Tectonic Review

8/20/2019 Tectonic Review

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Geosciences Journal

Vol. 12, No. 1, p. 7 -17, March 2008DOI 10.1007/s12303-008-0002-0

Tectonic setting of a composite terrane: A review of the Philippine islandarc system

Graciano P. Yumul, Jr.*

}}Tectonics and Geodynamics Group, National Institute of Geological Sciences, College of Science,

Carla B. Dimalanta

University of the Philippines, Diliman, Quezon City, Philippines

Victor B. MaglambayanEdanjarlo J. Marquez Department of Physical Sciences and Mathematics, University of the Philippines, Manila, Philippines

ABSTRACT: Features resulting from the interplay of arc magmatism,

ophiolite accretion, ocean basin closure and other subsequenttectonic processes are preserved in the Philippine island arcsystem. Subduction of ocean floor along the trenches surroundingthe Philippines is a major factor in shaping the geologic history ofthis island arc system. Stress-strain relationships, as manifest inboth the regional and local setting of the archipelago, are derivedfrom the interaction of at least four major plates: Sundaland, PhilippineMobile Belt, Philippine Sea and, to a certain extent, theIndo-Australian plate. Collision zones in this island arc system arecharacterized by the involvement of oceanic bathymetric highs(seamounts, spreading ridge, submerged continental fragment). Amajor strike-slip fault, the Philippine Fault Zone, with compressionaland extensional components, traverses the whole archipelago

where all excess stress not accommodated by the surroundingtrenches is taken up. Tholeiitic through adakitic to calc-alkalinerock suites characterize the different magmatic arcs. Exposed oceaniclithospheric fragments exhibit transitional mid-ocean ridge,back arc basin to island arc geochemical characteristics. Theobserved crustal thickness in the Philippines resulted from combinedmagmatic (volcanism) and amagmatic (ophiolite accretion)processes, with the former being the dominant factor.

Key words: tectonics, island arc, oceanic bathymetric highs, subduction,ophiolites, geophysics, Philippines

1. INTRODUCTION

Studies on how the Philippine island arc system has evolvedthrough space and time have been of interest among variousEarth scientists (e.g., Balce et al., 1976; Rangin et al.,1999a; Aurelio, 2000a; Milsom et al., 2006; Queaño et al.,2007). This is brought about by the unique setting that thisarchipelago occupies in the western Pacific region. ThePhilippines offers a glimpse, based on present-day land-ocean interactions, of what has transpired in the past (e.g.,Rangin et. al., 1990; Quebral et al., 1996; Dimalanta andYumul, 2003, 2004; Pubellier et al., 2003a, 2003b; Tamayo


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et al., 2004). Results of terrane accretion, ocean basin closure,arc formation, and indenter tectonics are some of thefeatures recognized in this island arc system. With theadvent of modern-day technology (e.g., global positioningsystem, mantle tomography) and recent onland and offshoredata, the tectonic evolution of the country is better understood(e.g., Besana et al., 1997; Bautista et al., 2001; Zamorasand Matsuoka, 2004; Galgana et al., 2007). The purposeof this review paper is to present an overview on the currentknowledge of tectonic setting of the Philippines. This informationwill contribute to the existing database of the geologicsetting and tectonic evolution of global island arc systems.

2. SURROUNDING MARGINAL BASINS ANDTRENCHESThe Philippine archipelago is bounded to the east andwest by subduction zones. A major left-lateral strike-slipfault zone longitudinally cuts the island (Fig. 1). West of thearchipelago are the east-dipping subduction zones composedof the Early Miocene Manila Trench, Middle MioceneNegros Trench, Late Miocene to Pliocene Sulu Trench andthe Cotabato Trench. The western boundary marks the subductionof the Early Oligocene to Early Miocene South

China Sea plate (Manila Trench), the Early to MiddleMiocene Sulu Sea plate (Negros and Sulu Trenches), andthe Eocene Celebes basin plate (Cotabato Trench) (e.g.,Hayes and Lewis, 1984; Mitchell et al., 1986; Rangin et al.,1999a,b). On the eastern boundary of the archipelago, theEocene West Philippine Sea plate, through oblique subduction,is being consumed along the west-dipping East LuzonTrough-Philippine Trench (e.g., Hamburger et al., 1983;Ozawa et al., 2004). All of these trenches are recognized asearthquake generation regions and are natural hazards to thecountry (Barrier et al., 1991; Bautista et al., 2001). The archi

pelago is also surrounded by marginal basins. These include

*Corresponding author: [email protected]

the South China Sea, Sulu Sea, Celebes Sea, Molucca Sea,

Also with Philippine Council for Industry and Energy Research and

and Philippine Sea. The Early Oligocene to Early Miocene

Development, Department of Science and Technology, Bicutan,Taguig, Metro Manila, Philippines South China Sea plate is made up of three subbasins: the


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Graciano P. Yumul, Jr., Carla B. Dimalanta, Victor B. Maglambayan, and Edanjarlo J. Marquez

Fig.1.The Philippine island arc system is surrounded by trenches, marginal basins andoceanic bathymetric highs (OBH). Moreover,the entire length of the archipelago is traversed by the left-lateral Philippine Fault Zone and some of its significant splays. Dashed linewith open triangle corresponds to the proto-Southeast Bohol Trench. Inset showsthe three major plates that bound the Philippinearchipelago Sundaland-Eurasian, Philippine Sea and the Indo-Australian plates (modified from map generated using the Online Map Creationat http://www.aquarius.geomar.de/). See text for discussion. SSSZ = Siayan-Sindangan Suture Zone; SCDL = Sindangan-Cotabato- DagumaLineament, T = Tablas, R = Romblon and S = Sibuyan.

NW subbasin, the SW subbasin, and the east subbasin. The the other hand, exposes E-W trending magnetic lineationseast and northwest subbasins exhibit, in general, a N-S (Hall, 2002). The Eocene West Philippine basin plate rep-opening direction (Taylor and Hayes, 1983; Briais et al., resents one of the mar

ginal basins of the Philippine Sea1993). The Early to Middle Miocene Sulu Sea plate is made plate. It includes the Shikoku basin, Parece Vela basin andup of two subbasins: the northwest subbasin underlain by the Marianas basin (Okino et al., 1999; Ohara, 2006; Ishiarcmaterial and the southeast subbasin underlain by oce-hara and Koda, 2007). Its spreading center, the Centralanic crust (Rangin, 1989). Magnetic lineations recognized, Basin Fault, preserves evidence of magmatism up to thethough not well-delineated, suggest N-S opening (Roeser, Pliocene (Fujioka et al., 1999). South of the Philippines in1991), which is interpreted to be a relict of the clockwise eastern Mindanao isthe northern extension of the Molucca

rotation the southeast subbasin had undergone. The original Sea basin. This basin had closed in the vicinity of easterntrend of the magnetic lineations is NE-SW (Yumul et al., Mindanao in a zipper-type fashion and is in the process of2000) consistent with the general tectonic trend in this part closure in relation to the subsequent collision of the Sangiheof the island arc system. The Eocene Celebes Sea basin, on and Halmahera islands (Bader and Pubellier, 2000). The


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Tectonic setting of the Philippines

Molucca Sea basin-derived Talaud ophiolite exposed inTalaud island has been dated as Eocene (Evans et al., 1983).

The left-lateral strike-slip Philippine Fault Zone, takes upwhatever stress that cannot be accommodated by the surroundingsubduction zones (Aurelio, 2000b) (Fig. 1). Thepresent-day East Luzon Trough is a rejuvenation of theproto-East Luzon Trough, which is responsible for the intrusionof Eocene to Oligocene magmatic rocks exposed in theNorthern Sierra Madre range (Hamburger et al., 1983). Thearchipelago is a composite terrane and the various geologicblocks have continental, oceanic, island arc or ophioliticaffinity (Karig, 1983; McCabe et al., 1985; Yumul et al., 1997;Pubellier et al., 2004). In general, the Philippines is madeup of the aseismic Palawan microcontinental block and theseismically-active Philippine Mobile Belt characterized byearthquakes and active volcanoes (Yumul et al., 2005, 2007).Indentation of the oceanic leading edge of the Palawanblock with the Philippine Mobile Belt and its subsequentsubduction possibly started during the Early Miocene withthe collision terminating by the Pliocene as exposed in Mindoro

island (e.g., Bellon and Yumul, 2001; Yumul et al.,2003a; Yumul et al., 2005 and references therein). This collisionresulted in the emplacement of an ophiolite, cusping,and microblock rotation among others (Rangin et al., 1985;Jumawan et al., 1998; Yumul et al., 2000, 2005). The continuingcollision between the Palawan block and the PhilippineMobile Belt is being taken up east of the Tablas,Romblon, and Sibuyan islands (e.g., Faure et al., 1989;Pineda and Aurelio, 1991; Yumul et al., 2003a) (Fig. 1).

In terms of major plates, Sundaland bounds the westernportion of the Philippines whereas the eastern side is definedby the Philippine Sea plate, specifically that of the West Philippine

basin plate. South of the archipelago is the Indo-Australianplate. Some parts of the Philippine Mobile Belt aremodeled to have been derived from the Indo-Australian margin(Pubellier et al., 2003a). Based on paleomagnetic data,most of the islands belonging to the Philippine Mobile Belthave been translated northwestward and rotated clockwiseduring the process (Hall, 2002).

3. OCEANIC BATHYMETRIC HIGHSDuring the northwest translation of the Philippine MobileBelt away from the Indo-Australian margin, its present-daywestern margin has acted as the leading edge whereas theeastern portion was the trailing edge (Yumul, 2007). In the

process, evidence of more tectonic interactions involvingcollision, subduction and suturing are found along the westernside of the archipelago (Yumul, 2007 and referencestherein). Such interaction is discernible in terms of the collisionof the Philippine Mobile Belt with several oceanicbathymetric highs along its western margin. There are at leastfour major oceanic bathymetric highs impinging on thePhilippines. These are the NW Luzon oceanic bathymetric

high, the Scarborough seamount, the Palawan microcontinental


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block, and the Zamboanga-Sulu Peninsula (Fig. 1).

In the north, the NW Luzon oceanic bathymetric highwas first recognized by Bautista et al. (2001). It is locatedoffshore of northwest Luzon (Fig. 1). This bathymetric highis believed to have caused cusping along the northernextension of the Manila Trench (Bautista et al., 2001). Furthermore,the extinct spreading center of the South ChinaSea basin represented by the Scarborough Seamount is sub-ducting beneath northern Luzon. Flat subduction, volcanicarc gap, and elevation of the forearc region along which theridge subducts are associated with this subduction (Yang etal., 1996). A more significant collision involves the interactionbetween the Palawan microcontinental block and thePhilippine Mobile Belt. Several indentation-related features(e.g., ophiolite emplacement, cusping, seismic gap and microblockrotation) are observed which are believed to be relatedto this collision (McCabe et al., 1985; Marchadier and Rangin,1990; Jumawan et al., 1998; Ramos et al., 2005). The southernpart of the Palawan microcontinental block defined by theCagayan de Sulu ridge is colliding with the southern part ofPanay (Bellon and Rangin, 1991). Dacite and andesite fromthe Cagayan de Sulu ridge has been accreted and emplacedonland of southern Panay. Another bathymetric high that

has collided with the Philippine Mobile Belt, specifically inMindanao, is the Zamboanga-Sulu Peninsula (Fig. 1). Havingcontinental, arc and ophiolite affinities (Querubin andYumul, 2001; Sherlock and Barrett, 2004), this block collidedwith central Mindanao during the Middle Miocene.Subsequent tectonic events include subduction, which wasconverted to strike slip fault motion along the Siayan-SindanganSuture Zone (Jimenez et al., 2002; Yumul et al., 2004)(Fig. 1). This suture zone is the northwestern extension ofthe Sindangan-Cotabato-Daguma Lineament as mapped byPubellier et al. (1991).

Based on offshore data and satellite images, two oceanic

bathymetric highs are recognized along the eastern boundaryof the Philippines. The Benham Plateau located offshoreof northeast Luzon has yet to subduct, contrary toprevious published claims (Barrier et al., 1991), as an oceanicbasin still exists between the plateau and Luzon (Fig.1). Along the eastern side of the Philippine Trench in thevicinity of eastern Mindanao, a small ridge chain can beidentified on the gravity anomaly map of Sandwell and Smith(1997), which will ultimately reach the trench and collidewith Mindanao. A common denominator when a ridge sub-ducts is that the trench is sediment-filled (e.g., De Jesus et al.,2000). The sediment acts as lubricant facilitating the ridgeto be subducted rather than accreted into the overlying plate.

4. MAJOR FAULTSThe Philippine Fault Zone is a left-lateral strike slip faultthat longitudinally cuts the whole island (Barrier et al.,


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1991) (Fig. 1). It has both transpressional and transtensionalcomponents (Aurelio, 2000a). Folding and faulting relatedto its transpressional feature are recognized whereas extensionaljogs along the fault zone are attributed to its transtensionalcomponent. The Philippine Fault Zone cuts Holocenesandstones exposed in Mati, Davao Oriental, southern Mindanao,demonstrating its active status. The Philippine FaultZone formed during the Middle Miocene (Aurelio et al.,1991) and is propagating southward. It is coupled with theyounging southward Philippine Trench (Quebral et al.,1996; Lallemand et al., 1998). Present-day evidence alsoshows that the fault zone is propagating northward (Yumulet al., 2005). For that matter, the northern and southern tipsof the Philippine Fault Zone are characterized by horse-tailstructures consistent with an actively propagating fault system(Pinet and Stephan, 1990) (Fig. 1).

Several subordinate faults are intimately linked to theevolution of the Philippine Fault Zone. The left-lateralLegaspi Lineament, acting as a transfer fault, connects the

Philippine Fault Zone with the Philippine Trench. An offshoreextension of the Philippine Fault Zone in the centralPhilippines is the left-lateral Sibuyan Sea Fault. In Mindanao,another left-lateral fault zone is present. It comprisesthe NW-trending Sindangan-Cotabato-Daguma Lineament,the northern extension of which connects with the Siayan-Sindangan Suture Zone (Fig. 1). This accommodates someof the stress that is not being accommodated by the surroundingtrenches in Mindanao. Aside from accommodatingexcess stress, these subordinate fault systems have alsoserved as hydrothermal fluid pathways manifest as ancientand present-day geothermal systems (e.g., Sillitoe andGappe, 1984; Mitchell and Leach, 1991; Sajona et al., 2002).

5. MAJOR VOLCANIC ARCSSeveral arc systems associated with the different surroundingtrench systems are found in the Philippines. Interestin these volcanic arcs is brought about by theirassociated mineralization, geothermal energy potential, andrelated natural hazards. The best studied arc in the Philippinesis the Luzon Arc (Fig. 2). It was formed as a result ofthe subduction of the South China Sea along the ManilaTrench beginning in the Early Miocene (Balce et al., 1982;Knittel and Defant, 1988; Teng, 1990; Maury et al., 1992;Yumul et al., 2003a). It consists of a 1200 km-long chain ofstratovolcanoes and volcanic necks extending from eastern

Taiwan to Mindoro (Marini et al., 2005; Castillo andNewhall, 2004). Tholeiitic through calc-alkaline to shoshoniticrock types have been erupted in these volcanic arcs(Defant et al., 1989; Maury et al., 1998; Polve et al., 2007).Adakitic rocks have also been identified among the older(>15 Ma) and younger volcanic centers (< 5 Ma) (Bellonand Yumul, 2000; Yumul et al., 2000; Jego et al., 2005).The Northern Luzon volcanic arc rocks have whole rock

K-Ar isotopic ages ranging from 32.3 to 5.6 Ma (Wolfe,


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1981; Bellon and Yumul, 2000). The K-Ar dates from theBataan front-arc volcanoes range from 7.0 to 0.2 Ma, whereasthe Bataan back-arc volcanoes including the volcanic rocksexposed in Mindoro range from 1.7 to 0.1 Ma (Defant et al.,1989; Yumul et al., 2000) (Fig. 2).

The Northern Sierra Madre arc trends N-S in its northernsegment and bends southwest joining the NW-SE trendingCaraballo Mountains (Sajona et al., 1994) (Fig. 2). TheNorthern Sierra Madre arc is made up of island arc rockswhich range in age from 49 to 43 Ma (whole rock K-Ardating by Wolfe, 1981). The arc is a result of ancient subductionalong the present-day East Luzon Trough. A younger(whole rock K-Ar age of 33-24 Ma) arc sequence (JapanInternational Cooperative Agency-Metal Mining Agency ofJapan, 1977), consisting mostly of volcanogenic sediments,basaltic flows and dikes in the southern portion of theNorthern Sierra Madre, shows island arc tholeiitic to calcalkalineaffinities. Quartz diorite and other intrusive rocksin the Caraballo Range possess primitive island arc characteristics,and their whole rock K-Ar ages range from 39to 27 Ma (e.g., Mitchell and Leach, 1991). On the otherhand, whole rock K-Ar ages of shoshonites and island arctholeiites in the Southern Sierra Madre-Polillo-Catanduanes

arc range from 36.9 to 1.2 Ma (Japan International CooperativeAgency-Metal Mining Agency of Japan, 1977).

Subduction of the Sulu Sea basin along the NegrosTrench produced the Negros calc-alkaline volcanic arc (Fig. 2).To the east, the East Philippine Arc extends from Bicol toeastern Mindanao (e.g., Andal et al., 2005a; McDermott etal., 2005) (Fig. 2). Rocks from the Bicol segment of the arcare dominantly medium- to high-K calc-alkaline, high-Albasalts and andesites. Magmatism was related to the westwardsubduction of the Philippine Sea plate along the PhilippineTrench (Weber and Knittel, 1990; Castillo andNewhall, 2004). In the Leyte segment, the volcanoes define

a 250 km-long NW-SE belt from Biliran to Panaon islands.Recent volcanism is linked with subduction along the PhilippineTrench (Sajona et al., 1994). The Plio-Pleistocenelavas are largely calc-alkaline with medium to high K contents(Ozawa et al., 2004). The Lower Oligocene-LowerMiocene rocks in the northeastern Mindanao segment haveerupted following an increase in the angle of subduction ofthe oceanic crust along the Philippine Trench (Mitchell etal., 1986). In the northern part of eastern Mindanao, severalandesitic rocks have been radiometrically dated as latePliocene-Quaternary (Mitchell and Leach, 1991). The CentralMindanao Volcanic Arc is referred to in earlier literatureas the Agusan-Lanao flood basalts (Balce et al., 1976).

This arc is interpreted to represent a magmatic response tothe collision between the western and eastern Mindanao ca.5 Ma (Pubellier et al., 1991; Castillo et al., 1999). The Plio-Pleistocene volcanoes are predominantly basaltic to basalticandesites with minor acidic rocks (Sajona et al., 1994). The


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Fig.2.The major magmatic arcs related to subduction along the different trenches surrounding the Philippine archipelago include:1 Luzon (Central Cordillera), 2 Northern Sierra Madre, 3 Southern Sierra Madre-Polillo-Catanduanes, 4 Negros, 5 EastPhilippines, 6 Central Mindanao, 7 Cotabato-Daguma and 8 Sulu-Zamboanga (modified from Mitchell and Leach, 1991). Insetshows that the Philippines is generally underlain by crust with a thickness varying from ~17 to 30 km. Two zones of thickened crustwere also identified (modified from Dimalanta and Yumul, 2004). Arrows show that the Sunda-Eurasian plate is moving southeastwardwhereas the Philippine Sea plate is moving northwestward (Kreemer et al., 2003).

subduction of the Celebes Sea plate along the CotabatoTrench resulted into the formation of the Cotabato arc(Aurelio, 2000b). The monzonitic-dioritic-granodioritic coreof the Daguma Range has whole rock K-Ar ages of ~30 Ma(Pubellier et al., 1991) (Fig. 2). The affinities of the rocks

vary from island arc tholeiite to tholeiite-calc-alkaline to

exclusively calc-alkaline varieties, and the Miocene rockshave adakite affinity (Sajona et al., 1994). The Pliocene-Quaternary Sulu-Zamboanga arc extends from the southernside of the eastern portion of the Zamboanga Peninsula toTawi-Tawi (Castillo et al., 2002; 2007) and is associatedwith the Sulu Trench through the subduction of the Sulu


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Sea plate. Older volcanic rocks in the Zamboanga Peninsulayield the whole rock K-Ar ages of 18.9 to 11.9 Ma and suggestthe existence of an Early Miocene proto-Sulu Trench(Sajona et al., 1994; Yumul et al., 2004). The majority ofthe rocks are arc tholeiites with subordinate high K calcalkalinevarieties.

In closing, the different volcanic arc rocks in the Philippinesmanifest geochemical signatures that show the influenceof the mantle, crust, subducted slab, sediments, andeven mantle plumes, similar to what has been observed inother parts of the world (e.g., Sajona et al., 2000; Yumul etal., 2003c; Macpherson et al., 2006; Currie et al., 2007; Ingleet al., 2007).

6. MAJOR OPHIOLITE BELTSOphiolites and ophiolitic rocks occur in many parts of thearchipelago (Nakagawa and Franco, 1996; Peña, 1996; Dimalantaet al., 2006) (Fig. 3). Amphibolite, quartz-albite-micaschist and serpentinite are mapped as metamorphic soles of

these exposed oceanic lithosphere fragments. Tectonic andsedimentary mélanges are associated with ophiolite andophiolitic suites (Yumul et al., 1997; De Jesus et al., 2000;Tamayo et al., 2001; Faustino et al., 2006). Majority of thesuites are classified as Tethyan (Moores, 1982). Balce et al.(1976) initially grouped ophiolites based on their geographicdistribution (e.g., eastern belt, western belt, central

belt) in the Philippines, whereas Tamayo et al. (2004)assigned the ophiolite complexes in the Philippines intofour belts based on age and geochemical data. Those classifiedare: Cretaceous sequences, Eocene complexes withstrong subduction signatures, ophiolites of varied ages and

ophiolite-arc components and younger sequences withweak subduction imprints which outcrop west of the WestPhilippine Suture Zone. Tamayo et al. (2004) suggest thatthe majority of the Eocene ophiolites possibly resultedfrom the rifting and spreading of Cretaceous ocean floor. Acorollary to this model is the idea that most of the Eoceneophiolites are autochthonous in origin (Encarnacion et al.,1993) (Fig. 3).

A new zonation for Philippine ophiolites and ophioliticcomplexes based on their ages and possible lithosphericsources has been proposed (Yumul et al., 2003b; Yumul,2007). The proposed zonation is consistent with the spatial

and temporal relationships among these complexes, whichare based on the available geological data (Fig. 3). Fromeast to west, belt 1 is composed of the eastern PhilippineLate Cretaceous ophiolite complexes with metamorphicsoles that served as the trailing edge of the proto-PhilippineSea plate (e.g., Andal et al., 2005b). Belt 2 corresponds tothe Early to Late Cretaceous Cordilleran suites withmélanges found along the entire length of the central Philippineswhich formed part of the leading edge of the proto-Philippine Sea plate (Encarnacion, 2004; Dimalanta et al.,


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Fig.3.Ophiolites and ophiolitic rocks found in different parts of the Philippine archipelago have been grouped according to: A. theirgeographic distribution (Balce et al., 1976). The ophiolites as grouped are found in: 1 Sierra Madre Range, 2 Zambales-Mindoro,3 Eastern Bicol-Eastern Mindanao, 4 Antique, 5 southern Palawan, 6 Sulu-Zamboana and 7 north-central Mindanao. B.ages and geochemical signatures (Tamayo et al., 2004). These are Belt I Cretaceous sequences; II Eocene complexes with subductionsignatures; III collision-related; IV young ophiolites west of the suture zone;and C. ages and possible lithospheric sourcesof these complexes (Yumul et al., 2003b; Yumul, 2007). Belt 1 corresponds to Late Cretaceous ophiolite complexes with metamorphicsoles whereas Belt 2 corresponds to the Early to Late Cretaceous ophiolites with mélanges; Belt 3 includes Cretaceous to Oligoceneophiolites along the collision zone and Belt 4 is made up of Sundaland-Eurasianmargin-derived ophiolites. See text for details.


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2006). This portion of the Philippines subsequently interactedwith the Sundaland-Eurasian margin during the northwestwardtranslation of the Philippines from the south(Pubellier et al., 2003a). Belt 3 consists of the Cretaceous toOligocene ophiolites and ophiolitic complexes that wereemplaced along the collision zone between the proto-PhilippineSea plate and the Sundaland-Eurasian margin (Ran-gin et al., 1985; Jumawan et al., 1998). Belt 4 correspondsto that of Tamayo (2001), and includes the Sundaland-Eurasianmargin-derived ophiolites emplaced in the Palawanand Zamboanga-Sulu regions (Tamayo et al., 2000; Yumulet al., 2004). The majority of these upper mantle-crustsequences have supra-subduction zone geochemical signatureswith a minority having mid-oceanic ridge signatures(Yumul et al., 1997; Dimalanta et al., 2006). Most of thesewere generated in fast spreading centers (Yumul, 2003b). Aminority were formed in intermediate spreading centers.Some of the mechanisms responsible for the emplacementof the Philippine ophiolites include onramping, subductionaccretion, and strike-slip faulting (Karig, 1983; Rangin etal., 1985; Mitchell et al., 1986; Yumul, 2007).

7. REGIONAL GEOPHYSICAL SIGNATURESObserved gravity anomalies in the Philippines are typicalof arc-trench systems in which island arcs are associatedwith large positive anomalies. Arcuate, low gravity anomaliescoincide with the major deep-sea trenches that surroundthe Philippine Archipelago (Hayes and Lewis, 1984).Mantle tomographic data have confirmed the presence of,and provided constraints on the location and depth of sub-ducted slabs along these trenches (Rangin et al., 1999a).Subducted slabs in the East Luzon Trough, PhilippineTrench, Sulu Trench, and Manila Trench are low attenuationfeatures (Besana et al., 2001). The slabs related to the

Manila and Philippine Trenches are estimated at 230 and290 kilometer depths, respectively (Besana et al., 1997).The presence of ancient subduction zones is also suggestedby offshore satellite bathymetry and gravity data. A relativelydeep (~2000 meters), NE-SW trending narrow basinwith a low gravity anomaly is observed between CentralVisayas and Mindanao, in the area now occupied by theBohol Sea. This feature is believed to represent an ancienttrench southeast of Bohol (Yumul et al., 2000). Oceanicbathymetric highs are also clearly defined by free-air gravityanomalies. The Scarborough Seamount, Cagayan deSulu Ridge, and Benham Plateau are distinctly marked onthe marine gravity map by high gravity anomalies. In addition,

the Reed Bank and Palawan microcontinental blockare characterized by similar, large amplitude gravity anomalies.The ~1200 kilometer-long Philippine Fault Zone canalso be traced on gravity and magnetic anomaly maps(Japan International Cooperation Agency Metal MiningAgency of Japan, 1990). The linearity of gravity contours

(30100 mGal) and the NW-SE orientation of magneticanomalies, ranging from 3900039400 gammas, serve todefine the Philippine Fault Zone in Leyte Island. The East


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Luzon Transform Fault, connects the East Luzon Troughand Philippine Trench (Fig. 1) and is seen on seismictomographic images as an east-west trending zone of highattenuation values (Besana et al., 1997). Bautista et al.(2001) have also pointed out a linear E-W trending zoneof seismicity that connects the East Luzon Trough to thenorthern end of the Philippine Trench. This feature isseen as a linear gravity low on the marine gravity anomalymap.

Aside from delineating major tectonic features and geologicstructures, available gravity data have also been usedto estimate the thickness of the crust. Gravity and seismicdata indicate that island arcs in the Western Pacific regionare much thicker than previously envisioned (Dimalanta etal., 2002). The crustal thickness of these island arcs havebeen estimated to range from ~20 to 35 kilometers (Dimalantaet al., 2002 and references therein). In the Philippines,the paucity of seismic refraction or OBS data has made anestimation of crustal thickness rather difficult. Recently,available geophysical (seismic refraction, seismicity, andgravity) and geochemical data (i.e., Plank-Langmuir systematics,and rare earth element ratios) were integrated andsynthesized to come up with an estimate of the crustal thickness

beneath the Philippine island arc. Results obtained showthat the Philippines is generally characterized by crustalthicknesses varying from ~17 to 30 kilometers. The centralportion of Luzon Island and the Bicol-Masbate-Panay-CentralMindanao area are interpreted to be made up of thickenedcrust (Fig. 2). More voluminous magmatism coupledwith ophiolite accretion may account for the thickened crustin this region (Dimalanta and Yumul, 2004; 2006).

Data obtained from the global positioning system networkswithin the Southeast Asian region have providedmeasurements of the convergence rate between the Sundaland-Eurasian margin and the Philippine Sea plate. On the

western side of the Philippine archipelago, the Sunda blockis found to be moving with respect to Eurasia from 10±1mm/year in the direction S78°E along its northern marginand ~6±2 mm/year towards S61°E along its southern margin(Kreemer et al., 2003). East of the archipelago, thenorthwest-drifting Philippine Sea plate is found to be movingapproximately 7 cm/year in the region northeast ofLuzon and around 9 cm/year southeast of Mindanao. Thesevalues were obtained using the HS2 NUVEL-1 model(Bautista et al., 2001) (Fig. 2). The GPS measurements andgeological observations allowed the subduction on eitherside of the Philippine Mobile Belt to be quantified. On theeastern side of the Philippines, the rate of subduction along

the Philippine Trench is observed to vary from north tosouth. Subduction takes place at 54 mm/year near 13°N and32 mm/year near 7°N. On the western side of the archipel


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ago, subduction along the Manila Trench has been estimatedto vary from ~90-100 mm/year to 50 mm/year northwest ofMindoro (Rangin et al., 1999b) (Fig. 2 inset).

8. SUMMARYa. Early Miocene to Pliocene east- and west-dipping subductionzones surround the Philippine archipelago. It isalong these trenches that the South China Sea plate, SuluSea plate, Celebes Sea plate, and West Philippine Sea plateare being consumed.b. There are several oceanic bathymetric highs which arecurrently impinging on the Philippines the NW Luzonoceanic bathymetric high, the Scarborough seamount, thePalawan microcontinental block, the Zamboanga-Sulu Peninsula,the Benham Plateau, and a small ridge chain east ofMindanao.c. The Philippine left-lateral strike-slip Fault Zone cutsthe entire Philippine archipelago from north to south. Severalmajor faults are linked to the evolution of the PhilippineFault Zone such as the Legaspi Lineament, Sibuyan

Sea Fault, and the Sindangan-Cotabato-Daguma Lineament.Both ends of the Philippine Fault Zone are characterized byhorse-tail structures consistent with active faulting.d. Several arc systems have been formed as a result ofsubduction along the trench systems surrounding the archipelago.Interest in these volcanic arcs is attributed to thesearch for mineral deposits, tapping of geothermal energyand the mitigation of associated natural hazards.e. Ophiolites and ophiolitic complexes are integral featuresfound in various regions of the Philippine archipelago.Currently available data allow these complexes to be groupedaccording to age, geochemical signatures, and possiblelithospheric sources. Most of these oceanic lithospheric

fragments are subduction-related and formed in fast-spreadingmarginal basins.f. Geophysical data (i.e., gravity, seismic tomography,seismicity, and magnetic) serve to delineate major tectonicfeatures and geologic structures such as trenches and faults.The information presented here helps us to recognizeimportant features developed over the course of the evolutionof the Philippine island arc system. More importantly,the implication of this knowledge allows us to better understandand recognize processes responsible for mineralization,energy source, and natural hazards. The informationpresented here may be useful in studies of other island arcsettings with similar geologic features.

ACKNOWLEDGMENTS: Most of the information presented herewas generated from the various projects and programs involving differentbranches of the geosciences. Financial and logistic supportextended by the Department of Science and Technology (DOST),DOST-Philippine Council for Industry and Energy Research and Development,University of the Philippines National Institute of GeologicalSciences, National Academy of Science and Technology, NationalResearch Council of the Philippines, DOST-Philippine Council for


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Advanced Science and Technology Research and Development, Commissionon Higher Education and the Mines and Geosciences Bureaucentral and regional offices are acknowledged. Support from theEmbassy of the Republic of France, Japan Society for the Promotionof Science, DUO-France Program, University of Tokyo, University ofHong Kong, University of Bretagne Occidentale, University of PaulSabatier, Kumamoto University and Okayama University are appreciated.Colleagues and students at the UP-NIGS and other institutionsare thanked for their unselfish contributions. Comments by Dr. J.I. Leeand an anonymous reviewer improved the presentation of this paper.We thank Professor Moonsup Cho for his encouragement to write thispaper. This is UP-NIGS contribution number 2008-01.

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