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Sediments and Structure of the

WILLIAM J. LUDWIG Lamont-Doberty Geological Observatory of Columbia University, Palisades, New York 10964 SADANORI MURAUCHI National Science Museum, Ueno Park, Tokyo, Japan ROBERT E. HOUTZ Lamont-Doberty Geological Observatory of Columbia University, Palisades, New York 10964

ABSTRACT INTRODUCTION Kula-Pacific ridge. Estimates of the time of formation of the sea range from the Cre- Seismic reflection (profiler) traverses of The -arc—deep-sea-trench systems taceous to the Oligocene, with final forma- the Japan Basin, Yamato Basin, and inter- of the western Pacific are related to tion by the early Miocene. In this paper, we vening Yamato Ridge reveal horizontally the small semi-isolated marginal or present a synthesis of Lamont-Doherty stratified sediments over weakly stratified back-arc basins such as the Japan Sea, Geological Observatory seismic data - sediments. The basement surface is rough in whose geologic structure and formative his- tained in the Japan Sea during 1969 and some places and smooth in others and rises tory are largely unknown. Hypotheses on 1971. These data provide another set of ob- with the topography of Yamato Ridge and the origin of back-arc basins include en- servations to the growing list of hard facts the lower continental slopes of and trapment of old oceanic crust by formation with which to consider fundamental ques- Japan. Compared to the Japan Basin, of the , generation of new crust by tions regarding the origin and development Yamato Basin has a shallower sea floor and arc migration (sea-floor spreading), subsid- of the sea. thinner sediments. In each basin, the sedi- ence or collapse of continental or quasi- ments decrease in thickness outward from a continental crust with attendant or subse- SYNOPSIS OF GEOLOGICAL center. quent oceanization, rejuvenation by intense AND GEOPHYSICAL PROPERTIES Wide-angle reflection and refraction data volcanism, and any combination of these OF THE JAPAN SEA from 65 sonobuoy stations made en route mechanisms. The genesis is believed to be give velocities in the sediments that range directly related to the formation of the bor- Submarine Morphology from 1.6 to 3.2 km/sec. Smooth oceanic dering island arc and continental margin. basement (or layer 2) has two refracting Kaseno (1969, 1970) has compiled a list The Japan Sea is separated by the layers, 3.5 and 5.8 km/sec; rough oceanic of publications that deal with the geological Plateau and Oki Bank-Yamato Ridge into basement is typified by the 5.8 km/sec veloc- and geophysical properties of the Japan the Japan Basin, the Yamato Basin, and the ity alone. Layer 2 is thicker in the Yamato Sea, an area of about 1 million km2 located (Fig. 1). The Japan Basin Basin than in the Japan Basin because of a between Japan, Korea, and the USSR (Fig. has water depths of 3,000 to 3,700 m; greater amount of 3.5-km/sec capping ma- 1). Asano and Udintsev (1971) edited an as- Yamato Basin and Tsushima Basin have terial. Layer 3, of velocity about 6.8 km/sec, semblage of papers summarizing the exten- depths of 2,000 to 2,500 m. In each basin, lies at nearly the same depth beneath the sive work of Japanese and Soviet inves- the sea floor is fairly smooth, except for oc- basins and Yamato Ridge. tigators. These studies have led to several casional seamounts and sea hills, and it dips The results of profiler-sonobuoy mea- divergent explanations for the origin of gently to the north. Northward-trending surements combined with the results of ear- the sea. promontories from subdivide the lier two-ship seismic refraction measure- According to Beloussov and Ruditch southeasternmost part of the sea into a ments indicate that the Japan Basin and (1961), Minato and others (1965), Belous- complex series of steep-sided ridges and in- Yamato Basin are underlain by oceanic sov (1968), and Minato and Hunahashi tervening troughs that trend approximately crust which in turn is covered by sediments (1970), the Japan Sea is a former landmass parallel to the coastline. Sado Ridge is a (and volcanics?) that have built a shallower that has subsided to its present depth by double ridge with a central (unnamed) sea floor than that in the western North crustal foundering or by oceanization. Vas- trough having an irregular longitudinal Pacific basin. Yamato Ridge appears to be ilkovsky and others (1971) share the view profile. In the central part of Mogami mainly a pile of volcanics resting on an that the sea represents a permanent ocean Trough, the sea floor is fairly flat; to the oceanic layer at normal depth. The crust of basin. The most popular hypothesis regard- north, it is more irregular and dips down Yamato Basin may also have been modified ing formation, however, is that the Japan toward Yamato Basin. Descriptions of the to the extent that it has a thicker than nor- Sea (notably the Japan Sea basin) is the various physiographic features are given by mal layer 3 and a low-velocity mantle. by-product of the southward drift of Japan Iwabuchi (1968) and by the explanatory Other profiler-sonobuoy data, gathered from (Murauchi, 1971; Matsuda and text of Japanese bathymetric chart 6302. in the strait between Japan and Korea and Uyeda, 1971; Hurley and others, 1973). Of particular interest to those working across the Japan Sea margin of Southwest Hurley and others contend that the Yamato on the geology of the Japan Sea is Oki Japan and Northeast Japan, are presented. Ridge and Yamato Basin are a submerged Bank—Yamato Ridge, an offshoot of Hon- A number of profiler crossings of section of continental crust. Hilde and shu that extends northward from the shelf Channel indicate its formation by turbidity Wageman (1973) consider that the Japan off Shimane Peninsula, curves clockwise currents. Turbidity flows in this channel Sea opened up from west to east, and they toward the Oga Peninsula, and almost and in other channels transport sediment to predict that the oldest basement is in the ex- closes the Yamato Basin. Yamato Ridge is the . Key words: marine treme western part of the sea. Uyeda and composed of several banks which are sepa- geophysics, seismic surveys, sonobuoys, Miyashiro (1974) have proposed that the rated from each other by transverse depres- marginal ocean basins, sediments, struc- Japan Sea opened up as the result of colli- sions. A longitudinal depression, the Kita- tural analysis. sion and subduction by the hypothetical Yamato Trough, divides the ridge into a

Geological Society of America Bulletin, v. 86, p. 651-664, 9 figs., May 1975, Doc. no. 50507.

651

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130° 135° 140° Figure 1. Physiography of the Japan Sea. Base is Japanese bathymétrie chart 6302. Nomenclature is adapted from the explanatory text of chart 6302 and Iwabuchi (1968). K-YB, Kita-Yamato Bank; YB, Yamato Bank; TB, Takuyo Bank.

northwestern part and a southeastern part. indicating that parts of the ridge may have eral physiography of the ridge and the pat- The crestal surfaces of the banks are often once stood near sea level and later subsided. tern of sedimentation over it and in the ba- gently arched and topped by terraces at 250 The discussion of the seismic reflection sins on either side. Various other topo- to 900 m below sea level (Iwabuchi, 1968), profiles that follows will compare the gen- graphic features, such as Toyama sub-

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marine channel, are similarly described in Dogo and Dozen (the Oki ) and plitudes of body waves for nearby and dis- relation to their seismic cross section. Utsuryo Island are essentially volcanoes of tant earthquakes recorded at several sta- alkaline rock, presumably built on plat- tions in Japan, Utsu (1966, 1967, 1969, Japan Sea Side of Honshu: forms of Paleozoic metamorphics (Hida 1971) and Utsu and Okada (1968) show Geologic Setting gneiss?). On Dogo Island, eruptions of that the body waves from intermediate to calc-alkaline of middle Miocene deep earthquakes beneath Japan and the A complete description of the geology of age were followed by eruptions of Japan Sea are attenuated much less than the lands bordering the Japan Sea is beyond trachybasalt. The volcanism that began in waves of the same type that propagate in the scope of this paper. Therefore, we will the early Miocene ended by the late the mantle at similar depths on either side restrict our discussion to the gross structure Miocene or early with uplift in the of the zone. In the anomalous zone, the ve- of the Japan Sea side of Honshu Island, as green tuff that has continued up to locity (V) is higher by about 6 percent and has been described by Saito and others the present. A resurgence of the volcanic ac- Q (the reciprocal of the attenuation factor) (1960), Takai and others (1963), and tivity, primarily basaltic, occurred in the is about ten times higher than in the adja- Minato and others (1965). Quaternary with most of the volcanos cent zone with low V and low Q values. The Japanese Islands are divided by a originating in the early Pleistocene (some of According to Utsu's (1971) model, the low zone of rupturing and volcanism, called the these are still active). V—low Q zone is 70 to 100 km beneath the Fossa Magna, into two main structural and 50 km or less beneath the units, Northeast Japan and Southwest Geological Sampling Sea of Japan. If the attenuation of seismic Japan, the latter is divided by the median waves is largely a function of rigidity, then dislocation line into an inner zone and an Dredging on Yamato Ridge and on the the upper parts of the zone of high V—high outer zone (Fig. 1). Southwest Japan is various banks and seamounts of the Japan Q may correspond to the lithosphere, and characterized by exposures of Paleozoic Sea has produced a wide assortment of those of the low V—low Q zone to the as- and Mesozoic rock, and limited occur- igneous rocks and the sands and gravels de- thenosphere. rences of Cenozoic rock, whereas North- rived from them (Hoshino and Homma, east Japan is widely covered by Cenozoic 1966; Iwabuchi, 1968). , granodio- Heat Flow rock. The major difference between the rite, and andesitic tuff breccia, inner and outer zones of Southwest Japan is , and were dredged from Heat flow in the Japan Sea is appreciably that the outer zone has east-northeast— three banks constituting the northeastern higher than that in the Pacific Ocean basin trending structures, ranging in age from half of Yamato Ridge. K-Ar age determina- (Yasui and others, 1968; Uyeda and Vac- Paleozoic to Paleogene, which become tions gave 197 m.y. and 220 m.y. (Triassic) quier, 1968). Values range between 1.3 and younger toward the Pacific side. For the for the ages of the granitic rocks dredged 5.3 HFU and average 2.2 HFU. The values most part, the outer zone is occupied by the from Kita-Yamato Bank and Takuyo Bank, tend to be higher in the deep northeastern Shimanto-Hidaka orogenic belts of Meso- respectively (Iwabuchi, 1968); 20 m.y. part of the Japan Basin than in the shal- zoic-Paleogene age. (early Miocene) for the average age of the lower Yamato Basin. Heat-flow measure- Tectonic movements related to the for- and andesites dredged from Yamato ments are difficult to obtain over Yamato mation of Japan as an island arc began dur- Bank; and 7.7 to 4.2 m.y. (late Ridge because of the hard bottom which ing the late Oligocene or early Miocene Miocene—late Pliocene) for the ages of prevents deep penetration of the thermistor with large-scale calc-alkaline volcanism andesites dredged from seamounts near probes. Measurements taken along the that took place along the Japan Sea side of Honshu (Ueno and others, 1971). flanks were high. In Kita-Yamato Trough, Honshu and , along the Fossa Quaternary-Holocene volcanism seems to however, the heat flow is comparatively Magna and Izu-Bonin Ridge, and along the have been completely confined to land areas low, averaging 1.8 HFU. western side of Kyushu. Prior to this event, because there are no known volcanos (sea- intrusions of granitic magma occurred re- mounts) of this age in the Japan Sea. Magnetics peatedly from Cretaceous to Paleogene time in the inner zone of Northeast Japan and in Seismicity Uyeda and others (1967) and Yasui and the inner zone of Southwest Japan (where others (1967) compiled all existing mag- are observed to intrude Cretaceous The Japan Sea is the location of inter- netic data on the Japan Sea and presented and older rocks). Granitic activity also oc- mediate to deep-focus earthquakes that charts of the total magnetic intensity and curred in the Korean Peninsula and in East- occur along a classic Benioff plane, plung- the total intensity magnetic anomaly ern Siberia. On Goto and Tsushima Islands, ing westward beneath Japan and the Japan (TIMA). In comparison with the typical granite has been intruded into the rocks of Sea. There are no shallow-focus - magnetic anomaly lineation patterns in the Oligocene age. quakes associated with the basins and western North Pacific basin, the amplitude Volcanism along the Japan Sea side of ridges of the sea. Sugimura (1960) and of magnetic anomalies observed at sea level Honshu was accompanied by subsidence others have presented contour epicentral in the Japan Sea (and Okhotsk Sea) is about and marine transgressions that resulted in maps showing curved seismic zones be- five times smaller, and the wavelength is accumulations of volcanogenic and neath Japan and the Japan Sea. Carr and about three times shorter (Isezaki, 1973; biogenic sediments more than 2,000 m others (1973) present a somewhat different Murauchi, 1972a). As a result, lineations of thick, referred to as green tuffs. These beds contour interpretation of the deep seismic 200 gammas in the anomaly pattern, if have been intruded by quartz diorite and zones, through the observation that there present, are most difficult to recognize. The granodiorite masses. The green tuff of are abrupt changes in the earthquake dis- TIMA contour maps show directions of consists of basaltic and tribution pattern along the strike of the elongation that are generally east- zones. Their contours, based on a model of andesitic agglomerates and lavas, dacitic or northeast, more or less parallel with the segmented blocks, approximate straight rhyolitic volcanoclastics, and dike rocks. trend of the Japan Arc. lines that divide the deep seismic zones into Andesites and , presumably of Isezaki (1973) recalculated the magnetic distinct segments. early Miocene age, are exposed on Noto anomalies, using a reference field that Peninsula, as well as the Hida metamorphic From studies of differences in earthquake matched more accurately the absolute val- complex of Paleozoic age. response and variations in spectral am- ues and trend of the observed total field

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and, by incorporating additional survey nov, 1965; Kovylin and others, 1966; ment in Yamato Basin are shallower than data, constructed a somewhat different Murauchi, 1966; Vasilkovsky and others, they are in the Japan Basin (in the areas not TIMA map than the earlier ones (also see 1971). complicated by the subbottom topography Isezaki and Uyeda, 1973). In Isezaki's map, of Yamato Ridge). the anomalies are corrected for about a NEW OBSERVATIONS Sediment Thickness. Isopachs in Figure — 100 gamma mismatch between the refer- 4 (compare Kovylin and Shayakhmetov, ence and observed magnetic fields, and the Seismic Profiler Data 1971) represent the "thickness" of the sed- pattern in no way resembles the lineated iments of velocity generally less than 3.5 patterns usually identified with sea-floor General. Continuous seismic reflection km/sec above acoustic basement in units of spreading. Relatively high amplitude measurements were made along the ships' reflection time. The northeast half of the anomalies, of about 500 gammas, are as- tracks shown in Figure 2. A number of gen- Japan Basin has thicker sediments than the sociated with sea peaks of Yamato Ridge eral observations can be made from the shallower Yamato Basin. In the Japan and seamounts in the basins on either side, profile sections (Fig. 3) that are particularly Basin, the sediments decrease in thickness but these also are much smaller than the important in evaluating the various expla- from a center outward. A similar pattern of anomalies associated with seamounts in the nations given for the origin of the sea. sediment distribution exists in Yamato western North Pacific basin. 1. The sediments above acoustic base- Basin, but the pattern is complicated by a ment (the deepest reflector observed) in the basement ridge (?) that marks an abrupt Gravity Japan Basin and Yamato Basin are com- change in level of the sea floor. The floor of posed of two main sections, an upper section the western part of Yamato Basin is deeper Free-air gravity anomalies in the deep- of highly stratified sediments (turbidites), than that in the eastern part by about 250 water basins of the Japan Sea are generally and a lower section of weakly stratified sed- m and is conspicuously flat (profile GH of positive, 10 to 20 mgal, and the field is iments (pelagics). The upper section has Fig. 3B). Furthermore, the sediments of smooth (Tomoda and others, 1970; Stroev, fairly uniform thickness over the entire re- Yamato Basin tend to abut the lower conti- 1971). There are no conspicuous negative gion, is horizontally bedded, and shows no nental slope of Northeast Japan. gravity zones in the sea except for a small signs of deformation. The lower section has Toyama Channel. Most of the smaller minimum located near the foot of the con- variable thickness and, presumably, is' also basins and troughs adjacent to Siberia and tinental margin of Siberia. Slightly higher undeformed. There are no obvious discon- Japan have canyons or channels through values of free-air gravity (up to +75 mgal) formities in either section. which sediment is carried by turbidity cur- occur over Yamato Ridge. 2. The acoustic basement of the Japan rents to the deeper basins (Iwabuchi, 1968). The simple Bouguer gravity anomaly of Basin and Yamato Basin is smooth and flat One of these is Toyama Channel extending the Japan Sea is 200 to 350 mgal less than in the center of the basins, as if distributed northward from Toyama Trough out to the that in the and western by a gravitational process, which indicates edge of the abyssal plain of the Japan Basin North Pacific basin (Dambara, 1968; the presence of high-speed sediments and (Fig. 5). The topographic profiles of Yoshii, 1972a). Yoshii (1972a) explains the (or) volcanic flows "ponded" over an un- Iwabuchi (1968) and our seismic profiler small Bouguer anomaly by assuming that derlying layer of rough topography. The traverses of the channel, reproduced in Fig- the density in the low V—low Q zone (as- rough and smooth basement areas do not ure 6, show rather conclusively that the thenosphere) is lower than that in the high produce detectable changes in the magnetic channel is an erosional-depositional feature V-high Q zone (lithosphere). In brief, his pattern observed at sea level. produced by the action of turbidity cur- calculations from gravity data support 3. The acoustic basement of Yamato rents. Toyama Channel has a V-shaped Utsu's (1971) model of a shallow low Ridge is characteristically rough. The asymmetric cross section and, outside of V-low Q zone beneath the Japan Sea (that sea-floor topography of the ridge has been Toyama Trough, is bordered on each side is, the Japan Sea has a thin lithosphere be- modified and greatly smoothed by deposi- by embankments with different heights that neath it). tional draping of pelagic sediment on resemble natural levees. The gentler slope basement. and higher levee of the channel is generally Crustal Structure 4. The distribution of weakly stratified situated on the eastern side, the result of (pelagic) sediments along Yamato Ridge is tilting of the upper surface of the chan- Several attempts have been made to map not uniform. The ridge flanks have varying nelized currents in response to Coriolis and the sediments of the Japan Sea by reflection amounts of pelagic sediments, whereas the centrifugal accelerations (Buffington, 1952; profiling (Hotta, 1967; Hilde and others, ridge crests seem to be fairly clean of them, Menard, 1955; Komar, 1969; Ness and 1969; Asanuma and Murauchi, 1970; perhaps due to the sweeping action of cur- Kulm, 1973). Hilde and Wageman, 1973), but acoustic rents. The gentler east slope indicates that the penetration of the sediments to basement 5. The acoustic basement rises toward current is tilted up toward the east, result- was hampered by lack of sound source of Yamato Ridge in the basins on either side. ing in greater erosion of the eastern side of sufficient power and (or) by electronic There is no structural or topographic de- the channel than of the western side at the problems with the hydrophone streamer. In pression bordering the ridge. same depth. The levees, of course, indicate the early Soviet seismic refraction mea- 6. The acoustic basement of the basins that turbid sediment-laden water overflows surements in the Japan Basin, oceanic layer rises with the topography of the continental the channel during times of flood. The ab- 3 was not detected; instead a thick layer slopes of Siberia and the western side of rupt decrease in velocity as the water leaves with a velocity of 6.1 to 6.4 km/sec was re- Northeast Japan. There are no declivities in the channel causes settling of the suspended ported to overlie mantle at normal depth layer 2 and no continental rise sedimentary sediment and formation of the levees. Be- (Andreyeva and Udintsev, 1958). This led prisms associated with these margins. In cause the mass of the flow is directed toward Menard (1967) and others to classify the most places off the western side of North- the east by Coriolis accelerations, the entire sea as having characteristics inter- east Japan, particularly where the lower levees built there are usually higher. Rever- mediate between continental and oceanic continental slope to the Yamato Basin is sals in the asymmetry between slopes of the crust. More recent Soviet and Japanese re- relatively steep, the sediments are of uni- channel (compare profiles N and P) or de- fraction measurements in the Japan Basin, form thickness almost to the base of the partures from the norm of higher or however, indicate that the crust is typically lower slope. broader levees on the east (see topographic oceanic in structure (Kovylin and Neproch- 7. The sea floor and the acoustic base- profiles of Iwabuchi, 1968) are probably

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45°

130° 135° 140° Figure 2. Location of seismic profiler traverses and sonobuoy stations in the Japan Sea. Heavy-lettered segments indicate record sections reproduced in Figure 3. Base is the Japanese bathymétrie chart 6302. Contours are in meters. Two-ship profiles are those of Murauchi (1966). For the convenience of drafting, the first digit (l or 3) has been deleted from most of the sonobuoy station numbers.

caused by centrifugal accelerations of the Japan Sea Margin of Japan. The profiles east Japan. Acoustic penetration of the sed- current (in bends of the channel) that act in of Figure 6 also show the sea floor mor- iments is quite limited with the seismic a direction opposite to that of the Coriolis phology (as already described) of the conti- profiler because of the wide distribution of acceleration. nental margin of the western side of North- sands and gravels (Iwabuchi, 1968) which

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* Assumed velocity

( ) Refraction velocity

Figure 3. Seismic profiler sections of the Japan Sea aligned from north to south. Pneumatic sound source. Passband 15—40 Hz. Vertical scale represents two-way reflection time. Horizontal scale varies with ship's speed, 6 to 8 knots, which causes a vertical exaggeration of about 25 to 1. All velocities are interval velocities from wide-angle reflection data, unless designated otherwise.

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greatly attenuate the seismic energy. Where penetrated by the seismic profiler, the sedi- ments appear to be moderately deformed (compare Hotta, 1967). The structure of the Japan Sea margin of Southwest Japan is similar to that off Northeast Japan, except that sedimentation has completely filled at least one of the offshore troughs, and indications are that others are being filled; the eventual result may be the construction of a shelflike plat- form such as that between Japan and Korea. The southernmost part of Yamato Basin (between Yamato Ridge and Oki Ridge) has thick sediments which have built up the sea floor (profile G-K of Fig. 3B). Oki Trough is almost completely filled at 40* the northern end (profile E-F of Fig. 3A). There is a buried ridge at the outer edge of the that trends roughly northeast between the Shimane and Noto peninsulas (profile E-F of Fig. 3A; profiles G-K and J-K of Fig. 3B) (Asanuma and Murauchi, 1970). Approximately 2 sec of sediment were measured at points K and F (Fig. 2) shoreward of the ridge. Presumably, the ridge, herein called Shimane Ridge, is the seaward border of a sediment-filled trough or coastal basin. Sediment-filled ba- sins with the outer ridge portion seemingly the continuation of coastal promontories are characteristic of the Philippine Sea side 35° of Southwest Japan (Ludwig and others, 1973).

Profiler-Sonobuoy Data

During the recording of vertical reflection data with a pneumatic sound source, radio-sonobuoys were launched at various localities (Fig. 2) to obtain simultaneously way reflection time (0.25-sec interval is used in thick sediments). Open and filled circles designate variable angle reflected and refracted waves sonobuoy stations. Actual thicknesses may be obtained by multiplying one-half the reflection time in- from the subbottom layers (Le Pichon and terval by the mean velocity in the sediment from sonobuoy solutions (Table 1). Note that the regres- others, 1968; Houtz and others, 1968, sion equation in Figure 8 can be integrated in time to yield thickness as a quadratic expression of 1970). Most of the lines of measurement one-way reflection time with sonobuoys were extended beyond the range of signals from the air gun by deto- (h = 1.43 + Ml T2) . nating 100 to 200-gram charges of explo- sives (Seismogel) every 45 sec or less. The Therefore, a 10-fold increase of reflection time results in a 16-fold increase in thickness, a property velocities and layer thickness computed that should be remembered when studying isopach maps contoured in reflection time. from the sonobuoy data are listed in Table 1 and are shown diagrammatically in Fig- ure 7. A plot of sediment-interval velocities as a function of one-way vertical travel time 5.8-km/sec lower part. In some localities, a The thinness of the 3.5-km/sec layer (along with the least-squares regression middle layer of velocity 4.5 km/sec is pres- (compared to the 5.8-km/sec layer) makes it equation) from the Japan Sea basins ap- ent. The top of the 3.5-km/sec layer repre- difficult to measure by first refracted arriv- pears in Figure 8. These results compare sents the smooth upper surface of the als because of the narrow range in which with the data published by Hamilton and basement. Where this layer is very thin or they occur. In the sonobuoy records from others (1974) from the Japan Sea and Tatar absent, the basement surface is usually the Japan Basin, first arrivals from the top Strait. rough and exhibits a velocity near of the 3.5-km/sec layer are usually not ob- Analysis of the sonobuoy data reveals the 5.8-km/sec. The 3.5-km/sec layer is thicker served; only reflected waves are recorded. basic velocity structure of the deep-water in Yamato Basin than it is in the Japan Therefore, it is often necessary to assume a basins of the Japan Sea. The sediments con- Basin. The additional resolution provided velocity in this layer if it cannot be com- sist of layers of velocity ranging from 1.6 by the sonobuoy technique shows a much puted from T2/X2 data. The necessity to as- km/sec to 3.2 km/sec (Figs. 7 and 8). more complicated and realistic structure sume a refraction velocity arises whenever Oceanic basement (or seismic layer 2) un- than was formerly believed (see Hilde and the basement refraction line falls well below derlies the sediments. It is divisible into a Wageman, 1973, for a recent summary the point where it should be tangent to the thin 3.5-km/sec upper part and a thick based on the earlier data). reflection curve formed by acoustic base-

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SR-Sado Ridge

WR- Wakasa Ridge 40" ir © MT- Mogami Trough

39°

37°

137° 138° 139° 140° Figure 5. Inset map of Figure 1, showing loca- tion of seismic profiler sections reproduced in Figure 6.

ment (see Houtz and others, 1970, for de- tails). If the gap between them is trivial, a velocity assumption may not be required, but gaps of 0.1 sec are commonly observed (about 2 wavelengths) and therefore require an assumed velocity. The depth to the top of layer 2 in the Japan Sea increases northward, from 4 km below sea level in Tsushima Basin, Yamato Basin, and Oki Trough to 6 km depth in the northeast part of the Japan Basin. This cor- responds to an increase in water depth and sediment thickness. In the Japan Basin, the 3.5-km/sec basement layer decreases in thickness rapidly with distance away from Yamato Ridge. Refraction measurements with son- obuoys indicate that material with oceanic crustal velocity, layer 3, lies at 6 to 7 km below sea level in Yamato Basin and at about 8 km depth in the Japan Basin. Depth to mantle was computed from the refrac- tion data to be 12 km in the Japan Sea Basin. The section in Figure 9 of the Japan Sea was composed from two-ship seismic re- fraction measurements made by a group of Japanese scientists in 1966. Although de- tails of the two-ship refraction profiles have not yet been published, the structure section Figure 6. Seismic profiler sections of derived from them has appeared in several Toyama Trough and Japan Sea margin of publications (Murauchi, 1966, 1972b; Northeast Japan. Explanation same as for Murauchi and Yasui, 1968; Yasui and Figure 3. Vertical exaggeration about 25 to 1.

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TABLE 1. RESULTS OF S0N0BU0V STATIONS. CONRAD CRUISE 12 (1969) AND VEMA CRUISE 24 (1971) IN THE SEA OF JAPAN

Velocity and Standard Deviation (S). km/sec Thicknesses (h). km Location Mean

Sonobuoy v V S Velocity Water h2 h, h. h5 h. 2 3 1 v4 V8 "8 Lat. (N) Long. (E) C12 324 1. 8» 4. 13 0. 12 0. 32 33°*51 129°11' 325 1. 65 2. 60 3. 64 0. 13 0. 70 0. 66 1. 62 34°05' 129°04' 326 2. 0» 4. 73 6. 21 0. 12 0. 57 0. 92 34°111 128°50.5' 327 1. 68 2. 68 4. 65 0. 15 0. 94 0. 68 1. 08 34°28' 129°09' 328 2. 82 3. 54 4. 51 0. 10 0.41 1. 21 34°51' 129°39' 329 2.65 3. 76 5. 02 5. 55 0. 13 1. 08 0. 56 2. 96 34°53' 129°54' 330 Insufficient data to compute 331 1. 97 0. 06 2. 35 3. 20 4.02 1. 27 0. 52 1. 36 0. 90 35°48' 130049' 332 2. 2« 2. 93 3. 72 4. 88 5. 50 1. 66 1. 87 0. 33 2. 11 36°04' 130°47' 333 1. 63 0. 11 2. 24 0. 06 3. 0» 5. 20 7. 22 2. 62 0. 29 0. 88 1. 64 370441 131°58.5' 334 2. 2« 4. 99 5.80 0. 34 0. 92 1. 55 37°16,5' 132°53' 335 1. 91 0. 07 1. 72 0. 10 3.04 3. 69 5. 20 2.09 1. 80 0. 92 0. 45 0. 61 36°53' 135°14' 336 1. 54 0. 07 1. 99 0. 21 3.4« 4. 58 5. 52 1. 75 2. 98 0. 42 0. 47 1. 71 37°38.5' 135°19' 337 Insufficient data-to compute 338 339 2. 2« 3.76 4.60 3. 05 0. 50 0. 77 0. 82 39°56' 132°39' 340 2. 0« 4.68 5.62 0. 87 0. 53 1. 20 39035.5' 133°46' 341 342 Insufficient data to compute 343 1.87 0.53 0.62 1.51 40°49' 134°37' Gii] 0.53 1.02 2.07 40°56' 134052' 346 Insufficient data to compute 347 348 1. 96 3.95 5. 60 1. 96 2.67 1. 00 1.49 0. 58 38°54' 136°31. 5' 1. 20 0. 72 0. 62 1. 02 39052' 135O48' [3491 1. 87 3. 61 5. 94 L 3SoJ 1. 20 0. 48 0. 38 1. 36 39044' 135°42' T3511 1. 89 2: 98 0. 60 0.82 1. 08 4OO37.5' 134°56' L352J 2.89 0. 60 0. 58 1. 09 40034' 134039' V28 124 1.90 0. 03 3. 08 0. 09 3.69 0. 19 0. 14 0.92 1. 41 1. 77 35013' 129°44' 125 1. 94 0. 01 2. 94 0. 12 4. 35 0. 38 0. 12 0. 79 0. 69 3. 43 35°20.5' 129°36' 0. 14 0. 51 0. 75 0. 27 0. 39 35038' 129°44' CS] 0. 20 0.43 0.52 0.76 0. 56 35°43. 5' 129045' 128 1. 63 0. 02 2. 35 0. 05 4.80 1.94 2. 16 0. 69 37008.5' I3OO5O.5' 129 1. 56 0. 02 2. 21 0. 08 2. 81 0.08 4.48 0.20 5.25 6.90 2. 16 2. 20 0. 55 0.55 0.86 1.70 1.45 17004' 131°06' 0. 18 0.60 36°34.5' 132°59' Efl 0. 28 1. 29 36°49.5' 132°59' 1. 39 2.06 36°34' 134°43' 2. 07 4. 38 GÏÎ] 1.46 2. 18 36037' 134°45' 134 2.0« 6. 23 2. 00 2.78 1. 04 38°37.5' 132°11 5' 0. 45 0. 35 0. 90 135 1.73 2.07 1 2. 02 0. 06 3. 34 0. 15 2. 35 2. 80 0. 42 38°37' 132°23.5' 136 1. 70 1. 95 2. 33 2. 95 0. 11 0. 47 0.40 0. 39 38°12' 138°51' 137 1. 76 0. 03 2. 61 0. 031 3. 5» 5. 97 2. 13 2. 33 0. 71 0. 82 1. 25 39009.5' 137°35.5' 138 1. 70 0. 06 1. 67 0. 291 2. 5* 3. 50 6. 00 2. 14 2. 36 0. 41 0. 38 0. 98 1. 50 39°14" 137°25' 139 1. 96 0. 05 2. 40 0. 091 6. 20 2. 12 2. 38 0. 91 0. 68 39°19. 5' 137°13' 140 Insuff cient d, ita to Icomput e 141 2. 23 0.07 2. 35 0. 101 2. 30 3.69 0. 62 1. 03 4IOII.5' 137039' 142 2. 29 0. 06 5. 16 2. 29 3. 67 1. 80 41°18' 137028' 3. 143 1. 76 0. 05 2. 64 0. 091 3. 5» 5. 80 7. 32 8. 00 2. 13 3. 61 0. 79 0. 86 0. 27 2. 16 81 42°23. 5' I35044.5' 144 2. 12 0. 07 2. 54 0. 081 3. 5« 5. 73 2. 30 3.60 0. 97 0. 91 0. 09 42°35.5' 135°27" 145 2. 13 0. 08 2. 50 0. 11 2. 69 0. 08 2. 47 3.47 0.64 0. 61 1. 21 41°14' I35O55' 146 1. 60 0. 05 2. 23 0. 07' 2. 97 0. 14 5. 81 7. 48 2. 23 3.47 0. 44 0. 50 0. 69 1. 67 41°08.5' 136°59.5' 147 1. 57 0. 05 2. 32 0. 041 3. 66 0. 06 6. 24 7. 42 2. 03 2. 58 0. 45 0. 74 0. 93 0. 83 390391 I3704O.5' 148 1. 86 0. 05 3. 41 0. 081 3. 75 5. 25 6. 80 2. 36 2. 31 0. 82 0. 72 1. 86 1. 88 39°06.5' I3702I' 149 1. 51 0. 13 2. 24 0. 041 3. 22 0. 08 5. 65 7. 10 1. 86 3. 54 0. 47 0. 66 0. 95 2. 02 41°01' 134°25' 150 1.64 0. 04 2. 21 0. 041 2. 42 0. 14 4. 20 0. 27 5. 26 2. 21 2. 92 0. 52 0. 50 0. 39 0.86 37°56' 135°30' 151 1.73 0. 05 1.83 0. 081 2. 95 0. 08 2. 26 2.87 0.48 0. 40 1. 05 37°42' 135038' 152 1.75 0. 16 2.04 0. 291 2. 72 0. 24 2. 74 0. 11 3. 5« 5. 58 6. 26 2. 08 0. 57 0. 23 0. 49 0.45 0. 63 39°17' I33056' 153 Insuff cient d ita to compute 154 1.68 0. 13 1. 82 0. 22: 2. 75 0. 03 3. 5» 6. 06 2. 36 3. 59 0. 42 0. 30 1. 84 1. 24 41 °4 2 ' 137°10' 155 1. 70 0. 06 2. 56 0. 091 2. 78 0.07 3.45 5.82 2. 47 3. 66 0. 40 0.64 1. 49 0. 25 42°05' 137°04' 156 1. 77 0. 10 2. 44 0. 281 3. 06 0. 05 4. 01 0. 11 4. 5* 5. 48 7. 03 8. 11 2. 22 3. 63 0. 39 0. 59 0. 58 1. 00 1. 25 3. 47 430II' 136°30' 157 1. 55 0.04 2. 54 0. 051 2. 62 0. 11 3. 99 0. 12 4.62 5. 23 6. 74 7. 22 2. 15 3. 58 0. 84 0. 57 0. 72 1. 85 0. 26 4. 21 43°26' 136°21' 158 1. 84 0. 13 2. 24 0. 111 2. 85 0. 04 2. 55 3.68 0. 34 0. 37 1.72 42°51' 137°47' 0. 09 159 1. 70 2. 44 0. 081 2. 66 0. 05 3. 5» 5. 80 6. 90 7. 80 2. 37 3.67 0. 35 0. 69 1. 07 0. 26 1. 31 4. 28 42°46.5' 137°52'

Notes. Standard deviation refers to the computed deviation of interval Velocity in the preceding column. Velocities not having a standard deviation are refraction velocities. In shallow water they compute by using the apparent velocity observed as the true velocity and assuming horizontal layers. In deep water the refracting layer is assumed to be parallel to the top of acoustic basement observed in the vertical profiler records. Asterisks denote assumed velocity of masked layer. Mean velocity refers to the computed mean velocity in the sediments above acoustic basement mapped in Figure 4. Brackets indicate reversed profiles.

others, 1968; Vasilkovsky and others, beneath Yamato Ridge is the same as that Kita-Yamato Trough has thick low-velocity 1971; Isezaki and Uyeda, 1973). Included beneath the basins on either side, about 8 sediments above a 3.5(?)-km/sec basement in the section of Figure 9 are the results km. Yoshii (1972b) has estimated from layer. from several of our sonobuoy stations. gravity data that the depth of the mantle Yamato Basin, on the basis of profiles 4 Many of the features of the composite beneath Yamato Ridge is about 23 km. The and 5 in Figure 9, seems to have a layer 3 section are readily apparent. Compared to major part of the topography of Yamato that is 3 to 4 km thicker than that in the the Japan Basin, Yamato Basin has a shal- Ridge is built of layers of velocity 3.5 and Japan Basin and a low-velocity mantle. The lower sea floor, thinner sediments (see Fig. 5.0 km/sec (compare Vasilkovsky and mantle beneath the Japanese islands also 4), shallower basement surface, and possi- others, 1971). The thickness of the 6.1 has relatively low seismic velocities (Aki, bly a thicker basement layer. Along this line km/sec crustal layer below does not differ 1971; Kanamori, 1968; Yoshii and Asano, of section, the depth of the top of layer 3 significantly from that in Yamato Basin. 1972).

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DISCUSSION (Scientific Staff, 1973). Pliocene-Pleistocene fact, velocities between 5.5 and 6.2 km/sec turbidites overlie late Miocene diatomites are quite characteristic of the lowermost The back-arc basin of the Japan Sea has and diatom oozes at sites 299 and 301 (Fig. unit of layer 2 in the western Pacific Ocean relatively shallow water depths, no shallow 3B). A show of gas at these two sites oc- (Murauchi and others, 1968; Den and focus earthquakes, high heat flow, short- curred in an interval corresponding to the others, 1968). A significant number of wavelength magnetic anomalies, and gen- level (500 m below sea bottom) between the sonobuoy stations in the northern Pacific erally positive free-air gravity anomalies. highly stratified turbidites and the underly- reported by Houtz and others (1970) gave Study of earthquake body waves and grav- ing weakly stratified diatomites. Site 300 velocities in this range. We suspect that the ity anomalies indicates that the thickness of was abandoned due to caving. Drilling at wide range of velocity measured in layer 2 the lithosphere of the Japan Sea may be site 302 on the Yamato Ridge (Fig. 3B) (3.5 km/sec to 6.4 km/sec) is largely the re- thinner than that of the western North penetrated 500 m of pelagic sediment and sult of discrete layering; that is, layer 2 may Pacific basin. bottomed in a green tuff unit that may be be either a one-, two-, or three-component The Japan Sea has several basins, each correlative with the early Miocene green layer. with seismic structure somewhat different tuffs on Honshu. Velocities near 5.8 km/sec are representa- from the others. The largest and deepest The 5.8-km/sec velocity measured in tive of the basement rock of the Japanese basin, the Japan Basin, is typically oceanic seismic layer 2 is not unusually high. In mainland (Hashizume and others, 1968) in structure. The shallower than normal sea floor is the result of thick sediments. Al- TSUSHIMA STRAITS TSUSHIMA BASIN though the sediments are generally thicker 25 [126-7] (324) (325) (326) (327) (328) (329) (331) (332) | 128 129 333 134 to the west, this may be a function of 2.0' m L9 1.7 1.7 2.7 provenance rather than older crust along 4.7 3.5 the western margin, as suggested by Hilde 27 U S5 3.6 6.2 4.7 4.5 2.3 1.6 1.6 2.2' IX and Wageman (1973). Depths to the top of 5.0 2.2 2.3 2.2 the crustal layers and mantle are similar to 6.2 6.0 3.2 Z2 2.8 4.0 3.0' those found in the western North Pacific 3.7 4.8 4.5 5.6 basin near Japan (Ludwig and others, gj) (5.3 1966; Den and others, 1968). The velocity 4.9 (7.21 [5.2 structure of Yamato Basin (and presumably 5.5 (6.9] that of Tsushima Basin) is typical of layer 2 and layer 3, but with thicknesses much J A P A B A S I N greater than normal. Depths to the top of (339) , 149 [ 344-5]p5l-2]| 145 146 144 142 141 I 157 layer 2 in Yamato Basin are shallower than those in the Japan Basin and main Pacific basin, largely because layer 2 has been built

up by the addition of the 3.5-km/sec sec- TT" 1.7 TB" 35 1.8 XE ZE tion. Profiles 4 and 5 in Figure 9 indicate a 2.5 2.6 2.6 3.1 Z& 2.8 2.8 thicker than normal layer 3 and relatively ia 4.0 77771 77TT7, WW low mantle velocities in the Yamato Basin, 45" 3.5" which distinguish it from the Japan Basin. (4.6 (55) [5B1 [6.11 The nearly identical gravity field strength over both basins does not indicate uniform- (6.7) 17.0) ity in structure but simply that the basins have the same total mass. Furthermore, the low gravity signature of Yamato Ridge in- 18.1) iTF dicates that its mass is compensated at OKI OKI - YAMATO RIDGE depth. Heat flow of both basins has no cor- Y A M A T 0 BASI I TROUGH relation with shallow subbottom structure, 150 151 348 148 139 138 D30-I] (334) 152 [349-Oj [132-3] 335

and so the heat source must originate 2.2' 2.2' deeper, perhaps in the upper mantle. 1.9 5.2 5J0 Higher values of heat flow are associated 1.8 3.6 1.6 TT 2.0 with thicker sediments in the Japan Sea. 5.8 e-9 T1 1.8 TTT7} Similar values of the total intensity mag- 2.9 S 13.91 4.2 TTTT. 5.6 netic anomaly in the basins seem to indicate (5Ü [5.3 (6.3 that the oceanic basement has just about (6.6) the same magnetization everywhere, re- gardless of elevation and topography of its upper surface. The 3.5-km/sec layer could represent a Figure 7. Velocity structure sections from sonobuoy solutions (Table 1) arranged according to wide range of consolidated sediments and physiographic province. Velocities in km/sec. Bracketed station numbers indicate a reversed pair of refraction profiles; hence, the velocities given are true velocities. Parenthesized station numbers indi- volcanics (green tuff?). JOIDES drilling in cate an unreversed refraction profile. They were computed by assuming horizontal layers. Similarly, the Japan Sea was hampered by the occur- parenthesized velocities are unreversed refraction velocities. They were computed by assuming that rence of natural gas in the sediments which the refractor is parallel to the top of acoustic basement observed in profiler records. All other velocities required cementing the boreholes long be- are interval velocities from wide-angle reflection data. Asterisks denote assumed velocity of masked fore the acoustic basement was reached layers. Hatching designates acoustic basement.

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and the continental margin on either side « (Ludwig and others, 1966; Yoshii and others, 1973; Fig. 9). However, the thick- Figure 8. Plot of sediment interval velocities ness of the continental basement is far versus depth of sediments in one-way reflection time. Vertical lines through data points represent greater than the thickness of the oceanic the standard deviation. The velocity line is the basement of similar velocity in the Japan least-squares fit of the data points and has not Sea. The velocity in the basement falls been constrained to pass through the ordinate at within the range of velocities measured for the velocity of the water/sediment interface continental quartzite, gneiss, serpentine, (compare Houtz and others, 1970). The data and granitic rock (Anderson and Lieber- points are restricted to interval velocities with mann, 1968) and for oceanic metabasalts in values less than 3.5 km/sec. This was done to re- the greenschist facies (Fox and others, duce the scatter in the plot caused by depth varia- 1973) and oceanic metadolerites in the tions to the 3.5 km/sec layer, whose interval ve- locity tends to be independent of the overburden greenschist facies (Christensen and Shaw, thickness. This important characteristic illus- 1970). Furthermore, the metabasalts trates that the 3.5-km/sec layer is probably com- dredged from the sea floor have a low posed of materials that are quite different from natural remanent magnetization (Fox and the overlying sediments. Opdyke, 1973), a possible property of the

Vertical x 15

-25

Northwest Southeast

Figure 9. Schematic structure section, from bathymétrie, vertical reflection, sonobuoy reflection and refraction, and two-ship refraction data. Velocities and layer thicknesses of two-ship profiles 1 through 5 (Murauchi, 1966,1972b) computed from the time-distance graphs. Circled profile numbers represent end-to-end pairs that compute using the average velocity of the apparent velocities observed in each direction as the true velocity and assuming horizontal layers. Parentheses indicate unreversed refraction measurements. Brackets denote velocity determination from later arrivals. Asterisks represent assumed velocity of masked layers. Sonobuoy 149 and refraction profiles 3 and 4 (locations in Fig. 2) appear outside the margins; they are not in the line of section and are shown for comparison.

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basement rocks in the Japan Sea basins due to potassium addition from sea water distribution and internal structure denote suggested by the subdued magnetic into the rock during exposure. Hida accumulation on a stable sea floor. Because anomalies observed over them. metamorphics are exposed on Oki Island, of the close affinity of Yamato Ridge to The morphology and structure of the Noto Peninsula, and along the northern Japan, it is possible that the ridge broke Japan Sea margin of Honshu, Japan, re- part of the Hida mountains near Toyama away from the Japanese mainland as a re- sembles the continental borderland off Bay. sult of the opening (spreading) of Yamato southern California (Emery, 1960). All the The intense Cretaceous-Paleogene tec- Basin. The offset position of the northern offshore ridges and troughs trend roughly tonism with granitic intrusions that oc- end of Oki Ridge and the southern end of parallel to trends on shore, indicating that curred in the peripheral lands of the Japan the basement ridge (?) found in Yamato they were formed contemporaneously with Sea may have been associated with tec- Basin may indicate that north-northwest— the present arcuate features of Japan. Be- tonism in the deep-water basins. In any south-southeast fault motion has taken tween Shimane and Noto Peninsulas, a event, material with velocity of about 3.5 place. Similarly, the eastern side of Oki former trough has been completely filled by km/sec, which may represent volcanics and Bank (trending north-northwest) may con- sediments. The other troughs are only par- volcanoclastics of early Miocene age, cov- tain a zone of shear that traces the relative tially filled. ers older oceanic sediments and basement movement of Yamato Bank and Japan. Sonobuoy refraction profiles made in the rock in the basins of the Japan Sea. straits between Korea and Japan (Fig. 7) Modification of an otherwise normal ACKNOWLEDGMENTS indicate considerable relief in the subbot- oceanic crust was more severe in Yamato tom. A sediment-filled trough lies im- Basin than in the Japan Basin largely be- Seismic data used in this report were col- mediately west of the Tsushima Islands. cause of the influence of the intervening lected during Conrad cruise 12 (leg 18) led These islands probably rise above the sur- Yamato Ridge, which allowed a much by the late Maurice Ewing and during face of a basement ridge. Sediments fill de- greater amount of the 3.5-km/sec material Vema cruise 28 (leg 15) led by W. J. Lud- pressions in the basement off the southeast- (green tuff?) to accumulate and build the wig. We are grateful to M. Abe, L. Garcia, ern coast of Korea. It seems probable that sea floor. The oceanic crust of Yamato E. Honza, S. Kamata, and R. Kuhlman for is a former ridge and Basin was also modified to the extent that it their assistance with the seismic work. S. L. trough province that has been covered with now has a thicker than normal layer 3 and a Eittreim and M. G. Langseth critically read sediments. low-velocity mantle, characteristics that re- the manuscript. late closely to Yamato Ridge and the This research has been supported by CONCLUSIONS Japanese mainland. Furthermore, Contract N00014-67-A-0 108-004 with volcanism seems to have been much more the U.S. Office of Naval Research, Depart- It is likely that the Japan Sea existed es- prevalent on Yamato Ridge and in Yamato ment of the Navy, and by Grant GA-27281 sentially in its present form prior to the be- Basin than in the Japan Basin. from the Oceanography Section of the Na- ginning of the movements (in late Results of JOIDES drilling (Scientific tional Science Foundation. Oligocene or early Miocene) that resulted Staff, 1973) indicate that the sediments of in the present arcuate structure of Japan. the Japan Sea basins are largely diatom-rich REFERENCES CITED Evidence of seismic velocity and layer pelagics (of probable Miocene age) capped thickness, rough local relief, large ampli- by Pliocene-Pleistocene turbidites. The tur- Aki, K., 1961, Crustal structure in Japan from tude magnetic anomalies compared to the bidites are distributed by channelized tur- phase velocity of Rayleigh waves: Tokyo basins, low value of free-air gravity bidity currents passing through Tartary Univ. Earthquake Research Inst. Bull., v. anomalies, dredged volcanics from the Trough (between Hokkaido and Siberia), 39, p. 255-283. crestal surfaces, and granitic rock from the Genzan Trough, Toyama Trough, Mogami Anderson, O., and Liebermann, R., 1968, Sound flanks, all indicate that Yamato Ridge is Trough, and through the various canyons velocities in rocks and minerals, in Physical acoustics: New York, Academic Press, p. primarily a large welt of volcanic material transecting Yamato Ridge and the conti- 330-472. intruded by granite on a thick layer of ma- nental margin of Siberia. Andreyeva, I. B., and Udintsev, G. B., 1958, Bot- terial with oceanic-type velocity. This can The hypothesis that the Japan Sea repre- tom structure of the Sea of Japan, from Vit- be interpreted to mean that Yamato Ridge sents subsided or oceanized continental yaz expedition data: Akad. Nauk SSSR Izv. is an integral part of the island-arc system crust is not supported by our data. Vertical Ser. Geol., p. 1-15 [in Russian]. of Japan, rather than a fragment of Siberia tectonics have probably played a role in the Asano, S., and Udintsev, G. B., eds., 1971, Island that has broken away and drifted out to its development of the ridge-trough system of arc and marginal sea: Tokai Univ. Press, present location. the Japan Sea margin of Japan and of 319 p. [in Japanese with English abstracts]. Asanuma, T., and Murauchi, S., 1970, Seismic The dredged basalt and andesite samples Yamato Ridge, but evidently these move- reflection studies of the sedimentary layers of Pliocene—early Miocene age from ments had little effect on the oceanic struc- of the sea bottom off Noto Peninsula by Yamato Ridge reflect the period of Neogene ture now observed in the basins. The Japan means of the seismic profiler: Natl. Science volcanism on the Japanese mainland. Late Basin and Yamato Basin have oceanic crust Mus. Mem., v. 13, no. 1, p. 77-82 [in Paleozoic—early Mesozoic plutonic intru- covered by varying amounts of Neogene- Japanese]. sives of Japan are thought to be related to Holocene sediments (and volcanics?) which Beloussov, V. V., 1968, Some problems of de- the regional metamorphism and tectonic have built the sea floor to depths shallower velopment of the earth's crust and upper movements that occurred during that time. than those in the Pacific Ocean Basin. 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