Neogene Strike-Slip Faulting in Sakhalin and the Japan Sea Opening Marc Fournier, Laurent Jolivet, Philippe Huchon, Konstantin F

Home , Aniva, Honshu, Kholmsk, Poronaysk, Tsushima Strait

Neogene strike-slip faulting in Sakhalin and the Japan Sea opening Marc Fournier, Laurent Jolivet, Philippe Huchon, Konstantin F. Sergeyev, Leonid Oscorbin

To cite this version:

Marc Fournier, Laurent Jolivet, Philippe Huchon, Konstantin F. Sergeyev, Leonid Oscorbin. Neo- gene strike-slip faulting in Sakhalin and the Japan Sea opening. Journal of Geophysical Research : Solid Earth, American Geophysical Union, 1994, 99 (B2), pp.2701-2725. 10.1029/93JB02026. insu- 00726734

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HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 99, NO. B2, PAGES 2701-2725, FEBRUARY 10, 1994

Neogenestrike-slip faulting in Sakhalin and the Japan Sea opening

Marc Fournier, Laurent Jolivet, and Philippe Huchon Laboratoirede G•.ologie,Drpartement Terre AtmosphereOcean, Ecole Normale Sul•rieure, Paris

KonstantinF. Sergeyevand Leonid S. Oscorbin Instituteof Marine Geologyand Geophysics, Far EastScience Center, Yuzhno-Sakhalinsk, Russia

We describestructural data from a 2000 km N-S dextral strike-slip zone extending from northem Sakhalin to the southeastcomer of the JapanSea. Satellite images,field data, and focal mechanismsof earthquakesin Sakhalinare includedin the interpretation.Since Miocene time the deformationin Sakhalin hasbeen taken up by N-S dextralstrike-slip faults with a reversecomponent and associateden 6chelonfolds. Narrow en 6chelonNeogene basins were formed along strike-slipfaults and were later folded in a second stageof deformation.We proposea modelof basinformation along extensional faults delimitatingdominos betweentwo major strike-slipfaults, and subsequentcounterclockwise rotation of the dominosin a dextral transpressionalregime, basins becoming progressivelyoblique to the direction of maximum horizontal cmnpressionand undergoingshortening. The associationof bothdextral and compressionalfocal mechanisms of earthquakesindicates that the sametranspressional regime still prevailstoday in Sakhalin.We presentfault set measurements undertaken in Noto Peninsula and Yatsuo Basin at the southern end of the Sakhalin-East JapanSea strike-slipzone. Early and middle Miocene formationsrecorded the sametranstensional regime as observedalong the west coastof NE Honshu.During the early and middle Miocene the strike-slip regime wastranspressional to the northin Sakhalinand Hokkaido,and transtensional to the southalong the westcoast of NE Honshuas far as Noto Peninsulaand Yatsuobasin. Dextral motion accommodated the openingof the JapanSea as a pull-apartbasin, with the Tsushimafault to the west.The openingof the JapanSea ceasedat the end of the middle Miocene when transtensionstarted to changeto E-W compressionin the Japan arc. Subductionof the JapanSea lithosphereunder the Japanarc started1.8 Ma ago. The evolutionof the stress regime from transtensionalto compressionalin the southernpart of the strike-slip zone is related to the inceptionof the subductionof the youngPhilippine Sea Plate lithosphereunder the Japanarc duringthe late Miocene. Subductionrelated extensionis a necessarycondition for the opening of the Japan Sea. Two possiblemechanisms can accountfor dextral shearin this area: (1) counterclockwiserotation of crustal blocksdue to the collisionof India with Asia, (2) extrusionof the OkhotskSea block squeezedbetween the North America and Eurasiaplates.

INTRODUCTION youngest dredged basaltic volcanics were dated 11 Ma [Kaneoka et al., 1990]. These agesare in good agreementwith Deformation resulting from the India-Asia collision is an earlier determination of the age of Japan Sea basement taken up by crustalthickening in the Himalaya-Tibet collision based on geophysicaldata betweev 30 and 10 Ma [Tamaki, zone and by geometric reorganization of continental blocks 1986, 1988]. Similar average heat flow value and basement accommodated by strike-slip motion along major faults depth in both Japan and Kuril Basins led Tamaki [1988] to [Molnar and Tapponnier, 1975; Tapponnier and Molnar, conclude to a simultaneousopening of the two basins in the 1976; Zonenshain and Savostin, 1981; Cobbold and Davy, late Oligocene to middle Miocene. Otsuki and Ehiro [1978] 1988; England and Molnar, 1990; Holt et al., 1991]. Along first outlined the role of strike-slip faults in the openingof the the southeasternlimit of Asia, strike-slip faults accommodated Japan Sea and proposed a drawerlike model of opening the opening of two marginal basins: the Andaman Sea in the between the left-lateral Tanakura tectonic line (TI'L) to the east prolongation of the Sagaing fault and Shan scarp system and the right-lateralTsushima fault (TF) to the west. Kimura et [Curray et al., 1979; Tapponnier et al., 1986], and possibly al. [1983] and Jolivet and Miyashita [1985] describedonland the SouthChina Sea along the extensionof the Red River fault transpressional deformation associated with N-S dextral [Tapponnier et al., 1982, 1986; Briais et al., 1993]. strike-slip motion in Sakhalin and Hokkaido during the In northeast Asia, at the end of the Tien Shan-Baikal- Oligocene and Miocene. Lallemant and Jolivet [1985] Stanovoy deformation zone, the Japan and Okhotsk Seas proposed a model of opening of the Japan Sea as a simple (Figure 1) openedin the Early and Middle Miocene. The oldest dextral pull-apart basin between this strike-slip zone to the oceanicbasalts drilled on the easternmargin of the JapanSea east, and the Tsushima fault to the west. Jolivet et al. [1991], during Ocean Drilling Program leg 127 have a radiometric using analogue laboratory modelling, refined the pull-apart 40Ar-39Arage of 24 Ma [Tamakiet al., 1992]and the model by introducing blocks rotations in the dextral shear zone, that only partly fit the paleomagnetic data. Jolivet [1986] associatedthe dextral motion to southward extrusion of the Okhotsk Sea block squeezedbetween the North America Copyright1994 by the AmericanGeophysical Union. and the Eurasiaplates. Kimura and Tamaki [1986] and Jolivet et al. [1990] finally related the opening of the Japan and Paper number 93JB02026. Okhotsk Seas to the motions of Asian microplates as a 0148-0227/94/93JB-02026505.00 consequenceof the India-Asia collision.

2701 2702 FOURNIERET AL.: NEOGENEDEXTRAL MOTION IN SAKHALINAND JAPAN SEA

• OCEANICBASEMENT

EURASIAI RECENT OFF-SHORE PLATE E-W COMPRESSION ZONE

OKHOTSK

KURIL BASIN

::JAPAN SEA-!

/PACIFIC PLATE

Fig. 1. Simplifiedstructural map of far eastAsia with topographyand bathymetry. Solid arrowsgive directionsof motionsof Pacificand Okhotsk plates relative to Asia.B is basin,IP is Izu Peninsula,MTL is mediantectonic line, R is ridge,S is strait,T is trench,TF is Tsushimafault, TI'L is Tanakuratectonic line, Y F is Yangsanfault, and ZR is Zenisu ridge.

The easternguide of the JapanSea dextral pull-apartis a describedby Lallemant and Jolivet [ 1985], Jolivet and Huchon 2000-km-long strike-slip zone extending from the northern [1989] and Jolivet et al. [1991]. In Sakhalin, field studies end of Sakhalin island to the north, to the southeast comer of showed that the island is cut by a N-S trending faults system the Japan Sea to the south (Figure 2)[Kimura et al., 1983; [Zanyukov, 1971; Rozhdestvensky, 1982] which is Lallemant and Jolivet, 1985]. The central part of the strike- responsiblefor the intense shallow seismicity of the island slip zone from Hokkaido to the west coast of NE Honshu is [e.g., Oskorbin, 1977; Savostinet al., 1983; Tarakamov and FOURNIER ET AL.: NEOGENE DEXTRAL MOTION IN SAKHALIN AND JAPAN SEA 2703

Kim, 1983]. Field observationsby Rozhdestvensky [1982] 1400 1450 55 øm describeddextral Miocene strike-slip motion along the faults. In this paper we describeboth the north and the south endsof the strike-slip zone, respectively, in Sakhalin and Central Japan. We first presenta study of the strike-slipzone in Sakhalin based on the structural interpretation of Landsat images, results of field work in 1989 and 1990 along the Tym- Poronayskfault and in the East Sakhalin Mountains, and fault plane solutionsfor major earthquakessince 1960 [this study; Fukao and Furumoto, 1975; Dziewonski et al., 1985; 1987]. We confirm the conclusionsof Rozhdestvensky[1982]: the island is a right-lateral strike-slip zone of Neogene age. Dextral motion is inferred from large-scale geological structuresobserved on the satellite images. Small-scale fault measurements in Neogene deposits provide maximal horizontal stress directions compatible with the dextral motion along the N-S faults. Seismologicaldata show that the dextral motion is still active today. We proposea model of progressive strike-slip deformation with block rotations about vertical axes to explain the later shortening of the Neogene dextral en 6chelonbasins. In a secondpart we infer the Miocene paleostress field from fault measurements undertakenin Noto peninsula and Yatsuo basin (central Japan, Figure 1) at the southernextremity of the Sakhalin-EastJapan Sea strike-slip zone. Miocene formations recorded there the same transtensionaldeformation as observed along the west coast of NE Honshu [Jolivet et al., 1991]. We conclude by discussing the evolution of the strike-slip zone from the 45 o-- Miocene to the present, e.g., from the pull-apart opening of the JapanSea to its present-dayincipient closure.

PRESENT-DAY GEODYNAMIC $ETITNG

The Japan island arc lies above the old Pacific slab to the east which is subductedwestward at a high rate of 10 cm/yr, and above the younger slab of the Philippine Sea Plate to the south which underthrustsJapan at a slower rate of a few centimetersper year (Figure 1) [Matsukata et al., 1991]. The Philippine sea plate (PHSP)/Eurasia (EUR) rotation pole is located immediately north of the PHSP/EUR/Pacific (PAC) triple junction [Seno, 1977; Ranken et al., 1984; tluchon, 1985]. The slightly oblique subductionof the PHSP in the Nankai trench is partitioned between compressional deformation in the trench and slow dextral motion along the median tectonic line (MTL) in southwest Japan. A large accretionary prism is progressively built up in the Nankai trench at the expense of trench turbidites [Le Pichon et al., 1987; Le Pichon, Kobayashi et al., 1992]. Close to the triple junction the Bonin arc collides with Central Japangiving rise to active thrustingnorth of the Izu Peninsulawith a fan-shaped stress pattern [Huchon, 1985]. Active compression propagatessouthward inside the Bonin arc and Shikoku Basin with the formation of crustal scale shorteningthrust (Zenisu ridge) [Lallemant et al., 1989; Chamot-Rookeand Le Pichon, 1989]. 35 ø The Pacific plate is consumed in the Bonin, Japan and Kuril trenches which are almost devoid of turbidites infill, and where fast erosion of the inner wall and tectonic erosion are the major phenomena[Cadet et al., 1987a, 1987b; Von Huene and Lallemand, 1990]. A compressionalstress field dominates in northeast Japan with an E-W direction of compression almost parallel to the PAC/EUR motion vector. Active shorteningis observedalong the easternmargin of the Japan Fig. 2. Tectonicmap of the Sakhalin-EastJapan Sea strike-slipzone. Sea as a zone of shallow seismicity (Figure 1) with large Solid arrows representearly and middle Miocene main stressfield directions,transpressional to the north in Sakhalinand Hokkaido,and transtensionalthe southalong the eastemmargin of the JapanSea (with dextral en 6chelonbasins). Open arrowsrepresent upper Miocene and Pliocenemain stressfield directions,still transpressionalin Sakhalinand regime from stress-freeto compressionalduring late Miocene. The purely compressionalin the Japanarc. Changesof stressfield directions possibleclockwise rotation of the stessfield in Sakhalinis discussedin in the southof the strike-slipzone correspond to changeof subduction the text. TTL is Tanakura tectonic line. 2704 FOURNIERET AL.' NEOGENE DEXTRAL MOTION 1N SAKHALIN AND JAPAN SEA earthquakes(M>7.5) evidencingE-W compression[Fukao and 1983]. The seismogeniczone extendsnorthward in Sakhalin Fururnoto,1975] taken up by thrustfaults [TarnaM and Honza, whereboth pure compressional and pure strike-slip events are 1984; Lallernandand Jolivet, 1985]. This active deformation recorded[Chapman and Solomon,1976; Sarostinet al., 1983; hasbeen interpreted as the incipient subduction of the young thisstudy]. Seismicity activity also occurs in thewestern part JapanSea lithosphereunder northeast Japan [Nakamura, of the JapanSea displayingdextral strike-slip events along NE trendingfaults in the Tsushimastrait [Jun, 1990]. Within the Asiancontinent dextral motions along NE trendingfaults control the formation of pull-apart basinsin the Bohai Gulf region [Chen and Nabelek, 1988].

TRANSPRESSIONALDEFORMATION IN SAKHALIN In order to get informationat the scale of the entire shear zonelocated east of the JapanSea, the precisegeometry of the whole strike-slip system in Sakhalin has to be known. For I I I

,,I,,49ø

q. ..:.;2.-i•?•.•%.'•½•i? '.'..';"•;.;;:::.•--•::::- .....'i. I

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.¾j?-;---.-•..•.... !;?-":.'".'.{' q. .'.•:...'..½.-. ..•:½-.• .•

. Fig.3. (a) SketchofLandsat images ofSakhalin. (b)Seismotectonic mapof Sakhalin. Forfault plane solution (lower hemisphere),Tquadrants (compressional quadrants)are shown inshaded areasand P quadrants (dilatational quadrants) are openareas. Solid T quadrants correspond tofault plane solutions determined byOscorbin, vertically ruled T quadrants correspondtofault plane solutions determined byFukao andFurumoto [1975], andhorizontally ruledTquadrants correspond tocentroid moment tensor determined byDziewonsla' etal. [1985, 1987]. Paleo-stress fieldhorizontal directions deduced from fault(c) Geologicalset analysis map. areplotted asconvergent (•1)or divergent (•3)arrows. Bis bay, Fis fault, GYR isGyrgylan'i-Ossoy fault. I 2705 142ø• 144ø c

i + + •o_

• • • %• •Beddin•_strike _ • • \ • Sealcling_dip ½t \ • , I ...... Miocene volcanics q8 • i •1 •Principals•ess directohs

-,2

w/ •, ...... /

SAKHALINSKIY ,•( •';"•[•"'- ' • , • • '15 I,• ,,•1 2 " •¾ 0 • 17 ',,, •,• / • ' ,,,, , ',•,• •l• I •-...-

"-... ,. • • • • --' •'?....

TERPENYA B,4y

MAKAROV

KRASNOGORSK

VOSTOCHNYY TPF9

'' ' '' '•PF 8 Fig. II --48 ø TPF õ .:•..,...,....•'.-_.f•:-\.!•.- '".• '-•

USUNAI COMPLEX •.L•s L"""'"'"'7'"""•'""'"•••.•,o-

// / / R USE

Fig. 3. (continued) 2706 FOURNIER ET AL.: NEOGENE DEXTRAL MOTION IN SAKHALIN AND JAPAN SEA

this purposewe studiedLandsat Thematic Mapper imageswith that bounds to the west a central depressionfilled with natural color on 240-mm paper at the scale of 1/1000000. The quaternarydeposits (see geological map Figure3c). To the east mosaic covers the entire island except the northern Schmidt of the depression,the East SakhalinMountains (in the north) peninsula(Figure 3c). Our map showsgeological objects such andthe Susunaimetamorphic complex (in the south)display a as sedimentarylayers with indication of dips, folds, faults, Paleozoicbasement metamorphosed during Mesozoic time, major volcanic intrusionsand sedimentarybasins. Despite a with eclogiticand high-pressure/low-temperatureparageneses. dense vegetation cover, the morphological signatureof the The basementis overlain by a pile of Mesozoic (Jurassic- geological formations allowed simple large-scalemapping. Cretaceous)deep sea sedimentsconsisting of a folded and Working with the 1/1000000 geological map of Sakhalin schistosed tectonic alternation of cherts, blackshales and [Geology of the USSR, 1970], it was possibleto draw a large- basalts, described as an ancient accretionary complex scale geologic map consistent with our observations. The [Zonenshain et al., 1990; Kimura et al., 1992]. These results are presentedin Figure 3a, the Landsatimage mosaic, formationsare cut by N-S trendingfaults boundingnarrow Figure 3b, a data map showingthe structuresobserved on the basins in which Neogene sedimentswere unconformably images, the paleo-stressfield directionsmeasured on the field depositedand later folded.To the west, the Tym-Poronaysk and the focal mechanismsof earthquakes,and Figure 3c, an fault bounds the West Sakhalin Mountains, folded strata of interpretative geological map. The geometry of the major Upper Cretaceousto Quaternary age. The thick Upper dislocation,the Tym-Poronayskfault, is detailed. Using six Cretaceous (Cenomanian to DanJan) sequence lays SPOT imageswe studiedat a smallerscale three regions located unconformably over a Pa!eozoic and Mesozoic basement along the Tym-Poronayskfault (see locationin Figure3c). For similar to that of the East Sakhalin mountains [Melnikov, each region we presenta sketchof two SPOT imageswith its 1987; Melnikov and Rozhdestvensky, 1989]. Three structural interpretation.In the central region, the geologic transgressivecycles are recognizedduring the Late Cretaceous map is compiled from K. F. Sergeyev (unpublished data, in thenorthern part of the westSakhalin basin (Figure 4), with 1980), the geologic map of Sakhalin, and our new deepening sequencesof terrigeneous deposits showing observations. successivelycoal-bearing continental deposits,sandstones and claystones.The base of the Paleogeneis marked by a Geological Setting conglomerateof variablethickness (up to 600 m), overlainby two transgressivesequences of terrigeneousdeposits, with a Sakhalinisland is 1000 km long and 30 to 180 km wide. It thick volcanogeniclayer at the top (Takarada¾and Maruyama is structuralydivided in two partsby the Tym-Poronayskfault formations).To the eastof the Tym-Poronayskfault (Figure4)

WEST SAKHALIN BASIN CENTRAL DEPRESSION Approximative thickness(m) West of the Tym-Poronaysk fault East of the Tym-Poronaysk fault

Age Formation Formation Pliocene L MARUYAMA

T.CI:•I•OV Neogene ':'Y..":':'• _•--•KLNEDUYSKA KHOLMSK 3••DUYSKA .:: PALEO-MESOZOIC BASEMENT Paleogene ...... I••DUYS • • • • • • • • • KAMIN

KRASNOYARKOV Lithology ......

Santonian . o .•a•a•9a,.s •-• Conglomerate ...... Tuffaceoussandstone • BIKOV l• Coalbearing sandstone Cretaceous / Sandstoneandconglomerate :!:• Silicifiedclaystone andsiltstone

Cenomanian

__ :::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::• • • • •'--+--'•Claystone Albian POBEDIN ""'•:"-"'•Volcanic flows and breccias

Fig.4. Detailedstratigraphic and lithological section of thewest Sakhalin Mountains and the Central Depression at Vakrouchev latitude. FOURNIERET AL.: NEOGENEDEXTRAL MOTION IN SAKHALIN AND JAPANSEA 2707

the Neogene depositslay directly on the Palezoic-Mesozoic constant in size. When two focal mechanisms were determined basement as shown by drilling in the central Neogene- for an earthquakeand its aftershock,we only plotted the focal Quaternary depression [Rozhdestvensky, 1982]. The mechanismcorresponding to the main event. The two centroid Cretaceoussequence to the west andNeogene formations to the moment tensorsdetermined by Dziewonskiet al. [1985, 1987] east are in contact acrossthe fault. Basic sills and dykes of correspond to two earthquakeswhose fault plane solutions Neogeneage are emplacedin the vicinity of the fault. have independently been determined in this study (for An E-W synthetic cross section of Sakhalin island is simplicity we only indicate our epicenter locations). The P presented in Figure 5 after Rozhdestvensky [1982], axes are almost similar in both cases, and the T axes of the Pushcharovskiyet al. [1983], and our field observations(see Dziewonski et al. [1985, 1987] focal mechanismsare steeper location of the cross section in Figure 3c). The Tym- so that they indicate compressionalmotion when ours indicate Poronaysk fault is localized along the boundary between a strike-slip motion. Mesozoic accretionarycomplex to the east and a sedimentary Most of the mechanisms determined along the Tym- basin of Upper Cretaceousage to the west. The sedimentary Poronaysk fault evidences dextral strike-slip motion on the Upper Cretaceousand Cenozoicsequence in this basin is about fault. The same observationapplies to the West Sakhalin fault 10 km thick. The Tym-Poronayskfault might have reworked which parallels the Tym-Poronaysk fault along the western an inherited Cretaceousstructure as a strike-slip reverse fault. coast. Focal mechanismsof the northernoffshore earthquakes Seismic data show that the off shore structure in the are not consistent with onshore focal mechanisms; they are vicinity of the island is similar to the onland structure.Along not even internally consistent and are not clearly related to the east coast of Sakhalin Gnibidenko and Svarichevsky surface structures. Savostin et al. [1983] saw evidence of [1984] and Gnibidenko [1985] describedN-S trending faults dextral motion along NNW trending faults in this region. At parallel to the Tym-Poronayskfault. The northeastcoast of island scales, two categories of focal mechanisms are Sakhalin is limited by off shore faults several hundred represented: strike-slip ones essentially located in the kilometerslong, boundingnarrow basinsfilled with Cenozoic vicinity of the Tym-Poronaysk fault and consistent with sediments more than 5000 m thick. The southwest coast is dextral motion along it, and compressionalones evidencing bordered by the West Sakhalin fault parallel to the Tym- E-W shortening taken up along N-S trending faults. We Poronaysk fault, and which is observed onland in therefore conclude that Miocene transpressionaldextral strike- Boshnyakovo region (Figures 2 and 3c). Along this coast slip motion continuesat present in Sakhalin. Antipov et al. [1979] describeden 6chelonnarrow basinswith Assuming that the regional stressfield is simple and does a north-northeast trend filled with 2000-m-thick middle not show major local variations of the directions of the Miocene and younger sediments.Such basins similar to these principal stressaxes, P and T axes plotted on a stereographic describedby Lallemandand Jolive [1985] alongthe northwest projectionmay be used to infer a regional stresstensor (Figure coast of Honshu may be interpreted as dextral en 6chelon 6). Hereafter,0.1, 0'2, and o' 3 will referto theprincipal axes of structures associated with the West Sakhalin fault. the stress tensor, with 0.1>0.2>0.3 . The best P and T axes directionsdetermined with the right dihcdra method IAngelier Active Strike-Slip Motion Along the Tym-PoronayskFault and Mechler, 1977] are assumedto be 0.1 and 0'3, respectively. The Tym-Poronaysk fault is seismically active and has The result show 0'1 almosthorizontal and trendingENE-WSW. caused most of the onland main earthquakesof Sakhalin Directions and dips of 0.2 and 0'3 are poorly constrainedas [Oscorbin, 1977; see Chapman and Solomon, 1976; Savostin both compressional and strike-slip focal mechanisms co- et al., 1983]. In contrast,the faults cutting through the East exist. Though it is questionablein generalthat a small number Sakhalin Mountains are seismically inactive. Fault plane of focal mechanismscan be used to computea mean regional solutions(Table 1) of shallow earthquakes(depth less than 30 stresstensor, the result we obtain is very similar to the results km) determinedin this study (by L. S. Oscorbin), Fukao and of fault sets analysis undertaken in Sakhalin and described Furumoto [1975], and centroidmoment tensors determined by Dziewonski et al. [1985, 1987] are plotted in Figure 3b. The later, with roughly the same trend of 0.1 though sometimes focal mechanismswere determinedby utilizing P wave first more northerly. This suggeststhat both Miocene and present- motions detectedby a Soviet regional seismologicalnetwork. day seismic deformationpatterns were governedby a similar The radius of the focal mechanisms is a function of the overall stress field. However, there is a possibility that 0'1 magnitude(surface waves) of the earthquakesexcept for those rotated slightly clockwise toward a more latitudinal taken from Dziewonski et al. [1985, 1987] which are kept orientation, as we will discuss later.

WESTSAKHALIN CENTRAL E TARTARY STRAIT MOUNTAINS I DEPRESSION i EAST SAKHALIN MOUNTAINS i OKHOTSK SEA I I i ' TPF ', LANGERI B. ' ,

1Okra , •

Metamorphic rocks Jurassic Cretaceous Paleogene Neogene Fault and thrust Fig. 5. East-westcross section of Sakhalin,localized in Figure3c. B is basin,TPF is Tym-Poronayskfault. 2708 FOURNIERET AL.: NEOGENEDEXTRAL MOTION IN SAKHALINAND JAPAN SEA

TABLE1. Listof SourceParameter Results for 24 EarthquakesPlotted in Figure3b. Number Date Latitude Longitude Depth,km Magnitude P Axis T Axis Reference I March12, 1962 47.1 143.2 30 4.8 233 38 126 23 1 2 April7, 1963 53.8 142.7 17 5.0 182 02 272 34 1 3 May10, 1964 46.6 142.4 13 5.0 301 01 211 28 1 4 Oct.2, 1964 51.9 143.3 10 5.8 243 50 126 22 1 5 Dec.24, 1967 54.7 143.6 20 5.5 335 30 245 00 1 6 Dec.29, 1967 54.7 143.7 20 4.3 238 03 148 17 1 7 Jan.6, 1970 49.6 142.3 25 5.5 220 26 312 05 1 8 Jan.6, 1970 49.6 142.4 22 4.7 074 01 169 80 1 9 Jan.10, 1971 55.1 142.1 20 4.9 220 56 116 11 1 10 Sept.5, 1971 46.45 141.24 22 7.1 099 06 236 81 3 11 Feb.6, 1973 49.2 141.9 10 4.7 069 28 175 27 1 12 April4, 1973 53.9 141.3 20 5.0 025 14 280 50 1 13 Aug.17, 1974 54.7 144.3 15 5.4 310 02 217 58 1 14 Aug.17, 1974 55.0 144.0 15 4.2 116 37 216 15 1 15 July25, 1977 51.8 143.0 10 4.6 294 00 207 73 1 16 July28, 1979 50.0 142.7 20 5.8 229 01 320 39 1 17 July28, 1979 49.48 142.41 15 260 06 157 62 2 18 March14, 1980 52.4 142.2 16 4.8 276 13 019 43 1 19 Marcch19,1982 52.3 142.7 5 4.1 302 01 207 70 1 20 March20, 1982 52.3 142.8 5 4.3 292 03 198 60 1 21 March17, 1983 51.5 142.4 20 5.2 242 02 336 45 1 22 July22, 1983 50.3 142.2 20 4.5 103 25 313 62 1 23 Dec.22, 1984 50.0 142.7 30 5.2 088 38 294 49 1 24 Dec.22, 1984 50.17 142.79 37.8 084 06 296 83 2 Referencesare (1) L. S.Oscorbin (this study), (2) Dziewonski etal. [ 1985,19871, and (3) Fukaoand Furumoto [19751.

NeogeneStrike-Slip DeformationAlong the Tym-Poronaysk with many dextrally shifting short segments.Bedding traces Fault are parallel to the long segmentsand are strainedby dextral shearalong the shortones. This is observedin the Yuzhno- TheTym-Poronaysk fault is themajor geological feature of Sakhalinskregion to the south,and in the Poronayskregion Sakhalin.It is 600 km long and disappearsto the north under in the centralpart of the island(Figures 3b and3c). The same Quaternarydeposits and to the southin the La P•rouseStrait geometry is observedwith the axes of en •chelon folds between Sakhalin and Hokkaido. It extends in Hokkaido as the affectingthe whole sedimentarysection (upper Cretaceous to Horonoba fault and the Hidaka mountains[Kimura et al., 1983; upperMiocene) on eachside of the fault. The fold axestrend Jolivet and Miyashita, 1985]. In Sakhalin,the fault trends N140øE to N180øE, roughlyparallel to the long segmentsof roughlyN-S, bracketedbetween 142øE and 143øE.The fault the fault, and are strained by dextral shear along the short trace consistsof a successionof nearly linear segments,the segments.There is a single exceptionin the Vostochnyy longerones trending N160øE to N180øEare shifteddextrally regionwhere a fold trendsnorthwesterly. This fold couldbe by shorterones trending N010øE to N045øE(Figures 3b and older than the others as it affects only Cretaceous and 3c). The ratio of long andshort segments determines the mean Paleogeneformations. Figure 7 showsa detail of the Landsat trend of the fault: N-NW from north of the island to Poronaysk imageof the southwesternpeninsula of the island.Four dextral city in the central part with only three short N010øE to en 6chelon folds in Miocene formationsare strainedby dextral N045øEtrending segments (Figure 3c), andN-NE to the south shearalong the Tym-Poronayskfault. Neogenedeposits of

P-axes T-axes Stress tensor

,..e o o o

Dihcdra contour I)ihcdra contour ß P-axes

Fig.6. P andT axesprojection on stereodiagramsfor 17 focalmechanisms (lower hemisphere). Seven focal mechanisms of the northernoffshore earthquakes are not taken into account as theyare not consistent with the other ones. Contours and grids are determinedwith the right dihedramethod. FOURNIER ET AL.: NEOGENE DEXTRAL MorION IN SAKHALIN AND JAPAN SEA 2709

\ \

F•zult Bed•ng trGce Fold oxe

,6 ø .5 km

Fig. 7. Landsatimage of the southwesternhorn of Sakhalin(localized in Figure 3c) showingthe clockwisecurvature of the dextral en 6chelonfold axis in the vicinity of the Tym-Poronayskfault.

northernSakhalin are affectedby similarN-NW trendingfolds the formation of these basins preceded their sublatitudinal [Rozhdestvensky, 1982, 1986]. The fold axes direction shortening.Assuming that they formed parallel to •YHmax as provides an east-northeast (N050øE to N090øE) trend of for extensionalcracks, their presenttrends provide a direction maximal horizontal stress, which is consistent with dextral for 6Hmax between N000øE and N030øE. Thus the maximal motion along the Tym-Poronayskfault, as well as the dextral horizontal stressapparently rotated clockwiseduring the late en 6chelonpattern of the folds and their sigmoidalshape. In Miocene from N000-N030øE to N050-N090øE. The the field, the fault is a steepstructure dipping sometimes to the significanceof this apparentrotation will be discussedlater. east and to the west, always showing a reverse motion Our interpretationof the SPOT images of the Yuzhno- component.This is illustratedin Figure 8 with compressive Sakhalinskregion in the southernpart of the fault (locationin structures, drag folds, and ramp and Hat structures in Figure 3c), is shownin Figure 9. The city is locatedin the Cretaceousformations along the fault. The overall structure,a central Quaternarydepression (Figure 3c) boundedto the east N-S strike-slipfault with a reversecomponent associated with by the Susunaimetamorphic complex and to the west by the N140øE to N180øE trendingfolds, results from deformationin Tym-Poronayskfault. The fault, parallel to beddingtraces in a transpressionalregime along the N-S discontinuity,with its northern part, is twice shifted dextrally by NE trending O'Hmax= o'1 trendingE-NE (O'Hmaxis the maximalhorizontal segmentsin the vicinity of Yuzhno-Sakhalinskand obliquely stress). The deformation is partitioned between pure cut Cretaceousformations (Figure 3c). Dextral shear is also compressionalong the N160øE to N180øE segments,and observedalong a NE trendingfault in the Susunaimetamorphic almost pure strike-slip shear along the N010øE to N045øE complexto the east, where lineaments,probably representing segments. the surfacetraces of the metamorphicfoliation, are curvedin a To the north,in the Aleksandrovsk-Sakhalinskiyarea, two dextral sigmoid. dextral en 6chelonMiocene basinstrend NNE (Figure 3c). Figure 10 shows the interpretationof the two northern These basins are similar to those described below in the East SPOT imagesalong the fault (see locationin Figure 3c). The Sakhalin Mountains and are consistent with Miocene dextral sharpnessof the Tym-Poronayskfault is pronouncedto the motion along the Tym-Poronayskfault. As we will see later, south,along the NE trendingsegment cutting through Lower 2710 FOURNIERET AL: NEOGENEDEXTRAL MOTION IN SAKHALINAND JAPANSEA

Fig. 8. Compressionalstructures (drag fold and rampand fiat structures)along the Tym-Poronayskfault in UpperCretaceous formations (central Sakhalin).

Late Cretaceousformations• The hangingwall to the west Early PlioceneMaruyama formation) is even less folded with showsdegraded triangular facets evidencing recent activity. limbs dippingnot more than20 ø . Thus thesefolds experienced Small-scalefield work was undertakenin the centralregion a continuous deformation during the Miocene and the located on the east coast of the island, between Gastello and Pliocene.Three Quaternaryterraces are superimposedon the Vostochnyy (SPOT images detailed in Figures 11a, l lb, and west coast of the island, the highest one (probably 500,000 1 l c, location on Figure 3c). On the SPOT images, the most years old by analogywith Hokkaido) is about500 m high, and prominent feature is the Santonian-Campaniancuesta marked there is only one low terrace or no terrace at all on the east by the thick terrigeneousCampanian depositsto the west (see coast. This is an indicatorof relative uplift of the west coast Figure 11c). To the south this limit makes the Vostochnyy with respect to the east coast. The Tym-Poronayskfault is fold (NE trending axis) standout on the images.To the west, likely to localize relative motion between the east and the the discordanceof the Paleogenebasal conglomerate(Figure west coasts,which would imply an intenseQuaternary activity 12a) onto Cretaceousstrata is clearly observedon the images: with a high reverse componentof motion [Zakharov and the conglomerate cuts through the underlying Cretaceous Yakushko, 1972]. However, it was not possible to observe beddingtraces. Along the east coast the Tym-Poronayskfault whether the Quaternarymarine terracesare folded or not. In is a sharpfeature particularly clear in its southernpart where it someplaces the Tym-Poronayskfault splitsin severalparallel divides the relief of Miocene terrigeneous and volcanic segmentsand could be the surfacemanifestation of a deep formationsto the east from a depressionfilled with Cretaceous compressionalflower structure. Volcanic rocks mapped in argilites (Bikov formation) to the west. As alreadynoticed on Figure 1lb were emplacedduring the Miocene as dykes and the Landsatimages, the Tym-Poronayskfault consistsof long sills in the vicinity of the fault, with a NNE trendingdirection roughly N-S segmentsdextrally shifted by short NE trending slightly oblique to the fault, compatible with dextral shear segments. Evidence for dextral Miocene motion along the along it. Large sills in Cretaceousformations are alsomapped fault is especially remarkable in the north where three en in Figure 1l a. These sills make the topographiccrests and 6chelon folds in Miocene formations are strainedby dextral determine the geometry of the hydrographicnetwork. One shear. A similar fold in the Neogene Kholmsk formation is suchsill is shownin Figure 12c in Miocene deposits. shown in Figure 12b outcropingalong the coastline, southof Volcanic rocks recorded well the brittle deformation and Yuzhno-Sakhalinsk. Figure 13 shows two cross sections are good candidatesfor stressorientation measurements along through the Tym-Poronaysk fault (see location of the cross the Tym-Poronaysk fault in both Cretaceous and Miocene sectionsin Figure 11c): the folds close to the fault evidence formations.Four stereogramsused to calculatethe paleostress the transverse shortening associated with strike-slip motion field directionsplotted in Figure 12c are shownin Figure 14. (see also Figure 10c showing vertical Miocene sedimentary The tensoranalysis follows the methoddeveloped by Angelier beds). The upper Miocene and Pliocene depositsare weakly [1984]. In the field we mostlyobserved compatible strike-slip folded and the youngerpart of the section(the late Miocene- and reverse faults. The direction of o'1 is bracketedbetween FOURNIER ET AL.: NEOGENE DEXTRAL MOTION IN SAKHALIN AND JAPAN SEA 2711

143 ø andthe corresponding stereo plots are shown in Figure15. We first focuson the southeastpeninsula of the islandnortheast of Aniwa bay where faults were measuredin Jurassic, • Metamorph•crocks Cretaceousand Paleogene rocks (see geological map in Figure • Fault 3c). Two sites (T3 and T5) have recordedtwo successive ...... Bedding trace • Fold episodeswith roughlyperpendicular cr1 (siteT4 alsorecorded twosuccessive episodes but it is disturbedby theemplacement of a Paleogenegranite in its vicinity). No obvious chronologyhas been observed in the field; the chronology

Fig. 9. Structuralinterpretation of a mosaicof two SPOT imagesof the Yuzhno-Sakhalinskregion along the Tym-Poronayskfault (localizedin Figure 3c).

N050øE and N090øE and it is alwaysperpendicular to the fold axes. We therefore assume that the measured stress field is associatedwith folding. The sites TPF7, TPF8 and TPF9 are located on an almost pure strike-slip segment of the fault, along which the fold axes are curved and suffered at least a 30 ø .., clockwise rotation (Figures 12c). The ty1 rotated clockwise with the fold axes near the fault. Thus the true trend of tYl ., associated with folding is likely to be bracketed between N020øE and N060øE. The superpositionof two distinct stress fields discernable chronologically was seldom observed. When observed,it showedthat ty1 was alwayshorizontal and trended first E-W and secondly NE-SW, which can be accommodated by a progressive clockwise rotation of structuresunder a single stressfield with cr1 trendingNE-SW. This NE-SW trend is consistentwith dextxalstrike-slip motion along the Tym-Poronayskfault. Fig. 10. Structuralinterpretation of a mosaicof two SPOT imagesalong Paleostress fields inferred from fault measurements the Tym-Poronayskfault (northernregion localizedin Figure 3c). Same undertakenin other parts of the island are plotted in Figure 3b legend as figure 9. F is fault. YA R.

MAKAROVA

LESNAYA R.

LOZAVAYA R.

Fig. 11. (a) Mosaic of two SPOT imagesalong the Tym-Poronayskfault (central region localized in Figure 3c)and (b) structuralinterpretation. Same legend as Figure9. Thicklines are fault traces and the main sills are mapped with thin lines.The arrowson the imagesshow the Tym-Poronayskfault trace.(c) Geologicalmap of the centralregion after K. F. Sergeyev (unpublisheddata, 1980) and our new observations.Key shows(1) Maruyamaformation and upperPliocene-Quatemary deposits,(2) Verkneduyskaformation, (3) Tcheckhovformation, (4) Kholmskformation, (5) Gastelloformation, (6) Paleogene basaldeposits, (7), (8), and(9) Bikov suite,(10) Miocenevolcanics, (11)mud volcano,(12) fault andsuspected fault (dashed line), (13) fold axis (synclineand anticline,respectively), (14) geologicalcontour and suspectedgeological contour (dashed line), (15) paleo-stressfield direction.The Santonian-Campaniancuesta is shownas the boundarybetween formations 7 and8. FOURNIERET AL.: NEOGENEDEXTRAL MOTION IN SAKHALINAND JAPANSEA 2713

given in Figure 3c is basedon the study area being analogous 143ø C to NE Japan[Jolivet et al., 1990]. Figure 15 illustratesphase separationfor the three sitesT3, T4 and T5. Phase 1 has o'1 trendingN020øE in good agreementwith most of the sites of the peninsula,and also with directionof •1 determinedin the •astello central region (see above). If we generalize these observations to the whole island, it is possibleto interpretall the computed paleo-stress-directions in one single stress system with horizontal•1 trendingNE-SW, and with local rotationsalong 49 ø the Tym-Poronayskfault. Phase2 with •1 roughlyE-W can be related either to late sublatitudinal shortening as observed in Hokkaido [Jolivet et al., 1990] or to local rotations as observedalong the Tym-Poronayskfault.

Geometry of the Neogene Strike-Slip Deformation in the East Sakhalin Mountains

In the East Sakhalin Mountains three parallel submeridian en- 6chelon faults cut through the Paleozoic and Mesozoic basement(Figures 3b and 3c): from west to east they are the Central, Pribrezhnaya (coastal) and Liman (estuarine) faults [Rozhdestvensky, 1982]. These faults are well expressedon the Landsat images in their northernparts (see a detail of the .'LADIAMt: \ Landsat mosaic in Figure 16) as they put into contact contrasted lithological formations, a high block to the west made of Mesozoic rocks cut by deep valleys, and a low one to the east made of Neogene depositswith smooth topography.

W Paleogenebasal E conglomerate

ROVA Mt

Makarov

':.:'/.:...... :'•5 :•!• 6

8 _• 9

•12 • 13

• 15

hnyi Fig. 12. (a) Paleogenebasal conglomerateon the E-W road to Boshnyakovo(central Sakhalin), lying unconformablyonto Upper Cretaceousformations (to the east). The thicknessof the conglomerate is variableand reaches600 m in someplaces. (b) Disruptedfold in NeogeneKholmsk formation south of Yuzhno-Sakhalinsk.(c) Sill in almostverficalized Miocene sedimentary beds along the Tym-Poronaysk fault in the Lozovayariver valley(mapped in Figure11 b). Coolingjoints Fig. 11. (continued) are perpendicularto the stratification. 2714 FOURNIER ET AL.: NEOGENE DEXTRAL MOTION IN SAKHALIN AND JAPAN SEA

Fig. 12. (continued) FOURNIERET AL.: NEOGENEDEXTRAL MOTION IN SAKHALINAND JAPANSEA 2715

SW NE

Vikhr Mt TPF Vakrouchev Okhotsk Sea , i i I 1000 m 1000m T • • I '[

ß 0 In 0m

W E -• MARUYAMA Makarov Mt Okhotsk Sea

1000 In • 1000In • TCHEKOV

0 In Om z .'::..:...:'•KHOLMSK

1 krn

Fig. 13. Cross-sectionsacross the Tym-Poronayskfault (localizedin Figure1 lc). TPF is Tym-Poronayskfault.

The faultsare indeedbordered to the eastby narrowbasins with a ridge for about 10 km (Figure 3b). According to a NNE trend, filled with Miocene and Pliocene deposits Rozhdestvensky [1982], en 6chelon folds with sigmoidal overlying unconformably the Paleozoic and Mesozoic bends evidence dextral motion along these faults. In the basement [Rozhdestvensky,1982]. The presenttrends of the Schmidt Peninsula the Cretaceous-Neogene contact is basins (N000øE to N030øE), assumedto be parallel to the dextrally offset by a strike-slip fault for 5.5 km, and another maximal horizontal stress direction, and their dextral en strike-slip fault cut through Neogene sediments with a 6chelonpattern are consistentwith dextral strike-slip motion displacement of up to 7 km [Rozhdestvensky, 1982]. A along N-S faults. A Neogenepull-apart basin along the Central summation of the strike-slip offsets of faults from the East fault is also an indicator of dextral motion, and to the south Sakhalin Mountains to the Schmidt peninsula therefore gives the sigmoidal shape of the Mesozoic crests between the a minimum value of dextral N-S relative motion of about 50 Pribrezhnayaand the Liman fault supportsdextral shearalong km since Miocene time. these faults. The three faults therefore experienceddextral strike-slipmotion with a normal componentin Neogenetime. Discussion: Progressive Strike-Slip Deformation From An evaluationof the displacementalong the Central fault is Miocene to Present given by the 25-km offset of the metamorphiccomplex in the southern part of the fault (Figure 3c). The Langeri graben The observations made at all scales reveal the (Figure 16) is filled with Mio-Plioceneformations up to 600 m juxtaposition and successionin time and space of pure strike- thick lying unconformably either on Paleozoic or Mesozoic slip structures,pure compressionalstructures, and extensional rocks [Rozhdestvensky, 1982]; the Miocene deposits are structures. At island scales, the western side of the Tym- folded and standalmost vertically on the east side of the basin. Poronaysk fault is characterizedby en 6chelon folds and the A late sub-latitudinal shortening thus affected the Miocene fault itself has a thrust component which indicates a basin. Similar observationsare made along the southerncoast transpressionalregime along 600 km from south to north. of the Aniva peninsula to the south, where the Mesozoic While the Upper Cretaceousto Neogene sedimentson the west basementwas thrustedeastward over Paleogenebasins folded side of the fault displays a distributed deformation into synclines (cross sections of Melnikov and symptomatic of a relatively soft medium, the more competent Rozhdestvensky[1989], and Rikhter [1986]). Mesozoic basementon the easternside of the fault displays en North of the East Sakhalin Mountains, the submeridian 6chelon narrow basins localized along strike-slip faults (in the Gyrgylan'i-Ossoy,Ekhabi-Pil'tun and Okha faults continuethe East Sakhalin Mountains), which had been in turns folded abovedescribed structures until the SchmidtPeninsula through showing an apparentrotation of cr1. Small-scale observations Neogene formations.We observedon the Landsatimages that shows that compressionis always perpendicular to the fold the northernpart of the Gyrgylan'i-Ossoyfault shiftsdextrally axes and that the E-W compressionseen in the microtectonic 2716 FOURNIER ET AL.: NEOGENE DEXTRAL MorION IN SAKHALIN AND JAPAN SEA

TPF5 TPF7

TPF8 TPFg

Fig.14. Fault set stereodiagrams (Schmidt projection, lower hemisphere) anddeduced paleostress field directions for four sites plottedin Figure12b. Dashed fines represent bedding planes. Three-branch staris e3.Four-branch staris e2-Five-branch star is el (el > 02 > 03).

analysesis the resultof clockwiserotations close to theTym- Extraspace in the shearzone would be accommodatedby the Poronayskfault. Thusat this scale,only onestress regime is formation of grabens along the cross faults. However, if required. It is possibleto reach the same conclusionat the transpressionprevails, the shear zone tends to narrow and scaleof the islandwith large-scalestructures. counterclockwiserotations will be accommodatedby dextral The overall structure of the East Sakhalin Mountains can motion along the crossfaults. E-W shorteningwill be taken be describedas a dominosystem in a shearzone between two up by counterclockwise rotations of the blocks and master faults, the Tym-Poronayskfault to the west and an compressionalstructures along their borders.Applying this offshorefault observedon seismicdata alongthe northeast modelto the EastSakhalin Mountains where the stressregime coastof Sakhalinto the east[Gnibidenko and Svarichevsky, is transpressionaland the cross faults are dextral, we should 1984; Gnibidenko, 1985], the Central,Pribrezhnaya, and expect counterclockwise rotations of the dominos. Liman faults being crossfaults which boundcrustal blocks The evolution of the East $akhalin Mountains can be (Figure 17). Such geometryimplies block rotationsabout explainedas follows:the narrowbasins first formedalong the vertical axes [Ron et al., 1984; Nur et al., 1986; Scotti et al., cross faults which were parallel to the maximum horizontal 1991]. Assumingthat the cross faults appear first as stress and then later progressively rotated in a extensionalcracks parallel to tYHrnaxand are laterrotated, the counterclockwise manner. The trend of the basins thus senseof rotationdepends upon the sign of the stresstensor becomesoblique to the maximumhorizontal stress and they componentperpendicular to the plane of the masterfaults. An were subjected to shorteningand folding. Takeuchi et al. overalltranstensional stress field wouldlead to a wideningof [1992] recently presentedpaleomagnetic results from the shear zone and favors clockwise rotation of blocks Hokkaidoand southernSakhalin, including several sites on accommodatedby sinistral motion along the cross faults. the westernside of the Tym-Poronayskfault. They obtained FOURNIER ET AL.: NEOGENE DEXTRAL MOTION IN SAKHALIN AND JAPAN SEA 2717

TPF3 T2

T3 (1) T3 (2)

Fig.15. Fault set stereodiagrams (lower hemisphere) anddeduced paleostress field directions for sites ploued in Figure3b. Two successivestress fields are observedon sitesT3, T4, and T5.

Middle to Late Miocene dextral rotations, compatible with the cross faults can lead to the distribution of pure strike-slip progressivedextral shearof the fold axes along the fault. They segments and mostly compressional segments actually unfortunatelyhave no data from the East Sakhalin Mountains, observed along the Tym-Poronaysk fault. This geometry can so the block model there cannot be tested with the be obtained provided that some of the cross faults can be paleomagnetic results obtained so far. The expected connected to some of the dextral offsets along the Tym- counterclockwiserotations are however small and might not Poronaysk fault through the basementunder the Quaternary be easily detected. depressionas shownin Figure 17b. Figure 17 shows the precise geometry imposed by this The observed clockwise rotations of fold axes caused by model. Figure 17a shows the simplified fault pattern in dextral simple shear along the Tym-Poronaysk fault can Sakhalin, which is isolated from the coast lines in Figure 17b. explain the E-W direction of compressiondeduced from fault The continuity of geological structuresin western Sakhalin set analysis:ty 1 was initially NE-trending and has been rotated shows that this region behaved as a single elongated block with the rocks. The block model with counterclockwise betweenthe West Sakhalinfault and the Tym-Poronayskfault. Continuousstrain within the block was accommodatedby en rotations in the East Sakhalin Mountains can explain the 6chelon folds leading to oblique shortening. In the East apparent succession of tectonic stages (basins formation Sakahlin mountains, however, the basement was cut by followed by transverse shortening) in the single several parallel faults and was divided into several dominos transpressionalstress field [Scotti et al., 1991]. This model which we predict have rotated counterclockwise.Figure 17c is integrates in a single stage a succession of superimposed a simplificationof the fault pattern with a block model. The E- structuresof various style and explains the geometry of the W shortening coeval with dextral strike-slip involves a Tym-Poronaysk fault, but it still needs to be tested with component of thrusting along the blocks boundaries, since paleomagnetic data. However, we cannot eliminate the the total width of the shear zone decreases. One can see that possibility of a change in stressconditions from tyI trending the counterclockwise rotation of blocks along the dextral NE-SW to nearly E-W (seeFigure 2). 2718 FOURNIER ET AL.: NEOGENE DEXTRAL MOTION IN SAKHALIN AND JAPAN SEA

T4 ('1) T4 (2)

T5 ('1) T 5 (2)

Fig. 15. (continued)

TRANSTENSIONAL DEFORMATION IN NOTO PENINSULA AND YATSUO by the dextral transtensionaldeformation described by Otsuki BASIN [ 1989, 1990] and Jolivet et al. [ 1991], distributed all along the west coast of NE Honshu down to Sado island to the south. We have describedSakhalin Island as a NeogeneN-S Yamaji [1990] describedbasins in the Uetsu district (along the dextral strike-slip zone. This continues to the south in the west coast of NE Honshu) formed between 18 and 15 Ma with HiddakaMountains (Hokkaido) reworking a major Mesozoic the samedextral en 6chelonpattern. To the souththe Tanakura suture zone that localized part of the strain during the Tectonic Line (Figure 2) localized part of the dextral motion Cenozoic [Kimura et al., 1983]. Strike-slip ductile duringthe Miocene [Otsukiand Ehiro, 1978; Koshiya, 1986]. deformation in high-temperature conditions [Jolivet and The transition from transpression to transtension is Miyashita, 1985] duringlate Oligoceneand early Mioceneis observed at the latitude of southwest Hokkaido. In order to see associatedwith en 6chelon folds and thrustsin the upper the transition between the dextral shear zone and the southern brittle crust[Jolivet and ttuchon,1989]. Fault set analysisin extensional margin of the Japan Sea [Tamaki, 1988], we lower and middle Miocene formationsprovides a direction of studiedthe brittle deformationin Noto peninsulaand Yatsuo horizontal compressioncompatible with folding that trends basin(Figure 18) at the southeastcomer of the JapanSea (see NE-SW. The samedextral transpressionalstress field therefore location in Figure 1). Volcanic and pyroclastic rocks of characterizes Sakhalin and Hokkaido during the early and Miocene age observedin Noto peninsulaand Yatsuobasin lay middle Miocene. directly on the Upper Cretaceousbasement. Upper Miocene Further south Miocene strike-slip extends along the formationsdid not record any significantbrittle deformation. easternmargin of the JapanSea. It is distributedin a transition Lower and middle Miocene formations are affected by zone betweenthe oceaniccrest of the JapanSea basin and the conjugatestrike-slip faults associatedwith compatiblenormal continentalcrest of the Japanarc. It is expressedoffshore by faults: fault sets analysis provides a direction of maximal dextral en 6chelon basins with a NNE trend [Lallemand and horizontal compressiontrending N030øE associatedwith a Jolivet, 1985; Tamaki, 1988; Jolivet et al., 1991] and onland perpendicularextension (Figure 18). On a regional scale, the FOURNIEREl' AL.: NEOGENE DEXTRAL MOTION IN SAKHALIN AND JAPAN SEA 2719

NeogeneBasin

Fault

% %

LANGERI BASIN

10km • , , •

Fig. 16. Detail of the Landsatimage of the EastSakhalin Mountains showing three paralld faultsbounding Neogene basins.

Yatsuo basin is borderedto the east and to the south by large regime with Crtlmax = crI trending E-W. The late E-W normal faults that probably have some strike-slip component compressionis recorded in late Miocene and Pliocene rocks in this transtensional context. The middle Miocene [Yamagishi and Watanabe, 1986; Jolivet and Huchon, 1989]: transtensional deformation corresponds to a late stage of it is related to an uplift of westernnortheast Japan since 10 Ma openingof the JapanSea. [Sugi et al., 1983; Jolivet and Tamaki, 1992]. The offshore Our results are in agreementwith previously published Miocene extensional structures have been reworked as paleostressfields [Otsuki, 1989, 1990; Jolivet et al., 1991] compressional ones [Tamaki and Honza, 1984; Tamaki, determinedalong the easternmargin of the Japan Sea from 1988], and Tamaki et al. [1992] showed that thrusting and northern Honshu to Sado island: we observed in Noto convergencealong the easternmargin began 1.8 Ma ago. The peninsulaand Yatsuo basin the same associationof strike-slip margin is seismically active and focal mechanisms of and normal faults as in Sado, with similar principal stress earthquakesindicate compressionalong N-S reverse faults directions. The same transtensionalconditions prevailed all [Fukao and Furumoto, 1975; Tatnaki, 1988](seealso Figure 3 along the margin during opening of the Japan Sea and the of Jolivet et al. [ 1992]). deformationof the strike-slipmargin continuedduring oceanic The stress field and the associated deformation evolved accretion. along the strike-slip zone (Figure 2). In early and middle Miocene time the stressfield was transpressionalto the north in Sakhalin and Hokkaido, and transtensional to the south DISCUSSION AND CONCLUSIONS where the Japan Sea opened behind the Pacific subduction Nakamura and Uyeda [1980] showedthat stressregime zone. During the late Miocene the stressorientation changed: evolved in NE Japanduring late Miocene from an extensional in the southcrHmax rotated clockwise fxom NE-SW to E-W (we regime with Crtlmax = cr2 trendingNNE to a compressional have seenbefore that we do not necessarilyneed a rotation of 2720 FOURNIERlET AL.: NEOGENEDEXTRAL MOTION IN SAKHALINAND JAPANSEA

ASA• 10km ,o I

Cretaceous

Middle Miocene

+ ++++++

+ ++++ +++++ +

++ ,-++++++++

Fig.18. Geological map of Noto Peninsula and Yatsuo basin and fault set measurements inlower and middle Miocene deposits. Strike-slipand normal faults are often associated, paleostress field is transtensionalwith oilmax trending NE. 2722 FOURNIER ET AL.: NEOGENE DEXTRAL MOTION IN SAKHALIN AND JAPAN SEA

-• 300

• 1Oø

/ 80 o 100o 120o 140 o

Fig. 19. Simplifiedmodel of deformationof Asia (obliqueMercator projection)[after Jolivet et al., 1990].ATF is Altyn Tagh fault, RRF is Red River fault, and TF is Tanlu fault.

tYHmax in Sakhalin), and tYHmax became tYl all along the accommodatedby rotation of large-scaledominos and dextral strike-slip zone. In the Japan arc the recent deformation is motion betweenthem. An additionalcomponent can be added: purely compressional with folds and reverse faults, while the North America-Eurasiapole of rotation is located in east dextral motion is still active in Sakhalin. We therefore Siberiabetween the LaptevSea and the OkhotskSea [Chapman observe a different behavior between the northern intra- and Solomon, 1976; Sarostin et al., 1983; Cook et al., 1986] continental part of the strike-slip zone, transpressionalsince and has always been in this region since the early Eocene. the Late Oligocene, and the southern part neighboring the This situation leads to extensional strain north of the rotation subduction zone. The Japan Sea opened in a transtensional pole along the Nansen Ridge and Lena river mouth, and regime when the subductionboundary was stress-free.It began compressionalstrain south of it. The compressionmight be to close when subductionimposed a compressionalregime partly taken up by the southeastwardextrusion of the Okhotsk perpendicularto the trenches [Nakamura and Uyeda, 1980] block producing transpressional dextral shear in Sakhalin probably linked with the inceptionof subductionof the young [Jolivet, 1986; Riegel et al., 1993]. oceaniclithosphere of the Philippine sea plate under the Japan To conclude,the Sakhalin-NE Japan strike-slipzone is a arc during the late Miocene [e.g., Jolivet et al., 1989]. The 2000-km-long crustal scale structure that penetratesinside geometry of the strike-slip zone is controlled by the Asia to the north and has guidedthe openingof the JapanSea variations of stress regime in the back arc region due to to the south along the Pacific subductionzone. Jolivet and changing geodynamic conditions along the subductionzone. Tamaki [1992] presentednew reconstructionsof the opening Thus the extensionalcomponent of the JapanSea openingis of the JapanSea that allow a crudequanfification of the finite likely to have been providedby back arc rifting probablydue dextral offset since 25 Ma: at least 400 km of relative motion to trench retreat [Chase, 1978; Molnar and Atwater, 1978; are necessaryto accommodatethe opening (Figure 20). It Uyeda and Kanarnori, 1979], and the strike-slipcomponent is seemsunlikely that this large offset has been accommodated linked to some other mechanism such as the India-Asia only by the Tym-Poronaysk fault: most of the offset was collision as discussedby Kimura and Tamaki [1986] and probably taken up by offshore faults in the Tartary strait Jolivet et al. [1990] (Figure 19). The model presentedby between Sakhalin and mainland Asia where the crust is the Jolivet et al. [1990] is derived from Davy and Cobbold's thinnest, and further south along the transition from [1988] experiments:it involves a wide left-lateral shear zone continental(NE Japanarc) to oceaniccrust (Japan basin). connecting the Pamir-Tien Shan ranges to the Baikal- In NE-Asia, dextral motion associated with extension is Stanovoy regions which evolves from transpression to also evidencedby Chen and Nabelek [1988] along the Tanlu transtension. In the transtensional domain left-lateral shear is fault in the Bohai Gulf (Figure 19). It is not yet possibleto FOURNIERET AL.: NEOGENEDEXTRAL MOTION IN SAKHALINAND JAPANSEA 2723

48 ø EARLY MIOCE 20 Ma

40ON YBK

PAC

32 ø

136 ø 140øE 136 140 ø /

Fig.20. (a) Present-daygeodynamic context of Sakhalinand Japan Sea region. Shaded area represents the oceanic crust of JapanSea and Kuril Basin. Darker shading shows the zone of activecompression of eastern Japan Sea. (b) Sameregion 20 Ma ago,reconstruction parameters after Jolivet et al. [1991]and Jolivet and Tamaki [1992]. TPF is Tym-Poronayskfault, HSZ is Hidaka ShearZone, MTL is medientectonic line, YBK is YamatoBank, YB is YamatoBasin, YF is Yangsanfault, TB is TsushimaBasin, and TF is Tsushimafault.

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