Abstract
We describe results of a paleomagnetic study for sedimentary rocks of the Plio-Pleistocene Sasaoka Formation in the Gojome area in Akita Prefecture, northeast Japan. Samples were collected from 26 sites over the Gojome Syncline, an open syncline with a nearly horizontal fold axis. Progressive thermal demagnetization revealed high-temperature characteristic remanences from 11 sites ; 5 sites on the lower part of the measured section had normal polarity, and 6 sites on the upper part were reversely magnetized. The incremental fold test showed that the best grouping of the remanence directions occurred when the syncline was 80% unfolded. Unfolding it at the same percentage climaxed the anti-parallelism of a set of normal remanences and that of reversed ones. The possibility that the rocks acquired the remanences during folding (synfolding remanence) seems to be unlikely from the stratigraphically ordered polarity sequence. Alternatively, it is suggested that strain has altered the initial angular relationship between the bedding plane and the remanence. For this reason the discussion on vertical-axis crustal rotation becomes only qualitative, but little rotation may be deduced from the northerly mean directions at all stages of unfolding. The normal-to-reversed polarity-reversal horizon at the middle part of the measured section is correlated, with the aid of available biostratigraphic information, to the Gauss-Matuyama chron boundary at about 2.6 Ma.
