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[SSS05-11] The thickness of plate boundary fault and its deformed zone in the subduction zone: Example of the Hanazono Formation in the Cretaceous Shimanto Belt, Kii Peninsula, SW Japan
★Invited Papers
Keywords:plate boundary fault, melange, Shimanto Belt, Kii Peninsula
The thickness of plate boundary fault and its deformed zone in the subduction zone is an important parameter for understanding fault-slip behavior, including fault strength and spatial heterogeneity. The fossil plate boundary fault and deformed zone are preserved respectively as the thrust and melange zone in an accretionary complex. Rowe et al. (2013) examined the variations of thickness of the plate boundary fault and deformed zone in response to depth at the plate boundary, and suggested homogeneity regardless of the depth. However, this integrates in several subduction zones and has not been considered the differences in their regional characteristics.
The Hanazono Formation (Yamamoto and Suzuki, 2012; Kurimoto et al., 2015) in the Shimanto Belt, Kii Peninsula, SW Japan, is an accretionary complex that formed in the eastern Asian subduction zone during the Santonian to Campanian (Cretaceous). This formation can be regarded as a deeper facies of the coeval Mugi Formation (Kimura et al., 2012) because the Hanazono Formation shows higher temperatures (280-290 °C: Awan and Kimura, 1996) than the Mugi Formation (150-200 °C; Ikesawa et al., 2005; Kitamura et al., 2005). In the present study, we conducted field survey and geological mapping for the Hanazono Formation to clarify its lithology and structure, and determine the thickness of thrust and melange zone.
The Hanazono Formation is divided into two units: coherent unit consisting of sandstone-mudstone alternation and melange unit containing lenses or blocks of sandstone, vari-colored shale, chert, and basalt in a muddy matrix. These units consist of a single package of structurally higher coherent and lower melange units, and the packages are juxtaposed by thrusts (with up to several tens of centimeters thick shear zone). The thickness of the melange unit can be estimated to be several kilometers, being thicker than that in the Mugi Formation (100 to 150 m: Ikesawa et al., 2005; Kimura et al., 2012). Our results suggest that the thickness of the deformation zone (melange) varies with the depth of the plate boundary.
The Hanazono Formation (Yamamoto and Suzuki, 2012; Kurimoto et al., 2015) in the Shimanto Belt, Kii Peninsula, SW Japan, is an accretionary complex that formed in the eastern Asian subduction zone during the Santonian to Campanian (Cretaceous). This formation can be regarded as a deeper facies of the coeval Mugi Formation (Kimura et al., 2012) because the Hanazono Formation shows higher temperatures (280-290 °C: Awan and Kimura, 1996) than the Mugi Formation (150-200 °C; Ikesawa et al., 2005; Kitamura et al., 2005). In the present study, we conducted field survey and geological mapping for the Hanazono Formation to clarify its lithology and structure, and determine the thickness of thrust and melange zone.
The Hanazono Formation is divided into two units: coherent unit consisting of sandstone-mudstone alternation and melange unit containing lenses or blocks of sandstone, vari-colored shale, chert, and basalt in a muddy matrix. These units consist of a single package of structurally higher coherent and lower melange units, and the packages are juxtaposed by thrusts (with up to several tens of centimeters thick shear zone). The thickness of the melange unit can be estimated to be several kilometers, being thicker than that in the Mugi Formation (100 to 150 m: Ikesawa et al., 2005; Kimura et al., 2012). Our results suggest that the thickness of the deformation zone (melange) varies with the depth of the plate boundary.