10:45 〜 12:15
[SIT20-P02] Quartz microstructures in the Sambagawa schist, southwest Japan memorize oblique subduction during the Cretaceous
キーワード：斜め沈み込み、三波川変成岩、石英c軸ファブリック、剪断センスの指標、白亜紀の左横ずれ断層運動テクトニクス、中央構造線
Plate movement directions are not orthogonal, but oblique relative to many convergent plate boundaries at present. So, oblique subduction of the oceanic plate is not a special, but common type of subduction. This is also the case in the geological periods if uniformitarianism is correct. Although oblique subduction is an important subject in plate tectonics, not many researches have been conducted so far. There are obviously arc normal and parallel components for oblique subduction. Fitch (1972) proposed that the arc-normal component is accommodated by mega-thrusting at the plate boundary, and the arc-parallel component by strike-slip faulting such as the Median Tectonic Line in southwest Japan (i.e. strain partitioning). This seems to be the case for a shallower brittle region of arc crust. In fact, mega-thrust earthquakes occur, while the forearc sliver (e.g. Tsukuda, 1992) is moving west bound by the Nankai trough as the southern boundary and the MTL as the norther boundary. On the other hand, oblique subduction is recorded in rocks which were situated at the subduction plate boundaries, which were then exhumed to the near surface in the past.
I have worked on microstructural analyses in the Sambagawa metamorphic rocks, southwest Japan, which were deformed and exhumed along the paleo-subduction interface in the Cretaceous. From these observations, I could conclude that the kinematics (direction and sense of shear) is in fact explained by the model of helical corner flow in subduction channel caused by oblique subduction (e.g. Otsuki, 1993). For the Sambagawa metamorphic rocks, we have analyzed microstructures in more than 100 quartz schist samples collected along the Asemi, Dozan, Saruta and Kamio River routes (Tagami Takeshita, 1998; Yagi and Takeshita, 2002; Takeshita and Yagi, 2004; Takeshita, 2021). First of all, note that the orientation of the stretching lineation of quartz (X) is consistently E-W and horizontal, which is rotated clockwise by c. 15 degrees from the direction of the southwest Japan arc trending in N75°E. We analyzed microstructures and the quartz c-axis orientations in the XZ-section, and projected them on it. It has been found that in almost all quartz schist samples, well-defined quartz c-axis fabrics are developed, which mostly show a type-I crossed girdle pattern with subordinate amount of small-circle or cleft-girdle one. These quartz c-axis fabrics are inferred to have formed at lower temperatures ranging between 300 and 400 °C (Takeshita, 2021, see references therein) than the peak-metamorphic temperatures up to 600 °C (Enami et al., 1994). Therefore, it can be safely concluded that the microstructures and quartz c-axis fabrics in the quartz schist recorded deformation during exhumation, neither the peak-metamorphism nor subduction. The quartz c-axis fabrics mostly show an asymmetric pattern (i.e. monoclinic symmetry), from which a top-to-the-west-sense of shear is always inferred. Further, the foliation defined by the long-axis of recrystallized quartz grains (Sq) is oblique to that by white mica (Sm) by up to 30 degrees in some of the quartz schist samples (called oblique foliation by Passchier and Trouw, 2005). From the oblique foliation, a top-of-the-west-sense of shear is also always inferred, indicating 100 % agreement between those inferred from the two shear sense indicators. In the MTL-related mylonite derived from the Ryoke granitoids formed in the Cretaceous (Bui, 2019, Ph. D. thesis), the shear sense was also a top-to-the-west-sense without exception. In quartz schist tectonic blocks from the Kamuikotan metamorphic rocks formed in the Cretaceous, shear deformation is dominant inferred from asymmetric quartz c-axis fabric patterns (Takeshita et al., 2018). In the presentation, sinistral faulting along the MTL and other major faults in east Asia during the Cretaceous to Paleogene periods will be also introduced in relation to oblique subduction of the oceanic plate.
I have worked on microstructural analyses in the Sambagawa metamorphic rocks, southwest Japan, which were deformed and exhumed along the paleo-subduction interface in the Cretaceous. From these observations, I could conclude that the kinematics (direction and sense of shear) is in fact explained by the model of helical corner flow in subduction channel caused by oblique subduction (e.g. Otsuki, 1993). For the Sambagawa metamorphic rocks, we have analyzed microstructures in more than 100 quartz schist samples collected along the Asemi, Dozan, Saruta and Kamio River routes (Tagami Takeshita, 1998; Yagi and Takeshita, 2002; Takeshita and Yagi, 2004; Takeshita, 2021). First of all, note that the orientation of the stretching lineation of quartz (X) is consistently E-W and horizontal, which is rotated clockwise by c. 15 degrees from the direction of the southwest Japan arc trending in N75°E. We analyzed microstructures and the quartz c-axis orientations in the XZ-section, and projected them on it. It has been found that in almost all quartz schist samples, well-defined quartz c-axis fabrics are developed, which mostly show a type-I crossed girdle pattern with subordinate amount of small-circle or cleft-girdle one. These quartz c-axis fabrics are inferred to have formed at lower temperatures ranging between 300 and 400 °C (Takeshita, 2021, see references therein) than the peak-metamorphic temperatures up to 600 °C (Enami et al., 1994). Therefore, it can be safely concluded that the microstructures and quartz c-axis fabrics in the quartz schist recorded deformation during exhumation, neither the peak-metamorphism nor subduction. The quartz c-axis fabrics mostly show an asymmetric pattern (i.e. monoclinic symmetry), from which a top-to-the-west-sense of shear is always inferred. Further, the foliation defined by the long-axis of recrystallized quartz grains (Sq) is oblique to that by white mica (Sm) by up to 30 degrees in some of the quartz schist samples (called oblique foliation by Passchier and Trouw, 2005). From the oblique foliation, a top-of-the-west-sense of shear is also always inferred, indicating 100 % agreement between those inferred from the two shear sense indicators. In the MTL-related mylonite derived from the Ryoke granitoids formed in the Cretaceous (Bui, 2019, Ph. D. thesis), the shear sense was also a top-to-the-west-sense without exception. In quartz schist tectonic blocks from the Kamuikotan metamorphic rocks formed in the Cretaceous, shear deformation is dominant inferred from asymmetric quartz c-axis fabric patterns (Takeshita et al., 2018). In the presentation, sinistral faulting along the MTL and other major faults in east Asia during the Cretaceous to Paleogene periods will be also introduced in relation to oblique subduction of the oceanic plate.