Marine Geosciences

OKINAWA TROUGH BACKARC BASIN: EARLY TECTONIC AND MAGMATIC EVOLUTION

Sibuet J.-C., Deffontaines B., Hsu S.-K., Thareau N., Le Formal J.-P., Liu C.-S. and the ACT party, 1998,  J. Geophys. Res., 103, pp: 30245-30267.

Abstract. The Okinawa Trough, lying between Japan and Taiwan, is a backarc basin formed by extension within the continental lithosphere behind the Ryukyu trench-arc system. Stress directions associated with the two last extensional phases in the southwestern Okinawa Trough have been deduced from a comparison with analog modeling: the direction of extension is N150° for the Pleistocene phase of extension (2-0.1 Ma) and N170° for the late Pleistocene-Holocene phase of extension (0.1-0 Ma). The present-day Ryukyu volcanic arc, a narrow continuous feature extending from Japan to Taiwan, is located on the eastern side of the Okinawa Trough, 80-100 km above the Wadati-Benioff zone, the minimum depth for emplacement of arc magmatism. Scarce present-day backarc volcanism appears in the middle and southern Okinawa Trough within linear en echelon bathymetric depressions. A N045° oriented seamount volcanic chain cuts across obliquely the southwestern Okinawa Trough and lies in the direct line of extension of the Gagua ridge, a N-S linear volcanic feature of the Philippine Sea plate. Associated with this extension of the Gagua ridge, a large reentrant located at the base of the Ryukyu prism, the uplift of part of the Nanao forearc basin and the deformation of the sedimentary arc suggest that the voluminous cross-backarc volcanism could be tied to the subduction of the Gagua ridge located there at a depth of 80-100 km beneath the backarc basin. A second area of anomalous volcanism has been identified in the middle Okinawa Trough in the ENE extension of the Daito ridge, a WNW-ESE 400-km-long volcanic feature of the Philippine Sea plate. We suggest that the Gagua and Daito ridges initially induced stress at the base of the arc which is still brittle and cracks propagated through the overlying brittle lithosphere, allowing magmas with arc affinities to erupt at the seafloor. This excessive magmatism reaches the seafloor through conduits which preferentially follow in their shallowest portion the crustal normal faults of the backarc rifts. The Okinawa Trough is consequently still in an early stage of evolving from arc type to backarc activity.

Plate 1. NE simulated hillshading of the southwestern Okinawa Trough established from raw data gridded every 75 m. The 1500-m isobath (blue to yellow boundary) underlines contours of the N085° backarc depression.

Plate 2. Three-dimensional view of the eastern portion of the southern Okinawa Trough bathymetric map (SW simulated hillshading). Note the presence of extinct channel meanders cut across by the N085° normal faults. Two types of volcanism are present in the backarc basin depression: elongated volcanic intrusions along N085° normal faults (backarc basin volcanism) and seamounts aligned along a NE-SW trend (cross-backarc volcanism).

Figure 2. Track lines of the ACT cruise and the preceding transit of the R/V l'Atalante (May-June 1996) in the southwestern Okinawa Trough with locations of portions of profiles displayed in the following figures. GG', Chinese Petroleum Company seismic profile [Huang et al., 1992].

Figure 3. Bathymetric map of the southwestern Okinawa Trough established from the swath-bathymetric EM12D and EM950 data. Mercator projection; isobath spacing, 50 m. Dots are locations of earthquakes from the Taiwan Telemetered Seismograph Network (S.-N. Cheng, personal communication, 1996). Lines A, B, C, and D locate major geological discontinuities across the region. Structural context is indicated by thick or thin lines with large thick marks for normal faults with large or small vertical offsets identified from the ACT survey; thick or thin lines with small thick marks for normal faults with large or small vertical offsets identified by Huang et al. [1992] on the northern margin; dark areas for volcanic outcrops or slightly buried volcanism.

Figure 14. Shaded bathymetric map of ACT (as well as preceding and following transits) data [Lallemand et al., 1997] showing the N358° trending Gagua ridge which subducts beneath the Ryukyu arc and forearc [Dominguez et al., 1996]. Associated surface manifestations in its northern prolongation are the large reentrant located at the base of the Ryukyu prism (Yaeyama ridge (YR), the uplifted part of the Nanao forearc basin (NB), the deformation of the sedimentary arc and the Okinawa Trough voluminous arc volcanism which is located in the direct prolongation. The two diagrams show the velocity vectors needed to create the N045° cross-backarc trail for both the Pleistocene and late Pleistocene-Holocene phases of extension.

Figure 15. Location of the two OT areas of large anomalous volcanism in the prolongation of the Gagua ridge (southwestern OT) and of the Daito ridge (VAMP area, middle OT). Detailed bathymetry of the VAMP area and projected magnetic anomalies [Sibuet et al., 1987] in the top left corner. Outside these two areas, there are only a few volcanic outcrops in the middle and southern OT interpreted as backarc volcanism (stars, from Sibuet et al. [1987]) and the Ryukyu volcanic arc shown as a continuous grey line between Japan and Taiwan.