Microstructures in Semiconsolidated Sediments of Deep Sea Drilling Project Leg 59 (Philippine Sea Transect) and Their Relation to Regional Tectonic Setting

Abstract

The purpose of this study is to describe the occurrence of microstructures in Leg 59 sediments and to interpret the tectonic setting of back-arc regions of the Philippine Sea. Some 24 holes were drilled as part of the South Philippine Sea transect (Legs 59 and 60); the regions drilled include the West Philippine Sea, the PalauKyushu Ridge, the Parece Vela Basin, and the West Mariana Ridge on Leg 59 and the Mariana Trough, the Mariana arc, and the Mariana Trench on Leg 60. One of the key objectives of the South Philippine Sea transect was to test the hypothesis (Karig, 1971, 1975) that backarc basins and remnant arcs were progressively formed by processes of island-arc sundering that left remnant arcs on the west while the active portion of arcs migrated toward the east, creating marginal-basin spreading centers between them. Thus far, shipboard results of Legs 59 and 60 have supported this conclusion (Kroenke, Scott, et al., 1978; Hussong, Uyeda, et al., 1978), demonstrating progressively younger marginal basins from west to east. In particular, the Parece Vela Basin appears to be a classic case of symmetrical spreading that took place between the remnant PalauKyushu Ridge to the west and the active West Mariana Ridge to the east during the Oligocene and Miocene. Similarly, the West Mariana Ridge is now the remnant arc west of the modern Mariana Trough spreading center and the modern Mariana arc. A study of the microstructures in sediments from the remnant and active arcs will reflect the local stress environment and may reflect the regional tectonic environment at the time of deformation, as was assumed by Choukroune et al. (1978) in part of the FAMOUS project.