Crustal-scale strike-slip deformation in Hokkaido, northern Japan

Field surveys in Hokkaido reveal that the entire tectonic belt in northern Japan was a right-lateral shear zone from Eocene to Middle Miocene time. This zone of deformation can be followed northwards in Sakhalin for more than 2000 km, and formed at the same time as the opening of the Japan Sea. A thrusting component during the strike-slip deformation and later deformation stages led to the uplift of the deep parts of the shear zone, exposing a complete section through the upper crust. The stress field inferred from the analysis of fault sets is consistent with the non-coaxial deformation observed in the deeper ductile parts of the shear zone. There is a transition from west to east across the width of the belt (100 km) from en échelon folds, thrusts and nappes at upper structural levels, through the brittle—ductile transition with an incipient vertical schistosity, into ductile deformation with a vertical foliation and metamorphic recrystallization in the amphibolite facies. Deep parts of the crust (granulites) were uplifted and retrogressed during the strike-slip movement. Fault-set analysis together with data concerning the direction and sense of nappe emplacement imply that thrusting, west of the ductile deformation zone, occurred during the strike-slip deformation. Minor flat-lying structures coexist with vertical pure strike-slip structures. This study and previous ones reveal a crustal-scale half-flower structure with superficial thrusting associated with a deep narrow ductile zone, with a high gradient of deformation and temperature.