Evolution of the southern Taiwan±Sinzi Folded Zone and opening of the southern Okinawa trough Fanchen Kong a, *, Lawrence A. Lawver b , Tung-Yi Lee c a Department of Geological Sciences and Institute for Geophysics, University of Texas, Austin, TX 78759, USA b Institute for Geophysics, University of Texas, Austin, TX 78759, USA c Department of Earth Sciences, National Taiwan Normal University, Taipei, 117, Taiwan, Republic Of China Received 3 March 1998; accepted 23 January 1999 Abstract Recent interpretation of seismic sections and free-air gravity anomalies in oshore northern Taiwan reveals that the southern Taiwan±Sinzi Folded Zone began to form in late Middle Miocene, though it was mainly constructed in the Late Pliocene with strong reverse faulting and folding. Two westward progradational sequences were deposited in the shelf basin with sediments supplied from the southern Taiwan±Sinzi Folded Zone and the southern Ryukyu Arc. These two structures are displaced by several northwest-striking dextral strike±slip faults that were active in the early Quaternary when the clockwise-rotated southern Ryukyu Arc and the folded southern Taiwan±Sinzi Folded Zone were broken. It is believed that recent extension in the southern Okinawa Trough started in the early Quaternary because uplift on the southern Taiwan±Sinzi Folded Zone continued to latest Pliocene±early Quaternary. Paleogene±Miocene sediments of the East China Sea Shelf in the western part of the southern Okinawa Trough Basin are interpreted to indicate that the East China Sea Shelf Basin extended to the east of the southern Taiwan±Sinzi Folded Zone. # 2000 Elsevier Science Ltd. All rights reserved. 1. Introduction The southern Taiwan±Sinzi Folded Zone (TSFZ) separates the East China Sea Shelf Basin (referred to as ``the shelf basin'') from the Okinawa Trough Basin in the East China Sea area (Fig. 1). The TSFZ is a basement high that is easily traced in satellite gravity data and runs NNE from Taiwan to about 338N between Korea and Kyushu, Japan. It was named the Taiwan±Sinzi Folded Zone by Wageman et al. (1970), and is often called the Diaoyudao Folded Uplift Belt (Zhou et al., 1989) or Diaoyu Island Uplift (Liu, 1989) by mainland Chinese geologists. The southern TSFZ is the roughly east±west trend- ing southern section of the zone and extends approx. 300 km from the northern tip of Taiwan. Although the evolution and tectonic framework of the southern TSFZ have been discussed in many studies (Sun, 1985; Huang et al., 1992; Chen and Watkins, 1994; Hsiao, 1997; Hsu and Sibuet, 1995; Sibuet and Hsu, 1997), several key problems remain unsolved. First of all, while previous publications show sinistral displace- ments of the southern TSFZ and the southern Ryukyu Arc (Zhou et al., 1989; Yang, 1989; Wang et al., 1995), our interpretations of seismic sections, free-air gravity anomalies, and bathymetry indicate that they are oset dextrally by a series of northwest-trending strike±slip faults (Fig. 2). Second, the outline of the southern TSFZ has not heretofore or previously been well delineated. Third, the sharp termination of the thick sedimentary zone near the southern TSFZ (Fig. 3) suggests that it may have formed within the thick sedimentary zone, and that the southern Okinawa Trough Basin may have also extended within these same shelf basin deposits. While Paleogene and lower Neogene shelf basin deposits may be found in the Journal of Asian Earth Sciences 18 (2000) 325±341 1367-9120/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved. PII: S1367-9120(99)00062-0 * Corresponding author. Present address: Maxus Energy Corpor- ation, Town Center Two Building, 1330 Lake Robbins Drive, Suite 300, The Woodlands, TX 77380, USA.