West Philippine Basin Research Articles

The West Philippine Basin: An Eocene to early Oligocene back arc basin opened between two opposed subduction zones

Based on geological and geophysical data collected from the West Philippine Basin and its boundaries, we propose a comprehensive Cenozoic history of the basin. Our model shows that it is a back arc basin that developed between two opposed subduction zones. Rifting started around 55 Ma and spreading ended at 33/30 Ma. The initial spreading axis was parallel to the paleo‐Philippine Arc but became inactive when a new spreading ridge propagated from the eastern part of the basin, reaching the former one at an R‐R‐R triple junction. Spreading occurred mainly from this second axis, with a quasi‐continuous counter‐clockwise rotation of the spreading direction. The Gagua and Palau‐Kyushu ridges acted as transform margins accommodating the opening. Arc volcanism occurred along the Palau‐Kyushu Ridge (eastern margin) during the whole opening of the basin, whereas the paleo‐Philippine Arc decreased its activity between 43 and 36 Ma. The western margin underwent a compressive event in late Eocene‐early Oligocene time, leading to the rising of the Gagua Ridge and to a short subduction episode along Eastern Luzon. In the western part of the basin, the spreading system was highly disorganized due to the presence of a mantle plume. Overlapping spreading centers and ridge jumps occurred toward the hot region and a microplate developed. Shortly after the end of the spreading, a late stage of amagmatic extension occurred between 30 and 26 Ma in the central part of the basin, being responsible for the deep rift valley that cut across the older spreading fabric.

Late amagmatic extension along the central and eastern segments of the West Philippine Basin fossil spreading axis

The spreading processes within the West Philippine Basin (WPB) remain partly unknown. This study presents an analysis of the tectono-magmatic processes that happened along its spreading axis during the conclusion of the last spreading phase at 33/30 Ma. We demonstrate that the late episode of N–S opening from an E–W-trending spreading system has been followed by a late tectonic event occurring in the central and eastern parts of the basin. This event was responsible for transtensional strain accommodated along the NW–SE faults cutting through the former E–W rift valley in the central part of the basin. In its eastern part, the same event occurred at a larger extent and led to the creation of a new NW–SE axis, obliquely cutting the older E–W spreading segments and their associated spreading fabrics. At this location, several tens of kilometers of slightly oblique amagmatic extension occurred following a ∼60° direction. We propose that this late event is associated with the onset of E–W opening of the Parece-Vela Basin located along the eastern border of the WPB at 30 Ma. Extensive stresses within this basin were probably transmitted to the hot and easily deformable rift zone of the WPB. The newly-created NW–SE axis most likely propagated from east to west, being responsible for scissors opening within the WPB. NE–SW extension ceased when well-organized spreading started at 26 Ma in Parece-Vela Basin, accommodating entirely the global extensive stress pattern.

A geologic test of the Kula-Pacific Ridge capture mechanism for the formation of the West Philippine Basin

In spite of nearly 30 yr of investigation, the origin of the West Philippine Basin remains elusive. This basin occupies nearly half of the Philippine Sea plate, which remains one of the most poorly understood of the major lithospheric plates on Earth. Establishing the origin of the West Philippine Basin is fundamental to deciphering the evolution of the western Pacific Basin, including the India-Asia collision and the opening of numerous marginal basins in the region. In this paper we combine new on-land kinematic data from southwest Japan with existing marine magnetic data from the ancient Kula-Pacific Ridge to test an early tectonic model that suggests that the West Philippine Basin arose by trapping of a part of the Kula-Pacific Ridge during the middle Eocene. The results suggest that this mechanism is viable, but that the ridge-capture event may have occurred during the middle Eocene, ∼10 m.y. earlier than originally proposed.

Archaeology of Archaea: geomicrobiological record of Pleistocene thermal events concealed in a deep-sea subseafloor environment.

A record of the history of the Earth is hidden in the Earth's crust, like the annual rings of an old tree. From very limited records retrieved from deep underground, one can infer the geographical, geological, and biological events that occurred throughout Earth's history. Here we report the discovery of vertically shifted community structures of Archaea in a typical oceanic subseafloor core sample (1410 cm long) recovered from the West Philippine Basin at a depth of 5719 m. Beneath a surface community of ubiquitous deep-sea archaea (marine crenarchaeotic group I; MGI), an unusual archaeal community consisting of extremophilic archaea, such as extreme halophiles and hyperthermophiles, was present. These organisms could not be cultivated, and may be microbial relicts more than 2 million years old. Our discovery of archaeal rDNA in this core sample, probably associated with the past terrestrial volcanic and submarine hydrothermal activities surrounding the West Philippine Basin, serves as potential geomicrobiological evidence reflecting novel records of geologic thermal events in the Pleistocene period concealed in the deep-sea subseafloor.

New insights on 3-D plates interaction near Taiwan from tomography and tectonic implications

Recent tomographic results are used to trace the South China Sea and Philippine Sea subducting slabs, south and northeast of Taiwan along the Manila and Ryukyu trenches, respectively. In particular, the 3-D plates interaction beneath Taiwan is discussed based on a close-up view of the tomographic sections together with earthquake hypocenters distribution. Our study indicates that: (1) the east-dipping South China Sea slab can be followed to the north, up to the latitude of Hualien, (2) the Eurasian plate subducts beneath most part of the Taiwan island down to the 670 km-depth discontinuity, (3) the north-dipping Philippine Sea slab can be followed slightly west of the longitude of Hualien. Both plates thus interact beneath northern Taiwan where the arc–continent collision is paroxysmal. (4) Slab detachment is envisaged at the northern edge of the subducted Eurasian plate beneath the Coastal Range of Taiwan, which may facilitate the northwestward motion of the Philippine Sea plate with respect to Eurasia. Slabs geometries obtained from tomographic sections allow us to reconstruct the Late Neogene plate kinematics and dynamics in this region. Our main conclusions are: (1) The size of the original South China Sea was about twice its present size. (2) The subducted part of the West Philippine Basin i.e. the largest oceanic basin of the Philippine Sea Plate, extends only 400 km north of the Ryukyu Trench. (3) Slab detachment might have occurred 3–5 my ago beneath the central and northern Ryukyu Arc along a weak zone that is aligned with the Gagua Ridge: an ancient plate boundary. (4) The Ryukyu Trench has propagated westward from 126°E of longitude (southeast of Miyako Island) to the locus of the present arc–continent collision, along a major lithospheric tear that cut through the continent–ocean boundary first, and then through the continental lithosphere. As a consequence, the southern Ryukyu margin developed progressively from east to west as a subduction zone during the last 8 my. It has been elaborated onto a passive margin which corresponded to the edge of the Eurasian continental shelf bordering the South China Sea prior to Pliocene. (5) The Southern Okinawa Trough rifted during the last 2±1 Ma following the propagation of the subduction zone. (6) Subduction of the Eurasian margin probably stopped recently beneath northern Taiwan and a flip of subduction from west to east is being initiated near Hualien.

Tectonic setting of Eocene boninite magmatism in the Izu–Bonin–Mariana forearc

Middle Eocene boninites were simultaneously generated over a large region during the early history of the Izu–Bonin–Mariana (IBM) arc. However, widespread boninite magmatism is not recognised in younger subduction zones of similar dimensions. This suggests that an additional tectonic or thermal factor influenced the generation of the IBM boninite suite. Regional uplift, ocean island basalt-style magmatism and high heat-flow also characterised the northern Philippine Sea plate (PSP) at the start of the Middle Eocene. These features are similar to those observed in large volume basaltic volcanic provinces such as the early Tertiary North Atlantic and suggest that the IBM boninite suite may have been produced because there was already a thermal anomaly in the mantle beneath the PSP. The reconstructed Middle Eocene location of the IBM arc and West Philippine Basin lies close to the present day Manus Basin where petrological and geochemical evidence indicate the presence of a mantle plume. A calculated plume track linking these locations through time also passes close to the Eauripik Rise, an aseismic ridge on the Caroline Plate, during the Oligocene and Miocene. Therefore, we propose that a thermal anomaly or mantle plume influenced the magmatic and tectonic development of the western Pacific from the Middle Eocene until the present day.

Evidence for Early Cretaceous oceanic crust trapped in the Philippine Sea Plate

The Huatung Basin is a small oceanic basin located east of Taiwan. Previous age estimates from magnetic lineation studies indicated an Eocene age for the basin, and formation from the Central Basin Spreading Center of the West Philippine Basin during the last phase of spreading in Middle Eocene. New Ar/Ar ages obtained on gabbros dredged on oceanic basement highs of the Huatung Basin are Early Cretaceous. These old ages are consistent with Early Cretaceous ages determined on radiolarian assemblages from Lanyu Island (Luzon Arc). We have performed magnetic anomalies modeling for an Early Cretaceous oceanic crust. Our results are in good agreement with new Ar/Ar ages determinations. The best fit is indeed obtained with an opening of the Huatung Basin during the Early Cretaceous from 131 to 119 Ma, with a half spreading rate varying between 25 and 30 mm/yr. The spreading center appears to be located south of the actual basin. The abnormal depth (5500 m instead of 5900 m) and thickness (∼12 km instead of 6 km) of the crust beneath the basin indicate that there was probably an excess supply of magma during its formation. We propose that the basin is a fragment of the former ‘proto-south China Sea’ or possibly the ‘New Guinea Basin’ that has been trapped by the Philippine Sea Plate.

The Philippine Sea: New survey results reveal the structure and the history of the marginal basins

The Hydrographic Department of Japan initiated a long‐term geological and geophysical survey of the Philippine Sea in 1983. We here summarize the bathymetry and magnetic anomalies of this area, focusing on the structure and the evolutionary process of the marginal basins. The Shikoku and Parece Vela Basins developed as one backarc system in the later phase of their formation; the significant difference between the two basins, however, is clearest near the extinct spreading centers. The steep Oki‐Daito Escarpment, located in the northern West Philippine Basin, was mapped for the first time during this survey. The N‐S trending spreading fabric north of the escarpment is in striking contrast to the NW‐SE to E‐W lineament in the south. Our data reveal a more complicated history of the western Philippine Sea than has been proposed in previous studies.

The last spreading episode of the West Philippine Basin revisited

In 1996, bathymetric data and backscattering imagery were gathered along a 20 km‐wide NW‐SE transect across the West Philippine Basin (WPB), between Taiwan and the Palau‐Kyushu Ridge. These data reveal fine‐scaled structures of the fossil spreading axis from which we infer episodes of oblique deformation and diminished magmatic supply resulting from cessation of spreading. An original NE‐SW seafloor fabric is observed in the basin northeast of the Benham volcanic plateau. Since this trend is oblique to the more common E‐W and NW‐SE seafloor fabrics known in the WPB, it could be the result of overlap of two segments of the spreading axis. Several rhomboidal structures exist in the vicinity of the rift valley, suggesting that dextral shear occurred along the central basin rift axis during the last spreading phase. The E‐W spreading segments associated with N‐S nontransform discontinuities that occur between 127°E and 132°E were probably cross‐cut during a final, largely amagmatic, extensional phase to produce a N130° ‐trending deep rift valley.

Enigmatic extinct spreading center in the West Philippine backarc basin unveiled

Research Article| December 01, 1999 Enigmatic extinct spreading center in the West Philippine backarc basin unveiled Kantaro Fujioka; Kantaro Fujioka 1Japan Marine Science and Technology Center, Yokosuka 237-0061, Japan Search for other works by this author on: GSW Google Scholar Kyoko Okino; Kyoko Okino 2Hydrographic Department of Japan, Tsukiji, Tokyo 104-0045, Japan Search for other works by this author on: GSW Google Scholar Toshiya Kanamatsu; Toshiya Kanamatsu 1Japan Marine Science and Technology Center, Yokosuka 237-0061, Japan Search for other works by this author on: GSW Google Scholar Yasuhiko Ohara; Yasuhiko Ohara 2Hydrographic Department of Japan, Tsukiji, Tokyo 104-0045, Japan Search for other works by this author on: GSW Google Scholar Osamu Ishizuka; Osamu Ishizuka 3Geological Survey of Japan, Tsukuba, Ibaraki 305-8567, Japan Search for other works by this author on: GSW Google Scholar Saturu Haraguchi; Saturu Haraguchi 4Ocean Research Institute, University of Tokyo, Nakano, Tokyo 164-8639, Japan Search for other works by this author on: GSW Google Scholar Teruaki Ishii Teruaki Ishii 4Ocean Research Institute, University of Tokyo, Nakano, Tokyo 164-8639, Japan Search for other works by this author on: GSW Google Scholar Geology (1999) 27 (12): 1135–1138. https://doi.org/10.1130/0091-7613(1999)027<1135:EESCIT>2.3.CO;2 Article history first online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Kantaro Fujioka, Kyoko Okino, Toshiya Kanamatsu, Yasuhiko Ohara, Osamu Ishizuka, Saturu Haraguchi, Teruaki Ishii; Enigmatic extinct spreading center in the West Philippine backarc basin unveiled. Geology 1999;; 27 (12): 1135–1138. doi: https://doi.org/10.1130/0091-7613(1999)027<1135:EESCIT>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract The Central Basin fault in the center of the West Philippine Basin was first discovered ∼50 yr ago. It is a 1000-km-long ridge oriented northwest to southeast and is cut by north-south–trending fracture zones. Hypotheses about the origin and development of the Central Basin fault have remained unresolved until recently. Submersible observations and SeaBeam surveys show that the Central Basin fault is a segmented spreading ridge having a morphology similar to that of a slow spreading ridge, with a nontransform offset, a nodal deep, and an inside corner high. The distance from the ridge versus the depth of the sea floor, the obliqueness of sets of small trough and ridge structures, and heat-flow values both of the crestal and off-axis areas of the Central Basin fault suggest that the fault is not a simple spreading center, but rather underwent multiple spreading episodes. The texture and chemistry of basalts obtained from the ridge suggest that the lavas were formed in a backarc basin setting. These data confirm that the Central Basin fault is a slow backarc spreading center that has a more complicated evolutionary history than previously realized. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Three-dimensional shear wave velocity structure in the upper mantle beneath the Philippine Sea region

The three-dimensional shear wave velocity structure in the upper mantle beneath the Philippine Sea was investigated with Rayleigh wave phase velocities in the periods 30–100 s. More than 900 Rayleigh wave phase velocity curves were obtained for this region with good path coverage. The phase velocity data were inverted for the phase velocity distribution maps in the Philippine Sea with 2-D tomographic technique without any a priori regionalization. The resolutions of the tomographic analysis were quite good in almost of the target region. The phase velocity maps were inverted for the 3-D shear wave velocity structure in the upper mantle down to 220 km. In the shallow depths lateral heterogeneities with short wavelength were seen in the shear wave velocity maps. This might be related with complicated surface structures. In the middle depths the shear wave velocity was well correlated to the main tectonic features seen at the surface and well explained by the evolution history of the Philippine Sea. The older western Philippine Sea had higher shear wave velocities than the younger eastern Philippine Sea. In the western Philippine Sea the central basin ridge, which is the youngest in this area, showed the low velocity anomaly. This is supported by the fact that the West Philippine Basin was formed in this area. In the depths 150–200 km the low velocity anomaly was dominant inside the Philippine Sea, which might suggest the existence of the mantle return flows. The thickness of the lithosphere in the south of the West Philippine Basin reached about 100 km, which is much thicker than the results of previous studies for this region.

KR98ー01によって西フィリピン海盆，セントラル-ベイズンフォルトとマリアナ海溝の地形とテクトニクス

KR98ー01によって西フィリピン海盆，セントラル-ベイズンフォルトとマリアナ海溝の地形とテクトニクス

40Ar/ 39Ar and K–Ar geochronological age constraints for the inception and early evolution of the Izu–Bonin – Mariana arc system

K–Ar and 40Ar/39Ar dates are presented for locations in the Izu–Bonin – Mariana (IBM) forearc (Ocean Drilling Program (ODP) sites 786 &amp; 782, Chichijima, Deep Sea Drilling Program (DSDP) sites 458 &amp; 459, Saipan), and Palau on the remnant arc of the Kyushu–Palau Ridge. For a number of these locations, the 40Ar/39Ar plateau and 36Ar/40Ar versus 39Ar/40Ar isochrons give older ages than the K–Ar results. The most important results are: (i) at site 786, initial construction of the proto‐IBM (now forearc) basement occurred at least by ca 47–45 Ma, consistent with the age of the immediately overlying sediments (middle Eocene nannofossil Zone CP13c); the younger pulse of construction dated at ca 35 Ma by K–Ar could not be confirmed by 40Ar/39Ar analysis; (ii) 40Ar/39Ar ages for the initial construction of the Mariana portion of the IBM system are as old as those of the Izu–Bonin portion, for example at site 458, initial construction commenced at least by ca 49 Ma and at ca 47 Ma at Saipan (Sankakayuma Formation); and (iii) a combination of K–Ar and 40Ar/39Ar ages indicate continued boninite magmatism in the Izu–Bonin forearc (and remnant arc at Palau) until ca 35 Ma. Subduction inception including boninite series rocks along most of the exposed length of the IBM system, clearly preceded by some 5 million years the Middle Eocene (ca 43.5 Ma) change in Pacific plate motion. Boninitic series magmatism persisted at locations now exposed in the forearc for ∼ 15 million years after arc inception concurrently with low‐K tholeiitic series eruptions from a subaerial arc system, established at ≥ 40 Ma, on the Kyushu–Palau Ridge. For the Mariana portion of the IBM system, reconstruction of the proto‐arc places this activity adjacent to the concurrent but orthogonally spreading Central Basin Ridge of the West Philippine Basin. It is possible that a combination of subduction of a young North New Guinea Plate beneath newly created back‐arc basin crust may account for some of the features of the Mariana system. It is clear, however, that the understanding of the processes of subduction initiation and early IBM arc development is incomplete.

New Gravity and Magnetic Anomaly Maps in the Taiwan-Luzon Region and Their Preliminary Interpretation

We have compiled new free-air gravity anomaly (FAA) and magnetic anomaly maps, shedding light on the tectonics in the Taiwan-Luzon region. To have a suitable datum level for both the available gravity and magnetic anomaly data, the set of data from an ACT cruise, conducted during May 27 to June 21, 1996, was chosen as a reference. Based on the cross-over error analysis, all the other data were adjusted accordingly. Some satellite-derived, airborne or land data were also added to the compilation to obtain better coverage. Several major new insights into the Taiwan-Luzon region are revealed by the new maps. (1) A prominent NE-SW trending belt of gravity and magnetic anomalies is present in the onshore and offshore areas of southwestern Taiwan. The Peikang High is located on this belt. (2) Located in the offshore region west of Taiwan and to the north of the belt described above, the Taishi Basin, in contrast, occupies a relatively low FAA area. It could be regarded as a flexural basin on account of the loading of a thrust-and-fold belt in western Taiwan. (3) A probable NW-SE trending old transform fault is well imaged off southwestern Taiwan, which separates the lithosphere (plate) of the South China Sea from a trapped piece of the Philippine Sea plate. (4) Located east of the Luzon Are, the Huatung Basin contains several E-W trending magnetic reversals and two N-S trending old fracture zones. The Huatung Basin is separated from the West Philippine Basin by the 123E Fracture Zone . Accordingly, the Gagua Ridge corresponds to a transverse ridge bounding the 123E Fracture Zone. (5) The Luzon Are is abnormally concave toward the Manila Trench and becomes wider toward the south. The internal deformation of the Luzon Are in terms of several NE-SW discontinuities could be related to its collision with eastern Taiwan, (6) Three previously proposed NW-SE trending strike-slip faults in the southern Okinawa Trough, west of 123...

A detailed study of the Gagua Ridge: A fracture zone uplifted during a plate reorganisation in the Mid-Eocene

Recent multibeam bathymetric and geophysical data recorded in the West Philippine Basin, east of Taiwan, reveal new information on the structure and the tectonic origin of the oceanic Gagua Ridge. This linear, 300 km-long, 4 km-high, north-south-trending ridge, is being subducted beneath the Ryukyu Trench along 123 E. This basement high separates two basins of different ages. Its summit is marked by two crests and an axial valley. A map of the basement top shows the region of the ridge to be composed of a set of linear and parallel ridges and troughs. All these elements suggest that the development of the ridge, and its surroundings, has been influenced by strikeslip deformation. Nevertheless, the height of the ridge indicates also an important compressive component in the deformation. Gravity models across the ridge show local compensation with a crustal root, indicating that an overthickening of the crust occurred when it was young and thus more easily deformable. This idea is strengthened with flexural modeling of the lithosphere that bends under the load of the ridge, indeed it indicates that the high probably formed when the underlying lithosphere was young. We interpret the Gagua Ridge as a fracture zone transverse ridge uplifted during a transpressive episode along a north-south -trending fracture zone in the middle Eocene time, if we accept Hilde and Lee’s (1984) model of magnetic lineations. This tectonic event could be contemporaneous with a change of the pole of rotation of the West Philippine Basin which occurred about 43/45 Ma ago.

Some features of the arc‐continent collision zone in the Ryukyu subduction system, Taiwan Junction area

Abstract To the northeast of Taiwan, northwestward subduction of the Philippine Sea plate is occurring beneath the Eurasian plate along the Ryukyu Trench. The Ryukyu Trench, which is well defined along the northeastern part of the Ryukyu arc, cannot be easily defined west of 123° east. This is an area where the Gagua Ridge (whose origin is controversial) enters the trench from the south. On the basis of the marine geophysical survey data the following results have been obtained. The structural elements associated with the Ryukyu subduction system deform and partially disappear west of 123° east. Among other things the Ryukyu Trench terminates close to the western slope of the Gagua Ridge. The Gagua Ridge is the result of tectonic heaping and is likely to be an uplifted sliver of oceanic crust. The interaction between the Ryukyu subduction system and the Taiwan collision zone encompasses a wide region from Taiwan to the longitude 124.5° east. The Gagua Ridge is a boundary between the active deformation zone related to the collision in Taiwan and the West Philippine Basin. It is proposed that there is a tectonic zone that can be traced from the Okinawa Trough on the north to the southern termination of the Gagua Ridge on the south.

The upper mantle beneath the Philippine Sea region from waveform inversions

We present a three‐dimensional S‐velocity model for the upper mantle beneath the Philippine Sea region. It was derived from inversions of 281 broad band vertical‐component seismograms recorded in the area at the Global Seismological Network (GSN) and SKIPPY portable array stations. We have been able to obtain high‐resolution tomographic images spanning the depths down to 200–300 km and locally down to the upper transition zone. High‐velocity subducting slabs and low‐velocity volcanic arc regions are the dominant features of the model. Fast, thin lithosphere of back‐arc basins is underlain by a prominent low‐velocity zone. Low velocities at lithospheric depths are observed beneath the extinct Central Basin Ridge in the West Philippine Basin and close to the Eauripik Ridge that separates the East and West Caroline basins. High upper‐mantle heterogeneity and resulting scattering presents a difficulty and limits the resolution, especially below 200–300 km. Explicit modeling of seismic wave diffraction may be necessary for a significant improvement in resolution.

Crustal Structure of the Northeastern Taiwan Area From Seismic Refraction Data and Its Tectonic Implications

The southwestern part of the Ryukyu arc-trench system to the west of126°E is obviously bent northwards horizontally.The mechanism of thishorizontal bending has been discussed in the literature,but it is still ambiguous.The West Philippine Basin(WPB)is generally believed to be produced by spreading along the Central Basin Ridge(CBR).However,theCBR does not extend to the northwestern corner of the WPB.Previousgeomagnetic and geomorphological studies suggested that the CBR terminates or is displaced at its northwestern end.However,the detailed pictureis still unclear.In this paper,we have examined bathymetry,gravity andseismicity in and around the northwestern corner of the WPB,and discovered a NE-trending right-handed strike-slip fault system which can be usedto interpret the horizontal bending of the southwestern part of the Ryukyuarc-trench system.A plausible model showing how the right-handed faultsystem results in the bending of the Ryukyu arc-trench system is demonstrated.As the southeastern part of the right-handed strike-slip fault system,the Luzon-Okinawa Fault Zone,which is an obvious fault zone between the Luzon and Okinawa islands,is described in terms of bathymetriccontours and profiles along some cruise lines.The Luzon-Okinawa FaultZone is the main cause of the termination of the CBR.

A Strike-Slip Fault System Trending NE in and Around the Northwestern Corner of the West Philippine Basin

The nature of the plate boundary between the Eurasian and PhilippineSea Plates changes from subduction to collision along the northern extension of the Manila Trench,north of 21°N,offshore southwest Taiwan.Todetermine whether the collision-formed deformation front passes throughthe Tainan Basin,a region of petroleum interest,seismic profiles that crossa proposed”deformation front”are interpreted.However,only a majornormal fault,that is a growth fault that forms the northwestern side of atilted horst structure can be positively identified.The nature of this normal faulting is also supported by the modeling of gravity and magnetic data.The observation of only tensional faults in the Tainan Basin suggeststhat the deformation front is located to the southeast,near Kaohsiung oreven further to the south.The economic basement in the Tainan Basin region,defined as pre-Tertiary rocks that underlie the thick Tertiary sequences along the seismiclines we examined,maintains a depth of 3 to 5 km in a tilted horst-like zone,known as the Central Uplift,and deepens in either direction away from it.The thick Tertiary deposits provide excellent potential for source rock andseal.Petroleum exploration in this area should be directed toward generating prospects related to normal faulting if structural closures can be identified.

Origin and motion history of the Philippine Sea Plate

The Philippine Sea Plate is the one major plate whose Tertiary motion is poorly constrained and whose origin is problematical. Its southern boundary is the Sorong Fault system which is part of a major left-lateral fault system at the northern margin of the Australian plate. The southern part of the plate in eastern Indonesia has been neglected in most syntheses but includes some of the oldest rocks within the plate which are separated from remnant arcs of the Daito Ridge province of the northern Philippine Sea by the West Philippine Central Basin. The east Indonesian islands of the Halmahera-Waigeo region contain a good Mesozoic and Tertiary stratigraphic record indicating a long arc history for the southern part of the plate. New palaeomagnetic data from these islands define two sub-areas: an area forming part of the Philippine Sea Plate north of the Sorong Fault, and an area within the Sorong Fault system. The area north of the fault records a long-term clockwise rotation history whereas that within the fault zone records local rotations interpreted as due to deformation at the plate edge. Rocks of Philippine Sea Plate origin within both areas record similar latitudinal shifts. The rotation of the area north of the Sorong Fault is considered to represent the motion of the southern part of the Philippine Sea Plate. The new data indicate large Tertiary clockwise rotations similar to earlier suggestions for other parts of the plate but record a discontinuous and more complex motion history than previously suggested. For the southern part of the plate there was 40° rotation with northward translation between 0 and 25 Ma, no significant rotation between 25 and 40 Ma, and there was 50° rotation with southward translation between 40 and 50 Ma. We show that the new palaeomagnetic data form part of a single set with earlier palaeomagnetic data from elsewhere in the plate. The translation history of the southern part of the plate in eastern Indonesia can be reconciled with northward motions recorded elsewhere and can be used to determine rotation poles for the plate (15°N, 160°E for the interval 5–25 Ma, and 10°N, 150°E for the interval 40–50 Ma). Reconstructions based on these poles predict that at ∼ 45 Ma the Palau-Kyushu Ridge had a WNW-ESE orientation which is very different from that postulated by many models used to explain the widespread boninite volcanism in the Izu-Bonin-Marians forearc at this time. The long arc history of the southern part of the plate and the reconstructions based on the rotation poles calculated from the palaeomagnetic data favour an origin for the West Philippine Basin by spreading in a backarc basin.