Paleomagnetic Study of Plio-Pleistocene Sediments in the Concentrated Deformation Zone Along the Eastern Margin of the Japan Sea

Abstract:

Along the eastern margin of the Japan Sea, there is a concentrated deformation zone in which Miocene and younger strata have been strongly folded and faulted within a Quaternary compressional stress regime. An interpretation that the zone is an incipient Eurasian-Okhotsk plate boundary has been proposed, based on several large-magnitude earthquakes occurring in this zone during historic times. In this study we conducted a paleomagnetic and rock magnetic investigation of Plio-Pleistocene sediments of the Sasaoka Formation in northern Honshu, Japan, in order to test whether the compressional deformation is accompanied by rotation about a vertical axis. Despite the possible presence of magnetic iron-sulfides in fine clastic sediments, pre-folding remanent magnetization is confirmed by a bootstrap fold test and reversals test. Recognizable felsic tuffs mostly have stable remanent magnetization carried by magnetite. Site-mean remanent magnetization directions were determined for 23 sites, which cover an interval from ca. 2.7 Ma to 1.7 Ma on the basis of magnetostratigraphic correlation. The resultant overall mean direction is indistinguishable from the geocentric axial dipole field direction, suggesting no rotation. Comparison of this result with Plio-Pleistocene directions reported from other areas illuminates no detectable rotation in and adjacent to the deformation zone, except for local rotations along strike-slip faults.