No abstract

The results of twenty‐eight seismic refraction profiles recorded in the various physiographic provinces of the Philippine Sea as part of the United States and Japan Science Cooperation Program are presented in four schematic structure sections. The basins of the Philippine Sea have fairly normal oceanic crust that includes, between the sea floor and layer 2, a layer of about 3.5‐km/sec velocity controlling the characteristic rough topography. Crustal thickening beneath the Nansei Shoto, Oki‐Daito, Kyushu‐Palau, and the Honshu‐Mariana ridges is associated mainly with an increase in thickness of the 3.5‐km/sec layer and a thick underlying section of material with a velocity between 5.5 and 6.0 km/sec. Beneath the Nansei Shoto trench and the Honshu‐Mariana trench, there is a tendency for layer 2 to increase and layer 3 to decrease in thickness as the trench is approached from the adjacent oceanic sector.

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.

Seismic‐refraction measurements made on Shatsky rise in the Northwest Pacific basin show crustal layering appreciably different from that in the basin on either side. Beneath the crestal zone of the rise and its broad western flank at water depths less than 5 km, oceanic basement (layer 2) is about 3–6 km thick and contains two refracting layers, 4.7 and 5.5 km/sec, overlying the main crustal layer (layer 3) of velocity averaging 7.0 km/sec. Pronounced crustal thickening beneath the rise is associated mainly with a 7.3‐ to 7.8‐km/sec layer intermediate between layer 3 and the upper mantle; the thickness of layer 3 is approximately normal. The depth to mantle is at least 22 km near the crestal zone and perhaps even greater below the crest where comparatively shorter range profiles did not determine it. Additional measurements between two large seamounts of the Emperor seamount chain, on the eastern flank of a low rise in the basin floor, also indicate two layers within layer 2, but with velocities different from those found beneath Shatsky rise. Here, layer 2 is 2–4 km thick and composed of material with velocities of 3.7 and 6.2 km/sec. Below these layers the crust seems normal, except that the average thickness of layer 3 is somewhat lower than regional average.

Evaluation and synthesis of seismic-refraction measurements made in the Japan trench in the light of the results of deep-sea drilling from Glomar Challenger Legs 56 and 57 result in a schematic section of the crustal structure. The Pacific Ocean crustal plate underthrusts the leading edge of the continental block of Northeast Japan at a low angle and follows at depth the upper seismic belt of the Wadati-Benioff zone. Overthrust continental crust extends to within 30 km of the trench axis; thenceforward the lower inner trench slope is formed by a thick, wedge-shaped body of low-velocity sediments. The volume of these sediments is too small to represent all the sediments scraped off the Pacific plate and transported from land. This indicates that substantial amounts of sediment are carried down along slip planes between the continental block and the descending plate. Dewatering of the sediments lubricates the slip planes. The slip planes migrate upward into the overlying continental edge, causing removal of material from its base. This tectonic erosion causes subsidence and landward retreat of the continental edge.