The models on the tectonic evolution of the Philippine Sea can be divided mainly into two:(1) entrapment of a normal ocean at 43Ma for the West Philippine Basin formation and back-arc basin formations caused by successive landward retreats of the upper Philippine Sea plate, and (2) back-arc basin formations caused by successive seaward retreats of the trench axis. These models are discussed on the basis of the presently available data. The island arc type volcanism at the northern half of the Palau-Kyushu Ridge since 48Ma, and the age-depth relation and chemistry of basalts at Sites 291 and 292 in the West Philippine Basin favor the back-arc spreading origin for this basin. In contrast, the chemistry of basalts at Site 447 and the large distance between the Central Basin Ridge and the northern Palau-Kyushu Ridge favor the entrapment origin for this basin. It is likely that the West Philippine Basin has formed under the unique situation of the ocean basin formation, i.e., with subduction at its northern margin and a spreading center far from this subduction zone. Subduction or collision of the aseismic ridges in and around the Philippine Sea is discussed in terms of the crustal structure of the ridges, seismicity, and onland geology. It is found that all these aseismic ridges except for Izu which has a thick 20-30km crust have been subducted, although many of them have formed a cusp or discontinuity at the trench. Elucidating the times of the bending of the Cretaceous-Paleogene terranes in central Honshu and in Kyushu is critical to discriminate the two models for the Shikoku Basin formation, i. e., the fixed trench model and the trench retreating model.
Geomagnetic survey by means of the proton magnetometer and the three component magnetometer was carried out by the RN 83 Cruise of the Nagasaki Maru to reveal Pleistocene igneous activities along the axial median valley (central rift) of the middle Okinawa Trough in 1983. Preliminary results are as follows: (1) Magnetic anomaly lineations were clearly recognized in the studied area, having approximately N80°E-S80°W trend nearly parallel to the narrow topographic depression (central rift). (2) Amplitudes of anomalies (peak to peak) are about 200-500nT and their wave lengths range from 20 to 30km. (3) The magnetic anomaly lineations were concentrated within the width of 20km from the central rift trending ENE-WSW. (4) Two dimensional block model based upon the geomagnetic, reversal time scale seems to well explain the magnetization pattern in the middle Okinawa Trough. Calculated result suggests that the major spreading of the Okinawa Trough had occurred between around 1.8Ma and 0.5Ma. (5) However, another geological evidence never shows oceanic condition in this area because the axial area of the middle Okinawa Trough has continental crust, and arc volcanisms have been occurred.
Manganese nodules from Komahashi-Daini Seamount, Amami Plateau, Daito Ridge, Okidaito Ridge and a seamount in a southern part of the Kyushu-Palau Ridge (SKPR) are similar in occurrence, morphology, surface texture, internal structure and chemical composition. SEM observation revealed a presence of numerous natural casts of calcareous nannoplankton fossils within metal oxide layers of these manganese nodules. Fecal pellets, coccospheres and tests of agglutinated foraminifers, also preserved as natural cast, were discovered commonly in the inner layers. 27 species were identified as a whole. Younger species were systematically found outwards within each manganese nodule. These observations ensure the use of biostratigraphic method to determine the formation ages of these nodules. A manganese nodule from SKPR grew on calcareous ooze in late Oligocene time, and was buried by pelagic clay of early Miocene in age. The rest of four nodules began to form on calcareous ooze in early to middle Pliocene with a rapid initial accretion followed by a slow growth (6.0-13.0cm to 0.6-1.7mm/Ma) which may have ceased in middle to late Pleistocene. The formation of these four manganese nodules with respect to the geologic history of the Philippine Sea can be summarized as follows: When the subsidence of the seamounts and ridges started during the late Mioce ne resulting from a differential deepening of this basin, pelagic condition became prevalent over the Philippine Sea region. However, the islands, i.e., the summits of seamounts and ridges, locally produced a surface condition favorable for planktonic organisms, resulting in the deposition of calcareous sediments during the Pliocene. As the subsidence continued, productivity in surface waters declined in early to middle Pliocene because of the reduction of the islands in scale. The resulted decreased sedimentation initiated the nodule formation with an initial rapid rate of accretion. As the islands continued to subside, the rate of nodule accretion decreased reflecting the lowered surface productivity. When the islands submerged completely below the sea level probably in a middle Pleistocene, the pelagic condition predominated this region. As the supply of calcareous material almost stopped, the growth of manganese nodules diminished substantially. A very local variation in nodule mineralogy and chemistry found on the Komahashi-Daini Smt. may be related to a depth dependent variation in marine environments during the susidence. The formation of the one from SKPR is also explained by the same schema, although the subsidence and nodule formation occurred in late Oligocene.
The climatological characteristics of heavy rainfalls in Hokkaido Island are investigated on the basis of areal frequency distribution of heavy rainfalls (the amount of daily precipitation above 100mm) and geographical distribution of the maximum daily precipitation. The causes of heavy rainfalls and the maximum daily precipitation are investigated on the daily synoptic weather charts, and their geographical distributions are also investigated. Relation between the regional characteristics of heavy rainfalls and the location of the Polar front is discussed. The statistical period is 25 years, from 1973 to 1977 and the data of 256 stations are analysed. As a result, Hokkaido Island is divided into two climatic regions, the one is ‘high frequency heavy rainfall region (HR)’ and the other is ‘low frequency heavy rainfall region (LR)’. The bordering line of these two regions runs approximately from ENE to WSW, from Shiretoko peninsula, via Shiranuka Hills, Ishikari Mountains, Hidaka Mountains, Iburi Mountains to the southern part of Oshima Peninsula. To the south of this line is the HR in which local extremly high frequency heavy rainfall regions are located on the eastern to southern slopes of the mountain ranges. In this region the frequency of heavy rainfalls is more than once per year, and the maximum daily precipatation exceeds 300mm. The main causes of heavy rainfalls here are extratropical cyclones passing to the south of Hokkaido Island from Japan Sea or from Pacific Ocean, most often in August and September. This types of heavy rainfalls are mainly caused by the moist air flow from these cyclones and the effect of orographic rifting strengthens the rainfalls. In these cases the Polar front is usually located to the south of Hokkaido Island. On the other hand to the north of the line, the frequency of heavy rainfalls decreases drastically to less than once per five years. Here the main causes of heavy rianfalls are fronts and tropical cyclones, mainly in July and August. This time the Polar front lies in the northern part of Hokkaido Island and this location corresponds to its mean northernmost position. The region in which the main cause of heavy rainfalls is such northernmost Polar front is classified into ‘frontal heavy rainfall region’. In addition, extremly low frequency heavy rainfall rigion in which no heavy rainfalls were recorded through whole investigated period are discerned.