Paleomagnetic study of Plio-Pleistocene sediments in the concentrated deformation zone along the eastern margin of the Japan Sea

Abstract

Paleomagnetic results are presented for a concentrated deformation zone (CDZ) in a convergent plate boundary region along the eastern Eurasian margin. We sampled late Pliocene–early Pleistocene tuffs and clastic sediments of the Sasaoka Formation (∼750 m thick) in northeastern Honshu, Japan, to test whether the late Pliocene–Quaternary crustal deformation within the CDZ along the eastern margin of the Japan Sea was accompanied by rotation about a vertical axis. Rock magnetic experiments suggest that the principal magnetic carrier is magnetite in the fine tuffs, and magnetic iron sulfide in the fine sandstones. Pre-folding characteristic remanent magnetization was confirmed using a positive bootstrap fold test. We obtained 21 acceptable site-mean characteristic directions that include our preliminary published results, and which cover an interval from ca. 2.7 to 1.7 Ma on the basis of magnetostratigraphic correlations. An updated age–depth model is given, and this allowed us to make numerical age estimates for key interbedded tuff beds (tephras). A positive fold test also suggests that the Gojome syncline began to develop after 1.7 Ma, which means the folding began long after the initiation of late Pliocene crustal shortening in northeastern Honshu. After 100% unfolding, the overall mean direction (D = 359.7°, I = 54.9°, α95 = 6.7°) is indistinguishable from the geocentric axial dipole field direction, indicating that the Gojome syncline, the most prominent structural element in the study area, developed without vertical-axis rotation. A comparison of our results with Plio-Pleistocene directions reported from other areas reveals no paleomagnetically detectable rotation in or adjacent to the CDZ, except for local rotation near strike-slip faults in central Honshu.