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I Philippine Trench 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. 7øN duringthe 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 the northernpart 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 shown by 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 Parece Benham Rise Vela 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
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