MARINE GEOLOGY OF NORTHWESTERN PACIFIC: DESCRIPTION OF JAPANESE BATHYMETRIC CHART 6901

Abstract

This paper is a geological discussion of the northwest Pacific, to accompany the Japanese Hydrographic Chart 6901 (a bathymetric chart of the northwest Pacific) which is presented as Plate 1. A special edition of this chart was published under an arrangement made by the writer between the Japanese Hydrographic Office and The Geological Society of America in the belief that it will provide useful topographic data in a previously little-known portion of the sea floor and that it will stimulate further interest in Pacific marine geology. A comparison of the land surface with the sea floor, both of which are contoured at 500-m intervals, emphasizes the variety of sea-floor features and the grand scale of this submerged topography where erosional leveling is subdued. The region covered by the chart can be divided into (1) the Pacific Basin proper, (2) the Philippine Sea Basin, including the arcuate ridges extending from Japan to Palau, and (3) the submerged portion of the Asia continental block, including the continental shelf and slope and the encompassed “intra-continental” basins. Large-scale undulations or swells and swales of low relief are the dominant structures of the Pacific Basin, but the five great groups of seamounts are the most striking features. A study of the original soundings shows some abrupt changes in level and grabenlike depressions which probably mark fracture zones in the earth9s crust. The seamounts are considered to be volcanoes, many of which are capped with coral. Smooth bottom characteristizes most of the Basin, suggesting a thick sedimentary blanket. Two of the seamount groups (the Emperor Seamounts and the Magellan Seamounts) are entirely submerged; another group pierces the surface at only two places (Marcus I. and Wake I.); the Caroline and the Marshall-Gilbert groups form numerous atolls. A large number of the seamounts have deeply submerged flat-topped summits and, thus, are guyots or tablemounts. However, because of the low seismicity and the rarity of raised islands, the Pacific Basin is considered a stable area, and the sinking of the guyots is thought to be a local isostatic subsidence in response to the load of the seamounts. The Philippine Sea Basin is bounded by the Asia continental block on the west and by the great geanticlinal ridges, extending from Japan to Palau, on the east. Trenches, presumably marking downbuckles in the earth9s crust, lie seaward of these ridges. These geanticlinal ridges are active belts surmounted by volcanoes and a string of raised islands. The Kyushu-Palau Ridge, another large geanticlinal ridge, bisects the basin. Soundings give little evidence of guyots on any of the ridges or in the Philippine Sea. The Daito Mountains, nested between the Kyushu-Palau Ridge and the Ryukyu Trench, are probably orogenic rather than volcanic. The Philippine Sea Basin appears to be non-sialic and a region with normal Pacific crustal structure which has been deformed by the same compressional forces that have affected the margin of Asia. The continental slope, marking the boundary of the Asia sialic block, extends from Kamchatka, along the Kurils, Japan, and the Ryukyus to Formosa, and probably along the east side of the Philippines. Extensive featureless shelves with shelf-breaks at normal depths fringe China and Siberia. The continental slopes are canyoned and irregular in detail. In the Okhotsk Sea there is a remarkable deep terrace which is probably a subsided normal shelf. The Kuril Basin, the Japan Sea Basin, and the Ryukyu Basin are similar abyssal pear-shaped basin within the continental framework. En bloc drifting of the associated island arcs, opening up simatic pools in the continental block, may account for these basins. Their smooth bottoms suggest that they are sediment traps for a great amount of sediment eroded from Asia.