R E V I EW AR T I C L E

Origin of the early Cenozoic belt boundary thrust andIzanagi–Pacific ridge subduction in the western Pacific margin

Gaku Kimura1 | Yujin Kitamura2 | Asuka Yamaguchi3 | Jun Kameda4 |

Yoshitaka Hashimoto5 | Mari Hamahashi6

1Department of Marine Environment and

Resources, Tokyo University of Marine

Science and Technology, Tokyo, Japan

2Department of Earth and Environmental

Sciences, Graduate School of Science and

Engineering, Kagoshima University,

Kagoshima, Japan

3Atmosphere and Ocean Research Institute,

The University of Tokyo, Chiba, Japan

4Department of Earth and Planetary Sciences,

Graduate School of Science, Hokkaido

University, Sapporo, Japan

5Department of Applied Science, Faculty of

Science, Kochi University, Kochi, Japan

6Earth Observatory of Singapore, Nanyang

Technological University, Singapore

Correspondence

Yujin Kitamura, Department of Earth Sciences,

Faculty of Science, Kagoshima University,

Korimoto 1-21-24, Kagoshima, 890-8580,

Japan.

Email: yujin@sci.kagoshima-u.ac.jp

Funding information

Japan Society for the Promotion of Science,

Grant/Award Numbers: Grant-in-Aid for

Scientific Research from MEXT of Japan,

15H05717, Bilateral Joint Research Project

with France

Abstract

The belt boundary thrust within the Cretaceous–Neogene accretionary complex of

the Shimanto Belt, southwestern Japan, extends for more than ~ 1 000 km along the

Japanese islands. A common understanding of the origin of the thrust is that it is an

out of sequence thrust as a result of continuous accretion since the late Cretaceous

and there is a kinematic reason for its maintaining a critically tapered wedge. The

timing of the accretion gap and thrusting, however, coincides with the collision of the

Paleocene–early Eocene Izanagi–Pacific spreading ridges with the trench along the

western Pacific margin, which has been recently re-hypothesized as younger than

the previous assumption with respect to the Kula-Pacific ridge subduction during the

late Cretaceous. The ridge subduction hypothesis provides a consistent explanation

for the cessation of magmatic activity along the continental margin and the presence

of an unconformity in the forearc basin. This is not only the case in southwestern

Japan, but also along the more northern Asian margin in Hokkaido, Sakhalin, and

Sikhote-Alin. This Paleocene–early Eocene ridge subduction hypothesis is also con-

sistent with recently acquired tomographic images beneath the Asian continent. The

timing of the Izanagi–Pacific ridge subduction along the western Pacific margin

allows for a revision of the classic hypothesis of a great reorganization of the Pacific

Plate motion between ~ 47 Ma and 42 Ma, illustrated by the bend in the Hawaii–

Emperor chain, because of the change in subduction torque balance and the

Oligocene–Miocene back arc spreading after the ridge subduction in the western

Pacific margin.

K E YWORD S

accretionary prism, East Asian margin tectonics, Izanagi–Pacific ridge, Japanese islands,

Shimanto Belt, subduction zone, Western Pacific

1 | INTRODUCTION

When describing mountain ranges, geologists have traditionally classi-

fied geological belts or terranes on the basis of differences in

lithology, age, and structure. Most orogen-parallel belt boundaries are

faults, regardless of the sense of shear. In the Japanese islands, geo-

logical descriptions were first completed during the late 19th century

by geologists mainly from the United States and Germany. The first

Received: 11 September 2017 Revised: 25 April 2019 Accepted: 20 May 2019

DOI: 10.1111/iar.12320

This is an open access article under the terms of the Creative Commons Attribution-NonCommercial-NoDerivs License, which permits use and distribution in any

medium, provided the original work is properly cited, the use is non-commercial and no modifications or adaptations are made.

© 2019 The Authors. Island Arc Published by John Wiley & Sons Australia, Ltd

Island Arc. 2019;28:e12320. wileyonlinelibrary.com/journal/iar 1 of 15

https://doi.org/10.1111/iar.12320

geological map of the Honshu mainland and Shikoku and Kyushu

islands was edited and published by Naumann (1890), following a map

of Hokkaido by Lyman (1874). Prior to 1965, the geological evolution

of the Japanese islands had mainly been discussed on the basis of the

classic geosyncline idea (e.g. Kobayashi, 1941; Minato, Gorai, &

Hunahashi, 1965). However, during the late 1970s and 1980s, it

became a matter of debate as to whether plate tectonic processes

were applicable or not to the geology of the Japanese islands, as the

geosyncline idea was supported by many geologists in Japan.

A breakthrough occurred through the detailed dating of terrige-

nous and exotic pelagic sediments. The geosyncline hypothesis pre-

mised mostly conformable relationships among the sediments,

regardless of their origins, whereas the accretionary prism hypothesis

derived from plate tectonics to explain the formation of the geological

belts inferred that terrigenous sediments were younger than accreted

pelagic and exotic sediments. Tiny radiolarian fossils documented by

Taira (1988) supported the accretionary prism hypothesis for the Cre-

taceous to Miocene Shimanto Belt, and those documented by

Matsuda and Isozaki (1991) supported that hypothesis for the

Jurassic–early Cretaceous Chichibu Belt. In Japan, geologists termed

such a revolutionary change in geological thought as the ‘radiolarian

revolution’ (Yao, Mizutani, & Kuwahara, 2001), illustrating the docu-

mentation of the age of each geological body through a tiny fragment

of chert at the hand sample scale.

Following the results of the ‘radiolarian revolution,’ the geological

survey of Japan revised the geological map of Japan and published an

integrated digitized map at a scale of 1:200 000 (Geol. Survey Japan, ,

2012). Alternative hypotheses of the geological history of Japan were

proposed as a result of the ‘radiolarian revolution’ and the theory of

plate tectonics (Maruyama, Isozaki, Kimura, & Terabayashi, 1997;

Taira, 1988, 2001). These new scenarios emphasized continuous

accretion since the Mesozoic period. Yamamoto, Senshu, Rino, Omori,

and Maruyama (2009) and Isozaki, Aoki, Nakama, and Yanai (2010),

however, recently revised the previous synthesis by assuming ‘tec-

tonic erosion’ for the age gap between the geological belt boundaries.

The Nobeoka thrust in Kyushu and its equivalents in southwest-

ern Japan form a large belt boundary fault separating the northern

and southern Shimanto sub-belts (Figure 1). This fault is traceable

for more than 1 000 km along the entirety of southwestern Japan.

The traditional hypothesis for the age gap between rocks on either

side of the belt boundary fault, contrasting with the recently-

proposed tectonic erosion hypothesis, was that out of sequence

thrusting occurred during continuous growth of the accretionary

wedge (Hamahashi et al., 2013; Kondo et al., 2005; Ohmori et al.,

1997). Possible alternatives to the hypotheses of an out of sequence

thrust origin during continuous accretion or an origin related to tec-

tonic erosion include the cessation of subduction–accretion or sig-

nificant changes in plate convergence and continental tectonics

affecting the forearc, such as trench advance or retreat. In this work,

we revisit the early Cenozoic belt boundary fault in the Shimanto

Belt and discuss the most plausible cause of the age and structural

gap. We review the early Cenozoic geology of southwestern Japan,

northeastern Japan, Hokkaido-Sakhalin and the east Asia continen-

tal margin, focusing on the accretionary complex, the unconformity

within the forearc basin deposits, and the age gap in the volcanic

activity of the magmatic arc.

F IGURE 1 Distribution map of the Cretaceous to early Cenozoic accretionary complexes in the Japanese islands: The Shimanto Belt in SWJapan, the Sorachi–Yezo–Hidaka Belt in central Hokkaido and the Tokoro Belt in eastern Hokkaido (a), and the Shimanto Belt in Kyushu, Shikoku,the Kii Peninsula in southwestern Japan and in central Japan (b). The Shimanto Belt is subdivided into two sub-belts: The Northern and Southernsub-belts

2 of 15 KIMURA ET AL.

2 | SUMMARY OF THE EARLY CENOZOICBELT BOUNDARY THRUST AND GAPS IN AGEAND STRUCTURE IN THE SHIMANTO BELT

The Shimanto Belt is a well-studied Cretaceous–Miocene accretionary

complex that yields an opportunity to investigate plate boundary pro-

cesses, including seismogenesis in subduction zones. The belt is exten-

sively exposed from central Japan through to the Kii Peninsula, Shikoku,

and Kyushu (Figure 1). The Shimanto Belt in southwestern Japan gener-

ally consists of a southward verging fold and thrust belt with the polar-

ity becoming successively younger southward and downward, which

was first noted by Kanmera and Sakai (1975) and later justified by dat-

ing, mainly using radiolarian microfossils (e.g. Taira, 1988, 2001).

2.1 | The Nobeoka thrust in Kyushu

The Shimanto Belt in Kyushu is subdivided into two sub-belts by the

Nobeoka thrust (Figure 1). Main lower part of the hanging wall block of

the thrust, the Makimine Group, constitutes the accretionary complex

with Santonian–Campanian (83.6–72.1 Ma) fossil ages. The complex was

subjected to greenschist to amphibolite facies metamorphism (Nagae &

Miyashita, 1999; Toriumi & Teruya, 1988) with K–Ar ages of 63.9 ±3.2.

The footwall block of the thrust, the Hyuga Group with Middle Eocene

to Oligocene fossil ages, was metamorphosed at temperatures of approx-

imately 260 �C–270 �C (Kondo et al., 2005) during the middle to late

Eocene (Hara & Kimura, 2008), shortly after deposition at

approximately 48–40 Ma. In northeast Kyushu, the Eocene complex of

the Kitagawa Group with K–Ar ages of 49.2 Ma ±2.5 Ma and zircon fis-

sion track ages of approximately 46–50 Ma (Hara & Kimura, 2008) was

added to the lowest portion of the hanging wall. Such geological relation-

ship presents that the Nobeoka thrust is branched from the main location

of the boundary between the Cretaceous and Eocene complexes to

within the Eocene complexes. Thus, in Kyushu, the age gap between the

Cretaceous and Eocene complexes across the main part of the Nobeoka

thrust is approximately 24 m.y. (million years) for the protoliths (the

youngest age for the hanging wall is 66 Ma and the oldest age for the

footwall is 41.3 Ma) and the metamorphic age gap suggested by the

annealing temperature of zircon fission track (~ 250 �C) corresponds to a

few m.y. around the middle Eocene (43 Ma). Raimbourg et al. (2014)

reported on the deformation of the Nobeoka thrust and the surrounding

Shimanto Belt and proposed that the middle Eocene Izanagi–Pacific ridge

subduction was followed by large-scale displacement along the Nobeoka

thrust in the middle Miocene.

2.2 | The Aki Tectonic Line in Shikoku

A belt boundary fault in Shikoku equivalent to the Nobeoka thrust is

the Aki Tectonic Line (ATL) (Taira, 1988). Ohmori et al. (1997) inferred

that the activity of the ATL occurred at approximately 43 Ma on the

basis of partial annealing of zircon fission tracks. This age corresponds

to the fission track age observed in Kyushu. The lowest geological unit

of the hanging wall of the ATL is composed of Shimanto Belt rocks,

including the Mugi Mélange in eastern Shikoku, of which the youngest

tuffs present a U–Pb age of approximately 62 Ma (Shibata, Orihashi,

Kimura, & Hashimoto, 2008). This age coincides with the fossil ages

(Kitamura et al., 2005). The fossil age of the footwall of the ATL is mid-

dle Eocene (Suyari & Yamasaki, 1987). Thus, the age gap between the

protoliths and the timing of the activity of the ATL are consistent with

those observed for the Nobeoka thrust in Kyushu. Deformation fea-

tures of the Shimanto Belt were reported by Kimura and Mukai (1991),

Onishi and Kimura (1995), Onishi, Kimura, Hashimoto, Ikehara-Ohmori,

and Watanabe (2001), Kitamura et al. (2005) and Ikesawa et al. (2005)

for the northern sub-belt, and for the southern sub-belt by Ditullio and

Byrne (1990) and Lewis and Byrne (2001). These studies suggest a

change in relative plate motion in the Eocene on the basis of change in

kinematic indicators from in map to macroscopic scales.

2.3 | The Gobo-Totsukawa Tectonic Line in the KiiPeninsula

The Shimanto Belt of the Kii Peninsula can also be subdivided into

two sub-belts as in other regions. However, the boundary between

the sub-belts and the ages remain controversial (e.g. Nakaya, 2012).

The main part of the southern sub-belt is characterized by a strongly

folded and thrust accretionary prism composed dominantly of late-

early Eocene to early Miocene sandstones and mudstones. The south-

ernmost portion of the northern Shimanto sub-belt is composed of a

Campanian complex including many fragments of oceanic crust

(Kimura, 1986). The northernmost part of the southern Shimanto sub-

belt of the Kii Peninsula (the Otonashigawa belt: Hatenashi Research

Group, 2012) resembles the Mugi region in Shikoku in regards to the

lack of thick pelagic sediments (Kimura et al., 2012). Its Paleocene age

(Hatenashi Research Group, 2012) is also similar to that of the Mugi

Mélange in Shikoku (Shibata et al., 2008).

Figures 2 and 3 show a simplified geological map and a cross-section

of the southern Shimanto Belt of the Kii Peninsula (Kimura et al., 2014).

At this scale, it can be seen that the fold axes and thrusts, corresponding

to the Gobo-Totsukawa Tectonic Line, are bent and there is a northward-

plunging antiform. In addition, smaller-scale conjugate sets of kink fold

structures have been identified (Nakaya, 2012). The timing of the activity

of the tectonic line is not currently clearly documented but the age gap

based on radiolarian stratigraphy between the early Paleocene and late-

early Eocene is the same as that in Shikoku and Kyushu. In this case, the

boundary thrust equivalent to the Aki Tectonic Line in Shikoku could be

along the Hatenashi thrust (Hatenashi Research Group, 2012).

3 | EARLY CENOZOIC GEOLOGICAL GAPOTHER THAN THE SHIMANTO BELT IN THEJAPANESE ISLANDS

The early Cenozoic geological gap is not only found in the Shimanto

Belt but is also found in other regions. A reconstructed location map

of the Japanese islands prior to the opening of the back arc basins of

the Japan Sea and the Kuril Basin is shown in Figure 4, although the

KIMURA ET AL. 3 of 15

details of the reconstruction remain controversial. Southwestern

Japan, northeastern Japan, central Hokkaido and western Sakhalin

were located along the continental margin but eastern Hokkaido was

separated from those locations as it was part of the Kuril Arc and was

located further to the northeast compared to its present location

(e.g. Kimura, 1994; Figure 4).

The uppermost Cretaceous to lower Cenozoic geological assemblages

are divided into three or four ancient settings on the basis of area, lithol-

ogy, and age. The settings of the geological assemblages are (i) a

subduction–accretion complex along the inner trench slope, (ii) forearc

basin sediments, and (iii) volcanic arc and intracontinental back arc rifts.

We briefly summarize the assemblages from (i) the continental

margin of southwestern Japan, (ii) the continental margin of north-

eastern Japan, Hokkaido-Sakhalin, and Sikhote-Alin, and (iii) the

southernmost Kuril Arc of eastern Hokkaido (Figures 1A, 5 and 6).

3.1 | The continental margin of southwestern Japan

Latest Cretaceous–Paleogene acidic volcano-plutonic rocks are

broadly distributed in southwestern Japan. The ages of the rocks

range from ~ 90 Ma to ~ 60 Ma (e.g. Nakajima, 1996) and a time gap

exists prior to the occurrence of Miocene volcanic rocks along the

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Late Cretaceous Shimanto Belt

Eocene~ealy Miocene Shimanto Belt

Middle Miocene felsic plutons

Middle Miocene felsic intrusive& extrusive rocks

Middle Miocene basic plutons

thrust

fold axis & map-scale kink-band

NNE -ward plunging antiform

Gravity anomaly contour (mgal)

with Higher (H) or Lower (L) anomalies inside

Gravity anomaly contour (mgal) higher than 60 mgal.

Median Tectonic Line

Aki Tectonic Line (Gobo-Totsukawa Thrust)

Butsuzo Tectonic Line

Cape Shiono

Kumano Basin

5.631E0.631EE135.5

N34.0

N33.5

4

su

A lin

e fo

r sim

plifie

d p

rofile

(Fig

. 3)

F IGURE 2 A simplified geologic map of the Kii Peninsula with the Bouguer gravity anomaly contours of Kimura, Hashimoto, Kitamura,Yamaguchi, and Koge (2014)

~90-65 Ma wedge~59-42 Ma wedge

~41-16 Ma wedge

Cape ShionoAki Tectonics LineButsuzo Tectonic Line

~100 Ma

wedge

wedge mantle

Nankai TroughMegasplay fault <2Ma

Kumano forearc basin < 2Ma

25 km

km

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0

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Chichibu &Sanbagawa Belt

brittle crust

ductile lower crust

Highly resistive, high-velocity body

F IGURE 3 A simplified profile of the Kii Peninsula from Suzuki (2012). The profiling line is shown in Figure 2

4 of 15 KIMURA ET AL.

Japan Sea side of the Honshu mainland. Several hypotheses have

been proposed for this magmatic activity. Uyeda and Miyashiro

(1974) ascribed this magmatic activity, expressed as broadly distrib-

uted acidic plutons along the Asian continental margin, to the subduc-

tion of the Kula-Pacific spreading ridge. This hypothesis has been

supported by many scientists (Kinoshita, 1995; Maruyama et al.,

1997; Nakajima, 1996; Takahashi, 1983), although an alternative

hypothesis suggests more common subduction-related magmatism,

paired with the formation of accretionary complexes exposed in the

outer zone of southwestern Japan (Matsuda & Uyeda, 1971;

Taira, 1988).

In northern Kyushu, Eocene strata unconformably overlie these

late Cretaceous volcanics. These strata include coal and fluvial beds

and shallow marine sediments (e.g. Aihara, 1989; Shuto, 1993). The

basin trapping the strata is a half graben bounded by a north-

northwest trending normal fault. The sediments grade into Oligocene

shallow marine sediments, indicating a transgressive environment.

Between the Eocene coal field basin and the Shimanto Belt of the

accretionary prism, there was an upper Cretaceous narrow forearc

basin (Miyata, 1990), which is inferred to have been a trench-parallel

pull-apart strike-slip basin associated with oblique subduction along

the Shimanto Belt (Taira, 1988). The basin dried up and was trans-

formed into the early Eocene basins after a sedimentary break

(Shuto, 1993).

Thus, there is an unconformity between the earliest Paleocene

and the early Eocene periods in the forearc to volcanic arc regions.

This gap is consistent with that existing between the previously-

described northern and southern Shimanto sub-belts (Figure 5). In

addition to these gaps, Raimbourg et al. (2014) recently suggested

that the southern Shimanto sub-belt in Kyushu can be further sub-

divided into four units and the early-middle Eocene evolution is char-

acterized by extension resulting from subduction of the Izanagi–

Pacific ridge.

Along the Asian continental margin, several Paleogene rifted

basins are recognized. Two of them are the Bohai Basin and the North

China Basin (Figure 6). The basins were pull-apart basins in association

F IGURE 4 Eocene configuration of the Japanese islands beforethe opening of the Japan Sea and before the collision in centralHokkaido and Sakhalin (modified from Kimura et al., 2014). The

locations of Eocene coal fields (C.F. in the figure) are shown.References for the coal fields are within the text

F IGURE 5 Plate configuration in the Pacific Basin at 60 Ma and 50 Ma (modified from Seton, Flament, Whittaker, Müller, & Gurnis, 2015).The locations of southwestern Japan and Hokkaido-Sakhalin before the opening of the Japan Sea are shown, along with the trench along thecontinental margin. The Kuril subduction zone was not shown in the diagram of Seton et al. (2015), but we tentatively place it on the basis of thereconstruction in Figure 4. Gray lines in the continent show the block boundaries

KIMURA ET AL. 5 of 15

with the northeast trending dextral strike-slip faults parallel to the

continental margin (Allen et al., 1998). Eocene alkaline basalts

intruded into the coal-bearing sediments suggest that an episode of

continental rifting occurred during this period. The basaltic activity

intermittently continued into the Oligocene–Miocene and is inferred

to have been related to the inception of the Japan Sea rifting

(Miyashiro, 1986; Tatsumi, Maruyama, & Nohda, 1990).

3.2 | The continental margin environment ofNortheastern Japan, Hokkaido, Sakhalin, andSikhote-Alin

Northeastern Japan, Hokkaido, Sakhalin, and Sikhote-Alin comprise

another active continental margin that was associated with oceanic

plate subduction from the Cretaceous to the Paleogene. The island of

Hokkaido formed by collision between the rocks of western and east-

ern Hokkaido, principally since the Miocene (e.g. Kimura, 1994).

Volcanic rocks in the Sikhote-Alin region of Russia were paired

with the upper Cretaceous to Paleogene accretion–subduction com-

plex in central Hokkaido to Sakhalin before the opening of the Japan

Sea (Kimura, 1994). Martynov and Khanchuk (2013) reported that

their ages were consistent with studies of paleomagnetism (Otofuji

et al., 1995). These ages range from around 80 Ma to around 50 Ma,

as do those in southwestern Japan. Cretaceous granitic rocks are also

recognized in southwestern Hokkaido and its southern extension to

the Tohoku region (Tsuchiya & Kanisawa, 1994). Okamura et al.

(1998), Tatsumi et al. (2000), and Okamura et al. (2005) suggested

that the subduction-related volcanism continued until the Oligocene,

when the rifting-related magmatism of the Japan Sea initiated,

whereas Grebennikov et al. (2016) suggested that the Paleocene–

Eocene bimodal volcanism was already related to asthenospheric

30Ma 40506070

Paleocene

(2) NE Japan, central Hokkaido, Sakhalin and Sikhote- Alin

(3)East Hokkaido and the southernmost Kuril Arc

Oligocene

(a) volcanic arc

intra-continental rift

(b) forearc basin

(c) trench wedge

(a) volcanic arc

(b) forearc basin

(c) trench wedge

(a)volcanic arc

(b)forearc basin

(c)trench wedge

Izanagi-Pacific ridge subduction

Izanagi-Pacific ridge subduction

Izanagi-Pacific ridge

Pacific Plate Motion @Hawaii

????

Hawaiian-Emperor Bend

~50-~42 Ma

Seton et al., 2015

Seton et al., 2015

acific-intermediate volcanics

EoceneCreta.

alkaline basalt-andesite basalt-andesite

basalt

bimodal magmatism

alkaline basalt

marine

marine

marine marine

marine

shelf marine

fluvial

tidal~fluvial

tidal~fluvial

accretion cease of accretion (Hokkaido, Sakhalin)

subduction resurgence (NE Japan)

accretion accretion

accretion accretion

acidic rocks

in-situ MORB

1

2

3

4

5

6

7

8

9

10

11

(12)

(1) SW Japan and continental margin

Gap

Gap

Gap

Gap

F IGURE 6 Timeline of the geological records and their gaps in (1) the continental margin of southwestern Japan, (2) northeastern Japan,Hokkaido, Sakhalin and Sikhote-Alin, and (3) eastern Hokkaido. The tectonic setting for each of these regions was (a) the volcanic arc, (b) theforearc basin, and (c) the accretionary prism of the forearc wedge. References for each setting are as follows: 1. Bohai region (Allen, Macdonald,

Xun, Vincent, & Brouet-Menzies, 1998); 2. southwestern Japan (Kinoshita, 1995; Nakajima, 1996); 3. (Aihara, 1989; Shuto, 1993); 4. ShimantoBelt (this paper); 5. Sikhote-Alin (Grebennikov, Khanchuk, Gonevchuk, & Kovalenko, 2016; Martynov & Khanchuk, 2013; Okamura, Arculus, &Martynov, 2005; Okamura, Martynov, Furuyama, & Nagao, 1998; Otofuji et al., 1995; Tatsumi, Sato, Sano, Arai, & Prikhodko, 2000); 6. (Ando,2003; Kato, Akiba, & Moriya, 1996; Komatsubara, 2004; Takashima et al., 2004; Takano & Waseda, 2003); 7. (Jahn, Usuki, Usuki, & Chung, 2014;Ueda, Kawamura, & Niida, 2000; Ueda & Miyashita, 2005); 8. (Ando, 2003; Kiminami, Shibata, & Uchiumi, 1990; Miyashita & Katsushima, 1986;Tajika, 1992); 9. (Kiminami & Kontani, 1983; Kimura & Tamaki, 1985; Morijiri & Nakagawa, 2014; Shibata, 1986); 10. (Kurita & Tajika, 1998;Naruse, 2003; Okada & Kaiho, 1992); 11. (Kiminami et al., 1990; Sakakibara, 1986, 1991). For each region, we also included the Izanagi–Pacificplates younger than ~ 80 Ma from Seton et al. (2015). The color index is the same as that used in Figure 5 and the red center of the time range isinferred to be the timing of the ridge subduction

6 of 15 KIMURA ET AL.

injection because of the opening of a slab window. Jahn et al. (2015)

suggested that the granitic rocks in Sikhote-Alin intruded mainly from

around 80 Ma to 56 Ma on the basis of U–Pb dating of zircons. This

age range is nearly the same as that in southwestern Japan.

Cretaceous to Paleogene accretionary-subduction complexes are

recognized in central Hokkaido to Sakhalin (e.g. Kiminami & Kontani,

1983; Kimura, 1994; Kiyokawa, 1992; Ueda, 2005; Ueda et al., 2000;

Ueda & Miyashita, 2005). Cretaceous accretionary complexes in Hok-

kaido are dominated by greenstone of various origins, such as sea-

mounts, normal oceanic crust, oceanic plateau material and island arcs

(Ueda & Miyashita, 2005). In contrast, the Paleocene–Eocene com-

plexes are dominated by terrigenous sediments (Kiminami & Kontani,

1983; Nanayama, 1992; Nanayama et al., 2018). The in situ eruption

of mid-oceanic ridge type basaltic magma (MORB) into the Paleogene

accretionary complex has been reported (Miyashita & Yoshida, 1994)

and it was suggested that the Kula-Pacific ridge–trench encounter

occurred during the Paleocene.

The accretionary complexes in central Hokkaido and southern

Sakhalin are intruded by granitic rocks with dates ranging from

approximately 45–46 Ma to approximately 37 Ma. The geochemical

characteristics of the granites suggest a lack of mixing of these

magmas with older crustal material (Jahn et al., 2014). The origin of

the granite is controversial, but one hypothesis suggests a relation to

the Kula-Pacific ridge encounter with the continental margin

(Maeda & Kagami, 1996).

The accretionary complexes in central Hokkaido are covered by a

thick pile of forearc basin deposits of Cretaceous to Paleogene age

(e.g. Ando, 2003; Ando & Tomosugi, 2005). A clear sedimentary gap

exists between the forearc deposits of the upper Cretaceous marine

sediments and the Eocene fluvial sediments including coal beds, from

central Hokkaido to Sakhalin (Ando & Tomosugi, 2005). The same set-

ting is also recognized in the Joban Coal Field in northeastern Japan

(Kato et al., 1996; Komatsubara, 2004). Transgression transformed

the fluvial environment to a marine setting in the Oligocene, creating

an unconformity. This transgression was in contrast to a global regres-

sive sea level change resulting from the growth of the ice cap in Ant-

arctica (Zachos, Dickens, & Zeebe, 2008) and is suggestive of tectonic

subsidence of the Asian continental margin (Takano & Waseda, 2003;

Takashima et al., 2004).

3.3 | The southernmost Kuril Arc of EasternHokkaido

Eastern Hokkaido is geologically a part of the Kuril Arc, which finally

collided with western Hokkaido during the Miocene (Kimura, 1994).

The volcanic arc and forearc basins of the late Cretaceous to Paleo-

gene are recognized in the present non-volcanic outer arc of the

Nemuro Peninsula to Shikotan Island. Alkaline basalts and andesites

are exposed and intercalated with forearc basin deposits (Kiminami &

Kontani, 1983). Eocene fluvial deposits with coal beds unconformably

overly the forearc basin, as in central Hokkaido, and again transition

to Oligocene marine deposits (Ando, 2003; Okada & Kaiho, 1992)

(Figure 6).

South of the present outer arc of the Nemuro Peninsula, in the

present forearc, an E–NE trending, magnetically anomalous area exists

(Morijiri & Nakagawa, 2014). This magnetic anomaly with an associ-

ated gravity anomaly is inferred to consist of serpentinite (Morijiri &

Nakagawa, 2014). The anomalous area changes its direction to the

northwest in the vicinity of central Hokkaido and is truncated by a

large terrane boundary fault (Kimura, 1981). There is no serpentinite

exposure on land but numerous basaltic rocks with upper Jurassic

pelagic limestones and cherts are exposed. This assemblage is inter-

preted as an accreted seamount (Sakakibara, 1986, 1991) and a ser-

pentinite body is inferred to be associated with its oceanward

extension in the present forearc. The accreted seamounts strike

northwards on land. This is a result of the clockwise rotation of the

western tip of the paleo-Kuril Arc during collision (Kimura, 1996;

Kimura, 1997). To the west of the accreted seamount, N–S striking

Paleocene turbiditic accretionary complexes that are eastward facing

with a westward vergence are exposed (Iwata & Tajika, 1992;

Kiminami et al., 1990).

4 | COMPARISON OF THE EARLYCENOZOIC GEOLOGICAL GAP ACROSSTHREE REGIONS

We now compare and contrast the observations from the three

previously-described regions, as shown in Figure 6.

4.1 | Accretionary complex

The accretionary complex in southwestern Japan ceased to develop

near the end of the Cretaceous to the earliest Paleocene, then re-

developed mainly during the middle Eocene. The complexes in central

Hokkaido and southern Sakhalin continued to grow until the middle

Eocene (Figure 6). In the southernmost Kuril Arc of eastern Hokkaido,

the Paleocene accretionary complex finally merged with that of cen-

tral Hokkaido sometime after the Oligocene as a result of the collision

with the Kuril and northeastern Japan arcs, although the detailed

timing of the collision remains obscure (Kimura, 1994).

4.2 | Forearc and intra-arc basin

The aforementioned different features relating to the development of

the accretionary prism are closely consistent with the differences

observed in the development of the forearc basin. The latest Creta-

ceous forearc basin in southwestern Japan, a narrow strike-slip pull-

apart basin of half graben (Noda, 2016; Noda & Toshimitsu, 2009;

Taira, 1988) stopped developing around the end of the Cretaceous

and new extensional basins bearing coal beds started to develop dur-

ing the Eocene in northern Kyushu. The continental margins of the

Bohai Basin and the northeastern China Basin also started to rift,

accompanied by alkaline basaltic activity (Figure 6). The previous set-

tings for these basins were the Cretaceous volcanic margins.

KIMURA ET AL. 7 of 15

In central Hokkaido, the late Cretaceous forearc basin continued to

develop before ceasing during the late Paleocene, as shown by the exis-

tence of an unconformity. During the middle Eocene, the basin formed

again and developed as a fluvial basin with accumulating coal beds.

The early Cenozoic unconformity gap in the forearc basin in east-

ern Hokkaido occurred during the late Eocene. This timing is much

later than that of the unconformities of central Hokkaido and north-

eastern and southwestern Japan (Figure 6). Continuous strata beyond

the Cretaceous–Tertiary boundary are only found in eastern Hokkaido

in the Japanese islands (Kaiho & Saito, 1986) as a result of this delay.

The forearc basin under a marine environment ceased to accumu-

late sediment, resulting in an unconformity, before sedimentation

started again. This sequence is similar to others along the Japanese

islands. However, the timing of the unconformity becomes succes-

sively younger towards the north from southwestern to northeastern

Japan and central and eastern Hokkaido (Figure 6).

4.3 | Volcanic activity

The early Cenozoic volcanism precedes the spreading of the Japan

Sea; therefore, a discussion of subduction-related magmatic activity

must include not only volcanic activity from the Japanese islands but

also from the continental margins. In southwestern Japan, late Creta-

ceous volcanic activity is well dated and is successively younger east-

ward and southward (Nakajima, 1996). The activity has been ascribed

to the subduction of the Kula-Pacific ridge (Kinoshita, 1995; Uyeda &

Miyashiro, 1974) or to the normal subduction of the oceanic plate

(Taira, 2001). After cessation of the magmatic activity during the

Paleocene, the Eocene basins started to develop with coal bed deposi-

tion and without any magmatic activity in southwestern Japan

(Figure 6). In the continent of eastern Asia, Eocene rifting with mag-

matic activity consisting of alkaline basalts occurred in the Bohai Basin

and northeastern China Basin (Allen et al., 1998) (Figure 6). The rifting

accompanied by magmatic activity is well known to have started dur-

ing the Oligocene in the Japan Sea region (e.g. Jolivet, Tamaki, &

Fournier, 1994), concurrently with intracontinental rifting and mag-

matic activity (e.g. Tatsumi et al., 1990). Recently, Eocene magmatic

activity in Sikhote-Alin has been ascribed to the Izanagi–Pacific ridge

subduction (Grebennikov et al., 2016), although Tatsumi et al. (2000)

inferred that the activity was related to normal subduction.

In northeastern Japan and westernmost Hokkaido, early Creta-

ceous and younger volcanic activity after the Miocene are clear but

no igneous rocks are paired with late Cretaceous and Paleocene–

Eocene accretionary-subduction complexes. Along the continental

margin of Sikhote-Alin, a large amount of late Cretaceous to early

Cenozoic igneous rocks are found paired with the accretion–

subduction complexes, as previously described.

5 | DISCUSSION

In order to examine the causes of the previously-described geological

gaps, recent discussions of plate tectonics in the western Pacific

region must be reviewed. The Hawaii–Emperor hot spot controversy

(e.g. Tarduno, Bunge, Sleep, & Hansen, 2009) and the tectonic inter-

pretation of mantle tomography (e.g. Honda, 2016; Liu, Zhao, Li, &

Wei, 2017; Seton et al., 2015；Wu, Lin, Flament, & Wu, 2018,

December) are important as they relate to the tectonic background of

the geological gaps present along the early Cenozoic belt boundary

thrust in the Japanese islands.

5.1 | Early Cenozoic Izanagi–Pacific ridge subductionin the western Pacific margin

Figure 5 shows the plate tectonic map of the Pacific Basin at 60 Ma

and 50 Ma by Seton et al. (2015). The map suggests that the Izanagi–

Pacific spreading ridge encountered the subduction zone in the west-

ern Pacific and was underthrust between 60 Ma and 50 Ma. The syn-

thesized works within the classic fixed hot spot reference framework

for the Pacific Basin (Engebretson, Cox, & Gordon, 1985; Woods &

Davis, 1982) proposed that the subduction of the Kula-Pacific ridge

occurred along the northern Pacific margin. Seton et al. (2015), how-

ever, separated the Kula Plate in the western Pacific from that in the

eastern Pacific and redefined the eastern section as the Izanagi Plate.

Thus, the previous hypothesis concerning the subduction of the Kula-

Pacific ridge is the same as the recently-proposed Izanagi–Pacific

ridge subduction by Seton et al. (2015) and others.

Many researchers have referred to the Kula (or Izanagi–Pacific)

ridge subduction along the western Pacific margin since the theory of

plate tectonics was established (Engebretson et al., 1985; Hilde,

Uyeda, & Kroenke, 1977; Honda, 2016; Isozaki et al., 2010; Kiminami,

Kashiwagi, & Miyashita, 1995; Kimura & Tamaki, 1985; Kinoshita,

1995; Liu et al., 2017; Maruyama et al., 1997; Maruyama & Seno,

1986; Nakajima, 1996; Seton et al., 2015; Taira, 1988; Uyeda &

Miyashiro, 1974; Whittaker et al., 2007). Among these studies,

Raimbourg et al. (2014) proposed the youngest age of around 43 Ma

for the Izanagi–Pacific ridge subduction. The age and geological evi-

dence for the ridge subduction are, however, different in each study

and ridge subduction remains an important controversy.

The geological consequences of ridge subduction were discussed

by Delong, Fox, and McDowell (1978); DeLong, Schwarz, and Ander-

son (1979), Farrar and Dixon (1993), and Thorkelson (1996). The con-

sequences were classified as (i) a thermal effect as a result of hot slab

subduction immediately before and after the ridge encounter, (ii) the

effect of magmatic and hydrothermal fluid injection due to a slab win-

dow opening and (iii) a tectonic effect resulting from the collision of

the topographic high of the buoyant ridge. The thermal effects and

magmatic/hydrothermal activity are recorded as a line of geological

evidence but the tectonic effect is inferred only from other geological

phenomena without direct evidence, such as regional uplift of the

forearc and/or a lack of accretionary prism due to a lack of sediment

supply into the trench, or subduction erosion due to the collision of

the topographic ridge. How long the effect of the ridge subduction

continued may depend on several factors, such as spreading rate, con-

vergence rate between the upper and subducting oceanic plates and

the geometry of the triple junction. The thermal effect resulting from

8 of 15 KIMURA ET AL.

the hot underthrust slab may have continued for more than

about 20 m.y., from before until after the ridge subduction. The mag-

matic and hydrothermal activity resulting from the opening of a slab

window may have been shorter than the thermal effect and may pre-

sent different chemical signals from normal subduction. For example,

the impact may have been broader beneath the upper plate and not

only limited to the volcanic arc and/or forearc near the trench. The

main tectonic effect of the ridge subduction may have been uplift

followed by subsidence in a broad forearc area, and the effect may

have been sustained for longer than a more discrete event such as a

seamount collision. Figure 7 presents the age gaps in the volcanic arc,

forearc, and accretionary prism, and these gaps may represent inte-

grated geological consequences of the effects prior to, coeval to, and

following the ridge subduction.

5.2 | Gaps in early Cenozoic geological events andtheir interpretation as related to the Izanagi–Pacificridge subduction in southwestern Japan

The volcanic activity gap, the break in forearc basin sediment deposi-

tion marked by an unconformity and the break in accretionary prism

development in the forearc are well recorded in (i) southwestern

Japan, (ii) northeastern Japan and Hokkaido-Sakhalin, and (iii) eastern

Hokkaido, as previously described (Figure 6). Figure 6 also shows the

timing of the subduction of the Izanagi–Pacific ridge and the time

range for the subduction of the hot and buoyant slab, which was

younger than ~ 20 m.y., from Seton et al. (2015).

The timing of these breaks in geological events appears to have

become later in a northward direction from region (i) to (iii). Regions

(i) and (ii) were located along the continental margin but region

(iii) was separated from them and was located along the Kuril Arc

(Figures 4 and 5).

The accretionary complex of the Shimanto Belt in southwestern

Japan extends more than 800 km (Figure 1), but the age gap between

the northern and southern sub-belts does not appear different. If this

gap is ascribed to the Izanagi–Pacific ridge subduction, such simulta-

neous phenomena suggest that the ridge may have been nearly paral-

lel to the trench as depicted in Figure 6. The oceanic plate was

buoyant as it was the thinnest dense plate, closest to the oceanic

ridges. The encounter of the oceanic ridges with the trench, therefore,

caused forearc uplift and a break in the development of accretionary

prisms, as observed at the modern Chile triple junction (Behrmann,

Lewis, & Cande, 1994). Because of this, ridge subduction can be con-

sidered as a candidate responsible for the early Cenozoic geological

gaps in southwestern Japan.

This early Cenozoic Izanagi–Pacific ridge subduction hypothesis,

however, must be discussed in relation to the previously-proposed

and much earlier Kula-Pacific ridge subduction hypothesis. Kinoshita

(1995) ascribed the late Cretaceous to Paleocene magmatic activity in

southwestern Japan to the Kula-Pacific ridge subduction. The Kula

Plate must be renamed if consensus is achieved with respect to the

redefined Izanagi Plate in the western Pacific. Kinoshita (1995), and

recently Iida et al. (2015), ascribed all the magmatism to the existence

of an open slab window. This hypothesis, however, conflicts with

observations of the accretionary complex of the Shimanto Belt. The

Shimanto Belt, especially the late Cretaceous northern sub-belt, pre-

sents the precise age of accretion and age of the subducted oceanic

plates (Taira, 1988). The age of the oceanic plate approaches the age

of accretion towards the end of the Cretaceous period but never

shows evidence of ridge subduction from ~ 80 to 90 Ma, although

Kinoshita (1995) and several authors (e.g. Kiminami, Miyashita, &

Kawabata, 1994; Kinoshita, 1995; Maruyama et al., 1997) have pro-

posed this. Onishi and Kimura (1995) and Kimura, Hashimoto,

Yamaguchi, Kitamura, and Ujiie (2016) supported the explanation

given by Taira (1988) that the age during the latest Cretaceous did

not correspond to the ridge subduction itself but subduction of young

and hot oceanic plate during the end of the Cretaceous. This explana-

tion is different from that of ridge subduction because no slab

Japan

Pacific

Pacific

trench retreat

Izanagi

Asia

Asia

~50 Ma

~35Ma

Lower mantle

Lower mantle

Upper mantle

window

rifting

rifting

Core

Core

Izanagi

F IGURE 7 Schematics of the east–west profiles of East Asia at~ 50 Ma and ~ 35 Ma. The Izanagi–Pacific ridge subduction isinferred to have occurred at ~ 55 Ma. The Eocene rifting, includingalkaline basalts in the continent, and the Oligocene rifting in the JapanSea region are shown. This cartoon was produced from the work ofSeton et al. (2015), Honda (2016) and Jolivet et al. (2018)

KIMURA ET AL. 9 of 15

window opens beneath the forearc, as in Kiminami et al. (1994). Late

Cretaceous metamorphism and exhumation under high temperature–

pressure conditions (Maruyama et al., 1997) could be the result of

young and hot oceanic plate subduction. Strictly speaking, this is dif-

ferent from ridge subduction defined by slab window opening and

asthenospheric upwelling (Thorkelson, 1996).

The Eocene alkaline basalt magmatism that accompanied rifting in

Bohai and the eastern China Sea represents interesting magmatic

activity related to the slab window of the Izanagi–Pacific ridge (Allen

et al., 1998). Alkaline basalt magmatism is a possible consequence of

the opening of a slab window; therefore, this activity needs to be fur-

ther examined in detail to confirm this hypothesis.

5.3 | Early Cenozoic event gaps and an interpretationof the Izanagi–Pacific ridge subduction in centralHokkaido, Sakhalin, and Sikhote-Alin

The Eocene ridge encounter in Hokkaido is shown more clearly than

in southwestern Japan, although the age gap in the accretionary com-

plex is not well documented because of difficult natural conditions for

field work and intensive deformation due to collisional tectonics since

the Miocene (Kimura, 1986; Kimura, Miyashita, & Miyasaka, 1983).

Studies by Miyashita and coworkers suggest that the in situ eruption

of mid-ocean ridge basalts into the Eocene accretionary prism

occurred in central Hokkaido, as previously mentioned. The same geo-

logical occurrence in Shikoku was challenged and a line of evidence

for seismogenic fluidization as an alternative to the magmatic injection

was proposed (Ujiie, Yamaguchi, Kimura, & Toh, 2007). More detailed

investigation of the ‘in situ’ occurrence of these basalts within the

Eocene accretionary prism in Hokkaido is necessary.

The unconformity between the late Cretaceous–Paleocene

forearc sediments deposited in a marine environment and the Eocene

tidal to fluvial sediments, followed by a return to marine deposits,

documents an uplift–subsidence tectonic cycle in the forearc setting.

A reconstruction of global sea level fluctuations resulting from climate

change (e.g. Zachos et al., 2008) does not match the record of the

regression and transgression in this forearc basin (Takashima et al.,

2004). A common interpretation for such rapid uplift and subsidence

in a forearc basin is the collision of a large topographic high on the

subducting oceanic plate, such as a ridge or plateau (e.g. Hamahashi

et al., 2017). Thus, the timing of the unconformity is consistent with

the subduction of the Izanagi–Pacific ridge during this period. The del-

ayed timing of the ridge–trench encounter in comparison to that in

southwestern Japan is also consistent with Seton's model (Figures 5

and 6).

Recently synthesized ages of volcanic rocks in Sikhote-Alin are

also consistent with the ridge subduction model. Grebennikov et al.

(2016) explained that the subduction-related magmatism was replaced

by rifting-related activity because of asthenospheric injection. The

timing of the rifting is the same as the occurrence of the magmatism

corresponding to the Eocene–Oligocene granite in central Hokkaido

(Jahn et al., 2015).

Tomographic images under the Asian continent, particularly to the

west of Sikhote-Alin (Liu et al., 2017; Obayashi et al., 2013) and the

interpretation of these images (Honda, 2016) are important because

the subducted slab from the trench along Sakhalin detached sometime

during the past and is now stagnant above the boundary between the

upper and lower mantle (Obayashi et al., 2013). These images are con-

sistent with the Eocene Izanagi–Pacific ridge subduction hypothesis

of Seton et al. (2015).

5.4 | A revised hypothesis: Ridge subduction, slabwindow opening and back arc spreading

During the early stage of the development of plate tectonic theory,

Uyeda and Miyashiro (1974) proposed a link between oceanic ridge

subduction, the occurrence of large amounts of magmatic activity and

resultant back arc spreading in the western Pacific. The timing of the

Japan Sea spreading was considered in the early 1970s to have been

during the Cretaceous period. Geophysical and geological data collec-

tion, however, rapidly progressed after the proposal of this hypothesis

and the results of ocean drilling programs clarified that the Japan Sea

rifting began during the Oligocene and the sea finally opened during

the middle Miocene, together with the drift of the Japanese islands

(e.g. Jolivet et al., 1994). However, the Late Cretaceous subduction

hypothesis concerning the Kula-Pacific ridge remained and was

applied to explain several geological phenomena (e.g. Isozaki et al.,

2010; Maruyama et al., 1997), together with a plate tectonic model

based on the fixed hot spot reference frame. The ridge subduction

hypothesis was, therefore, separated from the back arc spreading

kinematics.

However, as previously discussed, geological phenomena related

to the late Cretaceous ridge subduction are not as clear. The Paleo-

cene to early Eocene Izanagi–Pacific ridge subduction hypothesis

appears to explain more geological phenomena than the late Creta-

ceous hypothesis, particularly a set of phenomena including magmatic

activities in the continental margin, clear uplift/subsidence tectonic

movements in the forearc basins, extensional collapse of the accre-

tionary prisms, and the suggested injection of MORBs in Hokkaido, as

well as mantle tomographic images showing a detached slab under

the northern Asian continent. These observations are consistent with

the Izanagi–Pacific ridge subduction during the Paleocene to early

Eocene (Figure 7).

If this new hypothesis and data set for the early Cenozoic Izanagi–

Pacific ridge subduction is correct, Uyeda and Miyashiro (1974)‘s

pioneering work regarding back arc spreading should be revived.

Among the difficult questions related to this is the issue of why the

western Pacific margin was dominated during the Oligocene–Miocene

by back arc spreading. If this was a result of the preceding Izanagi–

Pacific ridge subduction during the Eocene and the oceanward retreat

of the trench and the slab of the Pacific Plate during the Oligocene–

Miocene, the space–time restriction is understandable (Figure 7).

10 of 15 KIMURA ET AL.

5.5 | Eocene reorganization of plate motion in thePacific Ocean

Another controversy in the Pacific Ocean is whether the Hawaii hot

spot has been fixed or has moved in the past (e.g. Tarduno et al.,

2003, 2009). The fixed hot spot hypothesis (Morgan, 1971; Wilson,

1963) has been the fundamental concept for the plate tectonic refer-

ence frame to define the absolute motion (Morgan, 1971) and the

past relative plate motion along the western Pacific margin was

inferred based on this reference frame (Engebretson et al., 1985; Mar-

uyama et al., 1997; Maruyama & Seno, 1986). The rapid change in rel-

ative motion between the Eurasian and Pacific plates was suggested

to have occurred at around 43 Ma in these studies, causing the bend

of the Hawaii–Emperor chain.

The timing of the bend now ranges from around 50 to 42 Ma

(O'Connor et al., 2013, 2015) and the hot spot is now inferred to have

moved over time (e.g. Tarduno et al., 2009). This age range coincides

with the start of the Eocene coal-bearing forearc basins in the conti-

nental margin, which is ascribed to subduction initiation of the Pacific

Plate after the subduction of the Izanagi–Pacific ridge, as previously

discussed. Even though the hot spot itself moved, the moving vector

of the Pacific Plate is inferred to have changed (O'Connor et al.,

2015). Thus, the Eocene tectonic events appear to be linked to each

other on a global scale.

6 | CONCLUSIONS

The compilation of the early Cenozoic geology of the Japanese islands

(southwestern Japan, northeastern Japan, Hokkaido-Sakhalin) and the

east Asia continental margin suggests that subduction of the Izanagi–

Pacific ridge occurred from the south to the north during the

Paleocene–Eocene. The central points of the compilation relate to the

ages and structural gaps within the accretionary complex, the uncon-

formity within the forearc basin deposits, and the age gap in the vol-

canic activity of the magmatic arc on the continental margin:

1. In southwestern Japan, the structural gap between the late

Cretaceous–Paleocene accretionary complex (the northern

Shimanto Belt) and the Eocene–early Miocene complex (the

southern Shimanto Belt) is clearly recognized as a tectonic line of

the terrane boundary and the age of this gap is well constrained.

There is no exposure of the Cretaceous–Cenozoic accretionary

complex on land in northeastern Japan but the northern equiva-

lent of the Shimanto Belt is traceable in central Hokkaido. The

structural gap in the accretionary complex and its age are unclear

in Hokkaido but the simultaneous eruption of mid-ocean ridge

related basalts with the occurrence of trench filling deposition has

been reported from part of the Eocene complex in central

Hokkaido.

2. The latest Cretaceous to early Paleocene forearc basin where sed-

imentation occurred in a marine environment dried up, resulting in

an unconformity, followed by a return to sedimentation. The

timing of the unconformity becomes successively younger

towards the north from southwestern Japan to northeastern

Japan and central and eastern Hokkaido.

3. Late Cretaceous subduction-related volcanic activity in southwestern

Japan ceased during the Paleocene. In the eastern Asian continent,

Eocene rifting accompanied by alkaline basalt magmatic activity

occurred. The rifting along with magmatic activity is well established

to have started during the Oligocene in the Japan Sea region.

4. The early Cenozoic geological events are explained by the subduc-

tion of the Izanagi–Pacific ridge and the subduction of a young and

hot, buoyant slab adjacent to the ridge, which migrated from south-

western Japan to northeastern Japan and then to Hokkaido during

the Paleocene–early Eocene period. The subduction of the Pacific

Plate may have started during the late Eocene–Oligocene period.

5. The timing of the Izanagi–Pacific ridge subduction inferred from

the on-land geology is consistent with the plate tectonic model

and the seismic tomography of the mantle beneath eastern Asia.

ACKNOWLEDGEMENTS

We gratefully acknowledge two anonymous reviewers for their thor-

ough reviews and constructive comments that greatly improved this

manuscript. We also thank Jonny Wu and Tim Byrne for many discus-

sions and suggestions. This research was financially supported by a

Grant-in-Aid for Scientific Research from MEXT of Japan (15H05717)

and by a Japan-France Integrated Action Program (SAKURA) Joint

Project from the Japan Society for the Promotion of Science.

ORCID

Gaku Kimura https://orcid.org/0000-0002-5049-1069

Yujin Kitamura https://orcid.org/0000-0001-8839-1189

Mari Hamahashi https://orcid.org/0000-0001-6400-9758

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How to cite this article: Kimura G, Kitamura Y, Yamaguchi A,

Kameda J, Hashimoto Y, Hamahashi M. Origin of the early

Cenozoic belt boundary thrust and Izanagi–Pacific ridge

subduction in the western Pacific margin. Island Arc. 2019;28:

e12320. https://doi.org/10.1111/iar.12320

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