**2. Opening of the Sea of Japan**

events in Northeast Japan based on reviews of recent high-resolution studies on modes and

154 Mechanism of Sedimentary Basin Formation - Multidisciplinary Approach on Active Plate Margins

**Figure 1.** Index map showing the tectonic setting of Northeast Japan. OBR: Ou Backbone Range, DH: Dewa Hills, KM: Kitakami Mountains, AM: Abukuma Mountains, HR: Hidaka Range, VF: Quaternary volcanic front (dotted line). ISTL: Itoigawa-Shizuoka Tectonic Line, TTL: Tonegawa Tectonic Line. Solid circle represents ODP sites with ages of basement rocks and MITI soma-oki well. Star represents location of the Yuda Basin. A hexagone and a dotted square denote the

development patterns of intra-arc basins.

areas shown in Figure 2 and Figure 4, respectively.

The Sea of Japan is a large marginal sea formed in the backarc side of the Southwest and Northeast Japan and comprises component backarc basins, the Japan, Yamato and Tsushima basins (Figure. 1). The Japan Basin is underlain by an oceanic-type crust 11-12 km thick, whereas the Yamato Basin is floored by a crust 17-19 km thick and is unlikely to be oceanic [6]. The component structure of the Sea of Japan likely constitutes a multi rift system [7]. The Japan Basin was dated around 24-17 Ma based on 40Ar-39Ar dating of basement basalt in Site 795 of ODP Leg. 127 (Figure. 1)[8]. The Yamato Basin yields younger age of 21-18 Ma in Site 794 and 797 of ODP Leg. 127 (Figure. 1)[8]. However, the timing and processes of the opening of the Sea of Japan has been a matter of debate. Two mechanical models for the formation of the Japan arc – the Sea of Japan system has been proposed; a double-door opening model and a pullapart basin model.

A double-door opening model is primarily based on the paleomagnetic evidence on the Japan arc. The model includes a clockwise rotation of the southwest Japan [9] and a counter-clockwise rotation of the northeast Japan [10-11]. This model implies a simultaneous occurrence of the rotation of the Japan arc and the opening of the Sea of Japan. Paleomagnetic data suggest a rapid 50° clockwise rotation of the Southwest Japan at around 15 Ma [12] and rather prolonged counter-clockwise rotation of the Northeast Japan from 17 to 14 Ma [13]. In terms of discrep‐ ancy between the timing of the opening of the Sea of Japan and that of the rotation of SW and NE Japan, Nohda [14] reassessed the 40Ar-39Ar age data of basement basalt with reference to Nd-Sr isotopic data in Site 797 of ODP Leg. 127. The basalts from the upper part of basements in Site 797 have not been dated and were overlain by a felsic tuff dated to be 14.86 Ma. He concluded that the upper basalts at Site 797 may be inferred to be younger than the lower basalts and that the inferred timing of volcanic activity in the Sea of Japan region (ca. 21-15 Ma) is consistent with the timing of rotational crustal movements inferred from palaeomag‐ netic studies in the Japanese arcs.

A pull-apart basin model is primarily based on the structural studies. In this model, the opening of the Sea of Japan was attributed to pull-apart basins formed by lateral displacement of the Japan arc associated with dextral transcurrent fault systems [15-17]. Jolivet et al. [18] revised their previous model and incorporated paleomagnetic evidence of rotation into their model; strike-slip displacement of the Japan arcs was associated by clockwise and counter‐ clockwise rotations of numerous blocks in southwest and northeast Japan, respectively.

There has been still no general agreement on the timing and processes of the opening of the Sea of Japan. However, Takahashi [5] proposed that the Early Miocene volcanic front of the northeast Japan was displaced more than 200 km toward east from that of southwest Japan because of the right lateral strike-slip motion of the Tonegawa tectonic line (TTL; Figure. 1). This suggests a differential rotation between southwest and northeast Japan accompanied by southward displacement of northeast Japan.
