Late Holocene tectonics inferred from emerged shoreline features in Higashi-Izu monogenetic volcano field, Central Japan

Abstract

Emerged shoreline features such as morphology, stratigraphy, and fossil assemblages are useful to reconstruct the history of vertical crustal movement in coastal areas. In particular, resolving the history of individual tectonic events can help understand regional seismo- and volcano-tectonic processes. In the eastern coast of the Izu Peninsula, central Japan, multiple levels of emerged shoreline features formed as a result of repeated uplift caused by the activity of Higashi-Izu monogenetic volcano field or related seismo-tectonic processes during the Late Holocene. To clarify the history of uplift events and the mechanism of regional tectonics, we surveyed these features with a focus on fossil sessile assemblages. Based on an analysis of their height and age, three zones associated with intermittent emergence events, denoted Zone 1 (1.05-m uplift in 595–715 CE), Zone 2 (1.33-m uplift in 1356–1666 CE), and Zone 3 (0.82-m uplift after 1830 CE), were identified. The interval between events was 400–800 years. Zone 3 can be regarded to have emerged via a stepwise but gradual uplift that lasted 130 years. This movement was caused by geodetically observed volcanic crustal deformation with earthquake swarms. We infer that zones 1 and 2 were uplifted by similar intervals of volcanic deformation, but cannot discard coseismic contributions from offshore faults as a possible explanation. The timing of these emergence events coincides with periods of both increased volcanic and seismic activities in the vicinity, suggesting that they are interrelated. Based on the height and age of the additional older assemblage denoted Zone 0 overlying Zone 1 and those of the post-glacial transgressive deposits in the adjacent areas, we suggest that the crustal uplift of the Izu Peninsula was initiated around 3000 years ago, which coincides with the largest eruption of Kawagodaira volcano. Therefore, volcanic activity is an important factor governing coastal uplift in this region.