4:00 PM - 4:15 PM
[SMP27-15] Deformation Microstructures and Olivine Crystal-fabrics of the Horoman Peridotite Complex, Hokkaido, Japan
Keywords:Horoman Peridotite Complex, crystallographic preferred orientation, peridotite, seismic anisotropy, Hidaka Metamorphic Belt
The Horoman Peridotite Complex (HPC) is a rock body that is subject to strong deformation and recrystallization throughout its entire body, and various deformation structures are observed. Because of the large exposure size (10×8 km) and minor serpentinization, various studies have been conducted to elucidate the mantle process. However, only a few studies have been focused on deformations (e.g., Niida, 1975; Takizawa, 1993; Sawaguchi, 2004). In particular, quantitative discussions on the degree of deformation and deformation processes of the entire HPC have not yet been progressed in recent years since Sawaguchi (2004). In this study, deformation microstructures and deformation processes of the HPC were considered along with crystallographic preferred orientations (CPOs) of the major constituent minerals.
In order to study from Lower to Upper Zone of the HPC (Komatsu and Nochi, 1966), we have selected two sections of Horoman River route and the trail of Mt. Apoi route, where 14 oriented samples have been collected. We have measured olivine mean grain sizes, J-indices, and olivine CPOs by the EBSD method. Microstructures of the Horoman River route varied from weakly mylonitized textures to porphyroclastic textures to mosaic textures towards the structurally upper section in the Upper Zone. Although the Mt. Apoi route is only in the Upper Zone, their microstructures have various textures from equigranular to lobate textures.
The olivine mean grain sizes were in the range of 300−500 µm and showed no systematic variations with respect to the structural unit defined by Sawaguchi (2004). The olivine CPOs were characterized by the Vp Flinn diagram of Michibayashi et al. (2006). It shows that E-type occurs in the lowermost part of the Lower Zone, A-type in the middle to upper part in the Lower Zone, and AG-type in the Upper Zone. We determined the shear sense of each sample based on the obliquity of the olivine CPOs against structural framework. Our results show that the shear senses were roughly classified into two groups: top-to-the south and top-to the north, which were in good agreement with Sawaguchi (2004).
E-type CPOs in the lowermost part of the HPC could be developed under hydrous conditions (Karato, 2008). The samples with E-type CPOs showed weak-mylonitic textures with higher J-index values. It indicates that intense shear deformation occurred in the lowermost part of the HPC under hydrous conditions, presumably due to the movement of the basal fault mentioned by Sawaguchi (2004). Combined with the thermal structure and uplift history of the HPC (e.g., Ozawa, 2004; Akizawa et al., 2021), we will have discussed the deformation process of the HPC.
In order to study from Lower to Upper Zone of the HPC (Komatsu and Nochi, 1966), we have selected two sections of Horoman River route and the trail of Mt. Apoi route, where 14 oriented samples have been collected. We have measured olivine mean grain sizes, J-indices, and olivine CPOs by the EBSD method. Microstructures of the Horoman River route varied from weakly mylonitized textures to porphyroclastic textures to mosaic textures towards the structurally upper section in the Upper Zone. Although the Mt. Apoi route is only in the Upper Zone, their microstructures have various textures from equigranular to lobate textures.
The olivine mean grain sizes were in the range of 300−500 µm and showed no systematic variations with respect to the structural unit defined by Sawaguchi (2004). The olivine CPOs were characterized by the Vp Flinn diagram of Michibayashi et al. (2006). It shows that E-type occurs in the lowermost part of the Lower Zone, A-type in the middle to upper part in the Lower Zone, and AG-type in the Upper Zone. We determined the shear sense of each sample based on the obliquity of the olivine CPOs against structural framework. Our results show that the shear senses were roughly classified into two groups: top-to-the south and top-to the north, which were in good agreement with Sawaguchi (2004).
E-type CPOs in the lowermost part of the HPC could be developed under hydrous conditions (Karato, 2008). The samples with E-type CPOs showed weak-mylonitic textures with higher J-index values. It indicates that intense shear deformation occurred in the lowermost part of the HPC under hydrous conditions, presumably due to the movement of the basal fault mentioned by Sawaguchi (2004). Combined with the thermal structure and uplift history of the HPC (e.g., Ozawa, 2004; Akizawa et al., 2021), we will have discussed the deformation process of the HPC.