Paleomagnetic evidence for Miocene transtensional deformations at the eastern margin of the Japan Sea

The opening of the Japan Sea backarc basin accompanied dextral, transtensional deformations in the NE Japan arc. To understand the intra-arc deformation we have studied paleomagnetism in the Uetsu area, NE Japan. Among the samples collected at 70 sites for paleomagnetic measurement reliable directions were obtained at 38 ones. About half of the sites yielded clockwise and the others show opposite declinations. Our data indicate that the northern Uetsu area rotated counterclockwise as a single block, whereas the southern Uetsu area was broken into a number of blocks that was rotated clockwise by the dextral transtension along the Nihonkoku-Miomote Line. Not only the Uetsu area but the entire eastern margin of the Japan Sea experienced such transtensional deformations in the Early to early Middle Miocene. The crust under NE Japan was broken into blocks but they rotated coherently with dextral transtension.