ALTERNATE SENSES OF DISPLACEMENT ALONG THE TSUSHIMA FAULT SYSTEM DURING THE NEOGENE BASED ON FRACTURE ANALYSES NEAR THE WESTERN MARGIN OF THE JAPAN SEA

The western margin of the Japan Sea is characterized by the existence of a N20 degrees E-trending transcurrent fault system in the Korea str ait between Korea and Kyushu. This fault system, called here the Tsush ima fault system, played a major role during the opening of the Japan Sea during the Tertiary. In order to understand its displacement histo ry, we analysed the deformation recorded by Neogene rocks exposed on t he Korea strait islands (Tsushima, Goto and Hirado islands). A synthes is of published stratigraphical and structural data together with our field observations reveals that two tectonic events occurred in succes sion in the study area: an Early Miocene NW-SE-oriented extension (eve nt I); and a Middle Miocene NW-SE-oriented shortening (event II). Two distinct families of fractures with contrasted stratigraphic occurrenc es have been recognized in the Neogene rocks exposed on the Korea stra it islands and allow us to define and date the stress fields character izing each tectonic event. Stress field I is of intermediate type (the vertical axis is either sigma(1) or sigma(2)) with a N45 degrees E si gma(Hmax) and a N135 degrees E sigma(Hmin) and was active between 22 a nd 16 Ma. Stress field II is compressional (the vertical axis is sigma (3)) with a N135 degrees E sigma(Hmax) and a N45 degrees E sigma(Haim) and was active between 15 and 10 Ma. The trends of stress field I pri ncipal axes imply that the displacement along the Tsushima fault syste m was dextral during the Early Miocene. Conversely, the trends of stre ss field II principal axes imply that the displacement along the same fault system was sinistral during the second half of the Middle Miocen e. The inferred displacement history agrees with: (1) a pull-apart mod el for the opening of the Japan Sea during the Early Miocene; and (2) transition from tensional to compressional stress fields in the Japane se are and back-are region at the end of the Middle Miocene.