Accurate determination of the Pleistocene uplift rate of the NE Japan forearc from the buried MIS 5e marine terrace shoreline angle

Abstract

We used tephra and phytolith stratigraphy to refine Pleistocene marine terrace chronology, with reference to reported eustatic sea levels associated with the dated marine terraces, and estimated uplift rates along the northern part of the northeast (NE) Japan forearc (the overriding plate in the northeast Japan subduction zone) by using elevation measurements of the buried shoreline angle beneath well-dated marine terrace surfaces to derive more accurate paleo-sea levels. On the Kamikita Plain, northernmost Honshu Island, the T4 and T3 marine terraces were correlated to marine isotope stage (MIS) 5e and MIS 7c or 7e, respectively, by the stratigraphic positions of the tephras Toya (MIS 5d) and WP (MIS 7b). The T2 terrace surface, which had previously been assigned to MIS 7, is buried by loess correlated to MIS 8 by its phytolith assemblage; therefore, this terrace can be reasonably re-correlated to MIS 9. The reported optically stimulated luminescence ages for Middle Pleistocene terrace deposits tend to be younger than these refined terrace ages. We used the buried shoreline angle elevation, determined from the reconstructed profile of the marine terrace deposits and paleo-sea cliff, to refine the uplift rate of T4 since MIS 5e. The accurate rate thus determined, 0.18–0.26 m/ky, is slower than the rates initially inferred from the inner edge of T4, but faster than the reported rate (0.14–0.19 m/ky, as recalculated here) inferred from the assumed MIS 5e shoreline elevation. In addition, it is roughly compatible with the rate inferred by using the top of T2 lagoon deposits to determine the shoreline elevation during MIS 9. Therefore, during the last 300 ka, the uplift rate of the Kamikita Plain has been constant at about 0.2 m/ky. This Pleistocene uplift is probably attributable to a combination of moment release on the megathrust, isostatic uplift driven by crustal thickening, and reverse faulting of the upper crust.