中新世における東北本州弧の沈降運動とそのメカニズム  [in Japanese] Miocene subsidence of the Northeast Honshu Arc and its mechanism  [in Japanese]

The Northeast Honshu Arc is covered by thick Neogene sediments. Accumulation of the sediments started under extensional tectonics in Early Miocene. The authors compiled stratigraphic data obtained from the land area to clarify the process of sedimentary basin formation on the island arc. Paleobathymetric reconstruction using benthic fossils separated the Miocene subsidence of the Northeast Honshu Arc into two stages, namely, very rapid (from 16 to 15 Ma) and thereafter slow subsidence (from 14 to 8 Ma). Magnitude of the former subsidence was about two or three kilometers in the backarc side and some hundreds meters in the forearc side. The rapid subsidence took place during the extensional tectonics, so that it was caused most likely by crustal attenuation through stretching of the arc lithosphere. Assuming the initial crustal thickness as 35 km and Airy-type local loading, the extension value (β) was estimated as 1.7〜2.3 from the magnitude of the initial subsidence of the backarc region. The stretching resulted in a few hundreds meters of thermal subsidence that agrees with the observed slow subsidence from 14 till 8 Ma.