[H-QR04] 第四紀：ヒトと環境系の時系列ダイナミクス

[HQR04-P05] Exposure ages of uplifted marine terraces along the coast of the Sea of Japan estimated from terrestrial in situ cosmogenic radionuclides dating, Fuka’ura and Murakami area, Northeast Japan

*馬 博文¹、若狭 幸²、石山 達也¹、越後 智雄³ (1.東京大学地震研究所、2.国立大学法人弘前大学地域戦略研究所、3.株式会社環境地質)

Northeast (NE) Japan is one of the most active convergence zones in the world where deformed marine terraces are well developed and potentially record long-term permanent deformation data of the overriding plate above subducting Pacific plate in 10⁵ to 10⁶ yrs timescales. Whereas ages of marine terraces along the NE Japan coasts are well defined based on tephrochronology (c.f., Miyauchi, 1988; Koike and Machida (eds), 2001), those lacking robust evidence for chronology complicate correlations of marine terraces and determination of long-term uplift rates. In this study, we carried out terrestrial in-situ cosmogenic ¹⁰Be and ²⁶Al dating to estimate exposure ages of strath surfaces of middle marine terraces and to determine uplift rates in the study region. We collected five samples at Fuka’ura and Murakami areas from middle marine terraces distributed along the coast of Japan Sea. These survey areas are suitable for ¹⁰Be and ²⁶Al dating because marine terraces are erosional surfaces commonly underlain by quartz-rich basement rocks. Before sample collection, we identified and mapped middle marine terraces using topographic maps (at a scale of 1:2,5000), stereopairs interpretations, and a 1 to 2 m grid Digital Elevation Model (DEM). At Iwagasaki site in Murakimi, we collected two samples of saprolites within weathered late Cretaceous granitic rocks from a 40-cm deep small pit excavated on the terrace surface. At Yukiaizaki site in Fuka’ura, we also collected three samples from outcrops on the middle marine terrace surfaces comprised by Miocene quartz-rich rhyolite. After several chemical processes including purification and physical processes, we will carry out ¹⁰Be/⁹Be and ²⁶Al/²⁷Al Accelerator Mass Spectrometry (AMS) for the samples using tandem accelerators. ¹⁰Be and ²⁶Al concentrations in quartz calculated from the measured ¹⁰Be/⁹Be and ²⁶Al/²⁷Al ratios will be used for estimating exposure ages whereby considering site effects of the sample sites. Our goal is to estimate exposure ages of the marine terraces based on ¹⁰Be and ²⁶Al dating and long-term uplift rates of these rocky coasts which could lead to understanding of permanent strains accommodated by underlying active thrusts in response to plate subduction processes.

References:
Miyauchi, T., 1988, Correlation and chronology of the late Pleistocene marine terraces in northern northeast Japan. Geographical Review of Japan, Ser. A, 61A, 404-422.
K. Koike, H. Machida (Eds.), Atlas of Quaternary Marine Terraces in the Japanese Islands, University of Tokyo Press, Tokyo (2001), p. 105(in Japanese)