10:45 AM - 11:00 AM
[HQR05-07] Holocene vertical deformation at lower reach of the Mano River, southern part of Pacific coast, Northeastern Japan
Keywords:crustal movement, Holocene, Pacific Coast NE Japan, sediment core, 14C ages
Temporal inconsistency in vertical deformation between long-term (> 103 years) uplift and short-term (101 – 102 years) subsidence along the Pacific coast northeast Japan has been discussed in relation to cycle of megathrust earthquakes (e.g., Ikeda et al., 2012, J. Geol. Soc. Jpn.). Estimation of millennial-scale crustal movement (e.g., Niwa and Sugai, 2020, Mar. Geol.) and re-examination of uplift on a timescale of 105 years (e.g., Miyazaki and Ishimura, 2018, Chigaku-Zasshi) along the northern part of the Pacific coast (Sanriku coast), which were conducted after the 2011 Tohoku-oki earthquake, have contributed to consideration of issues concerning the temporal inconsistency throughout providing new data on long-term vertical deformation. However, such study has not been fully conducted yet along the southern part of the coast except for report of northward decreasing of uplift rate during the past 100kyr on the basis of distribution of MIS 5.5 marine terrace (Suzuki, 1989, Geogr. Rep. Tokyo Metrop. Univ.).
This study estimates millennial-scale vertical deformation in the lower reach of the Mano River (Mano River Lowland), southern part of the Pacific coast northeast Japan, on the basis of well-dated Holocene sediment cores and existing boring logs. Analyses of existing boring logs indicate that buried terrace and main axis of incised valley are located at northern and southern parts of the lowland, respectively. Lithofacies, diatom assemblages, and total sulfur content of four sediment cores document the transgression and regression of back-barrier estuary in wave-dominated estuary system during the Holocene; incised valley fills consist of early Holocene marsh sediments, Middle Holocene sediments of central basin to bay-head delta in wave dominated estuary, Middle Holocene intertidal sediments, and subsequent Middle to Late Holocene fluvial sediments. Lowness of relative sea-level (RSL) inferred from height of intertidal sediments ca. 6000 cal BP relative to non-tectonic hydro-isostatic sea-level predictions (Okuno et al., 2014, Quat. Sci. Rev.) indicates millennial-scale subsidence trend of 0.2 to 1.1 mm/yr at the Mano River Lowland. However, the Middle Holocene RSL in the study area estimated from the sedimentary environment at that time is more than 8.2 m higher than that in the Tsugaruishi plain, Sanriku coast, where millennial-scale subsidence of 1.1 ~ 1.9 mm/yr was inferred (Niwa et al., 2017, Quat. Int.). This indicates that the Mano River lowland has been uplifted relative to the Tsugaruishi plain on a millennial scale (> ca. 1mm/yr). Distribution of MIS 5.5 marine terrace, which shows northward decreasing of uplift rate during the past 100kyr along the southern part of the Pacific coast, and positional relation that the Mano River Lowland is located at northern limit of MIS5.5 marine terrace distribution (Suzuki, 1989, Geogr. Rep. Tokyo Metrop. Univ.) indicates stable or slight uplift trend at the lowland during the past 100kyr. These imply that change of vertical deformation trend from stable or slight uplift to slight subsidence occurred in the Mano River Lowland at sometimes between Late Pleistocene and Early Holocene.
This study estimates millennial-scale vertical deformation in the lower reach of the Mano River (Mano River Lowland), southern part of the Pacific coast northeast Japan, on the basis of well-dated Holocene sediment cores and existing boring logs. Analyses of existing boring logs indicate that buried terrace and main axis of incised valley are located at northern and southern parts of the lowland, respectively. Lithofacies, diatom assemblages, and total sulfur content of four sediment cores document the transgression and regression of back-barrier estuary in wave-dominated estuary system during the Holocene; incised valley fills consist of early Holocene marsh sediments, Middle Holocene sediments of central basin to bay-head delta in wave dominated estuary, Middle Holocene intertidal sediments, and subsequent Middle to Late Holocene fluvial sediments. Lowness of relative sea-level (RSL) inferred from height of intertidal sediments ca. 6000 cal BP relative to non-tectonic hydro-isostatic sea-level predictions (Okuno et al., 2014, Quat. Sci. Rev.) indicates millennial-scale subsidence trend of 0.2 to 1.1 mm/yr at the Mano River Lowland. However, the Middle Holocene RSL in the study area estimated from the sedimentary environment at that time is more than 8.2 m higher than that in the Tsugaruishi plain, Sanriku coast, where millennial-scale subsidence of 1.1 ~ 1.9 mm/yr was inferred (Niwa et al., 2017, Quat. Int.). This indicates that the Mano River lowland has been uplifted relative to the Tsugaruishi plain on a millennial scale (> ca. 1mm/yr). Distribution of MIS 5.5 marine terrace, which shows northward decreasing of uplift rate during the past 100kyr along the southern part of the Pacific coast, and positional relation that the Mano River Lowland is located at northern limit of MIS5.5 marine terrace distribution (Suzuki, 1989, Geogr. Rep. Tokyo Metrop. Univ.) indicates stable or slight uplift trend at the lowland during the past 100kyr. These imply that change of vertical deformation trend from stable or slight uplift to slight subsidence occurred in the Mano River Lowland at sometimes between Late Pleistocene and Early Holocene.