Topographic and geomorphological features and tectogenesis of the southern section of the Kyushu-Palau Ridge (KPR) and its adjacent areas

Xu-wen Qin, Wei-dong Luo, Pan-feng Li, Hong-jun Chen, Xiao Xiao, Gang Hu, Yu-fang Tan,Run-lin Du, Mei-jing Sun, Jing-yi Cong, Xiao-san Hu, Kai Lu, Li-xing Wang,Huo-dai Zhang, Hao-yi Zhou

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

b Key Laboratory of Marine Mineral Resources, Ministry of Natural Resources, Guangzhou Marine Geological Survey, Guangzhou 510075, China

c Key Laboratory of Gas Hydrate, Ministry of Natural and Resources, Qingdao Institute of Marine Geology, Qingdao 266071, China

d Laboratory for Marine Mineral Resources, Qingdao National Laboratory for Marine Science and Technology, Qingdao 266071, China

Keywords:

Topographic and geomorphological features

Sedimentary features

Tectogenesis

Kyushu-Palau Ridge

Western Pacific Ocean

Marine scientific survey

A B S T R A C T

The Philippine Sea is the largest marginal sea in the Western Pacific Ocean and is divided into two parts by the Kyushu-Palau Ridge (KPR). The western part is the West Philippine Basin, and the eastern part consists of the Shikoku and Parece Vela basins. Based on surveyed data of massive high-resolution multibeam bathymetric data and sub-bottom profiles data collected from the southern section of the KPR from 2018 to 2021, this paper analyzes the topographic and geomorphological features, shallow sedimentary features, and tectonic genesis of the southern section of the KPR, obtaining the following conclusions. The southern section of the KPR has complex and rugged topography, with positive and negative topography alternatingly distributed and a maximum height difference of 4086 m. The slope of seamounts in this section generally exceeds 10° and is up to a maximum of 59°. All these contribute noticeably discontinuous topography. There are primarily nine geomorphological types in the southern section of the KPR, including seamounts, ridges, and intermontane valleys, etc. Among them, seven independent seamount groups are divided by five large troughs, forming an overall geomorphological pattern of seven abyssal seamount groups and five troughs. This reflects the geomorphological features of a deep oceanic ridge. Intramontane basins and intermontane valleys in the southern section of the KPR are covered by evenly thick sediments. In contrast, sediments in ridges and seamounts in this section are thin or even missing, with slumps developing locally. Therefore, the sediments are discontinuous and unevenly developed. The KPR formed under the control of tectonism such as volcanic activities and plate movements. In addition, exogenic forces such as underflow scouring and sedimentation also play a certain role in shaping seafloor landforms in the KPR.

1. Introduction

The Philippine Sea has attracted the attention of many researchers worldwide due to its unique tectonic and geomorphological features. Andrews JE et al. (1980)identified linear abyssal seamounts parallel and subparallel to the Central Basin Fault zone according to the sidescan sonar data obtained across the West Philippine Basin, opening up the study on the tectonic geomorphology of this area. Lin MH et al. (1998a, 1999) explored the deep trenches around the Philippine Sea and discussed the geomorphological features of the Ryukyu Trench and the Central Fracture Zone in the West Philippine Sea in detail. Li CZ et al. (2000) described the relief characteristics of the Philippine Sea and considered the relief of the deep seafloor as the reflection of tectonic movements. Tang Y et al. (2011) analyzed the topographic and geomorphological features of the Philippine Sea using gravity and magnetic data and concluded that the KPR is discontinuous in terms of natural topography. Based on the tectonic evolution study of the Philippine Sea Plate, Fang YX et al. (2011) concluded that the KPR is a ridge on an oceanic plate rather than a natural component of Japanese continental margins. Song WY et al. (2021) classified the geomorphology in typical areas in the south-central part of the Philippine Sea based on the Digital Elevation Model (DEM) data and proposed quantitative classification criteria of six fourth-order geomorphic units, namely seamounts, sea knolls, submarine rifts, intermontane valleys, intermontane depressions, and intermontane basins. Zhang J et al. (2012) studied the crustal structure of the KPR and its formation and evolution and summarized the differences in crustal thickness in the KPR.

Most previous studies focusing on the macroscopic topographic and geomorphological features of the Philippine Sea only cover a part of the deep crustal structure and tectonic units or local special landforms in the KPR (Li YX et al.,2006; Zhang B et al., 2014). There are few detailed studies on large-scale topographic, geomorphological, and sedimentary features of the KPR and their geneses, as well as inadequate comparative studies on the sedimentary features of typical geomorphic units in the KPR. Given this, this paper highlights the KPR-an important tectonic and geomorphic unit in the Philippine Sea-based on measured multibeam data and subbottom profiles. It deeply analyzes the topographic and geomorphological features of the southern section of the KPR and its adjacent areas (the study area is shown in Fig. 1).Meanwhile, it finely describes third-order geomorphic units,delineates fourth-order geomorphic units, and conducts a comparative study on the sedimentary features of typical geomorphic units in this area. All these will provide important references for improving the scientific understanding of the tectonic movement attributes of the KPR.

Fig. 1. Topography map of the Philippine Sea and the location of the study area (the red box denotes the location of the study area).

2. Regional geological setting

As the largest marginal basin in the Western Pacific Ocean, the Philippine Sea is surrounded by two island arcs,namely the Japanese Islands-Ryukyu Islands-Taiwan islands-Philippine Islands arc in the west and the Izu Islands-Bonin Islands- volcanic archipelago-Mariana Islands-Caroline Islands in the east and south (Yap Islands-Palau Islands;Sasaki T et al., 2014; Zhang J and Zhang G, 2020). The nearly-NS-strike KPR divides the Philippine Basin into two parts (Fig. 1). The part to the west of the KPR consists of the West Philippine Basin, the Daito Ridge, and the Palau Basin,all of which have complex evolutionary processes and are older than 30 Ma. In comparison, the part to the east of the KPR includes the Shikoku and Parece Vela basins, which were formed during 29-16 Ma and are typical back-arc spreading basins (have ceased spreading now; Gaina C and Muller D, 2007; Wu SG et al., 2013).

Karig DE et al. (1975) considered the KPR a remnant arc that was active between the Middle Oligocene and the Early Miocene. The time of volcanic activities in the remnant arc is critical to the evolution of the West Philippine Basin. Results obtained from expeditions DSDP 31 and 59 reveal the following formation process of the KPR. (1) At approximately 42 Ma, the subduction direction of the Pacific Plate was shifted from NNW to NWW. As a result, the large transform fault boundary on the east side of the West Philippine Basin was transformed into a subduction zone.Afterward, the volcanic activities started, and the KPR was initially formed on the top of a common oceanic crust (Hilde TWC et al., 1977). (2) During 42-32 Ma, approximately 2.5 km thick basement complexes of the KPR were accumulated on the top of a 2.9 km thick crust with medium seismic velocity. The petrological and geochemical analysis results of cores show that pillow basalts in the KPR are island arc tholeiites. However, the Task Group on Ophiolite of the International Geoscience Programme (IGCP; 1977) obtained igneous rocks with typical island arc petrological trend through trawling in the KPR. Based on this, it concluded that a few calc-alkaline volcanic activities also occurred in the KPR and that volcanism ceased during the Oligocene. (3) At approximately 32 Ma, volcanic activities began to decrease and ceased at 29 Ma, which may represent the rifting time of the KPR along volcanic arcs. The forearc area of the KPR moved eastward and became the predecessor of the West Mariana island arc. (4) Strata inclined by two stages. About 20° inclination occurred during the late stage of the Middle Oligocene after the arc basement complexes were deposited,and 20° inclination occurred from the Late Oligocene to the Early Miocene after interlayer tuff and chalk were deposited.(5) As indicated by nannofossil chalk and ooze, there is no sign of tectonic disturbance except for the slow subsidence occurring in the Middle Miocene (Karig DE et al., 1975; Scott R and Kroenke L, 1980).

The ages of basalts obtained from the KPR yield 34-32 Ma, the age of volcanic rocks obtained from the West Mariana Ridge yield an 11 Ma, and ages of the basement in the Parece Vela Basin younger than 17-24 Ma. This series of volcanic ages, combined with other oceanic drilling data,allow for the delineation of the evolutionary history of the island arc of the southern Philippine Sea. After the KPR formed, it separated from the active Izu-Bonin-Mariana island arc between the Middle Oligocene and the Early Miocene.This back-arc spreading event asynchronously started along the Oligocene island arc. First, the spreading event began at around 30 Ma, which occurred at the center of the Parece Vela Basin and propagated northward and southward later. As a result, the Parece Vela Basin became a bow in shape. The spreading event began to propagate at 27 Ma again, which occurred in the northernmost part of the Shikoku Basin and propagated southward later. The Parece Vela Basin and the Shikoku Basin joined at the current position of approximately 24°N at 27 Ma and shared a common spreading axis until 17-15 Ma when spreading ceased (Xia CL et al., 2017). The second spreading event began during the Late Miocene when the southern section split again. The KPR was subject to both vertical rotational stress and horizontal compression during the rifting, which is one of the main reasons for the formation of the current narrow geometry of the KPR (Qiu Y and Gao HF, 2017).

3. Data and methods

3.1. Data sources

Five intergrade geological and geophysical expeditions surveys were carried out in the Philippine Sea by two scientific research vessels of the China Geological Survey,namely R/V Haiyangdizhi 6 and R/V Haiyangdizhi 9 during 2018-2021. The data set used in this study originated from the high-resolution multibeam bathymetric data and subbottom profiles obtained from these expeditions.

3.2. Survey equipment and mapping software

Two multibeam bathymetry systems-Simrad EM122 and EM302 echo sounders introduced from Kongsberg Martine,Norway-were used in this study. The operating parameters of the EM122 echosounder include a frequency of 12 kHz and a depth range of 20-11000 m. It sends out the sound with a beam angle of 1°×1° and achieves an angular coverage sector of up to 150°. The beam number per swath is 288, and 432 soundings can be obtained in the high-density mode. In the dual swath mode, two swaths are generated per ping cycle,producing up to 864 soundings in the vertical plane perpendicular to the heading direction. As a result,bathymetric swaths with a certain width can be formed. The operating parameters of the EM302 echosounder include a frequency of 30 kHz, a beam angle of 1°×1°, a depth range of 5-7000 m, an effective pulse length of 700-200 ms, and a swath width of 5.5 times water depth.

The deep-sea parametric sub-bottom profiler ParaSound P70 produced by Teledyne Marine was adopted in this study.With a depth range of 10-11000 m, it is suitable for the survey and analysis of full ocean geological structures of the seafloor. It has multiple transmission modes, including the traditional monopulse mode and the advanced pulse-train and quasi-equidistant modes. Based on multiple pulse types such as continuous wave (CW) pulses, frequency modulated(chirp) pulses, and barker-coded pulses, combined with parametric-array difference frequency technology, ParaSound P70 can perform full ocean surveys with high sediment resolution and penetration.

The quality of multibeam data in this study meets the precision requirements of the International Hydrographic Organization (IHO). The multibeam data were processed using the CARIS 8 software. Collected multibeam data were processed through post-processing procedures (such as the bathymetric correction, system parameter correction, and ship attitude correction of full-depth sound speed profiles) and formed grid data. Finally, 3D topographic maps were developed using the Surfer10 and Global Mapper17 software.

3.3. Principles and methods for geomorphic classification

There are various methods for classifying submarine landforms. Among them, three general opinions are that submarine landforms should be classified based on endogenic forces, exogenic forces, and plate tectonics. Given the tectonic environment of the study area and referring to the requirements of Article 9.5 in GB/T 12763.10-2007Specification for Oceanographic Survey Part 10. Submarine Topography and Geomorphology, this paper adopts a comprehensive analytical method based on the theory of plate tectonics. In detail, it combines endogenic with exogenic forces, morphology with genesis, and classification with grading, and comprehensively considers the measured water depths and tectonic features in the study area. According to the primary and secondary factors of geomorphic genesis and the development scale of landforms, the landforms in the study area are classified as follows. The first-order landforms in the study area are all oceanic landforms. They were further divided into large-scale second-order landforms according to morphological features, geological structure, and exogenic factors. As a result, the second-order landforms in the study area mainly include abyssal basins. The second-order landforms were further grouped into third-order mediumsized landforms such as abyssal plains according to morphological features. The third-order landforms are composed of independent small-scale geomorphic entities,such as seamounts and sea knolls, which are fourth-order landforms.

4. Topographic and geomorphologic features of the seafloor

4.1. Topographic features

The 3D topographic map of the KPR is based on highprecision multibeam bathymetric data (Fig. 2). The study area has a complex topography, and the water depth in the study area is 522-6736 m, which varies greatly and has an average of approximately 4800 m. The study area is generally deep in the west and shallow in the east with the KPR as a boundary.

The southern section of the KPR exhibits complex and variable topography, which is long, narrow, rugged, and highly undulating overall (Fig. 2). The KPR is approximately 620 km long and 46-148 km wide, covering an area of 51×103km2. It can be roughly divided into two sections with 15°N as the boundary. The southern section of the KPR has a NE-SW strike, while the northern section has a nearly SN strike, and thus the KPR features a narrow northern and a wide southern section overall. Fig. 3 shows the diagram of the south-north topographic profile of the southern section of the KPR. According to this figure, the southern section of the KPR shows noticeably discontinuous topography. The water depth in this part is 522-6234 m. The portion to the north of 14°N has a low water depth on average and slightly undulating topography. In contrast, the portion to the south of 14°N possesses greatly undulating topography overall due to higher seamounts and large-scale and deep basins, with a maximum elevation difference of 4086 m. The lowest water level in the study area is at the seamount summit (Fig. 3). The seamount slopes or escarpments in the study area are steep,with a slope of more than 10° mostly and a maximum slope of 59°.

Fig. 2. 3D topographic map of the southern section of the KPR.

Fig. 3. Diagram of the north-south topographic profile of the southern section of the KPR.

The topographic slope on both sides of the southern section of the KPR is asymmetrical since the west slope is steep and the east slope is relatively wider and gentle. With the 3741-5577 m and 5000-5898 m isobaths as the boundaries, the east and west sides adjoin the Parece Vela Basin and the West Philippine Basin, respectively.Meanwhile, the west side at 14°-16°N adjoins the east side of the central rift zone.

The Parece Vela Basin has a water depth of 3741-5800 m(average: approximately 4900 m), an average slope of approximately 6.7°, undulating basin bottom in a hilly form,and en-echelon-shaped topography in the form of an NNE-trending arc. The West Philippine Basin has a water depth of 4000-6736 m (average: approximately 5400 m) and is dominated by the typical central rift zone in terms of topography.

4.2. Seafloor geomorphological features

The study area is composed of four orders of geomorphic units. Among them, the first- and second-order geomorphic units are oceanic landforms and abyssal basins, respectively.The second-order geomorphic units can be further divided into third-order landforms, including abyssal basins, deep oceanic ridges, deep-sea seamount groups, and fault trough ridge zones. Furthermore, the third-order geomorphic units are further divided into 12 types of fourth-order geomorphic units according to micro landform morphology. The determined fourth-order geomorphic units include seamounts,sea knolls, intramontane basins, deep oceanic ridges, and abyssal canyons. Table 1 and Fig. 4 show the classification and gradation of seafloor landforms in the southern section of the KPR and its adjacent areas.

Fig. 4. The seafloor landforms map in the southern section of the KPR and its adjacent areas.

Table 1. Classification of seafloor landforms in the southern section of the KPR and its adjacent areas.

As an important tectonic and geomorphic unit in the Philippine Sea Plate, the KPR is a remnant arc forming after the spreading of the Shikoku and Parece Vela basins (Ben-Avrabam E and Uyeda S, 1983). The southern section of the KPR has a complex topography and eight types of landforms,including seamounts, sea knolls, ridges, intramontane basins,and intermontane valleys, which are typical deep oceanic landforms. This part consists of seamount groups (composed of moniliform seamount chains, seamounts, and sea knolls),intramontane basins, and intermontane valleys, which are distributed in an alternating manner. Overall, it is present as seven independent seamount groups being divided by five large troughs. The seamounts have steep edges, with a maximum slope of up to 59°. Meanwhile, the height difference between the seamounts and the surrounding seafloor is up to 4086 m. Therefore, the southern section of the KPR shows an overall geomorphic pattern of “seven mountains and five troughs”. Meanwhile, the KPR narrows down or even interrupts and shows noticeable geomorphic discontinuity, reflecting the geomorphological features of a deep oceanic ridge.

The West Philippine Basin and the Parece Vela Basin on both flanks of the KPR are both typical deep-water oceanic basins originating from seafloor spreading. They differ significantly from the KPR in terms of landforms.Specifically, they are mainly characterized by the parallel alternating spreading of small ridges and troughs, with large water depth overall but a small height difference of fewer than 500 m mostly. Among them, the Parece Vela Basin on the east flank has a nearly SN strike, while the West Philippine Basin on the west flank has a nearly EW strike.

There are 111 geomorphic units in the KPR, including 45 seamounts, 16 sea knolls, 16 ridges, 26 intramontane basins, 1 intermontane valley, 2 escarpments, 4 submarine canyons and 1 crater. At present, the Sub-Committee on Undersea Feature Names (SCUFN) has named 72 seafloor geographic entities in the southern section of the KPR, which are dominated by seamounts, sea knolls, ridges, and intramontane basins and include 30 seamounts, 16 ridges, 4 sea knolls, 2 small sea knolls and 15 intramontane basins. To better depict the features and distribution pattern of typical topography and landforms in the southern section of the KPR, the negative topographic assemblages consist of intramontane basins and intermontane valleys that are continuously present from east to west are defined as large troughs. Three types of typical geomorphic units (i.e., typical large troughs, seamount groups, and intramontane basins) and some fourth-order geomorphic units were described in detail according to topographic profiles.

4.2.1. Large troughs

The three-dimensional topographic and geomorphological maps of the southern section of the KPR are shown in Fig. 2 and Fig. 4, respectively. According to these figures, the southern section of the KPR is divided by five large troughs at 12°N, 12.3°N, 12.7°N, 13°N, and 15.4°N, with a height difference of 1000-4086 m and noticeable discontinuity. The major topographic parameters of the five large troughs are listed in Table 2.

Table 2. Major topographic parameters of five large troughs in the southern section of the KPR.

The topographic and geomorphological features of large troughs are described in detail with trough No. 1 as an example. Trough No. 1 is located at 11.43°-12.16°N in the southern part of the southern section of the KPR (Fig. 5). On the northern side of the trough is a series of moniliform seamounts and ridges distributed in a nearly EW strike. The seafloor geographic entities on this side named by SCFUN include the Hahirohoshi seamount, the Nautilus seamount, the Cenotaph seamount, and the Bedaoch ridge from west to east,with a maximum height difference of 3088 m. On the southern side of trough No. 1 is a series of high-rising seamounts and ridges that are distributed in a NW-SE strike.The seafloor geographic entities on this side named by SCFUN include the Babeldaob ridge, the Rteluul seamount,the Ngedebuul seamount, and the Kerradel seamount from west to east. Trough No. 1 has a large size, spans the KPR from east to west, and consists of negative topographic assemblages such as intramontane basins and intermontane valleys that alternately appear from east to west. It has a NW-SE strike, a length of approximately 126.9 km, and a width range of 2.8-26.7 km, and covers an area of approximately 2006 km2. Furthermore, it has a maximum downcutting depth of 3088 m, a trough wall slope of approximately 2.5°-27°, and a water depth of 3503-5488 m at the trough bottom (Fig. 5a).

According to Section A-A′, the crest of the Bedaoch Ridge on the north side of trough No. 1 is shallow, with a depth of approximately 1813 m; the slopes on the north side of trough No. 1 show a stepped decrease in the slope gradient, with a slope of 17°-20°, and the Rteluul seamount on the southern side of trough No. 1 is deep, with a depth of approximately 2640 m and a slope of approximately 13°. Meanwhile, the trough bottom has a flat topography, a water depth of approximately 4400 m, and a width of approximately 8.5 km(Fig. 5b).

4.2.2. Abyssal seamount groups

Numerous seamounts are developed in the southern section of the KPR, and they are grouped into seven abyssal seamount groups according to their planar distribution characteristics in this study. These seamount groups each include 3-9 seamounts, showing different scales. Overall, the seamount groups in the southern part of the southern section of the KPR have larger sizes. The major topographic parameters of abyssal seamount groups are listed in Table 3.The seamounts in abyssal seamount groups Nos. 1, 3, and 4 are distributed in a moniliform shape, while seamounts in other abyssal seamount groups are relatively concentrated and are unevenly distributed.

Table 3. Major topographic parameters of abyssal seamount groups.

The topographic and geomorphological features of the abyssal seamount groups are described as follows with the southernmost abyssal seamount group No. 1 as an example(Fig. 2). Abyssal seamount group No. 1 is composed of ten geomorphic units, including six seamounts, one ridge, and three (small) sea knolls. Among them, the two ridges have the largest scales. They have a strike of NE-SW strike and are separated by intramontane basins. Meanwhile, the complexes consisting of seamounts and sea knolls roughly have the same strike as the ridges. Abyssal seamount group No.1 has a water depth range of 1000-5030 m, a maximum height difference of 2900 m, and steep seamount slopes with a maximum slope gradient of 26°.

4.2.3. Intramontane basins

A total of 26 intramontane basins of different sizes have developed in the southern section of the KPR, and their overall planar morphology roughly coincides with the nearly NS strike of the southern section of the KPR. In terms of planar distribution, intramontane basins in the north have a smaller quantity and size than those in the south, with seven and 19 basins in the north and south, respectively. Among them, the southernmost Hahirohoshi intramontane basin is the largest, which is surrounded by six seamounts, sea knolls, and ridges such as the Hahiroohoshi seamount and the Babeldaob ridge. It has an elliptic shape on a horizontal plane, with an area of approximately 717 km2, a water depth of 4500-5130 m and a maximum height difference of 3088 m between it and surrounding seamounts.

4.2.4. Fourth-order geomorphic units

Fourth-order geomorphic units can be finely depicted using high-resolution and high-precision multibeam data. In this study, 12 fourth-order geomorphic units are identified in the southern section of the KPR, including four submarine canyons, two abyssal fans, four seamounts, one sea knoll, and one ridge. Fig. 6 shows typical fourth-order geomorphic units in the southern section of the KPR and its adjacent areas. The figures exhibit clear spatial distribution characteristics of these landforms, especially those of canyons and abyssal fans in Figs. 6b-c, and vividly depict the migration channels and processes of abyssal volcanic clastic sediments in the southern section of the KPR.

Fig. 6. Fourth-order geomorphic units of the southern section of the KPR and its adjacent areas.

5. Sedimentary characteristics of typical geomorphic units

Deep-sea sediments in the southern section of the KPR and its adjacent areas mainly originated from marine organisms and seafloor volcanoes, with a small percentage of terrigenous sediments. During the spreading of the Parece Vela Basin, the sediments in the west are thin and are dominated by pelagic clay (Scott R and Kroenke L, 1980). As revealed by the analyses of high-resolution sub-bottom profiles, sediment thickness differs between adjacent typical geomorphic units.

The sub-bottom profile (BB′ in Fig. 2) has a E-W strike and spans the eastern part of the West Philippine Basin, the KPR, and the Parece Vela Basin from west to east, with the KPR lying in the middle part of the profile. This profile clearly reveals the differences in the reflection characteristics and thickness of sediments between the West Philippine Basin and the Parece Vela Basin on both sides of the KPR. Most importantly, multiple inter-ridge depressions are formed among seamounts and ridges on the KPR. Therefore, the KPR shows intermittent differential sedimentary characteristics.

5.1. Sedimentary characteristics of the KPR

The KPR is intermittent in the north-south direction due to discontinuously distributed ridges in the KPR. Moreover, it shows the irregular pattern of alternating uplifts and depressions in the E-W direction. The height difference between ridge crests and inter-ridge depressions is large and is approximately 2400 m. As a result, a broken pattern instead of a continuous band pattern is formed. The southern section of the KPR is separated by three large ridges (Fig. 7a), forming two inter-ridge depressions (Figs. 7b, c). Besides, a small trough lies between a ridge and a seamount in the east (Fig. 7d).The effective accommodation sedimentary space of the depressions and the trough gradually decreases from east to west, which is referred to be caused by the subduction from east to west. Specifically, far-end depressions of the eastern subduction have wide sedimentary areas (Fig. 7b), while depressions in near-end positions closer to the subduction have increasingly limited scope (Figs. 7c, d).

Fig. 7. Sedimentary characteristics spanning the KPR in the sub-bottom profile (refer to Fig. 2 for the location of the profile line BB′).

Reflection interfaces SQ0(seafloor) andSQ1are identified in the sedimentary areas with continuous reflection (Figs.7b,c). Among them, SQ1show continuous and high-amplitude reflections. As a result, Sequence A is bounded by reflection interfaces SQ0and SQ1as its top and bottom, respectively.Sequence A shows good continuity and has interbeds consisting of strata with medium-high-amplitude reflection and those with weak-amplitude reflection. The underlying strata of Sequence A show increasingly poor continuity,strong amplitude, and traces of effects of magmatic activities occurring at the bottom and its surrounding areas.

The reflection characteristics of the three depressions/trough b, c, and d in the KPR are as follows (Fig. 7). The sedimentary strata in depression b show good reflection continuity, high-amplitude reflection near the seafloor, and alternating thin-layered high-amplitude reflection and relatively thick-layered low-amplitude reflection downward below the seafloor. Sequence A in the area of depression b has a thickness of 28-39 m and shows the sedimentary characteristics of boat-shaped strata with a thick middle part and thin ends, suggesting a relatively stable deep-sea sedimentary environment. Depression c only shows a small scope of continuous and parallel reflection at the bottom and exhibits chaotic reflection at the slope toe, where igneous rock debris forming from the denudation and slump of ridges on both sides of depression c are accumulated. Trough d in the east only has a small area of sedimentary space and shows chaotic reflection characteristics, which is caused by denuded igneous rock debris that slumped to the trench bottom under gravity. In addition, the areas shown in Figs 7c-d show chaotic reflection. Therefore, it is difficult to identify the bottom of Sequence A in these areas. As a result, the thickness of Sequence A cannot be determined. Overall, the KPR shows complex tectonics and topography, where different scaled depressions/trough formed due to the separation of intermittent ridges. Moreover, it exhibits greatly different sedimentary characteristics, including reflection characteristics, sedimentary thickness, and sedimentation types.

5.2. Sedimentary characteristics of basins (valleys)

In the sedimentary area of the West Philippine Basin-a vast area on the west side of the KPR, Sequence A has a sedimentary thickness of approximately 26-30 m and good continuity and shows alternating high- and low-amplitude reflection. This suggests a relatively stable deep-sea sedimentary environment and higher continuity than the various separated depressions in the eastern part of the KPR(Fig. 7e). In addition, this area has a wider accommodation space and relatively weaker tectonism compared to adjacent areas. Overall, the sedimentary sequence in the central rift of the West Philippine Basin is unevenly distributed and thin,with a sedimentary thickness of fewer than 100 m mostly,reflecting the sedimentary characteristics dominated by pelagic sedimentation. However, sediments in local depressions or basins formed by seafloor spreading are thick,with a maximum thickness of up to approximately 300 m, and they are supposed to originate from surrounding pyroclastic materials (Ingle JC Jr et al., 1975). The seismic profile reveals that the KPR is composed of multiple uplifted volcanic ridges.Multiple basins were formed among these volcanic ridges and have thick sedimentary strata, with a thickness of up to 380 m(Dong DD et al., 2017). However, the sedimentary strata have uneven thickness inside the basins, and they are the thickest in the middle parts and gradually thin or even pinch out toward both sides (Fig. 7b).

5.3. Analysis of differences in sedimentary characteristics

Deep-sea sediments mainly originate from pelagic sedimentary provenances. Different flow rates lead to different deposition rates and transport rates of sediments,which further causes different sediment thicknesses at hilly undulations at different intervals. The seafloor topography and water depth vary greatly. The changes in seafloor topography and the induced changes in bottom currents may jointly have significant effects on the distribution of sediments (Rebesco M et al., 2008). In areas with high relief,sediments are further redistributed on the seafloor due to the dilution of altered materials from seamounts or ridges and the erosion of bottom currents. In contrast, in areas with low relief, sediments are well preserved since the surrounding higher topography and high-density water bodies decrease the dilution of materials from surrounding seamounts or ridges and the erosion of bottom currents. The sediments on the eastern and western ridges (thickness: ＜50 m) are obviously thinner than those in deep basins (50-250 m), and even the bedrock is exposed in some areas, indicating effects of seafloor topography and bottom currents on sediment distribution.

6. Tectogenesis of typical geomorphic units

Seamount chains are the products of magmatic activities after the seafloor spreading (Deschamps A et al., 1999, 2002a;Wang YJ et al., 2009; Yang SY et al., 2011). With the subduction of the Pacific Plate at approximately 50 Ma,numerous independent seamounts and seamount groups developed on collision margins, with the seamount groups distributed in a chain pattern as a whole (Haraguchi S et al.,2003). A series of linear seamounts and sea knolls distributed in nearly NS direction have been newly discovered outside the western boundary of the KPR, and they form the western boundary of abyssal basins. Among them, the seamounts are distributed in the direction roughly perpendicular to the ancient spreading ridge of the Philippine Sea. These seamounts and sea knolls in nearly NS direction may be formed due to magma upwelling in weak areas of transform faults (Klein GD et al., 1978; Marsh NG et al., 1980; Dick HJ et al., 1980; Hickey-Vargas R et al., 1991, 2008) and may be the products of two geological processes. Firstly, the western boundary of the KPR is the product of the initial formation stage of island arcs, and abyssal basins of the KPR may be onor inter-arc basins. This discovery provides a new perspective for studying the starting of the subduction of the Pacific Plate toward the Philippine Plate, which is a major geological science issue. Second, the seamounts and sea knolls are the products of the transform faults in the central rift zone of the Philippine Sea. They are a part of the fault trough ridge zone,and their morphologies are jointly controlled by westward compression of the KPR and the clockwise rotation of the Philippine Plate.

Studies show that the changes in seafloor topography and the changes in bottom currents they induce may greatly affect sediment distribution (Yang HL et al., 2021; Ming J, 2013;Tian JW and Qu TD, 2012; Rebesco M et al., 2008). Different types of geomorphic units have developed on the east and west flanks of the KPR, which is directly related to tectonic movements and the direction of bottom currents. Three large submarine canyons and two abyssal fans have developed from north to south at the junction of the KPR and abyssal basins in the west, while three large intramontane basins have developed from north to south at the junction of the KPR and deep-sea knoll groups in the east (Figs. 2, 4). Data on geological sampling and sub-bottom profiles show that the sedimentary thickness in the intermontane basins in the east is much thinner than that of abyssal basins and abyssal fans in the west (Divins DL, 2003; Dong DD et al., 2017). Survey results show that the bottom currents in the Parece Vela Basin on the east flank of the KPR originate from Antarctic bottom water (Lee I and Ogawa Y, 1998). Owing to the effects of regional topography, the bottom currents bypass obstacles and flow counterclockwise around the whole Parece Vela Basin.Deep-sea sediments mainly originate from pelagic sedimentary provenances. Different flow rates lead to different deposition rates and transport rates of sediments,which further causes different sediment thicknesses at hilly undulations at different intervals (Yang HL et al., 2021). The formation of sediments may be directly related to the flow direction of bottom currents in this area. The bottom currents flow from east to west, and thus the sediments are more liable to be deposited on the west flank and then transported to the abyssal basins through submarine canyons. As a result, the accumulation of landforms such as abyssal basins and abyssal fans are formed. In addition, the bottom currents become weak in terms of intensity and scouring effects when reaching the west flank of the KPR. This provides important support for studying the flow regularity of bottom currents and the sediment transport in the southern section of the KPR.

The KPR experienced continuous subsidence and showed stable sediments environment after the Parece Vela Basin spread (Dong DD et al., 2017). Abyssal basins exhibit a stepped descending trend from north to south. According to measured data, abyssal basins are distributed in an elongated pattern in the SN direction. With the KPR lying on the east side and the west side being obstructed by a series of linear seamounts and sea knolls, the abyssal basins have a length of 311 km, a width of 15-40 km, an area of 0.82×104 km2, and a water depth of 4423-6194 km. Furthermore, the overall topography of the abyssal basins shows a stepped descending trend from north to south, and the average height differences between the three steps from north to south and the deep-sea knoll groups on the west side are 620 m, 320 m, and 200 m,respectively. The stepped topography is related to the quantity of materials transported from the KPR. This coincides with the understanding that the late reactivity of the central rift zone in the West Philippine Basin induced massive volcanism. That is, a shorter distance from the rift zone indicates more transported materials and thicker sediments.

7. Conclusions

(i) The southern section of the KPR has complex, rugged,and hugely undulating topography, with a maximum height difference of 4086 m. This section consists of seamount groups (composed of moniliform seamount chains,seamounts, and sea knolls), intramontane basins, and intermontane valleys, which are distributed in an alternating manner from north to south. The slope of the seamounts is generally greater than 10° and is up to a maximum of 59°.Therefore, the topography of the southern section is noticeably discontinuous.

(ii) Surveyed data reveal 12 fourth-order geomorphic units in the southern section of the KPR, including four submarine canyons, two abyssal fans, four seamounts, one sea knoll, and one ridge. Moreover, three abyssal fans are developed at the exits of two large submarine canyons on the west side of the KPR, and intermontane basins and seamounts are developed on the east side. This finding helps to study the regularity of bottom currents and sediment migration in the southern section of the KPR.

(iii) There are primarily eight geomorphological types in the southern section of the KPR, including seamounts, sea knolls, ridges, intramontane basins, and intermontane valleys.Meanwhile, five large troughs have developed at 12°N,12.3°N, 12.7°N, 13°N and 15.4°N. All these create an overall geomorphic pattern of “seven mountains and five troughs”and reflect the geomorphological features of a deep oceanic ridge.

(iv) Basins and depressions in the southern section of the KPR are covered by evenly thick sediments. In contrast,sediments in ridges and seamounts in the southern section are thin or even missing, with slumps developing locally. Overall,sediments in the southern section of the KPR are discontinuous and unevenly developed, and the sedimentary environment varies with geomorphic units.

(v) The formation of the KPR is mainly controlled by tectonism such as volcanic activities and plate movements. A series of linear seamounts and sea knolls distributed in nearly NS direction exist outside the southwestern boundary of the KPR. They may form the western boundary of the KPR and the products of the initial formation stage of the KPR.

CRediT authorship contribution statement

Xu-wen Qin and Wei-dong Luo conceived the presented idea. Pan-feng Li and Hong-jun Chen developed the theory.Gang Hu and Yu-fang Tan verified the analytical methods.Xu-wen Qin encouraged Wei-dong Luo and Run-lin Du to investigate and supervise the findings of this work. All authors discussed the results and contributed to the final manuscript.

Declaration of competing interest

The authors declare no conflicts of interest.

Acknowledgment

This paper is funded by the National Special Program of China Geological Survey (DD20191002, DD20191003).Thanks to all crew numbers of R/V Haiyangdizhi 6, R/V Haiyangdizhi 9 for their hard work and valuable contributions in field surveys.