Field Survey of the Sambagawa Metamorphic Belt in Tokushima

The Sambagawa Metamorphic Belt is one of the most best-known high-pressure (HP) metamorphic belts in southwestern Japan. It is considered to represent a former subduction-zone complex that formed mainly during the Cretaceous and was subsequently exhumed to the Earth's surface. The belt extends for approximately 800 km from the Kanto Mountains through the Kii Peninsula and Shikoku to Kyushu. Most of the belt lies south of the Median Tectonic Line (MTL), a major tectonic boundary separating the Sambagawa Belt from the Ryoke Belt to the north.
The Sambagawa Belt consists mainly of pelitic, psammitic, siliceous, calcareous, and mafic schists, accompanied by smaller amounts of metagabbro, ultramafic rocks, chert, and eclogite bodies. These rocks originated from materials such as oceanic crust, basaltic volcanic rocks, seamounts, pelagic sediments, trench sediments, and continent-derived clastic sediments that were transported into a subduction zone and subsequently metamorphosed. The Sambagawa Belt therefore provides an important natural laboratory for investigating the transport and recycling of crust- and mantle-derived materials associated with subduction processes.
The Sambagawa Belt is widely exposed in eastern Shikoku, including Tokushima Prefecture. Professor Minami and first-year master's student Sato of the Archaeomagnetic Research Group of TranSEHA conducted a field survey around Mt. Kotsu in Tokushima Prefecture on September 3–4, 2026, together with Professor Azizi of the University of Kurdistan, Iran, Professor Anma and Associate Professor Aoya of Tokushima University, and Associate Professor Asahara of Nagoya University. During the survey, they collected eclogitic blueschist and greenschist from the Sambagawa Metamorphic Belt, as well as rocks of the Mikabu Greenstone complex, including metagabbro and mafic phyllite.
These schists are characterized by well-developed schistosity and tend to split readily into thin slabs, making them relatively easy to shape into flat forms. For this reason, they have frequently been found as raw materials for polished stone tools at archaeological sites dating to the Middle Yayoi period. Blueschist, in particular, is known to have been used as a high-quality stone material compared with greenschist. Greenschist, on the other hand, is more susceptible to weathering due in part to the presence of minerals such as chlorite, and this property may have affected its suitability as a raw material for stone tools. Future investigations of the petrological and geochemical characteristics of the collected samples and their relationship to their use as stone-tool materials are expected to provide further insights into the selection and use of these rocks, as well as their provenance and distribution.