Initial Reports of the Deep Sea Drilling Project - Volume 60

This volume covers Leg 60 of the cruises of the Drilling Vessel Glomar Challenger. A major goal of DSDP sampling of tectonically active ocean margins has been to obtain a complete transect of an interoceanic subduction zone- island arc-back-arc basin system. A classic example of such a system is in the western Pacific, where an apparent series of back-arc basins and volcanic arcs have formed behind (west of) the Mariana Trench. Two consecutive drilling legs (Legs 59 and 60) were assigned to drill an east-west transect of sites along latitude 18°N, from the Mariana Basin of the Pacific Ocean to the oldest inactive island arcs and back-arc spreading basins of the Philippine Sea. The western (older, inactive) half of this transect (Philippine Sea, Palau-Kyushu Ridge, Parece Vela Basin, and the West Mariana Ridge—Sites 447-451) was drilled during Leg 59. The objective of Leg 60 was to sample the tectonically active eastern portion of the transect in a closely spaced series of holes across the presently opening back-arc basin (Mariana Trough, Sites 453-456), the Mariana arc itself (Site 457), the Mariana fore-arc region (Sites 458 and 459), and deep within the Mariana Trench (Sites 460 and 461). In addition, one site (Site 452) was drilled on the Pacific plate seaward of the trench to obtain a reference section of sediments and oceanic crust being delivered to the trench. Although even this geographical concentration of drill sites provided sparse data in the complex arc-trench system, it was hoped that by drilling each of the primary structural elements of the area we could elucidate the spreading, subsidence, and sedimentation history of the Mariana Trough, the volcanic and tectonic history of the Mariana arc, and the tectonic history of the trench and fore-arc region. Leg 60 began on 15 March, 1978 at Apra, Guam and ended at the same location on 15 May, 1978. During Leg 60, the Glomar Challenger traveled 2445 nautical miles and attempted to drill 17 holes at 10 sites.