Parallel Stretching and Folding of Forearc Basins and Lateral Migration
TECTONICS, VOL. 18, NO. 2, PAGES 231-247, APRIL 1999
Trench-parallel stretching and folding of forearc basins and lateral migration of the accretionary wedge in the southern Ryukyus' A case of strain partition caused by oblique convergence
SergeLallemand, 1 Char-Shine Liu, 2 St•phaneDominguez, 1 Philippe Schntirle, JacquesMalavieille, 1 andthe ACT ScientificCrew 3
Abstract. Detailed seafloormapping in the area east of Taiwan 1. Introduction revealed trench-parallel stretching and folding of the Ryukyu forearc and lateral motion of the accretionary wedge under Strainpartition has been documented in manysubduction oblique convergence.East of 122ø40'E, a steep accretionary zoneswhere the convergencevector makes an anglewith the wedge is elongated in an E-W direction. A major transcurrent directionnormal to the trenchaxis [e.g.,Filch, 1972]. right-lateral strike-slip fault accommodatesthe strain Generally,transcurrent strike-slip faulting is observednear the partitioning caused by an oblique convergenceof 40ø. A volcanicarc because it corresponds to the weaker region of the spectacular out-of-sequencethrust may be related to the overridingplate. Prime examplesinclude the Great Sumatran subduction of a structural high lying in the axis of the N-S Faultin Indonesia,the Median Tectonic Line in Japan,and the trending Gagua Ridge. This asperity is likely responsiblefor PhilippineFault [e.g., Diament el at., 1992;Jarrard, 1986]. the uplift of the accretionarywedge and forearcbasement and Thesouthern Ryukyus offer an interesting geodynamic setting may have augmented strain partitioning by increasing the to studythis process because (1) theobliquity exceeds 40 ø, (2) coupling between the two plates. West of 122ø40'E, the low- the southernRyukyu Arc is nonvolcanic,and (3) the taper accretionarywedge is shearedin a direction subparallel overridingplate undergoes backarc extension as attestedby to the convergencevector with respectto the Ryukyu Arc. The the openingof the OkinawaTrough. bayonetshape of the southernRyukyu Arc slope partly results On the basis of the geology of the YaeyamaIslands from the recent(re)opening of the southernOkinawa Trough at [Kuramotoand Konishi, 1989] and on earthquakeslip vectors a rate of about 2 to 4 cm/yr. Right-lateral shearing of the [Kaoel at., 1998],some authors argue for strain partitioning sedimentary forearc with respect to the nonlinear Ryukyu alongthe RyukyuArc. However,there are absolutelyno backstop generatestrench-parallel extension in the forearc indicationsof obliquerifting or spreadingin the axisof the sediment sequence at dilational jogs and trench-parallel OkinawaTrough [Sibuet el at., 1995].In thisstudy, we present folding at compressivejogs. The Hoping Basin lies above a new data collectedalong the Ryukyu forearcnear Taiwan and diffuse trench/trench/fault(TTF) or TFF unstabletriple junction discussthese data with regardto the otherprevious studies. moving toward the south along a N-S transformzone which A joint French-Taiwanesecruise, called ACT cruise for accommodatesthe southward drift of the Ryukyu Arc with active collision in Taiwan,in May and June1996 on the R/V respectto Eurasia. L•ltalante provideddetailed structural images of the seafloor and crustaround Taiwan, in particularalong the westernmost 200 km of the Ryukyuforearc area [Lallemand et al., 1997a]. The data collected during this cruise helped answer the 1CNRS-Universit6Montpellier 2, Laboratoire G6ophysique et following questions:Does strain partitioningoccur in the Tectonique,Montpellier, France. forearc area, and if so, is the deformation localized or diffuse? 2Institute of Oceanography, National Taiwan University, Taipei, Doesdeformation affect the arc slope,the forearcbasins, or the Taiwan. accretionarywedge? Another topic addressed in this studyis 3j. Angelier,Laboratoire deTectonique Quantitative, Universit6 concerned with the interactionbetween the subductingGagua Pierre et Marie Curie;J.-Y. Collot,ORSTOM; B. Deffontaines,and M. Fournier,Laboratoire de TectoniqueQuantitative, Universit6 Pierre et Ridgeand the southernRyukyu margin: How does the Ryukyu Marie Curie; S.-K. Hsu, Instituteof Oceanography,National Taiwan forearcdeform during ridge subduction and does it affectslip University;J.-P. Le Formal, IFREMER Centre de Brest; S.-Y. Liu, partitioning? Instituteof Oceanography,National Taiwan University;C.-Y. Lu, Departmentof Geology,National Taiwan University; J.-C. Sibuet,and N. Thareau,IFREMER Centre de Brest;and F. Wang,Institute of Applied 2. Data Acquisition Geophysics,National Taiwan Ocean University. Thestudy area (see location in Figure1) wasmapped with Copyright1999 by the AmericanGeophysical Union. 100% bathymetricand backscatteringcoverage during the ACT cruise[Lallernand et al., 1997a].Tracks were generally Papernumber 1998TC900011. orientedparallel to the structuresalong the Ryukyuand Luzon 0278-7407/99/1998TC900011 $12.00 Arc slopes in order to keep the swath width constantand
231 232 LALLEMAND ET AL.: STRAIN PARTITION IN THE SOUTHERN RYUKYUS
120 ø 125 ø
- ======-?*•:':•:::•.?::•:•%•?7%•:•f•?•½?•?•::•?•.•:::::-•:::•:.::•:,• • •'}•• ß . • ...... OK IN AW A •- ,
25 ø
' ' ß •ff• ISLANS]• • (42) 20 ø
SEA2
I I I I I I I I I Magmaticbodies(extinct volcanoes, plateaus,ridges...) • Present-dayvolcanic arc •1•Volcanic activity below sea-level •1•Active volcanoes
Figure 1. Geodynamicsetting of the study area.The main tectonic featuresare represented,including the major resultsof the active collision in Taiwan (ACT) cruise.Magnetic lineations are fromHilde and Lee [1984] in the Philippine Sea and from Briais and Pautot [1992] in the South China Sea.Convergence arrow is 81 km long (1 m.y. of convergence)in the direction given by Global PositioningSystem (GPS) measurements[Yu et al., 1997] relativeto the SouthChina Block (SCB). The line A- A' indicatesthe locationof the lithosphericcross section of Figure 2. Deep Sea Drilling Programsite number293 is located in the PhilippineSea with absolutecrustal age in parentheses.Isobaths are every 500 m. becausemost previously acquired seismic lines were shot and with variations in the nature of the seafloor. Subbottom normalto the structures.The ship is equiped with SIMRAD (3.5 kHz) and reflection seismicprofiling and magnetic and EM12-Dual and EM950 (for depths shallower than 300 m) gravity data were also recorded along the 20-km-spaced multibeamsystems that enable swath mappingand side-scan parallel ship tracks for great depths and along more closely imageryover a maximum20-km-wide strip (151 simultaneous spacedship tracks for shallower depths. We deployed a six- soundings)of seabedin a single pass.The side-scanimagery channelstreamer with two 1230-cm3 generator/injector (GI) associatedwith EM.12-Dual gives detailed informationon the gunsat a pressureof 160 bars.The guns were fired in harmonic acousticreflectivity associatedwith fine bathymetricfeatures modeto gen6ratea sourcesignature centered on 20 Hz for deep LALLEMAND ET AL.' STRAIN PARTITION IN THE SOUTHERNRYUKYUS 233
penetration.Shot intervalswere- 50 n• Seismicdata were fracturezone [Mrozowskiet at., 1982; Deschampset al., 1997]. processedusing ProMAX softwareto obtain poststacktime- In addition to this prominent ridge, the PSP carries the migratedprofiles. Neogene Luzon volcanic arc, paired with the Manila subduction zone, as well as seamountsand oceanic plateaus. 3. Geological Setting of the Southern The PSP moves toward the northwest, with respect to the Ryukyus Near Taiwan Eurasian Plate as indicated by global kinematics [e.g., Seno, 1977]. Yu et al. [1997], on the basis of geodetic data issued The relativeplate convergencebetween Eurasia and the from a 5-year Global Positioning System (GPS) survey, PhilippineSea Plate (PSP) is obliquealong the two major proposea currentconvergence of 80 to 83 mm/yrin an azimuth plateboundaries which meet in Taiwan(Figure 1). One is the of N306ø + 1ø of the Lanyu Island (representativeof the PSP) N-S trendingManila subduction zone along which a transition relative to the Penghu Islands (representativeof the South occurs between an eastward dipping oceanic subduction China block (SCB)). Heki [1996] proposedthat the SCB was (SouthChina Sea) south of 21ø40'Nand southeast dipping extrudedfrom the stableAsian continentat a rate of 11 mm/yr in continentalsubduction (Chinese shelf)north of this latitude. the direction N112 ø, following the model of Tapponnier et al. The otheris the northdipping Ryukyu subduction zone along [1982]. After accountingfor the relative motion betweenthe which the PSP is subductingbeneath the Ryukyu Arc, rifled SCB and stable Eurasia, the relative convergencebetween the fi'omthe Chinesemargin. At the junction betweenthese two PSP and Eurasia is in excellent agreementwith Seno et al's oppositesubductions, the Taiwan orogen is activelygrowing. [1993] model.
3.1. Philippine Sea 3.2. Okinawa Trough and Ryukyu Arc
The oceanic crust of the PSP near Taiwan belongs to the The Chinese continental shelf suffered back arc extension PaleogeneWest Philippine Basin. East-westtrending from Taiwan to Kyushu (Japan)above the Philippine Sea slab. magneticlineations, observed in the smallHuatung Basin near The Ryukyu Arc is thus a rifted fragmentof continental crust. Taiwan,were tentatively identified by Hilde and Lee [ 1984] as Back arc extensionin the Okinawa Trough may have started as anomalies 16 to 19 on the magnetic reversal scale. They early as middle Miocene according to Park [1996]. Other representthe youngest stage (middle to late Eocene)in the authorsargued for a late Miocene first phaseof opening [e.g., openingof the West PhilippineBasin (Figure 1). Accordingto Letouzeyand Kimura, 1985]. In any case,Sibuet et at. [1995] the magneticlineations map in the West Philippine Basin suggestedthat the openingceased from 6 to 2 Ma and resumed (Figure1), the 37 Ma fossil spreadingcenter, north of anomaly since 2 Ma. On the basisof GPS measurementson the Yaeyama 16 in the Huatung Basin, should be buried beneath the Islandsover a few years [Imanishiet al., 1996], Yonaguni (the Ryukyu accretionarywedge near latitude 23ø30'N. It should closest island from Taiwan) is moving toward the south be noted that according to the satellite altimetry-derived (N184ø) at a rate of about 40 _+5 mm/yr,whereas Ishigaki and gravity map of Smith and Sandwell [1997], the grain of the Haterumaare moving at a lower rate of 22 and 24 _+5 mm/yr oceanic crust and the Central Riff Valley of the West toward the south(N172 ø) and SSE (N150ø), respectively,with Philippine Basin trend northwest-southeastrather than east- respect to the SCB, that is, after correcting for the relative west as proposedby Hilde and Lee [1984]. The Huatung Basin motion between the SCB and stable Eurasia given by Heki is separatedfrom the main Philippine Sea basin by the N-S [1996]. Such significant rates of opening necessarily have trending Gagua Ridge which probably originated from a important consequenceswith regard to the recent tectonic
ukyu OkinawaTrough Ryukyu Arc enchWest Philippine ! ©le Basin + + + •+ ß + + + + + + + PLATE EUR/xs\/x /
0 100 km i i
Figure 2. Interpretativegeological cross section A-A' (shownin Figure 1). Interpretationsinclude ACT cruiseresults such as strike-slipfaulting at the rearof the YaeyamaRidge or magmatismand activenormal faulting along the southernside of the Okinawa Trough [Sibuet et al., 1998]. 234 LALLEMANDET AL.: STRAINPARTITION IN THE SOUTHERNRYUKYUS
evolutionof the Ryukyu Arc [Lallemandand Liu, 1998], and 4. New Insights From the ACT Geophysical the relative convergencebetween the PSP and the Ryukyu Survey margin in the study area occursat a rate of 10.7 cm/yr in a N325 ø direction. 4.1 SubductingPhilippine Sea Plate The Ryukyu subductionstarted during the Late Cretaceous Detailed analysesof the ACT reflectionseismic lines, accordingto Lee and Lawyer [1995]. Ryukyu arc volcanismis togetherwith previouslyrecorded seismic profiles, reveal that expressedon the morphological arc north of Okinawa, and in the basementof the basinis very roughwith N-S trending the axis,or on the southernside of the Okinawa Trough fi'om ridgesand troughs [Deschamps, 1997]. The maximum depth of Okinawato Taiwan[Sibuet et al., 1987].The RyukyuArc is the basinfloor is 4800 rr[ It shallowsgently westward toward thus nonvolcanic south of Okinawa Island. The back arc basin the arc to a depth of 4000 + 500 rn,because all sedimentsare is about2-km deep near Taiwan, and seismic activity, faulting, suppliedfi'om the Taiwan mountainbelt throughnumerous andmagmatism presently occur along the southern part of the canyons(Figures 3 and 4). The meansediment thickness in the troughas revealed during the ACT cruise[Sibuet et al., 1998] Huatung Basin is 1600 rn, increasingto morethan 2000 m (seecross section A-A' on Figure2). Southof the Yaeyamatoward the mouthof the Hualienand Chimei Canyons (Figure Islands,the southern Ryukyu forearc system consists of a steep 4). arc basementslope, a seriesof forearcbasins (East Nanao, TheGagua Ridge rises above the surroundingseafloor by Nanao, and Hoping Basins),and an accretionarywedge up to 4 km. It is a 300- to 350-km-long,and 20- to 30-km-wide, (YaeyamaRidge). linear asymmetricridge. The highestpeak, 1520 m below sea
I
24'N
23øN
121øE 122øE I 123øE 124øE I I
Figure3. Multibeambathymetry ofthe study area at 200-m contouring. Inthe vicinity of the Yaeyama islands (upper right corner)and in theleft side of Figure 3, thegaps are filled using the available bathymetric data of the area. On land, the topographyis given at a resolutionof500 m with 200-m contouring. Plates 1 and2 andFigure 6, ACTseismic lines, and anotherseismic line (MCS 367-9)shown in thisstudy are located. LALLEMAND ET AL.' STRAIN PARTITION IN THE SOUTHERN RYUKYUS 235
24' 30' N
Iriomo• Island ß
Haterusa Island R•ukyuAreelope 24'00'N
East Nanao Basin
23' 30'N
23ø00'N
HuatungBasin 22' 30' N
122' 30'E 123' 00' E 123' 30' E
Plate 1. Shadedview of swath bathymetryacquired during the ACT cruise. See location of the box on Figure 3. Note the junctionof Hualien and TaitungCanyons in the Ryukyu Trench,just west of the Gagua Ridge, and the eastwardcourse of the resultingchannel after passinground the northernend of the ridge. Note also the prominentlinear strike-slip fault, paralleling the trenchat the rear of the accretionarywedge and the uplift of the accretionarywedge and forearcbasin in the axis of the Gagua Ridge. The differentmorphology of the accretionarywedge eastand west of 122ø40'N is clearly visible: to the west, a broad gentlewedge with closelytight spacedthrusts and to the east,a narrow,steeper wedge with wider thrust spacing.Seismic lines ACT 81 and ACT 92 shown in this studyare locatedon the map.
level, is locatednear 22ø05'N of latitude.The crestdeepens Basin.Dredged rocks recovered on the westernflank near 21 øN fromthis latitudetoward the northand south.The ridge dams includegabbros, amphibolites, and basalts with slickensides, the sedimentsand turbidites coming from the west,except in whichMrozowski et al. [1982]interpreted as an up-faulted the northwhere it vanishesin the RyukyuTrench (Plate 1). silverof oceaniccrust bounding a fracturezone. Thisfeature partly explains why the seafloor in the sedimentary Thenorthwesternmost partof the West Philippine Basin is basinsis 400 m deepereast of the ridge. Another reasonfor the very ruggedeast of the GaguaRidge, with little sediment.Some depth differencecould be relatedwith the 4 m.y. younger seamountscrop out through the 2-km-thick trench fill. The oceaniccrust of the HuatungBasin with respect to the adjacent Ryukyu trench axis beyond 200 km east of Taiwan is filled WestPhilippine oceanic crust [Hilde and Lee, 1984]. In fact,-with up to 2 kmof turbidites flowing from west of the Gagua theGagua Ridge parallels the N-S trendingfracture zones of Ridge(Figure 4 andPlate 1). Theseorogenic sediments are the WestPhilippine Basin (Figure 1). It thusprobably derived from the Central and Coastal Ranges mostly through originatedfrom an old fracturezone of the WestPhilippine theHualien, Chimei, and Taitung Canyons. 236 LALLEMAND ET AL.- STRAIN PARTITION IN THE SOUTHERN RYUKYUS
% YonaguniIsland / •• IriomoteIs. ..½ -.. // • •' • HaterumaIsl. Hoping 'qL'--- '...... \ • ] ":• (- Basin3000m) •t/ • • '"-.•"• •• X•• 24øN- ...... ''x •...... , •:: '."'...... % • • •t / "¾'•'•..... X%• Hateruma •x ....cast Hanao•f D.B •x,•X • I , e•nnBasin --• ß I• ml _ • •" ' •
j
- 5200 rn
-t • RyukyuTrench
23 ø N - I ./,.,_3oo Ij•. '•///.:Gagud !•idg•/ •Z L%• T2'•
121øE 122 ø E 123 ø E •.... Lichim&lange -•- -•- Anticlinal ridges i'• I •' I J,YaeyamaWestern Foothills/ Islands •. _ Troughs '....'..:•'-.:''. Hsiieshan Range I///1/1//tJKenting m61ange • Thrusts ß . . Normal faults • BackboneRange Volcanicrocks "• Active strike-slipfaults z,,-3soo,,,..Maximum (or minimum)local depth I+++ ++1 Central Range s.s. • Canyonsand channels .-27oo,... Localdepth ...... Limits of morphologicalunits 3600mJ.' Average depth
Figure4. Simplifiedstructural map of thestudy area offshore east of Taiwan,based on the analysisof all the ACT seismiclines, aswell as 1/100,000scale bathymetric charts. Simplified geology is given on land with moredetails along the CoastalRange. The stippledarea represents the uplifted part of theaccretionary wedge and forearc basin north of theGagua Ridge. Normal faults in the Okinawa Trough are basedon Sibuet et al. [1998]. HR is HsinchengRidge.
4.2. The Ryukyu accretionary wedge trenchwardand is about2 km high (Figures3 and 5). The front of the wedge is steeperand roughereast of the Gagua Ridge South of the Yaeyama Islands, a classical sequenceof arc (Figure 4). The morphologyof the lower slope in this area slope,forearc basins, and accretionarywedge is observed.For a suggestsmaterial collapse down to the trench (Figure 4 and long time, the wedge was improperlycalled "YaeyamaRidge." Plate 1). Oversteepeningof the frontalwedge and gravitational In fact, it was proven, after a cruise on board the R/V Jean collapse of the margin indicate either an increased effective Charcot, that the Yaeyama Ridge is a typical accretionary basalfriction along the dtcollement[Gutscher et al., 1998] or wedge (J.-F. Sttphan et al., unpublished POP2 cruise report, the recent subduction of an oceanic high [Lallernand et al., 1985). 1994]. According to the roughness of the oceanic basement 4.2.1. East of 122ø40'E. The accretionarywedge is 50 to south of the Ryukyu Trench (Plate 1)and to the existence of 60 km wide with numerousslope breaks correspondingto the positive magneticanomalies beneath the wedge [Hsu et al., emergenceof thrust ramps.There is a pronounced thrust front, 1996a], we proposethat oceanicasperities are responsiblefor that we interpretas an active out-of-sequencethrust within the both instabilities of the frontal accretionarywedge. North- wedge, along latitude 23ø20'N, east of the Gagua Ridge (see south reflection seismic profiles acquired during the the ACT line 92 on Figure 5). The frontal scarp dips 15 ø TAICRUST cruise [Schniirle et al., 1998] and the ACT cruise LALLEMAND ET AL.' STRAIN PARTITION IN THE SOUTHERN RYUKYUS 237
ACT 92 - PART A
V.E. = 5.7
0 5 10 km PartA , , • Part -4 mass-wasting -5 z,/:• major _6 /..._./ transcurr•nt- Y _8 •...•-..C•,•-.•.:.'_--555•, out-of:sequence _9 -,a•----?.._••••••••RyukyuTrenchii N,.. thrust _10 -• topof oceanic crust I s-twt
V.E.: 4
V.E. = 5.7
Figure5. Portionof N-Stime-migrated 6-channel seismic line ACT 92 acrossthe Ryukyu Trench and accretionary wedge, east of theGagua Ridge. Note the depth of thed6collement in the trench, which almost coincides with the top of the oceanicbasement. Theemergence of an out-of-sequence thrust is suspectedatthe base of themain inner wall scarp, and, on thebasis of bathymetric studies,the majortranscurrent fault is locatedat thetop of the wedge.Vertical exaggeration is -• 5.7 at seafloor.Location is shownin Figure 3 and Plate 1. 238 LALLEMAND ET AL.: STRAIN PARTITION IN THE SOUTHERN RYUKYUS
122ø30'E diamond-shapestructure •23øE 123ø30'E I. • • • I , , !
23o30,N
23oN
Figure6. Bathymetricmap of the Yaeyamaaccretionary wedge in the regionof apparentinteraction between the GaguaRidge and the margin(see location in Figure 3). Isobathsare every 20 • Note the strike-slip fault at the rear of the wedge (small. arrows)and the steepnessof deformationfronts in andeast of the reentrantalong the emergenceof the out-of-sequencethrust. A particularstructure is observedat therear of thewedge near 122.ø40'E, refered as the diamond-shaped structure in this paper(see explanationsin the text). Locationof Figure7 (box).
(Figure 5) both reveal that the entire sedimentarysection, 2- to splays,south of the main E-W trending shearzone (Figure 7c), 3-km-thick, of the Ryukyu Trench is presently offscrapedat the becausethe downthrown basementblock presumablybelongs front of the wedge east of the GaguaRidge. to the plate subducting northward. The upthrown basement Transcurrentfaulting is localized at the rear of the wedge block probably correspondsto the basementof the Ryukyu along a single major fault which is poorly resolved on seismics Arc. A local change in the dip of subductingbasement in the (Figure 5) but nicely imagedon bathymetry(Plate 1). Indeed, a vicinity of the shear zone (Figure 7c) is suggestedby the spectacularN95 ø trending strike-slip fault is observedat the associatedregional high in the accretionarywedge north of the rear of the accretionarywedge, east of the Gagua Ridge (Figure Gagua Ridge (Figures 4 and 6 and Plate 1). West of 123øE, the 4 and Plate 1). Its morphologicalexpression is a linear, 1 + 0.5- N95 ø trending strike-slip fault turns toward the north, km-wide, up to 650-m-deep, 100-km-long trough and reaching N33øW in azimuth (Figure 6). The curvature of the associatednarrow ridge to the north (Figures 6 and 7a). The fault is also markedby what could be a curved drag fold to the backscattering image shows a series of parallel fractures, north and a "diamond-shapedstructure" to the east (Plate I and interpreted as Riedel shears (Figure 7b), producing a right- Figure 6, crossingof a N-S and an E-W fold). lateral displacement across the main fault. Consequently, the The Ryukyu deformationfront has been indented about 15 accretionarywedge movestoward Taiwan with respect to the km in front of the Gagua Ridge. The reentrant consists to the Ryukyu Arc. Results fi'omsandbox experimentsindicate that east of a N30øW striking scarp, which is parallel to the oblique slip along a dipping basementfault with predominant terminationof the right-lateralstrike-slip fault at the rear of the strike slip and additional componentof reverse dip slip (by wedge (Figures 4 and 6 and Plate 1). The 3-km-high frontal opposition with pure strike slip) causes synthetic Riedel slope dips about 30ø , close to the angle of repose of the shears,with an upward convex shape, that are preferentially accretedsediment, and is consequentlyhighly unstable. A located above the downthrown side of the basement fault portion of the wedge about 30 km in diameteris uplifted by [Mandl, 1988, p. 142]. These experimentalobservations are in about1000 to 1500 m with respectto the surroundingseafloor very good agreementwith the existenceand location of most (Plate 1 and stippled area on Figure 4). The highest point, LALLEMANDET AL.' STRAINPARTITION IN THESOUTHERN RYUKYUS 239
Figure 7. (a)Bathymetric map of the uplifted region which is cut by the major strike-slip fault. Isobaths are every 20 nx (b) Onboard display of real-timebackscattering image of the major strike-slip fault located on Figure 7a. Note the Riedel shears mainly on the southern side of the shearzone. (c) Interpretative cartoon showing the geometry of the Riedels, adapted fi•om Mandl[ 1988].
1980 m deep, is located north of the main N95 ø shear zone Plate 1). A NW-SE trending right-lateral strike-slip fault is (Figures4, 6, and 7a). clearly visible in the bathymetry (Figure 9a)on shaded 4.2.2. West of 122ø40'E. West of the Gagua Ridge, perspectiveviews (Plate 2)and on the onboard display of evidence for active accretion at the trench is less convincing, reflectivity along the ship's route (Figure 9b). The fault can be except close to the ridge. The frontal taper is much lower traced over more than 40 km on the detailed bathymetric map (Figure 8)and the fromally accreted unit is crosscut by (Figure9a). Its precisetrend, that is, N313ø, is closeto that of meanderingchannels (Plate 1). As already observed during the convergencevector between PSP and Taiwan (SCB) [Yu et previous Taiwanesecruises [Liu et al., 1996, 1997], oblique al., 1997]. Becausethe strike-slipfaults are linear with respect thrusting and strike slip prevail in the wedge as attested by to the complexstructure of the adjacentaccretionary wedge and linear and lenticular structuresof troughsand anticlines which forearcbasin, we think that they recently cut through the are subparallelto the convergencevector (Figures4 and 8 and previousfeatures rather than reactivatingold thrusts. 240 LALLEMAND ET AL.: STRA1N PARTITION IN THE SOUTHERN RYUKYUS
SW
,0 5 1,0km MCS367-9
accretionarywedge -4 Confluence of Hualien anao forearc basin
&Chimei canyons and 5 - terminationofRyukyu _ _6;
Trench•_,.__• -•,• •-,_• -7
transcurrentfaulting /• --- •,-'---8 top of RyukyuArc basement •
s-twt V.E. = 5.4
Figure 8. Portionof the NE-SW time-migrated56-channel seismic line 367-9, acquiredin 1993 on boardthe TaiwaneseR/V Ocean ResearcherI, acrossthe westernmostpart of the Ryukyu trenchand accretionarywedge. Note the low taper compared with line ACT 92 (Figure5), andthe presenceof transcurrentdextral faults at the rearof thewedge. Vertical exaggeration is • 5.4 at seafloor.Location is shown in Figure 3.
The Hualien Canyon outlines the physical boundary 4), whereas the Hoping Basin deposits are now eroded and between the accreting Luzon Arc and the accretionary reworked [Lallernand et al., 1997b]. Little deposition occurs wedge.It deeplyincises the sedimentarymass of the wedge in the East Nanao Basin, and the HatemmaBasin may be fed by (Plate 2). sedimentscoming from the YaeyamaIslands through a N W trending valley (Figures 3 and 4 and Plate 1). The maximum 4.3. Ryukyu Forearc Basins thickness of 3.7 s two-way travel time (two of horizontal depositshas been observedin the Nanao Basin. In fact, there The ACT surveyrevealed what was previously considereda are two adjacentbasins, a westernone 3-s thick and an eastern single forearcbasin to be, in fact,a seriesof forearcbasins one 3.7-s thick, separatedby a 1.8-sdeep basement high, in the arrayed in an along-strike staircasegeometry. The basins E-W trending Nanao Basin [Schniirle et al., 1998]. Numerous shallow westward from 4600 m in the East Nanao Basin to normal faults, N-S to NNW-SSE striking, affect the forearc 3700 m in the Nanao Basin and to 3000 m in the Hoping Basin. basinssediment as seen on seismicline ACT81 (Figure 10) or They also shallow eastwardto 3400 m in the HatemmaBasin on other trench-parallelseismic lines alongthe forearcbasins. (Figures3 and 4 and Plate 1). This basinwas namedafter the There is a broad uplifted area rising 200 m above the 3700- ACT cruise,using the nameof the nearestJapanese island south m-deepNanao Basin (Plate 1 and Figure 10) and 1100 m above of Iriomote (Figure 4). the 4600-m-deep East Nanao Basin (Plate 1 and Figure 6) in On the basis of ACT reflection seismic lines, as well as on the axis of the ridge and uplifted wedge. previouslines [Liu et al., 1996, 1997;Schniirle et al., 1998], it The structureof the Hoping Basin appearsmore complex.It becamepossible to propose a relative chronology for the is likely controlled by the ongoing arc-continentcollision in tectonic evolution of the Nanao and East Nanao Basins. Taiwan. A tremendous,negative fre,e-air gravity anomaly of Present-daysedimentation in the Nanao Basin is primarily about -200 mGal and a sediment thickness of more than 4 s turbiditescoming from the Ilan area(Lanyang Hsi River, Figure (twt) characterizethe basin area [Lallernand et al., 1997b]. LALLEMAND ET AL.' STRAIN PARTITION IN THE SOUTHERN RYUKYUS 241
Figure 9. (a)Bathymetricmap in the region of N313ø-strike-slip faulting offshoreof Hualien. Location shown in Plate 2. Isobathsare every 20 m. (b) Onboarddisplay of real-timebackscattering image of the strike-slipfault locatedon Figure 9a.
NW-SE lineaments are observed across the E-W trending northeastward verging thrusts have been interpreted from Hsincheng Ridge in the Hoping Basin (Figure 4 and Plate 2) several reflection seismic lines (Figure 4). It is located and were reported following previous cruises [Lallemand et immediatelynorth of the Coastal Range and could thus be al., 1997b; Liu et al., 1996, 1997]. The ridge is nonmagnetic related with the subductionof the northermostportion of the [Hsu et al., 1996a], and both southwestward and volcanic arc. 242 LALLEMAND ET AL.' STRAIN PARTITION IN THE SOUTHERN RYUKYUS
South rn ...... Okirmwa Lowresolution d•le Tr ugh Ilmt PI•I
24ø30'N RyukyuAre
\
/ •• ' '•Hopillg• .•. B •n '•
. %%H i ch ,Ridge
L ,, ACT 65 24ø00'N
N i18o
Ryuk U •.(;mr riOfiery wed
23'30'N 121' 30' E 122' 00' E 122ø30'E
Plate2. Shadedview of swathbathymetry acquired during the ACT cruisein the Hoping Basinoff Hualien. See location of the boxin Figure3. Notethe E-W trendingHsincheng Ridge and the N313ø trendingshear zone. Location of Figure9a (box)and seismicline ACT 65 areshown in this study.Unfiltered swath bathymetric data were used in orderto enhancefine structural details;thus some north-southartefacts remain in the image.
4.4. Ryukyu Arc Slope From west to east, the following is known: 1. The N30øE to N45øE striking steep slope, north of the The Ryukyu Arc slope exhibits a bayonet geometryin map HsinchengRidge (Figures3 and 4), belongsto the Central view (Figures 3 and 4), with segmentsapparently offset right laterally along N-S to NW-SE strike-slipfaults. On the basis of Rangeas indicatedby the simplifiedgeological map of Figure this observation, Hsu et al. [1996b] interpreted the southern 4. It is very likely affectedby normalfaulting as observedby Ryukyu Arc and back arc as being dissectedby three N27øW authorsalong the coastalroad on land and as predictedby trending strike-slip faults controlling the location of canyons GPS measurements[Yu et al., 1997]. in the northern margin of the Okinawa Trough. The detailed 2. The detailedACT bathymetryallowed us to trace several swath bathymetryacquired during the ACT cruise confirmsat en 6chelon dextral strike-slip faults fromthe Okinawa Trough least part of this interpretationin the sensethat the southern axis to the Hopingforearc basin (Figure 4). One of the faults is arc slope shows N30øW trends north of the Nanao Basin associatedwith an east facing scarp in the Okinawa Trough interpreted as old strike-slip faults subparallel to those (24ø50'N, 122ø13'E,see the completeACT surveybathymetric observed in the accretionarywedge north of the Gagua Ridge mapof Lallemand et al. [1997a] or Sibuet et al. [1998]) and (Figures3 and 4 and Plate 1). The arc slope also exhibitsN-S with a right-lateral offsetof structuralhighs on both sides of to NNE-SSW spoon-shaped normal faults visible on the about 15 km (Plate 2). The fault continues southward across bathymetricmap (Figure 3) merging on WNW-ESE trending the arc basementand splits into severalN-S trending branches lineaments,subparallel to the convergencevector (Figure 4). in the HopingBasin (Figure 4). The isobathsare systematically LALLEMAND ET AL.: STRAIN PARTITION IN THE SOUTHERN RYUKYUS 243
Line ACT-81 conversely, evidence of southeastward motion in the northeastern area of Taiwan and on Yonaguni Island with respectto SCB [Yu et at., 1997; Imanishi et at., 1996] indicate ";•:'•:";•;'•:•'•;!•'•:•'•"':"•s '•?-•";•'•! ' +'•'•< •:'"•'-': ...... ,"•e• that a westward motion of a Ryukyu forearc sliver is incompatiblewith geodeticdata and stressregime of the upper plate. For instance,transcurrent faulting is well documentedin the 7 accretionary wedge, but it cannot have any influence on slip vectors. NW-SE diffuse shearing characterizes most of the upliftedarea E-•-• wedge west of 123øE, whereas a major E-W strike-slip fault, Nanao Basin Line ACT-65
••-- -•_-• /
4.0
- /• • • basement 4.2 aseme•t".'" / 9 •, 10km 7 0 5 km Figure 10. Portionof E-W time migrated6-channel seismic , , line ACT 81 across the Nanao forearcbasin and the uplifted W HopingBasin E area between the former basin and the East Nanao Basin. Note the normalfaults in the Nanao Basin and the progressiveuplift of the seafloorabove the subducting asperity as indicated by 4.5 onlapof sediments.Vertical exaggeration is ---6.5 at seafloor. I unconformity Location is shown in Figure 3 and Plate 1. 5.0 SuaoBasin•. l/ 5.5 deflectedalong longitude 122ø04'E between 23ø44'N and 24ø32'N, that is, over 85 km (Figure 3 and Plate 2). A flower 6.0 structureis seenalong the E-W line ACT-65 crossingthe shear zonein the HopingBasin on bothseismic and 3.5 kHz profiles 6.5 (Figure 11). /•_Strendihgshear zoneV.E.~5.2 3. The prominentN50øW trending spur and associated narrowvalley, south of IriomoteIsland (Figure 4 and Plate 1), has remainedenigmatic for a long time becauseit is highly obliqueto the strike and it seemsto offsetleft laterallythe RyukyuArc slope,that is, in a senseopposite to the other 4.5 offsets. 5.0 5. Discussion and Interpretations 5.5 5.1. Lateral Transport of the Ryukyu Accretionary Wedge Caused by High Convergence Obliquity 6.0 On the basis of GPS measurements,the present-day convergence between the PSP and the southern Ryukyu 6.5 margin in the study area has an obliquity of 40ø east of 122ø40'Eand 60 ø west of this longitude [Lallemand and Liu., Figure 11. (a)E-W time-migrated 6-channel seismic line 1998]. Slip partitioning between closer-to-trench normal ACT 65 across the southern part of the Hoping Basin. The thrusting and trench-parallel strike-slip faulting has been Hoping Basin sequencesare thinning toward the east above postulatedfrom the studyof earthquakeslip vectors[Kao et al., the highly deformedSuao Basin strata [see Lattemand et at., 1998] at least along the arc segment between 122øE and 1997b]. The vertical strike-slip faults might belong to the N-S transform zone which allows the Ryukyu Arc to drift 123øE. Because the selected earthquakes (interface seismic southwardwith respectto the SCB (see Figure 4). (b) A kind of zone) are located between 24øN and 24ø20'N beneath the flower structure is visible on the 3.5 kHz profile. The Ryukyu arc slope, transcurrentfaulting must occur landward, horizontal scale is the same as Figure 11a but vertical that is, in the arc itself or in the Okinawa Trough, to accountfor exaggerationsdiffer. Vertical exaggerationis • 5 at seaflooron the slip deflection.The lack of E-W compressionalstress at the the seismicline and is - 15 on the 3.5 kHz profile. See location western boundary of the presumedRyukyu forearc sliver and, on Figure 3 and Plate 2. 244 LALLEMAND ET AL.' STRAIN PARTITION1N THE SOUTHERNRYUKYUS
W E HopingB. NanaoBasin EastNanao B. •• •) ' + RyukyuAr;'baseme•t i L__v +.(
D I
i•Jence W E _a• Yeya ,..,out-o..f• Gagua ••_•/.•••thrust .,R•dge. v ©v A' :i il B' 122øE ',: 123oE Subductingasperity I I • I •
East out-of-sequence thrust Nanao Yaey.ama Ryuky.u $ N Nanao J Ryuky.u S ; ukw• Ridgel'rench basement. 'kr• ' •v ©• •,•y• 'Ar•+_l'r•©•,* •, (/• basement +Z v••• • 10km1 •',+,// 1,00km50V.E. = 2.7
Figure12. Four sections across and along the Yacyama accrctionary wedge and forearc basins. These interpreted sections arc basedon seismic line interpretations anddetailed bathymetry fi'omM. Ewing cruise [$chnfirle etal., 1998] and this cruise for shallowerparts. Interpretations atdeeper levels, such as the geometry ofthe subducting plate and out-of-sequence thrustare deducedfrom sandbox experiments [Lallemand etal., 1992]. The deepening oœthe subducting Philippine SeaPlate (PSP) away fromTaiwan issuggested bygeometrical considerations oœthePSP near Taiwan, seismicity [ICao et al., 1998], and refraction studies[11/ang et al., 1996]. associatedwith southwardthrusting prevails to the east(see Inasmuchas only 20% o• theconvergence between PSP and Figure4 andsynthetic cross sections A-A' andB-B' on Figure SCBis taken up on land in theHualien area [¾u et al., 1997], 12). thewestward displacement ofthe wedge, if significant,should Near 122ø40'E,the majorN95 ø transcurrentfault is deviated becounterbalanced by shortening in the wedge (see the N-S towardthe north and exhibits a N33øW segmentwhich offsets trendingfold east of theHualien Canyon, Figure 4 andPlate 2 therear of the wedge(Figure 6). It duplicatesthe changein andcross section D-D' on Figure12) and/orerosion by the strikeof the southernslope of the arc north of the Nanao Basin HualienCanyon (see the 20 ø dipping,1-kin-high east wall of (Figures3 and4 andPlate 1). Assuming that the Ryukyu Arc the canyon,Figure 3). basementacts as a rigidbackstop controlling the growth of the wedge,we supposethat its bayonetshape is reflectedin the 5.2. GaguaRidge Subduction and Degree accretionarywedge, especially at its rear.West of this segment of Slip Partitioning at the tip of the N95ø trendingstrike-slip fault, the diamond- shapedstructure probably partly took up someintrawedge Accordingto previousstudies on subductingasperities shortening.This particularstructure may also represent an [e.g.,Lallemand and Le Pichon,1987; Lallemand et al., 1990, overlapzone, which transfers the displacement,with lessclear 1994],indentation of thedeformation front by localblocking of trench-parallelstrike-slip faults to thewest (Figures 4 and 6). the d6collement.and interruption of frontal accretionwith These right-lateral transcurrentfaults accommodatestrain significantuplift of the marginare generallyinterpreted as partitioningwithin the accretionarywedge, parallel to the evidencefor subductionof a structuralhigh. In this case,the strikeof theadjacent trench section, that is, roughlyE-W east top of the subducting high should be located beneaththe of 122ø40'E and WNW-ESE west of 122ø40'E. highestpoint in thewedge (-1980 m, seeFigures 4, 6 and7). LALLEMAND ET AL.: STRAIN PARTITION !N TIlE SOUTHERN RYUKYUS 245
Becausethe more-than-300-km-longGagua Ridge seemsto toe of the rigidbackstop terminate at the trench (its height with respect to the surroundingbasement decreases from 5 km in the south to less than 1 km in the trench), we considerthat either (1) the ridge extendsto the north with a low at the trench or (2) the ridge ends at the trench and a subducted seamount lies in the axis of the ridge north of it. In any case, the subduction of a bathymetrichigh is certainlyresponsible for the indentation of the margin front. + +1 Now, we considerthe two extremecases with respectto the amount of strike slip absorbedon the N95 ø transcurrentfault. (1) If there is no lateral slip (degreeof partitioning is 0), then the trail of the subductedhigh must coincide with the azimuth releasingbend,•,,u,n,n• uena of present-day convergencebetween the PSP and the local Ryukyu margin, that is, N325 ø. (2) If the total lateral Figure 13. Perspectivecartoon illustrating the geometryof releasingand restraining bends of the toe of the rigid backstop componentof the oblique convergenceis accommodatedalong underlyingthe forearc basin sediments. The accretionarywedge the N95 ø transcurrentfault (degreeof partitioning is 1), then is movinglaterally along a majortranscurrent fault locatedat the trail of the subducted high must be normal to the strike of the rearof theprism. This motion induces some dextral shearing the trench, that is, N5 ø. within the forearcsedimentary section with stretchingabove As we can see on the bathymetric map (Figure 6), the the releasingbends and folding above the restrainingbends of reentrantis asymmetric,and the visiblepart of the Gagua Ridge the arc basement. is located on the western side of the reentrant. This means that there is somedegree of slip partitioning accommodatedalong the transcurrentfault. The distancebetween the asperity(below the highestpoint in the wedge) and the presentmargin front is moving toward the south(with respectto SCB) 1.6 to 1.8 cm/yr about 50 km. The magnitude of the normal-to-the-trench faster than the "Iriomote" Ryukyu Arc segment (IS) which componentof the relative convergencevector (PSP/Ryukyu moves toward the south-southeast. This present-day margin) is about 81 km/m.y. [Lallemand and Liu., 1998]. This kinematics is reflected in the bathymetry (Figure 3), as implies that the asperity passedthrough the deformationfront illustratedby the YS which is offset right laterally with respect about 0.6 m.y. ago and even less if we account for frontal to northern Taiwan along a N-S transformfault zone, whereas accretion during this period of time (see Figure 5). Thus, the IS is apparently offsetleft laterally with respectto the Y S during the last 600,000 years, the lateral slip along the N95 ø along a NW-SE fault zone (Figure 4). In fact, the N50øW strike-slip fault may have reacheda maximumof 42 km in the trending spur and associatednarrow valley north of the East case of total slip partitioning. Nanao Basin (Figure 4 and Plate 1) probably originate from On the basis of observationsin the wake of a subducting extension perpendicular to their trace according to the asperity,we have shown that strain partitioning was efficient respective motion of Hateruma with respectto Yonaguni. The within the wedge at least during the last 400,000 to 600,000 amount of extension has :beenestimated to be about 2.4 cm/yr years. Detailed morphological investigations supported by by Lallemand and Liu [1998]. The differential southeastward sandboxmodels are currently being performed[Dominguez et motion of both Ryukyu Arc segmentsresults in an apparent al., 1996] to propose a kinematic analysis of this sector and left-lateraloffset of the southernRyukyu Arc slope. estimatethe contribution of the subducting seamount/ridgein A right-lateralmotion of the sedimentaryforearc (basin and the coupling between the two converging plates and thus the wedge) with respect to an underlying bayonet-shapedrigid degree of partitioning. backstop may generate trench-parallel extension above releasing bends and compressionabove restraining bends as 5.3. Trench-Parallel Stretching and Folding in the Forearc illustrated on Figure 13. On the basis of the structural map, Basins and Relations With the Arc Framework dilational jogs or bends (incipient pull-apart) are found beneath the forearc basins near 122ø10'E, 122ø30'E, and The forearcbasins shallow systematically from 123ø30'E to 122 ø50'E where normal faults are observed, whereas a Taiwan. The samegeneral trend is observedfor the accretionary compressirejog (pop up) may exist at 123ø40'E between the wedge and the subductingoceanic basement (sections C-C' and East Nanao and Hatemma Basins, where a fold is observed in D-D' in Figure 12). The shallowing cannot be entirely caused the forearc sediments(Figures 3 and 4 and Plate 1). by the nearby collision zone where the PSP overthrusts the Somelineaments subparallel to the convergencevector are Eurasian Plate along the Longitudinal Valley Fault because observed in the Ryukyu Arc slope suggesting some discrete (1) the shallowing is not progressivebut occursstep by step, right-lateral motion within the arc. The geometryof the forearc (2) there are structural highs between the various basins, and basins, the numerous normal faults within the recent sediment (3) the easternmostHatemma Basin is 1200 m shallower than fill and on the slopesof the basinswhich connectwith NW-SE the East Nanao Basin. Boundaries between the basins are thus lineaments, and the associated free-air gravity minima are probably structurally controlled. interpreted as indications of a transtensional regime. It is We have seen that the forearcbasins are arranged in an en associated with subsidence of the forearc basins in a context of 6chelonarray, as is the Ryukyu Arc slope. GPS measurements right-lateralshearing or lateral displacementof the accretionary indicate that the "Yonaguni" Ryukyu Arc segment(YS) is wedge with respectto the arc basement. 246 LALLEMAND ET AL.: STRAIN PARTITION IN THE SOUTHERN RYUKYUS
Trench-parallel stretching has already been documented The Hsincheng Ridge is located exactly at the intersection along the Sumatraor Aleutians subduction zones [McCaffrey, of (1) the northernend of the LongitudinalValley Fault, (2) the 1991; Ryan and Coleman, 1992; Bellier and Sdbrier, 1995]. northwestern extension of the NW-SE strike-slip faults Such extension is generally caused by an increase of affecting the accretionarywedge, and (3) along a major N-S convergence obliquity and thus of the slip rate along the transform zone. This is the reason why it is so difficult to trench. This explanation can be invoked for the present study seismically image structural features it. it, even with becausethe convergenceobliquity increasesfrom 40ø east of multichannel seismics.Lallemand et al. [1997b] have shown 122ø40'Eto 60ø west of this longitude. The lateral component that the upper sequence of the ridge consists of deformed of the relative convergencevector thus increasesfrom 7 to 10 sediment of the Hoping Basin. The ridge is thus presently cm/yr for the case of total strain partitioning. This mechanism growing. Seismicdata and morphological studies allow us to supposesthat the degree of partitioning remainsmore or less infer that it undergoesstrong compression.The ridge is also constant along the accretionary wedge and it only explains locatedin the extensionof the CoastalRange, so that its deeper trench-parallel extension in the wedge, not in the forearc parts can be constituted of the Luzon Volcanic Arc, which basins. The particular geometry of the toe of the backstop could be downfaulted to the north along an E-W tear fault togetherwith the shearingof the overlying sedimentaryforearc [Lallemand et aL, 1997b]. is thus probably a second factor of trench-parallel extension and compression. 6. Conclusion It has been shown in this study that slip partitioning, 5.4. Hoping Basin: A Diffuse Triple Junction? causedby oblique convergence,was either localized along a The most prominentfeature which is associatedwith the major transcurrentfault at the rear of the accretionarywedge for right-lateraloffset of the arc basementis the N-S trending shear an obliquity of 40ø or distributed over the width of the zone closeto Taiwan (Figure4). On the basis of GPS and very sedimentary wedge when obliquity increased to 60ø . The long baselineinterferometry studies [Imanishi et al., 1996; Yu influence of a subducting ridge or seamounton the degree of et al., 1997; Heki, 1996], the present-day kinematics of the strain partitioning needs to be further investigated using southernRyukyu Arc areais suchthat about1.2 to 1.3 cm/yrof analog modeling, and no definitive conclusion can be reached right-lateraldisplacement must be absorbedalong a N170 ø yet. The forearcbasin sedimentsappear to be slightly sheared trendingtransform fault zonesomewhere between the Ilan Plain between the rigid underlying arc basement and the moving and Yonaguni(Figure 4) [Lallemandand Liu, 1998]. accretionaryprism. Trench-parallel stretching and folding is The deformedHoping Basin presentlytrends N-S or NNW- thus observedabove the bayonet-shapedtoe of the rigid arc. SSE rather than E-W like the other forearc basins. There are Unlike the Sumatra and Hikurangi caseswhere transcurrent severalreasons for this geometry,among which is its particular faulting also occurs onshore near the volcanic line, our locationat the complexjunction between (1) the westerntip of observations from the southern Ryukyu show that no the Ryukyu Trench,(2)the northernend of the Longitudinal significant transcurrentfeatures were revealedin the arc. This Valley Fault, known on land as the suture betweenPSP and absence within the southern Ryukyu Arc basement could SCB, and (3) the transformzone along which the Ryukyu Arc result from the lack of any volcanism (no weakness zone) moves southward with respect to SCB (Figure 4). If we and/orthe extensionalregime of the overriding plate (Okinawa considerthat these three featuresrepresent plate boundaries, Trough opening). then the basin is located on top of an unstable trench-trench- transformfault triple junction or a trench-fault-faulttriple Acknowledgments.We thank INSU-CNRS, the FrenchInstitute in junction,inasmuch as the LongitudinalValley Fault is rathera Taipei(Ministry of ForeignAffairs), andthe NationalScience Council high-anglereverse fault than a subductionthrust. The diffuse (Taiwan)for fundingand supporting this collaborative work; IFREMER for providingR/V L•talante shiptime and equipment;and GENAVIR junction migratestoward the south sincethe start of activity officers,technicians, and crew. We would also like to thank H. Kao, S.- along the N-S transformzone. After restoringthe Ryukyu Arc B. Yu, T.-K. Wang, Y. Fontand A. Deschampsfor their constructive to its former position (about 15 km back to the north discussionsthroughout these last years and the reviewers D. Scholl,Steve correspondingto the lateraloffset of structuralhighs on Plate Lewis, and an anonymousreviewer for their very usefulremarks. A. Delplanqueis acknowledgedfor preparingmost of the draftsof this 2, i.e., about 1 m.y. of activity at the presentrate), the Hoping paper.M.-A. Gutscherhelped improve the Englishof the manuscript. Basin unbendsand joins the E-W alignmentof the other forearc Figure3 and Plates1 and 2 were producedusing Wessel and Smith basins. [ 1991] GMT software.
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