The Okinawa Trough, extending from southwest Kyushu, Japan, to the Lan-Yang plain of northeastern Taiwan, is a tectonically active intracontinental backarc basin situated between the East China Sea continental shelf and the Ryukyu arc (Uyeda, 1977; Lee et al., 1980; Letouzey and Kimura, 1985; Sibuet et al., 1987, 1998; Sibuet and Hsu, 1997; Park et al., 1998). Opening of the Okinawa Trough commenced during the middle Miocene (~10–6 Ma) (Sibuet et al., 1995; Miki, 1995), but it became inactive after the arc-continent collision of Taiwan and the Philippine Sea plate. As opening ceased, deposits of the Shimajiri Group (mainly of Pliocene in age) accumulated over a wide area on the East China Sea continental shelf and the forearc region (Park et al., 1998). Between ~1.7 and 0.5 Ma, the Southern Okinawa Trough and Ryukyu arc was formed (Ujiié, 1994). Meanwhile, postcollisional lithospheric extension occurred in the northern Taiwan mountain belt, making room for southwestward propagation of backarc rifting (Teng, 1996; Wang et al., 1999; Shinjo et al., 1999; Chung et al., 2000). Based on magnetic and geochronological data, Miki (1995) proposed that the second phase of opening was characterized by wedge mode opening in the southern Okinawa Trough and parallel rifting in the central Okinawa Trough.
Sibuet et al. (1998) suggested that the initiation of subsidence and block faulting along the central axis of the Okinawa Trough took place at ~2 Ma, based on the supposed development of sedimentary basins between the Okinawa Trough and Kyushu (Letouzey and Kimura, 1985; Kimura, 1985) and the uplift of the Ryukyu arc during the Pleistocene between 1.7 and 0.5 Ma (Ujiié, 1994). Although the age of the penultimate rifting of Okinawa Trough is still open to further investigation, most researchers agree that the last extension has occurred during the past 0.1 m.y. (Furukawa et al., 1991; Sibuet et al., 1995; see also Sibuet et al., 1998, for a review) on the basis of seismic correlation with drilling results (Tsuburaya and Sata, 1985). Furthermore, Ujiié et al. (1991) and Ujiié and Ujiié (1999) suggested that there was a bridge connecting the central–southern part of the Ryukyu arc with Taiwan, preventing inflow of the Kuroshio Current to the Okinawa Trough during the last glacial stage.