Initial Reports of the Deep Sea Drilling Project - Volume 58

This volume covers Leg 58 of the cruises of the Drilling Vessel Glomar Challenger. Leg 58 began in Yokohama, Japan in December 1977 and terminated in Okinawa, Japan in January 1978. A total of 9 holes were drilled at 5 sites. Several objectives were developed by the JOIDES Active Margin Panel for Leg 58 in the north Philippine Sea. Drilling concentrated in the Shikoku Basin and the Daito Ridge and Basin province of the northwestern part of the Philippine Sea. The Shikoku Basin sites were selected to solve several problems concerning back-arc basins formed by ocean-floor spreading. It was expected that drilling would result in recovery of basement basalts to determine the age of oceanic crust, and to answer some petrogenetic questions. Also, it was expected that drilling might resolve some outstanding problems related to sediment evolution during the rifting of the basin, as well as some general problems related to paleocirculation in the basin. Therefore, our principal objectives in the Shikoku Basin were (1) to determine the age of the oldest sediment, so as to calibrate magnetic-anomaly ages and to provide a test for various spreading models suggested for the basin; (2) to investigate the mineralogy, petrology, and chemistry of basalt samples recovered by drilling, and to compare there basalts with those of mid-ocean ridges, so as to understand the nature and source of magmatic materials in these basins; (3) to determine the distribution of sediment types in time, and to relate that distribution to the tectonic history of the basin; and (4) to determine the paleocirculation within the basin during its initial rifting history and during its post-rifting history. Sites in the Daito Ridge and Basin province were drilled with various objectives relevant to the origin of back-arc basins and related tectonic features. Objectives of drilling in the Daito Ridge and Basin province were (1) to determine the age of acoustic basement, so as to establish the time of arc and back-arc-basin development; (2) to determine the mineralogy, petrology, and chemistry of acoustic-basement rocks and to compare them with mid-ocean basalts; (3) to determine the amount of subsidence of Daito Ridge since the Eocene; (4) to determine the relationships of sedimentation events to the tectonic evolution of this region; and (5) to investigate the paleoceanography of the region.