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Hidakasanmyaku Erimo Tokachi NP, Hokkaido
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Here, rocks form a near-continuous slice from upper mantle, across the mantle–crust boundary, into upper crust. An Environment Ministry expert review says only Pakistan’s Kohistan belt offers a comparable section.
At the Higashi Fuyushima Tunnel in Samani’s Hidaka Yabakei, a roadside outcrop reveals a branch of the 140-km Hidaka Main Thrust: dark serpentinite beside green metagabbro, once oceanic rock caught in the collision.
About 12–13 million years ago, a southwest-moving slice of the Kuril forearc began colliding with northeast Japan’s arc. The squeeze raised deep crust and mantle to build the Hidaka Range.
At the range’s southern end, the 8 × 10 km Horoman peridotite body brings upper-mantle rock to the surface. Its layered stack reaches 3 km thick, preserving clues to processes deep inside Earth.
Hidaka jade is a look-alike, not true jade: chromium gives its diopside the green color, rather than jadeite or nephrite. It was once made into jewelry and souvenirs, until scarcity ended the trade.
West of the Hidaka Range, blue-green blueschist in the Kamuikotan Zone records subduction: basalt transformed under high pressure but relatively low temperature, with blue amphibole minerals coloring the rock.
Seismic imaging places the 1970 M 6.7 Hidaka earthquake on the deep continuation of the Hidaka Main Thrust. Researchers interpret the earthquake and surrounding stresses as evidence of ongoing arc collision.
