Sediments and Structure of the Japan Sea 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 island-arc—deep-sea-trench systems taceous to the Oligocene, with final forma- the Japan Basin, Yamato Basin, and inter- of the western Pacific Ocean are related to tion by the early Miocene. In this paper, we vening Yamato Ridge reveal horizontally the small semi-isolated marginal seas or present a synthesis of Lamont-Doherty stratified sediments over weakly stratified back-arc basins such as the Japan Sea, Geological Observatory seismic data ob- 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 Siberia and trapment of old oceanic crust by formation with which to consider fundamental ques- Japan. Compared to the Japan Basin, of the island arc, 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 Korea 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 Tsushima Basin (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 Honshu 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 Asia (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 Toyama 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 abyssal plain. 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 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/86/5/651/3418392/i0016-7606-86-5-651.pdf by guest on 29 September 2021 652 LUDWIG AND OTHERS 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- Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/86/5/651/3418392/i0016-7606-86-5-651.pdf by guest on 29 September 2021 SEDIMENTS AND STRUCTURE OF THE JAPAN SEA 653 marine channel, are similarly described in Dogo and Dozen (the Oki Islands) 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 andesites 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 Pliocene with uplift in the of the zone. In the anomalous zone, the ve- of the Japan Sea side of Honshu Island, as green tuff regions 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).