Miocene Subduction of an Active Mid-Ocean Ridge and Origin of the Setogawa Ophiolite, Central Japan

The Setogawa subduction complex represents the subduction of an active mid-ocean ridge (the Kula-Pacific Ridge) under the Japan Arc about 20 m.y. ago in the early Miocene. The basalt is youngest in subzone B3 of the Setogawa Belt and becomes older in both landward and oceanward subzones. Accumulation time for pelagic sediments overlying the basalt lengthens in the same two directions. The basalt of subzones B2 and B3 is directly overlain by or contemporaneous with the hemipelagic sediments. In subzone B3, the Setogawa dismembered ophiolite, including basalt, ultramafic rocks, gabbro, tonalite, and trondh-jemite was emplaced from the mid-ocean ridge near the subduction zone.