5:15 PM - 6:30 PM
[SGL23-P04] Publication of the 1:50,000 geological map of the Ikeda District
Keywords:Geological map, Ryoke Complex, Izumi Group, Sanbagawa Complex, Mitoyo Group, Median Tectonic Line
Geological map of the Ikeda District, which is located in the northeastern part of Shikoku Island, has been published. The basement rocks are divided into the Inner and Outer zones of Southwest Japan by the Median Tectonic Line (MTL). The Inner Zone is composed of the Ryoke Complex, the Izumi Group (Upper Cretaceous), the Setouchi volcanic rocks (Miocene), and the Mitoyo Group (Lower Pleistocene). The Outer Zone comprises the Sanbagawa Complex (Upper Cretaceous–Paleogene) and the Dochu Formation (Lower Pleistocene). The southern margin of the Sanuki Mountain Range is bounded by active faults of the MTL, which include the Ikeda, Mino, and Hashikura faults, and they cut the lower terrace deposits (Upper Pleistocene) and the Holocene. The northern margin of the mountain range is bordered by Takenari, Ebata, and Kashihara faults; the Takenari Fault deformed the Mitoyo Group (Lower Pleistocene), and the Ebata Fault partly cut the lower terrace deposits (Upper Pleistocene). Landslide deposits are common in this district, especially in the southern part of the mountain range. Therefore, this district is an important area in terms of understanding of the basement geology as well as geohazards of earthquakes and landslides.
The Ryoke Complex comprises high-T/P metamorphic rocks, granitic rocks (Kan-onji Granite), and dikes. The protolith age of the metamorphic rocks is Late Permian based on the youngest peak age (258 Ma). The zircon U–Pb ages of granites and dikes range between 94 and 88 Ma. The Izumi Group unconformably overlies the Kan-onji Granite, which is composed of the northern marginal facies and the main facies. The northern marginal facies is characterized by conglomerate and massive mudstone, while the main facies is composed of alternating beds of sandstone and mudstone with felsic tuff beds. Ammonite fossils (Metaplacenticeras subtilistriatum zone) and zircon U–Pb ages (79–78 Ma) of felsic tuff beds indicate that the depositional age is the middle Campanian, Late Cretaceous. The Sanbagawa Complex is subdivided into the Itano, Minawa, and Koboke units from north to south. They are composed of mafic, pelitic, and psammitic schist. U–Pb ages of detrital zircons extracted from psammitic schists (93–81 Ma for Minawa unit and 76–74 Ma for Koboke unit) and K–Ar ages of phengite (74–72 Ma for Minawa unit and 65–62 Ma for Koboke unit). The Lower Pleistocene includes the Mitoyo Group (Saita, Koda, and Yakeo formations), Norouchi and Dochu formations. Formations of the Mitoyo Group are interfingered each other. Two widespread tephras constrain the depositional ages (2.4–2.3 Ma for the Yakeo and 1.75 Ma for the Saita). The Metasequoia sp. fossils also occur in the Mitoyo Group and the Dochu Formation.
Based on the above evidences, the geological history in this district can be summarized as follows. In the arc side, granitic magmas continuously provided to the lower crust that caused high-temperature and low-pressure metamorphism during the Late Cretaceous (94–88 Ma). At the same time (93–74 Ma), trench-fill deposits subducted beneath a forearc wedge to the depths of 20–35 km along the plate boundary, and then experienced low-temperature and high-pressure metamorphism at the later stage of subduction (74–62 Ma). In addition, the Kan-onji Granite was exposed on the seafloor and unconformably overlain by the Izumi Group (79–78 Ma). The simultaneous occurrences of magmatic, metamorphic, and depositional events recognized in this district imply an importance of flow/transport paths of materials in a forearc, which include supply of continuous magma to the lower crust, uplift and exhumation of the upper crust, sediment transport and deposition on the arc, subduction of trench-fill sediments beneath the arc. Finally, in response to the regional stress change in Southwest Japan since early Pleistocene, the Sanuki Mountain Range began to uplift and the Mitoyo Group and Dochu Formation deposited at the both sides of the mountain range.
The Ryoke Complex comprises high-T/P metamorphic rocks, granitic rocks (Kan-onji Granite), and dikes. The protolith age of the metamorphic rocks is Late Permian based on the youngest peak age (258 Ma). The zircon U–Pb ages of granites and dikes range between 94 and 88 Ma. The Izumi Group unconformably overlies the Kan-onji Granite, which is composed of the northern marginal facies and the main facies. The northern marginal facies is characterized by conglomerate and massive mudstone, while the main facies is composed of alternating beds of sandstone and mudstone with felsic tuff beds. Ammonite fossils (Metaplacenticeras subtilistriatum zone) and zircon U–Pb ages (79–78 Ma) of felsic tuff beds indicate that the depositional age is the middle Campanian, Late Cretaceous. The Sanbagawa Complex is subdivided into the Itano, Minawa, and Koboke units from north to south. They are composed of mafic, pelitic, and psammitic schist. U–Pb ages of detrital zircons extracted from psammitic schists (93–81 Ma for Minawa unit and 76–74 Ma for Koboke unit) and K–Ar ages of phengite (74–72 Ma for Minawa unit and 65–62 Ma for Koboke unit). The Lower Pleistocene includes the Mitoyo Group (Saita, Koda, and Yakeo formations), Norouchi and Dochu formations. Formations of the Mitoyo Group are interfingered each other. Two widespread tephras constrain the depositional ages (2.4–2.3 Ma for the Yakeo and 1.75 Ma for the Saita). The Metasequoia sp. fossils also occur in the Mitoyo Group and the Dochu Formation.
Based on the above evidences, the geological history in this district can be summarized as follows. In the arc side, granitic magmas continuously provided to the lower crust that caused high-temperature and low-pressure metamorphism during the Late Cretaceous (94–88 Ma). At the same time (93–74 Ma), trench-fill deposits subducted beneath a forearc wedge to the depths of 20–35 km along the plate boundary, and then experienced low-temperature and high-pressure metamorphism at the later stage of subduction (74–62 Ma). In addition, the Kan-onji Granite was exposed on the seafloor and unconformably overlain by the Izumi Group (79–78 Ma). The simultaneous occurrences of magmatic, metamorphic, and depositional events recognized in this district imply an importance of flow/transport paths of materials in a forearc, which include supply of continuous magma to the lower crust, uplift and exhumation of the upper crust, sediment transport and deposition on the arc, subduction of trench-fill sediments beneath the arc. Finally, in response to the regional stress change in Southwest Japan since early Pleistocene, the Sanuki Mountain Range began to uplift and the Mitoyo Group and Dochu Formation deposited at the both sides of the mountain range.