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Home , Amurian Plate, Baikal Rift Zone, Japan Trench, Northeastern Japan Arc, Okhotsk Plate, Tsugaru Strait

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 Central Belt and marine geology studies along the eastern margin of 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- rifting of a pull-apart basin accommodated along two large right-lateral shear zones, east of and west of northeast Japan and . It is followed, during Upper Oligocene/Lower , by the main opening of the Japan Basin as a mega pull-apart. Then a back-arc spreading probably related to the 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 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 Sea, the question then is whether the fragmentation of the Asiatic is an intracontinental deformation related process as proposed here or a subduction related one.

1. Introduction years later, Kobayashi and Isezaki [13] presented an evolutionary model of northwest Pacific with The is located at the junc- the southward drift of the Japanese islands from tion of four plates: the Amurian, Okhotsk, Pacific and Philippine Sea plates [1,2] (Fig. 1) and its complex evolution is governed by the relative mo- tion of these plates. The basement of the Japan Sea is, at least partly, oceanic [3-7]. In spite of Plate rather poor magnetic lineations [8], five small axes of symmetry roughly ENE-WSW in the Japan Plate Basin (Fig. 2) and NE-SW in the Yamato Basin have been recognized [9]. It is consequently highly probable that several spreading centers were in- volved in the creation of the deep basins (e.g. [10]). .,.,. Until now many models have been proposed to explain its formation by drifting of the islands, Almost all of these models deal with the southern part of the Japan Sea: the tectonic meaning of Tartary Strait and the drift of northeast Japan (north of Honshu and Hokkaido) are usually not Fig. 1. Localisation of plates in East adapted from considered, even the reconstruction took Sakhalin Zonenshain and Savostin [1]. The western boundary of the into account (e.g. [111). Murauchi et al. [12] pro- microplate of Seno [2] is represented in dotted lines, PhP posed a schematic pre-drift reconstruction. Ten .

0012-821X/86/$03.50 © 1986 Elsevier Science Publishers B.V. 376

the Asiatic continent, Sakhalin and the central model, adding a clockwise rotation of southwest part of Hokkaido during Eo-Oligo-Miocene. Re- Japan and great displacements along left-lateral cently, Kobayashi [14] reviewed their previous strike-slip faults in northeast Honshu but keeping

OKH

135 ° AMU

Okhotsk Sea

0 500 km I , . , i ! o

LSrnt

126 ° .40°--~ / i]I

Yellow Sea / t

, PAC

126 ° V i I , I 1.o

/ l East Oina Sea / ~ik~u Basin 4/ i /' PHS ',

Fig. 2. Structural context and physiography of Japan Sea and the Japanese islands. Plates: AMU = Amurian, OKH = Okhotsk, PAC = Pacific, PH.S = Philippine Sea. Tectonic features: MTL = Median Tectonic Line, KaTL = Kanto T.L., ISTL = Itoigawa- Shizuoka T.L., HSZ = Hidaka Shear Zone, KTL = Kamishiyubetsu T.L., ATL = Abashiri T.L., FIF = Fudzino Iman , CSAF = Central Sihkote Alin F., YF=Yangsan F., TF= Tsushima F. Hypothetical faults: EKF= East Korean Fault, OF= Old F., TF ~ Toyama F., OTF = Oga-Tartary F., SF = Sado F. Islands: G.Is = Goto is., T.Is = Tsushima is., O.Is = Old is., U.Do = Ullung Do is., S.Is = Sado is., Ok.Is. = Okushiri is. Seamounts: S.Smt = smt., B.Smt = Bogorov smt., V.Smt = Vityaz smt. Ridges: K.Y.Rd. = Kita-Yamato Ridge, O.Rd = Old Ridge, S.Rd = Sado Ridge. Banks: O.Bk = Old Bank, M.Bk = Musashi Bank. Plateaus: K.P1 = Korean Plateau, O.PI = Oshima Plateau. Troughs: G.Tr = Genzan Trough, O.Tr = Old Trough, T.Tr = Toyama Trough. Straits: T.Str = , Tg.Str = , S.Str = Soya Strait. Peninsulas: N.Pen = Noto Peninsula, O.Pen = Oga Peninsula, S.Pen = Shakotan Peninsula. 377 the same blocks as before. On the basis of the the Japan Basin and the structure of its margins strike-slip faults activity in the circum Japan Sea allow us to propose the idea of a pull-apart basin region from to Paleogene, Otsuki and opened between two right-lateral faults during the Ehiro [15] have also reconstructed the drift history Oligocene/Lower Miocene. of Japan. But these models do not fit with recent on-land 2. Morphological evidence for a pull-apart basin structural data. The main point is to reconcile the opening of Japan Sea and the contemporaneous Except for the Tartary Gulf, the coast lines of right-lateral movement along the Hidaka Shear Japan sea have a rhombohedral shape striking Zone [16,17] (Fig. 2) and the Yangsan Fault as will roughly N-S and WSW-ENE (Fig. 2). This is even be discussed later in this paper. Besides clearer for the outline of the Japan Basin. Its paleomagnetic studies have led to the proposal of topography is rather flat (3000-3700 m), except a clockwise rotation of southwest Japan around a for the presence of local highs like those of Bogorov pole located near the Tsushima Strait [18] and of and Siberia seamounts (1500 m deep). Three less the bending of Honshu [19]. These should give rise important basins or troughs, shallower than 2500 to considerable deformation contemporaneous m, extend the Japan basin northward (the Tartary with the rotations, especially in central Honshu, Gulf) and southward (the Yamato and Tsushima which have not been observed in the field, this Basins). All of them are elongated along a NE-SW conflict will also be discussed. direction. The main feature is the Yamato Ridge A new set of tectonic data brings new con- which overhangs the Japan Basin by 2500 m; it straints on the tectonic pattern of the eastern can be divided into two subridges trending NE- margin of the Japan Sea on-land and offshore SW: the Kita-Yamato and the Yamato Banks [16,20-23]. In addition, some Japanese data were separated by a narrow 2000 m deep. reinterpreted in light of the new hypothesis about Whereas the Siberian and the northernmost incipient subduction and obduction along this Korean margin of the Japan Sea is continuous and margin [24]. The present paper is an attempt to narrow, all the other margins are very complex. interpret these data in the framework of a new West of the Japan Sea lies the Korean Plateau model of the Japan Sea formation. (Korean Continental borderland of Mogi [25]) with In summary, the Japan Sea consists of several ridges and throughs striking NE-SW and N-S. deep basins with oceanic basement separated by This mosaic of peaks and depressions ends abruptly ridges of continental . The structure is very northward and southward against a N-S escarp- simple in the basins and complex on most of the ment corresponding to the continental slope (Fig. margins. The eastern and western margins both 2). show a particular structure: numerous N-S and The Japan sea ends to the south in the Tsushima NNE-SSW trending ridges bounding very narrow Strait between Korea and Kyushu. A NE-SW and rapidly subsiding basins filled with thick accu- channel limited by scarps runs on the plateau [5]. mulations of . The ridge and The linearity of the margins is interrupted by the basin structure can be followed along more than N-S Oki Bank and the extension of the Noto 1000 km along the eastern margin in the Tartary Peninsula. Their eastern flanks are respectively Gulf strongly suggesting that both margins are bounded by an escarpment and by a trough. We controlled by strike-slip faults. Considering the name these features the Oki Fault and the Toyama "en 6chelon" pattern of several troughs around the Fault (Fig. 2) based on the description given by Sado island or in the Tartary Gulf, the movement Ludwig et al. [4], Chihara [26] and others. Two could be right-lateral. This system is subparallel to other NE-SW trending blocks separate the Yamato the right-lateral Hidaka Shear Zone [16] which can Basin from the Oki Trough (the Oki Ridge) and be followed northward in Sakhalin. It is likely that from the Tsushima Basin creating a high between the two systems belong to the same wide right- the Oki Bank and the Yamato Ridge. These two lateral shear zone. Similarly we argue the fact that blocks might have drifted northward from south- the western margin of the Japan Sea is a right- west Honshu along the Oki and Toyama Faults. lateral strike-slip zone. The rhombohedral shape of Along the eastern margin, several ridges and 378

138 ° 140 °

44 °

%- KKAIDO Japan Basin '~ Seal ]) 42 °

.¢ v ~ ~9 ~

t- '." ~'k

Y.B, Tohoku

7

\ \

100 km HONSHU

"'" pi ""- Itoigawa-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. depressions follow one another with an "en Oga Peninsula: from NNE-SSW in the south to 6chelon" pattern from the Sado island to the lati- N-S in the north (Fig. 3) following the Mogami tude of Shakotan Peninsula off Hokkaido. A par- Trough and from south to north following the ticularity of this eastern margin is an abrupt change Nishi-Tsugaru Basin, the Okushiri Basin, the in the direction of isobaths at the latitude of the Shiribeshi Trough and two other small depressions 379 to the north. Many of these directions now coin- In Hokkaido, the first sediments which uncon- cide with active faults [24,27]. Only the Oshima formably overlie the Cretaceous Yezo Group are Plateau, where the Oshima caldera is located, does Paleogene non-marine and shallow marine de- not follow this regional trend, probably because it posits rich in coal beds [31,32]; they were de- did not yet exist during the movements responsible posited in strongly subsiding narrow basins [31]. for the N-S structuration of this margin. The Tar- Furthermore, on the basis of the plate thickening tary Gulf to the north lies between Sikhote Alin model in relation with the sedimentation, Takeuchi and the islands of Sakhalin and Hokkaido. It is an et al. [33] concluded for a marine invasion at elongated depression 1000-2000 m deep display- approximately 40 My B.P. ing several ridges striking NW-SE or NE-SW. Well-dated sections on-land allow to precise the Finally except for the Oki and Toyama Faults age of the syn- blocks movements on the trending NNW-SSE, the Japan Sea margins show margins. In eastern Korea, especially in the small two obvious directions: N-S for the east Korean Pohang Basin, marine deposits of the Yeonil Group continental slope and the Oga-Tartary line, and (Burdigalian to ) seal fault movements NE-SW to NNE-SSW, for the Tsushima Fault and which cut the continental Yangbug Group (Upper the Mogami Trough for example. Oligocene to Aquitanian). The NNE trending faults In summary, the Japan Sea displays a main give rise to an eastward downfaulting [34]. In rhombohedral basin and several subsidiary basins northern Honshu, in a section described in the bounded on both east and west sides by very long Oga Peninsula [35], the non-marine Daijima For- scarps systems striking N-S which evoke strike-slip mation (Lower Miocene) which overlies uncon- margins. formably the massive andesitic Green Tuffs de- posits (Upper Oligocene to Lowermost Miocene 3. Age of opening Nishioga and Monzen Formations) can be inter- preted as syn-rift and is covered by the littoral The magnetic lineations in the Japan Sea are neritic Nishikurosegawa Formation (Lower to not easily identified and the oldest sediments which Middle Miocene) indicating a marine transgres- overlie the oceanic crust have never been reached sion. Thus, major blocks movements occurred dur- by drilling, except for the northern flank of the ing uppermost Oligocene/Early Miocene both off Yamato Ridge where the top of the basement Korea and Honshu and ended in Middle Miocene. consists of Lower Miocene volcanic siltstones and A two stages opening with the creation of the Green Tuff formations, similar to those known in Yamato Basin during the last stages has been Honshu [4]; consequently there is no direct evi- proposed [13,14,36]. It is in good agreement with dence concerning the age of opening of the margi- the main opening at least of the Japan Basin nal sea. It is a subject which had been very con- during the Upper Oligocene/Lower Miocene, fol- troversial although a consensus seems to have been lowed by the opening of the Yamato Basin which reached now. may be associated with a small amount of rotation The high but uniform values of the heat flow of southwest Japan during Middle Miocene, as lead Watanabe et al. [28] to conclude that the well be discussed below. opening occurred necessarily before 20 My ago. We consider as a working hypothesis, a begin- Seismic profiles, DSDP results and onshore sec- ning of rifting during Eo-Oligocene followed by an tions in favor of a Lower Miocene age for the active tectonism along the eastern and western oldest sediments of the deep basins (probably margins during the Upper Oligocene/Lower Green Tuff formations or remobilized Green Tuffs, Miocene. From the Middle Miocene, the margins at least in the Yamato Basin). are no longer deformed except for the Tartary The earliest evidence for the presence of a basin Strait and the formation of small depressions along in the Japan Sea seems to be Eo-Oligocene in- the eastern margin. Of course, we leave the recent asmuch as non-marine and marine sediments, compressive tectonics along this margin out of showing northward paleocurrents [29,30] were de- account. Middle Miocene deposits seal the move- posited in northern Kyushu, indicating that a basin ment in the section recognized on-land. already existed north of Kyushu at that time [15]. 380

4. Strike-slip margins Hidaka metamorphics [16,42] are plotted. The fold axis pattern and their relation with the strike-slip Off northern Honshu and Hokkaido, the fault system led Kimura et al. [20] to conclude that Neogene and Quaternary sediments are distrib- the Sakhalin-Hokkaido system was a right-lateral uted in narrow basins controlled by faults [37]; the oblique collision zone during the Paleogene and thickness of the sediments can reach more than 3 Early Miocene. Jolivet and Miyashita [16] showed km in the Nishi-Tsugaru Basin [22] or 6 km off that the structure of the Hidaka metamorphic belt Tohoku [38]. A recent survey of the eastern margin is that of a right-lateral strike-slip shear zone of of the Japan Sea [22] reveals that the nature and Oligo-Miocene age. The structure of both Meta- thickness of the sediments in the graben-like basins Ophiolite Zone and Main Zone is consistent and north of the Oga Peninsula are very different from displays a vertical N-S foliation bearing an hori- those of the Japan Basin. These horst and graben zontal stretching lineation (earlier mapped by structures are pre- or syn-sedimentary, the sedi- Kizaki [42]). Non-coaxial deformation criteria al- ments being of probable Plio-Quaternary. They lowed them to propose that a right lateral strike-slip were particularly well developed along a weak synmetamorph movement was responsible for the zone such as the Oga-Tartary line (Fig. 3). Many formation of the shear zone. young eastward or westward reverse faults are Fig. 4 also shows the thickness of the sedimen- recognized along this margin and seaward tary cover along the Tartary Strait after Antipov et [23,24,27,39]. These observations indicate prob- al. [40]. Very narrow basins aligned N-S are filled ably the reactivation of ancient normal faults into with very thick sedimentary sequence (up to 8 km) reverse ones [22]. The Tartary Gulf shows the in the West Sakhalin Trough. On seismic profiles, same characteristics with two narrow depressions: such basins are bounded by vertical faults [6]. In the "West-Sakhalin Trough" (1000 km long, 60 the southern part of the area concerned by Fig. 4, km wide) which can be followed from the Sakhalin we plotted the axes of the "en 6chelon" troughs margin to the southern margin of Hokkaido across along the Oga-Tartary line. the island (the Sapporo-Tomakomai depression), All these features indicate that the entire area and the "Tartar Trough" (1500 km long, 200 km (Sakhalin, Hokkaido Central Belt and Tartary wide) which corresponds to the deepest part of the Strait) are different expressions of the same right- gulf. The thickness of the sediments varies between lateral strike-slip shear zone, the "' Hidaka- Tartary 3 and 8 km [40]. Shear Zone ". For the eastern margin, the idea that New data on the structure of the Hokkaido the structure on-land and in the Tartary Strait Central Belt have been obtained. Kimura et al. [20] belong to the same orogenic system has been pro- argued about right-lateral oblique collision during posed by Den and Hotta [43], the "Tartary- Tertiary. Jolivet and Miyashita [16] and Jolivet Komuikotan Line". However, because the char- [17] showed that the Central Belt acted during the acteristic structure of the Kamuikotan schists Oligocene and Lower Miocene as a right-lateral originated during older tectonic stages, in the up- strike-slip shear zone, the Hidaka Shear Zone. The permost Jurassic [44], and are not related to the amount of displacement is not directly known but strike-slip movement on land, it seems preferable is likely to be great, considering the width of the to use the words "Hidaka-Tartary Shear Zone" shear zone and the intensity of deformation. From (Fig. 4). the Middle Miocene, the movement became com- In the same way, off Korea, horst-like ridges of pressive in the southern part of the shear zone [41] the acoustic basement striking NNE-SSW are but was still a strike-slip in the north. The fault bounded on their eastern side by thick sedimen- system can be followed in Sakhalin [20]. In South tary basins with up to 2.5 km of Cenozoic sedi- Hokkaido, the shear zone is not expressed west of ments [45]. The Dolgorae-1 well (southern part of the Sapporo-Tomakomai depression. the Tsushima Basin), conducted by Korea In- Fig. 4 shows the different tectonic characteris- stitute of Energy and Resources (KIER) in 1982 tics of the eastern margin. On land tectonic fea- [46], was composed of 740 m of Pleistocene and tures such as the main strike-slip faults, Tertiary recent deposits and 3522 m of Miocene deposits fold axes [20] as well as stretching lineation in the made up of claystone, siltstone and . The 381

META-OPHIOLITE ZONE

,MAIN ZONE

~ /! ,,-- / U / / ,' , /' j' /" / ,

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

20 km [

! I / I

SHIRISESI TROUGH

OKUSHIRI ISOPACHS OF SEDIMENTARY 7" S ASIN" COVER (in km) I I .... MAIN REVERSE FAULTS NISHI-TSUGAR • , MAIN STRIKE -SLIP FAULTS BASIN ...... FOLD AXES

m STRETCHING LINEATION

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 the time of opening. On the east side, the Hidaka- above from deeper marine sediments below, mostly Tartary Shear Zone was active during Oligocene deposited on a continental slope. A few marine and Lower Miocene according to on-land data. On deposits of that period were recognized on land on the west side, the Yangsan Fault was active be- both sides of the strait. Thus the western margin of tween 46 My and the end of Oligocene and was the Japan Sea (Korean Plateau and Tsushima relayed by the Tsushima fault during Lower Strait) displays very deep and narrow basins filled Miocene. During this period, small amounts of with thick Cenozoic deposits which appear to be displacement also occurred along the Tanakura controlled by N-S to NNE-SSW faults. We think and the Itoigawa-Shizuoka Tectonic Lines [15]. that the East Korean Fault and the Tsushima We now tentatively define several compara- Fault acted right-laterally, even if the N-S trending tively undeformed continental blocks (Fig. 5). The faults show a left-lateral move- first one is the East Hokkaido block located east of ment [47]. As a matter of fact, every NNE-SSW the Hidaka Shear Zone. Inasmuch as the shear strike-slip faults on land, which were active during zone is wide, the western boundary of this block the Paleogene, like the Yangsan Fault or the was drawn in an arbitrary position between the Yeongyang Fault, are right-lateral [48] due to a Meta-Ophiolite Zone and the Main Zone. It in- NE-SW compression. Consequently, a problem re- cludes the eastern part of the Hokkaido Central mains concerning the undated left-lateral strike-slip Belt and the continental margin of the Kuril Basin. 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 south- west 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 con- tinental 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 Fig. 5. Schematic structural map showing the different con- tinenta] blocks which have drifted away from the Asiatic conti- The Japan Sea is flanked to the east and west nent. EKB = East Korean block, KYB = Kita Yamato block, by two N-S trending right-lateral shear zones at YB = Yamato block, SRB = Sado Ridge block. 383

It would be possible to subdivide it into several 5.2. Second stage: Upper Oligocene / Lower subblocks by the Abashiri Tectonic Line and the Miocene, main opening Kamishiyubetsu Tectonic Line (Figs. 2 and 4). The second one is the Tohoku-West Hokkaido block An important biomodal magmatism responsible between the Hidaka Shear Zone and the Tanakura for the Green Tuff accumulations occurs and marks Tectonic Line. The first two blocks extend north- the transition between a continental and an oce- ward into the Tartary Strait and Sakhalin island. anic rifting [4]. The right-lateral movement is very The third one is the Central Honshu block east of important on both sides of the Japan Basin. The the Itoigawa-Shizuoka Tectonic Line. The last two motion of the whole of Japan is mainly southward blocks are considered as behaving as an indepen- along the East Korean Fault and Oga-Tartary dent microplate by Seno [2]. Three possible loca- Fault, creating the Japan Basin with its rhombo- tions of the present Eurasian-North American hedral shape (Fig. 6). The Yangsan Fault becomes plate boundary or Amurian- inactive, whereas the Tsushima Fault relays the boundary (fig. 1) have been discussed: across Hok- East Korean Fault to the south. The Hidaka Shear kaido [50], at the base of the eastern margin of the Zone has been active until Sakhalin and Hokkaido Japan Sea extending southward in the Itoigawa- reached their present position in Middle Miocene. Shizuoka Tectonic Line [51] or both with an inde- We use the spreading axes direction in the Japan pendent microplate between them [2]. The fourth Basin of Isezaki [9]. Southwest Japan drift without main block includes the whole southwest Japan any rotation, the amount of displacement are equal bounded to the west by the Tsushima-Yangsan along both shear zones. Block movements occur fault system. It is possible to outline several micro- along the margins: strongly subsiding basins are blocks bounded by faults, such as the Kita-Yamato flanked by strike-slip faults. The "en 6chelon" and Yamato Banks, the high between the Oki troughs system along northeast Honshu and the Bank and the Yamato Ridge, the Oki, Sado and small N-S basins in the Korean Plateau are created, Okushiri Ridges and several blocks on the Korean or at least initiated as subordinate pull-aparts. Plateau among which the most important is rep- resented 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: /Lower Oligocene, before the main opening

During the Upper Cretaceous, the Japanese is- lands 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 metallo- genic belt are continuous from 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. 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 previous tripling of motion at 25 My B.P. [53]. Southward drift along the two N-S shear A back-arc spreading subparallel to the south- zones has stopped, or may be limited to narrow ern part of the Japanese arc (Fig. 6) occurs during zones, such as the deepest parts of the Tartary Middle Miocene, may be a consequence of the Gulf. The margins continue to subside and Middle

1 2 Eocene :~.~ ~.\ ~,;. Upper Oligocene , '~ Lower Oligocene "i-II~ f/'!"X

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/,-/ ~ Ii , Amurian i .~' \ • ~late's motion /r,', / (,.-~,, ,~ ~,. / ~' .~'~t "". 2.• ,\~" ~Z ,~,'*,-~,~*,'-J: ) / :~. 2,~E., ' I i~.,t<.l"

"Jr" closing axis I

Fig. 7. Reconstruction of the circum Japan Sea region from Eocene to Present. 385

Miocene seal up the prior deformation. After 21 closure of the Japan Sea. This evolution is prob- My B.P., the stress field is largely extensional with ably due to the counterclockwise rotation of the an axis perpendicular to the trend of the Japanese relatively to (Fig. 7.4, [1]). blocks (NW-SE extension in southwest Japan and The eastern margin of the Japan Sea is seismi- E-W extension in northeast Japan [54]). This strain cally active and 7 tsunamigenic shallow earth- field induces the opening of the Yamato Basin and quakes of magnitude up to 6.9 were registered other NE-SW trending basins inside the Korean since 1930 [57-59]. The focal mechanisms reveal Plateau, obliterating the previous N-S features. If thrust-type which occurred mostly fixing microblocks such as the Yamato Ridge or along east dipping fault planes subparallel to the the East Korean microblock, southwest Japan has trend of the margin, indicating an E-W compres- drifted toward the southeast guided by transform sion. On the basis of these earthquakes and other faults like the Oki and Toyama Faults, but also the seismic data, some authors [51,60,61] suggest that southern part of the East Korean Fault. This a nascent subduction zone, where the Japan Sea period also corresponds to the creation of horsts begins to subduct below northeast Japan, corre- and along the eastern margin of the Japan sponds to the present boundary between the North Sea, superimposed on the previous strike-slip fea- American plate and the since 1 or 2 tures. Some on-land faults, like the Tanakura My. No evidence of such a process has been Tectonic Line or the Itoigawa-Shizuoka Tectonic discovered during the recent ESTASE I survey of Line, may have been active left-laterally during the the base of the margin, although reverse faults opening of the Yamato Basin. It eastward spread- indicate obviously a convergence (Fig. 3) espe- ing prevailed (Yamato Basin) between 15 and 12 cially well expressed east on the ridges which My B.P., it could induce a rotation of southwest separate the graben like basins from the Japan Japan with respect to Korea, using the Itoigawa- Basin [22,23]. Thus compression certainly prevails Shizuoka Tectonic Line as a rather now along this margin, but direct evidence of than the Tanakura Tectonic Line. The deforma- eastward thrusting in the Japan Basin along some tion would become compressive along the Hidaka profiles off Hokkaido, makes the idea of an east- Shear Zone, probably due to the southwest motion ward subduction debatable. of the East Hokkaido block. The end of the back- Some earthquakes corresponding to strike-slip arc spreading at 12 My B.P. or earlier, may be faults with a N-S T-axis and an E-W P-axis; occur related to the collision of the Kyushu-Palau and along the southwest margin of the Japan Sea. proto-Izu-Bonin Ridges [55]. The location of the These data are in agreement with an E-W com- Oligo-Miocene trench off northeast Japan in Fig. pression since 1 or 2 My B.P. [54]. 7.3 is based on Cadet and Charvet's [56] paper. For this stage, an alternative process can be 6. Discussion and conclusion discussed: the movement along the Hidaka-Tar- i tary Shear Zone stops only along the Hidaka We describe the opening of the Japan Sea using Shear Zone s.s. where the deformation becomes mainly a pull-apart mechanism between two compressive. Strike=slip continues along the Tatar right-lateral strike-slip fault systems. It is clearly in Strait and Oga-Tartary Fault. This movement in- disagreement with most of previous ideas of recon- duces the opening of the Yamato Basin and struction of the archipelago before the opening of Tsushima Basin at ~the southern end of the fault, the marginal sea, but also with the mechanism. We leading to a slight rotation of southwest Japan. examine here the main opening sketches and then discuss each of them. 5.4. Fourth stage: Quaternary, compression (1) Numerous paleomagnetic studies, especially in southwest Honshu, indicate a clockwise rotation According to Nakamura and Uyeda [54], exten- of southwest Japan by an angle varying from 20 ° sion ceased 7 My ago and compression began 1 or [62] to 60 ° [63] depending on authors and data. 2 My ago. The recent thrust faults observed along Most recent results include those of Otofuji and the eastern margin of the Japan Sea (including the Matsuda [64] with a rotation of 55 ° from 15 to 12 Tartary Gulf) may express the beginning of the My B.P. and those of Torii [65] or Sasajima and 386

Torii [55] with an angle of 45 ° between 15 and 13 be considered after the formation of the Hidaka My B.P. and a pole located somewhere in Tsushima Shear Zone to accommodate the slight bending of Strait. the Hokkaido Central Belt. Any clockwise rotation Such an important rotation is in disagreement has to be older than Miocene. with the fit of the 2000-m isobaths (Fig. 6) of both Thus, the "banana-like" shape of Honshu would margins on one hand and with the constraints mostly pre-date the opening. given by the geology of northeast Japan and (3) Recently, Kimura and Tamaki [66] pro- Sakhalin on the other hand. It is easy to observe posed that the opening is related to the northward that in the few models which consider the whole retreat of the Amurian plate during Cenozoic time Japan Sea, including its northern part [11,13-15, due to the collision between India and Eurasia. 66], southwest Japan is rotated by only a small First, their model implies important NNW angle. This is due to a space problem. Otofuji and right-lateral strike-slip movements in the northern Matsuda (e.g. [67]) propose a reconstruction which Japan Arc during Miocene time that nobody has only takes into account the coast lines of Asia and observed [15]. Second, they use the extrusion southwest Japan. When taking into account the tectonic model of Tapponnier et al. [72], but the limits of the (Fig. 5 or see [68] for more right-lateral movement along the Hidaka-Tartary accuracy) it is not possible to rotate by more than Shear Zone began earlier (Eocene) than the north- 20 ° using the pole proposed by Otofuji and ward movement of the Amurian plate (Lower Matsuda. Thus, a rotation angle of 55 ° to 60 ° is Miocene [73]). So, in our opinion, the movement unrealistic if southwest Japan is undeformed; but along the Hidaka-Tartary Shear Zone is not re- nothing on land allow us to think that a deforma- lated to the same phenomenon but rather with the tion related to the individualisation of blocks oc- America-Eurasia relative motion for recent time curred at that time. [50] as well as for the Tertiary ([20], [74]). Our model supposes that the clockwise rotation (4) The synthesis of both on-land and offshore of southwest Japan and the counterclockwise rota- structural and morphological data allow us to pro- tion of northeast Japan [18,64,67,69,70] are not as pose a new interpretation of the Japan Sea forma- important as it was proposed on the basis of tion. Our model is the first which opens the Japan paleomagnetic measurements. Sea using a pull-apart mechanism between two (2) These rotations are generally supposed to "en 6chelon" strike-slip faults systems. Such a be contemporaneous with the bending of Honshu. model was first applied by Tapponnier et al. [72] Such a bending would imply a very important for the opening of the South China Sea. They deformation at the time of rotation which is not described it as a pull-apart basin on the termina- observed in the field. Furthermore, the earliest tion of the Red River Fault, related to the extru- structural directions (namely the direction of the sion of the China block due to the collision of stretching lineation in the Mesozoic high-pressure, India and Asia. low-temperature schists ) are the same in the The question, then, is whether the fragmenta- Sambagawa Belt [71] and in the Kamuikotan Belt tion of the Asiatic continental rim is the conse- [44]. It is thus unlikely that the agreement in the quence of back-arc spreading (a subduction-re- structural directions is coincidental and produced lated process as usually stated, e.g. [75]) or an by rotation. intracontinental deformation-related process. Our Kawai et al. [19] have proposed, based on model takes into account both processes: in- paleomagnetic data, that Hokkaido underwent a tracontinental deformation during the first stage clockwise rotation relative to the main island of of opening (partly due to the America-Eurasia Japan. We have seen before that the Hidaki Shear relative motion) and subduction during the second Zone can be followed northward with the same stage (Fig. 6). This qualitative model has to be strike in Sakhalin. A similar remark was made by tested by kinematic studies and further on-land Honza [6]. It is thus unlikely that a rotation oc- field work and oceanographical studies. curred recently. Only a slight counterclockwise rotation of the southern part of Hokkaido could 387

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