Origin of Philippine Sea Basins During Subduction Initiation in the Western Pacific

Understanding the age and dynamics of the overriding plates allows an assessment of competing subduction initiation hypotheses. The Izu‐Bonin‐Mariana margin in the Western Pacific is a key example of initiation and hence it is important to constrain the age and origin of the oldest igneous crust of the supra‐subduction Philippine Sea Plate. We present geochronological and geochemical data of igneous rocks from the oldest ocean basins of the Philippine Sea Plate: the West Philippine and Palau Basins. Basalts from these basins have enriched geochemical characteristics similar to the EM‐2‐like mantle component found in OIB‐like basalts associated with the Oki‐Daito mantle plume. Ages of basalts from the northernmost West Philippine Basin (WPB) and the Palau Basin range from 43.5 to 50.5 Ma, which is similar to the oldest samples associated with the Oki‐Daito mantle plume (48–50 Ma). This implies that the plume contributed to magmatism from the onset of basin formation. It also provides support for the proposition that rifting of the Mesozoic arc terrane and subsequent seafloor spreading of the WPB was triggered by the arrival of the Oki‐Daito mantle plume at the base of the lithosphere. The age of these Philippine Sea Basins implies that only the Mesozoic Daito Ridge Group and the Gagua Ridge existed as Philippine Sea Plate crust before subduction initiation. A major fault activity after 37 Ma in the northernmost WPB demonstrates that careful reconstruction of the Eocene Philippine Sea Plate is critical to understanding plate dynamics during subduction initiation in the Western Pacific.


Introduction
The mechanism that initiates subduction to create oceanic island arcs remains one of the important unresolved issues in plate tectonics (e.g., Lallemand & Arcay, 2021;Stern & Gerya, 2018).Understanding the birth of oceanic arcs leads to solving broader geological, volcanological and tectonic problems such as the origin and tectonic significance of supra subduction zone ophiolite sequences (e.g., Dilek & Furnes, 2014;Ishizuka et al., 2014;Stern & Bloomer, 1992;Taylor et al., 2022).The Izu-Bonin-Mariana arc is one of the most studied oceanic arcs in terms of its geologic and magmatic evolution and hence is a suitable target to examine subduction initiation (e.g., Ishizuka et al., 2006Ishizuka et al., , 2011aIshizuka et al., , 2020;;Reagan et al., 2010Reagan et al., , 2019;;Taylor et al., 1994).Understanding initiation in this region requires a robust tectonic reconstruction of the Philippine Sea Plate around ∼52 Ma.This will enable assessment of the competing hypotheses such as spontaneous or induced nucleation (e.g., Stern, 2004).The nature and origin of the overriding plates is especially important because plate density is a key parameter in numerical modeling of initiation (e.g., Leng & Gurnis, 2015).
There is increasing evidence that geological events which formed major structures of the Philippine Sea Plate took place about the time of subduction initiation ∼52 Ma (Ishizuka, Tani, et al., 2011;Reagan et al., 2019).Hence, the pattern and tempo of these geological events, particularly the duration and extent of seafloor spreading in the proto arc and its temporal relationship with spreading in the West Philippine Basin (WPB), are important in developing regional reconstructions.
Recent marine expeditions in the Philippine Sea investigated the origin and age of basins in and around the Daito Ridge (Figure 1a), the ridge being regarded as a Mesozoic remnant arc (e.g., Hickey-Vargas, 2005;Ishizuka, Taylor, et al., 2011;Tani et al., 2012).Potentially, these basins existed or were formed in the overlying plate during subduction initiation.Since gravitational instability between neighboring plates could be a critical factor for subduction initiation, it is important to understand the age and origin of the overriding plate.Recovery and characterization of the igneous crust of these basins provide constraints on the foundations of the arc and help evaluate initiation models.
A recent study of the Kita-Daito Basin, which separates the Amami Plateau and the Daito Ridge, has established that andesitic magmatism was widely distributed across this basin and the surrounding ridges (Figure 1a, Ishizuka et al., 2022).This magmatism is Eocene (46-41 Ma) and was not associated with ongoing subduction but related to the rifting/spreading event forming the Kita-Daito Basin (Ishizuka et al., 2022).The arc-like geochemistry of the volcanics (Northern Philippine Sea volcanics) is interpreted to indicate melting of the lithospheric mantle, which had been previously metasomatized by Mesozoic subduction of a plate with Pacific-MORB isotopic characteristics.On the other hand, basalts of 37.5-33 Ma in the Kita-Daito Basin without an arc signature mark the last stage of magmatism in the Kita-Daito Basin, and could have formed from low-degree decompressional melting of the asthenospheric mantle associated with the final stage of Kita-Daito Basin spreading (Ishizuka et al., 2022).Based on these results, the Kita-Daito Basin formed during WPB spreading but postdated subduction initiation.
The Amami Sankaku Basin (ASB), located in the reararc of the Kyushu-Palau Ridge, lies directly east of the Mesozoic Daito Ridge group (Figure 1a).Accordingly, this basin is thought to be the foundation on which the Izu-Bonin-Mariana arc was built (Arculus et al., 2015) and hence it is critical to know its age.IODP Exp.351 at U1438 successfully recovered the uppermost crust of the ASB.This basaltic material was found to share the geochemical characteristics of the Forearc basalts (FAB), which formed the spreading crust on the overriding Philippine Sea Plate at subduction initiation (Ishizuka, Tani, et al., 2011;Reagan et al., 2010).The basement basalts from the ASB gave well-defined and consistent 40 Ar/ 39 Ar ages of the Middle Eocene (48.7 Ma), which is in the age range of other Izu-Bonin-Mariana FAB (c.52-48 Ma).
Based on these age constraints and the geochemical similarity of the basement basalts to FAB, Ishizuka et al. (2018) proposed a tectonic model for the period of subduction initiation of the Izu-Bonin-Mariana arc.A key feature of this model is, based on the wide distribution of "forearc basalt" magmatism that the area affected by spreading at this time spanned from the forearc to the reararc area of the future Izu-Bonin-Mariana arc.In this scenario, most of the Izu-Bonin-Mariana arc volcanoes formed on post-initiation ocean crust.It is now clear that the Mesozoic terrane (Daito Ridges), thought to have been the main component of the overriding plate, was rifted prior to the onset of spreading at subduction initiation.This means that the overriding plate is not entirely ocean crust as has previously been assumed, and subduction began at least in some part of the plate margin between juxtaposed oceanic crust (Pacific Plate) and remnant arc terranes of Daito Ridges.A tectonic setting wherein a plate composed of buoyant Mesozoic remnant arcs faced an oceanic plate is more favorable for subduction initiation than the situation where both plates consist of oceanic crust (e.g., Stern & Bloomer, 1992).However, this model needs to be tested by detailed reconstruction of the Philippine Sea plate at around 50 Ma.
Despite recent progress in understanding basin formation in the northern Philippine Sea, the origin and history of the spreading of the WPB (Figures 1a-1c) and its adjacent basins, which occupies the rest of the Philippine Sea Plate remains poorly understood.Especially, understanding the temporal and tectonic relationship between the birth of the Izu-Bonin-Mariana arc and the WPB is critical for reconstruction of Philippine Sea plate tectonics, which closely links to subduction initiation of the Pacific Plate.This study reports geochronological and geochemical data from the potentially oldest northern and southernmost sections of the WPB as well as the Palau Basin (PB) (Figure 1c).These data put critical constraints on the timing of the major basinal formation in the Philippine Sea plate and their temporal relationship to subduction initiation and interaction with the Oki-Daito mantle plume (Ishizuka et al., 2013).

West Philippine Basin (WPB)
The origin of the WPB has been a long debate.Hilde and Lee (1984) published magnetic lineation data for this area (Figure 1a) and suggested that the WPB formed by spreading from the CBF Rift (Central Basin Spreading Center).The spreading direction was NE-SW between 60 and 45 Ma at a half rate of 4.4 cm/year.After 45 Ma, the spreading direction changed to a more N-S direction associated with a spreading center reconfiguration, and the rate decreased to 1.8 cm/year.This major phase of spreading is inferred to have finished at c. 35 Ma.In their model the PB is regarded as the oldest part of the WPB.Lewis et al. (1982) proposed that the WPB formed as a back-arc basin opened behind East Mindanao-Samar arc.In contrast, Seno and Maruyama (1984) suggested that this basin was formed behind the Kyushu-Palau Ridge.The idea that WPB is a back-arc basin has been further developed by recent works (e.g., Deschamps & Lallemand, 2002, 2003;Fujioka et al., 1999;Okino & Fujioka, 2003), primarily based on new bathymetric and geomagnetic data from around the CBF rift and the northern WPB.Combined with the notion that subduction initiation along the Izu-Bonin-Mariana arc was contemporaneous or preceded the opening of WPB, Hall et al. (1995), Hall (2002), Deschamps and Lallemand (2002) devised models where the WPB opened between the two subduction zones of east Philippines and the Izu-Bonin-Mariana arc.Deschamps and Lallemand (2002) proposed that rifting started at 55 Ma and spreading ended at 33-30 Ma with the spreading axis parallel to the east Philippine arc, but became inactive when a new spreading ridge propagated from the eastern part of the basin.Spreading of the basin occurred mainly from this axis, and the axis continuously rotated counterclockwise.
Another hypothesis for the origin of these basins is so called "trapped basin" model (e.g., Uyeda & Ben-Avraham, 1972).Le Pichon et al. (1985) proposed that the extinct spreading center of the WPB was a remnant of the North New Guinea-Kula spreading ridge, which was captured at 43 Ma.Jolivet et al. (1989) presented a similar but modified model insisting that the WPB is a remnant of Pacific-North New Guinea spreading ridge captured at 43 Ma by the inception of a new subduction zone (i.e., Izu-Bonin-Mariana arc) along a transform fault.Sasaki et al. (2014) compiled and interpreted magnetic anomaly data from the southern part of the WPB.They recognized magnetic anomalies corresponding to Chron C16r to 21n (c.36 to 46 Ma).Their interpretation indicates that the half spreading rate of the basin was constant throughout this period at c. 4.4 cm/year, and the spreading ceased progressively from the southeast to the northwest along the CBF rift at around 36 Ma.Okino and Fujioka (2003), Deschamps and Lallemand (2002), Deschamps et al. (2002Deschamps et al. ( , 2008) ) presented new data along the CBF rift.They showed that in the western part of the basin, the spreading system was highly disorganized and overlapping spreading centers and ridge jumps occurred toward the oceanic plateaus such as the Urdaneta Plateau.Deschamps et al. (2002) summarized the history of the CBF Rift.The spreading axis gradually rotated counterclockwise, and the spreading direction changed from NNE-SSW to NNW-SSE toward the end of spreading.After the cessation of spreading, sometime between 33/30 and 26 Ma, NE-SW amagmatic extension event occurred, which might be related to a major change in the regional tectonic setting such as E-W rifting in the Parece Vela Basin.This extension formed a deep rift valley that cut across the older spreading fabric.
The WPB contains some oceanic plateaus and numerous small seamounts mainly in its western to northern part (Figure 1a).The Benham Rise, Urdaneta Plateau and Oki-Daito Rise are the three plateaus, which are comprised of basalts with geochemical characteristics of ocean island basalts (OIB) (Hickey-Vargas, 1998a;Ishizuka et al., 2013), and are interpreted to have formed above a mantle plume.This plume, termed the Oki-Daito plume, remained fixed at the spreading center for ∼10 m.y., generating a series of age-progressive oceanic plateaus with an EM-2 mantle signature (Ishizuka et al., 2013).The first impact of the Oki-Daito plume with the lithosphere Geochemistry, Geophysics, Geosystems 10.1029/2023GC011291 ISHIZUKA ET AL. could have triggered the spreading of the WPB by uplifting and heating the overlying Mesozoic arc crust (Ishizuka et al., 2013).It is also proposed that the Oki-Daito plume is currently represented to the south of the Philippine Sea Plate as the Manus plume (e.g., Macpherson & Hall, 2001;Macpherson et al., 1998;Wu et al., 2016).

Palau Basin
The PB is separated from the WPB to the north by the Mindanao Fracture Zone (MFZ), which is composed of 4 main curvilinear strands (Sasaki et al., 2014, Figures 1a and 1c;Taylor & Goodliffe, 2004).The PB floor is generally shallower than the WPB and appears to have thicker pelagic sediment cover (Mrozowski et al., 1982).Hilde and Lee (1984) assigned anomalies 26 and 25 (approximately 59 to 56 Ma) inside the PB.However, Taylor and Goodliffe (2004) pointed out that magnetic anomalies 25 and 26 identified by Hilde and Lee (1984) are incorrect because ridge jump and several fracture zones were not taken into consideration.Deschamps and Lallemand (2003) proposed that PB (southernmost part of WPB in their figure) was formed by a back-arc spreading center parallel to the east Philippine arc.They regarded PB as the oldest part of WPB.This spreading appears to have ceased by 45 Ma when secondary spreading axis that formed NE-SW to NNE-SSW trending abyssal hills in WPB became active.Sasaki et al. (2014) analyzed bathymetry and magnetic anomaly data collected in the PB during the R/V Yokosuka YK10-14 cruise in 2010.They indicated that the PB was formed by E-W seafloor spreading.Based on preliminary age data on basalts from the northernmost PB, they interpreted magnetic anomalies to model a spreading half rate of ∼4.3 cm/year between approximately 35 and 39 Ma, which is slightly slower than that of the WPB.They speculate that the spreading center of the WPB and the PB were originally parallel and in the N-S direction, and the former center then rotated counterclockwise along the MFZ.
Geological information for the southern part of the WPB and PB is very sparse.No rock samples have been recovered from these areas so far.Some dredge sampling was conducted by a Russian ship in 1970s along the Kyushu-Palau Ridge that corresponds to the northeastern margin of PB (Malyarenko & Lelikov, 1995).This dredge sampling recovered shist, gneissoid granite, diorite and hornblende gabbro from 3 sites between 8°56.2′-8°58.1′N.K-Ar ages for these granitic rocks range from 86 to 125 Ma (Malyarenko & Lelikov, 1995), implying the presence of a Cretaceous basement for Kyushu-Palau Ridge.The Palau Islands, the only subaerially exposed section of the Kyushu-Palau Ridge, expose an Eocene to Oligocene volcanic section mainly composed of tholeiitic to calcalkaline rocks (e.g., Hawkins & Ishizuka, 2009).The northern extension of the Kyushu-Palau Ridge is composed of arc volcanics of the Eocene to Oligocene age (Ishizuka, Taylor, et al., 2011).

Samples Studied
Samples studied here were collected by (a) drilling using the Deep-sea Boring Machine System fitted to R/V Hakurei-maru No.2, (b) dredge sampling conducted by R/V Hakurei-maru No.2, R/V Yokosuka, and R/V Mirai, (c) diving survey using manned submersible "Shinkai 6500" equipped with 2 manipulators.Brief descriptions of the sample material including the recovery method are listed in the supporting information (Table S1).Drilling was normally conducted on flat sediment-covered ridge tops and seamounts or on gentle slopes (less than 10°).The depth of drilling penetration was normally 5-10 m.Dredge sampling and diving surveys were conducted at steep escarpments.

West Philippine Basin
Volcanic rocks collected from the WPB during this study were all basalt.Ocean crust of the northern WPB was mainly recovered along the Oki-Daito Escarpment and other major fractures (Figure 1b).Two submersible dives (6K#1547 and 6K#1548) observed and collected samples of the Oki-Daito Escarpment, as well as 3 dredge hauls (MR17-02D1, D2 and OK640AD01: Figure 1b) also recovered some fresh samples from this escarpment.One dredge haul and a drill site (07OK637AD01, OK436B01) along the Minami Okinawa Escarpment (Figure 1b) recovered basalt lava with some fresh glass.Sampling at the southern extension of the Minami Okinawa Escarpment (OK643AD03), and an escarpment west of the Urdaneta Plateau (OK636AD02: Ishizuka et al., 2013) also returned basalt samples from the ocean crust of the WPB.
MR17-02D4 was conducted on a seamount in the WPB just southeast of the Oki-Daito Ridge and west of the Kyushu-Palau Ridge.This seamount has been offset by the NW-SE trending scarp, and the dredge was conducted on this scarp.This dredge aimed to collect igneous rock samples from the fault scarp of this seamount to obtain age constraints on the formation of the NW-SE trending fault system, and the age and origin of magmatism which formed this seamount.Blocks of lapilli stone, and fragments of volcanic sandstone were mainly recovered.Lapilli stone is mainly composed of olivine basalt with subordinate amounts of plagioclase and clinopyroxene phenocrysts.Some clasts have chilled margins.
In the southernmost part of the WPB, a dredge station at YK10-14D4 was conducted between 6,063 m and 5,353 mbsl where an abyssal hill was dissected by a NW-SE trending scarp (Figure 1c).This dredge haul recovered pillow lava blocks consisting of aphyric basalt with trace amounts of olivine and plagioclase phenocrysts and a completely altered glass rind.Submersible dive 6K#1359 was conducted on the abyssal hill in a similar setting to the YK10-14D4 station, and pillow basalt lava blocks with fresh glass rinds were recovered (Figure 1c).

Palau Basin
Sampling of the ocean crust of PB along the MFZ was conducted in 2 regions (Figure 1c).One is at c. 130 o E, where the MFZ forms a single prominent ridge, and the PB floor is relatively deep (generally deeper than 6,000 m) and shows a series of abyssal hills trending N-S to NE-SW (Figure 1c).The 2 dredge hauls made in this region (YK10-14D1 and D2) recovered a small amount of basaltic lava and olivine-rich dolerite.D2 recovered radially jointed pillow lava block (olivine-clinopyroxene basalt) with the remaining fresh glass rind.Another dredge haul (D3) was made in the eastern part of the basin at c. 131°30′E along the MFZ, where the depth of the basin floor is shallower, c. 4,500-4,000 m.A submersible dive 6K#1358 observed the escarpment between c. 5,300 mbsl and 4,100 mbsl where the MFZ transects NE-SW trending abyssal hill of the PB (Figure 1c).The section that this dive observed corresponds to the representative ocean crust section of the eastern part of the PB.
A major fracture zone at 5 o N with WNW-ESE strike extends more than 160 km (Figure 1c).The northern cliff of the fracture zone between 5,200 and 4,800 mbsl was targeted by the dredge YK10-14D7 (Figure 1c).This dredge haul recovered about 40 kg of basalt breccia.Some basalt clasts in the breccia preserve fresh glassy rinds.Most basalts are vesiculated (10%-20% vesicles), and they are olivine basalts, aphyric basalts, and a plagioclaseclinopyroxene basalt.

Bathymetric Survey
New bathymetric data have been obtained in the northern WPB and the MFZ, including the northern PB.Data were collected by multi-narrow-beam echo-sounder systems: SEABEAM 2112 system (L3 Communications ELAC Nautik) for the YK10-14 cruise, and EM122 (Kongsberg Maritime) for the YK13-08 cruise by R/V Yokosuka and SEABEAM 3012 Upgrade Model (L3 Communications ELAC Nautik) on R/V Mirai.The results of the bathymetric and magnetic survey of the YK10-14 cruise by R/V Yokosuka in the PB, MFZ and the southernmost part of the WPB have been reported by Sasaki et al. (2014).
The northernmost WPB is characterized by the WNW-ESE-trending Oki-Daito Escarpment (Figures 1a and 1b), which lies south of Oki-Daito Ridge.The escarpment exceeds 1,000 m relief, with the basin floor north of the escarpment being shallower than to the south.Abyssal hills in the WPB are different north and south of the escarpment, with the hills to the north being dissected by the escarpment (Figure 1b).This makes interpretation of seafloor magnetic anomalies difficult to the north of the escarpment (Deschamps & Lallemand, 2002) without any direct age constraints on the ocean crust.Deschamps and Lallemand (2002) proposed that the Oki-Daito Escarpment is a manifestation of the transition of the WPB spreading from subparallel to the Philippine Trench to behind the Izu-Bonin-Mariana arc margin.
The WPB north of the escarpment is characterized by the occurrence of abundant small knolls and swells (Figure 1b).Basalts with geochemical characteristics similar to OIB have been reported from one of the knolls in the northernmost part of the WPB (Ishizuka et al., 2013).
In the southernmost part of the WPB, the major trend of abyssal hills is NW-SE, and makes high angles with the MFZ; however, the direction is variable near the MFZ (Figure 1c).In the MFZ, NW-SE trending lineament, and ENE-WSW trending ridges subparallel to the MFZ are dominant.
In the PB, the trend of abyssal hills is generally N-S, and clearly different from those in the WPB (Figure 1c).Their trend gradually changes from N-S to NE-SW toward the MFZ from south to north and forms a J-shaped pattern.The MFZ branches out into two or three scarps to the east of 130°E.On the other hand, west of 130 o E, the MFZ converges to an E-W-trending single prominent ridge (Figure 1c).

40 Ar/ 39 Ar Dating
Ages of the fresh volcanic rocks were determined using the 40 Ar/ 39 Ar dating facility at the Geological Survey of Japan/AIST.Details of the procedures are reported in Ishizuka et al. (2009Ishizuka et al. ( , 2018)).5-20 mg of phenocryst-free groundmass, crushed and sieved to 250-500 μm in size, was analyzed using a stepwise heating procedure.The samples were treated in 6N HCl for 30 min at 95°C with stirring to remove any alteration products (clays and carbonates) present in the interstitial spaces.After this treatment, samples were examined under a microscope.Sample irradiation was done either at (a) the JRR4 reactor (irradiation No. JRR4xxxx: Table S1), (b) the Kyoto University Reactor (KUR), (c) the CLICIT facility of the Oregon State University TRIGA reactor (irradiation No. OSUxx-xx).At the KUR (irradiation No. KURxxxx), the neutron irradiation was performed for 10 hr at the hydro-irradiation port under 1 MW operation, where thermal and fast neutron fluxes are 1.6 × 10 13 and 7.8 × 10 12 n/cm 2 s, respectively, or for 2 hr under 5 MW operation, where thermal and fast neutron fluxes are 8.15 × 10 13 and 3.93 × 10 13 n/cm 2 s, respectively.
Sanidine separated from the Fish Canyon Tuff (FC3) was used as flux monitor and assigned an age of 27.5 Ma, which has been determined against the primary standard for our K-Ar laboratory, Sori biotite, the age of which is 91.2 Ma (Uchiumi & Shibata, 1980).
A CO 2 laser (NEWWAVE MIR10-30) was used for sample heating.For the CO 2 laser heating system, a faceted lens was used to obtain a 3.2 mm-diameter beam with homogenous energy distribution to ensure uniform heating of the samples during stepwise heating analysis.Argon isotopes were measured in a peak-jumping mode on a VG Isotech VG3600 noble gas mass spectrometer fitted with a BALZERS electron multiplier for analysis No. U11xxx and U15xxx) and on an IsotopX NGX noble gas mass spectrometer fitted with a Hamamatsu Photonics R4146 secondary electron multiplier for the rest of the analyses.
Correction for interfering isotopes was achieved by analysis of CaF 2 and KFeSiO 4 glasses irradiated with the samples.The blank of the system including the mass spectrometer and the extraction line was 7.5 × 10 14 ml STP for 36 Ar, 2.5 × 10 13 ml STP for 37 Ar, 2.5 × 10 13 ml STP for 38 Ar, 1.0 × 10 12 ml STP for 39 Ar and 2.5 × 10 12 ml STP for 40 Ar with the VG3600 instrument, and 2.9 × 10 14 ml STP for 36 Ar, 1.4 × 10 13 ml STP for 37 Ar, 1.0 × 10 14 ml STP for 38 Ar, 1.2 × 10 14 ml STP for 39 Ar and 1.9 × 10 12 ml STP for 40 Ar with the NGX mass spectrometer.Blank analyses were conducted every 2 or 3 steps.All errors for 40 Ar/ 39 Ar results are reported at one standard deviation.Errors for ages include analytical uncertainties for Ar isotope analysis, correction for interfering isotopes, and J value estimation.An error of 0.5% was assigned to J values as a pooled estimate during the course of this study.Results of Ar isotopic analyses and correction factors for interfering isotopes are presented in the supplementary data (Table S2).
Plateau ages were calculated as weighted means of the ages of plateau-forming steps, where each age was weighted by the inverse of its variance.The age plateaus were determined following the definition by Fleck et al. (1977), but in two cases (Analyses U17105 and U18281) we adopted ages from steps which do not strictly satisfy the definition with reasonings detailed in Results section.Inverse isochrons were calculated using York's least squares fit, which accommodates errors in both ratios and correlations of errors (York, 1969).

Whole Rock Chemistry
About 10-20 g of rock chips were ultrasonically cleaned with distilled water, and then crushed with an iron pestle and pulverized using an agate mortar.Whole rock major elements were analyzed on glass beads prepared by fusing 1:10 mixtures of 0.5 g subsamples and lithium tetraborate.The glass beads were analyzed using a Panalytical Axios XRF spectrometer at the Geological Survey of Japan/AIST.External uncertainty and accuracy are generally <2% (2.s.d), but Na could have as much as ∼7% analytical uncertainty.The data for each element are in agreement with accepted values and uncertainties of international standards (Table S3).
The rare-earth elements (REE), V, Cr, Ni, Rb, Sr, Y, Zr, Nb, Cs, Ba, Hf, Ta, Pb, Th, and U concentrations were analyzed by ICP-MS on a VG Platform instrument and Agilent 7900, both at the Geological Survey of Japan/ AIST.About 100 mg of powder from each sample was dissolved in a HF-HNO 3 mixture (5:1) using screw-top Teflon beakers.After evaporation to dryness, the residues were re-dissolved with 2% HNO 3 prior to analysis.In and Re were used as internal standards, while JB2 with a similar level of dilution to the samples was used as an external standard during ICP-MS measurements.Instrument calibration was performed using 5-6 calibration solutions made from international rock standard materials (including BIR-1, BCR-1, AGV-1, JB1a, BEN).Reproducibility is generally better than ±4% (RSD) for the REE, and better than ±6% (RSD) for other elements except those with very low concentration and Ni (see BHVO2 analysis in Table S3).Detection limits vary from element to element, but for elements with low concentrations, such as REE and Ta, limits typically fall within a range from 0.2 to 2 pg g 1 .
Major element compositions of volcanic glass samples were determined using an electron microprobe analyzer (JXA-iPF200H) at GSJ-lab operated at 15 kV acceleration voltage and 10 nA current.Glass compositions were obtained for each polished section by averaging 20-50 spot analyses with a 5 μm beam.Trace element compositions of glass samples were determined by Laser Ablation-Inductively Coupled Plasma-Mass Spectrometry (LA-ICP-MS) using an Agilent 7900S coupled to an ESI NWR-193 laser ablation system at the Geological Survey of Japan.Each analysis was performed by ablating spots of 40-50 μm in diameter at 8 Hz with an energy density of 8 J/m2 per pulse.Signal integration times were 30 s for a gas background interval, and 30 s for an ablation interval.Calibration of trace element concentrations was performed using BCR-2g glass analyzed every 6 analysis and was monitored by analyses of NIST 610, NIST 612, and BHVO-2g, with a secondary correction made using the intensity of 44 Ca with known CaO content of the glasses determined by electron microprobe.Linear drift correction was applied between each BCR-2g analysis.The relative precision of trace element analyses is typically better than 2.5% (2σ), and accuracy is within 5% of recommended values for the reference materials (see BHVO2 analysis in Table S3).

Radiogenic Isotopic Composition
Isotopic compositions of Sr, Nd, and Pb were determined on 200-500 mg of hand-picked 0.5-1 mm rock chips.The chips were leached in 6M HCl at 140°C for 20-30 min prior to dissolution in HF-HNO 3 , except for the glass samples for which 5-10 min' leaching was applied.Sr, Nd and Pb isotope ratios were measured on a nine-collector VG Sector 54 mass spectrometer.Sr was isolated using Sr resin (Eichrom Industries, Illinois, USA).For Nd isotopic analysis, the REEs were initially separated by cation exchange before isolating Nd on Ln resin (Eichrom Industries, Illinois, USA) columns.Procedural Sr and Nd blanks were considered negligible relative to the amount of sample analyzed.Sr and Nd isotopic compositions were determined as the average of 150 ratios by measuring ion beam intensities in multi-dynamic collection mode.Isotope ratios were normalized to 86 Sr/ 88 Sr = 0.1194 and 146 Nd/ 144 Nd = 0.7219.Measured values for NBS SRM-987 and JNdi-1 (Tanaka et al., 2000) were 87 Sr/ 86 Sr = 0.710278 ± 19 (2 s.d., n = 33) and 143 Nd/ 144 Nd = 0.512104 ± 10 (2 s.d., n = 38) during the measurement period.All 87 Sr/ 86 Sr ratios were normalized to NBS SRM-987 87 Sr/ 86 Sr = 0.710248 (Thirlwall, 1991), and 143 Nd/ 144 Nd ratios were normalized to JNdi-1 = 0.512115 (Tanaka et al., 2000) as measured during the same analytical session.
The Pb isotopic compositions were determined at the Geological Survey of Japan/AIST.The average isotope ratio data from both laboratories was found to be within ∼1 s.d., and were within similar levels of uncertainty of the poly-spike SRM 981 values of Taylor et al. (2015).Consequently, the data presented are not internally adjusted or normalized.Pb separation was achieved using AG1-X8 200-400 mesh anion exchange resin.Procedural Pb blanks were <30 pg, and considered negligible relative to the amount of sample analyzed.Pb isotopic measurements were made in multi-dynamic collection mode using the double spike technique (Southampton-Brest-Lead 207-204 spike SBL74: Ishizuka et al., 2003;Taylor et al., 2015).Natural (unspiked) measurements were made on 60%-70% of collected Pb, giving 208 Pb beam intensities of 2.5-3 × 10 11 A. Fractionation-corrected Pb isotopic compositions and internal errors were obtained by a closed-form linear double-spike deconvolution (Johnson & Beard, 1999)

40 Ar/ 39 Ar Ages
Twelve samples have been dated by the laser-heating 40 Ar/ 39 Ar dating technique (Figure 1, Table 1: Age spectra and raw data are presented as Figure S1 in Supporting Information S1 and Table S2).
Four basalts from the uppermost part of the PB crust exposed along the MFZ gave an age range between 40.4 and 48.0 Ma.One sample (6K#1358R05) does not satisfy the criteria of age plateau in a strict sense, but the highly radiogenic character of the plateau-forming steps as well as total gas age indistinguishable from the plateau age strongly implies that the K-Ar system of this sample has not been significantly disturbed, and the obtained weighted mean age of the plateau steps is regarded as a reliable eruption age.
Basalt from the western scarp (YK10-14D1R01) showed a younger age than those from the eastern scarp.From a single section explored in one submersible dive (6K#1358), a basalt with more incompatible element-enriched characteristics returned a slightly younger age (6K#1358R05) than the more depleted one (6K#1358R02).
Six samples from the northern WPB gave ages around 47 to 50 Ma.Samples from a single dive track (6K#1548R13g and R18g) gave consistent plateau ages, and those from the same dredge haul with the same geochemical characteristics also showed identical ages within uncertainty, which indicate that these ages represent age of uppermost ocean crust of these locations.The western location (6K#1548) gave younger ages than the eastern sites.
Basaltic andesite clasts (MR17-02D4R02 and R03) from a seamount dissected by a major NW-SE fault scarp in the northernmost part of the WPB gave ages of 37.4 and 39.0 Ma, which are identical within 2σ error.These ages reveal the age of volcanism at this seamount, and also imply that the offset by the NW-SE trending fault was made after 37 Ma.

Whole Rock Chemical Compositions
Volcanic rocks from the WPB and the PB are tholeiitic basalts with variable K 2 O and TiO 2 (Figure 2, Tables S3  and S4).Basalt from the Oki-Daito Escarpment has K 2 O between 0.13 and 0.72 wt% and TiO 2 between 0.97 and 3.25 wt%.Other Northern WPB basalts have generally lower K 2 O (0.06-0.36 wt%), while basalts from the southernmost WPB (just north of the MFZ) have very low K 2 O between 0.14 and 0.17 wt% (whole rock) and around 0.02 wt% for glass (Figure 2a).The K 2 O content of the studied basalts is comparable to or significantly higher than those from WPB ocean crust collected by drilling, and also higher than basalts from the Eocene ocean crust in the Philippine Sea, such as FAB from the Izu-Bonin-Mariana forearc and the ASB (Figure 2a: e.g., Reagan et al., 2010;Shervais et al., 2019;Ishizuka, Taylor, et al., 2011, 2018).
Basalts from the PB have similar 0.2 to 0.4 wt% K 2 O to the basalts from the WPB (Figure 2a), but one sample shows much higher K 2 O (0.91 wt%).These basalts also have 0.85-1.5 wt% TiO 2. Glass samples contain 0.15-0.23 wt% K 2 O and 1.1-1.55wt% TiO 2 (Figure 2b), whereas glass samples from the central part of the PB (YK10-14D7) show a similar range of K 2 O of 0.21-0.25 wt%.
MORB-normalized trace element patterns of the basalts from the WPB and PB do not show enrichment in fluidmobile elements and Th, and depletion in high-field-strength elements (HFSE) (Figure 3).These basalts appear to be free from Th and Ba enrichment relative to global mantle melts (Figure 4).They do have variable incompatible  York (1969).Integrated ages were calculated using sum of the total gas released.λβ = 4.962ξ10 10ψ 1, λε = 0.581ξ10 10ψ 1, 40Κ/Κ = 0.01167% (Στειγερ & ϑαγερ 1977).Atmospheric 40Ar/36Ar: 295.5.Fitton et al., 1997) as a measure of plumerelated enrichment (Figure 5).Other basalts from the northern WPB generally show similar trace element characteristics to the Oki-Daito Escarpment, that is, higher Nb/Zr, Zr/Y, LREE/HREE relative to N-MORB and depleted Eocene Philippine Sea ocean crust (Figures 3 and 5), and have positive ΔNb.However, basalts from a deep fracture zone scarp (07OK643AD03) contrast in having highly depleted in incompatible elements relative to other basalts from the area, and show negative ΔNb (Figures 3 and 5).
Basalts from locations (YK10-14D4 and 6K#1359) in the southernmost part of the WPB are all highly depleted in incompatible elements, so are distinct from most of the northern WPB but similar to those from 07OK643AD03 and other Eocene Philippine Sea ocean crust (Figures 3 and 5).
Basalts from the PB generally show geochemical characteristics similar to those of enriched basalts from the northern WPB.They show elevated but generally slightly lower Nb/Zr, La/Sm and LREE/HREE compared to the enriched northern WPB basalts, and have both positive and negative ΔNb, but still have higher ratios than the depleted Eocene Philippine Sea ocean crust (Figures 3 and 5).These basalts are distinct from those from the adjacent southernmost WPB to the north of the MFZ, which are highly depleted.
There is no significant HREE depletion relative to MREE, implying that melting of garnet is not recognized in the basalts from the WPB and PB (Figure 3).

Radiogenic Isotopes
Pb isotope ratios of the basalts from the northern WPB form a trend above NHRL (Hart, 1984), that is, have the characteristics of Indian Ocean MORB (Figure 6).Basalts from the Oki-Daito Escarpment show 206 Pb/ 204 Pb of 18.4-18.8,which is significantly higher than the depleted basalts from the WPB, but comparable to those of the  Eocene ASB basalts and FAB from the Izu-Bonin-Mariana arc.These basalts partially overlap with but generally show slightly lower Δ 208 Pb/ 204 Pb compared to OIBs from the oceanic plateaus in the WPB (Figure 6).
They also have lower 143 Nd/ 144 Nd with higher 206 Pb/ 204 Pb and 87 Sr/ 86 Sr relative to the depleted WPB basalt, and their compositional range extends toward that of the WPB OIB (Figure 6).
Basalts from the PB show 206 Pb/ 204 Pb of 18.15-18.4with one exception of 18.8, that is, generally slightly lower than those from the northern WPB (Figure 6).They show slightly higher 143 Nd/ 144 Nd and lower 87 Sr/ 86 Sr relative to those from the northern WPB, and are isotopically intermediate between the enriched northern WPB and highly depleted WPB basalts (Figure 6).Nd and Pb isotopes also correlate with some trace element parameters, for example, 206 Pb/ 204 Pb and Δ8/4 are negatively and positively correlated with ΔNb and LREE/HREE, respectively (Figure 7).
Basaltic andesites from the northern WPB seamount have higher 206 Pb/ 204 Pb, 87 Sr/ 86 Sr and lower 143 Nd/ 144 Nd compared to depleted WPB basalts.These basaltic andesites also show enrichment of fluid-mobile elements such as Ba and Pb (Figures 3 and 4) and have higher Sr and Pb isotope ratios and fluid-mobile element/LREE ratios  (2018).Other data sources as Figure 6.
relative to depleted Philippine Sea MORB (Figure 7).High Th/LREE relative to the depleted WPB basalts was accompanied by lower 143 Nd/ 144 Nd.

Spatial Geochemical Variation
Some geochemical variation observed in the basalts from the WPB appears to correlate with location.In the northwestern part of the WPB, a mantle plume (Oki-Daito plume: Ishizuka et al., 2013) is predicted to have been fixed at a specific location relative to the WPB spreading center (CBF Rift), and produced a time-progressive chain of oceanic plateaus.For the sampling stations in the WPB, the distance from the plume at the time of its formation can be estimated assuming the location of the plume based on bathymetry.
Source enrichment caused by the effect of the Oki-Daito plume varies as a function of distance from the plume.
Trace element ratios such as Nb/Zr, ΔNb, LREE/HREE decrease with increasing distance from the plume (Figure 8).Isotopic compositions also show spatial variation.The minimum value of 143 Nd/ 144 Nd increases and the maximum value of 206 Pb/ 204 Pb decreases with distance from the plume in the WPB (Figure 8).

Age and Mode of Formation of the Oldest Basins in the Philippine Sea
Age of basalts from the northernmost WPB ranged from 47 to 50.4 Ma.The age range overlaps with that of the oldest samples, which were interpreted to be associated with the Oki-Daito plume (48-50 Ma: Ishizuka et al., 2013).Ages of basalts from the PB (40.4-48Ma) also imply that the basin is not significantly older than 50 Ma.These new results strongly imply that a major part of the ocean basins (with potentially older Huatung Basin: Deschamps et al., 2000) constituting the western half of the Philippine Sea Plate only formed after c. 51 Ma.A recent survey of the intervening basins within the Daito Ridge Group revealed that their rifting initiation commenced mainly after 45 Ma (Ishizuka et al., 2022).
These lines of evidence indicate that before 51 Ma, only a limited amount of ocean crust existed in the Philippine Sea Plate.Among the currently preserved terranes in the Philippine Sea Plate which convincingly existed prior to the Eocene, that is, before subduction initiation, are the Mesozoic remnant arcs of the Daito Ridge Group (e.g., Hickey-Vargas, 2005;Ishizuka, Taylor, et al., 2011, 2022) and Gagua Ridge (Deschamps et al., 1998;Zhang et al., 2022).This raises the possibility that at subduction initiation, the overriding Philippine Sea Plate was mostly composed of non-oceanic Mesozoic arc crust (Figure 9).This strongly implies that the subduction initiation which led to the Izu-Bonin-Mariana arc occurred between the ocean crust of the Pacific Plate and a plate mainly composed of buoyant arc crust.Based on numerical modeling, this arc-ocean juxtaposition is more favorable for subduction to initiate than where both plates are oceanic (e.g., Leng & Gurnis, 2015).The new data in this study further supports the model that the Izu-Bonin-Mariana arc is an "oceanic" island arc that was established on ocean crust, but its oceanic basement formed only after subduction initiation by rifting and seafloor spreading of the Mesozoic arc terrane (Ishizuka et al., 2018).
Considering the length of the subduction zone along the Izu-Bonin-Mariana arc margin (over 2,500 km), the existing overriding plate appears to be far too small.A counterpart of the Mesozoic Daito Ridge Group is expected to have existed to the south or west of the southern WPB and/or PB.This has potentially been subducted or accreted to the landward slope of the Philippine Trench, that is, it no longer exists in the Philippine Sea Plate.The eastern part of the Philippine archipelago is composed of ophiolitic terranes of mainly Cretaceous age (e.g., Deschamps & Lallemand, 2002), which may be remnants of the counterpart terrane.

Tracking the Effect of Plume-Induced Enrichment in the Sub-Philippine Sea Mantle
OIB-like basalts with EM-2 (enriched mantle 2)-like enrichment are found on the Urdaneta Plateau and Oki-Daito Rise and on seamounts across the northernmost WPB and Oki-Daito Ridge (Figure 9, Hickey-Vargas, 1998a;Ishizuka et al., 2013) Geochemistry, Geophysics, Geosystems 10.1029/2023GC011291 Basalts from the northern WPB are generally characterized by higher K 2 O, TiO 2 , Nb/Zr, ΔNb, LREE/HREE at similar MgO relative to the depleted MORB-type basalts from the other parts of the WPB.These basalts show no Th enrichment relative to MORB (Figure 4), and appear to be free from the contribution of slab-derived components such as enrichment in fluid-mobile elements and Th or crustal material.These basalts show intermediate trace element ratios between the depleted WPB MORB-type basalts and OIB-like basalts associated with the Oki-Daito plume (Figures 3-5).These enriched basalts can be explained by melting an Indian Ocean MORB-type source with the contribution of EM-2-like plume component.Isotopic characteristics of these basalts are also  intermediate between the depleted MORB-type basalts and OIBs, that is, lower 206 Pb/ 204 Pb of 18.4-18.6,higher 143 Nd/ 144 Nd of 0.5129-0.5130,but similar Δ 208 Pb/ 204 Pb of 30-60 to the OIB-like basalts (Figure 6).These isotopic characteristics can also be explained by the contribution of the Oki-Daito plume component to an Indian Ocean MORB-type source.Basalts from two locations in the northern WPB (OK436B01 and OK643AD03) and those from the southernmost WPB share depleted characteristics with those of Indian Ocean MORB-type basalts, that is, with little plume influence.
As Figure 8 shows, source enrichment of the WPB varies as a function of distance from the Oki-Daito plume.This enrichment is strongest near the plume location at a particular time and weakens with increasing distance, and this is applicable across the WPB on both sides of the spreading center.
WPB spreading has a contrasting fabric between its western and eastern parts (e.g., Deschamps et al., 2008).The western part shows complicated fabrics including several propagating rifts, which could relate to a mantle thermal anomaly and associated excess magmatism.However, the eastern part is characterized by a relatively constant trend of abyssal hills, indicative of stable spreading with no excess magmatism, matching the diminishing effect of the plume toward the east.
The PB shares the geochemical characteristics with the northern WPB.Accordingly, the PB basalts also have an influence from the Oki-Daito plume (Figures 5-7), even though the contribution of the plume in the PB is smaller than that in the northern WPB.The fact that basalts forming basin crust of both PB and WPB have influence of the Oki-Daito plume means that the activity of the plume had commenced at 50-51 Ma, and seafloor spreading to form these basins occurred along with the plume activity.This also has important implications for the origin of the PB.Based on the spatial variation of the plume component in the WPB basaltic crust, the plume appears to have spread along the spreading axis of the WPB.Even though the location of the PB spreading center is yet to be determined, if both basins initially formed from the single spreading center on which the plume was located, as proposed in the WPB (Figure 9, Ishizuka et al., 2013), the PB could have an effect on the plume at its formation before 40 Ma.This situation could explain why the influence of the Oki-Daito plume is spread across both basins.
Clear surface expression of the plume activity after the spreading of the WPB ceased has not been recognized even after collecting new data in this study.It is not clear whether this means that the plume had ceased to exist or melt production of the plume or ascent of melt ceased due to the disappearance of the decompression/extension environment.This makes it difficult to determine the link between the Oki-Daito plume and the Manus plume (e.g., Macpherson & Hall, 2001;Macpherson et al., 1998).

Origin of MFZ and Palau Basin
MFZ separates the PB and the southernmost part of the WPB.In the southernmost part of the WPB, the major trend of abyssal hills is NW-SE; however, the direction is variable near the MFZ.Based on our new bathymetry data, Figure 10 shows an interpretation of seafloor fabric, which has the following implications for the formation of the MFZ and PB: 1. J-type morphology of abyssal hills (e.g., Croon et al., 2010) along the MFZ in the northernmost PB and southernmost WPB implies that the fracture zone was active while both the WPB and PB were spreading (Figures 9 and 10).The strike of the abyssal hill of both PB and WPB appears to be similar in the area west of 130°30′E, that is, N-S to NE-SW, even though the trend in the southernmost WPB is highly variable (Figures 1c and 10a).This might indicate that both basins were spreading in the same direction, possibly from the same spreading center.In this case, the MFZ could have initially been a segment boundary of the spreading center, that is, a transform fault.2. At some point, the WPB and its spreading axis started rotating counterclockwise, but the portion which will become the PB did not follow this rotation (Figures 9 and 10).The MFZ accommodated the relative rotation between the two basins.Based on magnetic lineation identification, Sasaki et al. (2014) estimated left-stepping offsets of about 200 km.Both transpressional and transtentional stress regimes emerged depending on the location along the fracture zone (Figure 10b).3. West of 130°E, stress regime along the transform fault was transpressional, and a transpressional ridge formed (Figure 10b: e.g., Croon et al., 2010;Pockalny, 1997).East of 130°E, however, the fracture zone was transtentional, and locally a basin formed because the plates facing each other along the transform fault were moving in opposite directions.
4. As the rotation of the WPB progressed, the strike of the major transform in the MFZ also changed with time, from E-W to NE-SW (Figure 10).The curvilinear fractures (ridges) with varying strikes from E-W to NE-SW might have recorded changes in the transform fault as WPB rotated counterclockwise.NE-SW trending ridges (transform faults) are expected to be younger than the E-W trending ridges (Figure 10).
As a summary, the WPB and PB appear to have formed contemporaneously, possibly from the same spreading center (Figure 9).The onset of spreading might have been triggered by the arrival of a plume.Then, at some point, the segment forming the WPB rotated counterclockwise, and the MFZ formed as the WPB rotated against the PB (Figure 9).

Post Subduction-Initiation Tectonic Activity
Age of the volcano dissected by NW-SE trending escarpment in the northern WPB indicates that tectonic activity which formed NW-SE trending faults occurred after c. 39 Ma in the Eocene to Oligocene period.This implies that the major NW-SE trending lineaments and associated troughs formed on and around the Oki-Daito Ridge were contemporaneous (Figure 9).Combined with the period of formation of the Kita-Daito Basin between 45 and 33.5 Ma (Ishizuka et al., 2022), and the spreading and rotation of the WPB, significant tectonic activity and structural modification of the Philippine Sea Plate must have occurred after subduction initiation of the Izu-Bonin-Mariana arc.Temporal overlap between the formation of the Kita-Daito Basin and the counterclockwise rotation of the WPB could have caused shear, uplift and subsidence between the two terranes, that is, the boundary region between the Daito Ridge Group and the WPB, and formation of NW-SE trending scarp and ridges along the Oki-Daito Ridge.This hypothesis needs to be tested by sampling and structural survey in the southern half of the Oki-Daito Ridge and the northernmost part of the WPB.
The late Eocene-Oligocene tectonic activity in the Philippine Sea plate, which was not assumed to have occurred in the previous studies, requires consideration when reconstructing the overriding plate at subduction initiation.The overriding Philippine Sea plate mostly consists of an arc terrane, which is more buoyant than the previously assumed ocean crust.This arc terrane was potentially weakened, thinned and became more buoyant by heating due to the impact of the Oki-Daito plume.Unlike the subduction initiation in the late Cretaceous Caribbean margin (Whattam & Stern, 2015), there seems to be no geochemical evidence of plume components in the magmatism following subduction initiation, such as FAB, boninites and subsequent high-Mg andesites.They are characterized by a highly depleted mantle source (e.g., Hickey-Vargas et al., 2018;Ishizuka et al., 2020;Shervais et al., 2021).However, still upwelling of hot asthenosphere associated with the plume might contribute to trigger subduction initiation by changing mantle convection and causing weaking, rupturing and eventually sinking of the lithosphere (Whattam & Stern, 2015).
These considerations all affect density estimates of the overriding plate, which are critical in calculating the probability of subduction initiation between the two plates (e.g., Leng & Gurnis, 2015).Also, in some cases, interpreting paleomagnetic data from the Eocene-Oligocene terrane in terms of the tectonic reconstruction of the Philippine Sea plate requires a consideration of the tectonic movement inside the Philippine Sea plate.
Particularly Daito Ridge Group area, where basin formation by rifting occurred between c. 46 Ma and 33 Ma (Ishizuka et al., 2022) including the neighboring Kyushu-Palau Ridge, was clearly affected by tectonic movement inside the Philippine Sea.Relative rotation between the WPB and the PB and axis rotation of the WPB also affected older parts of the basin independent of the Philippine Sea plate movement.Paleomagnetic data of the samples taken from these areas are expected to record the movement of the Philippine Sea plate as well as local tectonics.

Conclusion
This investigation of the oldest Philippine Sea Plate basins was conducted to elucidate and reconstruct the tectonics before and after subduction initiation in the Western Pacific.The key conclusions are 1.Basalts from the northernmost WPB are 47-50.4Ma, which overlaps with the oldest samples associated with Oki-Daito plume activity (48-50 Ma: Ishizuka et al., 2013).Ages of basalts from the PB ocean crust (40.4-48Ma) also imply that they are not significantly older than 50 Ma.These new results indicate that the oldest major Philippine Sea Plate basins formed after c. 51 Ma. 2. This provides support for a model where the Izu-Bonin-Mariana arc was established on ocean crust which formed during subduction initiation as opposed to ocean crust that existed prior to initiation (Ishizuka et al., 2018).This also confirms that the Philippine Sea Plate was the overriding plate during initiation.3. Basaltic ocean crust of both the PB and WPB has an influence from the Oki-Daito mantle plume.This implies that the plume-related source affected the spreading center of both basins.This could be explained if the spreading centers for the 2 basins were initially contiguous, and this center was influenced by outflow from the Oki-Daito plume.Potentially, the plume outflow might have been channeled beneath the spreading axis (Ishizuka et al., 2013).4. Bathymetric data reveals that the MFZ was active while both the WPB and PB were spreading.Subsequently, the WPB started rotating counterclockwise, but the portion which will become the PB did not rotate as much as the WPB did.Hence, the MFZ appears to have accommodated the relative rotation between the two basins.5. Enrichment of the WPB basalts from the Oki-Daito plume varies as a function of distance from the plume.
When compared at the similar age of formation, enriched trace element (La/Yb, ΔNb) and isotopic signatures are most significant in the vicinity of the oceanic plateaux, that is, the plume location at each point in time, and diminish as the distance from the plume increases.This indicates that the degree of source enrichment observed in the WPB basalts is spatially controlled rather than temporally variable, with plume activity persisting from the onset of the basin formation until around 35 Ma.This spatially controlled plume effect can be explained by lateral transfer of plume-derived asthenosphere beneath the CBF Rift spreading center (Ishizuka et al., 2011b(Ishizuka et al., , 2013)).6.Late Eocene tectonic activity formed NW-SE trending lineaments and basins on the northern margin of the WPB and adjacent Oki-Daito Ridge.This activity may result from the accommodation of WPB rotation along the MFZ.

Figure 1 .
Figure 1.(a) Overview of the bathymetry of the Philippine Sea Plate and location of the studied area, (b) Detailed bathymetric map of the northern West Philippine Basin.Locations of submarine sampling stations are shown with the symbols used in the geochemical plots.Age data (shown in Ma) is based on results of this study and a previous study (ages shown in parenthesis are from Ishizuka et al., 2013).(c) Bathymetric map of the Mindanao Fracture Zone and the Palau Basin.The assumed age progressive track line of the Oki-Daito plume (Ishizuka et al., 2013) is shown in a pink dashed line.Major seafloor fabrics are shown in panels (b) and (c).Age data is based on the results of this study.

For 5
glass samples, Pb isotopic compositions were determined at the School of Ocean and Earth Science, University of Southampton UK.Pb isotope ratios were measured by a Thermo Neptune MC-ICP-MS in the School of Ocean and Earth Science, University of Southampton UK, using a double spike run of each sample to correct for instrumental mass fractionation.The 207 Pb-204 Pb SBL74 spike was added such that 204 Pb sample/ 204 Pb spike was 0.09 ± 0.03.Procedural blanks ranged between 30 and 105 pg Pb.NBS SRM 981 values achieved during the measurement period were 206 Pb/ 204 Pb = 16.9404 ± 24, 207 Pb/ 204 Pb = 15.4969 ± 26, 208 Pb/ 204 Pb = 36.7169± 66 (2 s.d., n = 44).

Figure 4 .
Figure 4. (a) Th/Yb-Ta/Yb and (b) Ba/Yb-Ta/Yb plots to identify Th and Ba enrichment in the studied rocks relative to mantle-derived melts (MORB and ocean island basalts).The line representing the array of global MORB-OIB composition is from Pearce et al. (2005).Data sources are as Figure 2.

Figure 5 .
Figure 5. Incompatible trace element ratios for the studied igneous rocks.Variation of (a) Nb/Zr, (b) La/Sm, (c) La/Yb with MgO wt.%,(d) Dy/Yb with Nb/Yb, and (e) Zr/Y-Nb/Y relationship.Two solid lines in panel (e): reference lines mark the limits of Icelandic basalt data by Fitton et al. (1997), effectively showing the ocean island basalts field.Data sources are as Figure 2.

Figure 8 .
Figure 8. Variation of trace element ratios and isotopic compositions of basalts from the West Philippine Basin with distance from the assumed location of the Oki-Daito plume.The distance from the sampling location to the assumed age progressive track line of the Oki-Daito plume shown in Figure1ais regarded as the distance from the plume to each sampling location at the time of formation of basalts.The source of uncertainty of the distance could be the rearrangement of the spreading axis due to the axis rotation and change in spreading direction.But it is estimated to be unlikely that it affects the order of the distance for each location; thus, it does not affect the general trend we see in this figure.Data source: Oki-Daito plume OIB:Ishizuka et al. (2013).

Figure 9 .
Figure 9. Schematic tectonic history of the Philippine Sea Plate including the West Philippine Basin (WPB) and the Palau Basin (PB) (shown as PB in this figure).(1) Before 48 Ma: Arrival of Oki-Daito plume caused basaltic volcanism with geochemical characteristics of ocean island basalts, which formed small volcanic edifices on the Mesozoic terrane.Spreading of the WPB might have begun triggered by upwelling of this plume.The WPB might have formed in the rear of the paleo Philippine arc.(2)48 Ma: Subduction initiation to form the Izu-Bonin-Mariana arc has occurred at 52 Ma, while the WPB and the PB continued to spread in the Mesozoic arc terrane, possibly from the continuous spreading center.Seafloor spreading took place along the Pacific plate margin associated with subduction initiation and formed ocean crust.Volcanism associated with the Oki-Daito plume continued, and focused on the spreading center of the WPB where an oceanic plateau formed, such as the Oki-Daito Rise.(3)After 46 Ma, rifting initiated in the Mesozoic terrane, and the Kita-Daito Basin started to develop.Rifting caused melting of the lithospheric mantle due to decompression, and volcanic activity resulting in the Northern Philippine Sea volcanics (Ishizuka et al., 2022) occurred in the Mesozoic terrane as well as the basin floor of the Kita-Daito Basin.(4)By 40 Ma, counterclockwise rotation of the WPB occurred.As the WPB rotated, possible transform faults developed as Mindanao Fracture Zone, and the spreading center became discontinuous between the WPB and the PB.Possible interaction between the spreading and rotating WPB and the Daito Ridge Group terrane might have caused activity of major faulting between the two terranes and potential uplift or obduction of the WPB crust against the Oki-Daito Ridge.

Figure 10 .
Figure 10.Schematic model of formation of the Mindanao Fracture Zone (MFZ).(a) Both basins were spreading in the same direction, and the MFZ was the segment boundary of the spreading center, that is, transform fault.(b) The spreading axis of the West Philippine Basin (WPB) started rotating counterclockwise.The former transform fault evolved to the MFZ.This fracture zone accommodated the relative rotation between the WPB and the Palau Basin.Both compressional and extensional stress regimes emerged depending on the location along the fracture zone.