Varied responses to subduction in Nankai Trough and Japan Trench forearcs

Abstract

Responses of sediment entering trenches range from accretion at the foot of the inner slope to subduction into the mantle, with inner trench slopes variously displaying uplift, subsidence, growth and retreat. Leg 87 is the latest of the DSDP voyages which have provided many clues to the behaviour of subduction zones, and was devoted to the study of subduction processes in two trenches off Japan. The Nankai Trough (Fig. 1) on the basis of very high quality reflection profiles1,2, is accreting most or all its trench fill, which is being stripped off the underlying Shikoku Basin haemipelagites along a decollement near their contact (Fig. 2a). Because these seismic profiles define the large scale geometry so clearly, the objective of the Leg 87 drilling programme was to investigate the structural fabric and physical properties of sediments as they enter the subduction zone. Using these data which we now report, we hoped to deduce or constrain the stress conditions accompanying deformation.