Pre-Eocene Synmetamorphic Structure in the Mindoro-Romblon-Palawan Area, West Philippines, and implications for the history of southeast Asia Michel Faure, Yves Marchadier, Claude Rangin
To cite this version:
Michel Faure, Yves Marchadier, Claude Rangin. Pre-Eocene Synmetamorphic Structure in the Mindoro-Romblon-Palawan Area, West Philippines, and implications for the history of southeast Asia. Tectonics, American Geophysical Union (AGU), 1989, 8 (5), pp.963-979. 10.1029/TC008i005p00963. insu-00716213
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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. TECTONICS, VOL. 8, NO. 5, PAGES 963-979, OCTOBER 1989
PRE-EOCENE SYNMETAMORPHIC STRUCTURE IN THE MINDORO-ROMBLON-PALAWANAREA, WEST PHILIPPINES, AND IMPLICATIONS FOR THE HISTORY OF SOUTHEAST ASIA
Michel Faure
D&partment des Sciences de la Terre, Universit& d'Orl&ans, Orleans, France.
Yves Marchadier and Claude Rangin
D&partement de G&otectonique, Universit• Pierre et Marie Curie, Paris, France.
Abstract. The structure of the pre-Eocene which is correlated with the olistostromes rocks, considered as the "basement" of the found in Calamian, North Mindoro, Carabao, Philippines, has been investigated in the and Buruanga peninsula (North Panay). The Mindoro-Lubang, Romblon-Tablas-Sibuyan, microstructural features and the presence and North Palawan areas. In the former two of the olistostrome suggest that the North areas the same pre-Eocene succession of Palawan, Mindoro, Tablas, Romblon, units is recognized from top to bottom: Sibuyan, and Carabao islands belong to the (1) a pre-Eocene olistostrome; (2) an same North Palawan block of Hamilton ophiolitic nappe; (3) a schistose sequence (1979) which is a continental fragment (pelites, sandstones, basic schists and rifted from Asia in Cenozoic times. It is marbles); and (4) a gneissic unit. The assumed that the Western Panay and nature of the contact between the Zamboanga areas, which are characterized olistostrome and the underlying units is by Mesozoic ophiolites and metamorphic unclear, but the ophiolite and the rocks, also belong to the North Palawan schistose sequence form two thrust sheets Block. All these islands experienced, to of oceanic origin thrust upon the gneissic some extent, the same Mesozoic geohistory: unit that is considered a part of a the south verging thrusting is interpreted continental basement called the West as the result of an oblique collision of Philippines Block. Small-scale structures the West Philippines Block with Asia. The show that the early deformation stage is contemporaneous left-lateral strike-slip characterized by a submeridian (0ø-N40øE) faulting and the calc-alkaline magmatism lineation formed in greenschist to widespread along the Chinese margin are amphibolite facies conditions during the also included in the geodynamic model. thrusting. Kinematic analysis show that the thrusting was from north to south. In North Palawan, metamorphic rocks with INTRODUCTION similar microtectonic and kinematic characteristics are found. They are overlain by a Late Jurassic olistostrome The Philippine Islands lay at the convergent boundary between the Eurasian margin and the Philippine Sea plate. From Copyright 1989 Taiwan to the Celebes Sea several domains by the American Geophysical Union. can be distinguished (Figure 1), namely, (1) a collision area in Taiwan between Paper number 89TC00709. eastern Asia and the northern end of the 0278/7407/89/89TC-00709510. O0 Luzon arc, (2) an oceanic subduction area 964 Faure et al.: West Philippines Tectonics
in the South China Sea west of the Luzon arc along the Manila trench, (3) a complex collision area with transcurrent motion, between the islands of Palawan on one hand and Mindoro-Panay on the other hand, (4) an oceanic subduction area of the Sulu Sea below the central Philippines along the Negros trench, and (5) an oceanic subduction area of the Celebes Sea below Mindanao. __2 o_•__ i[ SEAPLATE The geology of the Palawan-Mindoro area has been studied by many workers (e.g., Hamilton, 1979; Hashimoto, 1981; Holloway, 1982; McCabe et al., 1982, 1985; Taylor and Hayes, 1983; Karig, 1983; Rangin et al., 1985; Stephan et al., 1986; Mitchell Mindoro et al., 1986; Sarewitz and Karig, 1986; Marchadier, 1988). According to their conclusion, the structure of this area is the result of a collision of the Palawan microcontinent with an eastern microblock Calarnian called the East Mindoro Block. The former microcontinent includes,in addition to the Palawan northern part of Palawan, the Calamian Islands, the western part of Mindoro, and the Buruanga peninsula in western Panay ZarnbOan (Figure 1). It was rifted from SE China in Borneo Oligocene-early Miocene times when the South China Sea opened. The East Mindoro Block includes, at least, eastern Mindoro, Lubang, Golo, and Ambil Islands (Figure Fig. 1. Present geodynamic framework of 2). The eastern limit of this block is the Philippine archipelago. The dot unclear. The origin of the East Mindoro pattern in the marginal seas stands for block is also disputed. For Karig (1983), oceanic crust. McCabe et al. (1982, 1985), Mitchell et al. (1986), and Sarewitz and Karig (1986) Because of the complex Cenozoic it is an "allochtonous terrane" traveling geodynamic evolution of the region, the from some remote southern region. For correlations between each group are not Hashimoto (1981), Holloway (1982), Rangin settled. This paper deals with the et al. (1985), and Marchadier (1988), the tectonics of the Mindoro, Romblon, and East Mindoro Block is a small piece of the Palawan metamorphics. The regional Palawan microcontinent which has been structure, deformation stages, and slightly pulled apart in Oligocene times synmetamorphic kinematics are described, from the main part of the Palawan and a correlation is proposed. Assuming microcontinent and rewelded during Middle that these pre-Eocene rocks belong to the Miocene times. Palawan block, a geodynamic model Pre-Tertiary rocks have been recognized accounting for the pre-South China Sea for a long time in the central Philippines geohistory of Southeast Asia is discussed. (e.g., Irving, 1950; Gervasio, 1967; Bureau of Mines, 1963). These metamorphic THE MINDORO METAMORPHICS rocks presently form three geographic groups: (1) the Palawan metamorphics found The Mindoro metamorphics form the in North Palawan and Calamian; (2) the backbone of the island, culminating in Mindoro metamorphics found in East Mt. Halcon (Irving, 1950; Teves, 1953; Mindoro, Lubang, Golo, and Ambil; and (3) Hashimoto and Sato, 1968). These rocks the Romblon metamorphics found in Tablas, are well exposed on the north coast of Romblon, Sibuyan, Carabao, and Panay Mindoro. Though separated by strike-slip (Figures 1 and 2). These basement rocks faults (Figure 3), the metamorphics are composite. Some are really outcropping in the islands of Lubang, metamorphosed while some are only weakly Golo, and Ambil belong also to the same transformed or virtually unmetamorphosed. group. Faure et al.: West Philippines Tectonics 965
q;•. • -- LUBAN6 I -'dk'i---x-• AMBIL I2I I I22 N
Gneissicbasement
• UitramaficsSchistseries
a ROMBLON ß. TABL ?•.,/Tertiarygranite SiBUYANr•'""•• Mesozoic • • otistostrome-. • EARABAO•" •
Fig. 2. Structural map of the Mindoro-Lubang and Romblon-Tablas areas. VPF, Verde Passage fault. Arrows represent average stretching lineation trends, with shear sense of the upper block with respect to the lower block. Simplified from Bureau of Mines (1963).
sw NE
X•oA•o'' v ••• --x-X x + + PALAWA•/• • MINDORO Io-M-PJOEgoHio - Ptiocene •j;•j•;¾Z;;•Utframafics&hibo,fe sore
'•EocenecJasfics • Gneiss.Hindoro basemenl' undifferenfiafed
•g. 3. •tesent ctustal-scale •ntetpte•at•ve ctoss section o•
Lithology and Age serpentinized ultramafics (mainly harzburgite) found in Puerto Galera, Four lithologic groups are Golo, and Ambil Islands (Caagusan, 1966; distinguished from top to bottom: (1) Andal and Caagusan, 1968; Karig, 1983; olistostrome with chert blocks poorly Malicse and Yumul, 1986; McCabe et al., exposed near Puerto Galera, (2) 1985) (they are not studied here) (3) a 966 Faure et al.: West Philippines Tectonics sedimentary sequence formed by pelitic for most of the Mansalay formation. The schists, metasandstone, and basic Mesozoic fossils are probably reworked schists: volcanoclastites, gabbroic either in themselves or together with sandstone, and breccia, other facies such their surrounding rocks as olistoliths. as metachert and marble are also found, Therefore the unconformity, if real, is and the association of terrigenous probably younger than Mesozoic. Moreover, sediments together with chert and basic as the main outcrops of the Mansalay schists (indicating a kind of ophiolitic formation lay on west Mindoro, the detritism) suggests that they formed in metamorphic pebbles might also derive from an active margin environment, (4) a the Palawan metamorphics. gneissic sequence outcropping in Mindoro, The Mindoro metamorphics are along the Odalo river, (Caagusan, 1966) undoubtedly unconformably covered by and in east Lubang and west Golo hemipelagic siltites and sandstones (the (unreported until now). Several gneissic Lasala formation) of middle Eocene age facies are recognized: arkosic or acid (Hashimoto, 1981; Sarewitz and Karig, volcano-detrital rocks, biotite bearing 1986; Marchadier, 1988). As already paragneisses, orthogneisses noticed (Karig, 1983; Sarewitz and Karig, (metagranodiorite) crosscut by 1986) there is a metamorphic and leucocratic gneisses (probably dykes), structural gap between the well-foliated and metadioritic gneiss and xenolithic basement and the Eocene cover, which is schists. There is no radiometric data, affected only by a slaty cleavage. In but a reasonable interpretation is to western Lubang (at Binacas) we found a consider the gneisses as a sialic reef limestone containing the following basement formed by magmatic rocks and a foramminifera: Pellatispira mirabillis sedimentary cover. (Umbgrove), Operculina cf. saipanensis, The Mindoro metamorphics lack fossils Amphistegina radiata, Rotalidae sp., and or isotopic data, and hence the age is Spherogypsina sp. determined by J. unknown. Caculitan et al. (1978) assume Butterlin (Marchadier, 1988) to be of that they are Jurassic or Cretaceous. On late Eocene age. In conclusion the the basis of Sr isotopic composition of Mindoro metamorphics are not younger than the marbles, Knittel and Daniels (1987) Eocene and probably not older than late have proposed an early Paleozoic age for Paleozoic. the protolith, though according to their own data a late Paleozoic age ranging between middle Carboniferous to early Regional Structures Permian seems more likely in agreement with the geological environment. Recent K- The structures are obviously due to a Ar datings on hornblende separates from polyphase deformation. In the Calavite amphibolite and garnet amphibolite rocks area (Figure 4) the metamorphic rocks from Mindoro indicate a 59 Ma age of overthrust Eocene slates (Rangin et al., metamorphism (R. McCabe, written 1985). In the slates the slaty cleavage communication, 1989). According to Karig bears an E-W trending mineral lineation (1983) and Sarewitz and Karig (1986), of phyllites and a stretching lineation upper Cretaceous basalts overlay the of pull-apart and elongated clasts in Mindoro metamorphics. However, the pebbly mudstones, which is in agreement contact is never observed, and the with the Oligo-Miocene tectonics. The precise fauna list is not given. It is kinematics of this phase have not been often argued that the Mindoro metamorphics studied. are of Paleozoic or at least pre-late In addition to these Cenozoic Jurassic age (e.g., Hashimoto and Sato, structures, two ductile deformation 1968; Hashimoto, 1981) because they are phases affect the metamorphic rocks. The unconformably overlain by the Mansalay second one is responsible for kilometer formation which is a turbiditic sequence scale upright antiforms (Figures 2, 4, and with intraformationnal conglomerate- 5). They trend close to N-S in Central bearing pebbles of crystalline schists and Mindoro and Wawa area, and they are Callovian-Oxfordian ammonites (Sato, 1961; deflected to the WNW-ESEtrend in Lubang Andal et al., 1968). However, objections (Caagusan, 1966; Hashimoto and Sato, 1968) arise, first, the unconformity is unclear by the left-lateral Lubang fault and second, micropaleontological studies (Marchadier, 1988). At the meter scale the (Marchadier, 1988) indicate an Eocene age foliation (S1) is microfolded and cut by a Faure et al.: West Philippines Tectonics 967
LUBAN' • ' s , • •
Puerto Galera N. MINDORO •
lO km
t01igo-Hio-P[iocene '•.... Ulframaficsandamphibotife -•-•• Schlsfandmarb[e .'$•...Mesozoic olisfosfrome 1•, I gneiss Sfref[:hingiineafion wifh Shearsense • posf-Eocenefhnusf -'" anfiform •'•• synforrn • pre-Eocenefhrusf possibly neused Fig. 4. Lineation mapand cross section of the North Mindoro-Lubangarea. Arrows represent average stretching lineation trends, with shear sense of the upper block with respects to the lower block. Simplified from Bureau of Mines (1963). Numbers6A, 6B, and 6C are the locations of Figure 6 photographs. discrete crenulation cleavage (S2) which upon the schists is described near Puerto is axial planar to parasitic minor folds Galera (McCabe et al., 1985; Malicse and associated with the large-scale antiforms. Yumul, 1986). At the microtectonic scale Along the S2 surfaces, opposite offsets the thrusting is responsible for the and curvature of S1 are observed on both regional foliation S1 observed in every limbs of the fold. The apparent lithology except the granitic core of the displacement is to the north in Lubang and antiforms. S1 bears a mineral and to the south in Calavite. Shear criteria stretching lineation L1 which is well such as pressure shadows are lacking, it marked in all rocks except some massive is likely that the offsets are mainly marbles and the peridotites. L1 is also apparent due to volume change caused by refolded by the crenulation folds. flattening. The regional major structure, prior to Metamorphismand Kinematics of the First the folding and faulting events, Phase corresponds to a stack of thrust sheets. Generally, the precise contacts are not A cursory examination of the observed, but they are well exposed on metamorphic minerals present was carried the south coast of Lubang and Golo. The out. In pelitic schists the most common gneissic series are thrust by the schist assemblage is quartz, albite, biotite, and series which in turn is thrust by the garnet; stilpnomelane is conspicuous in serpentinized peridotite (Figures 2 and gneisses; actinolite and epidote are 4). A similar thrust of the ultramafics present in basic schists. As these phases 968 Faure et al.: West Philippines Tectonics belong to the epidote-amphibolite facies, pressure and temperature during the synmetamorphic deformation are estimated around 5 kbar and 450ø-500øC, respectively. As already stated, L1 is a mineral and stretching lineation. In the field it is also marked by cigar-like quartz rods in pelitic schists, biotite streaks in gneisses, and elongated pebbles of conglomerate. The L1 trend (Figures 4 and 5) is dominantly submeridian: NW-SE in NORTH TABLAS Lubang, Golo, and Calavite areas; N-S near Puerto Galera, and E-W west of Wawa. The last trend, mainly found just below the Eocene slate, could be due to a local reorientation when the Eocene rocks were deformed. The deformation regime has been investigated by looking at the asymmetric structures in sections perpendicular to S1 and parallel to L1 (e.g., Eisbacher, 1970; Etchecopar, 1977; Simpson and Schmid, 1983; Passchier and Simpson, 1986; Cobbold SOUTH TABLAS et al., 1987). The most commoncriteria are asymmetric pressure shadows (Figures 6b and c) which are widespread around clasts in sandstone and impure marble. These shadows are also found around metamorphic minerals (albite, garnet) in pelitic schist and around pyrite or epidote in basic schist. Sigmoidal biotite (Figure 6a) are found in gneisses. The quartz C axes preferred orientation have been used also. The senses of shear
(Figure 4) along the lineation show a LUBANG consistent shear from the north toward the south. Assuming that the lineation reflects the nappe transport direction for a shear intensity (•) greater than 5 (e.g., Escher and Watterson, 1974; Mattauer et al., 1981; Malavieille et al., 1984) the nappe emplacement occurred from north to south. The geodynamic significance of the motion will be discussed below.
NORTH MINDORO COAST THE ROMBLON METAMORPHICS East of Mindoro, "basement rocks" are Fig. 5. Stereographic diagrams of recognized in the islands of Tablas, Lubang, North Mindoro coast, and north Romblon, Sibuyan and west Panay (Figures and south Tablas areas. The squares 2 and 3). In the following we deal only represent pole projections of foliation with Tablas, Romblon, and Sibuyan areas. planes. The triangles represent the It is sometimes argued (e.g., Karig, 1983; projection of stretching lineations. The Mitchell et al., 1986) that a major strike- N150øEtrending line on the foliation slip fault, the "Tablas lineament", pole diagram of Lubangis the axis of the separates Tablas and Romblon. On the east upright fold. Numbers of measurements coast of Romblon, no microstructure given next to each lower hemisphere favoring such a fault has been observed. projection. Faure et al.: West Philippines Tectonics 969
Fig. 6. Microscopic shear criteria in Mindoro-Lubang and Palawan areas. Scale bar is 0.5 mm. Photo localities are shown on Figures 4 and 10. (a) Sigmoidal muscovite in gneiss, Lubang. (b) Epidote with asymmetric pressure shadows in mafic schist, Mindoro. (c) Clast with asymmetric pressure shadows in pelitic schist, Lubang. (d) Quartz clast with asymmetric biotite pressure shadows, West of Caramay, Palawan.
However, since sea bottom deepens very LitholoRy and ARe quickly, such a fault may exist but is of minor importance as far as the basement Three series are distinguished from geology is concerned. The equivalence top to bottom, namely, (1) an ophiolitic between the metamorphic rocks of Tablas, series underlain by an amphibolite sole Romblon, and Sibuyan can be safely commonin other ophiolitic overthrusts considered as verified by lithologic, (e.g. Oman, Newfoundland, etc), (2) a metamorphic, and structural similarities. schist series formed by pelitic schists, In the following they will be referred to metasandstone, quartzite, and marble as the Romblon metamorphics, since these (very abundant in Romblon); several kinds rocks are best exposed on the island of of basic schists: lavas, tuffs, basic Romblon. In Tablas the metamorphic rocks siltites, gabbroic sandstones, and outcrop only in the north, east, and south conglomerates belong to this series, and areas (Figure 7). The Romblon metamorphics (3) a gneissic series formed by acidic are similar to the Mindoro metamorphics. volcaniclastic rocks is found only in However, here we discuss the typical Sibuyan. Some weakly or unfoliated features of the Romblon metamorphics granitoids found as float in northern separately. Tablas might belong to this group. 970 Faure et al.: West Philippines Tectonics
TABLAS• • ROMBLON
7C SIBUYAN
• 12 30 7A
•---•J01Jgo - Mio - Ptiocene .••__]ø 0tisfostrome 12'15 :'"'"""•Utframafics &hibotife sore ._-•Peliticschists withmarbles andbasic schist intercatation FT• 6neissicbasement
;• 122'•I CARABAO -I-L_-l-__lTertiary granite '12'•ø 1•km
Fig. 7. Lineation mapof the Romblonmetamorphics. Arrows represent average stretching lineation trends, with shear sense of the upper block with respect to the lower one. Simplified from Bureau of Mines (1963).
However, these rocks could also belong to The ductile structures are grouped into the Paleogene granites widespread in this two phases. The youngest deformation is area (Metal Mining Agency, 1983). Neither responsible for upright folds which are stratigraphic nor radiometric ages are well observed in Romblon and Tablas. available except the Permian limestone of Their axes are submeridian in average, Carabao island (Andal et al., 1968) which with local arcuation (Figure 7). A is an olistolith. The only chronological crenulation cleavage develops which is constraint is the Oligocene-early Miocene axial planar of meter and centimeter arc sequence found to unconformably cover scale parasitic folds. The first phase the metamorphic rocks in Tablas, Romblon regional-scale structure cannot be and Sibuyan (Marchadier, 1988). Moreover, inferred from one cross section, but from the metamorphics are intruded by the bulk geometry of the area and undeformed granitoids of unknown age in correlations between several cross Tablas but supposed to be of Paleocene age sections one can consider the stack of in Sibuyan (Metal Mining Agency, 1983). thrust sheets as the major structure. The ophiolite sequence overthrusts the schist series which in turn overthrusts the The Polyphase Deformation gneissic series (Figure 8). This main phase occurred in a metamorphic context and is responsible for the regional Only the ductile structures, are foliation (S1) and stretching lineation considered here, (a discussion of brittle (L1). The lineation trend (Figures 2, 5, structures is given by Marchadier 1988). and 7) shows that the submeridian Faure et al.: West Philippines Tectonics 971
Fig. 8. Interpretative schematic cross section from Tablas to Sibuyan along the line shown in Figure 7 (same symbols). direction (from NE-SW to NW-SE) is extinction, plane joint boundaries, triple dominant except in east and south Tablas junctions) indicate a post tectonic where it is E-W. In southern Tablas a annealing which is likely related to post-S1 thermal event (discussed below) intrusive rocks that have not been found has erased early structures. In Romblon in the field. Along streams, biotite the mineral lineation is strengthened by granite and pegmatite boulders are the axes of isoclinally folded quartz conspicuous. lenses which display sheath fold structures characteristic of non coaxial deformation (e.g., Cobbold and Quinquis, Kinematics of the First Phase 1980).
Asymmetric quartz lenses or large- Metamorphisms scale pressure shadows have been used directly in the field as shear indicators. At the microscopic scale the same criteria The type of metamorphism of Romblon as those used for the Mindoro metamorphics -Sibuyan differs from the metamorphism have been systematically employed: observed on Tablas. On Romblon-Sibuyan the asymmetric pressure shadows, sigmoidal assemblages of quartz-muscovite-biotite - phyllosilicates, and quartz C axis graphite in pelitic schist and actinolite- preferred orientation (Figure 9a and 9c). chlorite-epidote in basic schist indicate In south Romblon, S1 is cut by oblique low-pressure greenschist facies surfaces, corresponding to a kind of conditions. On Tablas there is a crenulation cleavage, the spacing is conspicuous thermal event. In south between 3 and 10 mm. There is no Tablas, though no granitoids have been microfolding of S1 between two shear found except one meter scale dyke, the surfaces, these cleavage zones end by sandstones and pelites are transformed curving and merging with S1, and there is into biotite hornfelses, suggesting the no obvious chemical differentiation in existence of an underlying mass. In east the cleavage surface (thus the volume is Tablas, quartz-biotite-garnet-staurolite conserved). These lines of evidence show assemblage is sporadically found in that these bands are small-scale shear pelitic schists, indicating higher - zones with a normal offset (Figure 9b). pressure and temperature conditions, about These shear bands indicate a real 4-5 kbar and above 550øC respectively. The displacement of S1, with extension garnet, biotite, and staurolite parallel to L1. The downfaulted block is porphyroblasts have a larger size than the always the southeast block. Such a matrix minerals upon which they overprint, microstructure is similar to the as shown by the subautomorph shape of the extensional crenulation cleavage (Platt garnet and the inclusions in staurolite. and Vissers, 1980) often found near the However, these rocks also experienced a mylonitized contacts. Though not always ductile deformation, indicated by the unequivocal kinematic indicators, the quartz pressure shadows around garnet and asymmetry of the Romblon extensional biotite or kinked staurolite with folded crenulation cleavage is in agreement with inclusions. Because of this thermal event, a shear directed toward the southeast all the eventual previous metamorphic along the lineation. The differences of assemblages which might have appeared the S2 cleavage, in which microfolded S1 related to the thrust emplacement are surfaces and pressure solution erased. In north Tablas the deformation are shown by concentrations microstructures of the hornblende crystals of ferromagnesian particles argue against in the amphibolite sole (e.g., sharp a shear displacement along S2 planes. 9?2 Faure et al.: West Philippines Tectonics
Fig. 9. Microscopic shear criteria in Romblon-Sibuyanarea, located on Figure 7. (a) Sigmoidal chlorite in quartz schist, Romblon.Scale bar = 1 mm. (b) Shear bands in pelitic schist, Romblon. Scale bar = 1 mm. (c) Hornblende with asymmetric chlorite pressure shadows in basic schist, Romblon. Scale bar = 0.5 mm. (d) Shear bands in volcanoclastic rock, Sibuyan. Scale bar = 1 mm.
A bulk southward shear is clearly structure and related synmetamorphic inferred in Sibuyan and Romblon (Figure deformation is erased by a thermal 7). As already stated, the metamorphism, probably due to granitic microstructures of south and east Tablas intrusion. were formed during a high-temperature (HT) metamorphism. Therefore they are not AN OUTLINE OF THE NORTH PALAWAN GEOLOGY related to thrusting. In east Tablas the shear criteria along a high-angle dipping Field Observations foliation indicate an eastward motion of , the upper part, while in south Tablas, where the foliation is on average Field observations in North Palawan subvertical, no shear sense has been lead us to consider the following observed. When the foliation is flat lithostratigraphic and tectonic units lying, the N120•E trending lineation is (Figures 10 and 11). Though partly similar refolded by N70•E overturned folds, but to previous divisions (Bureau of Mines, no shear sense can be found. 1963, Bureau of Mines and Geosciences In conclusion, two distinct domains (BMG) 1981; United Nations, 1985; Raschka belong to the Romblon metamorphics: a et al., 1985, Wolfart et al., 1986), they northern area interpreted as a stack of differ on some lithologic and synmetamorphic thrust sheets, emplaced in stratigraphic attributions. Several small ductile conditions from north to south, Neogene granitic rocks cross cut the flat - and a southern area where the thrust lying formations of North Palawan. From Faure et al.: West Philippines Tectonics 973 top to bottom the following sequence is marbles, and quartzite are found at the recognized. base of the thrust. Early Cretaceous An ophiolitic sheet composed of fossils have been found in cherts or red harzburgite, serpentinite, gabbro, pillow mudstones covering the volcanic rocks basalt, and pelagic sediments forms the (Benard and Muller, 1986; BMG, unpublished uppermost unit. Locally, this unit has an data, 1988). The ophiolitic sheet thrusts amphibolite sole, and some micaschists, a massive late Eocene to early Miocene reef limestone (St. Paul's, E1 Nido and Pabellon formations) which in turn unconformably overlies a clastic sequence. This clastic sequence is the most 1 widespread unit. Several facies such as 1119 turbidite, slump deposit, pebbly mudstone, (•) with exotic and thick sandstone are recognized. The "Sagasa tectonic complex" (United Nations, blocks 1985) is in fact formed by synsedimentary Sound disrupted sandstone lenses and scarce chert and pillow lava olistoliths. An Miocene œa•wag upper Cretaceous age is obtained from granit e (•pe• microfossils (Grellman and Newton, 1980; Saldivar-Sali et al., 1981; Wolfart et al., 1986; Benard and Muller, 1986). The "Concepcionpebbly phyllite" and the "Tinitian creek conglomerate" (United TerNary timestone Nations, 1985) are pebbly mudstone formation. The northernmost end of Palawan eramayslump deposit was thought to represent the normal pebblymudsforte stratigraphic sequence of the Palawan ;andstoneUpper Ere•a[eous- Block, ranging from Permian to Jurassic Eocene (e.g., Hashimoto and Sato, 1973). However,
Bay faul! sedimentological observations have shown , 30 km that it is an olistostrome, bearing late Jurassic or early Cretaceous radiolarians Fig. 10. Schematic structural map of into the matrix (Faure and Ishida, 1989). North Palawan. E-W bars, Miocene The lowermost unit is a metasandstone stretching lineation without shear sense; unit called the "Caramay schists" (United N-S arrows, late Cretaceous stretching Nations, 1985). It consists of schistose lineation showing southward movement of sandstone and phyllite metamorphosed into upper versus lower material. greenschist facies conditions. The North Reinterpreted from Bureau of Mines (1963) Palawan area is often considered as and United Nations (1985). extensively covered by the "Barton
W E :SW NE
UtuganBay Fautt '! Dumaranweft I •z.... ."•:•?!•.
I' -t-] Miocenegranite • Tertiarylimestone • Herasandstone UpperEretaceous-Eocene • Palawanbasement dasti( formation
.o•o,.'.•, o[istostromeJurassic - early Cretaceous Fig. 11. Interpretative schematic cross section of North Palawan, along line shown in Figure 10. 9?4 Faure et al.: West Philippines Tectonics metamorphics" (Reyes 1941, in Bureau of contain the same lithological Mines 1981). However, except for the assemblages, namely the pelitic schist- Caramay schists and the amphibolite sole limestone series of Puerto Galera and below the ophiolite, the North Palawan Romblon; basic schists of Mindoro, rocks do not seem to have experienced any Lubang, Romblon, and Sibuyan, metamorphism stronger than some epizonal paragneisses of Mindoro, Lubang and recrystallization of phyllites. Sibuyan. Second, with the exception of the anomalous HT area of SE Tablas, similar metamorphic conditions around Regional Structure of North Palawan and 450ø-550øC and 4-5 kbar are found in both Thrust Kinematics , areas. Third, the same regional stack of thrust sheets, olistostrome, ophiolite, and schist is observed. Finally, in both Hamilton 1979, Holloway 1982, Hinz and areas, the thrust displacement sense Schl•ter (1985), and Mitchell et al. inferred from microtectonics is toward (1986), suggest that the ophiolites the south. Radiometric data in both areas overthrust the clastic sequence and are lacking. Another indirect piece of probably the limestone. One interpretation evidence is that the same early Miocene is that this middle Miocene deformation is volcanic arc overlies both the Mindoro relevant of the "Mindoro collision" and Romblon metamorphics, showing that (Rangin et al. 1985; Marchadier, 1988). they have been put together at least at The conspicuous E-W trending lineation and the end of Paleogene (Marchadier, 1988). microscopic shear criteria suggest a Resemblances are less clear between Miocene thrusting from west to east. This the Mindoro metamorphics and the phase is a secondary one, an earlier metasandstone unit of Palawan. However, ductile deformation is recognized both in the same order of units and thrust the Caramay biotite schists and the vergence suggest that pre-Eocene amphibolite sole. It is characterized by a deformation in both areas may belong to N-S trending mineral and stretching the same event. lineation (L1) and isoclinal folds with axes parallel to L1. Asymmetric pressure The Palawan Block shadows around quartz clasts indicate a , southward shear (Figure 6d). This phase corresponds to the initial synmetamorphic Additional evidence that the Palawan thrusting from north to south of the and Mindoro-Romblon metamorphics belong ophiolitic sheet upon the Caramay schists. to a single basement unit is the The fragments of micaschists and olistostrome. Though not explicitly quartzites found below the amphibolite recognized as olistostrome, this might be scrapped parts during the formation has been found in North Palawan thrusting. Moreover, drilling has shown (Faure and Ishida, 1989), Calamian that around Dumaran and Paly islands (Fontaine, 1979; Fontaine et al., 1979; (Figure 10) the olistostrome is underlain Isozaki and Amiscaray, 1987), Buruanga by serpentinized ultramafics (Grellman and peninsula (Fontaine et al., 1983), Newton, 1980). These lines of evidence Carabao and North Mindoro (M. Faure et al. show that the structure of North Palawan, unpublished data 1987). This olistostrome prior to the Miocene tectonics, appears as a key formation for the corresponds to a similar stack of thrust recognition of the North Palawan Block of sheets as observed in the Mindoro and Hamilton (1979) and Holloway (1982). As Tablas areas (Figure 11). the distributions of the olistostrome and metamorphics are overlapping in the study area, the combined use of these two DISCUSSION AND INTERPRETATION formations allows us to enlarge the area of the North Palawan Block. As a working hypothesis, we propose ComparisonsBetween the Mindoro• Romblon• that the North Palawan Block can be and Palawan Metamorphics extended to (1) South Palawan, since the Ulugan Bay fault (Figure 10) is a minor Several lines of evidence suggest that structure, and the same formations are the Mindoro and Romblon metamorphics were found on both sides (Wolfart et al., formed during the same event. First, they 1986); (2) West Panay where early Faure et al.: West Philippines Tectonics 9?5
Cretaceous ophiolites and metamorphics are and Hayes, 1983; Tapponnier et al., 1986; reported (Florendo, 1981; McCabe et al., Ru and Pigott, 1986) if we close the 1982); and (3) Zamboangaarea, where pre- South China Sea, the presently Eocene serpentinites, marbles, pelitic, submeridian lineations keep the same and basic schists with submeridian trend or become close to NW-SE. lineations are described (Antonio, 1972). We propose the following scenario. During late Jurassic to early Cretaceous (Figures 12 and 13) an oceanic area was A Pre-Cenozoic GeodynamicModel closing by subduction below Asia. Assuming that the thrusting direction In Palawan, Mindoro, and Romblon areas inferred from lineations corresponds the pre-Eocene structure corresponds to a approximately to the plate motion vector stack of thrust sheets formed by at that time (e.g., Mattauer, 1975; synmetamorphic thrusting of oceanic Shackleton and Ries, 1984) the West series (i.e., ophiolites and schists) Philippines Block moved from S-SE to N- above a continental basement. The nature NE. At that time the South China margin (microcontinent, island arc, etc) and is characterized by an voluminous calc- extension of this basement is not known alkaline magmatism. In Taiwan and along exactly known, but it included at least the SE coast of Fujian province, Rb-Sr Palawan, Mindoro, Romblon, and Zamboanga ages of granites range between 120 and 90 areas. We propose to call it the West Ma (Jahn et al., 1976). Very often there Philippines Block. The thrusting occurred is a progressive transition from before Eocene and after early Cretaceous, magmatically oriented granites to which is the age of the sedimentary part plastically deformed orthogneisses and of the ophiolite. As this tectonism mylonites. Mapping, microtectonic predates the Oligo-Miocene opening of the analysis, and strain studies show that South China sea (Taylor and Hayes, 1983, the final emplacement of plutons Pautot et al., 1986), the ophiolites initiated by subduction is related to the which are significantly older than the left-lateral strike-slip faults parallel age of the South China Sea cannot be to the continental margin (Xu et al., obducted parts of the South China Sea, 1987; J. Charvet and M. Faure unpublished and any geodynamic model must take into data, 1988). A submeridian convergence account the contemporaneous events oblique to the SE China margin is in occurring along the Chinese margin. agreement with the synchronous left- Though the opening mechanism of the South lateral displacement along the faults China Sea is still disputed (e.g., Taylor (Figure 13).
SE CHINA W. PHILIPPINES synkinematic magmatism BLOCK el olistostrome
::".i:!:i:i:i:i:!:i ':':'"'"•
:i:i:!:i:i:!:i:i:!...... •..•;:•:•!.!.::!:!:i:i:i:i:!:!:i:!;•:!:•:•:i:•::':• LATEJUR :.:""..'.•i!![ii!i:.?.. EARLY CRETACEOUS
thrusting ophiolit•chi st LATE CRETACEOUS
...... ,.....;...... • ...... :.:.:.:.:.:.:.
Fig. 12. Geodynamic evolution model of SE China-West Philippines area. 9?6 Faure et al.: West Philippines Tectonics
EARLYCRETACEOUS / PALEOGENE
/
PHILIPPINES I • •_•CK
Fig. 13. Schematic tectonic map of the SE China-West Philippines area in pre-Eocene times. Early Cretaceous represents the oblique convergence stage. NC, normal component of subduction; LC, longitudinal component of subduction of the West Philippines Block with respect to Eurasia. Along the Chinese margin, left-lateral shearing and synkinematic magmatismoccur. Paleogene shows that the collision is completed. A suture zone extends along the southeast China margin.
At the end of the late Cretaceous the IFP. This is the CNRS-INSU DBT programm continental subduction of the West contribution 90. Philippines Block leads to collision and stacking of thrust sheets upon the block. REFERENCES Southward thrusting also occurred in Hong Kong where the lower Cretaceous red beds Andal, D. R., and N. L. Caagusan, Geology (Port Island formation) are cleaved of the iron deposits of northern (e.g., Ruxton, 1956; M. Faure, unpublished Mindoro, paper presented at the 2nd data 1988) and in Fujian province (J. Geological Convention, Geological Charvet and M. Faure, unpublished data Society of the Philippines, Manila,109- 1988). The nature of the Mesozoic SE China 120, 1968. margin is commonly considered of Andean Andal, D. R., J. S. Esguina, W. type, however according to this Hashimoto, B. P. Reyes, and T. Sato, collisional model it has to be revised. The Jurassic Mansalay formation, southern Mindoro, Philippines, Geol. Paleontol. Southeast Asia• 4, 179-197, 1968. Acknowledgments. We are deeply Antonio, L. R., Geology of the east indebted to G. Balce, R. Quebral, J. A. -central Zamboanga peninsula, Mindanao, Magpantay and all their colleagues from 87 pp., Bureau of Mines and the Bureau of Mines and Geosciences for Geosciences, Manila, 1972. their kind welcome and efficient support Benard, F., and C. Muller, Synthetic during our fieldwork in the Philippines. geological map obtained by remote J. Butterlin identified the foramminifera sensing, an application to Palawan from Lubang island. R. McCabe, Z. Ben- island, in Symposiumon Remote Sensing Avraham, and D. Sarewitz are thanked for for Resources Development, edited by their comments and advices on way to M.C.J. Damen, G. Sicco Smit, and H.T. improve the manuscript. Reasearch was Verstappen, pp., 103-109, Balkema, supported by grants from the CNRS-INSU and Rotherdam, Netherlands, 1986. Faure et al.: West Philippines Tectonics 9??
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Nations Development Program, New York, M. Faure, CNRSURA 1366, D&partement 1985. des Sciences de la Terre, Universit• Wolfart, R., P. Cepek, F. Gramann, E. d'Orl•ans, B.P. 6759 Orl&ans Cedex 2, Kemper, and H. Porth, Stratigraphy of France. Palawan island, Philippines, Newsl. Y. Marchadier and C. Rangin, Stratigr. • 16, 19-48, 1986. D•partement de G•otectonique, Xu, J., G. Zhu, W. Tong, K. Cui, and Q. Universit& Pierre et Marie Curie, Liu, Formation and evolution of the Paris VI, France. Tancheng-Lujiang wrench fault system: a major shear system to the northwest of (Received October 26, 1988 the Pacific ocean, Tectonophysics• 13•, revised March 13, 1989, 273-310, 1987. accepted April 5, 1989.)