Résumé
One of the objectives of Ocean Drilling Program (ODP) Leg 126 was to investigate the origina nd evolution of the Izu-Bonin arc and forearc, both products of the subduction of Pacific lithosphere under the Philippine Sea Plate. Within the forearc basin, a full set of downhole measurements was recorded in two deep holes (792E and 793B). In addition, borehole electrical images were obtained (for the first time in the context of ODP) with the Formation MicroScanner (FMS*).
The main result of the drilling is that the forearc basin formed between 31.0 and 24.0 Ma by separation of a formerly contiguous frontal and outer arc high. The cored material shows a characteristic pattern of volcanogenic input, from turbidites and debris flows produced by volcanism and erosion of surrounding highs. The short rifting period is characterized by high sedimentation rate (300 m/Ma). In this context, the high resolution of FMS images was used to analyze the sedimentary processes associated with the deposition of deep-water volcaniclastics. The images reveal fine details of turbidite sequences that dip at low angles due to recent tectonics.
An FMS-based sedimentary log was calibrated from cores and prepared for each of the two holes, providing continuous bed-by-bed sections and permitting the investigation of trends in bed thicknesses. Palaeocurrent data were obtained from the analysis of ripple marks. During early basin history (30.2 to 29.5 Ma), the main sediment source was located to the east, in the vicinity of the modern outer arc high, with a secondary transport-mode oriented northward, along the basin axis. In the shallower section, emplaced at a lower rate from 28.9 to 27.3 Ma, axial transport from the north dominates a small component of flow from the western margin of the basin.
Initiated by rifting of the arc during the Oligocene time, basin development was followed by periods characterized also by extensional tectonics. Postdepositional extensional deformations such as normal microfaults, conjugate high-angle fractures, and dewatering veinlets were identified in the core and on FMS images. The orientation of the stress field within the arc and forearc was obtained from the analysis of borehole ellipticity. The results confirm models of stress distribution in forearc-arc-back arc regions. In particular, a rotation of the maximum horizontal stress trajectory in the overlying plate was observed, in a direction orthogonal to the plate boundary. In spite of a 90° clockwise rotation of the Philippine Sea plate since Oligocene time, the orientation of the stress field seems to have remained stable with respect to the trench axis over this period.