundary sliding (disGBS) occurs. In the shear zone centers exhibiting Y-max, microstructural lines of evidence for GBS such as quadruple junctions and aligned grain boundaries were observed. This suggests that the CPO initially recorded type I cross-girdle under high strain conditions. Subsequently under hydrous conditions, strain localization occurred, facilitating disGBS and leading to a transition from basal<a> to prism<a> slip, overlapping single-girdle and Y-max fabrics.
The older Ryoke granite in southern Teshima Island has undergone regional ductile shear deformation and exhibits weak mylonitization. Its foliation trends ENE–WSW with a steep dip and shows a sinistral shear sense. Most small-scale shear zones develop along the southern coastline, trending WNW–ESE with steep dips. Lineation plunges 10–20° toward the ESE, indicating dextral shear. Their orientation resembles dextral shear zones along Late Cretaceous granite dikes in Awaji Island (Kano & Takagi, 1996), suggesting formation under a similar stress field. The shear zones range from 1–100 cm in thickness, while their total lengths remain unknown due to outcrop constraints. The foliation of surrounding granite exhibits locally symmetric sigmoidal drag. Some shear zones contain quartz veins, whereas others without veins often have a dark-colored center (a dark gray, high-strain zone <30 cm thick). In shear zones with quartz veins, multiple vein generations exist from pre- to late-stage deformation. Most veins occur in the shear zone center and are intensely deformed, suggesting that fractures formed before mylonitization, allowing fluid infiltration and quartz precipitation. This suggests the deformation of brittle-ductile transition regime. Additionally, low-angle quartz veins branching from the shear zone center show weaker deformation, suggesting formation during mylonitization. Furthermore, quartz veins adjacent to the dark-colored center exhibit minimal deformation, indicating late-stage formation.
In the small-scale shear zones, CPO in the shear zone center exhibits Y-max, while surrounding quartz displays type I cross-girdle and single-girdle patterns. If these CPO patterns reflect stress-dependent deformation, then the shear zone center suggests lower stress conditions. However, the mean grain size of dynamically recrystallized quartz decreases toward the center, implying increasing differential stress (e.g., Stipp & Tullis, 2003; Cross et al., 2017). This apparent contradiction, as noted by Nagara & Hara (1993), is likely due to increasing shear strain (Heilbronner & Tullis, 2006; Muto et al., 2011; Kilian & Heilbronner, 2017). Shear strain (γ) can be determined using cotβ = cotβ' + γ, where β' is the angle between the C-plane and S-plane, and β is the angle between the foliation of the sinistral older Ryoke granite and the C-plane (Ramsey, 1980). With β' nearly 0°, shear strain was high.
Additionally, Tokle et al. (2019) demonstrated that during the transition from basal<a> to prism<a> slip, dislocation-accommodated grain boundary sliding (disGBS) occurs. In the shear zone centers exhibiting Y-max, microstructural lines of evidence for GBS such as quadruple junctions and aligned grain boundaries were observed. This suggests that the CPO initially recorded type I cross-girdle under high strain conditions. Subsequently under hydrous conditions, strain localization occurred, facilitating disGBS and leading to a transition from basal<a> to prism<a> slip, overlapping single-girdle and Y-max fabrics.