Identification of the youngest activity of the Median Tectonic Line at Hiji outcrop, Ina City, Nagano Prefecture

Abstract

　 This paper presents the results of geological investigation of newly excavated Hiji and Mizoguchi-kita outcrops along the Median Tectonic Line in Ina City, Nagano Prefecture. From the excavation, terrace deposit unconformably overlying fractured basement rocks along the MTL dividing of the Sanbagawa belt and the Ryoke belt clearly appear. Collapse of terrace deposit gravel into the basement rocks along newly reactivated fault can be observed in the Mizoguchi-kita outcrop, and upward extension of one of the MTL faults crosscut into the terrace deposit in the Hiji outcrop. Although organic material or tephra deposit cannot be found in the terrace deposit, we found very small amount of volcanic glass shards in silt and sand layer intercalated within the gravel. Bulk chemical composition of the glass fragments clearly show that they can be correlated with the volcanic glass of Ontake Daiichi Tephra (On-Pm1) which was erupted at c. 100 ka. Accordingly, the terrace deposit of the Hiji outcrop was deposited after c. 100 ka and the fault along the MTL is probably active fault (the fault activated after 120―130 ka in a narrow definition) even though little topographic evidence of dextral displacement can be identified in the Ina area. The fault gouge zone along this fault in the Hiji outcrop clearly show dextral shear sense which is concordant with the evidence of topographic displacement along the MTL to the south of the Ina area.