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The clay mineralogy and morphology of DSDP Leg 87 sediments, studied from 200 samples and considered together with data from DSDP Legs 57 and 58, are discussed and interpreted in terms of paleoenvironment. Terrigenous influence represents the main factor responsible for clay deposition in the Nankai Trough and Japan Trench, both marked by proximity to Japan, high submarine relief, and high sedimentation rates. The argillaceous diagenesis is of local or minor importance and includes the readjustment of fleecy to lathed smectites. The volcanic influence consists chiefly of a reworking of smectite developed by subaerial alteration of island arc rocks and secondarily of a local alteration of ash beds. The late Cenozoic clay sedimentation in the Nankai Trough (Sites 582 and 583) underwent a major change during the early Pleistocene. Hemipelagic sediments characterized by a wide range of minerals, derived from various sources characterizing the whole northern Philippine Sea (Leg 58 data), were replaced by turbiditic sediments with clay associations typical of a restricted southern Japanese origin (chiefly illite and chlorite). The mineralogic transition, which roughly parallels the lithologic one, occurs by short successive steps. It appears to express indirectly the tectonic activity of the southern margin of Japan, marked by the subduction of the Shikoku Basin and the formation of the Nankai Trough. Middle Miocene to Pleistocene sedimentation on the inner part of the Japan Trench (Site 584) corresponds to a regular detrital supply from Japan, with dominant smectite associated with various minerals. Small mineralogic variations occur, determined by the late Cenozoic climatic variations and possibly by the volcanic activity. The Site 584 clay stratigraphy parallels the successions recorded at Sites 438 and 440 (Leg 57), without significant evidence of differential settling. The first deposits recovered at Site 584 correspond to the end of an important geodynamic history, well recorded in the clay stratigraphy of Sites 438 and 439: (1) existence during the Late Cretaceous of a sloped continental landmass, located somewhere east of the drilling area; (2) proximity of the actively eroded "Oyashio landmass" during the late Oligocene; (3) probable initiation of the collapse of this landmass during the latest Oligocene after a strong stage of volcanic activity; and (4) increase of the influence of Japan on sedimentation during the early-middle Miocene.

Nankai Trough and Japan Trench Late Cenozoic Paleoenvironments Deduced from Clay Mineralogic Data