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Kinematic analyses and radiometric dating of the Paleogene two-phase and large-scale faulting along the Median Tectonic Line, south-west Japan
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| Title: | Kinematic analyses and radiometric dating of the Paleogene two-phase and large-scale faulting along the Median Tectonic Line, south-west Japan |
| Other Titles: | 西南日本中央構造線に沿う古第三紀の２回の大規模断層活動の運動像と放射年代の解析 |
| Authors: | 窪田, 安打 Browse this author |
| Keywords: | kinematic history | | Median Tectonic Line | | normal faulting | | inistral-reverse faulting | | K-Ar geochronology | | Paleogene |
| Issue Date: | 23-Mar-2017 |
| Publisher: | Hokkaido University |
| Abstract: | The Median Tectonic Line (MTL) in southwest Japan, a major east–west-trending arc-parallel fault, has been defined as the boundary fault between the Cretaceous Sambagawa metamorphic rocks and Ryoke granitic and metamorphic rocks, which are unconformably covered by the Upper Cretaceous Izumi Group. For the long time span of the Paleogene period (66-23 Ma), the fault movement along the MTL has not been fully clarified. Based on detailed fieldwork and previous studies, the authors reconstruct the kinematic history along the MTL during the Paleogene period. The authors proposed in the previous studies that the Paleogene kinematic history of the MTL is divided into the Ichinokawa phase and Pre-Tobe phase: the Ichinokawa phase is large scale normal faulting accompanied by a wide fracture zone along the MTL, while the Pre-Tobe phase is defined as the activation of of the parallel faults to the MTL in the Upper Cretaceous Izumi Group, which are perhaps connected to the MTL at depth. In this study, the author has analyzed the kinematic movement along the MTL at the Pre-Tobe phase, based on the field survey and microstructural analyses in fault rocks. We found that en échelon folds oriented clockwise for the fault of map-scale were developed along the parallel faults, which are accompanied by a cataclasite zone of 60 to 25 m wide consisting of foliated cataclasite, cataclasite, and fault gouge zone. The map-scale sorth-vergent folds in the disturbed zone and microstructures (e.g. composite planar structures) in the cataclasite zone which accompanies the parallel faults strongly suggest that they formed due to sinistral and reverse faulting with a top-to-the-SW sense along the MTL. Moreover, based on previous studies and the K-Ar age measurements of authigenic illite from fault gouge along the parallel faults and MTL in this study, it has been found that the MTL was activated at two discrete stages of 60-57 Ma and 41 Ma. A combination of kinematic analyses with K-Ar age dating of authigenic clay minerals from the cataclasite zones allows us to reconstruct the kinematic history of the MTL: normal faulting during 60-57 Ma (Ichinokawa phase) and sinistral and reverse faulting during 41 Ma (Pre-Tobe phase). The MTL as a large-scale, arc-parallel fault records the movements of subducting oceanic plate relative to continental plate. According to previous studies, the movement direction of the subducting Pacific oceanic plate relative to proto-southwest Japan arc had changed from almost that normal to the arc at the Ichinokawa phase to that oblique to the one to generate sinistral slip along the MTL at Pre-Tobe phase. Therefore, the change in kinematic movement along the MTL and large-scale arc-parallel faults in fact reflects the movements of subducting oceanic plate relative to continental plate. |
| Conffering University: | 北海道大学 |
| Degree Report Number: | 甲第12693号 |
| Degree Level: | 博士 |
| Degree Discipline: | 理学 |
| Examination Committee Members: | (主査) 教授 竹下 徹, 教授 永井 隆哉, 准教授 亀田 純 |
| Degree Affiliation: | 理学院（自然史科学専攻） |
| Type: | theses (doctoral) |
| URI: | http://hdl.handle.net/2115/68523 |
| Appears in Collections: | 学位論文 (Theses) > 博士 （理学）
課程博士 (Doctorate by way of Advanced Course) > 理学院(Graduate School of Science)
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