Japan Sea: a pull-apart basin?

Earth and Planetary Science Letters, 76 (1985/86) 375-389 375 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

151

Japan Sea: a pull-apart basin?

Serge Lallemand 1 and Laurent Jolivet 2

s Laboratoire de Gbodynamique, Dbpartement des Sciences de la Terre, Universitb d'Orlbans, ERA CNRS 601, 45046 Orlbans Cedex (France)

2 Laboratoire de Gbologie, Ecole Normale Sup~rieure, LA CNRS 215, 46 Rue d'Ulm, 75230 Paris (France)

Received May 1, 1985; revised version accepted October 30, 1985

Recent field work in the Hokkaido Central Belt and marine geology studies along the eastern margin of Japan Sea in addition to previously published data lead us to propose a new model of opening of the Japan Sea. The synthesis of both on-land and offshore structural data gives new constraints about the structural evolution of the system. The rhombohedral shape of the Japan Basin and the particular tectonic behaviour of the margins on both east and west sides can be explained by an early Eo-Oligocene rifting of a pull-apart basin accommodated along two large right-lateral shear zones, east of Korea and west of northeast Japan and Sakhalin. It is followed, during Upper Oligocene/Lower Miocene, by the main opening of the Japan Basin as a mega pull-apart. Then a back-arc spreading probably related to the subduction process, induced the creation of the Yamato and Tsushima Basins at the end of Lower Miocene and in Middle Miocene. Clockwise rotation of southwest Japan larger than 20 ° or major bending of Honshu mainland deduced from paleomagnetic studies is unlikely at this time. Since 1 or 2 My B.P. to Present, compression prevails along the eastern margin of the Japan Sea. The generation of marginal basins as pull-apart basins along intracontinental strike-slip faults is a mechanism which has been proposed by other authors concerning the South China Sea, the question then is whether the fragmentation of the Asiatic continent is an intracontinental deformation related process as proposed here or a subduction related one.

1. Introduction

The Japanese archipelago is located at the junc- tion of four plates: the Amurian, Okhotsk, Pacific and Phil ippine Sea plates [1,2] (Fig. 1) and its complex evolut ion is governed by the relative mo- t ion of these plates. The basement of the Japan Sea is, at least partly, oceanic [3-7]. In spite of rather poor magnet ic l ineat ions [8], five small axes of symmetry roughly E N E - W S W in the Japan Basin (Fig. 2) and NE-SW in the Yamato Basin have been recognized [9]. It is consequent ly highly probable that several spreading centers were in- volved in the creat ion of the deep basins (e.g. [10]).

Unt i l now m a n y models have been proposed to explain its format ion by drif t ing of the islands, Almost all of these models deal with the southern par t of the Japan Sea: the tectonic mean ing of Tar tary Strait and the drift of nor theast Japan (nor th of H o n s h u and Hokkaido) are usual ly no t considered, even the reconstruct ion took Sakhalin in to account (e.g. [111). Murauchi et al. [12] proposed a schematic pre-drift reconstruction. Ten

0012-821X/86/$03.50 © 1986 Elsevier Science Publishers B.V.

years later, Kobayashi and Isezaki [13] presented an evolutionary model of northwest Pacific with the southward drift of the Japanese islands from

Plate

Plate

. , . , .

Fig. 1. Localisation of plates in East Asia adapted from Zonenshain and Savostin [1]. The western boundary of the microplate of Seno [2] is represented in dotted lines, PhP Philippine Sea plate.

376

the Asiatic continent, Sakhalin and the central part of Hokkaido during Eo-Oligo-Miocene. Re- cently, Kobayashi [14] reviewed their previous

model, adding a clockwise rotation of southwest Japan and great displacements along left-lateral strike-slip faults in northeast Honshu but keeping

A M U 135 °

O K H

Okhotsk Sea

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126 °

. 4 0 ° - - ~

Yellow Sea

LSrnt

/ I i]

/ t

, PAC

126 °

East Oina Sea

V i I , I 1.o

/ l / ~ i k ~ u Basin 4 /

i

PHS /' ' ,

Fig. 2. S t ruc tu ra l con t ex t a n d p h y s i o g r a p h y of J a p a n Sea a n d the J a p a n e s e is lands . Plates: A M U = A m u r i a n , O K H = O k h o t s k ,

P A C = Pacif ic , P H . S = Ph i l ipp ine Sea. Tectonic features: M T L = M e d i a n Tec ton ic Line, K a T L = K a n t o T.L. , ISTL = I to igawa- S h i z u o k a T.L. , H S Z = H i d a k a Shea r Z o n e , K T L = K a m i s h i y u b e t s u T.L. , A T L = A b a s h i r i T.L. , F I F = F u d z i n o I m a n Fau l t , C S A F =

C e n t r a l S ihkote Al in F., Y F = Y a n g s a n F. , T F = T s u s h i m a F. Hypothetical faults: E K F = Eas t K o r e a n Fau l t , O F = O l d F. , T F ~ T o y a m a F., O T F = O g a - T a r t a r y F., S F = S a d o F. Islands: G.Is = G o t o is., T . Is = T s u s h i m a is., O. Is = O l d is., U . D o = U l l u n g

D o is., S.Is = S a d o is., Ok.Is . = Okush i r i is. Seamounts: S.Smt = S iber ia smt. , B .Smt = B o g o r o v smt. , V .Smt = Vi tyaz smt . Ridges: K . Y . R d . = K i t a - Y a m a t o Ridge , O . R d = Old Ridge , S .Rd = S a d o Ridge . Banks: O . B k = O l d B a n k , M . B k = M u s a s h i Bank . Plateaus: K.P1 = K o r e a n P la teau , O.PI = O s h i m a Pla teau . Troughs: G . T r = G e n z a n T r o u g h , O . T r = O l d T r o u g h , T .T r = T o y a m a T r o u g h .

Straits: T.St r = T s u s h i m a Strai t , Tg .S t r = T s u g a r u Strai t , S.Str = Soya Strai t . Peninsulas: N . P e n = N o t o Pen insu la , O . P e n = O g a

Pen insu la , S .Pen = S h a k o t a n Pen insu la .

377

the same blocks as before. On the basis of the strike-slip faults activity in the circum Japan Sea region from Cretaceous to Paleogene, Otsuki and Ehiro [15] have also reconstructed the drift history of Japan.

But these models do not fit with recent on-land structural data. The main point is to reconcile the opening of Japan Sea and the contemporaneous right-lateral movement along the Hidaka Shear Zone [16,17] (Fig. 2) and the Yangsan Fault as will be discussed later in this paper. Besides paleomagnetic studies have led to the proposal of a clockwise rotation of southwest Japan around a pole located near the Tsushima Strait [18] and of the bending of Honshu [19]. These should give rise to considerable deformation contemporaneous with the rotations, especially in central Honshu, which have not been observed in the field, this conflict will also be discussed.

A new set of tectonic data brings new constraints on the tectonic pattern of the eastern margin of the Japan Sea on-land and offshore [16,20-23]. In addition, some Japanese data were reinterpreted in light of the new hypothesis about incipient subduction and obduction along this margin [24]. The present paper is an attempt to interpret these data in the framework of a new model of the Japan Sea formation.

In summary, the Japan Sea consists of several deep basins with oceanic basement separated by ridges of continental crust. The structure is very simple in the basins and complex on most of the margins. The eastern and western margins both show a particular structure: numerous N-S and NNE-SSW trending ridges bounding very narrow and rapidly subsiding basins filled with thick accumulations of Cenozoic sediments. The ridge and basin structure can be followed along more than 1000 km along the eastern margin in the Tartary Gulf strongly suggesting that both margins are controlled by strike-slip faults. Considering the "en 6chelon" pattern of several troughs around the Sado island or in the Tartary Gulf, the movement could be right-lateral. This system is subparallel to the right-lateral Hidaka Shear Zone [16] which can be followed northward in Sakhalin. It is likely that the two systems belong to the same wide right- lateral shear zone. Similarly we argue the fact that the western margin of the Japan Sea is a right- lateral strike-slip zone. The rhombohedral shape of

the Japan Basin and the structure of its margins allow us to propose the idea of a pull-apart basin opened between two right-lateral faults during the Oligocene/Lower Miocene.

2. Morphological evidence for a pull-apart basin

Except for the Tartary Gulf, the coast lines of Japan sea have a rhombohedral shape striking roughly N-S and WSW-ENE (Fig. 2). This is even clearer for the outline of the Japan Basin. Its topography is rather flat (3000-3700 m), except for the presence of local highs like those of Bogorov and Siberia seamounts (1500 m deep). Three less important basins or troughs, shallower than 2500 m, extend the Japan basin northward (the Tartary Gulf) and southward (the Yamato and Tsushima Basins). All of them are elongated along a NE-SW direction. The main feature is the Yamato Ridge which overhangs the Japan Basin by 2500 m; it can be divided into two subridges trending NE- SW: the Kita-Yamato and the Yamato Banks separated by a narrow graben 2000 m deep.

Whereas the Siberian and the northernmost Korean margin of the Japan Sea is continuous and narrow, all the other margins are very complex. West of the Japan Sea lies the Korean Plateau (Korean Continental borderland of Mogi [25]) with ridges and throughs striking NE-SW and N-S. This mosaic of peaks and depressions ends abruptly northward and southward against a N-S escarpment corresponding to the continental slope (Fig. 2).

The Japan sea ends to the south in the Tsushima Strait between Korea and Kyushu. A NE-SW channel limited by scarps runs on the plateau [5]. The linearity of the margins is interrupted by the N-S Oki Bank and the extension of the Noto Peninsula. Their eastern flanks are respectively bounded by an escarpment and by a trough. We name these features the Oki Fault and the Toyama Fault (Fig. 2) based on the description given by Ludwig et al. [4], Chihara [26] and others. Two other NE-SW trending blocks separate the Yamato Basin from the Oki Trough (the Oki Ridge) and from the Tsushima Basin creating a high between the Oki Bank and the Yamato Ridge. These two blocks might have drifted northward from southwest Honshu along the Oki and Toyama Faults.

Along the eastern margin, several ridges and

378

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.¢ v ~ ~ 9 ~

1 3 8 °

Japan Basin

1 4 0 °

KKAIDO

4 4 °

4 2 °

t - '." ~ ' k

Y.B ,

7

T o h o k u

HONSHU

\ \

1 0 0 k m

" ' " i " " - p I to igawa-Shizuoka Tectonic Line

Fig. 3. Structural features on the eastern margin of the Japan Sea. ST = Shiribeshi Trough, OI = Okushiri Island, OkB = Okushiri Basin, OP = Oshima Plateau, NTB = Nishi-Tsugaru Basin, MT = Mogami Trough, SR = Sado Ridge, SI = Sado Island, TT = Toyama Trough, YB = Yamato Basin, YR = Yamato Ridge, Pen = Peninsula. In dotted lines, the hypothetical strike-slip or transform faults active during Oligo-Miocene period. In full lines: present convergence front along reverse faults and thrusts.

dep re s s ions fo l l ow o n e a n o t h e r w i th an " e n

6 c h e l o n " p a t t e r n f r o m the S a d o i s land to the lat i-

t u d e o f S h a k o t a n P e n i n s u l a o f f H o k k a i d o . A pa r -

t i cu la r i ty o f this eas te rn m a r g i n is an a b r u p t c h a n g e

in the d i r ec t i on o f i soba ths at the l a t i t ude o f the

O g a Pen insu la : f r o m N N E - S S W in the sou th to

N - S in the n o r t h (Fig. 3) fo l l owing the M o g a m i

T r o u g h a n d f r o m sou th to n o r t h fo l l owing the

N i s h i - T s u g a r u Basin , the Okush i r i Basin, the

Shi r ibesh i T r o u g h and two o the r smal l dep re s s ions

to the north. Many of these directions now coin- cide with active faults [24,27]. Only the Oshima Plateau, where the Oshima caldera is located, does not follow this regional trend, probably because it did not yet exist during the movements responsible for the N-S structuration of this margin. The Tar- tary Gulf to the north lies between Sikhote Alin and the islands of Sakhalin and Hokkaido. It is an elongated depression 1000-2000 m deep display- ing several ridges striking NW-SE or NE-SW.

Finally except for the Oki and Toyama Faults trending NNW-SSE, the Japan Sea margins show two obvious directions: N-S for the east Korean continental slope and the Oga-Tartary line, and NE-SW to NNE-SSW, for the Tsushima Fault and the Mogami Trough for example.

In summary, the Japan Sea displays a main rhombohedral basin and several subsidiary basins bounded on both east and west sides by very long scarps systems striking N-S which evoke strike-slip margins.

3. Age of opening

The magnetic lineations in the Japan Sea are not easily identified and the oldest sediments which overlie the oceanic crust have never been reached by drilling, except for the northern flank of the Yamato Ridge where the top of the basement consists of Lower Miocene volcanic siltstones and Green Tuff formations, similar to those known in Honshu [4]; consequently there is no direct evidence concerning the age of opening of the marginal sea. It is a subject which had been very con- troversial although a consensus seems to have been reached now.

The high but uniform values of the heat flow lead Watanabe et al. [28] to conclude that the opening occurred necessarily before 20 My ago. Seismic profiles, DSDP results and onshore sections in favor of a Lower Miocene age for the oldest sediments of the deep basins (probably Green Tuff formations or remobilized Green Tuffs, at least in the Yamato Basin).

The earliest evidence for the presence of a basin in the Japan Sea seems to be Eo-Oligocene inasmuch as non-marine and marine sediments, showing northward paleocurrents [29,30] were deposited in northern Kyushu, indicating that a basin already existed north of Kyushu at that time [15].

379

In Hokkaido, the first sediments which uncon- formably overlie the Cretaceous Yezo Group are Paleogene non-marine and shallow marine deposits rich in coal beds [31,32]; they were deposited in strongly subsiding narrow basins [31]. Furthermore, on the basis of the plate thickening model in relation with the sedimentation, Takeuchi et al. [33] concluded for a marine invasion at approximately 40 My B.P.

Well-dated sections on-land allow to precise the age of the syn-rift blocks movements on the margins. In eastern Korea, especially in the small Pohang Basin, marine deposits of the Yeonil Group (Burdigalian to Pliocene) seal fault movements which cut the continental Yangbug Group (Upper Oligocene to Aquitanian). The NNE trending faults give rise to an eastward downfaulting [34]. In northern Honshu, in a section described in the Oga Peninsula [35], the non-marine Daijima For- mation (Lower Miocene) which overlies uncon- formably the massive andesitic Green Tuffs deposits (Upper Oligocene to Lowermost Miocene Nishioga and Monzen Formations) can be interpreted as syn-rift and is covered by the littoral neritic Nishikurosegawa Formation (Lower to Middle Miocene) indicating a marine transgres- sion. Thus, major blocks movements occurred during uppermost Oligocene/Early Miocene both off Korea and Honshu and ended in Middle Miocene.

A two stages opening with the creation of the Yamato Basin during the last stages has been proposed [13,14,36]. It is in good agreement with the main opening at least of the Japan Basin during the Upper Oligocene/Lower Miocene, followed by the opening of the Yamato Basin which may be associated with a small amount of rotation of southwest Japan during Middle Miocene, as well be discussed below.

We consider as a working hypothesis, a beginning of rifting during Eo-Oligocene followed by an active tectonism along the eastern and western margins during the Upper Oligocene/Lower Miocene. From the Middle Miocene, the margins are no longer deformed except for the Tartary Strait and the formation of small depressions along the eastern margin. Of course, we leave the recent compressive tectonics along this margin out of account. Middle Miocene deposits seal the movement in the section recognized on-land.

380

4. Strike-slip margins

Off northern Honshu and Hokkaido, the Neogene and Quaternary sediments are distrib- uted in narrow basins controlled by faults [37]; the thickness of the sediments can reach more than 3 km in the Nishi-Tsugaru Basin [22] or 6 km off Tohoku [38]. A recent survey of the eastern margin of the Japan Sea [22] reveals that the nature and thickness of the sediments in the graben-like basins north of the Oga Peninsula are very different from those of the Japan Basin. These horst and graben structures are pre- or syn-sedimentary, the sediments being of probable Plio-Quaternary. They were particularly well developed along a weak zone such as the Oga-Tartary line (Fig. 3). Many young eastward or westward reverse faults are recognized along this margin and seaward [23,24,27,39]. These observations indicate probably the reactivation of ancient normal faults into reverse ones [22]. The Tartary Gulf shows the same characteristics with two narrow depressions: the "West-Sakhalin Trough" (1000 km long, 60 km wide) which can be followed from the Sakhalin margin to the southern margin of Hokkaido across the island (the Sapporo-Tomakomai depression), and the "Tar tar Trough" (1500 km long, 200 km wide) which corresponds to the deepest part of the gulf. The thickness of the sediments varies between 3 and 8 km [40].

New data on the structure of the Hokkaido Central Belt have been obtained. Kimura et al. [20] argued about right-lateral oblique collision during Tertiary. Jolivet and Miyashita [16] and Jolivet [17] showed that the Central Belt acted during the Oligocene and Lower Miocene as a right-lateral strike-slip shear zone, the Hidaka Shear Zone. The amount of displacement is not directly known but is likely to be great, considering the width of the shear zone and the intensity of deformation. From the Middle Miocene, the movement became compressive in the southern part of the shear zone [41] but was still a strike-slip in the north. The fault system can be followed in Sakhalin [20]. In South Hokkaido, the shear zone is not expressed west of the Sapporo-Tomakomai depression.

Fig. 4 shows the different tectonic characteristics of the eastern margin. On land tectonic features such as the main strike-slip faults, Tertiary fold axes [20] as well as stretching lineation in the

Hidaka metamorphics [16,42] are plotted. The fold axis pattern and their relation with the strike-slip fault system led Kimura et al. [20] to conclude that the Sakhalin-Hokkaido system was a right-lateral oblique collision zone during the Paleogene and Early Miocene. Jolivet and Miyashita [16] showed that the structure of the Hidaka metamorphic belt is that of a right-lateral strike-slip shear zone of Oligo-Miocene age. The structure of both Meta- Ophiolite Zone and Main Zone is consistent and displays a vertical N-S foliation bearing an hori- zontal stretching lineation (earlier mapped by Kizaki [42]). Non-coaxial deformation criteria al- lowed them to propose that a right lateral strike-slip synmetamorph movement was responsible for the formation of the shear zone.

Fig. 4 also shows the thickness of the sedimentary cover along the Tartary Strait after Antipov et al. [40]. Very narrow basins aligned N-S are filled with very thick sedimentary sequence (up to 8 km) in the West Sakhalin Trough. On seismic profiles, such basins are bounded by vertical faults [6]. In the southern part of the area concerned by Fig. 4, we plotted the axes of the "en 6chelon" troughs along the Oga-Tartary line.

All these features indicate that the entire area (Sakhalin, Hokkaido Central Belt and Tartary Strait) are different expressions of the same right- lateral strike-slip shear zone, the "' Hidaka- Tartary

Shear Zone ". For the eastern margin, the idea that the structure on-land and in the Tartary Strait belong to the same orogenic system has been proposed by Den and Hotta [43], the "Tartary- Komuikotan Line". However, because the characteristic structure of the Kamuikotan schists originated during older tectonic stages, in the uppermost Jurassic [44], and are not related to the strike-slip movement on land, it seems preferable to use the words "Hidaka-Tartary Shear Zone" (Fig. 4).

In the same way, off Korea, horst-like ridges of the acoustic basement striking NNE-SSW are bounded on their eastern side by thick sedimentary basins with up to 2.5 km of Cenozoic sediments [45]. The Dolgorae-1 well (southern part of the Tsushima Basin), conducted by Korea In- stitute of Energy and Resources (KIER) in 1982 [46], was composed of 740 m of Pleistocene and recent deposits and 3522 m of Miocene deposits made up of claystone, siltstone and sandstone. The

381

META-OPHIOLITE ZONE

,MAIN ZONE

~ /! / U / / ,'

, /' j ' / , /"

/ ~ . / / " / , / / / " .," / , / / ' ' / .

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I I . . . . MAIN REVERSE FAULTS

• , MAIN STRIKE - S L I P FAULTS

. . . . . . . FOLD AXES

m STRETCHING L INEATION

Fig. 4. The Hidaka-Tartary Shear Zone. Fold axes and main thrusts are adapted from Kimura et al. [20], stretching lineation in the Hidaka Shear Zone is from Jolivet and Miyashita [16] and Kizaki [42] and thickness of sediments after Antipov et al. [40]. For explanation see text.

382

1850 m depth separating shallow marine sediments above from deeper marine sediments below, mostly deposited on a continental slope. A few marine deposits of that period were recognized on land on both sides of the strait. Thus the western margin of the Japan Sea (Korean Plateau and Tsushima Strait) displays very deep and narrow basins filled with thick Cenozoic deposits which appear to be controlled by N-S to NNE-SSW faults. We think that the East Korean Fault and the Tsushima Fault acted right-laterally, even if the N-S trending Tsushima island faults show a left-lateral movement [47]. As a matter of fact, every NNE-SSW strike-slip faults on land, which were active during the Paleogene, like the Yangsan Fault or the Yeongyang Fault, are right-lateral [48] due to a NE-SW compression. Consequently, a problem re- mains concerning the undated left-lateral strike-slip faults exposed on Tsushima island. They may be due to a more recent stress-field, It is clear that the shear sense on the Korean margin needs to be improved by future surveys.

Sillitoe [49] noticed that metallogenic belts, which can be followed from Korea to southwest Honshu are offset right-laterally by 250 km on both sides of the Yangsan-Tsushima fault system. Consequently, he used that fault system as a guide for a post-46 My B.P. southward drift of southwest Japan, 46 My being the age of the youngest considered metallogenic belt.

It is noticeable that the eastern and western margins of the Japan Sea display offshore a fault pattern which strikes subparallel to well expressed on-land strike-slip faults. Both systems (on-land and offshore) belong to the same shear zones. We assume that the N-S trending East Korean continental slope associated with ridges and troughs systems of the Korean plateau, subparallel to the Yangsan and the Tsushima Fault, can be interpreted as a strike-slip fault which we name the "East-Korean Fault" (Fig. 2). In a similar way, the Oga-Tartary line as well as the ridges and troughs between the Sado island and the Oga peninsula, can be interpreted as strike-slip faults which we name the Oga-Tartary and Sado Faults (Fig. 2).

5. Model

The Japan Sea is flanked to the east and west by two N-S trending right-lateral shear zones at

the time of opening. On the east side, the Hidaka- Tartary Shear Zone was active during Oligocene and Lower Miocene according to on-land data. On the west side, the Yangsan Fault was active between 46 My and the end of Oligocene and was relayed by the Tsushima fault during Lower Miocene. During this period, small amounts of displacement also occurred along the Tanakura and the Itoigawa-Shizuoka Tectonic Lines [15].

We now tentatively define several compara- tively undeformed continental blocks (Fig. 5). The first one is the East Hokkaido block located east of the Hidaka Shear Zone. Inasmuch as the shear zone is wide, the western boundary of this block was drawn in an arbitrary position between the Meta-Ophiolite Zone and the Main Zone. It includes the eastern part of the Hokkaido Central Belt and the continental margin of the Kuril Basin.

Fig. 5. Schematic structural map showing the di f ferent con- tinenta] blocks which have dr i f ted away f rom the Asiatic continent. EKB = East Korean block, K Y B = K i ta Yamato block, YB = Yamato block, SRB = Sado Ridge block.

It would be possible to subdivide it into several subblocks by the Abashiri Tectonic Line and the Kamishiyubetsu Tectonic Line (Figs. 2 and 4). The second one is the Tohoku-West Hokkaido block between the Hidaka Shear Zone and the Tanakura Tectonic Line. The first two blocks extend northward into the Tartary Strait and Sakhalin island. The third one is the Central Honshu block east of the Itoigawa-Shizuoka Tectonic Line. The last two blocks are considered as behaving as an indepen- dent microplate by Seno [2]. Three possible loca- tions of the present Eurasian-North American plate boundary or Amurian-Okhotsk plate boundary (fig. 1) have been discussed: across Hok- kaido [50], at the base of the eastern margin of the Japan Sea extending southward in the Itoigawa- Shizuoka Tectonic Line [51] or both with an inde- pendent microplate between them [2]. The fourth main block includes the whole southwest Japan bounded to the west by the Tsushima-Yangsan fault system. It is possible to outline several microblocks bounded by faults, such as the Kita-Yamato and Yamato Banks, the high between the Oki Bank and the Yamato Ridge, the Oki, Sado and Okushiri Ridges and several blocks on the Korean Plateau among which the most important is represented in Fig. 5. In the following, we propose a model of drift of these blocks away from the Asiatic continent (Figs. 6, 7).

5.1. First stage: Eocene/Lower Oligocene, before the main opening

During the Upper Cretaceous, the Japanese islands are still part of the Asiatic continent. They are part of an active margin (the Okhotsk- Chukotka volcanic belt, [52]) and undergo a strong deformation along NE-SW left-lateral strike-slip faults, such as the Sikhote-Alin Central Fault (Fig. 2) and the Tanakura Tectonic line. The metallogenic belt are continuous from South Korea to southwest Honshu [49]. Starting in Middle Eocene, a right-lateral movement begins along the Yang- san-Tsushima fault system and the Tartary-Hidaka Shear Zone. The whole of Japan and Sakhalin begin to drift southward. Consequently, the first basins open along shear zones, north of Kyushu and in Hokkaido as small pull-aparts.

383

5.2. Second stage: Upper Oligocene / Lower Miocene, main opening

An important biomodal magmatism responsible for the Green Tuff accumulations occurs and marks the transition between a continental and an oceanic rifting [4]. The right-lateral movement is very important on both sides of the Japan Basin. The motion of the whole of Japan is mainly southward along the East Korean Fault and Oga-Tartary Fault, creating the Japan Basin with its rhombohedral shape (Fig. 6). The Yangsan Fault becomes inactive, whereas the Tsushima Fault relays the East Korean Fault to the south. The Hidaka Shear Zone has been active until Sakhalin and Hokkaido reached their present position in Middle Miocene. We use the spreading axes direction in the Japan Basin of Isezaki [9]. Southwest Japan drift without any rotation, the amount of displacement are equal along both shear zones. Block movements occur along the margins: strongly subsiding basins are flanked by strike-slip faults. The "en 6chelon" troughs system along northeast Honshu and the small N-S basins in the Korean Plateau are created, or at least initiated as subordinate pull-aparts.

Fig. 6. Schematic model of opening of the Japan Sea; 1 = first opening, pull-apart mechanism; 2 = second opening, back-arc spreading.

384

5. 3. Third stage." Middle Miocene, second opening

A back-arc spreading subparallel to the southern part of the Japanese arc (Fig. 6) occurs during Middle Miocene, may be a consequence of the

previous tripling of Pacific plate motion at 25 My B.P. [53]. Southward drift along the two N-S shear zones has stopped, or may be limited to narrow zones, such as the deepest parts of the Tartary Gulf. The margins continue to subside and Middle

1 Eocene :~.~ ~ . \ ~,;. Lower Oligocene "i-I I~

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t

Fig. 7. Reconstruction of the circum Japan Sea region from Eocene to Present.

Miocene seal up the prior deformation. After 21 My B.P., the stress field is largely extensional with an axis perpendicular to the trend of the Japanese blocks (NW-SE extension in southwest Japan and E-W extension in northeast Japan [54]). This strain field induces the opening of the Yamato Basin and other NE-SW trending basins inside the Korean Plateau, obliterating the previous N-S features. If fixing microblocks such as the Yamato Ridge or the East Korean microblock, southwest Japan has drifted toward the southeast guided by transform faults like the Oki and Toyama Faults, but also the southern part of the East Korean Fault. This period also corresponds to the creation of horsts and grabens along the eastern margin of the Japan Sea, superimposed on the previous strike-slip features. Some on-land faults, like the Tanakura Tectonic Line or the Itoigawa-Shizuoka Tectonic Line, may have been active left-laterally during the opening of the Yamato Basin. It eastward spreading prevailed (Yamato Basin) between 15 and 12 My B.P., it could induce a rotation of southwest Japan with respect to Korea, using the Itoigawa- Shizuoka Tectonic Line as a transform fault rather than the Tanakura Tectonic Line. The deformation would become compressive along the Hidaka Shear Zone, probably due to the southwest motion of the East Hokkaido block. The end of the back- arc spreading at 12 My B.P. or earlier, may be related to the collision of the Kyushu-Palau and proto-Izu-Bonin Ridges [55]. The location of the Oligo-Miocene trench off northeast Japan in Fig. 7.3 is based on Cadet and Charvet's [56] paper.

For this stage, an alternative process can be discussed: the movement along the Hidaka-Tar- tary Shear Zone stops only along the Hidaka Shear Zone s.s. where the deformation becomes compressive. Strike=sli p continues along the Tatar Strait and Oga-Tartary Fault. This movement induces the opening of the Yamato Basin and Tsushima Basin at ~ the southern end of the fault, leading to a slight rotation of southwest Japan.

5.4. Fourth stage: Quaternary, compression

According to Nakamura and Uyeda [54], extension ceased 7 My ago and compression began 1 or 2 My ago. The recent thrust faults observed along the eastern margin of the Japan Sea (including the Tartary Gulf) may express the beginning of the

385

closure of the Japan Sea. This evolution is probably due to the counterclockwise rotation of the Amurian plate relatively to Eurasia (Fig. 7.4, [1]).

The eastern margin of the Japan Sea is seismi- cally active and 7 tsunamigenic shallow earthquakes of magnitude up to 6.9 were registered since 1930 [57-59]. The focal mechanisms reveal thrust-type earthquakes which occurred mostly along east dipping fault planes subparallel to the trend of the margin, indicating an E-W compression. On the basis of these earthquakes and other seismic data, some authors [51,60,61] suggest that a nascent subduction zone, where the Japan Sea begins to subduct below northeast Japan, corresponds to the present boundary between the North American plate and the Eurasian plate since 1 or 2 My. No evidence of such a process has been discovered during the recent ESTASE I survey of the base of the margin, although reverse faults indicate obviously a convergence (Fig. 3) especially well expressed east on the ridges which separate the graben like basins from the Japan Basin [22,23]. Thus compression certainly prevails now along this margin, but direct evidence of eastward thrusting in the Japan Basin along some profiles off Hokkaido, makes the idea of an eastward subduction debatable.

Some earthquakes corresponding to strike-slip faults with a N-S T-axis and an E-W P-axis; occur along the southwest margin of the Japan Sea. These data are in agreement with an E-W compression since 1 or 2 My B.P. [54].

6. Discussion and conclusion i

We describe the opening of the Japan Sea using mainly a pull-apart mechanism between two right-lateral strike-slip fault systems. It is clearly in disagreement with most of previous ideas of reconstruction of the archipelago before the opening of the marginal sea, but also with the mechanism. We examine here the main opening sketches and then discuss each of them.

(1) Numerous paleomagnetic studies, especially in southwest Honshu, indicate a clockwise rotation of southwest Japan by an angle varying from 20 ° [62] to 60 ° [63] depending on authors and data. Most recent results include those of Otofuji and Matsuda [64] with a rotation of 55 ° from 15 to 12 My B.P. and those of Torii [65] or Sasajima and

386

Torii [55] with an angle of 45 ° between 15 and 13 My B.P. and a pole located somewhere in Tsushima Strait.

Such an important rotation is in disagreement with the fit of the 2000-m isobaths (Fig. 6) of both margins on one hand and with the constraints given by the geology of northeast Japan and Sakhalin on the other hand. It is easy to observe that in the few models which consider the whole Japan Sea, including its northern part [11,13-15, 66], southwest Japan is rotated by only a small angle. This is due to a space problem. Otofuji and Matsuda (e.g. [67]) propose a reconstruction which only takes into account the coast lines of Asia and southwest Japan. When taking into account the limits of the continents (Fig. 5 or see [68] for more accuracy) it is not possible to rotate by more than 20 ° using the pole proposed by Otofuji and Matsuda. Thus, a rotation angle of 55 ° to 60 ° is unrealistic if southwest Japan is undeformed; but nothing on land allow us to think that a deformation related to the individualisation of blocks occurred at that time.

Our model supposes that the clockwise rotation of southwest Japan and the counterclockwise rotation of northeast Japan [18,64,67,69,70] are not as important as it was proposed on the basis of paleomagnetic measurements.

(2) These rotations are generally supposed to be contemporaneous with the bending of Honshu. Such a bending would imply a very important deformation at the time of rotation which is not observed in the field. Furthermore, the earliest structural directions (namely the direction of the stretching lineation in the Mesozoic high-pressure, low-temperature schists ) are the same in the Sambagawa Belt [71] and in the Kamuikotan Belt [44]. It is thus unlikely that the agreement in the structural directions is coincidental and produced by rotation.

Kawai et al. [19] have proposed, based on paleomagnetic data, that Hokkaido underwent a clockwise rotation relative to the main island of Japan. We have seen before that the Hidaki Shear Zone can be followed northward with the same strike in Sakhalin. A similar remark was made by Honza [6]. It is thus unlikely that a rotation occurred recently. Only a slight counterclockwise rotation of the southern part of Hokkaido could

be considered after the formation of the Hidaka Shear Zone to accommodate the slight bending of the Hokkaido Central Belt. Any clockwise rotation has to be older than Miocene.

Thus, the "banana-like" shape of Honshu would mostly pre-date the opening.

(3) Recently, Kimura and Tamaki [66] proposed that the opening is related to the northward retreat of the Amurian plate during Cenozoic time due to the collision between India and Eurasia.

First, their model implies important N N W right-lateral strike-slip movements in the northern Japan Arc during Miocene time that nobody has observed [15]. Second, they use the extrusion tectonic model of Tapponnier et al. [72], but the right-lateral movement along the Hidaka-Tartary Shear Zone began earlier (Eocene) than the northward movement of the Amurian plate (Lower Miocene [73]). So, in our opinion, the movement along the Hidaka-Tartary Shear Zone is not related to the same phenomenon but rather with the America-Eurasia relative motion for recent time [50] as well as for the Tertiary ([20], [74]).

(4) The synthesis of both on-land and offshore structural and morphological data allow us to propose a new interpretation of the Japan Sea formation. Our model is the first which opens the Japan Sea using a pull-apart mechanism between two "en 6chelon" strike-slip faults systems. Such a model was first applied by Tapponnier et al. [72] for the opening of the South China Sea. They described it as a pull-apart basin on the termina- tion of the Red River Fault, related to the extrusion of the China block due to the collision of India and Asia.

The question, then, is whether the fragmentation of the Asiatic continental rim is the consequence of back-arc spreading (a subduction-related process as usually stated, e.g. [75]) or an intracontinental deformation-related process. Our model takes into account both processes: intracontinental deformation during the first stage of opening (partly due to the America-Eurasia relative motion) and subduction during the second stage (Fig. 6). This qualitative model has to be tested by kinematic studies and further on-land field work and oceanographical studies.

Acknowledgements

T h e au tho r s a re g rea t ly i n d e b t e d to P ro fessors

X. Le P ichon , J. A u b o u i n , J.P. Cade t , K.

K o b a y a s h i a n d S. U y e d a , a n d Dr . M. F a u r e for

d i s cus s ion a n d cr i t ica l r e a d i n g of the m a n u s c r i p t .

W e t h a n k also P ro fes so r s V. Cour t i l l o t , K.

N a k a m u r a and A. T a i r a for the i r cr i t ica l review.

T h e w o r k has b e e n s u p p o r t e d by the F r e n c h

M i n i s t r y o f fo re ign af fa i rs for S.L., by a g r a n t

f r o m C N R S " G 6 o l o g i e et G 6 o p h y s i q u e des

O c 6 a n s " p r o j e c t fo r L.J . ; I F P ( Ins t i tu t F ran~a i s du

P6trole) a n d C N R S - I N A G : " B l o c s et C o l l i s i o n s "

p ro j ec t p r o v i d e d f inanc ia l s u p p o r t to b o t h au thors .

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Japan Sea: a pull-apart basin?