GROWTH OF A FORELAND FOLD-AND-THRUST BELT ON THE WEST OF THE HIDAKA COLLISION ZONE, HOKKAIDO, JAPAN

The Kuril arc in the North American (Okhotsk) plate has been colliding westward against the Northeast Japan arc in the Eurasian plate since Middle Miocene, forming the Hidaka Collision Zone (HCZ) (Kimura,1981,1986). However, neither the total shortening length associated with the collision nor the colliding process remains solved. A 70-km-wide foreland fold-and-thrust belt on the west of the HCZ is expected to present essential information on the unsolved problems.Fortunately recent release of both seismic reflection and well data for oil and gas exploration makes it possible to reveal structural characteristics of the belt as follows: 1) The Hidaka Basal Thrust (HBT) piles the pre-collisional accretionary complexes (Jurassic to Paleogene) on the Northeast Japan arc basement, forming a typical W-verging fold-and-thrust belt. 2) The HBT is bifurcated from the Hidaka Main Thrust (HMT) which juxtaposes the Kuril arc with the Northeast Japan arc, functions as a basal detachment at a depth of about 5km, and emerges at the western front of the fold-and-thrust belt. 3) The activity of the fold-and-thrust belt has been migrating westward forming a foreland depression at the front of the belt. The present activity is concentrated in the frontal part of the belt where a lot of active surface faults occur. These characteristics indicate that the fold-and-thrust belt has been growing by the development of the HBT directly bifurcated from the HMT in the HCZ. The total shortening length is evaluated to be at least 50km in the fold-and-thrust belt based on the restored whole profile of the belt. This means the average shortening rate is 3.5 mm/y since the beginning of the collision in Middle Miocene. This rate represents 40% of the convergent rate between the Okhotsk and the Eurasian plates (Seno et al., 1996).