** Progress in Earth and Planetary Science is the official journal of the Japan Geoscience Union, published in collaboration with its 50 society members.

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    • Progress in Earth and Planetary Science
    • Progress in Earth and Planetary Science
    • Progress in Earth and Planetary Science
    • Progress in Earth and Planetary Science
    Progress in Earth and Planetary Science

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    Solid earth sciences

    High-density surveys conducted to reveal active deformations of the upper forearc slope along the Ryukyu Trench, western Pacific, Japan

    Arai K, Inoue T, Sato T

    Normal fault, Active deformation, Ryukyu Trench, Subduction, High-resolution seismic profile

    Schematic illustration of trench-parallel extension caused by active back-arc rifting that stretches the forearc area. For details of the arc extension, see Kubo and Fukuyama (2003). Note the similar stratigraphy of growth strata (colored in the seismic reflection profiles) in the back-arc side and under the upper forearc slope.

    In this study, which was conducted to investigate present-day geological deformations occurring off the central Ryukyu Arc along the Ryukyu Trench (Okinawa-jima) in the vicinity of Japan, we obtained and examined dense, high-resolution, 2D grid (high-density) multichannel seismic reflection data. The forearc slope of the central Ryukyu Arc lacks a large forearc basin but has a steep narrow slope that dips to the southeast in the direction opposite to the plate convergence. However, while the seismic profiles obtained in our investigation do not reveal any active structures indicative of compressional stress in this convergent subduction zone, recent tectonic deformations are characterized by normal faults that strike perpendicular to the Ryukyu Trench axis. Examinations of high-density geological data show that the recent relative motion of the overriding plate has resulted in an active, arc-parallel extensional field. Furthermore, the results of our observations indicate two stages of extensional stress beneath the forearc slope of the central Ryukyu Arc that resulted from back-arc rifting in the Okinawa Trough.