[H-GM03] Geomorphology

[HGM03-P01] Exposure ages of middle-late marine terraces deduced from terrestrial in situ cosmogenic radionuclides dating in Sanriku coast, Northeast Japan

*Sachi Wakasa¹, Tatsuya Ishiyama², Daisuke Hirouchi³, Nobuhisa Matsuta⁴, Natsuko Fujita⁵, Tomoo Echigo⁶ (1.Institute of Regional Innovation, Hirosaki University, 2.Earthquake Research Institute, University of Tokyo, 3.Faculty of Education, Shinshu University, 4.Okayama University Graduate School of Education, 5.Tono Geoscience Center, Japan Atomic Energy Agency, 6.Kankyo Chishitsu Co., Ltd)

Keywords:Marine Terrace, Terrestrial in situ Cosmogenic Radionuclide dating, 10Be, Sanriku

Flights of Pleistocene marine terraces along the Sanriku area (eg., Miura, 1968; Miyauchi, 1985) has been widely interpreted as geomorphic manifestation of long-term coastal uplift along the Pacific coast associated with subduction of the Pacific plate. However, mainly due to insufficient age constraints on these geomorphic markers, long-term rates and mechanisms of coastal uplift still remain poorly resolved. In this study, we newly estimated surface exposure ages using terrestrial in situ cosmogenic radionuclide dating of exposed bedrock surfaces to define more accurate formations ages and long-term rates of coastal uplift of marine terraces along this coastal region. Based on reinterpretation of distribution of marine and fluvial terraces using stereopairs of aerial photos, we collected samples at two sites (Samuraihama in Kuji and Yoshihama in Ofunato) located along the northern and southern Sanriku coast, respectively. These sites are expectedly suitable for applying this dating method where marine and fluvial terraces are commonly in nature of bedrock strath surfaces comprised by Cretaceous quartz-rich granodiorite. At Samuraihama site, we collected three samples from outcrops of pairs of marine and fluvial terraces distributed on the east-facing flank of the northeastern Kitakami Mountains. In contrast, at Yoshihama site where bedrock surfaces are poorly exposed, we sampled vertically from weathered granite rocks on small trench walls excavated on the middle marine terrace along the Yoshihama Bay. After pretreating processes including mineral separation and purification of quartz to generate targets for accelerator mass spectrometry, we carried out ¹⁰Be/⁹Be mass spectrometry for the samples at the Tono Geoscience Center, Japan Atomic Energy Agency (JAEA) and Purdue Rare Isotope Measurement Laboratory (PRIME Lab), Purdue University using tandem accelerators. ¹⁰Be concentrations in quartz calculated from the measured ¹⁰Be/⁹Be ratios are then used for estimating surface exposure ages of the sample sites, considering site effects. In this presentation, we will mainly discuss on age estimates of the marine terraces constrained by our new ¹⁰Be exposure ages. This research was partly funded by the JSPS Grant-in-Aid for Scientific Research (Challenging Sprouting Research, PI: T. Ishiyama).

References:
Koike, K and Machida, H. eds., 2001, Marine terraces Atlas of Japan. 115 pp. University of Tokyo Press, Tokyo (in Japanese with English Abstr).
Miura, O., 1968, Rias coast and coastal terraces of sanriku, northeastern Japan. Geographical Review of Japan, 41, 732-747 (in Japanese with English Abstr).
Miyauchi, T., 1985, Quaternary crustal movements estimated from deformed terraces and geologic structures of the Kamikita coastal plain, northeast Japan. Geographical Review of Japan, 58 (Ser. A)-8, 492-515 (in Japanese with English Abstr).