Genetic Relations Between the Central and Southern Philippine Trench and the Sangihe Trench Serge E

Genetic relations between the central and southern Philippine Trench and the Sangihe Trench Serge E. Lallemand, Michel Popoff, Jean-Paul Cadet, Anne-Gaëlle Bader, Manuel Pubellier, Claude Rangin, Benoit Deffontaines

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Serge E. Lallemand, Michel Popoff, Jean-Paul Cadet, Anne-Gaëlle Bader, Manuel Pubellier, etal.. Genetic relations between the central and southern Philippine Trench and the Sangihe Trench. Journal of Geophysical Research. Oceans, Wiley-Blackwell, 1998, 103 (B1), pp.933-950. hal-01261614

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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. JOURNALOF GEOPHYSICALRESEARCH, VOL. 103,NO. B1,PAGES 933-950, JANUARY 10,1998

Genetic relations between the central and southern Philippine Trench and the Sangihe Trench 1 . 2 3 SergeE.Lallemand, ß 4 Mxchel Popoff,. 4 Jean-Paul Cadet, Anne-Gaelleß 3 Badertl Manuel Pubelher, Claude Rangre, and Benoit Deffontmnes

Abstract.We surveyedthe junction between the central and southern Philippine Trench and the SangiheTrench near 6øN using swath bathymetry, gravity, and magnetics. These data, along with seismicity,allow us to discussthe genetic relations between these trenches and the forces acting onconverging plates. Our final model favors the northern extension of theHalmahera Arc upto 8øN,with threesegments offset left-laterally along NW-SE transformfaults. Accretion of the northernsegment to MindanaoIsland 4 to 5 m.y. agoresulted in thefailure within the Philippine SeaPlate east of the arc.Initiation of thePhilippine Trench between 7øN and 10øN agrees with the maximumrecorded depth of the PhilippineTrench floor (10,000m belowsea-level) and PhilippineSea slab (200 km). South of 6øN(trench junction), another segment of thearc is being subductedbeneath the Sangihemargin, while southof 3øN,the southernsegment of the HalmaheraArc is still active.The rapidsouthward shallowing of the trenchfloor alongthe southernPhilippine Trench, the type of faultingaffecting both sides of thetrench, the lack of significantinterplate seismicity, and the concentration of the seismicity beneath the Miangas- TalaudRidge are interpretedas a slowingdown of thesubduction along this branch of the PhilippineTrench compared with therest of thesubduction zone. The Sangihe deformation front hasbeen recognized up to 7øN but seemsactive only southof 6øN.

1. Introduction in the Molucca Sea (Figure 2), where it is impossible,on the basis of global plate kinematics,to estimatethe relative plate In May 1994, the R/V Ltatalante surveyedthe area southof convergence individually along each subduction zone. 7øNduring the MODEC cruise(see locationon Figures1 and Preliminary results from Geodynamics of the South and South 2). Swath bathymetry, sonar imagery, six-channel reflection East Asia (GEODYSSEA) Global Positioning System (GPS) seismics,gravity, and magnetic anomalieswere recordedin measurementsindicate that the "Sundalandblock" is rotating thenorthern part of the Molucca Sea and along the Philippine clockwise with respectto Eurasia around a pole located south Trenchup to 11øN off Leyte (Figure 2). of Australia [Chamot-Rookeet al., 1997]. The convergence The Philippine Sea Plate (PSP) is mostly surroundedby between Sundaland and PSP is distributed over a 600 km wide trenches(Figure 1), except along a short segmentin the south zone from Sulawesi to L'uzon islands. Only 45% of the wherethe Ayu spreadingcenter forms the boundarybetween convergence,i.e., -- 3 cm/yr, is accommodatedalong the the PSP and the Caroline Plate [Fujiwara et al., 1995]. Philippine Trench in the southernPhilippines and probably Furthermore,the convergenceis partitionedalong its western even less along the southernbranch of the Philippine Trench boundary,so that slip vectors cannot be simply used for accordingto Ranginet al. [1997] (no definiteconstraints). constrainingits motion [McCaffrey, 1996]. Consequently, The convergenceacross the Philippine Trench may decrease theexact azimuth and magnitude of platemotion along its rim in magnitude south of 6øN, as indicated by shallowing remaincontroversial [Seno, 1977; Karig and Cardwell, 1986; bathymetryand the disappearanceof the accretionarywedge Huchon, 1986; Ranken et al., 1984; Seno et al., 1993; from 4 ø to 2øN [Nichols et al., 1990]; symmetrically, the McCqffrey,1996]. The numeroussubduction zones fringing same situationoccurs north of 15øN as shownby the scarce thewestern boundary of the PSPon eachside of the Philippine earthquake hypocenters and diminution of converging Islandsrender the southwesternplate boundaryparticularly character of the inner wall as seen on reflection seismics complex.This problemis especiallytrue southof Mindanao [Lewis and Hayes, 1983]. So'uthof Mindanao,the bathymetric trough in the prolongationof the Philippine Trench deviates toward the southeastby about30 ø (Figures 1 and 2). The trench fringesthe SnelliusRidge to the east [Ranginet al., 1996] and IUMRCNRS-UM2 5573, Laboratoire de G6ophysiqueet shallows from 9500 m at 6øN (junction between central and Tec.tonique,ISTEEM Montpellier France. southernPhilippine Trench) to 5000 m at 3øN, where the last ZUMRCNRS-UN•A 6526 G6•sciences Azur,Valbonne, France. seismic evidence of thrusting is observed [Nichols et al., 3URACNRS-UPMC 1759,D6partement deG6otectonique, Pads. 1990]. Immediately south of 3øN, the East Morotai Plateau aURACNRS-ENS 1516,D6partement deG6ologie, Pads. clogs the bathymetric trench. Part of the 1994 survey, discussedhere, was devotedto the tectonicsignificance of the Copyright1998by the ganerican Geophysical Union. junctionnear 6øN betweenthe centraland southernbranches of Papernumber 97JB02620. the Philippine Trench and the possible relations with the 014õ-0227/98/97JB-02620509.00 northern termination of the Sangihe Trench. In fact, the

933 934 LALLEMAND ET AL.: PHILIPPINE-SANGIHE TRENCHES GENETIC REI JATIONS

EURASIAN P^C}Fc

Shikoku "_•- PLATE P-LATE Basin•

Amami Plateau Plat.eau

/ ur•a•a 0 / Plateau

20'N

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Basin

10'N

Sulu

CAROLINEPLATE

[AHERA MelanesianTrenchlO•

120OE 1130OE PLATE-• 1140'E• )"""'--..•• 1150'E '•

Figure 1. Tectonicsetting of the PhilippineSea Plate modifiedafter Hall et al. [1995]. The stippledarea of the West Philippine Basin is the area produced by spreading after anomaly 20 (-- 45 Ma); anomaly identifications in the West Philippine Basin are from Hilde and Lee [1984]. The box outlines the location of Figure 2. deformationfront of the Sangihe subductionzone has been 2. Geodynamic Background tracedduring the MODEC cruise[Rangin eta/., 1996]up to the junctionbetween the centraland southernPhilippine Trench. 2.1. Converging Plates The deformationis thus distributed in the northern Molucca Seaover the Cotobato,Sangihe, and Philippine subduction The Molucca Plate, between Halmahera and zones,the latter two joining near 6øN. Sulawesi/Sangiheisland arcs on Figure2, has no surface LALLEMANDET AL.:PHILIPPINE-SANGIHE TRENCHES GENETIC RELATIONS 935

15 ø N

WEST HILIPPINE BASIN

/ 10 ø N

MINDANAO south- segm.

o o

5øN

/-%

0 ø N 125ø E 130 ø E Figure2. Mapshowing 'the main structural features in the vicinity of thePhilippine Trench. Bathymetry fromGeneral Bathymetfic Chart of the Oceans (GEBCO) database except inthe area surveyed during MODEC andDAVAPUS oceanographic cruises (dashed domain). Isobaths every 500 m. The thick solid arrow represents theconvergence ofthe PSP relative tothe fixed Eurasia plate after Seno etal. [1993]. The length ofthe arrow exactlyrepresents 90km, i.e., 1 m.y.of convergence. Magnetic lineations (16to 26) and transform faults (dashedlines) are from Hilde and Lee [1984]. The Philippine Trench has been cut into four segments (segm.) (seetext for details). T, trench; P,plateau; E MOR P, East Morotai Plateau; SR, Snellius Ridge, M T, Marl Fault,D G,Davao Gulf. Solid dots are deep-sea drilling sites, numbered DeepSea Drilling Project (DSDP) or OceanDrilling Project (ODP) sites and the absolute age(Ma) of the oceanic crust, e.g., 292 (49). 936 LALLEMAND ET AL.: PHK,IPP]NE-SANGIHE TRENCHF_S GENETIC RELATIONS expressionexcept a collisional complex pinched between pole of •'otationbased on best fit to slip vectorsat trench, thesetwo active volcanicarcs [Silver and Moore, 1978]. Three variesfrom about 30 ø to 35ø in the northernand southern majorlithospheric plates interact in the MoluccaSea southof segmentsof the trench, to 20ø near 10øN and, locally, 0o Mindanao, in the vicinity of the southern branch of the between6øN and 7øN, according to McCaffrey[1996] (about PhilippineTrench (Figure 1), The Eurasiaplate to the west, 20ø moreusing tl•e pole of Senoet al. [1993]). composedof marginal basins (Celebes, Sulu, or South China Sea) separatedby continental fragments drifted from the 3. Gravity, Magnetics, and Seismicity Eurasia continent, convergesobliquely with the PSP to the of the StudyArea east, along the so-called Philippine mobile belt. The mobile belt consistsof sliverarc terranesfringed by N-S subduction 3.1. Gravity zonesand laterally displacedbetween the former EurasiaPlate and PSP [Rangin and PubeIlier, 1990; Pubellier et al., 1992]. A free-air gravity anomaly map was produced after the To the south,the Australianplate movesnorthward relative to cruise[Rangin et al., 1996]. Becauseof the large width of the Eurasiaand interactscomplexly with the NW-SE convergence swath, ship tracks were spaced every 10 nautical miles on between the Eurasian Plate and the PSP. average,so that geophysicalmeasurements such as gravity, magnetics,or reflection seismics were not optimum for 2.2. Philippine Fault generating a homogeneous grid. However, we observe a remarkablecorrespondence between our data and the satellite- The Philippine Trench and fault systemsouth of Luzon is derivedfree-air gravity database of Smithand Sandwell[1995] often cited as a case study for strain partitioning [e.g., Fitch, (Figure 3). This comparisongives more confidence in the 1972; Pinet, 1990; Aurelio, 1992; Quebral, 1994; McCaffrey, gravity values, taken from the global database,along the 1992].The centralpart of the sinistralPhilippine Fault, south trenchaxis. Equidistant bathymetric and gravimetric transects of Luzon, formed in Plio-Pleistocenetimes [Aurelio, 1992; (every150 kin), normalto the trench,are plottedon Figure4. •uebral, 1994]. On the basisof field observations•Quebral et The first observationis that the minimumfree-air anomaly al. [1996] show that the fault reactivates preexisting (FAA = -260 mGaD occurswhere the trenchis deepest(-10100 collisional features.Its southwardpropagation is related to the m) near 10øN(transects 4 and 5 on Figure4) but alsoover a completion of the collision. Motion along the fault broad area west of the junction near 6øN, where the lower diminishesprogressively in Mindanaoin a WNW-ESE trench slope is 6500 _+3000 m deep (tranSect7 on Figure4). trending. horse-tail system (Figure 2). Detailed seafloor There the width of the gravity low (less than -200 regal) is imagery allowed us to map the southernmostoffshore trace of 100 km, yet it is only 30 km near 10øN. The minimum (-260 the PhilippineFault, i.e., the Mati Fault near6ø30'N (Figure2) regal) is shifted westwardby about 25 km with respectto the [Ranginet al., 1996].The fault does not extend in theDavao morphologicaltrench (-220 regal). This has been interpreted Gulf southof Mindanao (Figure 2), as previouslysupposed as (1) low-densitYmaterial belongingeither to the Sangibe [•uebralet al., 1996].;Fhe N-S trending Miangas-Talaud Ridge accretionarywedge or to a subductingbasin of the northern lies in the southernprolongation of the PhilippineFault and Halmahera arc [Rangin et aI., 1996]; (2) the result of has been interpreted as the emerged backstop of the west- extension due to a horsetail term.ination of the Philippine vetgent Sangihe subductionzone [Rangin et al., 1996]. Both Fault (M. Pubellieret al., Upper platedeformation induced by N-S trendingthrusts and NW-SE strike-slip faults characterize subduction of large asperities:Molucca Sea and Mindanao this seismicaJlyactive ridge [Quebral, 1994; Quebral et al., (Philippines-Indonesia)submitted tO Tectonophysics,1997, 1996; A.-G. Bader et al., Active silvering of oceanic crust here. in after referred to as Pubellier et al., submitted anongthe Molucca Ridge (Indonesia-Philippine): Geodynamic manuscript,1997); or (3) causedby the verticaloffse t of the implic•ition, submittedto Tectonics, 1997, here in after subductingPSP alonga N-S trendingwest dippingintraplate referredto as Bader et al., submittedmanuscript, 1997]. thrustlocated beneath the eastern flank bf theMiangas Ridge (Bader et al., submittedmanuscript, 1997). 2.3. Philippine Trench Second,the FAA associatedwith the outer rise (bending)of The origin of the Philippine Trench has been relatedto the the Philippine Sea Plate before •ntering the subductionzone flip of subductionfrom west to east of the Philippine Islands increasesboth in magnitudeand width when approachingthe after the locking of convergenceto the west causedby the junctionnear 6øN. This probably reflects the flexuralresponse collisionof the Palawanand Zamboangacontinental blocks of the PSP at a convextrench junction [Deplus and Dubois, [Barrier et al., 1991]. The collision of the Palawan block with 1989]. Mindorois dated8 to 9 Ma [Ranginet al., 1989;Marchadler Third, the PhilippineTrehch gravity low diverges andRangin, 1990] and a largeamount of shortening(50%) southwardof the junction(Figure 3) into two branches.The could thereafter have been accommodated by intra-arc westernbranch extendssouthward along 127ø+20'E(across a deformation.In this hypothesis,the Philippine Trench is thus steepESE-WNW gradientnear 5øN) throughoutthe Molucca younger than 8 to 9 Ma. The oldest recorded age for the Sea,reaching -180 mGal between the Halmahera and Sulawesi volcanicactivity related to the subductionof the PSPis 2.5 volcanicarcs. This low has b6en attributedto the expression Ma in Leyte [Sajona et al., 1994], which means that of theSangihe and/or Halmahera Trenches [Moore and Silver, subduction along the Philippine Trench probably started 1983].The easternbranch extends southeastward at an angle before4 Ma (takinginto account the minimum time required withthe N-S centralPhilippine Trench. It correspondsto the between initiation of subductionand volcanism). morphologicalsouthern branch of thePhilippine Trench. This The obliquity (defined as the angle between the plate low decreasessignificantly southward reaching 0 mGal.at convergencevector and the direction orthogonalto the trench) 3ø30'N,where the water depth is still6000 m. Theflexure of of plate convergenceacross the Philippine Trench, from a the PSP, when approachingthe PhilippineTrench, is LALLEMAND ET AL.:PHILIPPINE-SANGIHE TRENCHES GENETIC RELATIONS 937

122 ø 124 ø 126 ø 128 ø 130" 132 ø 134 ø

Figure3. Free-airgravity anomaly map from Smith and Sandwell [1995], contours every 20 mGal. Bathymetricandgravimetric transects (1to 9) of Figure 4 shown. T,trench. 938 LALLEMAND ET AL.: PHILIPPINE-SANGIItE TRENCHES GENETIC REI,ATIONS

West PhilippineBasin

m•"'""•v-"--"'-'•2000i x•400o s_9oo Samar6000 -,.•- _:•- • '

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. '•-•=oao •- 4000 • 5000 -Ni•oo•- ' ' ..... ' ' -- • --"6000 Mindanao b 80009000 ';/"X2oo -•1oo © "N-....i.o7 • .,-••- __o • ~200 •2oo• --¾ 4000 • 50o0 N6.Oo•- ' ß -6000 _•;z,• 80009000 Mindanao •200.... .• _• •1oo• ...... -•_ -lOO

-•ooo•-•t• so•gOO ...... •ooo_• aooo Davao Basin '

.... 2• "••000 .• •o'•ø ...... -,ooo•ooo SnelliusRidge ß -.•100i '--- .. ---100 -%•-•oo • -200 . •_.... ,•ooo•i :•• , -••oooToNRidge __,o• ...... lOO __1oo • --.lO• •-200 - ,•000 -- 1000 .?•.• _ ...... •7oo0, ..... ••ooo 5000 5•0

Figure 4. Superimposed(bottom) bathymetric and (top) gravimetrictransects normal to the Philippine Trench.See Figure 3 for location.VE = 10 (bathymetryin metersextrapolated from Mammerickx et al. [1976] and MODEC survey).Gravity anomaliesare expressedin milliGals. The morphologicaltrench is always at a distanceof 250 km from the beginning of the transect.The first 250 km to the west of the transects correspondto the overridingplate, and the 600 km to the rightare the unsubductedPhilippine Sea Plate. LALLEMANDET AL.:PHILIPPINE-SANGIHE TRENCHF3 GENETIC RELATIONS 939 expressedboth in bathymetryand gravity at, andto thenorth al. [1996] as the emergence of the Sangihe backstop. of, thejunction (transects I to 7, Figure4), but it is mainly Earthquakedistribution suggests that much more convergence expressedin the gravity 150 km southof the junction is accommodatedbeneath the Miangas-TalaudRidge (Sangibe (transect8, Figure4) anddisappears south of 3ø30'N(transect subduction) than beneath the inner wall of the southern branch 9, Figure4). Consequently,the PhilippineTrench losesits of the Philippine Trench. gravity/morphologysubduction character south of 3ø30'N, exceptthe local low of- 50 mGalobserved immediately east of Halmaheraisland between 1 and 2øN. 4. MorphologyRevealed by SwathBathymetric Survey 3.2. Magnetics We dividethe 1800 km long PhilippineTrench into four Very low amplitudemagnetic anomalies were recordedon segments,mostly based on depth and the azimuthof the trench the outer wall of the Philippine Trench during the MODEC axis (Figures2 and 7). cruise.Mrozowski et al. [1982] suggestedthat the subducting PhilippineSea crust, south of Luzon,was not typicaloceanic 4.1. Northern segment crest but rather a thickenedcrust as indicatedby slightly highergravity anomaliesand the lack of clear magnetic The northernsegment of the trench, between 15ø30'Nand lineations.They distinguishedthe "main Philippine Basin" 12øN,has a meandepth of 6500 m (seeFigure 2). The Benham from the "southernsubbasin" along latitudes 7 + 1ø N. This Plateau clogs the trench to the north so that the trench axis southern sub-basin is = 100-200 m shallower than the main locally trends N60øW, whereas it trends N25øW south of the basin.This is due both to a slightly greater sedimentcover in plateau.The obliquityof the convergencebetween the lbrearc thesubbasin and to a shallowerdepth of the top of its acoustic sliverand the PSP could be lessthan 30 ø accordingto thehigh basement.Reconstructions of the PSP generally take the level of slip partitioningat theselatitudes [McCaffrey, 1996]. southernsubbasin to be older than 50 Ma [e.g., Hall et aI., 4.2. North-Central Segment 1995].NW-SE trendinganomaly 21 has been drilled at Deep SeaDrilling Project (DSDP) site 291 giving an Eoceneage of This segment of the trench, between 12øN and 8ø40'N, 48 Ma (Figure 2). South of there, the crust is older and of trends roughly N18øW. Figure 8 gives a sample of the uncertainorigin [Hall, 1996]. Anomaly 26 (60 Ma) has been bathymetricmap obtainedalong the ship'smute, on which the tentatively identified by Hilde and Lee [1984] between depth of the trench floor is often 10000 m; 10100 m was latitudes7 and 8øN just off the trench, as shown on Figure 2. reachedat 9ø22'N,which ranksthe PhilippineTrench among Using extrapolationsfrom their magnetic map, the structural the deepestin the world.The slabreaches 200 km depthin the grainof the crust, parallel to the magneticlineations, should southernpart of this segment(Figure 5). strikeN130 ø + 20ø dependingon the latitude, while fracture The mean dip of the PSP, just before entering the zones should trend N35 ø _ 5 ø. subductionzone, is 6ø. Normal faulting is observednear the trench with maximumvertical scarp offsetsreaching 1500 m 3.3. Seismicity at 10ø30'N.Mean verticaloffsets generally range between 250 The Benioffzone of the west dippingPhilippine Sea slabis and 500 m. Scarpstrend between N30øW and N-S (see rose well definedbeneath the PhilippinesIslands from 15øN(at the diagram, Figure 8) slightly oblique to the N30 ø to N50øW latitude of south Luzon) to 7øN (the latitude of South oceaniccrust fabric (seemagnetic lineations, Figure 2). It thus Mindanao)reaching 200 km in depthfrom 7øN to 10øN.South seems that most of the faults were formed recently as a of 7øN, seismicityis inadequateto see the slab geometry consequenceof the strongbending of the PSP. They mostly [Cardwellet al., 1980] becausethe PhilippineSea slabis too face the trench, but some face eastward. The trench axis short and the contorted Molucca slab interferes. Cardwell et al. appearsas a successionof en 6chelon narrow basins,each 30 [1980]defined a tracefor the PhilippineSea slab as far south to 60 km long, trendingN25øW, regularly offset along N-S as 3øN off Morotai. Cross sectionsof seismicitybetween segments10 km long. latitudesIøN and 10øN(Figure 5) illustratethe complexityof The lowermostinner slope dips about 20ø all in the survey earthquakedistribution, especially between 4øN and 7øN. area (between8ø50'N and 10ø40'N) and is deeperthan 8000 m. Earthquakehypocenters reflect the mechanicalidentity of the Such a high taper (20ø of surface frontal slope + 6ø of PSPlithosphere under compression,west of Halmahera subducting plate slope) necessarily implies inner slope Trench,down to IøN (Figure5), but interplateseismicity is instabilities and gravitational collapse of material down into scarce,as indicatedby a plot of focal mechanismsof shallow the trench, using reasonableparameters of friction and pore earthquakes6xtracted from the Harvard centmidmoment tensor fluid pressuresin, and at the base of, the margin[Lallemand et (CMT)data set (Figure 6). al., 1994]. As no filling of the trench is observed(V-shaped ThePSP is undergoingtrench-perpendicular bending north crosssection), we concludethat the entire volume of slumped of 6øN,along or. immediately east of'the trench axis, whereas material is continuously underthrust beneath the inner trench no bendingrelated extension is observedin the southwithin wall. The steepnessof the lower part of the inner wall may the samepl'ate. This couldreflect the existenceof E-W result from the high interplate macrofrictiongenerated by the cornpressivetectonic forces acting on the subductingPSP seafloorroughness (scarps) but also to the subcrustalremoval alongthe southernbranch of the trenchwhich inhibits any of the marginnear the toe. shallowextension within the plate. 4.3. South-Central Segment It is clearfrom seismicity records (Figure 6) thatthe main interplateseismogenic zone, south of 6øN,corresponds to the In Figures9 and 10, we did not separatethe south-central N-SMiangas-Talaud Ridge which is interpretedbyRangin et (between8ø40'N and 6ø10'N) and southernsegments (between 940 LALLEMAND ET AL.: PHILIPP•SANGIHE TRENCHF• GENETICRELATIONS

PhilippineT. PhilippineFault ru SangiheT. I '- 500! I I I71 I I;• L1500 J km '- I I I 50•0, _ I I I' I I I'."•' 0 .,..I.[ [ [ ] 0 ' ß ß' ':.'"•,•..'::•.- ß a,.: %1. •"..... ":•:- ß ß ':t?.•...:'.' ...%.?• ' ;t'•..ß '".- %-'½ %

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PhilippineT. PhilippineFault SangiheT.[tu I ,, o• CotobatoZ.[ I • mo NegrosT. .PhilippineZ.• = 500 L L L •5oo•m ! ! ! ! I I ITI I ß I ß! !• [_J 0 I i I .15•1001! I I I I I:_I ..:.. . •L•-•.•. . .

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PhilippineFault I • PhilippineT. •>

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Philippin,eFault • CotobatoT.Philippine T. 0 .I i ] I .I' I ' I 1•:•'1151O0 •. 57•..•.•!•ß '•. ,.•.?'.•- ' m km : .: .•'.

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HalmaheraT. PhilippineFault m Sangih m m CotobatoT.Ph(lip.•ne T.• • 500 m 1500km • 500 • km ß I .i_..]I I I ] I I ] I I_'•'lJ I [ .]. I I I I I I ] ] ]. ];'?' 0 . ß.;¾,.:: .....•::o'.:,' :.-•.:•=•.•: "::.:'- 0 .... . •.,..'...,,.,.•...... ¾.-.....:?: .:- ß ;;" ;.

. -500 • -500 ••. km km .-'

Figure 5. Cross sections,down to 700 km depth, with earthquakehypocenters projected along vertical planesat latitudes1 øN to 10øN,with a width of 1o of latitude,between longitudes 118øE (0 km) and 130øE (1332 km). Data sourcescome from (1) the Council of the National SeismicSystem which use data from the NorthernCalifornia Earthquake Data Centerfrom 1964 to 1977 (magnitudesMw = 4 to 7.9) and (2) Harvard CentroidMoment Tensor (CMT) hypocentersfrom 1977 to 1994 (magnitudesMw = 4.9 to 7.7). T, trench. LALLEM,•qD ET AL.: PHILIPPINE-SANG1HE TRENCHES GENETIC RELATIONS 941

125 126 127 128 129 130 11 11

Focal mechanismsof earthquakes shallower than 60 km (January1977 to December 1994; Harvard CMT data set) lO 10

--½'" 9 043

15 15

.,,.,, 8

..,..

•...'•,_ ".z. PujadaP trench •"'~7•,•-,*•-, r• _MODEC_ 4: .- ß junction

5 • • ß/ . •.15-33• • • 5 Ta,aud&iI /

2 'b.•mJ--•••/ -••( .:.'/Morotai / I 2 125 126 127 128 129 130

Figure6. HarvardCMT solutions for shallow earthquakes (0-60 km) along the Philippine Trench (1977 to 1994).Size of beachballsis a functionof earthquakemoment. MODEC label indicates sections of plate boundarystudied during MODEC cruise [Rangin etal., 1996]. Numbers arecentroid depth of earthquakes (km) givenfor some events. Note the frequent shallow seismicity beneath the Philippine margin north of 6øNand beneaththe Pujada-Talaud Ridge south of 6øN. There are very few shallow earthquakes onthe west side of the southernbranch of thePhilippine Trench, indicating that seismic coupling is very low compared to the north. Mostearthquakes aredeeper than 100 km and probably correspond tothe Molucca Sea Benioff zone dipping in the oppositedirection. P, peninsula.

LALLEMANDET AL.: PHILIPPINE-SANGIHE TRENCHES GENETIC RELATIONS 943

E 126' 30' E 127' I 9800m• • 9600m?

, , !0000m ",

IOOOOm

9900

-t- N I0' ; --- -I-'N I0

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10050m +N 9'30'

3020 '•A ' ' %' ,O,00m'.. Om

+N9' + -., "':½'-? N 9 E 126' 30' E 127'

Figure 8. Bathymetricmap obtainedusing a singletrack along the Philippine Trench axis afterthe MODEC cruise(DAVAPUS cruise). Isobathsevery 100 m. Structuralinterpretation is shownat the samescale on the left, with dashedline for the axisof the trench,solid line for the deformationfront, and triangleson the overridingplate side. Barbs are on the downfaultedside alongthe normalfaults of the subductingplate. Diagramof oceanicfaults direction is alsoshown (azimuth/cumulate fault lengths).

5. StructuralSynthesis and Discussion oldest. One can thus imagine that subduction first initiated 5.1. PhilippineTrench From North to South ne,tr 9øN and then propagatedto both north and southat some point, at least4 m.y. ago accordingto the oldestage of the Thegeometry of the PhilippineSea slab seems rather volcanic arc. To the north, the convergencebetween the symmetricalboth north and south of 9øN.The slabis longest Eurasiaplate and the PSP is transferredfrom the westdipping v•herethe trench is deepestand where arc magmatism is the PhilippineTrench to the eastdipping Manila Trench(Figure 944 LALLEMAND ET AL.: PHILIPPINE-SANGIHE TRENCHES GENETIC RELATIONS

trench I EI2T • axis El2 • 3,2,

25 km (• I3f02/93 outer 's\o•e slope

26111/8118 km

26/05/81 10 km

trench unction

20/07183 03 slump 17 km 0 scar 4--- r- :1,- ; 22/04183 36 km (

Eurasia ',,: Philippine Sea -- plate plate

"' 05106187 15 krn ,-t"',- ___1. i'Dri-'- i

3305/06187 km • i 05•06•87 16 krn

..r...•.---Sangihe •:" ';'~' Snellius' • :-' "•'\ •'.c_'•,'.•,; \ ex /-z. \ outer slope deformation front Ridge Figure 9. Bathymetric map obtained during the MODEC cruise in the vicinity of the trench junction southeastof Mindanao. lsobathsevery 100 m. Stars representsepicenters of shallow earthquakesfor which focal mechanismsare available (see Figure 6). Date and depthof eventsare indicatednear the beachballs.

1). Both subductionzones overlap between 13øN and 16øN. To The importantpoint is that seismicityoccurs down to 200 km the south,the convergenceis transferredfrom the Philippine over a broad area between latitudes 4øN to 8øN and between Trench mainly to the Sangihe Trench as attested by the 125øEand 128øE.This is the area of the junction betweenthe regional seismicity (Figure 6). central and southernPhilippine Trench and the Sangihe Trench.Interpretative E-W crosssections will be furthergiven 5.2. Molucca Sea and Philippine Sea Slabs (Figure 14), based on the detailed bathymetryfrom the We have used closely spaced latitudinal cross-sectionsof MODEC cruise and onland geology, in agreementwith seismicity (Figure 5) to propose an isopach map of the seislnicity and gravity data. Benioff zones of the Molucca Sea and Philippine Sea slabsin 5.3. The 20 to 30 m.y. Long-Lived Sangihe the study area (Figure 13). The doubly verging Molucca slab, Trench describednear Mayu and Tifore Islands(see locationon Figure 2) by McCa.[fi'eyel al. [ 1980] is recognizedup to 4øN with a The west dippingMolucca Sea slab reachesthe mantle NNE-SSW trending hinge. The west dipping slab reaches discontinuitybeneath the Celebes Basin, attestingthat at almost 700 km beneath the Celebes Basin (see the cross least1000 km of plateconvergence has been accommodated section at 7øN on Figure 5) and 200 km east of Halmahera acrossthe SangiheTrench. This is clearlythe deepestslab island(between IøN and 3øN on Figure5). The contourlines known along the Philippine mobile belt. Late Miocene differ slightlyfrom the geometryproposed by Cardwellet al. volcanicsattributed to the Sangihesubduction were found in [1980], especially between 4øN and 8øN, where it seems .st•uthwesternMindanao [Pubellier et al., 1991].Tholeiitic and pointless to choose between the various slab combinations. calc-alkalinemagmatism of early Miocene (19 and 22 Ma) and LALLEMAND ET AL.' PHILIPPINE-SANGIHETRENCHES GENETIC RELATIONS 945

?--9100m ••'•-• 9500m 9600rn•,•"•t

+ N7'

\ 9700 rn

I ••tII! TRENCHJUNCTION

c• 8600 rn --l*'--perspective shaded c .•,,: viewof figure 11 -...-,Q + N 5'30'

bathymetric map of figure 12 9000m [ • \ ? %,•, %//750m1

8500 m 750m 8250 m E 127' •3 E 127' 30' E 128' E 128' •0' I I I l,

Figure 10. Structural interpretation of bathymctricmap (Figure 9) with dashed line for the axis of tile trench, solid linc for the deformation front, and triangles on the overriding plate side. Barbs are on the downfaulted side along the normal faults of tile subductingplate. Diagrams of fault directions are also sllown for both south-centraland southernsegments. The four cornersdelineate tile location of tile area used for tile shadedperspective view of Figure 11. Thin dotsoutline the rectangulararea of Figure 12.

even Oligocene(28 Ma) have also been reportedin tile 5.4. The l0 m.y. Long-Lived Halmahera northernarm of Sulawesiby Priadi [1993]. Presentvolcanic Subduction Zone and Its Supposed activityalong the Sangihevolcanic arc armthe highlevel of Northern Extension seismicitybeneath tile Sangibelorearc attest to convergence beingstill active along this subduction zone. The 1000km of The east dippin,, Molucca Sea slab reaches200 km depth convergenceduring 25 m.y. gives a mean convergencerate of belween latitudes I and 3øN (Figures 5 and 13). Tile volcanic 4 cm/yr, which is rensonable.This is a minimum estimate arc, relatedto eastwardsubduction of the Moluccu Sea plate at becausewe ignore the possibility of theslab passing tllrougl'• the Halmahera Trench was initiated at tile be2innin,, of the the"6'70 kin" mantle discontinuity. Tile SangihcTrench has late Miocene lSt(t)•i HaLira and Hall, 1991'1, with a brief beentraced east of tileTalaud Ridge by theMODEC team cessatio• of volcanism durin,,z-- the Pleistocene IHa!l, 1987]ß [Ranginet al., 1996]and might extend up to 7øNfollowing The 3u0 km long, at maximum, east dipping slab obtained thebase of tile steep middle slope (Figures 2 and 9). duri!lg 10 m.y. years gives amax.imum mean rate of 3 cm/yr 946 LALLEMAND ET AL.: PHILIPPINE-SANGIHE TRENCHES GENETIC RELATIONS

rhombohedral '- small basin

',• -- normal zone of • "' trench faults accommodation

depth •n meters

• • • • axis

Fi•ur• ]l. Perspective•hadcd vicw of par[ of the somhcmPhilippin• Trench. Vicw looMn• toward th• •W. The frontal •ccfion cro•c• the 5an•ihc deformation front, •l•c 5nc]]iu• Ridge, and •hc lower PhilippineTrcnch. Wc•t t•ci• •carps of normalt'au]ts arc clc•rly ob•crvcdon •h• oumr wall. Th• castward t'l•nk (•carp) of the 5ncllius Ridge is observable in cms• section. 5mall rhomboh•dral detected on thc low•r •]op• just north of [his scarp. The •-5 [r•ndin• 5an•ih• ticformation I•11owcdon the wc•t •idc of the diagram. Vertical cx•crmion = x 5.

for convergence across the Halmahera Trench. The present deformation of the forearc basement rather than to the deeper deformation front turns eastward just north of Morotai island Molucca Sea plate. The questionof whether or not the Molucca (Figure 2) as revealedduring the MODEC cruise [Rangin eta!., plate is faulted at its hinge is not critical for the present 1996]. However, two possible lines of evidences are present, model. According to Hall's [1996] Cenozoic reconstruction pertaining to a northern extension of the Halmahera arc: (1) model of SoutheastAsia, the Molucca Sea was a very wide area based on stratigraphic correlations, Hall [1987] considered formed by trapping of Indian Ocean lithosphereat 25 Ma. Its that the basement of East Mindanao and the Halmahera aoe should thus be pre-Tertiary. Such an old and dense, basementcomplex were part of a single arc and forcare of Late oceanic lithosphere should sink into the mantle after arc-arc Cretaceous - Early Tertiary age; (o) according to collision rather than be accreted, in agreement with the morphological and •ravimetric data recorded during the deepeningof the slab northwestward{,Figure 13). Depending MODEC cruise, the Snellius Plateau, which is pinchedbetween on the uppermantle viscosity, the invertedU-shaped slab will the Sangihe and the southern Philippine Trenches, might sink vertically(low viscosity)or will sink towardthe WNW in represent a relict of the northern Halmahera Arc between the direction of the longest slab (high viscosity plus slab Morotai and Mindanao. In this hypothesis, the former pull) requiring the unbendingof the opposite-vergingslab. Halmahera Trench, north of 3øN, should have been subducted beneath the Sangihe deformation front. 5.6. Progressive Subduction and Accretion of the Northern Extension of the Halmahera Arc 5.5. Fate of the Molucca Sea Slab From south to north, we observe (l) the active Halmahera We have tentatively interpreted three latitudinal cross volcanic arc associated with a clear deformation front sections of seismicity in the junction area between the (Halmahera Trench) for which, probably recent, northern Philippine, Sangibe, and former Halmahera Trenches (Figure terminationhas been tracedduring the MODEC cruiseuntil 14). We did not cut the Molucca Sea slab (west dipping high- 3øN; (2) a remnant of the Halmahera Arc (Snellius Ridge) angle reversefault) along its hinge as proposedby Cardwell et entering the Sangihesubduction zone; and (3) an exotic al. [1980] and McCaffi'eyet al. [1980] for the southernpart of terrane,which shouldrepresent the northernextension of the the Molucca Sea and by Bader et al. (submittedmanuscript, Italmahera Arc, accretedeast of the Philippine t;ault in 1997) for the northern part, becauseit was not necessarily lXlindanao. Intensedeformation and stackingof lithospheres requiredto fit the seismicityor gravity data. Inasmuchas the are evidencedfrom seismicitydistribution between 4øN and Miangas-Talaud Ridge could represent the basement of the 8øN (Figure5). The threeinterpretative cross sections attempt Sangiheforearc of oceanicorigin [Ranginet al., 1996] (Bader to reconcile the various data in this complex area. et al., submittedmanuscript, 1997), we think that it is simpler 5.6.1. The 5øN section. At this latitude, the to relate the high-angle reverse faulting mechanismsto the MoluccaSea slab is sufficientlyshallow to be responsiblefor LALLEMANDET AL.:PHILIPPINE-SANGIHE TRENCHES GENETIC RELATIONS 947

E127ø50 ' E128 ø

NORMAL N5ø30 '

LOWER SLOPE

N5o20 '

isobaths each 20 rn

0 5 km I ! I

Figure 12. Bathymetricmap focused on themain N60øW Fault Zone, showing both normal and left-lateral strike-slip components(see text for details).

the Sanoihe Arc volcanism The Snellius Rid.o,c which a/., 1991]. A swarmof earthquakesoccurred at aboutthe same represents the relict of the northern extension of the clcpthas 1ø to the southwithin the subductingPSP, suggesting Halmahera Arc, and the lorearc domain are subducted beneath a strong intraplate deformation. the Sangiheforearc, causing the uplift of Talaud island.The formerHalmahera Trench is nowburied underneath nearly 100 km of lithosphere, and the relative motion between the 5.7. Transfer of Convergence From the Sangihe subductedHalmahera forearc and the westdipping Molucca Trench to the Philippine Trench and the Origin of slabcould now occurin the othersense, compared with the the Philippine Trench prexiousconvergence between the arc and the eastdipping Combiningdata from varioussources, we proposea simple slab.According to seismicitydistribution, the PhilippineSea geodynamic story for the study area. We consider that the slaband the east dipping Molucca slab should join at depth. presentsituation at IøN prevailed 6 m.y. ago at 8"N. At that 5.6.2. The 6øN section. Earthquakesdeeper than time, the Sangihe Trench and the Halmahera Arc extended 200km mightreflect the presenceof the •inkingMolucca northward until 8øN. The Halmahera Arc consisted of en slab.We interpretthe 200-kin-thickseismic zone belowthe dchelonN-S trendingarc segmentsoffset left-laterally along Miangasislands as relatedto the stackingof the Sangibe NW-SE transformfaults. The cross-sectionalong 5øN {,Figure lorearcand HalmaheraArc and forearclithospheres. The 14) could be representativeof the situationalong 8øN, 4 m.y. positivebuoyancy of arc andforearc material prevents their ago. The northern segmentof the Halmahera Arc was accreted subductiondeeper in the mantle.Their subcrustalaccretion to the Eurasiamargin and the subductionjumped eastwardfrom maycause the local steepeningof the PhilippineSea slabwith the old SangiheTrench to the nascentPhilippine Trench. The eventualintraplate shearing associated with the 50- to 80-km- failure of the Philippine Sea lithosphere just east of the deepswarm of earthquakes.The eastwardverging thrusts Italmahera Arc initiated either at 8øN or slightly north, as associatedwith the SaranganiPeninsula and the IVliangas suggestedby the deepesttrench floor and presentPSP Benioff Ridgewere reported by Rangin et al. [1996]. 7one and rapidly propagated both northward and southward. 5.6.3. The 7øN section. At this latitude, the The deformed suture zone between the host margin and the •1oluccaSea slab is onlydetectable byearthquake hypocenters :.tccreted tcrr:tne was then reactivatedinto a strike-slipfault: deeperthan 400 km.The plate configuration is similar with the PhilippineFault, as a responseof slip partitioningunder thoseof theprevious section at 6øN.The section crosses the oblique convergence.Today, the old Sangihe Trench is ß',outhernmost termination of the PhilippineFault which inactive north of 6øN, because the Philippine Trench and boundsthe accreted Halmahera Arc lHa!!,1987; Pubellier et PhilippineFault can accomn•odatemost of the convergence, 948 LALLEMAND ET AL.: PHILIPPINE-SANGIHE TRENCHES GENETIC RELATIONS

Present Area of the Moluccaslab Contactzone

Hingeof the Moluccaslab Depth contoursof the Moluccaand Philippine slabs in km

Mostdeformationactive fronts Accommodationzone Activevolcanoes

Figure 13. Interpretative structuralmap of the surveyedarea. The grey pattern correspondsto the surface area of the Molucca slab subductingwestward beneath the Sangihe Arc and eastward beneath the Halmahera Arc. Its lateral extent and depthcontours of its Benioff zone estimatedfrom crosssections of seismicity(see Figure 5). The solid arrow representsthe plate convergencevector betweenPhilippine Sea and Eurasia plates. Its length is 90 km, i.e., I m.y. of convergence.Depth contoursof the Philippine Sea slab beneathMindanao are also plotted. The hatched are correspondsto the joining of the east dipping Molucca slab and the west dipping Philippine Sea slab at depth (contactzone). The three E-W transectslocate the interpretativecross sectionsof Figure 14.

whileit is still activeto the southbecause the PhilippineSea 3. The strong bending of the PSP at the trench north of slab joined the east dipping slab at depth soon after its 6øN, as indicated by earthquake solutions and vertical offsets subduction.A possible scenario for thenear future in thestudy of normal faults, can be caused by accreted terranes which area could be the following: (1) the Philippine Fault will locally steepenthe subductingPSP. propagatealong the Talaud-MiangasRidge, (2) the collision 4. The N60øW zone of accommodation could be inherited complex located between the ridge and the trench will be from an ancient transform zone between the northernmost upliftedand eroded, allowing the SnelliusRidge to outcrop, segmentof the HalmaheraArc and the "Snelliussegment of the and (3) the sinking of the Molucca Sea slab in the north will arc," which is reactivated near the trench as the PSP subduction unblockthe presentlydecelerating southern PSP subduction. is slowed down south of 5øN. 5. The wide gravity low observedbeneath the middle and lower slopesbetween 5 ø and 7øN correspondsto the areawhere 5.8. Agreement of the Geodynamic Model With the hinge of the Molucca slab detachesfrom the overlying the MODEC Observations lithospheres(Figure 14). This processstarts near 5øN in the 1. The deepestsegment of the PhilippineTrench near 9øN, northwesternextent of the accommodationzone, suggestinga as well asthe maximumdepth reached by thePSP slabbetween possiblecommon origin. Once the Molucca slab hasdetached, 7øN and 10øN,coincides with the areaof possibleaccretion of one may expect an isostaticrebound and thus uplift of the the northernsegment of the HalmaheraArc 4 to 5 m.y. ago, overlying lithosphere, as observed north of 7øN. andsubsequent initiation of the PhilippineTrench. 2. The middle slope outlines the Sangihedeformation 6. Conclusion front, called the Sangihe Trench south of 6øN where the Sangihesubduction is still active,and the Sangihesuture The detailedanalysis of the geophysicaldata acquired zone,north of 6øN,between the accretedHalmahera Arc (lower duringthe MODEC cruise,combined with otherregional data, slope)and the deformed Sangihe accretionary wedge. allowed us to better understand the distribution of the LALLEMANDET AL.:PHILIPPINE-SANGIHE TRENCHES GENETIC RELATIONS 949

middleslope (Sangihe suture zone) CotobatoTrenchCotobatoArc PujadaPeninsula •, transferzone Philippine(Mati)I Fault• PhilippineTrench

200km ,, , ,

middleslope SaranganiPeninsula (Sangihesuture zone) CotobatoTrench • MiangasRidge ß I PhilippineTrench

•:: -' • - • ß ---- .....

Sangihe SangiheTrench Cotobato Trench forearc basin SnelliusRidge

Figure 14. Interpretativelithospheric cross-sections along 5øN (southernsegment), 6øN (trenchjunction), and 7øN (south-centralsegment). These sectionsare based on seismicity(grey dots), global geodynamic context, and surface observationsfrom the MODEC cruise. Thick lines are plate boundaries.Depths are in kilometers.See text for detailsand Figure 13 for location. deformationin the vicinity of thePhilippine Trench junction northern part of the Halmahera arc and forearc to Mindanao near6øN. The originof thePhilippine Trench is controversialduring early Pliocene triggeredthe failure of the PSP along the in thatit is commonlyattributed to the8 to 9 Ma collision Philippine Trench. The trench has propagatedboth north and betweenthe Palawan and Zamboanga continental terranes, south soon after and was recently slowed down along the whilearc volcanism and slab length suggested a younger age southernbranch because the PSP slabjoined the MoluccaSea of about4 to 5 Ma. We suggestthat. the accretionof the slab at depth. 950 LALLEMANDET AL.: PHILIPPINE-SANGIHE TRENCHES GENETIC RELATIONS

Acknowledgments. We acknowledgeS. Dominguezand Y. Font MoluccaSea. in The Tectonicand GeologicEvolution of Southeast for their assistancein using GMT softwareand providingthe basic AsianSeas and Islands,Part II, Geophys.Monogr. Ser., vol. 27, editedby D. E. Hayes,pp. 360-371, AGU, WashingtonD.C., 1983. documentsfor somefigures. J.-F. Brouillet and A. Delplanqueredrew Mrozowski,C. L., S. D. Lewis, and D. E. Hayes, Complexitiesin the someof the figures.R. Russoand C. Wibberleyare acknowledgedfor tectonicevolution of thewest Philippine Basin, Tectonophysics, 82, theirimprovement of theEnglish in the firstand second versions of the 1-24, 1982. manuscriptrespectively. J. Ch6ry discussedsome problems related to Nichols,G., R. Hall, J. Milsom,D. Masson,L. Parson,N. Sikumbang,B. the mechanicsof the lithospherewith us. We thank B. Taylor, R. Dwiyanto, and H. Kallagher, The southerntermination of the McCaffrey,and D. Jurdyfor their constructivereviews. CNRS-INSU PhilippineTrench, Tectonophysics, 183, 289-303, 1990. andIFREMER providedgrants for participatingin the MODEC cruise Pinet, N., Un exemple de grand d6crochementactif en contextede and financialsupport for the data reductionand analyses.The R/V subduction oblique: La faille Philippine dans sa partie septentrionale(inFrench with 'Englishabstract), Doc. Travaux,8, L'Atalante crew is thankedfor their very professionalwork. We also 391 pp., 1990. acknowledgeour Filipino and Indonesiancolleagues who were on Priadi,B., G6ochimiedu magmatismede l'ouestet du nordde Sulawesi, boardand with whom we had very interestingdiscussions. Contribution Indon6sie--Tra•agedes sourceset .implicationsg6odynamiques, UMR "G6ophysique& Tectonique,""G6osciences Azur nø160," th•se,240 pp.,Univ. PaulSabatier, Toulouse, France} 1993. "G6otectonique,"and "G6ologie de I'ENS". Pubellier,M., R. Quebral,C. Rangin, B..Deffontaines,C. Muller, J. Butterlin, and J. Manzano, The Mindanao.collision zone: A soft References collisionevent within a continuousNeogene strike-slip setting, J. SE Asian Earth Sci., 6, 239-248, 1991. Aurelio, M. A., Tectoniquedu segmentcentral de la Faille Philippine-- Pubellier,M., N. Pinet,and C. Rangin,Docking of the Philippinemobile Etudestructurale, cin6matique et 6volutiong6odynamique(in French belt againstthe Eurasianmargin: Closure of the SouthChina Sea, in with English abstract),Mdm. Sci. Terre Univ., 92-22, 420 pp., 1992. Marine Geologyand Geophysicsof the SouthChina Sea, edited by Barrier, E., P. Huchon, and M. Aurelio, Philippine fault: A key for Jin X., H. R. Kudrass,and G. Pautot,pp. 71-82, China OceanPress, Philippinekinematics, Geology, 19, 32-35, 1991. Hangzhou,China, 1992. Cardwell,R. K., B. L. Isacks,and D. E. Karig, The spatialdistribution of Quebral,R. D., Tectoniquedusegment meridional deia Faille earthquakes,focal mechanismsolutions, and subductedlithosphere Philippine,Mindanao oriental, Philippines: Passage d'une zone de in the Philippine and northeastern Indonesian Islands, in The collision /tune zone de d6crochement (in French with English Tectonic and Geologic Evolution qf SoutheastAsian Seas and abstract),Mdm. Sci. Terre Univ., 94-00, 1994. Islands, Part L Geophys.Monogr. Ser., vol. 23, editedby D. E. Quebral,R., M. Pubellier,and C. Rangin,The onsetof movementon the Hayes,pp. 1-35, AGU, WashingtonD.C., 1980. PhilippineFault in easternMindanao: A transitionfrom a collisionto Chamot-Rooke, N., C. Vigny, A. Walpersdorf, X. Le Pichon, P. strike-slipenvironment, Tecumics, 15, 713-726,1996. Huchon, and C. Rangin, Sundaland motion detected from Rangin,C., andM. Pubellier,Subduction and accretion of PhilippineSea GEODYSSEA GPS measurements,Part I, Implicationsfor motionat Platefragments along the Eurasianmargin, in Tectonicsqf Circum- Sundatrenches, paper presented at GEODYSSEA (Geodynamicsof Pact•ficContinental Mart,ins, edited by J. Aubouinand J. Bourgois, the Southand SouthEast Asia) ConcludingInternational Symposium, pp. 139-164, VSP, Utrecht,1990. EC-ASEAN, Penang,Malaysia, April 14-18, 1997. Rangin,C., M. Pubellier,and L. Jolivet,Collision entre les margesde Deplus,C., and J. Dubois,Flexural responseof the oceaniclithosphere l'Eurasieet de l'Australie:Un processusde fermeturedes bassins at an arc-arcjunction: Implication for the subductionof aseismic marginauxdu Sud-EstAsiatique, C. R. Acad.Sci., Ser. II, 309, 1223- ridges,Tectonophysics, 160, 63-73, 1989. 1229, 1989. Fitch, T. J., Plate convergence, transcurrent faulting and internal Rangin,C., D. Dahrin,R. Quebral,and the MODEC ScientificParty, deformation adjacent to southeastAsia and western Pacific, J. Collision and strike-slip faulting in the northern Molucca Sea Geophys.Res., 77, 4432-4460, 1972. (Philippinesand Indonesia): Preliminary results of a morphotectonic Fujiwara,T., K. Tamaki, H. Fujimoto, T. Ishii, N. Seama,H. Toh, K. study,in TectonicEvolution of SoutheastAsia, edited by R. Hall and Koizumi,C. Igarashi,J. Segawa,and K. Kobayashi,Morphological D. Blundell,Geol. Soc. Spec. Publ., 106, 29-46, 1996. studiesof the Ayu trough, Philippine Sea-CarolinePlate boundary, Rangin,C., M. Pubellier,C. Vigny, X. Le Pichon,M. Aurelio,R. Geophys.Res. Lett., 22, 109-I 12, 1995. Quebral,and A.-G. Bader,Distribution of deformationacross the Hall, R., Plate boundaryevolution in the Halmaheraregion, Indonesia, Sundaland/PhilippineSea Plate boundaryfrom GPS results,paper Tectonophysics,144, 337-352, 1987. presentedat GEODYSSEA(Geodynamics of the Southand South Hall, R., ReconstructingCenozoic SE Asia, in Tectonic evolutionof East Asia) Concluding International Symposium,EC-ASEAN, SoutheastAsia. edited by R. Hall and D. Blurtdell, Geophys.Sot:. Penang,Malaysia, April 14-18, 1997. Spec.PubL, 106, 153-184,1996. Ranken,B., R. K. Cardwell, and D.E. Karig, Kinematicsof the Hall, R., J. R. All, C. D. Anderson, and S. J. Baker, Origin andmotion PhilippineSea Plate, Tectonics, 3, 555-575,1984. historyof the PhilippineSea Plate, Tectonophysics,251, 229-250, Sajona,F. G., H. Bellon,R. C. Maury,M. Pubellier,J. Cotten,and C. 1995. Rangin,Magmatic response to abruptchanges in geodynamic Hilde, T. W. C., and C.-S. Lee, Origin and evolution of the West settings:Pliocene-Quatemary calc-alkaline and Nb-enrichedlavas PhilippineBasin: A new interpretation,Tectonophysics, 102, 85-104, fromMindanao (Philippines), Tectonophysics, 237, 47-72, 1994. 1984. Seno,T.,. The instantaneousrotation vector of the PhilippineSea Plate Huchon,P., Commenton "Kinematicsof the PhilippineSea Plate" by B. relativeto theEurasian Plate, Tectonophysics, 42, 209-226,1977. Ranken, R. K. Cardwell, and D. E. Karig, Tectonics, 5, 166-168, Seno,T., S. Stein,and A.E. Gripp,A modelfor the motionof the 1986. PhilippineSea Plate consistent with NUVEL-1 and geological data, Karig, D.E., and R. K. Cardwell,Reply, Tectonics,5, 169-170,1986. J. Geophys.Res., 98, 17,941-17,948,1993. Lallemand,S. E., P. Schniirle,and J. Malavieille, Coulombtheory Silver,E. A., andJ. C. Moore,The Molucca collision zone, Indonesia, J. applied to accretionary and nonaccretionarywedges: Possible Geophys.Res., 83, 1681-1691,1978. causesfor tectonic erosionand/or frontal accretion,J. Geophys. Smith,W. H. F., and D. T. Sandwell,Marine gravity field from Res., 99, 12033-12055, 1994. declassifiedGeosat and ERS1 altimetry,Eos Trans.AGU, 76(46), Lewis, S. D., and D. E. Hayes, The tectonicsof northwardpropagating Fall Meet. Suppl.,F156, 1995. subructionalong easternLuzon, PhilippineIslands, in The Tectonic Sufni Hakim, A., and R. Hall, Tertiary volcanicrocks from the and GeologicEvolution of SoutheastAsian Seas and Islands,Part II, HalmaheraArc, EasternIndonesia, J. SEAsian Earth Sci., 6, 271- Geophys.Monogr. Ser., vol. 27, editedby D. E. Hayes,pp. 57-78, 287, 1991. AGU, Washington,D.C., 1983. Mammerickx,J., R. L. Fisher,F. J. Emmel,and S. M. Smith,Bathymetry A.-G.Bader, M. Pubellier,and C. Rangin,URA CNRS-ENS1516, of the east and southeast Asian seas, Scripps Institution qf D6partementde G6ologie,21 rue Lhomond,75231 Parisc6dex 5, Oceanography,La Jolla, Map, 1976. France. Marchadier,Y., and C. Rangin,Polyphase tectonics at the southerntip J.-P. Cadet, and B. Deffontaines,URA CNRS-UPMC 1759, of the Manila trench, Mindoro-Tablas Islands, Philippines, D6partementde G6otectonique,T 26-0, El, boite129, 4 placeJussieu, Tectonophysics,J83, 273-287, 1990. 75252 Paris c6dex 5, France. McCaffrey, R., E. A. Silver, and R. W. Raitt, Crustal structureof the Molucca Sea collision zone, Indonesia, in The Tectonic and S. E. Lallemand,UMR CNRS-UM25573, Institut des Sciences de la GeologicEvolution q[ SoutheastAsian Seas and Islands, Part L Terre,de l'Eauet de l'Espace(ISTEEM), Laboratoire de G6ophysique Geophys.Monogr. Ser., vol. 23, editedby D. E. Hayes,pp. 161-177, et Tectonique,C.C. 60, place E. Bataillon,34095 Montpellier c6dex 5, AGU, Washington, D.C., 1980. France.(e-mail: [email protected]) McCaffrey, R., Oblique plate convergence,slip vectors,and forearc M. Popoff,UMR CNRS-UNSA6526 G6osciencesAzur, 250 rue deformation,J. Geophys.Res., 97, 8905-8915, 1992. Albert Einstein, 06560 Valbonne, France. McCaffrey, R., Estimatesof modem arc-parallelstrain rates in fore arcs,Geology, 24, 27-30, 1996. (ReceivedJanuary 30, 1997;revised July 2, 1997; Moore, G, F., .andE. A. Silver, Collision processesin the northern acceptedSeptember 12, 1997.)