TECTONICS, VOL. 14, NO. 5, PAGES, 1117-1132, OCTOBER 1995 Cenozoicmotion of the Philippine Sea Plate: Palaeomagnetic evidence from eastern Indonesia Robert Hall SE AsiaResearch Group, Department of Geology,Royal Holloway, London University, Egham, England United Kingdom Jason R. Ali Departmentof Oceanography,Southampton University, Southampton, England, United Kingdom Charles D. Anderson Departmentof GeologicalSciences, University of California,Santa Barbara Abstract. The history of motion of the Philippine Sea Plate is understandingits evolution (Deep Sea Drilling Project poorly known becauseit is isolated from the oceanic ridge (DSDP)/OceanDrilling Program(ODP) Legs 58, 59, 60, 125, system.Interpretation of palaeomagneticresults from the plate and 126). Despitethis, it remainsa platewhose Tertiary motion has been controversial because declination data have been is poorlyknown. The principalobstacle to deducingthe history obtainedonly from the easternmargin where subduction-related of motion is the isolationof the PhilippineSea Plate from the tectonicprocesses may have causedlocal rather than plate-wide global plate circuit since it is surroundedby subductionzones. rotations.New palaeomagneticdata relevantto the problemhave For this reasonits origin and movementhistory have remaineda been obtainedfrom 34 sites north of the SorongFault and 29 sourceof controversy.It has been suggested[Uyeda and Ben- sites within the Sorong Fault system.These sites record south- Avraham, 1972] that the plate was formed when subductionof ward movement during the Eocene and northward movement the Pacific Plate was initiated at -45 Ma along transformfaults duringthe Neogene.Sites within the SorongFault systemrecord within old Pacific ocean floor, trapping the former spreading both counterclockwiseand clockwiserotations interpreted as the centrein the West PhilippineBasin, and this view has continued resultof Neogeneblock movementsat the southernboundary of to be cited in interpretationsof the plate's history[e.g., Stem and the Philippine Sea Plate. North of the Sorong Fault, all sites Bloomer, 1992]. record clockwise declinations. Neogene rocks have small Palaeomagnetismmay help providea solutionto the problem. deflections consistent with rotation about the present-day Palaeomagneticdata indicate northwardmovement of the plate Eurasia-Philippine Sea Plate pole. Oligocene-middle Eocene although the amount of rotation is less certain. The rotation rocks show consistent clockwise declination deflections of-40 ø. historyof the plate has a bearingon its origin, on the origin of Declinations of lower Eocene rocks indicate -90 ø of clockwise subduction-relatedmagmatism, and on its movement history, rotation.We proposethat the entire area north of the Sorong because there are hemisphere ambiguities in palaeolatitudes Fault in eastIndonesia has always been part of the PhilippineSea obtained from inclination data especiallyfor low-latitude sites. Plate and that the whole plate has rotated clockwise in a Most ocean drilling data lack declinationinformation, and land discontinuousmanner by approximately90 ø since the early palaeomagneticdata from the plate remain scarce[Haston and Eocene.The new data from north of the SorongFault providea Fuller, 1991]. Although many land studies have reported basis for determining rotation poles which satisfy all the declinationshifts implying clockwiserotations, the interpretation palaeomagneticdata from the PhilippineSea Plate and permit its of theseresults has been disputed.All samplingsites have been at reconstruction. the eastern edge of the plate where local subduction-related tectonic processesprovide a potential explanationfor observed Introduction rotations [Larson et al., 1975; McCabe and Uyeda, 1983] becauseseveral episodes of back arc spreadinghave extendedthe The Philippine Sea Plate (Figure 1) has been the sourceof plate's eastern margin since the Eocene. Distinguishinglocal numerousmodels associated with subduction-relatedprocesses of tectonic from plate-wide movementshas proved controversial. plate tectonics.Different partsof the plate have beenused as Haston and Fuller [ 1991] arguedthat since the middle Eocene examples for tectonic processesincluding initiation of the Philippine Sea Plate has behavedas a rigid unit with little subduction,development of intraoceanicsubduction zones, arc local tectonic deformation along its eastern margin, rotated rifting, back arc spreading,and forearc extension.There have continuouslyclockwise through 90 ø- 100 ø, and moved northward been two major phases of ocean drilling devoted to about 15ø-20ø. Koyama et al. [1992] reviewed tectonic models which could accountfor their own and older data. They accepted northwardmotion of the plate but concludedthat more data were Copyright1995 by theAmerican Geophysical Union. required to distinguish between models which accounted for Papernumber 95TC01694. declinationdeflections by rotation of the whole plate and those 0278-7407/95/95TC-01694510.00 which attributedpart of the deflectionsto local deformation. 1117 1118 HALL ET AL.: CENOZOICMOTION OF THE PHILIPPINESEA PLATE EURASIAN 30'N Shikoku PLATE PACIFIC Dakin Donin I•iand• Plateau, PLATE Plateau Plateau 20'N • • Pamce • • Vela Dakin II # PhilippineBasin 10øN Sulu Caroline ,Sea O Pala Ridge CAROLINE Sea PLATE Eauripik Ridge reo .........•...... Di.•rnarck DandaSea AUSTRALIAN sea . PLATE Figure 1. Principaltectonic features of the PhilippineSea Plateand surroundingareas. Double lines represent activeor formerspreading centres within the plate. The presentboundary between the PhilippineSea Plate and the AustralianPlate is a complexstrike-slip plate boundaryzone, with principallysinistral motion, which has been muchsimplified on the map. In this study we presentnew palaeomagneticdata from the tectonicsyntheses of the regionalthough the Indonesianislands southernpart of the Philippine Sea Plate in easternIndonesia between Halmahera and Waigeo together make up the largest which are relevantto the historyof motionof the plate duringthe land area within the plate: -5x 104km2 spreadover past 50 m.y. This part of the plate has largely been ignoredin -25 x 104km 2. Our new data support large clockwise rotations of HALL ET AL.: CENOZOIC MOTION OF THE PHILIPPINE SEA PLATE 1119 the whole plate but indicate a more complex history of rotation Molucca Sea collision complex(Figure 2) where the Halmahera and latitudinalmovement than previouslysuggested. Using our and Sangibe Arcs are actively converging.The Molucca Sea new data, earlier palaeomagneticdata, and geologicalarguments, Plate has an inverted U-shapedconfiguration [Hatherton and it is possibleto estimate the position of rotation poles which Dickinson, 1969; Katili, 1975; Hamilton, 1979; Cardwell et al., permit reconstructionsof the Philippine Sea Plate and adjacent 1980; McCafire.v,1982] and dips eastunder Halmahera and west regions[Hall et al., 1995b]. The resultsof the modellingprovide under the Sangibe Arc. Regional seismicity[McCafire.v, 1982] a basis for estimating how much of the clockwise rotation suggeststhat the east dipping slab extends200-300 km beneath observedat the eastern plate margin is due to local tectonic Halmahera.The west dipping slab can be identifiedto 600 km effects. beneaththe Celebes Sea [Cardwell et al., 1980]. PresentTectonic Setting of Eastern Indonesia The Philippine Sea Plate is currentlyrotating clockwise with respectto Eurasiaabout an Euler pole closeto its northernedge, Geologicalknowledge of easternIndonesia has increased basedon geological,geophysical, and seismologicalobservations considerablysince Hamilton's [1979] review of Indonesian along the plate margins [Ranken et al., 1984; Huchon, 1986; tectonics.Interpretations of present-daytectonics are basedon Send et al., 1987; Send et al., 1993]. The rate of convergence marine[e.g., Silver and Moore, 1978; Moore and Silver, 1983; increasessouthward, resulting in westward subductionat the Silver et al., 1983] and regionalseismic [Cardwell et al., 1980; PhilippineTrench, which terminatesjust north of Halmaheraat McCaffre.v, 1982] investigations.The IndonesianGeological 2ø50'N. The Philippine Trench formed later than 5 Ma and is Research and Development Centre (GRDC) has carried out associatedwith less than 150 km of subductedlithosphere mappingwith internationalcollaboration, and knowledgeof the [Cardwell et al., 1980; Karig, 1975] implying that before the geologicaldevelopment of easternIndonesia is based on this Pliocene, parts of the east Philippines formed part of the work [e.g., Dow and Sukamto,1984; Pigram and Davies, 1987; Philippine Sea Plate. Hamilton [1979] joined the Philippine Hall, 1987; Hall and Nichols, 1990] andour unpublishedresults. Trenchto the New GuineaTrench with a strike-slipfault eastof EasternIndonesia includes the junctionbetween three major Halmaheraand placedHalmahera on a microplateseparate from plates(Figure 1). Relativeto Eurasia,treated as a singleplate, the the Philippine Sea Plate. However, recentstudies of seismicity AustralianPlate is moving NNE at about75 mm/yr [DeMets et and marine geology [Cardwell et al., 1980; McCaffre.v, 1982; al., 1990], and the Philippine Sea Plate movesWNW at about Moore and Silver, 1983; Nichols et al., 1990] indicate that the 100 mm/yr [Send et al., 1993]. The marginsof Eurasiain SE Halmahera-Waigeoislands form part of the PhilippineSea Plate Asia mustbe consideredas involvingnumerous small platesand [Hall and Nichols, 1990]. The southernboundary of both the plateboundaries that haveshifted