Geological Summary of the Shikoku Basin and Northwestern Philippine Sea, Leg 58, DSDP/IPOD Drilling Results

Leg 58 drilled three sites (442, 443, 444) in the Shikoku Basin and two (445, 446) in the Daito Ridge and Basin province (BRBP). At the Shikoku Basin sites, drilling penetrated hemipelagic clays and underlying tholeitic pillow basalts and sills. Basalt sills intruded both sediments and pillow lavas. The K-Ar age on pillow basalt is at variance with one magnetic anomaly interpretation, but in agreement with another. Rifting of the Shikoku Basin was either symmetrical or asymmetrical; off-ridge volcanism in the basin has made basement age resolution ambiguous. Rocks at the DRBP sites (445, 446) consist of an upper pelagic interval and a lower section of volcaniclastic turbidites and debris flow sandstones and conglomerates (intruded by 26 sills at 446). Magnetic inclination data suggest that the DRBP drifted north 2,000 km from an equatorial latitude over the past 53 my. The age of the oldest sediment recovered, 52-53 my, indicates that this area formed in the earliest stages of rifting of the west Philippine Basin