Lu-ning Shang, Pan-feng Li, Run-lin Du, Feng-long Bai, Gang Hu, Wen-hao Lü, Xia Li,Xi Mei, Tian-yu Zhang, Hou-zhen Cao, Jing-yi Cong, Xian-yao Shi,*

a Qingdao Institute of Marine Geology, China Geological Survey, Ministry of Natural Resources, Qingdao 266071, China

b Laboratory for Marine Mineral Resources, Pilot National Laboratory for Marine Science and Technology, Qingdao 266000, China

c China Geological Survey, Ministry of Natural Resources, Beijing 100037, China

Keywords:

Kyushu-Palau Ridge

Central Basin Rift

Tecto-morphological features

Gravity and magnetic anomalies

Back-arc spreading

Structural discontinuities

Philippine Sea Plate

Marine scientific survey

A B S T R A C T

The intersection of the Kyushu-Palau Ridge (KPR) and the Central Basin Rift (CBR) of the West Philippine Basin (WPB) is a relic of a trench-trench-rift (TTR) type triple-junction, which preserves some pivotal information on the cessation of the seafloor spreading of the WPB, the emplacement and disintegration of the proto-Izu-Bonin-Mariana (IBM) Arc, and the transition from initial rifting to steadystate spreading of the Parece Vela Basin (PVB). However, the structural characteristics of this triplejunction have not been thoroughly understood. In this paper, using the newly acquired multi-beam bathymetric, gravity, and magnetic data obtained by the Qingdao Institute of Marine Geology, China Geological Survey, the authors depict the topographic, gravity, and magnetic characteristics of the triplejunction and adjacent region. Calculations including the upward continuations and total horizontal derivatives of gravity anomaly are also performed to highlight the major structural features and discontinuities. Based on these works, the morphological and structural features and their formation mechanisms are analyzed. The results show that the last episode amagmatic extension along the CBR led to the formation of a deep rift valley, which extends eastward and incised the KPR. The morphological and structural fabrics of the KPR near and to the south of the triple-junction are consistent with those of the western PVB, manifesting as a series of NNE-SSW- and N-S-trending ridges and troughs, which were produced by the extensional faults associated with the initial rifting of the PVB. The superposition of the above two reasons induced the prominent discontinuity of the KPR in deep and shallow crustal structures between 15°N-15°30′N and 13°30′N-14°N. Combined with previous authors’ results, we propose that the stress produced by the early spreading of the PVB transmitted westward and promoted the final stage amagmatic extension of the CBR. The eastward propagation of the CBR destroyed the KPR, of which the magmatism had decayed or ceased at that time. The destruction mechanism of the KPR associated with the rifting of the PVB varies along strike the KPR. Adjacent to the triple-junction, the KPR was destroyed mainly due to the oblique intersection of the PVB rifting center. Whereas south of the triple-junction, the KPR was destroyed by the E-W-directional extensional faulting on its whole width.

1. Introduction

The Kyushu-Palau Ridge (KPR) is an intra-oceanic ridge which separates the West Philippine Basin (WPB) and the Shikoku-Parece Vela Basin in the Philippine Sea Plate (PHP)(Fig. 1). It was suggested to be a remnant arc left by the backarc opening of the Shikoku-Parece Vela Basin, with its conjugate part lies in the Izu-Bonin-Mariana (IBM) Arc which rims the eastern margin of the Philippine Sea Plate(Hilde TWC and Lee CS, 1984; Lallemand S, 2016; Wu J et al., 2016). The KPR intersects with the fossil spreading center of the WPB, the Central Basin Rift (CBR), near 15°N latitude and formed a “pseudo” triple-junction exhibiting a distinct triangular morphology composed of several bathymetric highs(Fig. 1). According to the evolutionary models proposed by Hilde TWC and Lee CS (1984), Deschamps A et al. (2002a,2002b), Ishizuka O et al. (2011), Lallemand S (2016), and Wu J et al. (2016), this “pseudo” triple-junction may have evolved from a trench-trench-rift (TTR)-type triple-junction whose eastern two arms were corresponding to the trenches formed by the subduction of the Pacific Plate and the western arm was corresponding to the CBR of the WPB.

Fig. 1. a-Topographic features and plate tectonic setting of the Philippine Sea Plate (after Lallemand S, 2016). Active and inactive spreading centers are denoted by solid and dashed red double lines, respectively. Solid red single lines represent main fracture zones. The Kyushu-Palau Ridge is highlighted by the dashed yellow line. b-present-day relative plate motions across the PSP boundaries calculated from MORVEL (after Wu J et al., 2016). AP-Amami Plateau; BH-Bird’s Head minor plate; BR-Benham Rise; CBR-Central Basin Rift; DR-Daito Ridge;GR-Gagua Ridge; Hal-Halmahera; HB-Huatung Basin; LOFZ-Luzon-Okinawa Fracture Zone; MFZ-Mindano Fracture Zone; MS-Molucca Sea minor plate; ODE-Oki-Daito Escarpment; ODR-Oki-Daito Ridge; UR-Urdaneta Rise.

During the emplacement of the KPR, an immense bathymetric high might have formed by the excessive magma poured out through the CBR which may have served as the preferential conduit for magma upwelling. With the opening of the Parece Vela Basin (PVB), the proto- Izu-Bonin-Marianan (IBM) Arc which consisting the present KPR and IBM Arc was disintegrated, with the active IBM Arc drifted eastward, while the KPR became an inactive remnant arc. The extensive stress produced by the spreading of the PVB led to the reactivation of the CBR, and a NW-SE-trending deep rift valley was formed by the NE-SW-directional amagmatic extension (Deschamps A et al., 1999, 2002a, 2002b; Okino K and Fujioka K, 2003). This final stage amagmatic extension might have propagated eastward and altered the structure of the triple-junction.

The evolutionary history of the KPR-CBR triple-junction may have been affected by different geodynamic processes successively or simultaneously, including plate subduction,arc emplacement, arc disintegration, and amagmatic rifting.Therefore this triple-junction is ideal for studying the interactions between multiple tecto-magmatic processes occurring near the plate boundary. However, our understanding on the structure and evolution of the KPR-CBR triple-junction have been limited due to the lack of highquality and full-covered marine geophysical data, leaving many unresolved problems. One of the main questions is how the final stage amagmatic seafloor spreading of the CBR and the initial rifting of the PVB affected the structure of the KPR near the triple-junction.

Based on the newly acquired bathymetric, gravity, and magnetic data obtained by the Qingdao Institute of Marine Geology, China Geological Survey during 2018-2020, this paper studies the seafloor topography and shallow and deep structural characteristics of the KPR-CBR triple-junction. By describing the submarine topographic features and the interpretation and calculation of gravity and magnetic anomalies, the authors depict the deep and shallow structural characteristics at the triple-junction and explore their genetic mechanisms. The authors propose that the last episode amagmatic extension along the CBR and the initial rifting of the PVB have jointly destroyed the KPR, resulting in the structural discontinuity of the KPR in this area.

2. Geological setting

The Philippine Sea Plate is a roughly rhombic plate surrounded by subduction zones (Fig. 1). On its east side, the Pacific Plate subducts beneath the Philippine Sea Plate along the IBM Trench. On the southeast side, the Caroline Plate subducts beneath the Philippine Sea Plate along the Yap Trench and Palau Trench. On the northwest side, the Philippine Sea Plate subducts beneath the Eurasian Plate along the Ryukyu Trench and Nankai Trough. On the southwest side, the Philippine Sea Plate subducts beneath the Eurasian Plate along the Philippine Trench. On the west side,the South China Sea subducts beneath the Philippine Sea Plate along the Manila Trench.

The main part of the Philippine Sea Plate is divided into the WPB on the west and the Shikoku-Parece Vela Basin on the east by the nearly N-S-trending KPR. With a total length of about 2600 km, the KPR extends from the Palau Islands northward to the Nankai Trough offshore Shikoku Island,where it subducts beneath the Eurasian Plate. The KPR is commonly divided into three segments according to the variations of its strike, topography, and crustal structure.Previous results suggest that the significant changes in topography and crustal structure of the KPR from north to south are mainly associated with the different properties of the original crust in which it was emplaced (Yen HY et al., 2015;Nishizawa A et al., 2016). The northern segment of the KPR was emplaced in the Amami-Daito-Oki-Daito Province where thick crust was developed. The middle segment and the northern part of the southern segment were emplaced in the thin oceanic crust of the WPB. The southernmost part of the KPR (south of the Mindano Fracture Zone) was emplaced in the Palau Basin which has complex crustal structure.

The KPR is considered to be a remnant arc formed by the disintergration of the proto-IBM Arc induced by the opening of the Shikoku-Parece Vela Basin promoted by the rollback of the subducted Pacific Slab and the trench retreat. The eastern conjugate part of the KPR is located in the forearc area of the currently active IBM Arc. The proto-IBM Arc (composed of the KPR and the IBM) is believed to have formed at approximately 50 Ma and evolved from an ancient transform fault when the movement direction of the Pacific Plate deflected westward and the convergence angle between the Pacific Plate and the Philippine Sea Plate increased (Seton M et al., 2015; Lallemand S, 2016). The volcanism of the KPR ceased at approximately 25 Ma. Most volcanic rocks collected along the KPR have ages of 25-30 Ma, representing the products of the last-stage magmatic activities of the ridge(Ishizuka O et al., 2011).

As indicated by the distributions of magnetic lineations and the ages of seafloor sediments and rocks obtained through scientific drilling, the main part of the WPB to the west of the KPR (except for the Amami-Daito-Oki-Daito Province to the north of the Oki-Daito Escarpment and the Palau Basin to the south of the Mindanao Fracture Zone) was produced by nearly N-S-directional seafloor spreading during 45-32 Ma.However, the seafloor spreading direction was clockwise rotated from nearly N-S to NNE-SSW at approximately 35 Ma (Hilde TWC and Lee CS, 1984; Wu J et al., 2016). During 32-28 Ma (or later), the WPB underwent the final stage of amagmatic extension along the CBR after the initial rifting of the PVB, leading to the formation of a deep rift valley and the nearly NW-SE-trend seafloor fabric therein (Deschamps A et al., 2002a, 2002b). The tectonic and magmatic activities in the KPR gradually ceased after the establishment of stable seafloor spreading in the Shikoku-Parece Vela Basin. Despite the latest paleomagnetic evidences suggest that the Philippine Sea Plate has clockwise rotated significantly after the extinction of KPR (Yamazaki T et al., 2021), the structural and morphological characteristics of the KPR and the WPB were almost fixed and have remained until today.

3. Geophysical data

3.1. Data acquisition and processing

During 2018-2020, the Qingdao Institute of Marine Geology, China Geological Survey conducted a large-scale scientific investigation at the KPR-CBR triple-junction and its adjacent region using R/V Haiyangdizhi 9. High resolution geophysical data including multibeam bathymetric, gravity,and magnetic data were collected along E-W main survey lines with an interval of 2 km and N-S crosslines with an interval of 10 km within the area of 132.5°N-136°N and 13.2°E-16.9°E. The total length of the survey lines is over 90000 km.

During data acquisition, high-accuracy navigation and positioning with a horizontal error of less than 10 cm and vertical error of less than 15 cm was provided by the CNav3050 satellite-based differential GPS system. The EM302 multibeam echosounder system was employed for bathymetric surveys. The KSS32-M marine gravimeter was used to collect marine gravity data. The SeaSPY2 proton precession magnetometer and the Sentinel Subsea observation instrument were used to collect the magnetic data and observe the diurnal variations of geomagnetic field, respectively.

The QPS Geodetic Package was used to process the multibeam data. The main processing flow includes draft correction and sound velocity correction, and the false bathymetric data points were manually removed. Full coverage bathymetric data with a grid spacing of 100 × 100 m were obtained after processing. Gravity data processing included instrument position correction, drift correction, tidal correction, Eotvos correction, and latitude correction. After these corrections, the free-air gravity anomalies along individual survey lines were obtained. Combined with the rock density and bathymetric data, the Bouguer gravity anomalies were calculated. The crossover errors of intersection points in the survey network were calculated, and the survey adjustment was carried out. As a result, free-air gravity anomaly and Bouguer gravity anomaly data with a grid spacing of 1 km×1 km were obtained. The magnetic data processing included transducer head position correction,geomagnetic reference field correction, ship magnetism correction, and diurnal correction, as well as manual removal of false data points caused by equipment and positioning reasons. After these corrections, the magnetic anomalies along individual survey lines were obtained, and then the magnetic anomaly grid data with a resolution of 1 km × 1 km were obtained after survey adjustment. Finally, the authors obtained the most detailed multibeam, gravity, and magnetic data in this area. These high quality data provide an opportunity to study the seafloor topography, shallow and deep structures,and tectonic evolution of the KPR-CBR triple-junction.

3.2. Calculations

Calculations were performed based on the geophysical data for further analysis of the deep and shallow structures in the study area. Slope gradient was calculated using the bathymetric data to delineate the linear seafloor fabrics.Upward continuation of the free-air gravity anomaly was conducted to suppress shallow anomalies and highlight the medium and deep structures. In order to trace the mediumlarge scale linear structures and the major geological boundaries, the total horizontal derivatives of free-air gravity anomalies, Bouguer gravity anomalies, and 5 km upward continued free-air gravity anomalies were calculated.Combined with the results of seafloor topographic analysis,the major linear structures and boundaries are identified, and the location and scale of geological discontinuities are defined.

4. Results

4.1. Seafloor topography

Various types of seafloor morphological features can be observed near the KPR-CBR triple-junction (Fig. 2). The seafloor fabrics in the PVB, the KPR, the WPB, and the CBR show different strikes and relative heights. The PVB is characterized by alternating ridges and troughs with nearly NS strikes. Seafloor fabrics in the eastern and southwestern parts of the PVB have N-S strikes, while those in the northwestern part have NNE-SSW strikes. Owing to this difference, the linear topographic features in the western part of the basin show a southward divergent pattern. The WPB is dominated by alternating ridges and troughs with nearly E-W strikes, but the strikes of the linear features slightly change from the deep-sea basin area on both sides of the CBR to the rift axis. Approximately 100 km away from the CBR axis, the strikes of the linear features change from ENE-WSW on the outside to nearly E-W on the inside.

Fig. 2. Bathymetric map of the KPR-CBR triple-junction and its surrounding areas. Major submarine topographic features are denoted and labeled. The KPR is divided into three segments (S1-S3) according to the water depth and direction of each segment. Escarpments on the KPR(E1-E3) and those bounding the CBR (NM and SM) are delineated by solid red lines. Topographic highs including the KPR and the edifices on both sides of the eastern CBR are outlined by dotted white lines. Five small depressions are identified in the central segment of KPR (D1-D5)and outlined by dotted red lines. The arcuate lineament feature is identified and delineated by a dotted black line (AL). The black rectangle (G1)represents the topographic gap which almost completely disconnected the KPR. NM-Northern Margin, SM-Southern Margin, AL-Arcuate Lineament.

The CBR is about 50 km wide and bounded by nearly EW-trending ridges on its both sides. The relative height difference between the ridges and the bottom of the rift valley is over 3000 m. The topographic fabrics inside the CBR trend in ESE-WNW direction, oblique to the E-W- trending edges of the rift valley. Two large-scale topographic highs composed of small ridges and seamounts have developed on the south and north sides of the eastern end of the CBR. The ridges and seamounts in the topographic highs have nearly EW strikes, which are consistent with the general seafloor fabrics of the WPB. These two topographic highs are not connected with the KPR, with the northern and southern bathymetric highs are separated from the KPR by the NNESSW-trending linear escarpment E3 and the flat depression with a width of approximately 20-30 km, respectively. A small depression with a water depth of over 5000 m and a maximum bathymetric drop of about 2000 m develops in the easternmost part of the CBR. It was formed by the connection between the western parts of small depressions D1 and D2.

The KPR in the study area generally manifests as a topographic high extending in nearly NS direction. Its morphology varies significantly along strike from north to south and can be roughly divided into northern, middle, and southern segments (S1-S3), with the boundaries located at approximately 14.5°N and 15.5°N latitudes. The northern segment runs in NNE-SSW direction and has uniform width confined by straight edges on the east and west sides. The southern end of this segment is terminated at 15.5°N latitude by an E-W trending depression (D1), which has the maximum bathymetric drop of over 1400 m. The NNE-SSW-trending middle segment has a complex topography. The most distinct feature in this segment is the small triangular depression (D5)with a maximum bathymetric drop of over 2000 m relative to the surrounding bathymetric highs. Four E-W-trending depressions (D1-D4) develops in the western part of the middle segment, cutting this area into fragmented morphologic features.

Three prominent linear escarpments (E1-E3) can be delineated in the northern and middle segments of the KPR.Among them, the E1 forms the eastern edge of the northern segment of the KPR and extends southwestward along the western edge of the small triangular depression (D5). With a total length of over 200 km, this escarpment almost cuts through the KPR on its whole width. Due to the presence of the depression protruding westward from the PVB, the KPR narrows rapidly at approximately 14.5°N, and the middle segment of the KPR terminates here. The southern segment has a nearly N-S strike, with a prominent topographic gap(G1) occurs between 13.5°N and 14°N. G1 is composed of 3-4 pairs of N-S-trending ridge-trough combinations, and the bathymetric highs on its south and north sides are bridged only by a narrow linear ridge with a width of less than 10 km.Overall, the topography features of the eastern part of the KPR are consistent with those of the PVB to the east. In contrast, the topography feature of the western part of the KPR, especially those in the west part of the middle segment are concordant with those of the CBR.

4.2. Gravity and magnetic anomalies and calculation results

4.2.1. Free-air gravity anomaly

The free-air gravity anomalies of the KPR-CBR triplejunction and its surrounding areas are generally correspond to the seafloor topography (Fig. 3). Topographic highs are associated with high anomalies, while topographic lows exhibit low anomalies. The KPR manifests as a positive high gravity anomaly belt, while the PVB and the WPB have longwavelength and low-amplitude anomalies. A distinct exception is the negative anomaly belt to the west of the KPR,where there is no significant topography low can be observed.The southern and northern segments of this belt manifest as continuous negative anomaly zones, while the area near the triple-junction shows two separate negative anomaly circles.The CBR manifests as a negative anomaly belt bounded by the positive high-amplitude gravity anomalies corresponding to the topographic highs on its both sides. Apparent free-air gravity discontinuities, which roughly correspond to the topographic discontinuities, can be delineated along the KPR.The high positive gravity anomaly belt associated with the KPR narrows at 15.5°N, 14.5°N, and 13.5°-14°N and are even entirely severed by transverse low positive or negative anomalies. The escarpment E1 manifests as a distinct gradient zone on the free-air gravity anomaly map, and has leftlaterally offset the positive anomalies of the KPR on the two sides.

Fig. 3. Free-air gravity anomaly of the KPR-CBR triple-junction and surrounding regions. Dotted red lines are the inferred structural boundaries. Topographic elements (E1-3 and G1) are identical with those in Fig. 1.

4.2.2. Bouguer gravity anomaly

On the Bouguer gravity anomaly map (Fig. 4), the WPB is dominated by positive high anomaly with amplitude significantly higher than that of the PVB. The KPR exhibits a significant low anomaly zone limited by steep gradient zones on both sides. The amplitude of the Bouguer gravity anomaly of the CBR is slightly lower than that of the basins on the south and north sides, and shows a decreasing trend from west to east. The Bouguer gravity lows associated with the bathymetric highs on both sides of the eastern end of CBR are connected with the Bouguer gravity low of the KPR. Bouguer gravity anomaly of the western part of the middle segment of the KPR which located on the extension of the CBR is slightly higher than that of the rest part of the KPR. In particular, the D5 between E1 and E2 appears as the center of high Bouguer gravity anomalies. Two minimum centers (MD1 and MD2) of the Bouguer gravity anomaly occur near 14°30 ′N and 13°30 ′N, and they topographically correspond to seafloor volcanic complexes. In the G1 region between 13.5°N and 14°N, the Bouguer gravity low associated with the KPR is significantly narrowed, forming a saddle that separates the two low-amplitude anomaly centers to the south and the north. The negative free-air gravity anomaly belt to the west of the KPR does not show noticeable anomaly on the Bouguer gravity anomaly map. Since the Bouguer gravity anomalies mainly reflect the undulation of Moho, it can be speculated that the negative free-air gravity anomaly in this belt is primarily induced by the deeper buried igneous basement.

Fig. 4. Bouguer gravity anomaly of the KPR-CBR triple-junction and surrounding regions. Dotted red lines are the inferred structural boundaries. Topographic elements (E1-3 and G1) are identical with those in Fig. 1.

4.2.3. Magnetic anomaly

The WPB is dominated by E-W-trending, shortwavelength, and high-amplitude magnetic lineations, which roughly consistent with the distributions of the topographic fabrics (Fig. 5). The PVB is dominated by two arrays of S-N-trending alternating positive and negative magnetic anomalies, but their amplitudes are lower than those in the WPB. The KPR is generally characterized by a highfrequency and high-amplitude magnetic anomaly belt with alternating positive and negative anomalies. The trend of its single anomaly is near E-W, which is consistent with those in the WPB, except that the positions of the extreme values of the positive and negative anomalies are offset along the west side of KPR, and the magnetic anomaly amplitude of KPR is slightly higher than that of the WPB. In general, the distribution pattern of the E-W-trending magnetic lineations seems to have extended eastward from the WPB to the KPR,or even further east between 14°30′N-16°N. Since KPR is dominated by a series of isolated magnetic anomalies, it is difficult to determine whether the overall magnetic anomaly trends on the east side of KPR are consistent with those in the WPB (E-W direction) or with those in the PVB (NNE-SSW direction). Another distinct phenomenon is that positive highamplitude magnetic anomalies have developed in areas where the KPR significantly narrows, such as the east side of E1 between 15°30 ′N and 16°N, the depressions near 14°30 ′N,and G1.

Fig. 5. Map showing the total-field magnetic anomalies of the KPR-CBR triple-junction and its adjacent areas (Topographic features are identical with those in Fig. 1).

4.2.4. Calculation results

The 5 km upward continuation of the free-air gravity anomaly effectively suppress shallow anomalies and highlight middle-deep anomalies (Fig. 6a). According to the calculation result of upward continuation of the free-air gravity anomaly,there are three local maximum centers at 16°30′N, 14°30′N,and 13°30′N on the KPR and they topographically correspond to major seamount complexes. The western part of the middle segment of the KPR shows apparent low-amplitude anomalies, which are integrated into the low-amplitude anomaly zone of the CBR. The topographic highs on both sides of the CBR also show high-amplitude gravity anomalies,whose amplitude, however, is significantly lower than that of the KPR. Escarpment E1 dislocates the two positive highamplitude anomaly areas in the northern and middle segments of the KPR. Low-amplitude and negative anomaly areas with a width more than 100 km exist between the two local maximum centers of positive high-amplitude gravity anomalies at 13°30 ′N and 14°30 ′N. Among them, there is only a nearly N-S-trending low-amplitude anomaly zone with a width of approximately 10 km, which ends at G1 due to the presence of the negative anomaly. The PVB and the WPB show long-wavelength and low-amplitude negative anomalies or positive low anomalies. A distinct high-amplitude negative anomaly zone exists on the west side of the KPR, but its amplitude subdues at the intersection of the CBR and the KPR.

Fig. 6. Calculation results of free-air and Bouguer gravity anomalies. a-5 km upward continuation of free-air gravity anomaly. Topographic elements are identical with those in Fig. 1. Major gravity highs are outlined by solid black lines. b-total horizontal derivative (THD) of the 5-km upward continued free-air gravity anomaly. c-total horizontal derivative (THD) of free-air gravity anomaly. d-total horizontal derivative(THD) of Bouguer gravity anomaly. Maximum belts which may represent major structural boundaries are highlighted by solid red lines in (a),(b), and (c).

The calculation results of the total horizontal derivative of free-air gravity anomaly (Fig. 6c), Bouguer gravity anomaly(Fig. 6d), and the 5 km upward continued free-air gravity anomaly (Fig. 6b) effectively highlight the major structural boundaries. The maxima of the total horizontal derivative represent the major discontinuities such as faults and the boundaries of large features. However, the maximum zones may deviate slightly from the actual positions of the structural boundaries due to the changes in their burial depth. The apparent topographic escarpments (e.g., E1-E13, NM, and SM) are all corresponding to the calculated maximum zones.There are long maximum zones with high-amplitudes in the northern and southern segments of the KPR. The local maximum zone in the northern segment roughly coincides with the edges of the KPR. Several en echelon NEE-SWW-trending local maximum zones develop on the western edge of the KPR. The maximum zones in the southern segment of the KPR trend in nearly N-S, which are consistent with the topographic features. However, the maximum zones in the middle segment of the KPR are short, dominated by NNESSW and E-W directions, and characterized by low amplitude and variable strikes. Among them, the E-W-trending maximum zones are primarily located on the prolongation of the CBR.

5. Discussion

5.1. Structural characteristics of the KPR-CBR triple-junction

The KPR-CBR triple-junction is far away from the continent. After the cessation of tecto-magmatic activity, due to the lack of input of terrigenous clastic materials, the sedimentation rate is slow, and the thickness of sediments accumulated on the igneous basement is thin. Therefore, the present seafloor morphological characteristics of this area mainly reflect the results of the last stage of tecto-magmatism,and also retain the traces of multi-stage tecto-magmatic activities.

Although there is no distinct bathymetric depression, a remarkable negative free-air gravity anomaly can be observed to the west of the KPR. This is mainly due to the presence of a thick volcaniclastic apron on a deeply buried igneous basement. The calculation results of the 5 km upward continuation of free-air gravity anomaly and Bouguer gravity anomaly mainly reflect the medium to deep crustal structures.Because Bouguer gravity anomaly value decreases with the increase of Moho depth and vice versa, the Moho depressions(MD1, MD2, the northern segment of the KPR, and topographic highs on both sides of the eastern CBR) can be outlined by the Bouguer gravity anomaly lows. They are also corresponding to the large-scale seamount groups and the positive regions of the 5 km upward continued free-air gravity anomaly. In contrast, areas showing high Bouguer gravity anomalies (the CBR, the western part of middle KPR, and G1) are corresponding to the major depressions and the negative regions of the 5 km upward continued free-air gravity anomaly. Therefore, in the study area the Moho is roughly mirror symmetrical with the seafloor topography. The prominent linear topographic features, such as escarpments E1-E3, are mainly manifests as gradient zones of free-air gravity anomalies or boundaries separating extremal regions of anomalies. These linear structures have been further verified and highlighted by the calculation results of the total horizontal derivative of both free-air and Bouguer gravity anomalies. Therefore, it can be inferred that these linear structures may represent the deep-rooted faults cutting down to the upper mantle.

As an intra-oceanic remnant volcanic arc emplaced in the oceanic crust of the WPB, the KPR exhibits alternating E-W-trending linear magnetic anomalies which are consistent with those magnetic lineations originated from seafloor spreading in the WPB rather than severely disturb them. Thus it seems that the distribution pattern of the E-W-trending magnetic lineations in the West Philippine Sea has extended to the KPR or further east. However, the linear magnetic anomalies of the KPR are disconnected and offset with those of the WPB,making it difficult to directly correlate them. Furthermore, the magma could have been intruded or extruded along multiple discrete centers during the formation of the KPR. Therefore,the emplacement process of the KPR cannot be determined only by the analysis of magnetic data. However, the phenomenon that the E-W-trending linear magnetic belts extend from the CBR to the east side of the KPR between 14.3°N-16°N imply that the KPR-CBR triple-junction might have been affected by the eastward propagation of the N-S-directional spreading along the CBR.

Based on the analysis and calculation results of the seafloor topography and gravity and magnetic anomalies, the authors compiled a structural map to show the major structural features and discontinuities in and around the KPRCBR triple-junction (Fig. 7). Major structural units associated with the triple-junction include the KPR, the CBR, and the topographic highs on both sides of the eastern CBR. The CBR runs in nearly E-W direction and has an average width of approximately 50 km. The internal structural fabrics in the western CBR trend in NWW-SEE direction, but are counterclockwise rotated into E-W direction in the eastern CBR. The CBR extends eastward and terminates against the fault which corresponding to the escarpment E1. Thus the western part of the KPR in the triple-junction is essentially belonging to the eastern end of the CBR, which exhibits much more fragmented topography consisting several depressions(D1-4). The topographic highs (the Northern and Southern edifices) on both sides of the CBR are almost symmetrically distributed. The internal topography fabrics and magnetic lineations of these topographic highs are consistent with those of the WPB, indicating that they were formed along with the seafloor spreading of the CBR. Their large volume may be associated with the excessive magma drained along the preexisting CBR during the emplacement of the KPR.

Fig. 7. Schematic structural map of the KPR-CBR triple-junction and surrounding region. Bathymetric highs are outlined by dotted black lines. Linear seafloor fabrics are delineated by thin black lines. Thick solid red lines represent major faults those are evidenced by bathymetry,gravity, magnetic data and their calculation results. The Central Basin Fault and its eastern prolongation are colored into yellow. Dark grey areas represent major volcanic complexes outlined by 5 km upward continuation of free-air gravity anomalies. The northern, middle, and southern segments of KPR separated by major discontinuities are colored into green, light blue, and dark blue, respectively. Note that the structural boundary between the southern and middle segment is located near G1 area, slightly different from the topographic boundary of these two segments.

The KPR can be roughly divided into three structural segments (Fig. 7), which differs slightly from the topographic segmentation. The northern segment is separated from the middle segment by the CBR and E1. The middle segment is separated from the southern segment by the G1. Bounded by large faults E1 and E3 on both sides, the northern segment of KPR has straight edges and uniform width. In contrast, the middle and southern segments vary significantly in width and structure because they are incised by linear structures in different directions. The calculation results of gravity data indicate that the E1 may be a deep-rooted fault extends down to the upper mantle. Therefore, the middle and northern segments of the KPR are discontinuous on the crustal scale.To the southeast of E1, the KPR is dominated by NNE-SSW and N-S trending topographic and structural fabrics, which are consistent with those of the western PVB, indicating that their formations were mainly influenced by the rifting of the PVB.The most prominent region represents this influence is the G1 region located between 13°30′N-14°N. Here, the original structure of KPR was completely destroyed, forming the latticed morphology and structure.

5.2. Genetic mechanism of the tecto-morphological features

Through examine the bathymetric data and backscattering images, Deschamps A et al. (1999, 2002a, 2002b) and Okino K and Fujioka K (2003) inferred a last episode NE-SW opening of the WPB from 30 Ma to 26 Ma. They proposed that this event was associated with the onset of the E-W trending opening of the Parese Vela Basin and ceased when normal seafloor spreading established at 26 Ma in PVB.Extensive stresses associated with the opening of the PVB were probably transmitted to the WPB and promoted the rejuvenation of the weak and deformable CBF (Deschamps A et al., 2002a). Since the renewed age data of KPR rocks suggest that the volcanism along the KPR ceased at about 25 Ma after the initial rifting of the PVB (Ishizuka O et al.,2011), this last episode opening of the WPB must have occurred later than the time proposed by Deschamps A et al.(1999, 2002a, 2002b), perhaps at about 25Ma. Occurred during the cessation stage of spreading, this event was characterized by diminished magma supply or amagmatic extension, and produced a deep rift valley along the axial part of the CBR (Deschamps A et al., 1999).

Although the most prominent appearance of the deep valley of the CBR presents in 127° E-132° E (Deschamps A et al., 1999, 2002a, 2002b), the newly acquired data clearly show the existence of a deep valley of the CBR in the triplejunction area, suggesting an intense amagmatic extension in this region. To the east of 132°30′E, the direction of CBR is counterclockwise rotated into E-W, whereas the intra-rift topographic fabrics trend approximate NWW-SEE, which is consistent with those to the west of 132°30 ′E. Because the amagmatic extension of the CBR was occurred synchronous or post of the extinction of arc volcanism, without sufficient magma supply, the KPR located on the eastern prolongation of the CBR was destroyed along with the eastward propagation of the CBR, produced a series of extensional faults and depressions (D1-4). The strikes of these extensional structures are nearly E-W, which have counterclockwise rotated relative to the NEE-SWW structural lines in the rift to its west. This rotation may be related to the sinistral strike-slip along the faults E1 and E3 under the control of the stress field generated by the spreading of the PVB.

To the southeast of the E1, the trends of the topography and structures of the KPR coincide with the rifting-induced linear fabrics in the PVB, indicating that the development of the structural and topographic fabrics are controlled by the rifting of the PVB. Both the strikes of the structural fabrics in KPR and PVB change from NNE-SSW to N-S from north to south. At the transition zone near the triple-junction, the KPR was obliquely incised by the initial rifting center of the PVB,which may have contributed to the formation of the E1. As indicated by the discrete N-S-trending topographic and structural features in the southern segment of the KPR,especially those latticed ridges and troughs in the G1 area, the southern segment of the KPR might has been destroyed on its whole width by the rifting of the PVB.

Based on the above discussion, we proposed that the stress field generated by the initial seafloor spreading of the PVB transmitted to the west and facilitated the last stage amagmatic extension of the CBR. The eastward propagation of this amagmatic extension destroyed the original structure of the KPR, of which the magmatism had decayed or ceased at that time. The destruction mechanism of the KPR associated with the rifting of the PVB varies along strike the KPR. Adjacent to the triple-junction, the KPR was destroyed mainly due to the oblique intersection of the PVB rifting center. Whereas south of the triple-junction, the KPR was destroyed mainly due to the E-W- direction extensional fracturing on its whole width.

6. Conclusions

Through carefully examine the newly acquired multibeam bathymetric data and gravity and magnetic data, the authors depicted the detailed morphological features and gravity and magnetic anomaly patterns of the KPR-CBR triple-junction and adjacent areas in the Philippine Sea. Then the authors analyzed the structural characteristics their genetic mechanisms combined with the calculation results of the gravity data and some previous results. The main conclusions are listed as follow:

(i) As a relic of an ancient TTR triple-junction, the KPRCBR intersection exhibits unique morphological features and gravity and magnetic anomalies. The CBR shows a deep rift valley, which extend eastwards to the KPR. Two bathymetric high developed on both sides of the eastern end of the CBR.Morphological features vary significantly along-strike the KPR. Near and south of the triple-junction, the morphology and structure are complicated by the existence of abundant linear escarpments, depressions, and troughs.

(ii) The CBR extended eastward to the triple-junction,incised into the KPR and destroyed it. The structural fabrics of the KPR near and to the south of the triple-junction are consistent with those of the western PVB, manifesting as a series of NNE-SSW- and N-S-trending ridges and troughs,which were produced by the extensional faults associated with the initial rifting of the PVB. The above two reasons induced the prominent discontinuity of the KPR between 15°N-15°30′N and 13°30′N-14°N.

(iii) The extensive stress field generated by the initial seafloor spreading of the PVB transmitted to the west and facilitated the last stage amagmatic extension of the CBR. The eastward propagation of this extension destroyed the original structure of the KPR, of which the magmatism had decayed or ceased at that time. The destruction mechanism of the KPR associated with the rifting of the PVB varies along strike the KPR. Adjacent to the triple-junction, the KPR was destroyed mainly due to the oblique intersection of the PVB rifting center. Whereas south of the triple-junction, the KPR was destroyed by the E-W-direction extensional fracturing on its whole width.

CRediT authorship contribution statement

Lu-ning Shang and Xian-yao Shi conceived of the presented idea. Lu-ning Shang and Xian-yao Shi wrote the manuscript with support from Feng-long Bai and Wen-chao Lü. Lu-ning Shang, Pan-feng Li, Run-lin Du, Xia Li, and Gang Hu developed the theory and performed the computations. Tian-yu Zhang and Jing-yi Cong verified the analytical methods. Xi Mei and Hou-Zhen Cao helped supervise the project. All authors discussed the results and contributed to the final manuscript.

Declaration of competing interest

The authors declare no conflicts of interest.

Acknowledgment

This study was funded by the projects initiated by the China Geological Survey (DD20191003, DD20190236 and DD20190205).