Abstract

Quantitative evaluation of the changes in mode and rate of Quaternary crustal movement is important but is generally difficult to reconstruct with high time resolution. It is also difficult to evaluate to crustal subsidence. In this study, we demonstrate a method to reconstruct vertical crustal movement during the middle to late Quaternary on the basis of sedimentary facies analysis. The method is applicable to the cyclothemic shallow marine to nonmarine sequences such as those in the Oga Peninsula, northern Japan, whose ages have been estimated in detail. Assuming eustatic sea level is known for the last 450 ky, we estimate the temporal variation in vertical crustal movement using reconstructed temporal changes in depositional depth and accumulated sediment thickness. In the study area, a change in mode of vertical crustal movement from subsidence to uplift occurred at approximately 130 ka. Combining the results of reconstruction of vertical crustal movement at two neighboring sites, crustal movement is reconstructed, which involves a combination of tilting and uplift in the studied case. Subsidence was caused by simple crustal tilting with a fixed tilting hinge associated with folding, perhaps since ca. 450 ka. Rapid regional uplift started ca. 130 ka, all over the northeast Oga Peninsula, including the study area.

This case study demonstrates that (1) using this method, crustal subsidence is reconstructed with certainty, (2) this method is especially useful to reconstruct change in mode of vertical crustal movement from subsidence to uplift, and (3) combining results of reconstruction, it is possible to separate vertical movement from tilting. Combining this method and conservative methods using marine terraces and raised coral reefs, crustal movement in Quaternary orogenic zones will be understood more systematically.

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