Age and Deformation of Marine Terraces on the Ashizuri Peninsula and its Vicinity, Southwestern Japan

Abstract

This paper aims to establish the late Quaternary chronology of marine terraces and pattern and rate of late Quaternary tectonic deformation of a coastal area over ca. 50 km long from Cape Ino to Cape Ashizuri along the Tosa Bay, based on late Pleistocene and Holocene marine terrace data. This area is close to the Nankai Trough on the boundary between the Philippine Sea plate and Eurasia plate, and is known as a seismically active area.
Marine terraces are classified into Hi, H2, H3, M and L terraces in descending order. The M terrace is identified as the last interglacial terrace that corresponds to oxygen isotope stage 5e with an age of ca. 125 ka, judging by the following evidences : 1) This terrace is the most continuous, and has the best-preserved surface ; 2) It is underlain by relatively thick (up to 20 m) transgressive deposits in the vicinity of large river mouths, indicating that it formed in association with a rising sea level ; 3) Amino acid ratio of shells in the transgressive deposits implies an age of ca. 138ka (Mitsushio et al., 1989); and 4) Dune sand overlying the terrace surface includes K-Tz tephra of ca. 75-95 ka BP, demonstrating that the emergence of the M terrace surface was older than this age.
A eustatic origin is also inferred for the formation of H2 and H1 terraces, which are continuously traced within the study area. Judging by the close positive relationship between the former shoreline height of M terrace and that for H1 and H2 terraces, the ages for H1 and H2 terraces are estimated to be ca. 270 ka and 176 ka, respectively, although no direct evidence was obtained for the age determination of these high terraces. L terrace is Holocene in age and is subdivided into at least 3, and possibly 4 steps, implying that episodic uplift has repeated during the Holocene. Li, the highest Holocene terrace, represents the culmination of Holocene sea level rise and L3 is dated at ca. 500-700 yr BP, older than the last 1946 coseismic uplilt of ca. 0.4 m along the western part of the Ashizuri Peninsula.
The former shoreline of the M terrace now ranges in height from 20 m to 75 m with generaly northwestward (landward) downtilting. The study area can be subdivided into two subregions, Cape Ino subregion and Ashizuri subregion, both of which also tilt northwestward, when we consider that the abnormally low height at the northern margin of the Ashizuri Peninsula is an extension of Cape Ino fault, which represents one of subsidary faults from the main subduction thrust (Kato, 1983). The causative subsidary fault which is responsible for the tilting of Ashizuri subregion is not identified, however. Progressive northwestward tilting is obvious from the former shoreline height of the older terraces. L terrace also tilts northwestward. Therefore it can be said that this coastal area has been affected by the same pattern of deformation by movements of at least two subsidary faults for the last ca. 300 ka, since the estimated time of formation of the oldest terrace (H1).