Sediments and structure of the Japan Trench

Abstract

Seismic reflection profiles recorded east of Honshu show a fairly uniform thickness of acoustically transparent and presumably homogeneous sediment along the outer ridge and seaward slope of the Japan trench. The sediments continue to the bottom of the trench, where they abut the foot of the landward slope. In several localities the transparent sediments of the seaward slope end abruptly as a perched ledge shortly before the bottom of the trench is reached, suggesting post-depositional subsidence or extension of the sea floor near the trench axis. Seismic refraction measurements indicate that the seaward slope of the trench is a normal ocean floor that has been depressed. A succession of grabens and step faults observed along the entire seaward slope by the reflection technique suggests that the process which formed the trench is still going on. The faults are interpreted to be normal-antithetic and caused by tensional forces introduced in the convex side of the oceanic crustal plate as it is being further depressed, possibly in response to the load exerted by the weight of the island margin. Refraction profiles recorded along the upper landward slope (continental slope) show that its foundation is composed of material with seismic velocity about 5.8 km/sec; the depth to the Mohorovicic discontinuity is approximately 26 km. A tentative interpretation of one profile recorded along the lower landward slope of the trench indicates that the contact between the rocks of the island arc and the oceanic section lies west of the present topographic axis. A thick wedge of low-velocity sediment measured near the foot of the landward slope suggests that the topographic axis of the trench has been displaced seaward by outbuilding of the island margin, decreasing the maximum depth of the trench.