Previous deep-sea drilling sites along the Japan Trench are shown in Figure F2. During DSDP Legs 56 and 57, Sites 436 (Leg 56), 434 (Leg 56), 441, 440, and 435 (Leg 56) were drilled along seismic Line JNOC-2 across the trench at 3945N. Sites 438 and 439 were drilled along seismic Line JNOC-1 across the trench at 4040N (Scientific Party, 1980). During DSDP Leg 87, Site 584 was also drilled in the area (Kagami, Karig, Coulbourn, et al., 1986). These legs were focused on the study of the mechanism and dynamics of plate convergence and their effects on sedimentation. It is presently widely accepted that little tectonic accretion is occurring. Instead, a massive subsidence has been taking place along the Japan Trench. This was a surprising finding substantiated by past drilling. Another surprise was the unexpected discovery of andesitic volcanic rocks from Site 439, 90 km from the trench axis, which indicated that there is an offset of ~200 km between the present arc and where Oligocene volcanism occurred.

The Cretaceous unconformity widely recognized on land was observed at Site 439 and could be further extrapolated seaward by the aid of seismic records suggesting that a pre-Oligocene forearc once extended at least to the present midslope terrace where Site 440 is located. The objective at Site 584 was to further confirm and detail the findings of Legs 56 and 57. Site 584, at the outer slope, reached sediment of middle Miocene age, confirming persistent subsidence during the Miocene. It was suggested that extensional tectonics continued from the middle Miocene until the early Pliocene. Numerous ash layer records from all the sites suggest that onshore volcanic activity increased near the end of the late Miocene and continued through the early Pliocene.

Leg 186 was planned to establish borehole geophysical observatories that could monitor the ongoing tectonic processes at two sites (Sites 1150 and 1151) ~25 km north and south of 39N and slightly eastward of Site 439 in longitude.