Mid-Holocene Underwater Jomon Sites along the West Coast of Kyushu, Japan, Hydro-isostasy and Asthenospheric Viscosity.

Abstract

Mid-Holocene underwater Jomon sites typically observed along the west coast of Kyushu, Japan, are reasonably explained by the hydro-isostatic adjustment due to the last deglaciation. The distribution of these underwater Jomon sites is very sensitive to the rheological structure above 250km depth, and comparisons between observations and predictions of sea-level variations indicate a lithospheric thickness of 30-50km and an effective asthenospheric viscosity (-200km thickness) of (8-20)×1019Pa s. The viscosity contrast between the asthenosphere and the underlying upper mantle is less than 10, and a well defined low-viscosity asthenosphere may not exist even in the island arc regions such as the Japanese Islands. Thus regional postglacial sea-levels are spatio-temporally variable and dependent on crustal tilting caused by hydro-isostatic adjustment, and they might have constrained the development of prehistoric human settlements and the way of lives.