Paleomagnetic studies for igneous and sedimentary rocks of the latter part of the Tertiary have been facilitated in the Japan arc since the beginning of 1980. We review the paleomagnetic works of Northeast Japan and Southwest Japan, which have been interpreted that the Northeast Japan arc has rotated counter-clockwise through 47° around a vertical axis while the Southwest Japan arc has rotated clockwise through 56° between 21Ma and 11Ma. The rotation process of both arcs yields reasonable estimates of timing and mode of the opening of the Japan Sea.

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Paleomagnetic measurements have been made on tuff and mud samples from 25 sites of the Morozaki Group and the Mizunami Group, distributed in the eastern part of Southwest Japan. Stability tests by alternating field and thermal demagnetization reveal the existence of characteristic remanent magnetization, of which directions show significant clockwise deflection; the mean declination shift exceeds 50° in the late Early Miocene members (N6-N8), and is less than 30° in the early Middle Miocene members (N9). This result, compared with the other paleomagnetic data from the Setouchi Miocene Series and the volcanic rocks in the San'in district, suggests that coherent rotation of Southwest Japan began between the late Early Miocene and the early Middle Miocene. Nearly a half of the rotation might have been attained in the stage of Zone N9, which possibly corresponds to the period of the opening and increasing subsidence in the Japan Sea.

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Paleomagnetic studies of the Tertiary volcanic rocks were carried out in the four regions of the Noto Peninsula, the Sado Island, Niigata Prefecture and Fukushima Prefecture located on both sides of the Itoigawa-Shizuoka tectonic line in central Japan.
Rocks older than 16 Ma show a clear contrast in their declinations between the sides of the line. The paleomagnetic mean declination for the Noto region shows no significant deflection while those of the Sado and Fukushima regions are deflected westerly by 17° to 27°.
Moreover, as for the mean declinations for the period younger than 14Ma, there is a slight difference recognized between the data obtained from Niigata and Fukushima regions. It is concluded that the rotational tectonisms were still continuing until the late Miocene time in the eastern part of central Japan, and that central Japan was not a single tectonic block but has divided into several local sub-blocks.

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A paleomagnetic study has been carried out on the Miocene volcanic and sedimentary rocks from the Yatsuo area, the northern part of the Chubu district. Stable primary magnetizations are recognized from 8 sites among 36 sites ranging from 17 to 7Ma in age. Eight sites are grouped into two by their declinations; the lower 4 sites show easterly deflected declinations (mean direction: D=12.6°, 1=47.1°, α₉₅=7.0°) and the upper 4 sites show no significant deflection from the axial geocentric dipole field (mean direction: D=-2.5°, I=55.6°, α₉₅=6.6°). This result suggests that the Yatsuo area was subjected to a clockwise rotation about 13° between 15 and 12Ma, possibly synchronous with the rotation of Southwest Japan. The smaller rotation angle than that estimated for Southwest Japan (47°) suggests the differential rotation between the block which contains the Yatsuo area and the main part of Southwest Japan.

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Paleomagnetic measurements were made on the early Miocene marine sediments in the Chichibu Basin of the Kanto Mountains, Central Japan. The mean paleomagnetic direction, D=94°, I=53° (α₉₅=8.3°), indicates that the Kanto Mountains have not changed their latitude significantly, but have rotated clockwise through about 90° since the middle Miocene. The rotation can be explained by the opening of the Japan Sea and the collision of the Tanzawa Block.

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A paleomagnetic study has been made on 14 dikes emplaced at two formations in the Oga Peninsula. Stable remanences of seven dikes are normally magnetized, six are reversely magnetized, and one has an intermediate direction. No significant difference in the directions is observed between the normally and the reversely magnetized rocks and between the intruded formations. The veins of dikes trend northeast with the standard deviation of 4°. Mean inclination and declination of remanence for 13 dikes excluding an intermediate one are calculated to be 58.3° and -2.8°, respectively. The pole position corresponding to the mean field direction is close to that observed at the Middle Miocene formations in the peninsula and in the northern part of NE Honshu Island. Dike rocks indicate the bimodal volcanism characteristic in Middle Miocene time. The direction of stable remanences and the bimodal distribution suggest that the dikes intruded in Middle Miocene time. The dominant orientation of strikes in the Oga dikes represents one of the general trend of the Middle to Late Miocene stress field after the tectonic rotation of NE Honshu Island had completed.

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Cretaceous and early Tertiary rocks have been collected, by many workers including the present authors, from about two hundred sites in the Japanese Islands and South Korea for paleomagnetic investigations. The area mean direction of magnetization for the Oshima Peninsula and Okushiri Island of Hokkaido was 30.0° in declination and 58.4° in inclination. It differs distinctly from the area mean direction for granitic rocks from the Kitakami and Abukuma mountains. The area mean direction of magnetization obtained from the northern Kitakami belt was extremely westerly (D=-81.1°, I=22.3°) and that from the southern Kitakami belt was northwesterly (D=-44.4°, I=51.7°). The area mean from the Abukuma belt was also northwesterly (D=-31.9°, I=54.6°). The site mean directions of magnetization for granitic rocks in the Uetsu Province were not significantly grouped in the area. In Southwest Honshu, the area mean direction of the normal samples from various kinds of Cretaceous rocks was 50.2° in declination and 60.5° in inclination and that of the reversed ones was -114.80° and -55.8°, respectively. The normal and reversed magnetizations of early Tertiary rocks have shown similar directions to those of the Cretaceous rocks. The area mean of the normal samples was D=37.6°, I=52.3° and that of the reversed ones was D=-131.4°, I=-44.8°, respectively. The site mean directions of magnetization for Cretaceous rocks in South Korea appear to be slightly northeasterly at each sampling site. The remarkable discrepancy between these area mean directions of magnetization is interpreted as being due to regional tilting movements which occurred in the Japanese Islands since the Cretaceous.

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Paleomagnetic results have been obtained from ten Korean sedimentary rock formations ranging in age from Permian to Cretaceous. The magnetizations of almost all the rocks from Permian to Jurassic formations have been severely overprinted. Estimation of paleomagnetic direction of the period between Permian and Jurassic is hindered by this. The Cretaceous rocks from the Gyeongsang Supergroup, however, have recorded the paleomagnetic direction at the period of formation of sedimentary rocks during Cretaceous. Some strata in the Hasandong Formation of lower part of the Gyeongsang Supergroup show the reversed magnetization which is presumably ascribed to reversed magnetic polarity epoch of M-series in Mesozoic polarity scale. Paleomagnetic direction of the upper part of the Gyeongsang Supergroup (Middle to Late Cretaceous) is estimated to be Dec=28.4°, Inc=58.2° and α₉₅=6.4°. The pole position of Middle to Late Cretaceous obtained for the Korean Peninsula (202°E, 67°N) is in good agreement with other Cretaceous data for the Asian continent, implying that the Korean Peninsula has not been subjected to rotational movement relative to the Asian continent since Cretaceous.

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A magnetic anomaly contour map (contour interval is 50nT) is compiled with all the available data of the total intensity of geomagnetic field. Data are very dense in the coastal areas of the Japanese islands so that contour control is good while in the coastal areas of U. S. S. R. and the Korean Peninsula, data are not so dense. The amplitudes of anomalies are generally small compared with that in the northwestern Pacific Ocean. The magnetic anomaly lineations exist in the southern half of the Japan Basin. From identification of the lineations, the Japan Basin stopped opening 15Ma BP on the assumption of two-limbed opening and the spreading rate of 3.4cm/yr. There are many seamounts and uplifts which have anomaly patterns suggesting their reversed magnetization. This seems to be a crucial point towards understanding the tectonics of the Japan Sea.

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Geomagnetic anomaly profiles obtained by Isezaki and Uyeda are reanalysed to infer the mode and time of opening of the Sea of Japan, especially in the Japan Basin. The anomalies show a roughly ENE-WSW trend which is subparallel to the coastlines of Honshu island and Sikhote Alin. It is inferred from the analysis that Vine-Matthews type magnetic anomalies are present at least in the Japan Basin. Possible reasons why the anomalies could not be correlated in the previous analyses with the geomagnetic reversal time scale are also examined. One possibility is that the anomaly patterns in the Sea of Japan is not a product of simple sea-floor spreading but is obscured by the complexity in the mode of opening or in the tectonic evolution. However, the most plausible explanation is that the assumption of symmetric spreading about some spreading center was wrong. The shape analysis shows that a better fit is obtained for one-sided spreading with a spreading center near the coast of Sikhote Alin. A satisfactory fit is obtained for a magnetic profile with a model generated for the period of 10-15Ma.
This timing for the opening of the Sea of Japan is broadly consistent with the paleomagnetic data from the northeastern part of Honshu island. It is suggested that the original Vine-Matthews anomaly pattern was “contaminated” by the off-ridge volcanism which is also responsible for the creation of the abundant seamounts in both the Japan Basin and Yamato Basin.

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Ages of the Japan and the Yamato Basins in the Japan Sea were examined on the basis of three independent methods; stratigraphic consideration of the basin sediments, age-depth correlation, and age-heat flow correlation. Basement depths of the Japan and the Yamato Basins after sediment loading correction were compared with those of other marginal seas in the Western Pacific whose ages are well constrained. The age estimations by age-depth correlation and age-heat flow correlation show good coincidence in the Japan Basin. The age estimation of the Yamato Basin, however, shows a discrepancy among the three methods; the basement depth shows an age range from 6Ma to 0Ma, sediment stratigraphy shows that of about 10Ma, and heat flow data show the age from 30Ma to just over 10Ma. This discrepancy can be explained by the thick accumulation of Neogene volcaniclastics which are represented as acoustic basements on seismic reflection records with a refraction velocity of 3.5km/sec. The final estimated age of the Japan and the Yamato Basins is identical with a range of 30Ma or older to 15Ma or slightly younger.

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Horizontal profiles of geomagnetic transfer function were obtained at the periods of 15, 30, 60 and 120 minutes by analysing 3-component geomagnetic variation data at 21 sites in the central part of Japan and 7 sites on the Philippine Sea floor. A two-dimensional conductivity model was constructed, by use of the finite element method, to represent the spatial and period dependence of these transfer functions.
Characteristics of the observed transfer functions on the Pacific coast and continental shelf of the Philippine Sea were well explained by introducing a highly conductive layer beneath the Philippine Sea plate at a depth of about 30km in addition to a large conductivity contrast between land and seawater. In order to account for the anomalous phase variation observed at the northern half of the central Japan sites, conductive lower crust was required beneath this region. This lower crustal conductor also explained the reduction of the coast effect on the Japan sea coast, and the existence of free water in the lower crust was considered as its likely cause. Our result did not require the conductive upper mantle beneath the Japan Sea, at least 200km away from the coast, to explain the data, which may suggest that the plate motion is the possible cause of highly conductive asthenosphere as well as the partial melting.

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Based on available radiometric ages on igneous rocks recovered from the Japan Sea area, the constraints on the time of the evolution of the Japan Sea floor are discussed. To evaluate the reliability of radiometric ages on submarine rocks, some general problems related to each dating method are also referred.
As far as volcanic rocks recovered from seamounts in typical oceanic regions such as the Yamato Basin and the Japan Basin are concerned, they show relatively young K-Ar ages of less than 20Ma. However, rocks recovered from the Yamato Bank area, which is regarded to be a remnant land mass, suggest the occurrence of definitely older volcanic activity more than 20Ma ago. In the southwestern part of the Japan Sea, which has a continental crust, granitic rocks of more than 100Ma together with Precambrian gneisses were recovered. Thus radiometric age data support a relatively young formation of the typical ocean floor in the Japan Sea area.

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A sharp changeover of benthic molluscs from warm water to cold water faunas is widely recognized in the Japan Sea coast early Middle Miocene deposits dating from about 15 Ma ago and lying within the Blow's N. 9 planktonic foraminiferal zone. The change in the molluscan faunas is attributable to a regional change in paleogeography around the Japan Sea. Recent paleomagnetic work shows that the clockwise rotation of Southwest Japan took place within a very short interval, probably between 15 and 14Ma ago, at about the same time as the faunal change. The sharp change in marine faunas may be reasonably explained by the rotation and opening of the Japan Sea. As Southwest Japan rotated clockwise around the pivot located in the northern East China Sea, the seas around the Tsushima Straits, a passageway of the warm Kuroshio Current, might be closed. The Japan Sea was turned to open toward the north, and thus brought the fauna under the influence of cold, northern water.

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On the basis of compilation of paleomagnetic direction data, the mid-Cenozoic geography and tectonic stress field of the northeastern and southwestern Japan were restudied. Variations in paleomagnetic directions among the three tectonic provinces (northern, central and western Japan) cannot adequately be explained only by the abrupt 15Ma rotation of the two halves of Honshu arc, but can be satisfied by the preceded tectonism associated with initial spreading of the Japan Basin. The major new points are: (1) counterclockwise rotation of northeastern Japan had already occurred continuously during 32-23Ma, probably followed by another counterclockwise rotation associated with the opening of the Yamato Basin around 15Ma. (2) It is until the start of 15Ma clockwise rotation of western Japan that the three paleomagnetic provinces belonged to an extensional stress field on the elongated Honshu arc along a subduction zone of the Pacific plate. (3) The later local rotation might be probable in Northeastern Honshu during 15Ma to 7Ma concerning the development of intra-arc basin.

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