The Mindanao collision zone: a soft collision event within a continuous Neogene strike-slip setting

Journal of Southeast Asian Earth Sciences, Vol. 6, No, 3/4, pp. 239-248, 1991

0743-9547/91 $3.00 + 0.00 Pergamon Press Ltd

Printed in Great Britain

The Mindanao Collision Zone: a soft collision event within a continuous Neogene strike-slip setting M . PUBELLIER,* R . QUEBRAL,~" C. RANGIN,* B. DEFFONTAINES,* C. MULLER,~ J. BUTTERLIN [I a n d J. MANZANOt *Laboratoire de Geotectonique, Universit6 Pierre et Marie Curie, 4. pl. Jussieu, 75252 Paris Cedex 05, France; "['Bureau of Mines and Geosciences and Mineral Resources, Manila, Philippines; J~I, rue Martignon, 92500, Rueil Malmaison, France and I[Institut de Physique du Globe, Paris, France

(Received 31 August 1990; accepted for publication 5 May 1991) Alan.act--Two volcanic belts are presently juxtaposed on Mindanao Island in the southern Philippines. Southward, the collison is still active in the Molucca Sea which is commonly regarded as a region of doubly verging subduction, plunging eastward below the Halmahera arc and westward below the Sangihe arc. In the MoUuca Sea, tectonic features related to the incipient collision appear only in the very thick sediments of the basin, and the morphology of the parallel Halmahera, Talaud and Sangihe ridges is closely controlled by recent N-S strike-slip faults. Among these faults, the Philippine Fault is a neotectonic feature crosscutting the Agusan-Davao Basin which seals tectonic events not younger than Eocene. In addition, the Central Cordillera shows strong similarities with the Pacific Cordillera for both stratigraphy and tectonic evolution, and several indications favour a Eurasian margin affinity for the Daguma Range (Southern and Eastern Kudarat Plateau that may be part of the Sangihe arc, as inferred for the Zamboanga Peninsula and the Northern Arm of Sulawesi. Thus the island of Mindanao can be divided into two composite terranes, the western one (northward extension of the San#he arc) being restricted to the Kudarat Plateau and the Zamboanga Peninsula. The apparent continuation of the Sangibe arc into the Central Cordillera of Mindanao is thus the result of post collision tectonics. The portion of the suture where the collision is completed curves westward north of the southern peninsula and extends beneath the sediments of the Cotabato Basin or the volcanic plateaus of the Lanao-Misamis-Bukidnon Highlands. In the northern part, the contact is linear and suggests, together with the absence of compressional deformation, a docking of the eastern oceanic terrane (Philippine Mobile Belt-Halmahera arc) against the western continental terrane (Zamboanga-Daguma) in a strike-slip environment. Prior to Early Pliocene, the eastern and the western terranes were subject to different tectonic regimes with direction of extension perpendicular to the present one. From Late Pliocene to present, both terranes are affected by NNE and E-W compression.

INTRODUCTION

ORIGIN OF MICROBLOCKS

THE PHILIPPINE archipelago is generally regarded as a complex plate boundary (Fig. 1) resulting from successive collages of exotic terranes (Karig 1983, Karig et al. 1986, McCabe and Almasco 1985). The oblique convergence of the Philippine Plate relative to the Eurasian Plate (Uyeda and Ben Avraham 1972, Cardwell et al. 1980, Seno and Maruyama 1984) has been accommodated by large strike-slip (Karig et al. 1986, Sarewitz and Karig 1986) and collision zones (Rangin et al. 1985). The orogenic belt that resulted from this tectonic framework extends from Taiwan (Davis et al. 1983, Pelletier et al. 1985), through Luzon where both strike-slip and shortening have occurred (Bachman et al. 1983, Haeck and Karig 1985), Mindoro and Panay (Rangin et al. 1985, Marchadier and Rangin 1989), to Mindanao (Hawkins et aL 1985) and the Molucca Sea (Moore and Silver 1983). Vertical strike-slip boundaries may have evolved to shallow-dipping thrust zones (Karig et al. 1986). Some may have been relayed spatially by subduction or collision zones (Marchadier and Rangin 1989). Evidence of Miocene strike-slip deformation is pointed by microtectonic analyses of the metamorphics rocks either of the Philippine arc (Geary and Kay 1989) and of the basin edges in Luzon, Mindoro (Bachman et al. 1983, Maleterre et al. 1988), and Mindanao (Pubellier and Rangin 1989).

A close look at the geology of the microblocks jammed in the N-S plate boundary between Taiwan and Sulawesi (Fig. 1) shows that they can be attributed to either the continental Eurasian Plate or the Philippine Plate of oceanic and island arc origin (Rangin et al. 1989a). Such a pattern has been used as a basis for recent reconstructions (Daly 1987, Jolivet et al. 1988, Rangin et al. 1990). Basement of continental origin exists in the Palawan block, which was rifted away from the China margin by the opening of the South China Sea (Taylor and Hayes 1980, Holloway 1982). Similarly, opening of the Celebes and Sulu marginal basins within the free edge of Eurasia has isolated continental fragments (Rangin et al. 1989b). High to medium grade metamorphics are found in Palawan, east Mindoro and Panay (Irving 1950, Teves et al. 1953, Mitchell et al. 1986), in Romblon and Sibuyan (Corby 1951, Faure et al. 1989), in Zamboanga Peninsula (Santos Ynigo 1953, Ranneft et al. 1960, Antonio 1972) and in the north arm of Sulawesi (Ratman 1976). To the east, the Philippine Mobile Belt is composed of a large variety of blocks lacking evidence of continental origin (Mitchell et al. 1986). Basement is represented by ultramafic or metavolcanic rocks (Hamilton 1979, Moore and Silver 1983). A volcanic arc was developed

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upon this oceanic and island arc basement (Wolfe 1981, Lewis and Hayes 1983, Maleterre et al. 1988). Stratigraphic records indicate that the Early Neogene is also remarkably homogeneous all along the Philippine arc on which a tremendous amount of Early Miocene coarse clastics grading upward to finer terrigenous material are deposited (Bachman et al. 1983, Rangin et al. 1985, Aurelio et al. 1990, Pubellier and Rangin 1989). These

basins generally exhibit a Late Miocene unconformity followed by a renewal of coarse clastic deposition or in places, reefal limestones. The clastics mostly contain peridotites and volcanic pebbles and grains, and are devoid of continentally derived materials like the gneisses, micaschists and quartz sand known on the Eurasian fragments (Faure et al. 1989, Marchadier and Rangin 1989).

The MindanaoCollisionZone

241

MOLUCCA SEA AND PHILIPPINE FAULT

GEOLOGY OF MINDANAO

Situated south of Mindanao, the Molucca Sea is accepted as an example of an active arc-arc collision zone between two arcs facing each other (Hatherton and Dickinson 1969, Roeder 1977, Silver and Moore 1978, Cardwell et al. 1980, McCaffrey et al. 1980, Moore and Silver 1983). Westward, the Sangihe Arc is created by the west-dipping subduction of the Molucca Sea Plate (Krause 1966) and eastward, the Halmahera arc is the result of the at least Pleistocene (and probably Late Miocene) to present subduction of the east-dipping Molucca Sea (Hall et al. 1988, Hall and Nichols 1990). Based on the absence of apparent correlations across the Agusan-Davao valley, Hawkins et al. (1985) suggested that the basin may represent the northward extension of the Molucca Sea collision zone. However, there is very little evidence in Mindanao for the postulated subduction in either direction (Mitchell et al. 1986), despite the trace of a west-dipping slab beneath Mindanao Island, which is distinct from the one being subducted in the Philippine trench. The lack of constraint upon the different models is largely due to the paucity of reliable data concerning the geometry of the major faults in Mindanao. The Agusan-Davao Basin is crosscut by the active transcurrent Philippine Fault (Willis 1937), that passes west of the East Mindanao Ridge (Ranneft et al. 1960, Allen 1962, Philippine Bureau of Mines 1963). Although the fault is well manifested on seismic reflection profiles in the north (Bischke et al. 1988), it is difficult to trace it beneath the Quaternary alluvium and some authors doubt its extension in central Mindanao (e.g. Hamilton 1979). Various attempts to estimate the motion along the fault have been performed, the latest of which indicates a velocity of 2.5 cm/y (Barrier et al. 1991). The Cotabato Trench represents the subduction of the Celebes Sea beneath the Kudarat Plateau and the Daguma Range at a depth not greater than 85 km (Acharya and Aggarwal 1980). Focal mechanism solutions indicate east-west compression, but the width of the accretionary prism decreases towards the south along the Sangihe Ridge (Moore and Silver 1983), suggesting that the subduction vanishes southward where vertical motion along N-S faults prevails. The Sulu Trench is almost aseismic in its southern and central segments. Deformation occurs at the foot of the inner wall (Mascle and Biscarrat 1978, Hinz and Block 1990) and seismicity is more significant in the northern part where it deepens before connecting to the Negros Trench. Large volcanoes also exist in their fumarolic stages in Basilan Island on the Sulu Ridge and other recent volcanoes exist in northeast Zamboanga. The southern margin of the Sulu Ridge may be controlled by tilted blocks as suggested by the geometry of the southern coastline of Zamboanga and by a unique seismic profile (Rangin and Silver 1990).

Mindanao Island may be divided into two large possibly composite terranes (Figs 2, 3): an eastern terrane located east and north of the Cotabato Basin which also includes the northernmost part of Zamboanga, and a western terrane comprising most of Zamboanga, the Kudarat Plateau and possibly the southern Davao Del Sur Peninsula. The eastern and central part of Mindanao consist (Fig. 2) of the Agusan-Davao Basin which is an elongated trough tending roughly N-S and extending from Butuan City in the north down to Davao City in the south. It is bordered on the east by the East Mindanao Ridge or Pacific Cordillera (Irving 1952, Teves et al. 1953, Ranneft et al. 1960, Santos et al. 1962, UNDP 1984) that lies between the Surigao and the Pujada peninsulas, and to the west by the Central Cordillera of Mindanao. West of the Central Cordillera are the Lanao-Bukidnon Highlands (Ranneft et al. 1960, Villamor and Marcos 1981, Hawkins et al. 1985) composed principally of Pleistocene and Holocene volcanic and volcaniclastic rocks blanketing most of the underlying formations. Ultramafic basement crops out extensively in Dinagat island and the Surigao peninsula (Hawkins et al. 1985, Figs 2 and 3) and is overlain by a very thick conglomerate with reworked carbonates bearing Upper Eocene foraminifera near Rizal village south of Surigao. Basement of the Pacific Cordillera also includes greenschist facies metavolcanics, mostly meta tufts with sericite, chlorite and occasionally quartz (Villamor and Marcos 1981). The age of the limestone is consistent with the ones obtained on the western flank of the AgusanDavao Basin (Ranneft et al. 1960) from a thick limestone unit that can be traced on seismic lines acquired for AMOCO (Moore and Silver 1983). However the oldest ages we could find in the Central Cordillera are only as old as Late Oligocene-Early Miocene (Figs 2 and 3). In both Central and Pacific Cordilleras, these Early Miocene limestones overly the volcanic series and are always followed by clastic deposits (Villamor and Marcos 1981). On the west flank of the Pacific Cordillera, the stratigraphic sequences are dismembered by Pliocene to Quaternary faulting (Fig. 3), but calcareous sediments with nannofossils have been found within the clastic deposits yielding ages of Late Miocene. Early Pliocene and Late Pliocene. In the Malaybalay-Valencia area, the series are less affected by tectonics and late Early Miocene limestones with foraminifera overlie massive andesites having K/Ar ages of 20-17 Ma (Bellon and Rangin in press). The limestone is overlain by greywackes, cemented conglomerate bearing reworked Early Miocene carbonates and older volcanics near Managok and Conception villages on the flanks of recent tilted blocks (Fig. 3). Cross-bedded stratification is present in the clastics as well as synsedimentary normal and possibly strike-slip faults. These coarse sediments grade to finer ones in the central part of the stratigraphic column where siltstone and

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mudstone turbidites dominate. The upper part of the section which yielded an Early Pliocene age around Cabadbaran south of lake Mainit in Surigao, becomes coarse again. The Early and Middle Pleistocene also show clastic sedimentation intercalated with lava flows. A K/Ar age of 2.5 Ma was found on an andesite at Mount Balibayo in Surigao and ages of 1.15, 0.4 and 0.25 Ma were found on basalts of the Western flank of the Central cordillera between Valencia and Quezon. In northernmost Zamboanga (Figs 2, 3), a very similar stratigraphy is recorded along the coast between Sindangan and Dipolog. Massive volanics are overlain by Early Miocene limestones. Clastic sedimentation begins with thick conglomerates very well exposed on the road from Dipolog to Sergio Osmena and flysch type deposits dated as nannoplankton zone NN5 (Middle Miocene) north of Sindangan. The upper part is finer and shows white sandstones and marls bearing ages of N N l l to NN13 (latest Miocene). They are occasionally interstratified with thin volcanic flows. This upper part of latest Miocene age extends eastward to Cagayan de Oro (Fig. 2) area where the similar Opol Formation was dated at NNI1 (Late Miocene). The basement of Western Mindanao crops out in Zamboanga Peninsula (Figs 2, 3) and differs drastically from that of the eastern province by the presence of quartz bearing micaschists intruded by mid-Oligocene monzonites. It is very well exposed north of Zamboanga City and southwest of Labason where fresh tracks have been made for logging. Basement rocks include micaschists, slates and serpentinized peridotites crosscut by thick quartz veins. Phyllites engulf round boulders of marmorized limestones. Gneisses are also mentioned by Santos Ynigo (1953). In some places this basement exhibits in the upper part a less metamorphosed series of metavolcanics under greenschist faces. Metagreywackes of this type are exposed on the eastern flank of the Daguma Range along the track between Suralla and Bagumbayan village 50 km south of Cotabato (Fig. 3). Metatuffs crop out extensively in northern Zamboanga west of Labason (Figs 1 and 3). In East-Central Zamboanga sandstone intercalated with basalt sheets and thin limestone lenses of Eocene age are reported by Antonio (1972). The Daguma Range is cored by a large monzonitic and granodioritic batholith in which two K/Ar ages of 30 Ma were found. Early Miocene carbonates are very minor compared to the eastern Mindanao platform, and are composed only of black limestone lenses bearing corals, algae and foraminifera. In southern Zamboanga, Langhian limestone and NN5 calcareous mudstone overlie directly the metamorphic basement (Fig. 3B) and imply that this part was emerged during early Neogene. The volcanic series, well exposed in East-Central Zamboanga, around Santa Clara (Figs 2, 3) records a short-lived volcanic arc. Data in southern Zamboanga even suggest that the arc developed within the NN5 nannozone (early Middle Miocene). These data are in good agreement with the ages obtained in the pyroclastic flows drilled in the Sulu Sea (Rangin e t al. 1990). The Middle and Upper

Miocene are represented in Zamboanga by fine clastics with coal veins and rare pyroclastic flows deposited in SW-NE grabens parallel to the Sulu Ridge while other parts are emergent like Central and Southern Zamboanga. Offshore, in the Sulu Sea, pyroclastic deposits dated NN9 (early Late Miocene) also bear abundant reworked coal fragments (Rangin e t al. 1990). Late Pliocene and Early Pleistocene (NN19) ages have been found in shallow water clastics overlain by a thick tabular apron of basalt between Pagadian and Aurora. Basement rocks were not found in the Davao Del Sur peninsula. The oldest rocks found are late Early Miocene limestones which are overlain by volcanic rocks dated at 9.3 and 10.6 Ma (K/Ar) along the eastern coast north of Malita. Between Sulop and General Santos, volcanic and volcaniclastic rocks are covered by marly sand with N N i l (Late Miocene) ages. These volcanic sequences that extend through Late Miocene may represent the northward extension of the Sangihe arc dying out in the Early Pliocene. In the Daguma Range, volcanic activity was dated 16.7 My (K/Ar) and is overlain by tufts and greywackes which are older than the Late MioceneEarly Pliocene marls.

NEOGENE TECTONICS A detailed study of the drainage pattern and its discontinuities have been combined with radar and geophysical data to understand the distribution of the fractures that led to the present geometry of the Cotabato and Agusan-Davao basins, as well as the cordilleras and plateaus (Campbell 1896, Parvis 1950, Morisawa 1957, Allen 1962, Howard 1967, Argialas e t al. 1988, Deffontaines and Chorowicz 1988). First, we performed a descriptive analysis of the drainage network including classification and frequency (number of streams per unit area). This analysis allows recognition of subhomogeneous drainage areas that might correspond to structural domains, and also the major drainage perturbations in these domains. They are interpreted in this particular case in terms of structure and lithology, excluding climatic effects and results of human activity. Then, we analysed in detail each river or stream in order to identify minor discontinuities (anomalies, Fig. 4). Two distinct trends connect in eastern Mindanao. Microtectonic analyses performed in the field in the eastern Mindanao terrane (on the western flank of the Central Cordillera) indicate NE-SW extension in the Early or Middle Miocene, creating synsedimentary faults in the lower part of the basin. In southern Surigao near Cabadbaran, normal faults with NNW direction are assumed to have been produced by this phase of extension. They have been reactivated as left lateral strike-slip or reverse faults during the Pliocene and the Quaternary. Miocene tectonics in the Zamboanga area (Western terrane) are characterized by NE-SW horsts and grabens related to a SE-NW extension. This phase is difficult to date since most of the area was emergent at this time.

The MindanaoCollisionZone

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generating left lateral faulting. Microtectonic measurements were done on Pleistocene sediments and overlying basalts. East-west compression seems to be the active pattern based on one in situ stress measurement in the Malangas coal mines which indicates maximum stress orientation of N100 ° and minimum stress orientation of N007 ° (PNOC unpublished data). East-west normal faults accommodate the strike-slip motion and create pull-apart basins west of Aurora in Northern Zamboanga. Pliocene strike-slip faults oriented NW (N130 °) along the Daguma Range have been reactivated as normal faults with considerable offset southeast of Cotabato. Some of these normal faults crosscut the Parker Volcano in Southern Kudarat Province and cut the Quaternary reefal terrace south of General Santos. However, they can be interpreted as a back arc extension behind the Cotabato Trench. Extension is very active in the Lanao Lake area as pointed out by Ranneft et al. (1960). Quaternary volcanics are tilted along 060 to 080 normal faults. This volcanism is still active in Central Mindanao. In Eastern Mindanao the Philippine Fault diversifies into several splays that crosscut the Pacific Cordillera towards the Philippine Trench. Curved segments correspond to reverse faulting and may represent transpressive portions. Reverse faults belonging to a flower structure appear on an E-W seismic line transverse to the Agusan-Davao Basin (Bischke et al. 1988). North-south structures have been noticed by Ranneft et al. (1960) and are present as diffuse wide zones of

246

M. PUBELLIERet al.

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strike-slip faults. The easternmost zone lies parallel to the Agusan-Davao Basin and intersects the NNW Philippine Fault in a marshy zone of active subsidence underlined by 120° confluence angle of tributaries. In the

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247

The Mindanao Collision Zone

volcanic cones are present along the fault the Central Cordillera, some of them dated between 1.15 and 0.25 Ma (K/Ar). This fault zone may extend offshore along the western flank of the Sangihe Arc where N-S faults are mentioned by Moore and Silver (1982). In East-Central Zamboanga, between Diplahan and Imelda. North-south faults appear left lateral in the field. These faults, motion on which is supported by shallow focal mechanisms, crosscut and offset the tilted blocks in the area between Imelda and Ipil (Figs 1, 3). The associated NW-SE extension also remobilizes the Miocene tilted blocks of the Sibuguey Gulf. The largest cluster of shallow earthquakes occurs in this area.

CONCLUSIONS Two major composite terranes can be differentiated on the basis of their stratigraphic record. An eastern one, on the margin of the Philippine Plate, is dominated by an intraoceanic and island arc basement on which an Eocene and Oligocene volcanic arc is developed. Neogene clastic basins controlled by NW faults were emplaced above the arc. A western terrane with continental basement correlated to the Palawan block (Faure et al. 1989) was distended during Early and Middle Miocene along NE-SW faults during opening of the Sulu Sea. Volcanic activity continued into the Late Miocene in the Davao Del Sur Peninsula and the Daguma Range which may constitute the northward extension of the Sangihe Arc. The terranes were sutured by the Early Pliocene without major deformation. This age is consistent with the one deduced from the length of subducted slab (Roeder 1977). However, collision is still in progress in the Molucca Sea which is seismically very active (Cardwell et al. 1980, McCaffrey et al. 1980, Silver and Moore 1978, Moore and Silver 1983). The NNW faults, interpreted as older and related to the initial rifting of the Agusan-Davao Basin, were probably remobilized during oblique docking of the Philippine Mobile Belt against the Eurasian margin. Although some of these faults are still active and they have responded to the influence of the Philippine Plate, most of the decoupling now occurs along the Philippine Fault. North-south faults may reflect the incipient collision of the Indo-Australian Plate and accommodate differential northward motion of extruded blocks. Once the terranes had been sutured, they underwent similar sedimentation and tectonic history with abundant clastics deposited in shallow water. In northeast Zamboanga, Lanao and Central Mindanao, this sedimentation is associated with abundant volcanic flows that we interpret as the products of the mantle metasomatized during the subduction of the Molucca Sea lithosphere, erupted along the N-S strike slip faults and the E-W to SW-NE normal faults of the Lanao area. The location of the suture zone beneath the Cotabato Basin and in the Sindangan-Cotabato region would have migrated westward relative to the trace of the Molucca Sea slab, following the continuous oblique

convergence of the Philippine Plate relative to Eurasia. In addition, the suture known in Mindoro and Panay dated at Late Miocene (Marchadier and Rangin 1989) would be younger southward in Mindanao where it occurred sometime during Early Pliocene and still active further south. This process of soft collision, marking a short perturbation within the continuous rotation of the Philippine plate since the Eocene (Karig 1975, Ranken et al. 1984), illustrates one possible evolution of the oblique collision of a volcanic arc against a continental margin. Acknowledgements--This paper is part of the results of the fruitful cooperation between the Philippine Bureau of Mines and Geosciences, Philippine Institute of Volcanology and the University P. & M. Curie in Paris. We are very grateful to the Phil. Nat. Oil Comp. (P.N.O.C.) for giving access to some unpublished data, especially to the P.N.O.C. MALANGAS COAL MINE for their help and accommodation. Potassium/Argon ages have been provided by H. Bellon of the Universit6 de Bretagne occidentale in Brest (France), and seismicity data extracted with the help of M. Gaulon of the Institut de Physique du Globe in Paris. We are grateful to A. Mitchell and anonymous reviewer for their correction and constructive remarks on an earlier version of the manuscript. This work has been funded by the CNRSINSU project "Philippine Faults" and the Minist6re des Affaires Etrang6res.

REFERENCES Acharya, H. K. and Aggarwal, Y. P. 1980. Seismicity and Tectonics of the Philippine Islands. J. geophys. Res. 85, (B6), 3239-3250. Allen, C. R. 1962. Circum-Pacific faulting in the Philippine-Taiwan region. J. geophys. Res. 67, 4795-4812. Antonio, L. R. 1972. Geology and mineral resources of East-Central Zamboanga Peninsula. Report of the Philippine Bureau of Mines, 1972. 87pp. Argialas, D. P., Lyon, J. G. and Mintzer, O. W. 1988. Quantitative description and classification of drainage patterns. Phot. Eng. Rein. Sensing 54, 505-509. Aurelio, M. A., Rangin, C., Barrier, E. and Muller, C. 1990. Tectonique du segment central de la faille Philippine. C.R. Acad. Sci. Paris 310, Serie II, 403-410. Bachman, S. B., Lewis, S. D. and Schweller, W. J. 1983. Evolution of a forearc basin, Luzon Central Valley, Philippines. Am. Assoc. of Petrol. Geol. 67, 1143 1162. Barrier, E., Huchon, P. and Aurelio, M. 1991. The Philippine fault: A key for Philippines kinematics. Geology 19, 32-35. Bellon, H. and Rangin, C. in press. K-Ar isotopic dating of magmatic activity in Sabah (Borneo) and the Central-Southern Philippine archipelago. Initial Reports of the Ocean Drilling Program 12411. Bischke, R. E., Suppe, J. and Del Pilar, R. 1988. Implications of a newly discovered branch of the Philippine fault system. In: International Symposium on the Geodynamic Evolution of Eastern Eurasian Margin (Abstracts), Paris 13--20 September 1988. Campbell, M. R. 1896. Drainage modifications and their interpretation. J. Geol. 4, 567 581, 657 678. Cardwell, R. K., Isacks, B. L. and Karig, D. E. 1980. The spatial distribution of earthquakes, focal mechanism solutions and subducted lithospheres in the Philippines and Northern Indonesian region. In: The Tectonic and Geologic evolution of Southeast Asian Seas and Islands (Edited by Hayes, D. E.) Geophys Union, Mon. 23, 1-35. Corby, G. W. 1951. Geology and oil possibilities of the Philippines. Dept Agric. and Nat. Res. Tech. Bull. 21, 363pp. Manila. Daly, M. C., Hooper, B. G. D. and Smith, D. G. 1987. Tertiary Plate tectonics and basin evolution in Indonesia. Proc. Indon. Petr. Assoc. 6th Ann. Conv. 399-428. Davis, D., Suppe, J. and Dahlen, F. 1983. Mechanics of foldand-thrust belts and accretionary wedges. J. geophys. Res. 889 1153-1162.

Deffontaines, B. and Chorowicz, J. 1988. Principe d'analyse des r6seaux hydrographiques fi partir de donn6es multisources. Applications aux structures de bassins: Zaire-Foss6 rh6nan. Proc. Coll. Int. Nbotectonique B.R.G.M., 3 5 Oct. 1988, p. 15.

248

M. PUBELLIERet al.

Faure, M., Marchadier, Y. and Rangin, C. 1989. Pre-Eocene synmetamorphic structure in the Mindoro-Romblon-Palawan area, West Philippines, and implications for the history of Southeast Asia. Tectonics 8, 963-979. Geary, E. E. and Kay, R. W. 1989. Identification of an Early Cretaceous ophiolite in the Camarines Norte-Calaguas islands basement complex, Eastern Luzon, Philippines. Tectonophysics 168, 109-126. Haeck, G. D. and Karig, D. E. 1985. Strike-slip genesis of the Ilocos Norte melange. EOS 64, 871. Hall, H., Audley-Charles, M. G., Banner, F. T., Hidayat, S. and Tobing, S. L. 1988. Late Palaeogene-Quaternary geology of Halmahera, Eastern Indonesia: Initiation of a volcanic island arc. J. Geol. Soc. Lond. 145, 577-590. Hall, H. and Nichols, G. 1990. Plate boundary evolution in the Halmahera region, Indonesia. Tectonophysics 181, 207-272. Hamilton, W. 1979. Tectonics of the Indonesian Region. U.S. Geol. Survey Prof. Paper. 1078, 345pp. Hatherton, T. and Dickinson, W. R. 1969. The relationship between andesitic volcanism and seismicity in Indonesia, the Lesser Antilles and other island arcs. J. geophys. Res. 74, 5301-5310. Hawkins, J. W., Moore, G. F., Villamor, R., Evans, C. and Wright, E. 1985. Geology of the composite terrane of East and Central Mindanao. In: Tectonostratigraphic Terranes of the Circum-Pacific Region (Edited by Howell, D. G.) Circum-Pacific Council for Energy and Mineral Resources, Earth Sciences Series 1,437-463. Hinz, K. and Block, M. 1990. Summary of the Geophysical data from the Sulu and Celebes seas. Initial Reports of the Ocean Drilling Program 124, 87-92. Holloway, N. H. 1982. The North Palawan block, Philippines: its relation to the Asian mainland and its role in the evolution of the South China Sea. Am. Assoc. of Petrol. Geol. Bull. 66, 1355--1383. Howard, A. D. 1967. Drainage analysis in geologic interpretation: a summation. Am. Assoc. Petrol. Geol. 51, 2246-2259. Irving, E. M. 1950. Review of Philippine Basement and Its Problems. Philippine J. Sci. 79, 267-307. Irving, E. M. 1952. Physiographic observations on Mindanao, by aerial reconnaissance, and their geological interpretation. Philippine J. Sci. 81, 141-169. Jolivet, L., Huchon, P. and Rangin, C. 1988. Tectonic setting of Western Pacific marginal basins. Tectonophysics 160, 23-47. Karig, D. E. 1975. Basin genesis in the Philippine Sea. Initial Reports of the Deep Sea Drilling Project 31, 857-879. Karig, D. E. 1983. Accreted terranes in the northern part of the Philippine archipelago. Tectonics 2, 211-232. Karig, D. E., Sarewitz, D. R. and Haeck, G. D. 1986. Role of strike-slip faulting in the evolution of allochthonous terranes in the Philippines. Geology 14, 852-855. Krause, D. C. 1966. Tectonics, marine geology and bathymetry of the Celebes-Sulu Sea Region. Bull. Geol. Soc. Am. 77, 813-831. Lewis, S. D. and Hayes, D. E. 1983. A geophysical study of the Manila Trench, Luzon, Philippines; forearc basin structural and stratigraphic evolution. J. geophys. Res. 89, 9196-9214. Maleterre, P., Stephan, J. F., Andreieff, P., Bellon, H., Chorowicz, J., Boirat, J. M. and Balce, G. R. 1988. The Southern Central Cordillera of Luzon; a multistage Upper Eocene to Pleistocene arc deformed on the northern end of the Philippine strike-slip fault. In: International Symposium on the Geodynamic Evolution of Eastern Eurasian Margin (Abstracts), Paris 13-20 September 1988. Marchadier, Y. and Rangin, C. 1989. Passage subduction-collision et tectoniques superpos6es ~t l'extremit6 de la fosse de Manille (Mindoro-Tablas, Philippines). C.R. Acad. Sci. Paris 302 (II), 1715 1720. Mascle, A. and Biscarrat, P. A. 1978. The Sulu Sea, a marginal basin in Southeast Asia. Am. Assoc. Petrol. Geol. Mem. 29, 373-381. McCabe, R. and Almasco, J. N. 1985. Terranes of the Central Philippines. In: Tectonostratigraphic Terranes of the Circum Pacific Region (Edited by Howell, D.) Circum-Pacific Council for Energy and Mineral Resources, Earth Science Series 1, 421-436. McCaffrey, R., Silver, E. A. and Raitt, R. W. 1980. Crustal structure of the Molucca Sea collision zone, Indonesia. In: The Tectonic and Geologic Evolution of Southeast Asian Seas and Islands (Edited by Hayes, D. E.) Geophys. Union Mon. 23, 161-177. Mitchell, A. H. G., Hernandez, F. and de la Cruz, A. P. 1986. Cenozoic evolution of the Philippine archipelago. J. SE Asian Earth Sci. 1(1), 3-22. Moore, G. F. and Silver, E. A. 1983. Collision Processes in the Northern Molucca Sea. In: The Tectonic and Geologic Evolution of

Southeast Asian Seas and Islands (Edited by Hayes, D. E.) Geophys. Union Mon. 27, 360-372. Morisawa, M. E. 1957. Accuracy of determination of stream lengths from topographic maps. Trans. Am. Geophys. Union 38(1), 86-88. Parvis, M. 1950. Drainage pattern significance in air photo identification of soils and bedrocks. Highway Res. Board, Nat. Research Council Bull. 28, 36-62; Photogram. Eng. 16(3), 387-409. Pelletier, B., Stephan, J. F., Blanchet, R., Muller, C. and Hu, H. N. 1985. L'6mergence d'une zone de collision active fi la pointe sud de Tai'wan (P6ninsule d'Hengshun): tectoniques superpos~es et mise en 6vidence d'une tectonique mioc6ne moyen. Bull. Soc. Gdol. France 8, 161-171. Philippine Bureau of Mines 1963. Geological Map of the Philippines, Geological Survey Division, Manila, Philippines. Pubellier, M. and Rangin, C. 1989. The Philippine Mobile Belt from Bicol to Mindanao (Philippines); old structures and neotectonics. EOS Trans. Am. Geophys. Union 70, 1365. Rangin, C., Stephan, J. F. and Muller, C. 1985. Middle Oligocene Oceanic Crust of the South China Sea, jammed into Mindoro Collision Zone, Philippines. Geology 13, 425-428. Rangin, C., Silver, E. A. et l'equipe du leg 124. 1989a. Forages dans les bassins marginaux du S. E. Asiatique; r6sultats pr~liminaires du Leg 124 (O.D.P.), C.R. Acad. Sci. Paris 309, S6rie II, 1333-1339. Rangin, C., Pubellier, M. and Jolivet, L. 1989b. Collision entre les marges de rEurasie et de l'Australie: un processus de fermeture des bassins marginaux du Sud-Est asiatique. C.R. Acad. Sci. Paris 309, Serie II, 1223-1229. Rangin, C., Jolivet, L. and Pubellier, M. 1990. A simple model for the tectonic evolution of southeast Asia and Indonesian region for the past 43 My. Bull. Geol. Soc. France 8, VI (6), 889-905. Rangin, C. and Silver, E. 1990. Geological setting of the Celebes and Sulu seas. Initial Reports of the Ocean Drilling Program 124. Rangin, C., Silver, E. and Leg 124 Scientific Party 1990. Lithostratigraphy of site 768. Initial Reports of the Ocean Drilling Program 124A, 35-42. Ranken, B., Cardwell, R. K. and Karig, D. E. 1984. Kinematics of the Philippine Sea plate. Tectonics 3, 555-575. Ranneft, T. S. M., Hopkins, R. M. J., Froelich, A. J. and Gwinn, J. W. 1960. Reconnaissance geology and oil possibilities of Mindanao. Am. Assoc. Petrol. Geol. Bull. 4, 529-568. Ratman, N. 1976. Geological Map of the Tolitoli Quadrangle, North Sulawesi, Geological Survey of Indonesia, Map Series 1/250,000, Ministry of Mines. Bandung, Indonesia. Roeder, D. 1977. Philippine arc system--Collision or flipped subduction zone? Geology 5, 203-206. Santos, Ynigo. 1953. Geology of southern Zamboanga Province. Philippine Geologist 7, 45-64. Santos, R. R., Obial, R. C. and Zerda, R. R. 1962. Preliminary Report on the Geologic Reconnaissance of Surigao Peninsula and Vicinity. Report of the Philippine Bureau of Mines, 83pp. Sarewitz, D. and Karig, D. E. 1986. Processes of allochthonous terranes evolution in Mindoro Island, Philippines. Tectonics 5, 525-552. Seno, T. and Maruyama, S. 1984. Paleogeographic reconstructions of the Philippine Sea. Tectonophysics 102, 53-84. Silver, E. A. and Moore, J. C. 1978. The Molucca Sea collision zone, Indonesia. J. geophys. Res. 83, B4, 1681-1691. Tator, B. A. 1954. Drainage anomalies in coastal plain region. Phot. Eng. 20, 412-417. Taylor, B. and Hayes, D. E. 1980. The tectonic evolution of the South China Sea Basin. In: The Tectonic and Geologic Evolution of Southeast Asian Seas and Islands (Edited by Hayes, D. E.) Am. Geophys. Union Mon. 23, 89-104. Teves, J. S., Vergara, A. A. and Badillo, N. 1953. Report on the geologic reconnaissance of Agusan. Philippine Geologist 5, 24-50. United Nations Development Programme. 1984. Geology of Northern Agusan, Mindanao. In: Strengthening the Geological Survey Dept. of the Bur. of Mines and Geosciences. Ministry of Nat. Res., Tech. Rep. 2, New York. Uyeda, S. and Ben Avraham, Z. 1972. Origin and development of the Philippine Sea. Nature Phys. Sci. 240, 176-178. Villamor, R. T. and Marcos, D. M. 1981. Preliminary Report on the Geology of Tankulan, Dumalaguin, Sumilao and Kalasungay Quadrangle. Report of the Mines and Geosciences Reg. Office No. X, 36pp. Willis, B. 1937. Geologic observations in the Philippine islands. Nat. Res. Council Phil. Bull. 13, 127pp. Wolfe, J. A. 1981. Philippine geochronology. J, Geol. Soc. Philippines 35, 1-35.