DELAMINATION-WEDGE-SUBDUCTION SYSTEM IN THE HIDAKA COLLISION ZONE (HCZ) OF ARC-ARC TYPE, HOKKAIDO, JAPAN: A FACTORY OF CONTINENTAL CRUST

The Kuril arc in the Okhotsk plate has been colliding westward against the Northeast Japan arc since Middle Miocene, forming the Hidaka collision zone (HCZ) of arc-arc type. A series of deep seismic reflection studies have revealed the fundamental structures in the HCZ as follows: (1) The lower crust of the Kuril arc is delaminated at about 23 km deep. The upper half (upper crust + upper portion of lower crust) of the Kuril arc thrusts over on the Northeast Japan arc along the Hidaka Main Thrust (HMT), whereas the lower half (lower portion of lower crust + upper mantle) is descending down. (2) The wedge of the Northeast Japan arc is intruded into the delaminated Kuril arc. (3) The Pacific plate is subducting northward beneath the structures mentioned in (1) and (2). These structures function as a system necessary for continuation of the arc-arc collision, otherwise the collision must be interrupted. We call it "Delamination-wedge-subduction system". This system must provide the following mechanism to grow continental crust in the HCZ. (1) The lower portion of lower crust which is the most mafic in the whole crust is split from the main body of the Kuril arc by the delamination, and transported into mantle by the subducting Pacific plate. This means that the ratio of mafic material is relatively reducing during the collision. (2) The collision brings crustal thickening by the growth of the delamination-wedge structure. Thus a mafic-poor merger of the Kuril and the Northeast Japan arcs is thickening, and growing up into a nucleus of continental crust. The HCZ provides a good model of a factory of continental crust through an arc-arc collision.