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.

The J Anomaly Ridge is a structural ridge or step in oceanic basement that extends southwest from the eastern end of the Grand Banks. It lies beneath the J magnetic anomaly at the young end (M-4 to M-0) of the M series magnetic anomalies. Its structural counterpart beneath the J anomaly in the eastern Atlantic is the Madeira-Tore Rise, but this feature has been overprinted by post-middle Cretaceous deformation and volcanism. In order to study the origin and evolution of the J Anomaly RidgeMadeira-Tore Rise system, we obtained seismic refraction and multichannel reflection profiles across the J Anomaly Ridge near 39øN latitude. The western ridge flank consists of a series of crustal blocks downdropped along west-dipping normal faults, but the eastern slope to younger crust is gentle and relatively unfaulted. The western flank also is subparallel to seafloor isochrons, becoming younger to the south. Anomalously smooth basement caps the ridge crest, and it locally exhibits internal, eastward-dipping reflectors similar in configuration to those within subaerially emplaced basalt flows on Iceland. When isostatically corrected for sediment load, the northern part of the J Anomaly Ridge has basement depths about 1400 m shallower than in our study area, and deep sea drilling has shown that the northern ridge was subaerially exposed during the middle Cretaceous. We suggest that most of the system originated under subaerial conditions at the time of late-stage rifting between the adjacent Grand Banks and Iberia. The excess magma required to form the ridge may have been vented from a mantle plume beneath the Grand Banks-Iberia rift zone and channelled southward beneath the rift axis of the abutting Mid-Atlantic Ridge. Resulting edifice-building volcanism constructed the ridge system between anomalies M-4 and M-0, moving southward along the ridge axis at about 50 mm/yr. About M-0 time, when true drift began between Iberia and the Grand Banks, this southward venting rapidly declined. The results were rapid return of the spreading axis to normal elevations, division of the ridge system into the separate J Anomaly Ridge and Madeira-Tore Rise, and unusually fast subsidence of at least parts of these ridges to depths that presently are near normal. This proposed origin and evolutionary sequence for the J Anomaly Ridge-Madeira-Tore Rise system closely matches events of uplift and unconformity development on the adjacent Grand Banks.

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.