ABSTRACT: In Tsushima Island, it is interpreted that the NS or NNE-SSW trending tensional stress field induced the dextral strikeslip movement of the Tsushima-Goto faults during Late Oligocene and that initial rifting of the basin occurred. And the tensional stress was changed into NW-SE in direction at the early Early Miocene. At about 16 Ma, the collision of the northing Philippine Plate and the southwestern most part of Japanese Arc caused clockwise horizontal rotation of the southern part of Honshu, which generated NNW-SSE trending compressional stress field and made various compressional structures including folds and reverse faults in the Tsushima Island. At that time, the folding of the San'in folded zone was commenced. The folds in the Tsushima Island was almost completely formed at about 15 Ma and then an extensive intrusion of felsic magma occurred in the southern part of the island, which led the island to be tilted about 20° into northeast. Simultaneously, the Tsushima-Goto faults were reactivated as major sinistral strikeslip faults owing to the accumulated NNW-trending compressional stress. The adjacent blocks to the faults were rotated about 28° counterclockwise caused by the sinistral strike-slip movement. Resultantly, the western part of the San'in folded zone was dragged counterclockwise.
1. INTRODUCTION The Cenozoic Tertiary strata of Korea are mainly distributed in southeastern part of Korean Peninsula. These strata consist of several sedimentary basins and show the products of various crustal deformation accompanying volcanism. It is necessary to study the crustal deformation history of Tsushima Island in Japan in order to understand the tertiary geological characteristics of Korea. This study is important in solving the controversy about the East Sea (Sea of Japan) opening mode and in providing crustal information for establishing the Cenozoic crustal evolution history in the East Asia. It is also expected to provide basic geological information to develop natural resources in the vicinity of East Sea. Tsushima Island is located in the Korea Strait between the Korean Peninsula and Kyushu (Fig. lA). The NNEtrending Tsushima-Goto fault system passes through the western seashore of the island, which play an important role in changes of the stress regime in the study area. Various brittle ~ semi brittle structural elements and their cross-cutting relationships data were collected in the southern Tsushima Island. These field data and previously published geological data were synthetically analyzed in order to interpret the histories of paleostress fields and crustal deformations in the Tsushima Island and its vicinities during the Cenozoic Tertiary.
2. GEOLOGICAL BACKGROUND Late Oligocene - Early Miocene strata, called the Taishu Group, are exposed widely on the Tsushima Island (Kanno, 1955; Masuda, 1970; Shimada, 1977). Taishu Group is sedimentary strata and consists of shale, sandstone, mudstone and pebbly sandstone (Fig. 1). It is divided into three formations (Upper, Middle, and Low Formation) and Low Formation partially shows contactmetamorphosed features by granitic magma (Takahashi, 1969; Sakai and Kawahara, 1998). Taishu Group shows well-developed bedding plane and features of intertidal and shallow marine environment (Kanno, 1955; Ibakari, 1994; Nakazo and Maejima, 1998).