Estimating the amount of uplift from the current elevation of strata at the Middle Miocene Climatic Optimum: A case study of Kibi Plateau in Southwest Japan

The Middle Miocene Climatic Optimum (MMCO) formed shallow marine strata on continents and island arcs at ~15 Ma, and some of the MMCO strata later experienced uplift. The amount of uplift corresponded to sea level rise after the MMCO period, but ancient sea level could not be observed. This study uses a deductive approach to estimate the amount of uplift. The elevation of the youngest marine sedimentary layer of the MMCO strata approximates the sea level at the MMCO. The current elevation was reconstructed by adding the amount of denudation from the MMCO to the current elevation of the denuded MMCO strata at the outcrops, considering isostatic compensation. The amount of denudation since the MMCO period was estimated to be 180 m based on the global average denudation rate (Willenbring et al., 2013), and the estimated isostatic rebound was 153 m. By comparing the current, albeit virtual, elevation of the youngest marine sedimentary layer in the MMCO strata with sea level at the MMCO, the amount of uplift was estimated. For the MMCO sea level, a 66 m ice-free line was adopted. This method was applied to the Kibi Plateau in Southwest Japan, where the MMCO strata are distributed. The reliability of the denudation amount of 180 m for the MMCO strata was confirmed by paleo-water depths estimated using benthic foraminifera and mollusk fossils. The largest uplift of 504 ± 27 m occurred at the Jinseki-Kogen since 15 Ma. A positive correlation was observed between the amount of uplift and the current altitude of the Kibi Plateau. The Kibi Plateau's topography most likely evolved after the MMCO. It is concluded that a new perspective on estimating the uplift was obtained by taking denudation into account.