The Deformations of Holocene Marine Terraces in Southern Kanto, Central Japan

Abstract

In order to clarify the late Holocene crustal deformations in the coastal area of southern Kanto, central Japan, the distribution, formative ages and height of Holocene marine terraces in the southern part of the Bose Peninsula, Miura Peninsula and Oiso-Kozu area were investigated.
Distinct Holocene marine terraces can be divided into four levels (the Numa I-IV) in the southern part of the Boso Peninsula, three levels (the Nobi I-III) in the Miura Peninsula, and three levels (the Nakamura-hara, Maekawa and Oshikiri) in the Oiso-Kozu area. The ages of the Holocene marine terraces in the Miura Peninsula (the Nobi I-III) which had not been well known previously proved to be around 6, 000 y. B. P., 4, 600 y. B. P. and 3, 100 y. B. P. in descending order and they are considered to be correlative to the Numa I-III terraces. However, the fact that the terrace correlative to the Numa IV was not found in the Miura Peninsula and Oiso-Kozu area suggests that the emergence of the Holocene marine terraces did not always occur simultaneously throughout southern Kanto.
The height distribution of the highest Holocene marine terrace surface (the Numa I, Nobi I and Nakamura-hara terrace surfaces), which were formed at the maximum rise of the Holocene transgression (ca. 6, 000 y. B. P.), shows warpings of WNW-ESE axes superposing upon the landward tilting. The height distribution cannot be explained by only coseismic uplift of 1703 and 1923 types.
Holocene terrace surfaces are displaced by active faults in the Miura Peninsula and the Kozu-Matsuda fault. This displacement indicates that these faults have been active even in the late Holocene. The average uplift rates estimated from the height of the Holocene shorelines are larger than those estimated from the height of Pleistocene shorelines, therefore, it can be concluded that the crustal deformation in the Holocene is more active than that in the late Pleistocene.