Tectonophysics 384 (2004) 23–53 www.elsevier.com/locate/tecto

Formation of arcs and backarc basins inferred from the tectonic evolution of Southeast Asia since the Late Cretaceous

E. Honzaa,*, K. Fujiokab

a Graduate School of Science and Technology, Kumamoto University, Kurokami, Kumamoto 860-8555, Japan b Japan Marine Science and Technology Center, Natsushima, Yokosuka 237-0061, Japan Received 23 July 2002; accepted 10 February 2004 Available online 10 June 2004

Abstract

Results of the geological and geophysical surveys in the Daito ridges and basin in the northern West Philippine Basin suggest that the Daito Ridge was an arc facing toward the south from the Late Cretaceous to the Early . The Late Cretaceous and Tertiary history of Southeast Asia is evaluated based on these data in the Daito ridges and basins and reconstructed based on overall plate kinematics that have operated in this area. During the Late Cretaceous, the Daito Ridge and the East Philippine Islands were positioned along the boundary between the Indian and Pacific Plates. The western half of the Philippines setting on the Indian Plate approached from the south and collided with the East Philippine–Daito Arc either during the latest Paleocene or the earliest . It is inferred that the bulk of the Philippine archipelago rotated clockwise and spun counterclockwise during the Tertiary. From the reconstruction, the formation of backarc basins and their spreading direction are assessed. As a result, some primary causes and significant characteristics are suggested for the opening of backarc basins in Southeast Asia. First, opening of some backarc basins commenced with or was triggered by collisions. Second, backarc basins opened approximately parallel to oceanic plate motion. Third, the formation of some backarc basins was triggered by the approach of a hot spreading center. Fourth, the spreading mode or direction of backarc basins was greatly affected by the configuration of the surrounding continent and was also rearranged to spread approximately parallel to oceanic plate motion. The formation of backarc basins and their spreading direction can be reasonably explained by plate kinematics. However, the generative force responsible for their formation is possibly within the system, particularly to form horizontal tensional force in backarc side. D 2004 Elsevier B.V. All rights reserved.

Keywords: Daito ridges and basins; Arc and backarc basin formation; SE Asia; Reconstruction

1. Introduction Plates, forming an area of enigmatic plate boundaries with collision and subduction zones (Fig. 1).Its Southeast Asia is composed of many small plates modern configuration, with complicated sutures, and strewn between the Pacific and the India–Australia arc–backarc basin formations was initiated by the northward movement of India in the Late Cretaceous * Corresponding author. Tel.: +81-96-342-3415; fax: +81-96- and by the northward translation of Australia since the 342-3411. Eocene (e.g. Norton and Sclater, 1979; Mutter et al., E-mail address: [email protected] (E. Honza). 1985; Powell et al., 1988; Royer and Sandwell, 1989).

0040-1951/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.tecto.2004.02.006 24 E. Honza, K. Fujioka / Tectonophysics 384 (2004) 23–53 E. Honza, K. Fujioka / Tectonophysics 384 (2004) 23–53 25

There already are a number of regional reconstruc- Paleomagnetic work in Southeast Asia suggests tion models for Southeast Asia based on geological that the West Philippine Basin has rotated clockwise evidence, magnetic anomaly identification in marginal by approximately 80j since the earliest Tertiary basins and paleomagnetic analyses. These varied (Louden, 1977; Kinoshita, 1980; Haston and Fuller, models have some very basic difference. For example, 1991). From the work in Halmahera, Hall et al. (1995) one of their distinct disagreements is in the recon- suggested approximately 90j clockwise rotation of struction of the Philippine Plate, which is a conse- the Philippine Plate since the Eocene. This result is quence of how they isolate that from the surrounding based on an assumption that Halmahera is a part of the major plates. Some authors propose counterclockwise Philippine Plate since the Early Tertiary. The Celebes rotation (Holloway, 1982; Karig, 1983; Stephan et al., Basin is considered to have rotated counterclockwise 1986; Longley, 1997) while others suggest clockwise approximately 60j (Shibuya et al., 1991). Counter- rotation (Daly et al., 1987; McCabe and Cole, 1987; clockwise rotation of approximately 90j is postulated Hall et al., 1995). Despite some differences, a few for Borneo since the latest Cretaceous (Haile et al., elements are common among the models. Most of 1977; Haile, 1978; Sasajima et al., 1980; McCabe and them suggest that Philippine islands were initially on Cole, 1987; Schmidke et al., 1990; Fuller et al., 1991, the Pacific Plate located approximately to the north of 1999). Other paleomagnetic work suggests that there Australia. Formation of the West Philippine Basin is was no rotation of western Kalimantan during the also widely interpreted to have been associated with Tertiary (Lumadyo et al., 1993). Borneo is considered northwestward movement of the Philippines along to be a part of Sundaland behaved as a single tectonic sinistral strike slip faults (Seno and Maruyama, unit with no rotation observed in western Sundaland 1984; Sarewitz and Karig, 1986; Jolivet et al., 1989; (e.g. Hamilton, 1979; Hutchison, 1996). From the 20j Rangin et al., 1990; Honza, 1991). to 33j counterclockwise rotation of Malay Peninsula Some marginal basins age-formation was estab- during the Tertiary, possible rotation of Sundaland lished by magnetic anomaly identifications and by was suggested (Richter et al., 1999). Sundaland itself DSDP and ODP drillings. Results suggest that most does not appear to behave as a single block based on marginal basins in Southeast Asia were formed in paleomagnetic studies. It seems more logical to divide the and appear to have an intimate rela- that region into at least two parts: the northern Indo- tionship with subduction events that also corresponds China and southern Sundaland blocks. in duration to the formation of the marginal basin The Daito ridges and basins in the northern margin (Tamaki and Honza, 1991). This relationship is also of the West Philippine Basin are in the NWW–SEE well demonstrated in the backarc basins of the trend, approximately parallel to the magnetic character Tertiary arc chain along the central and southwestern of the basin but different from the surrounding trends of rim of the Pacific Plate (Honza, 1991). From these the Ryukyu Trench and Kyushu-Palau Ridge. This reconstructions and models for backarc basin forma- suggests that the Daito ridges and basins were formed tion, it is inferred that during the Cenozoic, some arc at a much earlier stage than the formation of the West regions in Southeast Asia were under tensional Philippine Basin. They are postulated to have formed in forces that led to backarc basin formation. This the Late Cretaceous and Early Tertiary (e.g. Uyeda and contrasts with Mesozoic arcs that were dominantly Ben-Avraham, 1972; Mizuno et al., 1978; Klein and formed under compression. Kobayashi, 1980; Shiki et al., 1985; Tokuyama, 1995).

Fig. 1. Tectonic setting of Southeast Asia. Thick lines with closed triangles are modern active trenches and those with open triangles are inactive. Thick arrows show the direction of plate movement in a fixed hotspot reference frame after Engebretson et al. (1985) and Royer and Sandwell (1989). Length of bar is motion for 10 Ma. Spreading centers are shown in double lines. Thin lines are active structural boundaries and thin broken lines are traces of offshore or buried structural boundaries. Abbreviation: T: Trench or Trough, JB: Japan Basin, NA: Nankai Trough, OT: Okinawa Trough, DRB: Daito ridges and basins, SB: Shikoku Basin, OD: Ogasawara Depression, MA: Mariana Trough, PB: Parece Vela Basin, WPB: West Philippine Basin, SCB: South China Basin, AB: Andaman Basin, PA: Palawan Trough, NE: Negros Trench, SU: Sulu Basin, CO: Cotobato Trench, CB: Celebes Basin, NS: North Trench, MB: Makassar Basin, BB: Banda Basin, MO: Molucca Collision, SO: Sorong , CE: Ceram Trough, CAB: Caroline Basin, NG: New Guinea Trench, WE: West Melanesia Trench, NB: New Britain Trench and TR: Trobriand Trench, RF: Red River Fault, SF: Seribu Fault. 26 E. Honza, K. Fujioka / Tectonophysics 384 (2004) 23–53

These data and some unpublished data are assessed lites and asterocyclina studies (Mizuno et al., 1975). to establish a possible formation history of the ridges Layer N in the basin is intercalated by basalt lavas and and basins. In this study, the tectonic evolution of the diabase sills and reworked Paleocene foraminifera Philippines and SE Asia is deciphered from the suggesting an age that is a little older than that from evaluation of the Daito ridges and basins, and the the ridges (Shipboard Scientific Party Leg 31, 1975; possible mechanism to form backarc basins is dis- Shipboard Scientific Party Leg 58, 1980; Shiki et al., cussed based on these results. 1985). Draped on Layer N and abutting the foot of the ridges is the to Layer J whose age was determined using its nannofossil and radiolarian 2. Results of the geological and geophysical contents. Layer J exhibits a distinct alternation pattern surveys in the Daito ridges and basins in the and is intercalated by turbidites in basins and in northern Philippine Sea benches on slopes. The topmost Layer O is pelagic clay similarly distributed in the basins and on the The Daito ridges and basins in the northern margin ridges and ranges from the Middle Miocene to Qua- of the West Philippine Basin consist of the Amami ternary (Figs. 3 and 4). Basin, the Amami Plateau, the Kita-Daito Basin, the Most ridge and basin areas, Layer N in the basin is Daito Ridge, the Minami-Daito Basin and the Oki- traced to abut the foot of some highs, suggesting Daito Ridge. The Oki-Daito Escarpment (Lapu-Lapu formation of the latter prior to the deposition of Layer Ridge) is at the south of the Oki-Daito Ridge. These N, such as at the north and south foots of the Amami ridges trend approximately perpendicular to the slant Plateau (Fig. 3, P1 and P2). However, the upper part of the Ryukyu Trench and to the Kyushu-Palau Ridge of the layer on the slope of the ridge is traced to the (Fig. 2). The Amami Plateau has a few peaks on its foot of the basin, suggesting syn-depositional uplift central part, whereas the Daito Ridge has a flat top at occurred, such as at the southern foot of the Daito 1500 m deep and the Oki-Daito Ridge has a peak with Ridge (Fig. 4). Layer N in the Minami-Daito Basin a narrow central depression (Fig. 3). The Kita-Daito shows the same acoustic pattern as that on the ridges Basin has a smooth horizontal seafloor. Sporadic and is thicker in the basins. Layer N in the West seamounts and highs are within these basins. The Philippine Basin is thinner than that in the northern Daito ridges and basins were formed in the Late basins and shows a clear alternation pattern, suggest- Cretaceous to Early Tertiary. The Daito Ridge is ing formation with turbidites (Fig. 3,P3).Atthe interpreted to have been an arc, and the Oki-Daito ridges, this layer is eroded to form a flat top, as on Ridge to be an oceanic ridge from the sampled results the Daito Ridge (Fig. 4). Based on the occurrence of and strontium isotope content and ratios in basalt nummulites, Layer N was deposit in relatively shal- (Matsuda, 1985; Tokuyama, 1995). The north and lower environment. An observed wave-cut base at south sides of the Oki-Daito Ridge are postulated to 1500 m deep on the Daito Ridge also suggests have formed by ocean floor spreading of a ridge shallow one. It is inferred that the basins deepened between the Kula and Australia–Antarctica Plates only after the deposition of Layer N. (Tokuyama, 1995). Reinterpreting a multi-channel seismic profile of Seismic profiles in the northern West Philippine the Daito ridges and basins, a possible northward Basin area (Honza, 1976; Mizuno et al., 1975, 1978) subduction south of the Daito Ridge is suggested are assessed and evaluated structure of the Daito (Fig. 4). This is also observed in a single-channel ridges and basins. Three sedimentary units are noted seismic profile (Fig. 3, P2). The Minami-Daito Basin in the seismic profiles, the ages of which are identified has a smooth seafloor tilted towards the north from the from dredge samplings and DSDP drillings. Layer N foot of the Oki-Daito Ridge to the foot of the Daito manifested by a week and layered reflection pattern Ridge. The basement traced from the Oki-Daito Ridge found in both ridge and basins is the bottommost dips relatively steeply at the south flank of the ridge sedimentary deposit upon the acoustic basement where thicker sedimentary layers are also observed. (Figs. 3 and 4). This layer is identified to be of the These features are similarly observed in modern arc Eocene age from radiolarians, nannofossil, nummu- and trench areas. E. Honza, K. Fujioka / Tectonophysics 384 (2004) 23–53 27

Fig. 2. Bathymetry of the Daito ridges and basins. Thick broken lines are reflection profiles in Figs. 3 and 4. Dotted lines in the northern Philippine Basin are topographic lineations based on a swath survey by Kasuga and Ohara (1997). 28 E. Honza, K. Fujioka / Tectonophysics 384 (2004) 23–53

Fig. 3. Interpretation of reflection profiles in the Daito ridges and basins. The surveyed lines are shown in Fig. 2. E. Honza, K. Fujioka / Tectonophysics 384 (2004) 23–53 29

Fig. 4. Interpretation of a reflection profile by Mizuno et al. (1977) in the Daito ridges and basins. The surveyed line is shown in Fig. 2. Possible subduction towards the north is observed at the southern foot of the Daito Ridge.

The Amami Plateau consists of aphyric basalt, Volcanic rocks in the Daito ridges and basins are plagioclase–augite–olivine phyric basalt, two-pyrox- approximately in the same age range of 85–60 Ma ene andesite, hornblende tonalite and trachyandesite (Fig. 5). The oldest rocks are the Amami Plateau arc indicating two type volcanism in the Middle Creta- tholeiites and tonalites (82–85 Ma) and the Late ceous characterized by immature arc activity produc- Cretaceous arc volcanism products composed of an- ing tholeiites, calc-alkaline rocks and plagiogranites. desite, granodiorite and sedimentary rocks in the Alkaline products characterize the Middle Eocene Daito Ridge (Ozima et al., 1980; Matsuda, 1985; volcanism (Table 1). The Daito Ridge consists of Shiki et al., 1985; Tokuyama, 1995). Younger volca- intrusives, metamorphic and sedimentary rocks com- nic event occurred in the Late Paleocene and Eocene. posed of granodiolite, gabbro, hornblende schist, Most rocks dredged from the Kita–Amami Basin, the serpentinite and hornfels. The Daito Ridge also had Amami Plateau and Kita-Daito Basin are from 85 to two major volcanic events in the Late Cetaceous 60 Ma in age, whereas those from the Daito ridge, characterized by arc volcanism of two-pyroxene an- Minami-Daito Basin and the Oki-Daito Ridge are desite and granodiorite. The Paleocene activity in the from 85 to 49 Ma in age (Fig. 5). area is characterized by tholeiitic and alkalic volca- The age of the Huatung Basin, a small basin east of nism associated with plagioclase-phyric basalt, pla- Taiwan, is postulated to be around the Early Creta- gioclase–clinopyroxene-phyric basalt and micro- ceous based on the ages of dredged gabbros from the dolerite. Igneous rocks in the Oki-Daito Ridge are site (Deschamps et al., 2000). However, magnetic divided into ophitic to subophitic alkali dolerite and lineations suggests that the basin was formed approx- mugearite. Petrographical evidence suggests that the imately from 50 to 40 Ma and that the Early Creta- volcanism on the Oki-Daito Ridge has closer affinity ceous rocks suggest, not the age of the basin itself, but to a seamount chain or an active spreading ridge rather the age of ridges (Sibuet et al., 2002). To the south of than an arc. the Oki-Daito Ridge, different morphological linea- 30 E. Honza, K. Fujioka / Tectonophysics 384 (2004) 23–53

Table 1 Summary of results of samples from the Daito ridge and basin area Number Sample Lat. (N) and Long. (E) Rock Fossil Age (Ma) number Amami Plateau 1 GDP-11-8 28–03.0, 131–34.8 Andesite tuff, Basalt 2 GDP-11-9 28–04.0, 131–37.8 Basalt, Andesite tuff Nummulites 85.1F2.1, 82.4F2.0 3 GDP-11-10 27–55.5, 132–05.0 Basalt 4 GDP-11-17 28–05.0, 132–01.4 Gabbro, Tonalite, Granodiorite, 75.1F2.4 Basalt, Andesite 5 KH72-2-46 28–05.9, 131–38.0 Andesite 6 KH76-2-1 27–51.0, 132–58.6 Carbonate, Dacite 7 KH82-4-12 28–08.3, 132–07.3 Trondhjemite, Basalt

Daito Ridge 8 GDP-15-1 25–39.5, 133.18.0 Black Schist, Granodiorite, Gabbro, Andesite, Dolerite 9 GDP-15-2 25–40.9, 133–22.7 Andesite, Basalt, Andesite Tuff 10 GDP-15-3 25–41.3, 133–20.3 Gabbro, Andesite, Andesite Tuff 11 GDP-15-4 25–27.8, 132–53.1 Limestone Nummulites U. Eocene 12 GDP-15-6 25–32.7, 132–32.9 Sedimentary rocks Nummulites U. Eocene 13 GDP-15-7 25–16.2, 133–14.5 Green Schist, Dolerite, Tuff Nummulites 14 GDP-21-3 26–18.9, 131–30.3 Basalt 15 GDP-24-13 25–41.1, 134–10.5 Chalk Oligocene 16 GH74-7-180 25–02.5, 133–20.5 Andesite 17 GH74-7-183 25–23.0, 133–32.0 Green Schist, Peridotite 49.0F3.7 18 KH72-2-48 26–11.6, 131–33.3 Andesite 19 KH72-2-50 25–59.4, 131–21.4 Andesite 20 KH72-2-51 25–56.9, 131–21.4 Andesite Tuff

Oki-Daito Ridge 21 GDP-15-10 23–03.3, 134–45.5 Sedimentary rocks Nummulites U. Eocene 22 GDP-15-11 22–47.5, 134–28.7 Basalt Discocyclina 23 GDP-15-13 22–58.0, 134–32.5 Sedimentary rocks Nummulite U. Eocene 24 GDP-21-5 23–45.0, 133–02.3 Dolerite, Basalt, Andesite Nummulite U. Eocene 25 GH74-7-160 23–28.0. 133.03.0 Basalt, Dolerite 26 GH74-7-167 24–20.0, 131–42.2 Limestone Nummulite U. Eocene 27 KH72-2-52 24–29.5, 131–00.0 Andesite

Kyushu-Palau Ridge 28 GDP-8-12 29–55.6, 133–18.5 Trondhjemite, Tonalite, 37.4F6.4 Granodiorite, Andesite 29 GH74-7-150-1 29–52.0, 133–17.0 Tonalite 37.5F1.9 30 GH74-7-150-2 29–53.0, 133–19.5 Tonalite 31 GDP-11-2 28–04.8, 134–39.8 Dolerite 32 GDP-11-3 28–05.5. 134–37.5 Andesite, Basalt Miogypsina U. Oligocene– Spiroclypeus M. Miocene 33 GDP-24-1 28–15.4, 134–32.5 Basalt Tuff 34 GH74-7-184 24–51.0, 134–34.7 Basalt, Tuff 35 GH74-7-175 26–06.0, 135–52.7 Granodiorite, 48.5F1.4 Basalt, Hornfels Numbers beside the sample numbers correspond to the site locations in Fig. 2. U is Upper and M is Middle in age. Samples are from the GDP cruise (Shiki et al., 1985), the GH cruise (Mizuno et al., 1977) and the KH cruise (Fujioka et al., 1980). Other data are in text. E. Honza, K. Fujioka / Tectonophysics 384 (2004) 23–53 31

Fig. 5. Ages of the Daito ridges and basins, and the surrounding area. Ages of the Daito ridges and basins are based on results by Mizuno et al. (1977), Fujioka et al. (1980), Shiki et al. (1985) and Matsuda (1985). Ages in the surrounding areas are in Table 1 and in the text. tions are preserved on either side of the Oki-Daito Oki-Daito Ridge collided. Spreading at the Oki-Daito Escarpment (Kasuga and Ohara, 1997; Okino et al., Ridge possibly formed the north side of the escarp- 1998). Lineations to the south of the escarpment are ment, where a different lineation is observed. NWW–SEE, parallel to the magnetic lineation iden- tified by Hilde and Lee (1984), whereas those on the north side are NW directed (Fig. 2). A different 3. Initial positions and rotations of the Daito morphological lineation suggests a different magnetic Ridges, Philippines and Borneo lineation from the interpretation by Hilde and Lee (1984), indicating different stages of formation for the Many workers have suggested that the Late Creta- basins. The escarpment is dated at approximately 52 ceous position of the Philippines must have been in Ma (anomaly 23 by Hilde and Lee, 1984). From the the southern Pacific, or at the boundary between the ages of intrusive and volcanic rocks, it is deduced that Pacific and North New Guinea Plates (Maruyama et the subduction at the south side of the Daito Ridge al., 1989). In these cases, the initial position of the commenced in the Late Cretaceous, at approximately Daito Ridge and the Philippines could have been on 85 Ma and ceased between 55 and 50 Ma, when the the north or northeastern side of Australia. A possible 32 E. Honza, K. Fujioka / Tectonophysics 384 (2004) 23–53 arc was postulated between the Izanagi and the Cretaceous. West Philippines are considered to have Farallon Plates in the Early Cretaceous. However, been in the south in the northern part of the Indian or there is a lesser possibility for arc formation between Neo-Tethys Plate. the Pacific and the Farallon Plates in the Late Creta- The West Philippine Islands might have collided ceous deduced from their motions. Nevertheless, it is with the East Philippine–Daito Arc at approximately suggested that a possible arc existed in the northern from 55 to 52 Ma. Spreading of the Oki-Daito Ridge Pacific, between the Pacific and Kula Plates, suggest- ceased and arc polarity was reversed. A new spread- ing subduction along 40j or 50j N latitude (Enge- ing center was formed along the Central Basin Fault, bretson et al., 1985). along with the formation of the central portion of the In the Early and Middle Cretaceous, spreading axes West Philippine Basin associated with subduction at formed triple-point junction boundaries between the the Kyushu-Palau Trench on the northern side. Neo (Ceno)-Tethys, Izanagi, and Phoenix or Aus- The clockwise rotation of the Philippine Plate with tralia–Antarctic Plates to the south of Southeast Asia respect to the hot spot reference frame and sift to the (e.g. Hilde et al., 1977; Norton and Sclater, 1979; north during the Tertiary is suggested in the compiled Audley-Charles, 1988; Sengo¨r et al., 1988; Sengo¨r paleomagnetic data (e.g. McCabe and Cole, 1987; and Natal’in, 1996). In the Late Cretaceous, the Schmidke et al., 1990; Fuller et al., 1991, 1999). From Pacific Plate on the eastern side and the Indian Plate these work, the West Philippine Basin was at the on the western side occurred in Southeast Asia. The equator or only several degrees south of it during boundary between these plates was a subduction zone the Eocene (Louden, 1977; Kinoshita, 1980). From as deduced from the northward movement of the Neo- the paleomagnetic work of the Tertiary volcanic rocks Tethys and Indian Plates, and the northwestward and limestones, Halmahera is suggested to have been movement of the Pacific Plate (Engebretson et al., at approximately 10j S in the Early Eocene and to 1985; Powell et al., 1988; Royer and Sandwell, 1989). have rotated clockwise since then (Hall et al., 1995). One more possible subduction was in central Pacific Finally, the Philippine Plate rotated clockwise, with a between the Pacific and Kula Plates. Euler pole in the southern Kuril Trench and has The Daito Ridge is considered to be the subduction subducted into the Philippine Trench since the latest boundary between the Indian and Pacific in Western Miocene, or the earliest at 6 or 4 Ma (e.g. Pacific deducing from their corresponding age. Other Seno et al., 1993). For the clockwise rotation of the possible arcs in the region are Sumatra and the Philippine Plate during the Tertiary, a few models are Philippine Islands, but the West Philippines are amal- proposed to explain the regional geological nature of gamation of several blocks having different trends of Southeast Asia, the temporal shift of the pole to the ophiolite and arc volcanism (e.g. Philippines Bureau east of the plate (Hall et al., 1995) or northward shift of Mines and Geo-Sciences, 1982; Wolfe, 1995; of the pole along the north side of the plate (Sibuet et Sajona et al., 1997). The only recognized arc is the al., 2002). East Philippines. Before the West Philippine Basin After the inception of the Daito Ridge, the next opened, the Daito ridges were neighbored, possibly stage is the formation of the East Philippine–Daito along or to the north side to the East Philippines. Arc along the boundary between the Indian and Pacific When there was a spreading axis at the Oki-Daito Plates (A–B in Fig. 6). The positions of Eurasia and Ridge, the northern and southern margins of the West Australia are obtained from the calculations for the Philippine Basin (prior to the formation of the West major plate motions (Engebretson et al., 1985; Powell Philippine Basin) are interpreted to have been initially et al., 1988; Royer and Sandwell, 1989). From the located more to the south, possibly in the Indian or in episodic arc volcanism in the Daito ridges, the East the Neo-Tethys Plate. The Oki-Daito Ridge was Philippine–Daito Arc is suggested to have shifted possibly a spreading center between the northern towards the northeast, associated with counterclock- Indian Plate and the southern Australian Plate. A wise rotation of Borneo during the latest Cretaceous subduction towards the north had occurred on the and Early . On the other hand, the Indian south side of the Daito Ridge forming a plate bound- Plate moved at a rate that ranged from 50 to 165 mm/ ary between the Pacific and Indian Plates in the Late year, with faster velocity during its earlier stage of E. Honza, K. Fujioka / Tectonophysics 384 (2004) 23–53 33

Fig. 6. Rotation of Philippines and the Daito Ridge since the Late Cretaceous. Points A and B are the NW and SE margin of the East Philippine–Daito Trench. Points C and D are the northern and southern margins of the Kyushu-Palau Ridge in the modern stage Number in point is age in Ma for each stage in Fig. 7. Black is the present position. Trace of the Kyushu-Palau Ridge is shown after the cessation of the opening of the West Philippine Basin (since 35 Ma). translation (Lee and Lawver, 1995). When a north- but has been bending since 6 Ma. The Euler pole for eastward shift at a rate at 100 mm/year for 20 million the clockwise rotation of Philippines and the West years is assumed for this plate, the total movement that Philippine Basin prior to 6 Ma is obtained from the it would have undergone is approximately 20j to have positions of the islands at 52 Ma and at 6 Ma to the jives with its position between Borneo and northwest- southeast for the basin to have a clockwise rotation. ern Australia. From these motions, especially from the This resulted in a pole position at 0j, 150j E (Fig. 6). motions of Borneo and Australia, these jives resulted Rotation angle in each stage during the Tertiary is in a rotation pole at 85j N, 20j W. The position of the based on measurements in the Daito ridges and basins. Kyushu-Palau Ridge at 6 Ma is obtained from its Inclination measurements at DSDP Sites 445, 446 present rotation direction with the assumption that suggest that the Daito ridges were at the equator 40 Philippines have not laterally shifted a large distance Ma and shifted 11j to the north 30 Ma ago, and 34 E. Honza, K. Fujioka / Tectonophysics 384 (2004) 23–53 decreasing the northward shift at 5j between 30 and 20 Sedimentary Basin between Sumatra and Java since Ma (Kinoshita, 1980). Inclination measurements at the Eocene (Cole and Crittenden, 1997; Wight et al., DSDP Site 292 in the Benham Rise also suggest the 1997). same trend of increased rotation rate during the Late Eocene to Oligocene and approximately half rate in the Miocene (Louden, 1977). This resulted to approxi- 4. Reconstruction of arcs in Southeast Asia since mately doubled rotation angle from the period 52 to 25 the Late Cretaceous Ma as compared with that from 25 to 6 Ma. This decreased rotation is also consistent with the tectonic Utilizing results presented above, complicated activities of the surrounding region. The difference in sutures and the formation of arcs and backarc basins rotation ratio was possibly caused by the formation of in Southeast Asia are reconstructed since the Late double subduction lines at the south of the Philippines Cretaceous. Evidence from geological data, overall in the later stage. It especially allowed for the opening plate kinematics and paleomagnetic data are made of the South China Basin by increasing freeboard use of to decipher major plate motions and backarc space. The final stage involves the formation of the basin formation during arc collision. Major plate Philippine Trench with a Euler pole in the southern motion values employed are from calculations by Kuril Trench area at about 6 Ma. The Central Philip- Engebretson et al. (1985) for the Pacific and Eurasia pine Fault was also generated after the amalgamation Plates, Powell et al. (1988), Royer and Sandwell of the East and the West Philippines. Maximum (1989), and Lee and Lawver (1995) for the Indian, displacement along this structure from since the Plio- Australian and India–Australia Plates. Igneous activ- cene is estimated to have already reached several tens ities are mainly cited by Nozawa (1975) for Japan, of kilometers (Rangin et al., 1999). Shiki et al. (1985) and Bloomer et al. (1995) for the The rotation angle in the early stage (52–25 Ma) of Daito, Izu–Bonin and Mariana Arcs; Wolfe (1995) the clockwise rotation of the Philippines was approx- and Sajona et al. (1997) in the Philippines, Polve et imately 1.1j per Ma, 0.5j per Ma in the later stages al. (1997) and Wilson and Moss (1999) in Sulwesi, (25–6 Ma) and was 1.5j per Ma in the final stage since McCourt et al. (1996) for Sumatra, and Honza et al. subduction of the Philippine Plate into the Ryukyu and (1987) for Papua New Guinea. The reconstruction of Philippine Trenches at 6 Ma. A different rotation ratio arcs facing the Pacific Plate is revised by Honza between the early and the late stages from the Eocene (1991). to the Miocene is consistent with the double suduction Most arcs and backarc basins in Southeast Asia in the south of the Philippines and with surrounding appear to have commenced forming during the Ter- tectonics, especially in relation to the opening of the tiary. Stages of their formation histories are discussed South China Basin from 32 to 18 Ma. in the succeeding sections. A few arcs were initiated To resolve the discrepancy between the paleomag- in the Late Mesozoic, including arcs in Japan, South netic results and the tectonic interpretation of south- China, Sunda and western New Guinea. They were eastern Sundaland, we divided Sundaland in two formed by movements of the Izanagi, Kula, Pacific, blocks, which are the northern Indo-China and the Neo-Tethys and Indian plates that occupied the sites southern Sundaland blocks. Borneo, with respect to of the Pacific and Indian Ocean at that time. Sundaland, rotated approximately 90j during the Tertiary (e.g. Fuller et al., 1991, 1999) with a nearby 4.1. Formation of the East Philippine–Daito Arc from Euler pole to the west side (on Bangka Island). It has the Late Cretaceous to Earliest Eocene also been suggested that Borneo underwent bending (Honza et al., 2000). Eocene bending of Borneo has Arc configuration from the Late Cretaceous to the resulted into a thick accretionary complex known as earliest Tertiary is defined by two linear chains along the Rajang Accretionary Complex in northwestern the eastern margin of Southeast Asia, one traced from Borneo (e.g. Tan, 1982; Honza et al., 2000). It also Japan to Borneo (the Japan, South China and Borneo produced a thick syn- sedimentary deposit along Arcs) and the other along the southern Sumatra (the the north–south trending Seribu Fault in the Sunda Sunda and East Philippine–Daito Arcs) (Fig. 7A). E. Honza, K. Fujioka / Tectonophysics 384 (2004) 23–53 35

Fig. 7. (A) Reconstruction of Southeast Asia in the Late Cretaceous. Points A and B are the initial margin of the East Philippine–Daito Trench. Point C is the northern margin of the Kyushu-Palau Ridge in the modern stage. A subduction zone has been formed between the Pacific and the Indian Plates in the Late Cretaceous. Plate kinematics in Southeast Asia was changed by collision of West Philippines in the middle Eocene. Other symbols are the same as those in Fig. 1. (B) Reconstruction of Southeast Asia in the earliest Eocene. Symbols are the same as A. (C) Reconstruction of Southeast Asia in the Middle Eocene. Symbols are the same as A. (D) Reconstruction of Southeast Asia in the earliest Oligocene. Symbols are the same as A. (E) Reconstruction of Southeast Asia in the latest Oligocene. Symbols are the same as A. (F) Reconstruction of Southeast Asia in the Middle Miocene. Symbols are the same as A. (G) Reconstruction of Southeast Asia in the earliest Pliocene. Symbols are the same as A. 36 E. Honza, K. Fujioka / Tectonophysics 384 (2004) 23–53

Fig. 7 (continued).

This configuration is also suggested by the alignment reconstructed on the basis of the paleomagnetic and of the Cretaceous intrusives on the eastern margin of geological data. During the tectonic evolution stage the Eurasian Plate traced from Japan, South China, in the Cretaceous, the intrusives of Borneo were South Vietnam and to South Borneo (e.g. Hamilton, nearer the Pacific side (Williams et al., 1988; Honza 1979; Xiong and Coney, 1985). et al., 2000). To the south of Borneo, subduction was Southern Borneo has rotated approximately 90j taking place along the Daito Ridge and the East counterclockwise since the Cretaceous (e.g. Haile et Philippines forming the East Philippine–Daito al., 1977; Fuller et al., 1991, 1999). Borneo is Trench. The Trans-Himalayan (Gangdese) Arc, the E. Honza, K. Fujioka / Tectonophysics 384 (2004) 23–53 37

Fig. 7 (continued). western extension of the , moved north- slow rifting from Antarctica in the Late Cretaceous. ward with the northward movement of the Neo- Full-scale rifting during the latest Paleocene pro- Tethys and the Indian Plates (Patriat and Achache, duced the Australian Plate (Powell et al., 1988; 1984; Klootwijik et al., 1985; Audley-Charles, 1988; Royer and Sandwell, 1989). Sengo¨r and Natal’in, 1996; McCourt et al., 1996). Subduction halted at the East Philippine–Daito Australia which was, until the Middle Cretaceous, Trench after the West Philippines, which was part of relatively immobile with respect to Antarctica the Indian Plate collided during the Eocene as formed the Australia–Antarctic Plate. It commenced deduced from the age of the Daito Ridge (Fig. 38 E. Honza, K. Fujioka / Tectonophysics 384 (2004) 23–53

Fig. 7 (continued).

7B). The Oki-Daito Ridge, an oceanic ridge be- at the East Philippine–Daito Arc, the Philippine– tween the northern Indian and the southern Austra- Daito block commenced to move towards the Pa- lian Plates, collided with the eastern margin of the cific side and a new reversed subduction zone East Philippine–Daito Trench by northward move- formed in the north, the Kyushu-Palau Trench. ment. This motion brought the ridge to the northeast The West Philippine Basin commenced opening at side of West Philippines as deduced from the the southern side of the Oki-Daito Ridge, in reaction intercalation of oceanic floor materials at the south to the northward movement of the Kyushu-Palau side of the Ridge. With the cessation of subduction Arc approximately at 52 Ma. E. Honza, K. Fujioka / Tectonophysics 384 (2004) 23–53 39

Fig. 7 (continued).

During the Paleocene, the Proto-New Guinea Arc 1979). Rifting appears to have been initiated by the extended to the west along the boundary between the subduction of the Pacific Plate. Another possible Pacific and the Australian Plates as suggested by the stimulus is the wedging of the North New Guinea volcanic and sedimentary deposits of the Auwewa Plate between the Australian and Pacific Plates (Seno, Formation in Irian Java (Milsom, 1985). A backarc 1985; Maruyama et al., 1989). basin also formed in relation to this arc in the later The Zambales ophiolite in Luzon Island (Hawkins phase. The Coral Sea Basin opened on its southeastern and Evans, 1983; Karig, 1983; Schweller et al., 1983) extension between 62 and 56 Ma (Weissel and Watts, was possibly a part of the Celebes Basin or Indian 40 E. Honza, K. Fujioka / Tectonophysics 384 (2004) 23–53

Fig. 7 (continued). oceanic crust that got accreted during an eastward the northeastward movement of the Kyushu-Palau Arc subduction under Luzon. (Fig. 7C). The southern trace of the Kyushu-Palau Trench appears to be initially a transform fault related 4.2. Rotation of the Philippine Plate from the Eocene to the formation of the West Philippine Basin and was to the Miocene converted to a subduction zone in the Eocene (Hawkins et al., 1984; Honza, 1991). Halmahera remained in the The Eocene western Pacific is characterized by the Philippine side and did not sift northward with the formation of the West Philippine Basin, associated with Daito ridges. Spreading pattern of the West Philippine E. Honza, K. Fujioka / Tectonophysics 384 (2004) 23–53 41

Fig. 7 (continued).

Basin changed in the Middle Eocene from a northwest presence of the Cotabato intrusives suggesting sub- to an east–west trending axis (Hilde and Lee, 1984; duction along that area. Western Mindanao was pos- Fujioka et al., 1999). sibly adjacent to the Celebes Basin prior to the The Celebes Basin also formed in the Eocene occurrence of subduction along Cotabato. The forma- (Weissel, 1980; Shipboard Scientific Party Leg 124, tion of the Celebes Basin was probably associated 1990) and has not separated from northeastern Borneo with the development of a new subduction on the since the basin was formed. Western Mindanao is not southwestern side of the Philippines due to the north- part of the basin but is separated from it by the eastward migration of the India–Australia Plate. Hall 42 E. Honza, K. Fujioka / Tectonophysics 384 (2004) 23–53

(2002) modeled the Celebes Basin to be the western southern Kyushu-Palau Arc formed at this time and extension of the West Philippine Basin formed in subsequently further south (Fig. 7D). The Caroline between the northern and southern Philippines. How- Basin formed on the Pacific side from the Early to ever, if we take the rotation of the Philippines from the Middle Oligocene (Bracy, 1975; Weissel and Ander- boundary along the Indian and the Pacific Plates to son, 1978; Hegarty et al., 1983; Altis, 1999). At the present position, such a case did not occur since the beginning, the Caroline Basin may have been associ- Middle Tertiary but the Celebes Basin developed ated with the southeastward-facing Caroline Arc but separately from the West Philippine Basin adjacent counterclockwise rotation and opening of the basin to northeastern Borneo and southwestern Mindanao, shifted it to the northeast. and subduction along the West Philippine Trench The South China Basin opened in the Early Oli- developed in the Late Oligocene and Early Miocene. gocene (Taylor and Hayes, 1983; Pautot et al., 1986; The Celebes Basin at present is narrowing due to Ru and Pigott, 1986; Briais et al., 1993; Zhou et al., subduction from the east, north and south. Initially, 1995) and by this time, spreading in the Celebes Basin the basin was wider, extending from eastern Borneo to has ceased. The opening of the South China Basin the northern part of New Guinea. might have been triggered by the approach of the hot In the Early Eocene, a subduction complex formed region responsible for the spreading center in the West along the eastern margin of Borneo (in the northwest Philippine Basin. The Proto-South China Basin was part at present) and after the termination of subduction commonly assumed to be located the northwest of in the Late Eocene was uplifted (Hutchison, 1996; present Borneo to accommodate the formation of the Honza et al., 2000). The cessation of subduction of South China Basin in the later stage and to explain a the Pacific Plate in Borneo is interpreted to be due to subduction complex in northwest Borneo (e.g. Hollo- the formation of the West Philippine Basin and the way, 1982). Our present interpretation to explain how Celebes Basin on the eastern side of the subduction the spreading in the South China Basin was accom- complex. As a consequence of the counterclockwise modated is by the bending of Borneo and by con- rotation of Borneo, a thick accretionary complex in sumption of the Celebes Basin. Spreading of the northwestern Borneo was formed. West Sulawesi Makassar Basin appears to have initiated in the rifted from Borneo in the Middle Eocene. The initi- Middle Eocene and considered until the Oligocene. ation of the opening of the Makassar Straits is A new reversed subduction of the West Melanesia demonstrated by the deposition of syn-rift sediments Trench occurred where the Central New Guinea from the lower to upper Eocene in eastern Kalimantan Trench collided with southern New Guinea (Honza, (Letouzey et al., 1990; Chambers and Daley, 1997; 1991) and along side it, the Solomon Basin began to Moss et al., 1997; Todd et al., 1997; Cloke et al., form. Two possible ages for the Solomon Basin have 1999) and North Sulawesi Arm was possibly in the been proposed on the basis of the magnetic anomaly northern margin of the Celebes Arc at this time. pattern (Joshima et al., 1987). One is from 39 to 36 The western Sunda Arc is thought to have been Ma and the other from 34 to 28 Ma. The earlier age active throughout the Cenozoic and the eastern Sunda coincides with the collision of the New Guinea Arc Islands were not formed at this time (e.g. Hamilton, with the Australian Continent (Central Orogenic Belt) 1979; Stauffer, 1985; McCourt et al., 1996). The and that the formation of the basin is inferred to have Central New Guinea Arc (the Proto-North New Guin- occurred soon after the amalgamation. The basin was ea Arc by Honza, 1991), on the eastern side of the more wide-ranging at this later stage (Honza et al., Celebes Trench, was formed associated with a re- 1987). These suggest that the Solomon Basin may versed southward facing subduction zone that com- have formed from the latest Eocene or Early Oligo- prise the Central Orogenic Belt (Dow and Sukamto, cene to Late Oligocene. 1984; Milsom, 1985; Pigram and Symonds, 1991) The North Palawan Block migrated to the south as after the collision with North New Guinea continental an eastern extension of the North Palawan Arc where block either in the Early or Middle Oligocene. to the north of it, the South China Basin was formed The West Philippine Basin ceased opening in the (McCabe et al., 1985; Faure et al., 1989) (Fig. 7E). Earliest Oligocene (Hilde and Lee, 1984) and the The formation of the South China Basin is deemed to E. Honza, K. Fujioka / Tectonophysics 384 (2004) 23–53 43 be associated with the left-lateral movement of the counterclockwise rotation of Malay Peninsula and a Red River (Song Ma) Fault on the northern boundary sinistral movement with a slight clockwise rotation of of the Indo-China Block (e.g. Tapponnier et al., 1986; the Red River Fault. Peltzer and Tapponnier, 1988; Leloup et al., 1995), The Shikoku and Parece Vela (West Mariana) although there are some disagreements with this Basins commenced opening in the Late Oligocene interpretation (e.g. Tri et al., 1973; Longley, 1997). (Okino et al., 1994; Okino, 2000). The Solomon and If the Fault is considered not to have played a role in Caroline Basins ceased opening with the Solomon the opening of the South China Basin, the retreat of a Basin having achieved its maximum width also at this trench consuming the southern oceanic crust and time. collision of the North Palawan block to the Philip- The late Middle Miocene time, the spreading of the pines appear preferable for its formation rather than Shikoku and Parece Vela Basins have come to an end, the fixed north-facing trench along the northern Bor- while the Japan Basin opened from the latest Oligocene neo model consuming the Proto-South China Basin as or from the Early Miocene (Fig. 7F). Otofuji and cited in some papers (e.g. Holloway, 1982). The Matsuda (1984) suggested the clockwise rotation of spreading in the South China Basin can be attributed Southwest Japan between 15 and 14 Ma based on their to the sinistral movement of the Red River Fault and paleomagnetics. However, other studies on magnetic formation of the North Palawan Arc. In this model, anomalies of the Japan Basin suggest a slightly earlier the Celebes Basin is being subducted instead of the formation (Kobayashi and Isezaki, 1976; Isezaki, Proto-South China Basin under the North Palawan 1986). Kaneoka (1986) and Tamaki et al. (1992) Trench. The same model is adopted for the formation interpreted the age of the basement rocks in the Japan of the Sulu Basin in the Miocene. Basin to be 24–17 Ma. The opening of the Japan Basin A new subduction zone, which consumes the is related to dextral strike-slip movement to account for Celebes Basin, was formed on the southwestern part the temporal difference between formation and rotation of the West Philippines as suggested by the episodic of the basin (e.g. Tamaki et al., 1992; Jolivet et al., arc volcanism in the Philippine Islands (Hashimoto, 1994; Honza et al., in preparation). We interpret the 1981; Philippines Bureau of Mines and Geo-Sciences, opening of the Japan Basin to be from 25 to 14 Ma. 1982; Karig, 1983; Gallagher, 1987; Saldivar-Sali et The South China Basin ceased opening in the al., 1987; Wolfe, 1995; Sajona et al., 1997).Asa Middle Miocene. The Nankai Trough, the Ryukyu result of subduction, Borneo and the Celebes Basin Trench and the Luzon Trench were formed in the rotated counterclockwise (Shibuya et al., 1991). East Middle Miocene but their subduction rates might Sulawesi, on the Indian Plate moved towards the have been slow as deduced from the rotation rate (Mubroto et al., 1994) and possibly of the Philippines at this stage. The southern part of collided with West Sulawesi along the corner of the the North Palawan Arc collided with Borneo result- Sunda and the Celebes Trenches in a little later phase ing in the reversal of arc polarity to form the Palawan in this stage (Polve et al., 1997). Trench. The Sulu Basin began spreading associated The boundary between Indo-China and Sundaland with the formation of the Palawan Trench while on is along the Three Pagodas and Ranong Faults in the its southern part, the Sulu Trench was formed (Ship- southern Thailand and extends to the Gulf of Thai- board Scientific Party Leg 124, 1990). The North land. The Malaya Basin opened as a pull-apart basin Palawan block, on the other hand, commenced col- where the faults were formed by compression in the lision with the western Philippines in the Late Mio- north–south direction in the Middle or the Late cene or in the Pliocene (McCabe et al., 1985; Oligocene (Todd et al., 1997; Leo, 1997; Cole and Gallagher, 1987; Marchadier and Rangin, 1990; Bar- Crittenden, 1997). This opening was associated with a rier et al., 1991). slight clockwise rotation (Haile, 1979) or counter- In the Middle Miocene, the West Halmahera and clockwise rotation (Longley, 1997; Richter et al., Sangihe Trenches were formed in the southern West 1999) of Sundaland. Other models suggest that the Philippine Basin and in the southern Celebes Basin, faults in the Malaya Basin were related the movement respectively. The Molucca Basin subducted under the of the Red River Fault. We however, prefer a slight Sangihe Trench that separates it from the Celebes 44 E. Honza, K. Fujioka / Tectonophysics 384 (2004) 23–53

Basin. The formation of these trenches was possibly Ma (Honthaas et al., 1998). Inversion of the Upper related to the collision of East Sulawesi with West Eocene syn-rift sediments in the Makassar Straits is Sulawesi. The E–W trending Sangihe Trench possi- considered to have occurred in the earliest Middle bly extended along North Sulawesi and becoming a Miocene (Chambers and Daley, 1997; Moss et al., N–S trending structure that defines the boundary 1997). between North and West Sulawesi. This boundary is The Australian continental margin collided with also observed in the seismic tomography in this area the Banda Arc at the end of the Miocene (Harris, (Hafkenscheid et al., 2001). Otofuji et al. (1981) 1991; Snyder and Barber, 1997) and left-lateral trans- proposed the bending of northern Sulawesi in the current faults occurred from east Sulawesi to western Middle Miocene. However, this is in conflict with New Guinea (Katili and Hartono, 1983). The Solo- the counterclockwise rotation of the Celebes Basin mon Basin was consumed by a new subduction zone and Borneo, even if, southward subduction at the that fromed along the Trobriand Arc, where volcanism North Sulawesi Trench occurred at a later phase. was active since the Middle Miocene (Milsom, 1985), Arc volcanism in the Halmahera Arc was active in in the southern part of the basin (Honza et al., 1987). the latest Cretaceous, in the Early Eocene and in the Further west, the Andaman Basin opened since the Oligocene, and again was active since the Middle Late Miocene (Curray et al., 1979). Miocene (Sukamto, 1989; Hall et al., 1995).Arc volcanism in the latest Cretaceous and the Eocene 4.3. Formation of the Philippine Trench in the was possibly generated by of the Pacific Pliocene or India–Australia Plate, and since the Middle Miocene by the northeastward subductions at the West Philip- Taiwan collided with continental China since the pine and the Halmahera Trenches. The opening of the Late Miocene to the Early Pliocene (e.g. Teng, 1990; Ayu Trough in Miocene is not taken into consideration, Sibuet et al., 2002) that possibly triggered subduction on account of indistinct magnetic anomalies. along the Philippine Trench (Fig. 7G). The Philippine The jump of subduction to the south of East Sea was isolated to form the Philippine Plate, which Sulawesi formed the eastern Sunda (Banda) Arc experienced a clockwise rotation since 6 or 4 Ma (Audley-Charles, 1986). This resulted from the colli- (Bowin et al., 1978; Cardwell et al., 1980; Hamburger sion of East Sulawesi with West Sulawesi, related also et al., 1983; Karig, 1983; Barrier et al., 1991; Seno et with amalgamation of Buru and Seram Islands from al., 1993). The southern trace of the Luzon Trench was the Late Oligocene to the Miocene. The collision also on the northern side of the Cagayan Ridge (Hinz and terminated the subduction in the Celebes Trench. East Block, 1990). A slow subduction commenced in the Sulawesi, Buru and Seram Islands were initially in the Cotobato Trench in the Late Pliocene possibly is Indian Ocean or in the northwest corner of Australia reactivation of the West Philippine Trench. The initial (e.g. Audley-Charles, 1988; Lee and Lawver, 1995) depression of the Okinawa Trough was also formed in but the jumping of the eastern Sunda Trench (Celebes the Pliocene (Kimura, 1985; Letouzey and Kimura, Trench) resulted in a new subduction to be formed in 1986; Miki, 1995; Park et al., 1998; Sibuet et al., 1998; the north side of Buru and Seram. The trench-jump- Kong et al., 2000) and the spreading was accelerated ing created a captured oceanic crust which was on its southern part since 2 Ma. Slight subduction of variously interpreted to have been formed in Meso- the South China Basin along Palawan and northwest zoic (Lapouille et al., 1985; Lee and McCabe, 1986), Sarawak occurred in the Pliocene (Tongkul, 1991). in Paleogene (Barber et al., 1981) or in The Molucca Basin, in the south, has been sub- (Hamilton, 1979; Silver et al., 1985; Fortuin et al., ducting under both the eastern Halmahera Arc and the 1990; Hall, 2002). Charlton (2000) proposed a push- western Sangihe Arc, forming an inverted V-shaped ing scenario along the western margin of northwest slab with the longer slab on the western side (Silver Australia instead of trench-jumping. Dredged result and Moore, 1978; McCaffrey et al., 1980; Moore and from the South Banda Basin suggests a multi-rift Silver, 1983). The collision of East and West Halma- opening for the eastern Wetar Basin and as a single- hera occurred in the Pleistocene based on the studies rift opening for the western Damar Basin from 6 to 3 on the basement ophiolitic and volcanic rocks in this E. Honza, K. Fujioka / Tectonophysics 384 (2004) 23–53 45 area (Hall and Nichols, 1990; Hall et al., 1991) and Kyushu-Palau Arc by arc reversal. The Kyushu-Palau that both the Solomon and Molucca Basins have Arc associated with the formation of the West Philip- collided along their northern and western margins, pine Basin was succeeded to the Izu–Bonin and respectively. A westward dipping slab along the Mariana Arcs associated with the formation of the Halmahera Trench is suggested in the northern junc- Shikoku and Parece Vela Basins since the Middle tion between Mindanao and Halmahera (Lallemand et Oligocene. al., 1998). However, more data are required to deter- The reconstruction presented above reveal that mine the timing of collision such as identification of there are some primary causes for the opening of the opening age of the Ayu Trough (Fig. 1). In this backarc basins in Southeast Asia. The opening of paper, we prefer a simple eastward subduction of the marginal basins in Southeast Asia appears to be Halmahera Arc based on seismic tomography evi- triggered initially by collisions, such as the collision dence (Hafkenscheid et al., 2001). of the West Philippines with the Daito Arc for the The Sula Microcontinent moved westward along opening of the West Philippine Basin in the Earliest the Sula–Sorong left lateral faults (Silver et al., 1983; Eocene, the Central New Guinea Arc to the southern Pigram et al., 1985; Fortuin et al., 1990).This New Guinea for that of the Solomon Basin in the movement caused the bending of the Seram Arc Earliest Oligocene, East Sulawesi to West Sulawesi for (Audley-Charles, 1986) and movement ceased as the the capture of the Molucca Basin, Sangihe to East Sula microcontinent collided central Sulawesi. The Sulawesi for that of the Sunda Basin in the Early North Sulawesi Trench was formed in the northern Miocene, the Trobriand Arc to North New Guinea for part of North Sulawesi by the cessation of the move- the opening of the Manus Basin and the North Palawan ment in the Sula–Sorong faults. The Trobriand Arc Arc to North Borneo for the opening of the Sulu Basin collided with New Guinea, forming the northern in the Late Miocene to Early Pliocene (Fig. 8). range, except for its eastern margin. Opening of the The opening may also be accelerated by oblique Manus and Woodlark Basins occurred in the Pliocene, subduction at trenches, and basins are formed on the associated with the formation of the New Britain backarc side of the arc (backarc basin) approximately (Bismarck) Arc (Taylor, 1979; Weissel et al., 1982; parallel to oceanic plate motion. Prime examples of Cooper and Taylor, 1987; Honza et al., 1987). The these are the West Philippine Basin in the Eocene, the Mariana Trough and the Ogasawara Depression were Makassar Basin in the Earliest Oligocene (Fig. 7C), formed in the later phase of this stage (Shipboard the Shikoku and Parece Vela Basins, and the Solomon Scientific Party Leg 31, 1975; Bibee et al., 1980; Basin in the Latest Oligocene (Fig. 7E). Spreading of Shipboard Scientific Party Leg 60, 1981; Honza and backarc basins are distributed approximately from Tamaki, 1985; Taylor, 1990), from approximately 3.5 NNW to SSE on the Pacific side and from NNE to Ma (Yamazaki et al., 1993). SSW on the Indian side, parallel to the motion of the Pacific and Indian Plates, respectively. The formation of backarc basins also has been 5. Formation of Backarc Basins triggered by the approach of the hot spreading center. Opening of the South China Basin was possibly From results presented above, complicated sutures activated by the approach of the spreading axis of and the formation of arcs and backarc basins in the West Philippine Basin (Fig. 7D) while the forma- Southeast Asia are reconstructed since the Late Cre- tion of the Japan Basin appears to have been triggered taceous showing active periods in lines (Fig. 8). For by the approach of the Shikoku Basin to the south of instance, the Japan Arc had been active since the Late Japan (Fig. 7E). Cretaceous but segmented to form the Tohoku, SW The spreading mode changed when the spreading Japan and Ryukyu Arcs in the Middle Miocene. The axis approached the corner of continent, as is observed Japan Basin was formed from the Late Oligocene to in the West Philippine Basin. Timing of this change the Middle Miocene. The East Philippine–Daito Arcs corresponds to the time of the approach of the Kyushu- shifted from the Sunda Arc in the Paleocene and Palau Trench at the corner between Borneo and South ceased its activity in the Late Paleocene to form the China (Fig. 7C). This also corresponds to the change of 46 E. Honza, K. Fujioka / Tectonophysics 384 (2004) 23–53

Fig. 8. Formation of arcs and backarc basins in Southeast Asia based on the tectonic evolution presented in this paper. Thick line with oblique bar shows branched or segmented arc activity. Thick line with bars shows arc activity with limited duration and thick line with arrow indicates continuous activity since then. Thin line shows backarc opening. Broken line is estimated event. R with circle is reversal of arc polarity due to collision.

Pacific Plate motion. This further suggests that a We cannot find any trigger for the opening of change of spreading mode can also be triggered by the Celebes and Caroline Basins. One possibility is the change of oceanic plate motion. In the Shikoku and that the Late Cretaceous subduction zone was Parece Vela Basins, it was adjusted to approximately reactivated in the later phase in West Philippines. parallel to the direction of motion of the Pacific Plate. Another possibility is that opening of basins is a E. Honza, K. Fujioka / Tectonophysics 384 (2004) 23–53 47 result of some amalgamations in the Philippines, faulting. Especially marginal basins beside Eurasia which cannot be detected at this time. The Early Continent, the Kuril, Japan and South China and Oligocene Caroline Trench and the adjacent West Andaman Basins are postulated to have formed by Melanesia Trench faced each other across a narrow the motion of microcontinents or blocks in Eurasia extension of the Pacific Plate (Fig. 7D). This may Continent (e.g. Tapponnier et al., 1982; Kimura and suggest that a portion of the Pacific Plate has a Tamaki, 1985). Some difference in the rotation rate southwestward component that differs from the of the Philippines might be resolved by introducing main motion, to explain the subduction into the sinistral transcurrent movement, although we have Caroline Trench. In the later phase, the Caroline not considered any transcurrent movement in the Basin appears to have opened, associated with the Philippines, except along the Philippine Fault in counterclockwise rotation of the Caroline Arc. the final stage. The generative force to form backarc basins is possible within subduction system due to horizontal 6. Discussion and summary tensional forces on the backarc side. These forces are considered not strong enough to overcome surround- The reconstruction since the Early Tertiary is ing compressive force prevailing in an arc system. based on the clockwise rotation of the West Philip- Backarc basins are possibly formed associated some pine Basin and on the counterclockwise bending at additional tensional force in backarc side (Tamaki and Borneo. If the Philippine were on the southern Honza, 1991). margin of the North New Guinea Plate, the subduc- The model for the paleogeographic reconstruction tion might have faced toward the north or the of the Latest Mesozoic and Cenozoic Southeast Asia northeast. This conflicts with the observed direction presented above is, first, based on fixing the initial of the subduction in the Daito Ridge. The initial position of Philippines and the Daito ridges and basins position of East Philippine Islands and the Daito in and along the boundary of the Indian, Neo-Tethys Ridge along the boundary between the Pacific and and Pacific plates. The second, the West Philippines Indian Plates eventually requires clockwise rotation riding on the Indian Plate approached from the south of Philippine Islands and the West Philippine Basin and collided with the East Philippine–Daito Arc in in the subsequent phase. The counterclockwise rota- the latest Paleocene, or in the earliest Eocene. The tion of Borneo can explain the thick accretion in third, clockwise rotation of Philippines and the West northwest Borneo without need to assume the pres- Philippine Basin and counterclockwise rotation of ence of a Proto-South China Basin. The paleolatitude Borneo are taken into consideration with references of the Philippine Plate is around 0j to 10j S in the of regional geological evidences and major plate Eocene (Louden, 1977; Kinoshita, 1980; Hall et al., kinematics. The forth is that the subduction at the 1995; Hall, 2002) which is concordant with our Philippine Trench makes the final stage in the paleo- interpretation except rotation angle since the Eocene. geographic reconstruction of Southeast Asia. Some Paleolatitude in the Late Cretaceous was approxi- captured oceanic crusts form small basins that may mately at 20j S in our interpretation. There may be a explain enigmatic tectonic settings, such as the shift of the rotation pole for the Philippine Plate by Molucca, Banda and Trobriand Basins. However, thechangeintherotationrateat25Maasis rotation of Borneo precludes the need for a Proto- suggested by Hall et al. (1995), nevertheless, we South China Basin. do not adopt any shift of the rotation pole for both of The reconstruction presented reveals some primary them. These will be resolved by paleomagnetic causes for the opening of backarc basins in Southeast measurements in the Philippines. Asia. In this paper, backarc basins are interpreted to have been formed by rotation around a Euler pole 1. Opening of some backarc basins was triggered by with respect to surrounding plate. Some backarc collisions. basins are interpreted to have formed associated 2. Backarc basins were opened approximately parallel not only with rotation but also with transcurrent to oceanic plate motion. 48 E. Honza, K. Fujioka / Tectonophysics 384 (2004) 23–53

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