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

Abstract

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.

The block rotation in the Uetsu area, Northern Part of Niigata Prefecture, Japan

“…However, the CCW rotation is supposed to have occurred in this area, based on the paleomagnetic data from the Atsumi area which also reports the NW-trending magnetizations (Yamaji et al, 1999).…”

“…Geological map of the Uetsu area with paleomagnetic directions reported by preceding studies. Bars and arrows with symbols indicate the paleomagnetic directions obtained by Yamaji et al (1999) and Otofuji et al (1985), respectively. Symbols mean their ages, and solid and open ones indicate paleomagnetic data from normal-and reversed-polarity sites.…”

“…The tectonic rotation is the most likely explanation of these CW deflections for the following reasons: (1) the area-mean Yamaji et al (1999) and Otofuji et al (1985), respectively. Solid circles show locations of sampling site with no reliable paleomagnetic data.…”

“…Paleomagnetic studies have shown that NE Japan consisted primarily of counter-clockwise (CCW) rotating blocks when the Japan Sea opened (Sasajima, 1981;Otofuji et al, 1985Otofuji et al, , 1994Hoshi and Matsubara, 1998;Yamaji et al, 1999). However, clockwise (CW) deflections are reported from the Takadate, Yanagawa, Matsushima areas, indicating CW rotations (Oda et al, 1989;Yamazaki, 1989).…”

“…1). They are ascribed to CW block rotations accompanied by strike-slip faultings between CCW rotating blocks (Otofuji et al, 1985;Yamaji et al, 1999). However, their paleomagnetic data are obtained mostly from a single geologic unit that covers a short period of time.…”