11:00 AM - 1:00 PM
[SCG44-P09] Geological characteristics and stress distribution in deep slow earthquake domain: evidence from the subduction-type Sanbagawa metamorphic belt
Keywords:ETS, stress estimation, dislocation creep, dynamic recrystallization, quartz
Episodic tremor and slow slip (ETS), a type of deep slow earthquake, is thought to be caused by combined brittle and ductile deformation occurring under low differential stress, high fluid pressure conditions. Developing a quantitative model for ETS requires a combination of geological methods, real-time observations and physical modeling. Geological observations can provide information on otherwise inaccessible depths of the subduction system.
The Sanbagawa belt, SW Japan provides a window into regions deformed at depths and conditions in subduction zones that are similar to where ETS is common in modern SW Japan and is well suited to studies of this type.The main rock types are mafic, quartz and pelitic schists derived from subducted oceanic crust. These are associated with less common but widespread serpentinite derived from the mantle wedge. The boundary represents a former plate boundary. Shirataki unit of Shikoku is particularly well exposed.
Quartz crystallographic preferred orientation (CPO) patterns develop as a result of deformation by dislocation creep and opening angles of cross girdle type quartz c-axis pole figures can also be used as a thermometer, allowing a link to be made between deformation and metamorphic conditions. A compilation of reported data shows a good correlation between CPO and Raman carbonaceous thermometry. This correlation indicates that the observed microstructures formed close to peak metamorphic conditions. Locally lower temperatures shown by CPO thermometry, indicate exhumation-related localized deformation.
The recrystallized grain size of quartz can be used in combination with flow laws to estimate stress and strain rates. The strain rate of rocks along the subduction boundary depends on the imposed shear rate of the subducting plate and the thickness of the deforming region. For the Sanbagawa belt, the subducting plate is the Izanagi plate with a well-known high rate of subduction. This information combined with our strain rate estimates allows the thickness of the subduction channel to be estimated. Applying this method suggests the channel width was several kilometers at relatively high P-T (500℃, 0.9GPa). However, unrealistically large widths are estimated at lower P-T (300-350℃, 0.6GPa), indicating that a different deformation mechanism was dominant. Pressure-solution is one possibility as witnessed by the development of crenulation cleavage and dissolution selvages widely observed in low-grade areas.
Geological structures proposed to account for ETS include mélange block-in-matrix; quartz-filled crack seal shear veins; and shear folds with pressure solution. Serpentine blocks are present scattered throughout the area, but these blocks are absent in the low-grade region and other lithologies show good continuity implying the mechanical characteristics of mélange do not play an important part in controlling how this region deformed. The formation of both quartz-filled shear cracks and crenulation cleavage involves a brittle fracturing stage with dilation followed by an inflow of fluid and deposition of quartz. Both styles of deformation are promoted by high fluid pressure making them good candidate structures for the origin of ETS. However, no good evidence for shear cracks was observed. In contrast, crenulation cleavage and related folds on various scales are widely observed. Our results suggest that models should be considered where ETS-related deformation occurs in a broad subduction channel of several kilometers width. The development of trains of folds forming at various scales including crenulation cleavage can account for the main features of ETS and is compatible with our observations.
The Sanbagawa belt, SW Japan provides a window into regions deformed at depths and conditions in subduction zones that are similar to where ETS is common in modern SW Japan and is well suited to studies of this type.The main rock types are mafic, quartz and pelitic schists derived from subducted oceanic crust. These are associated with less common but widespread serpentinite derived from the mantle wedge. The boundary represents a former plate boundary. Shirataki unit of Shikoku is particularly well exposed.
Quartz crystallographic preferred orientation (CPO) patterns develop as a result of deformation by dislocation creep and opening angles of cross girdle type quartz c-axis pole figures can also be used as a thermometer, allowing a link to be made between deformation and metamorphic conditions. A compilation of reported data shows a good correlation between CPO and Raman carbonaceous thermometry. This correlation indicates that the observed microstructures formed close to peak metamorphic conditions. Locally lower temperatures shown by CPO thermometry, indicate exhumation-related localized deformation.
The recrystallized grain size of quartz can be used in combination with flow laws to estimate stress and strain rates. The strain rate of rocks along the subduction boundary depends on the imposed shear rate of the subducting plate and the thickness of the deforming region. For the Sanbagawa belt, the subducting plate is the Izanagi plate with a well-known high rate of subduction. This information combined with our strain rate estimates allows the thickness of the subduction channel to be estimated. Applying this method suggests the channel width was several kilometers at relatively high P-T (500℃, 0.9GPa). However, unrealistically large widths are estimated at lower P-T (300-350℃, 0.6GPa), indicating that a different deformation mechanism was dominant. Pressure-solution is one possibility as witnessed by the development of crenulation cleavage and dissolution selvages widely observed in low-grade areas.
Geological structures proposed to account for ETS include mélange block-in-matrix; quartz-filled crack seal shear veins; and shear folds with pressure solution. Serpentine blocks are present scattered throughout the area, but these blocks are absent in the low-grade region and other lithologies show good continuity implying the mechanical characteristics of mélange do not play an important part in controlling how this region deformed. The formation of both quartz-filled shear cracks and crenulation cleavage involves a brittle fracturing stage with dilation followed by an inflow of fluid and deposition of quartz. Both styles of deformation are promoted by high fluid pressure making them good candidate structures for the origin of ETS. However, no good evidence for shear cracks was observed. In contrast, crenulation cleavage and related folds on various scales are widely observed. Our results suggest that models should be considered where ETS-related deformation occurs in a broad subduction channel of several kilometers width. The development of trains of folds forming at various scales including crenulation cleavage can account for the main features of ETS and is compatible with our observations.