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 southern Okinawa 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