Asymmetric Seafloor Spreading along the Central Mariana Trough: Insights from Sidescan Sonar and Multibeam Bathymetry Data

The Mariana Trough is one of active and slow-spreading (2~4 cm/yr) backarc basin. The basin exhibits remarkable bow-shape and extremely asymmetry geometry. Spreading axes of the trough are distributed near the Mariana volcanic arc, eastern part of the basin rather than the centerline. To elucidate a seafloor spreading styles of the Mariana Trough, the KR03-12 cruise of R/V Kairei was carried out along the spreading axes of the central Mariana Trough October to November in 2003. The main tool of the cruise was WADATSUMI sidescan sonar system operating at a frequency of 100 kHz for high-resolution observation of the seafloor. Detailed observation of the sidescan sonar imagery exhibits that two direction of faults and fissures, and related hummocky ridges within axial valley are developed on axial valley floor. One group trends parallel to the axial valley wall, and another group trends ~15° oblique to the valley wall. The obliquely trending group is obviously underlain by the parallel-trending one, and developed only on the eastern half of the axial valley. On the other hand, multibeam bathymetry data obtained during our cruise exhibit the highest backscattering strength at the eastern off-axis of the spreading axis, indicating the recent volcanic activities. Both of the newest tectonism and volcanism found on the eastern off-axis area indicate that the most active part of the ridge has moved eastward from the centerline of the axial valley. The overview of bathymetry around spreading axes seems very symmetric although some features such as inclination of axial valley walls and spacing of larger faults exhibit asymmetric character. Based on our observation, we propose that the _gridge jump event_h has occurred after MOR-like normal and symmetric seafloor spreading. Based on the distance between the centerline of the axial valley to the most active volcanic zone (detected by multibeam bathymetry data), the distance of the latest jumping event is estimated as 0~5.5 km. Magnetic polarity data [Deschamps et al., 2005] indicate that the jump has occurred at least twice since 0.78 Ma.