Geology of the Idonnappu Belt, central Hokkaido, Japan: Evolution of a Cretaceous Accretionary Complex

The Cretaceous Idonnappu Belt, located along the western Hidaka Mountains of central Hokkaido Island in Japan, records evidence of west to northwest directed underthrusting of oceanic crust. The Idonnappu fault divides the Idonnappu Belt into two subbelts; the western Oku‐niikappu (ON) subbelt and the eastern Koiboku (KO) subbelt. The ON subbelt is dominated by a melange facies. It includes various thicknesses of pillow basalts, bedded radiolarian cherts, limestones, and greenish siliceous shales, all of which are intermixed with a highly sheared shaley matrix that displays a scaly cleavage. The KO subbelt is dominated by a thick flysch sequence and alternations of sandstone and shale with thin tectonic melange. The stratigraphic sequences within these belts usually young toward the west and display westward vergent structures. Outcrop‐ to microscopic‐scale structures in the melange zone of the Idonnappu Belt, however, suggest eastward vergence. Detailed biostratigraphic studies show that structural packages young toward the east ranging in age from Lower to Upper Cretaceous. These observations are consistent with an accretionary prism model in which oceanic crust is underthrust toward the west or north west. In the middle Miocene, a change to westward vergence was caused by uplift of the Hidaka Mountains.