Zircon fission-track and U-Pb ages of the Green Tuff in Nishiwaga Town, Iwate Prefecture, and their implications

Abstract

We carried out fission-track (FT) and U-Pb double dating on zircon grains from six Green Tuff samples collected from Nishiwaga Town, Iwate Prefecture (northeast Japan). Based on these data, our revised stratigraphy is as follows: the Oarasawa Formation (the early Miocene ca. 20-18 Ma), which unconformably overlies the Paleozoic and Mesozoic basement; the main part of the Oishi Formation (16-15.4 Ma), which unconformably overlies the Oarasawa Formation; the Kawashiri Tuff Member (15.4-13.6 Ma) of the upper part of the Oishi Formation; and the Kotsunagizawa Formation (13.6-12 Ma), which conformably overlies the Oishi Formation. These results, which are consistent with the recently revised stratigraphy for the type section of the Green Tuff on Oga Peninsula, indicate that a tectonic change which is coupled with rapid tectonic subsidence started at ca. 16 Ma, accompanied by active felsic volcanism and counter-clockwise rotation of the region at ca. about 15.4-13.6 Ma, and then ended at ca. 12 Ma in the study area.