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The Islaizd Arc (1996) 5, 289-320
Thematic Article
Anatomy and genesis of a subduction-related orogen:
A new view of geotectonic subdivision and
evolution of the Japanese Islands
YUKIO ISOZAKI
Department of E a r t h and Planetary Sciences, Tokyo Institute of Technology, 0-okayama,
Megur.0, Tokyo 152, Japari
The Japanese Islands represent a segment of a 450 million year old subduction-related
orogen developed along the western Pacific convergent margin. The geotectonic subdivision of
the Japanese Islands is newly revised on the basis of recent progress in the 1980s utilizing
microfossil and chronometric mapping methods for ancient accretionary complexes and their
high-P/T metamorphic equivalents. This new subdivision is based on accretion tectonics, and it
contrasts strikingly with previous schemes based on ‘geosyncline’ tectonics, continent-continent
collision-related tectonics, or terrane tectonics. Most of the geotectonic units in Japan are
composed of Late Paleozoic to Cenozoic accretionary complexes and their high-PIT metamorphic
equivalents, except for two units representing fragments of Precambrian cratons, which were
detached from mainland Asia in the Tertiary. These ancient accretionary complexes are identified
using the method of oceanic plate stratigraphy. The Japanese Islands are comprised of
12 geotectonic units, all noted in southwest Japan, five of which have along-arc equivalents in
the Ryukyus. Northeast Japan has nine of these 1 2 geotectonic units, and East Hokkaido
has three of these units. Recent field observations have shown that most of the primary
geotectonic boundaries are demarcated by low-angle faults, and sometimes modified by secondary vertical normal and/or strike-slip faults. On the basis of these new observations, the tectonic
evolution of the Japanese Islands is summarized in the following stages: (i) birth at a rifted
Yangtze continental margin at ca 750-700 Ma; (ii) tectonic inversion from passive margin to
active margin around 500 Ma; (iii) successive oceanic subduction beginning at 450 Ma and
continuing to the present time; and (iv) isolation from mainland Asia by back-arc spreading a t ca
20 Ma. In addition, a continent-continent collision occurred between the Yangtze and SinoKorean cratons a t 250 Ma during stage three. Five characteristic features of the 450 Ma
subduction-related orogen are newly recognized here: (i) step-wise (not steady-state) growth of
ancient accretionary complexes; (ii) subhorizontal piled nappe structure; (iii) tectonically
downward-younging polarity; (iv) intermittent exhumation of high-P/T metamorphosed accretionary complex; and (v) microplate-induced modification. These features suggest that the
subduction-related orogenic growth in Japan resulted from highly episodic processes. The
episodic exhumation of high-P/T units and the formation of associated granitic batholith (i.e.
formation of paired metamorphic belts) occurred approximately every 100 million years, and the
timing of such orogenic culmination apparently coincides with episodic ridge subduction beneath
Asia.
Abstract
Key words: accretionary complex, Japan, microfossil mapping, microplate modification, oceanic
plate stratigraphy, orogeny, paired metamorphic belts, ridge subduction, subhorizontal nappe,
Yangtze.
Accepted for publication April 1996
290
Y. Isoxaki
INTRODUCTION
The modern Japanese Islands geographically comprise five island arcs: the Kurile, Northeast Japan,
Izu-Bonin, Southwest Japan, and Ryukyu arcs,
showing complex patterns common in the western
Pacific. Four of these form segments of active island
arcs between the Eurasian continent and the Pacific
Ocean where the seafloor is currently subducting
westward beneath Asia; the Izu-Bonin arc is the
exception and forms an intra-oceanic arc (Fig. 1).
Ongoing subduction processes along these margins
add materials to and modify tectonic features of the
Asian continental margin. Geological studies in the
1980s revealed that the Japanese Islands evolved
under similar tectonic processes to those active
today, and that major geologic units exposed on the
islands are subduction-related products of the Late
Paleozoic to Cenozoic orogenies.
It appears that the orogenic belts in Japan have
widened oceanward by about 400 km in 450 million
years, by virtue of long-term subduction of the
Pacific seafloor along the Yangtze (South China) and
Sino-Korean (North China) continental blocks. According to the classic categorization of orogenic
belts by Dewey and Bird (1970), this 450 million
year old orogen of Japan corresponds to a typical
example of a Cordilleran-type orogen between a
converging pair of continental and oceanic plates,
that features a subduction complex, high-P/T
schists and coeval granitic batholiths. The Cordilleran-type orogen is thought to originate from
steady-state subduction of an oceanic plate beneath
a continental plate, and contrast was emphasized
between the Cordilleran-type and the Alpine-Himalayan style (or continent-continent collision-type)
orogen. Although great variation in orogenic styles
and possible plate tectonic mechanisms within the
so-called Cordilleran orogens has since been described, the distinction between the Cordilleran-type
and collision-type orogens still appears justified.
Geologic and tectonic studies in Japan during the
last two decades have clarified several new and
significant aspects of oceanic subduction-related
(or classic Cordilleran-type) orogens. New observations are highlighted in the following five characteristic tectonic features of Japan: (i) intermittent
growth of accretionary complexes; (ii) subhorizontal nappe structure; (iii) downward younging polarity; (iv) episodic formation of a tectonic sandwich
with a high-PIT unit; and (v) microplate modification. Particularly noteworthy is the episodic (nonsteady-state) growth pattern of a subductionrelated orogen without a collision of continent or
arc because it clearly contrasts with the previous
understandings on steady-state growth of the
Cordilleran-type orogen.
This article reviews the latest version of the
geotectonic subdivision of the Japanese Islands in
view of these developments and discusses its tectonic implications. The implications of this review
appear to impact also on subduction-related orogens in general. The new geotectonic subdivision is
fundamentally adopted from the summary by Isozaki and Itaya (1991) and Isozaki and Maruyama
(1991) published in Japanese, and is slightly modified to accommodate recent information. A historical review of studies of orogeny and geotectonic
subdivision in Japan is also given in the appendix.
OCEANIC PLATE STRATIGRAPHY (OPS) ANALYSIS FOR
ANCIENT ACCRETIONARY COMPLEX (AC)
Nearly 90% of the shallow-level crust of the Japanese Islands is occupied by Late Paleozoic to Mesozoic accretionary complexes and granitic batholiths. This observation suggests that the major
orogenic framework of the islands formed a t this
time, and that the geotectonic subdivision of the
Japanese supracrust depends mainly on the 3D
configuration of the accretionary complexes and
their high-PIT metamorphosed equivalents.
In this article, the term accretionary complex (AC)
is strictly used in the following sense: an AC is a
geologic entity which grows in situ in trench and
trench inner wall in an active subduction zone as a
result of subduction-driven layer-parallel shortening
and vertical stacking/thickening of trench-fill materials usually composed of oceanic sediments and underlying volcanic rocks. This definition excludes continental blocks or island arcs, even though they once
occurred in an oceanic domain prior to arrival a t the
continental margin. The term OPS is an acronym of
‘oceanic plate stratigraphy’ and this represents a
sequence of sediments and volcanic rocks accumulated primarily on an oceanic plate prior to subduction-accretion a t trench (Fig. 2). A full spectrum
of an ideal OPS is comprised of, in ascending order:
(i) MORB basalt; (ii) pelagic/hemipelagic sediments;
and (iii) trench-fill turbidites, similar to the rocks
recovered from drilling through modern trench
floors. In modern examples, almost identical OPS is
often preserved also in imbricated thrust packages
in the trench inner-wall, that is, the youngest part in
an AC. This unique stratigraphy was once called
Anatomy and evolution of Japanese Islands
/II \
291
N. American Piate
Fig. 1 Plate tectonic framework around the
present day Japanese Islands Major plate interactions are oceanic subductions along the
Nankai trough off southwest Japan and along
the Japan trench off northeast Japan The
former is an accretionary margin while the
latter is an erosional margin with a high convergence rate and rugged-surfaced oceanic
plate The Mariana trench represents another
non-accretionary margin According to the oblique subduction of the Pacific and Philippine
Sea plates southwestward moving fore-arc
slivers develop in 3 domains adding secondary across-arc structural features The Miocene
back-arc basin Japan Sea, is under destruction along the convergent plate boundary between Eurasian and North American plates
while a new back-arc basin is emerging in the
Ryukyus associated with rifting of the icontinental crust
‘plate stratigraphy’ (Berger & Winterer 1974), however, the adjective ‘oceanic’ is added later in order t o
exclude sediments derived and accumulated exclusively on continental plate. Details of the concept of
OPS and the practical example of the OPS analysis
can be found in Isozaki et al. (in press b) and Matsuda and Isozaki (1991).
It is generally easy to recognize the occurrence of
modern AC by seismic resea,rch simply because they
occur immediately next to active trenches. Ancient
examples exposed on land, on the contrary, have
usually lost contact with their primary tectonic setting nor wedge-like 3D geometry through later overprinting processes, thus their recognition requires
more specific information, not by external geometry
but by internal structure and composition. The most
reliable way to identify ancient AC, and to distinguish them from neighboring ones, is the ‘OPS analysis’ refined mainly in Japan in the 1980s.
Ancient AC exposed on land also possess OPS
which usually consists of a sequence in ascending
order of greenstones (less than several tens of
metres thick), deep-sea pelagic chert (less than
200 m thick), hemipelagic siliceous claystone (less
than 100 m thick), and terrigenous clastic rocks
such as mudstone and sandstone (more than 200 m
thick). As most AC formed by the same teconic process from deep-sea rocks and sediments, they are
rands1 lmudst
I C ~ O U S mudstone
bedded c h e r t
Fig. 2 Simplified ridge subduction system and the concept of Oceanic Plate
stratigraphy (OPS) (modified from Matsuda & lsozaki 1991) Note the age gap
between two distinct horizons (solid triangle the horizon between pillowed MORB
greenstone and pelagic chert marking the
birth of oceanic plate open triangle the
horizon between hemipelagic mudstone
and terrigenous clastics marking the arrivdl
at trench) which represents the total travol
time of the subducting oceanic plate from
mid-oceanic ridge to trench in o t h u
words the age of the subducting oceanic
plate at trench
I
demlae a t
s ~ b d u c l l o nzone
t
auctlon zone
Wedge mantle
-
birth at MOR
292
Y. Isoxaki
best distinguished by accurate age determination.
Documenting a well-dated OPS for an ancient AC
provides significant information on the timing of accretion and the age of the subducted ancient oceanic
plate. Microfossil (conodont and radiolaria) dating of
component sedimentary rocks is particularly effective in OPS analysis for ancient AC exposed on land
because it reveals (i) the accretion age a t trench
( = age of the horizon between hemipelagic sediment
and trench-fill turbidite); and (ii) an age of subducted oceanic plate responsible for the accretion
( = duration of pelagic + hemipelagic deposition)
(Fig. 2). Thus if documentation of a unique OPS is
available with precise dating, it can provide a prime
identity for each ancient AC unit that appears as a
look-alike to neighboring units.
The OPS analysis for on-land exposed ancient AC
has played the main role in the drastic change in
geological studies in Japan in the 1980s (Ichikawa et
al. 1990), and the research style of detailed field
mapping in 1:5000 scale combined with microfossil
dating is here called ‘microfossil mapping’ of ancient
AC. An interval of one microfossil zone is less than 5
million years in average for the late Paleozoic to
Mesozoic, although their resolution for dating is less
precisely controlled with few tie points in time scale.
However, it is still generally quite useful to distinguish neighboring units in the field.
In addition, ages of metamorphosed AC of the
‘grey zone’ (see Appendix) that make microfossil
dating difficult are determined radiometrically using
various methods (K-Ar, Ar-Ar, Rb-Sr, and fission
track). In some cases, an AC is accurately dated by
two ‘radio ages’ (i.e. radiolarian-based accretion age
and radiometrically dated age of subduction-related
regional metamorphism). The subduction-related regional low-grade metamorphism usually occurred
-10-20 million years later than the former (Takami
et al. 1990, 1993; Kawato et al. 1991; Isozaki in
press b), and it can add another reference feature for
comparison in OPS. This research style combining
geochronology dating and detailed field mapping is
here called ‘chronometric mapping’ of ancient AC.
Practical examples of these research styles applied to the Permian and Jurassic AC in SW Japan
are reviewed in detail in Isozaki (in press a,b) and
Kimura (1996).
in southwest Japan, the Ryukyus, northeast Japan,
and east Hokkaido that are a t present separated
physiographically and/or tectonically by secondary
transverse faults. The fundamental scheme of this
subdivision is after Isozaki and Maruyama (1991),
and is slightly modified according to the latest information. The Izu-Bonin Islands are not described in
this article, as they form a young island arc of intraoceanic nature and have lesser significance to the
primary orogenic framework along the Asian continental margin.
In southwest Japan, including Kyushu, Shikoku
and western Honshu Islands, the Cenozoic volcanosedimentary covers are thinner than those in other
domains due to the rapid uplift in the Quaternary,
and this allows extensive exposure of Late Paleozoic t o Mesozoic AC and their metamorphic equivalents. The Ryukyus and northeast Japan can be
essentially treated as lateral extension of southwest Japan, however, these domains were considerably modified and dislocated by secondary tectonism including movement of fore-arc sliver, back-arc
spreading, and arc-arc collision. Thus these two
domains will be briefly explained after southwest
Japan as its lateral equivalents. The island of
Hokkaido is characterized by a unique setting with
an arc-arc collision between the Northeast Japan
arc and Kurile arc. East Hokkaido that belongs to
Kurile arc will be mentioned separately.
In the course of explaining geotectonic subdivision, the term ‘belt’ is used in this article to describe
the distribution of geotectonic units in two dimensions. When a three-dimensional geologic entity is to
be described, non-genetic terms like unit, body,
block, complex, and nappe are used. The term ‘terrane’ is avoided here because it may be confused
with a prejudicial connotation of allochthoneity (Coney et al. 1980; Howell 1985), which has been suggested by Sengor and Dewey (1991) and Hamilton
(1991). Refer to Ichikawa et al. (1990) for more
detailed descriptions and relevant references of preCretaceous belts; and to Taira et al. (1988,1989) for
those of much younger belts. In this long description
section, readers interested in tectonics rather than
local geology of the islands may read only the description of southwest Japan and skip those of the
Ryukyus, northeast Japan, and East Hokkaido because the latter areas represent lateral equivalents
of southwest Japan.
GEOTECTONIC SUBDIVISION
The newly proposed geotectonic subdivision of the
Japanese Islands based on these methods (Figs 3,4)
is described here in four sections, based on domains
SOUTHWEST JAPAN
In southwest Japan the two-dimensional east-west
trending zonal arrangement of various geologic
Anatomy and evolution of Japanese Islands
293
Fig. 3 New geotectonic subdivision of the Japanese islands (modified from lsozaki & Maruyama 1991). The geotectonic subdivision of the Ryukyus and their
correlation are shown separately in Fig. 8. Explanatory symbols for geotectonic units used in the figure and text are as follows: Southwest Japan [Rn. Renge belt
Sn: Sangun b ; Ak: Akiyoshi b , Mz. Maizuru b ; UT- Ultra-Tanba b (included in Mz in text), M-T. Mino-Tanba b., Ry. Ryoke b ; Sb: Sanbagawa b ; Ch: Chtchibu
b (including Northern Chichibu b.; Kurosegawa belt, and Southern Chichibu belt in text); Sh; Shimanto b. (divided into Northern and Southern Shimanto belts in
text]; Northeast Japan [Ht-Tk: Hitachi-Takanuki b. ( = Hida b.); Gs. Gosaisho b. ( = Ryoke b.), M M : Matsugataira-Motai b. ( = Renge b ); SK. Southern Kitakamt
b. ( = Oki b.); NK. Northern Kitakami-Oshima b. ( = Mino-Tanba b.); Kk: Kamuikotan b. ( = Sanbagawa b.), Hdk. Hidaka b. ( = Shimanto b.), Tokoro b
( = Sanbagawa b. + Shimanto b.); Nm: Nemuro b.]. Solid black area represents ophiolitic zone
~
units is more obvious if surface cover and granitic
intrusions are removed. Southwest Japan comprises 12 distinct geotectonic units (Fig. 5); from
oldest to youngest; a 2.0 (;a-250 Ma gneiss complex; a 230 Ma intermediate-pressure type metamorphic complex; a 580-450 Ma ophiolite; a 400300 Ma high-P/T schist; a 250 Ma AC; a 230200 Ma high-P/T schist; a 180-140 Ma AC; a
120-100 Ma low-P/T metamorphic complex; a
100 Ma high-P/T schist, an 80 Ma AC; and a
40-20 Ma AC. These units are distributed in 15
belts, that is, from the Japa.n Sea side to the Pacific
side: Oki belt; Hida b.; 0-eyama b.; Renge b.;
Akiyoshi b.; Sangun b.; Maizuru ( + Ultra-Tanba)
b.; Mino-Tanba b.; Ryoke b.; Sanbagawa b.;
Northern Chichibu b.; Kurosegawa b.; Southern
Chichibu b.; Northern Shimanto b.; and Southern
Shimanto b. Repeated occurrence of the same unit
in more than two belts (i.e. outliers in the form of
klippes and/or tectonic windows as shown in
Fig. 6) in southwest Japan causes a mismatch in
the number of belts and geotectonic units. For
example, the Jurassic AC apparently occur in three
belts, that is, the Mino-Tanba b.; N. Chichibu b.;
and Southern Chichibu b., separated from each
other for up to 50 km, although these are identical
in terms of OPS.
The three belts along the Japan Sea side, that is,
the Oki, Hida and 0-eyama belts, are intimately
linked to Precambrian crusts in nature, and they
form the core of the Phanerozoic orogen in southwest Japan (Fig. 3). On the other hand, the other
12 belts surrounding the above three represent
zones of subduction-related accretionary growth
that account for the 450 million year-long widening and thickening of southwest Japan.
The Oki belt of continental affinity is a t present
isolated from mainland Asia, as it was rifted and
294
Y. Isoxaki
A
A’
Hokkaido
B’
NE Japan
/
/
SW Japan
C’
1 lOkm
h
/
100km
/
/
Granites
S Ryukyus
a
D’
D
\fib
Cretaceous and
younger ACs
J
Jurassic ACs
P
Pre-Jurassic ACs
Pacific Ocean
C’
Continent
/
/
/
Shelf sediments
Fig. 4 Across-arc geologic profiles of the Japanese islands along four transects Hokkaido NE Japan SW Japan and southern Ryukyus (modified from lsozaki &
Maruyama 1991) Abbreviations for major boundary faults are as follows (I) Thrust (bold) HMT Hidaka Main Thrust HyTZ Hayachine tectonic zone Ng-Hm TL
Nagato-Hida marginal tectonic line, I-KTL Ishigaki-Kuga tectonic line BTL Butsuzo tectonic line (11)Strike-slip faults ATL Abashiri tectonic line, TTL Tanakura
tectonic line HTL Hatagawa tectonic line H-KTL Hizurne-Kesen nurna tectonic line, M T L Median tectonic line
detached by the Miocene opening of a back-arc
basin, the Japan Sea (Jolivert et al. 1994; Otofuji
1996, this issue). Judging from the lithologic/
chronologic similarity to the Precambrian rocks of
the Sobaesan massif in South Korea, the Oki belt is
regarded as an eastern extension of the Yangtze
(South China) craton (Sohma et al. 1990; Isozaki
& Maruyama 1991) which represents one of the
continental pieces rifted apart from the supercontinent Rodinia a t 750-700 Ma (Powell et al. 1993;
Li et al. 1995).
On the other hand, the kyanite-bearing Hida
metamorphic rocks are regarded as the northeastern extension of the ultrahigh-pressure to highpressure metamorphic rocks along the QinlingDabie suture (230 Ma continent-continent collision
zone) in central China between the Yangtze and
Sino-Korean (North China) blocks (Wang et al.
1989; Maruyama et al. 1994; Cong & Wang 1995).
The unique occurrence of Middle to Late Paleozoic
shelf strata with Boreal fauna in the periphery of
the Hida belt (Igo 1990; Kato 1990) suggests a
strong link between the Hida belt and Sino-Korean
block. The present position of the Hida belt in the
central part of Japan is due to later across-arc
contraction and juxtaposition (Komatsu 1990), and
the primary contact with the Oki belt (the Yangtze
block) has been lost. Concerning the geotectonic
correlation of the Oki and Hida belts with continental blocks and later tectonic juxtaposition, refer to
Isozaki (in press a).
The 450-580 Ma ophiolite of the 0-eyama belt
along the southern margin of the Oki belt is the
oldest oceanic material in Japan. As its easterly
extension in northeast Japan has mid-Paleozoic
sedimentary cover of continental shelf facies
Anatomy and evolution of Japanese Islands
295
Fig. 5 Geotectonic subdivision of southwest Japan (modified from lsozaki & ltaya 1991) Hatched areas represent occurrence of Paleozoic AC, and solid black
areas indicate their tectonic outliers on the Pacific side ( = Kurosegawa b) The Nagato-Hida Marginal tectonic line (b) separates the continental Oki belt from the
younger Paleozoic-Mesozoic AC belts The lshigaki-Kuga tectonic line (f) divides Paleozoic AC belts and Mesozoic ones in southwest Japan Note its winding
surface trajectory indicating a low-angle nature as shown in Fig 6
(Ozawa 1988), the 0-eyama ophiolite probably
represents a remnant of the initial oceanic crust
that formed through the break-up of Rodinia a t ca
750-700 Ma. Prior to the subduction regime
started from 450 Ma a t the latest, a piece of the
initial Pacific ocean floor was likely attached to the
rifted continental margin of the Yangtze block;
that is, the Oki belt (Isoza,ki & Maruyama 1991).
The occurrence of the medium-pressure-type amphibolite in the 0-eyama belt probably suggests its
involvement in the 250 Ma collision event (Isozaki
in press a). The lithologic assemblage, age, and
other characteristics of these three units, with
continental affinity, are briefly described.
1, Hida belt (Hd); 1.1 Ga to 250 Ma mediumpressure-type metamorphic rocks and 180 Ma
granites (Sohma & Kunugiza 1993). The highest
metamorphic grade reaches the upper amphibolite
facies, only locally to the granulite facies. Protoliths of paragneiss are composed of sedimentary
rocks that most likely accumulated along the passive continental margin. These include peraluminous siliciclastic rocks and impure carbonates with
a minor amount of mafic igneous rocks. Non- to
weakly metamorphosed Middle to Late Paleozoic
shelf sequences occur fragmentally in the periphery
of the belt; 2, Oki belt (Ok): 2.0 Ga to 250 Ma
medium-pressure-type gneiss and granite complex
(Suzuki & Adachi 1994). The highest metamorphic
grade reaches the upper amphibolite to granulite
facies. Protolith is continental sedimentam rocks
with minor amount of mafic igneous rocks; 3,
0-eyama belt (Oe): 450 to 580 Ma dismembered
ophiolite composed mostly of serpentinized ultramafic rocks (lherzolitic.harzburgite),metagabbros,
and a fragment of medium-pressure-type amphibolite (Arai 1980; Kurokawa 1985).
The belts 4-15 listed below are zones of
subduction-related accretionary growth that practically account for the 450 million year-long orogenic widening and thickening of southwest Japan
(Figs 5,6) together with underlying granitic batholiths emplaced later. These AC belts including
metamorphosed equivalents were successively
added to the southeastern margin of the continent,
in particular around the Yangtze block. A summary
on OPS for these AC units in southwest Japan is
shown in Fig. 7. It is noteworthy that the timing of
accretion is generally getting younger oceanward
from the 400 Ma meta-AC to the Miocene AC, and
that the age of the subducted oceanic plate responsible for accretion has been considerably variable;
from -160 million years old for the Jurassic AC to
almost zero for the Southern Shimanto AC. For a
more detailed description of belts 5-7 and 12, refer
to Isozaki (1996a); for a description of belts 8, 11
and 13 refer to Isozaki (in press b). 4, Renge belt
(Rn): 400-300 Ma high-P/T schists and associated
serpentinite (Nishimura 1990). The highest metamorphic grade reaches the high-pressure amphibolite facies through glaucophane schist facies. The
protolith is an AC of unknown age composed of
296
Y. Isoxnki
presentj
12. Ks
-,
Pre-Jurassic complexes
of the Inner Zone
~
Tectonic Outlier
Naoato- Hida
S
N
1.
- -.
INNER ZONE
L A
OUTER ZONE --
B
Fig. 6 Geotectonic profile of southwest Japan (modified from A lsozaki & ltaya 1991, B lsozaki eta/ 1992) Profile A shows the subhorizontal nature of the piled
nappes of AC with klippes and tectonic windows on the surface Note the downward younging polarity among the AC Block diagram B is an enlargement of central
portion of profile A showing correlation of geotectonic units and boundaries across the Median Tectonic Line ( M T L ) Concerning the two-fold nature of the M T L
refer to text and Figs 17b c and 18b
greenstones, siliciclastics and chert; 5, Akiyoshi
belt (Ak): Late Permian (250 Ma) AC composed of
oceanic greenstones mostly of oceanic island basalt
(OIB) origin, chert, reef limestone, and terrigenous
clastics (Kanmera et al. 1990). This unit suffered
from low grade regional metamorphism up to the
lower greenschist facies a t 220 Ma; 6, Sangun belt
(Sn): 230-210 Ma high-PIT schists (Nishimura
1990). The highest metamorphic grade reaches the
high-pressure amphibolite through the glaucophane schist facies. The protolith is an AC,
probably containing a part of the 250 Ma Akiyoshi
AC. Neighboring schist unit with problematic 200180 Ma ages (probably secondarily annealed) are
also included here; 7, Maizuru belt (Mz): MiddleLate Permian AC with 280 Ma ophiolite (Hayasaka
1990; Ishiwatari et al. 1990). The unit sometimes
discriminated as the ‘Ultra-Tanba belt’ (Caridroit
et al. 1981; Ishiga 1990) is included here. The
ophiolite suite is dismembered but its primary
thickness is estimated to be -25 km; 8, MinoTanba belt (MT): Jurassic AC with a minor amount
of latest Triassic and earliest Cretaceous parts
(200-140 Ma) (Wakita 1988; Nakae 1993). This
unit is composed of oceanic greenstones of OIB
origin, deep-sea pelagic chert, reef limestone, and
terrigenous clastics. Secondarily mixed AC (olistostromes and melanges) occur commonly. This AC
unit is tentatively subdivided into three parts; (i)
Early Jurassic part (accreted a t 200 Ma; metamorphosed a t 170 Ma); (ii) Middle Jurassic part (accreted a t 170 Ma; metamorphosed a t 140 Ma); and
(iii) Late Jurassic part (accreted a t 140-150 Ma;
metamorphosed a t 120 Ma); 9, Ryoke belt (Ry):
120-100 Ma low-PIT metamorphic rocks and associated granites (Nakajima in press). The highest
grade part includes sillimanite-bearing gneiss. Protolith is mostly composed of Jurassic AC with
lesser amount of the pre-Jurassic AC and sediments. Age of granites ranges mostly in 120-
Anatomy and evolution of Japanese Islands 297
Fara /lon
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I
A
Fig. 7 Oceanic plate stratigraphy (OPS) of AC units in southwest Japan and age of subduction-related regional metamorphism (modified from lsozaki &
Maruyama 1991) Note the oceanward and tectonically downward younging polarity not only in accretion timing (age of the horizon between hemipelagic mudstone
and terrigenous clastics) but also in high-P-T and relevant low-grade metamorphism (shown by asterisk)
70 Ma with eastward younging polarity along the
Southwest Japan arc; 10. Sanbagawa belt (Sb):
high-PIT metamorphosed Early Cretaceous AC
(Banno & Sakai 1989; Takasu & Dallmeyer 1990),
well known as the Sanbagawa schists. The highest
grade reaches the high-pressure amphibolite facies
and radiometric ages concentrate in 100-80 Ma.
The high-PIT Sanbagawa belt and the low-P/T
Ryoke belt (9) form paired metamorphic belts
(Miyashiro 1961); 11, Northern Chichibu belt (Cn):
Latest Triassic to Middle Jurassic AC equivalent to
the older part of the Jurassic AC in the MinoTanba belt (8) (Hada & Kurimoto 1990). This unit
is regarded as forming a tectonic outlier of the
Jurassic complex of the Mino-Tanba belt; 12,
Kurosegawa belt (Kr): Fault-bounded mixture of
the pre-Jurassic elements (Yoshikura et al. 1990;
Isozaki et al. 1992). Components of abovementioned belts 3 to 7 occur chaotically as slivers,
lenses and/or blocks of various sizes and shapes,
enveloped within serpentiriite matrix. As a whole,
this unit represents a tectonic outlier of the preJurassic rocks, which occurred on the Asian continent side; 13, Southern Chichibu belt (Cs): Early
Jurassic to earliest Cretaceous AC equivalent.to the
younger part of the Jurassic AC in the Mino-Tanba
belt (8) and partly to that in the Northern Chichibu
belt (11) (Matsuoka 1992); 14, Northern Shimanto
belt (Shn): Late Cretaceous scarcely metamorphosed AC composed mostly of terrigenous elastics
with lesser amount of oceanic rocks (Taira et al.
1988). Sporadically intervened are thin tectonic
slices of melanges that include oceanic greenstones
and bedded chert within scaly argillaceous matrices; 15, Southern Shimanto belt (Shs): Paleogene
and Miocene little metamorphosed AC composed
mostly of terrigenous elastic rocks (Taira et al.
1988). A minor amount of tectonic melanges occur
in this belt.
Major geotectonic boundaries in southwest Japan
(Figs 5,6) are listed with their nature. Also noted in
parentheses a r e well-documented examples of these
boundary faults: (a) boundary between belts 1 and 2
(Hida b./Oki b.): low-angle thrust? (Unazuki suture)
activated probably in late Mesozoic; (b) boundary
between belts 2 and 3 (Oki b./O-eyama b.), and
between belts 2 and 4 (Oki b./Renge b.): low-angle
thrust (Nagato-Hida marginal tectonic line);
(c) boundary between belts 4 and 5 (Renge b./
Akiyoshi b.): low-angle thrust (Toyogadake thrust;
298
Y. Isoxaki
Kabashima et al. 1994); (d) boundary between belts
5 and 6 (Akiyoshi b./Sangun b.): low-angle thrust
(Kitayama thrust), (e) boundary between belts 6 and
7 (Sangun b./Maizuru b.): unexamined; (f) boundary between belts 7 and 8 (Maizuru b./Mino-Tanba
b.): low-angle thrust (Ishigaki-Kuga tectonic line);
(g) boundary between belts 8 and 9 (Mino-Tanba
b./Ryoke b.): high-angle normal fault (ArimaTakatsuki line, Iwakuni fault); essential contact:
gradual metamorphic aureole; (h) boundary between
belts 9 and 10 (Ryoke b./Sanbagawa b.): primary
low-angle thrust (Paleo-Median Tectonic Line activated in the Tertiary) and secondary high-angle
strike-slip fault (Neo-MTL active in Quaternary). (i)
boundary between belts 10 and 11 (Sanbagawa
b./N. Chichibu b.): low-angle thrust (Sasagatani
fault; Kawato et al. 1991); 0') boundary between
belts 11 and 12 (Northern Chichibu b./Kurosegawa
b.): low-angle thrust (Agekura thrust, Nakatsu
thrust) = oceanward extension of the boundary fault
f (Ishigaki-Kuga tectonic line); (k) boundary between belts 12 and 13 (Kurosegawa b./Southern
Chichibu b.): low-angle thrust (Kanbaradani
thrust) = oceanward extension of the boundary fault
f (Ishigaki-Kuga tectonic line). (1) boundary between belts 13 and 1 4 (S. Chichibu b./N. Shimanto
b.): low-angle thrust (Butsuzo Tectonic Line, Tsuburo thrust; Sasaki & Isozaki 1992). (m) boundary
between belts 1 4 and 15 (N. Shimanto b./S. Shimanto b.): low-angle thrust (Nobeoka thrust, Aki
tectonic line).
THERYUKYUS
On the basis of strong similarity of components,
the Ryukyu Islands are basically regarded as the
southwestern extension of southwest Japan
(Fig. 8), however, a north-south trending transverse fault clearly separates the Ryukyus from
Southwest Japan. This fault in west Kyushu Island
sharply cuts off the zonal arrangement of southwest Japan. Right-lateral off-set of the Late Cretaceous high-P/T unit in mid- to west Kyushu
suggests the boundary fault between southwest
Japan and the Ryukyus belongs to the right-lateral
strike-slip fault system as well as the Tsushima
fault and Yangshan fault in southeast Korea (Yoon
& Chough 1995), probably formed in relation to
the Miocene opening of the Japan Sea along its
western margin.
Geological information on the Ryukyu Islands is
limited by the lack of exposures but some geotectonic units are well correlated with those in southwest Japan. Most of the units in this domain are
composed of Late Paleozoic and Mesozoic AC and
their metamorphic equivalents. There is no geotectonic unit with continental affinity in this domain
except Early Paleozoic ophiolite in west Kyushu. According to dredging data, the East China Sea is underlain by Precambrian rocks that probably belong
to the Yangtze (South China) craton.
The geotectonic units hitherto known from
Ryukyus are listed below. For convenience, the
numbering and symbols that were used for units in
southwest Japan are also used to describe units in
the Ryukyus: 3, metagabbro from Nomo point:
450-580 Ma ophiolite ( = Oe) (Igi & Shibata 1979;
Nishimura 1990); 6, Tomuru metamorphic rocks:
220 Ma high-P/T schists ( = Sn) (Nishimura 1990);
8, Fusaki Formation: Jurassic AC ( = MT) (Isozaki
& Nishimura 1989); 10, Yuan Formation and
Takashima schists: Early Cretaceous AC and 6090 Ma high-P/T metamorphic equivalents ( = Sb)
(Ujiie & Hashimoto 1983); 1 4 , 15, Kunchan Group:
Cretaceous and Paleogene AC ( = Sh) (Osozawa
1984).
The following two belt boundaries have been
examined on land in Ryukyus; (f) boundary between belts 6 and 8 (Sn/MT): low-angle thrust
(Ishigaki-Kuga tectonic line); and (1) boundary between belts 10 and 1 4 (SbIShn): high-angle fault
(Butsuzo tectonic line).
NORTHEAST JAPAN
Northeast Japan comprises northeastern Honshu
and the western Hokkaido Islands, and is separated from southwest Japan by a left-lateral
strike-slip fault called Tanakura tectonic line
(T.T.L.), and from eastern Hokkaido by a northsouth trending fault in central Hokkaido, respectively (Fig. 3). Northeast Japan is also characterized by an apparent zonal arrangement of several
geotectonic units on the surface (Fig. 9). This
domain has been intensely modified by secondary
tectonism, however, in particular by left-lateral
strike-slip faults relevant to the Miocene opening
of the Japan Sea. This faulting leads to uncertainty
in the primary geometry and nature of contact
among the component units. However, a comparison with southwest Japan can help in the reconstruction of primary features of this domain. Component units of northeast Japan are listed through
comparison with those in southwest Japan, and
short comments will be added for recent advances.
As well as the Hida, Oki and 0-eyama belts in
southwest Japan, the three belts in northeast
Japan, that is, the Hitachi-Takanuki b., Southern
A n a t o m y m i d evolution of Jupanese I s l a n d s
lshigaki island
(6)
2 2 0 ~ ~.
(8)
Jurassk AC
I SSM.IS
T
/
active trench
/
Ishigaki-Kuga Tectonic Line
299
Fig. 8 Geotectonic map of the Ryukyus and their link to southwest Japan (modified from lsozaki & Nishimura 1989) Inset map ( B ) of East Asia shows potential
along-arc continuity of the Jurassic AC belt for example for -5000 km from west Philippines to Chind/Russia border Another 1000 km northerly extension IS
examined recently in the Kamtchatsuka Koryak region northeast Russia (refer to lsozaki in press b)
Kitakami b., and Miyamori-hayachine b., have
strong continental affinities, in particular to the
Yangtze craton and the collisional suture between
the Yangtze and Sino-Korean cratons. The rest are
AC units which later accreted to northeast Japan.
1, Hitachi-Takanuki belt (Ht-Tk): 250 Ma
medium-pressure metamorphics (Tagiri 1973; Hiroi
& Kishi 1989). The protoliths include Late Paleozoic sedimentary rocks accumulated on the continental shelf and volcanic rocks of bimodal characteristics ( = Hd). Granite-related thermal overprint
occurred regionally at 110 Ma which is correlated
to the Ryoke metamorphism in southwest Japan
( = Ry); 2, Southern Kitakami belt (SK): 440 Ma
granite and gneiss, 350 Ma and 250 Ma granites
( = Ok) with Middle to Late Paleozoic sedimentary
covers characterized by marine fauna of Australian
(Gondwanan) affinity (Suzuki & Adachi 1993;
Kawamura et al. 1990; Kato 1990); 3, MiyamoriHayachine belt (MH): 450 Ma ophiolite ( = Oe) with
Paleozoic sedimentary covers (Ozawa 1988; Okami
& Ehiro 1988); 4, Matsugataira-Motai belt (MM):
300-400 Ma high-PIT schists ( = Rn) (Maekawa
1981); 8, Northern Kitakami-Oshima belt (NK0s): Jurassic AC ( = MT) + Early Cretaceous AC
( = Sb) (Minoura 1990; Okami & Ehiro 1988); 9,
Gosaisho belt (Gs): 110 Ma low-PIT metamorphic
rocks and granites ( = Ry). Protoliths include components of the Jurassic AC ( = MT) (Tagiri et al.
1993); 10, Sorachi-Yezo belt (SY): Early Cretaceous AC + 100 Ma high-PIT (Kamuikotan) schists
associated with (Horokanai) ophiolite ( = Sb); 14,
300
Y. Isoxaki
Geotectonic subdivision
of Northeast Japan
Pacific O c e a n
Mino -Ta n ba b.)
Paleozoic shelf sediments
High-P/T schists
itachi -Takanuki b.
Idonnappu belt (Id): Early Cretaceous to early Late
Cretaceous AC ( = Sh); 14, 15, Hidaka belt (Hdk):
Late Cretaceous to Paleogene AC ( = Sh) partly
metamorphosed into low- to medium-pressure type
metamorphics associated with 50 Ma migmatitegranite.
Although several units are well correlated to their
counterparts in southwest Japan, some of the units
Fig. 9 Geotectonic map of northeast Japan
(modified from lsozaki & Maruyama 1991)
Names in parentheses indicate correlative units
in southwest Japan Note that the primary subhorizontal geotectonic boundaries are cut by a
series of left-lateral strike-slip faults that were
activated by the back-arc basin (Japan Sea)
opening in the Miocene
in southwest Japan are apparently missing in northeast Japan, such as Permian AC ( 5 ) , 200 Ma highP/T schists (6), and 280 Ma ophiolite (7). Their
counterparts, however, may possibly be found also
in northwest Japan in future as subsurface underlying units bounded by blind thrusts, owing to the
subhorizontal structure mentioned below. The geotectonic boundaries between these units in northeast
Anatomy and evolution of Japanese Islands
Japan are listed here: (i) boundary between belts 1
and 2 (Hitachi-Takanuki b.lSouthern Kitakami b.):
unknown; (ii) boundary between belts 2 and 3
(Southern Kitakami b./Miyamori-Hayachine b.):
vertical fault but primarily low-angle thrust (western margin of the Hayachine tectonic zone); (iii)
boundary between belts 3 and 8 (MiyamoriHayachine b./Northern Kitakami b.): vertical fault
but primarily low-angle thrust (eastern margin of
the Hayachine tectonic zone; Tazawa 1988).
The boundaries (ii) and (iii) appear almost vertical
in outcrop but the large-scale sinuous trajectory on
the surface suggests a potentially low-angle nature
in deeper levels (Fig. 10). The zone between these
two faults has been traditionally called the Hayachine tectonic zone because serpentinized ophiolite
occurs in an apparently narrow belt. The eastern
margin of this zone ( = b) corresponds to the Ishigaki-Kuga tectonic line ( f ) in southwest Japan.
(iv) boundary between belts 1 and 9 (HitachiTakanuki b./Gosaisho b.): east-dipping low-angle
fault probably activated before the intrusion of the
Cretaceous granite; (v) boundary between belts 9
and 4 (Gosaisho b./Matsugataira-Motai b.): leftlateral strike-slip fault (Hatagawa tectonic line);
(vi) boundary between belts 4 and 8 (MatsugatairaMotai b./Northern Kitakami-Oshima b.): leftlateral strike-slip fault (Futaba fault); (vii) boundary between belts 8 and 10 (Northern KitakamiOshima b./Sorachi-Yezo b ): unknown (concealed
beneath Quaternary sediments); (viii) boundary between belts 1 4 and 15 (Idonappu b./Hidaka b.):
east-dipping high-angle thrust (Hidaka Main
Thrust) on the surface that translates into a lowangle one in deeper level (Ikawa et al. 1995).
The faults (v) and (vi) are typical examples of
sinistral strike-slip faults as well as the HizumeKesen’numa tectonic line that activated during the
Miocene opening event of the Japan Sea along its
eastern margin. These faults clearly cut the primary
sinuous boundary faults such as the faults (ii) and
-SW
Japan---+l*
(iii). The fault (viii) (Hidaka Main thrusts) represents another example of the secondary modification
upon the primary structure, and this transpressional
fault has been activated probably by ongoing westward collision of the Kurile fore-arc sliver (Kimura
1985, 1996).
Due to severe secondary modifications, primary
orogenic structures in northeast Japan have not
been fully clarified, however, an analogy in rock
type, age and OPS of AC to southwest Japan suggests that a similar subhorizontal piled nappe structure also predominates in northeast Japan which
may be represented by a series of subsurface blind
thrusts (Fig. 10).
EAST HOKKAIDO
East Hokkaido has a rather complicated tectonic
history compared to the rest of the Japanese
Islands, probably reflecting its peculiar geotectonic
condition, sandwiched between two Cenozoic backarc basins, that is, the Japan Sea and Kurile basin
(Fig. 1). The domain boundary between northeast
Japan including West Hokkaido and East Hokkaido
is inferred in central Hokkaido in the name of the
Tokoro fault or Shibetu fault, however, its precise
position, geometry and nature are unknown owing
to thick Quaternary covers in between. Three geotectonic units are recognized in East Hokkaido, as
described below (Fig. 11). There is no Paleozoic
and Early Mesozoic unit in East Hokkaido, and this
implies a unique geohistory for East Hokkaido: 13,
Yubetsu belt (Yb): Cretaceous AC; 10, 1 4 , Tokoro
belt (Tk): Cretaceous AC and high-P/T metamorphic equivalents; 14, 15, Nemuro belt (Nm):
Cretaceous-Tertiary shelf sequences with unknown
basement probably composed of MesozoicCenozoic crystalline rocks of arc affinity.
These three belts are separated from each other
by north-south trending faults. The fault between
the Tokoro and Nemuro belts has a strike-slip
I
NE Japan
I
I
Hayachine T Z
I
I
Halaaawa
( 3 O e + 4 R n i 6 Sn?)
NE
..
Fig. 10 Profile of northeast Japan Note
the left-lateral strike slip faults that cut primary subhorizontal piled nappe structure
that includes inferred blind thrusts
301
10 Sb
302
Y.Isoxaki
\
\
Northeast Japan
__
-\-
ldonappu belt
14,(Shimantob.)
3
,
\
East Hokkaido
~
North
\
i
\
Sorac hi-Yezo b.
10.(Sanbagawa b.)
--+
~
Yubetsu b. (Shimanto b.) 14.
younging polarity
Northern KitakamiOshima b.
8. (Mino-Tanba b.)
\
'
V
\
\,Hidaka b?.
\\
(shimanto b.)
14, 15.
100 km
Fig. 11 Geotectonic map of Hokkaido (modified from Kimura unpubl data, 1996) Note the polarity reversal of younging direction in AC units across the Hidaka
Main Thrust which represents a collision suture between the Kurile arc to the northeast Japan arc
nature, and this fault, called the Abashiri tectonic
line, has activated in a right-lateral manner during
the opening event of the Kurile back-arc basin.
Refer to Kimura (1996) for further details.
The main problem in Hokkaido lies in how to
interpret the origin of the parallel-running two
coeval blueschists belts, that is, the Kamuikotan b.
of northeast Japan and the Tokoro belt of East
Hokkaido. The opposite younging polarity in AC
between northeast Japan and East Hokkaido
(Fig. 11) suggests that a secondary collision/
amalgamation has doubled the primarily linear
blueschists belt along the Asian margin. Several
other interpretations were also proposed but a final
agreement has not yet been reached.
GEOTECTONIC HISTORY OF THE JAPANESE ISLANDS
The time-space relationships among the orogenic
units described here suggest that the Japanese
Islands have grown oceanward by almost 400 km
across the arc since the mid-Paleozoic. On the basis
of the new geological data and the revised subdivision of the Japanese Islands, the geotectonic
history of the Japanese Islands is summarized
below. The history began with rifting of a supercontinent in the late Neoproterozoic, and was followed by a tectonic inversion shifting from an
extensional (Atlantic-type) regime to a convergent
(Pacific-type) regime around 500 Ma. Since then an
oceanic subduction-related accretion regime has
Anatomy and evolution of Japanese Islands
persisted, and is responsible for the oceanward
growth of the islands. Figure 12 summarizes this
history from the birth of the islands a t ca 700 Ma
to their separation from Asia in the Miocene.
BIRTHPLACE OF PROTO-JAPAN
The Oki belt in southwest Japan and the Southern
Kitakami belt in northeast Japan represents part of
_ -,-
303
the core of Japan, which began as a segment of the
Yangtze continental margin. In addition, a fragment of the Sino-Korean continental block also
participated in the orogenic growth of Japan after
the 250 Ma collisional event (Isozaki in press a).
These two continental blocks, particularly the
Yangtze block, have played the most important roles
in constraining the configuration of the Japanese
Phanerozoic orogen.
I
li
rifling
Asthenospheric mantle
I 1
I 1.0-07Ga
I
I
Break-up of supercontinent Rodinia
and birth of Pacific (750-700 Ma)
(b)
Y ~ J W ~ ' Passive marain sediments
Remnant of orimilive
Omanward accretionay growth
in the Paleozoic (400-250 Ma)
lCollision of YangtzeISino-Korea
Hida gneiss
(collision comDlex)
Ok
Oe Rn Ak 6n.c:
Mz V.T Sb
SIB
Onset of initial oceanic subduction
(tectonic inversion) (-500 M a )
Calc-alkaline
volcanic arc
0-eyama fore-arc ophtolite
--,
,
,
system along continental
Fig. 12 Series of simplified cartoons (a-g) showing 700 million years of geotectonic evolution of the Japanese Islands (a) The birth of Japan along a Proterozoic
rifted continental margin of Yangtze by the break-up of the supercontinent Rodinia and the birth of the Pacific Ocean at 750-700 Ma. (b) After widening of the
proto-Pacific Ocean basin, tectonic inversion occurred to initiate an intraoceanic subduction zone off the rifted Yangtze margin around 500 Ma, leaving a small fragment
of primitiveoceaniccrust (0-eyamaophiolite). (c) Thearc-trench system wasestablished by ca450 M a a n d gaveacalc-alkalineoverprint on thefore-arcophiolite. (d)
The subduction zone matured to accommodate the oldest AC which corresponds to the protoliths of the 400 Ma high-P/T Renge schists. The high-P/T Renge belt
together with coeval granite belt (fragmented) form the oldest set of paired metamorphic belts. (e) Successive oceanic subduction widened the accretionary edifice
during the Late Paleozoic (the Renge and Akiyoshi belts). On the opposite side of the Yangtze block, another continental block (Sino-Korea) collided and thrust over the
Yangtze at around 250 Ma. (f) Oceanward growth on the Pacific side continued during the Mesozolc and Cenozolc to widen and thicken the accretionary edifice in the
form of subhorizontal piled nappes. The accretionary growth was punctuated by the episodic arrival of mid-oceanic ridge roughly every 100 milion years, which
generated 3 to 4 sets of sandwich structure of high-P/T nappe between low-pressure AC units above and below (e.9. Sangun and Sanbagawa belts; see text for details).
(9) By the back-arc opening of the Japan Sea at 20 Ma, Japan detached from mainland Asia; a continental arc becamean island arc The Hida belt represents a remnant
of the 250 M a collision suture between the Yangtze and Sino-Korean blocks that partly features an ultrahigh-pressure metamorphic belt.
304
Y.Isoxuki
The relation of the Yangtze and Sino-Korean
continental margins to protoJapan goes back to
nearly 750-700 Ma when it is believed that the
supercontinent Rodinia started rifting apart (Hoffman 1991; Dalziel 1992; Powell et al. 1993). Judging from the patterns of radial dyke swarms and
rifted basins (Park et al. 1995; Bond et al. 1984),
the breakup of Rodinia was probably triggered by a
superplume rising to the surface from the mantle/
core boundary. Consequently, the proto-Pacific
ocean was born in 750-700 Ma by the rifting of
Rodinia, and several continental fragments, including the Sino-Korean and Yangtze blocks were
dispersed in various directions (Fig. 12a). Due to
similarities in Neoproterozoic to Cambrian stratigraphy, Laurentia (North America) and Australia
are probably the conjugate continental block(s) to
the rifted Yangtze margins (Li et al. 1995). However, the paleoposition of the Sino-Korean block
with respect to Rodinia remains highly enigmatic.
The birthplace of Japan was probably located somewhere in the northern periphery of Rodinia in midlatitudes a t about 700 Ma. Most of the dispersed
continental fragments once again assembled to
form another (semi-)supercontinent, Gondwanaland, about 500 Ma (Hoffman 1991; Dalziel 1992),
however, some large blocks such as Laurentia, Siberia and Baltica were isolated from the supercontinental mass. Likewise, the absence of evidence for
late Neoproterozoic to Cambrian collision in China
suggests that both the Sino-Korean and Yangtze
blocks, including proto-Japan, were also isolated
from Gondwanaland, although Early to mid-Paleozoic faunal provincialism suggests that the Yangtze
block (including proto-Japan) and Australia were
close neighbors (Burrett et al. 1990; Kato 1990).
Due to limited exposure and later tectonic modification in Japan, there is little evidence to conclude
that continental rifting, such as extensional fault
system, rift-related bimodal volcanism, and riftrelated sedimentary sequences were active a t this
time. Stratigraphical and paleontological studies
suggest that the small distribution of Early to
Middle Paleozoic (Ordovician to Devonian) terrigenous clastic/carbonate sequences in the periphery of the Hida belt, Hitachi-Takanuki belt,
Southern KitakamiIMatsugataira-Motai belt, and
the Kurosegawa belt all represent remnants of
continental shelf facies accumulated along the
rifted Proterozoic continental margins. The Korean
peninsula and/or mainland China may have preserved such features of Rodinian rifting, however,
further structural and stratigraphical analyses are
needed.
INVERSION FROM PASSIVE TO ACTIVE MARGIN
With the exception of Precambrian gneissic clasts
in younger sediments, the oldest unit of oceanic
affinity in Japan is the 580 Ma ophiolite in the
O-eyama belt, southwest Japan. Its occurrence in
the periphery of the Oki belt (ancient Yangtze
margin) suggests that this unit is a remnant of
proto-Pacific oceanic crust. The O-eyama ophiolite
and its equivalent in the Miyamori-Hayachine belt
in northeast Japan have a bimodal distribution of
radiometric ages; one a t 580 Ma and another a t
480-450 Ma (Ozawa 1988; Nishimura & Shibata
1989). Isozaki and Maruyama (1991) explained the
age distribution of the oldest ophiolite in Japan in
the following way.
The tectonic history of the O-eyama ophiolite is
two-fold: (i) following rifting with the emplacement
of nascent oceanic crust, a MORB-like oceanic
crust formed at the proto-Pacific mid-oceanic ridge
around 580 Ma (Fig. 12a); and (ii) these rocks
were then intruded around 450 Ma by calc-alkaline
volcanism of arc affinity when a new intra-oceanic
subduction was initiated (Fig. 12b,c).
O-eyama is regarded as the only example of a
fore-arc ophiolite in Japan, while other ophiolitic
rocks are regarded as accreted fragments of ancient seamounts, rises and plateaus (Isozaki et al.
1990b; Kimura & Maruyama in press). Between
580 Ma and 480 Ma, tectonics in proto-Japan
changed dramatically from riftlridge-related extension to subduction-related compression, and
passive margins changed rapidly to active margins.
This tectonic inversion probably corresponds to
global plate reorganization, in particular to the
opening of the proto-Atlantic (Iapetus) ocean on
the opposite side of the globe. Accretion of the
O-eyama ophiolite can be explained by either of the
following two mechanisms: (i) initiation of a landward dipping subduction zone within the primary
oceanic crust (see Fig. 12b,c); or (ii) collision of an
island arc system from the ocean side and a
reversal of subduction polarity.
ACCRETIONARY GROWTH OF THE JAPANESE ISLANDS
After the tectonic inversion around 500 Ma, protoJapan began a state of accretionary growth that
persists today. Within 50 million years after initiation of intra-oceanic subduction, the arc-trench
system featured AC, high-PIT schists (metamorphosed AC), and granitic batholiths (Fig. 12d,e)
that is, in the Renge belt plus Kurosegawa belt
(=tectonic outlier of the former) in southwest
Amtomy a n d evolution of Japanese Islands
Japan, and in Southern Kitakami and Matsugataira-Motai belts in northeast Japan. In particular,
the oldest AC in Japan is the protolith of the 450400 Ma high-P/T schists. The high-P/T schists and
coeval granitic rocks are elements of a paired
metamorphic belt (with a high-P/T belt on the
ocean side and a low-P/T belt on the continent
side; Miyashiro 1961).
Following the oldest 450 Ma unit, Late Paleozoic, Mesozoic and Cenozoic AC were formed
through subsequent subduction. At least several
major oceanic plates have subducted beneath the
Yangtze margin, leaving more than 10 distinct AC
belts. Numerous oceanic fragments derived from
subducted oceanic plates, including deep-sea sediments and seamount-derived basaltslreef limestone, were accreted to Japan. Details of the
accretion processes during the Permian and Jurassic periods are reported in Isozaki (in press a,b).
Accretionary growth apparently has been not
continuous. Including the youngest AC now under
construction along the Nankai trough off southwest
Japan, total accretionary growth is nealy 400 km
in across-arc width (Fig. 12f), not taking into
account the material loss by subduction-erosion.
Thus the overall AC-dominated orogen in Japan
has grown oceanward for almost 400 km during
the 450 million years (-100 km per 100 million
years).
As all of the AC units in Japan were formed in
situ by subduction along the Yangtze (South
China) continental margin, they are autochthonous
to Asia, with the exception of small oceanic fragments peeled off from subducting oceanic crust and
accreted landward into AC (Isozaki et al. 1990b).
It thus appeared that the Japanese islands do not
represent a collage of ‘suspect or exotic terranes’
that had existed prior to subduction and accretion
processes (Coney et al. 1980).
It was a mere 20 million years ago when the
Japanese Islands obtained their present configuration through back-arc spreading (Fig. 12g). However, the accretionary growth of the Japanese
Islands will likely continue until other continental
blocks, such as Australia or North America, collide
against Asia to form a future supercontinent
(Maruyama 1994).
REMNANT OF CONTINENT-CONTINENT COLLISION
While most of the geotectonic units in Japan are
the results of oceanic subduction, the Hida belt and
the Hitachi-Takanuki belt are remnants of
continent-continent collision. At about 250 Ma, the
305
Sino-Korean and Yangtze blocks collided along the
Qinling-Dabie suture, generating an ultrahighpressure metamorphic belt, best preserved in the
Dabie mountains and Shandon province in China
(Wang et al. 1989; Maruyama e t al. 1994; Cong &
Wang 1995). The region of ultrahigh-pressure
metamorphism appears to be restricted to the
central part of the 3000 km long suture, suggesting that a promontory collision may have locally
generated ultrahigh-pressure conditions (Isozaki in
press a). East of the Shandon peninsula, the coeval
250 Ma regional metamorphism is detected solely
in the Hida and Hitachi-Takanuki belts in Japan;
they are characterized by medium-pressure-type
metamorphism. Although these belts lack ultrahigh-pressure metamorphism, from a geographical
viewpoint (Fig. 13), they may also correspond to an
eastern extension of a collision-related metamorphic belt (Sohma et al. 1990), including the Imjingang or Ogchon zones in Korea (Ernst et al. 1988;
Lan et al. 1995). The Hida belt and its equivalent
occur a t the eastern terminal of the suture. Data
from this belt indicate a decrease in grade and
magnitude of a collision-related metamorphic belt.
After 250 Ma, accretionary growth in Japan radiated from a core composed of the amalgamated
Sino-Korea plus Yangtze blocks. Thus while minor
in extent, the Japanese Islands do contain elements
of a non-accretionary, continent-continent collisiontype orogen.
DISCUSSION
The newly revised geotectonic subdivision and reconstructed tectonic history of the Japanese Islands clarifies the properties of the Cordilleran-type
orogenic growth. Five tectonic features are described following these observations in Japan. They
are: (i) step-wise growth of AC; (ii) subhorizontal
nappe structure; (iii) downward younging polarity;
(iv) a tectonic sandwich of high-P/T metamorphic
units; and (v) secondary tectonic modifications.
These may represent the principal characteristics
of subduction-related orogenic belts in general.
STEP-WISE GROWTH OF AC UNITS
The 450 million-year-old subduction-related history
of the Japanese Islands is characterized by the
intermittent formation of AC, clearly detected by
OPS analysis (Fig. 7). Several intervals lack AC
(Carboniferous-Early Permian, Early-Middle Triassic, and late Early Cretaceous), and this empha-
306
Y. Isoxaki
Jurassic AC
sizes the overall zonal (piled nappe) arrangement
of AC in Japan (Figs 5,6,8-11). Secular changes in
relative plate motion between the Asian continent
(or its precursory fragment) and subducted oceanic
plates may be responsible for such episodicity
because AC cannot form from a highly oblique
subduction or in an along-arc strike-slip regime
(Maruyama & Sen0 1986).
On the other hand, extensive modern AC can be
constructed where a large amount of sediments are
supplied and the subduction rate is moderately low.
No accretion or tectonic erosion (subductionerosion) occurs when a trench is starved of sediments or the subduction rate is too high (von
Huene & Lallemand 1990; von Huene & Scholle
Fig. 13 Tectonic framework of 250 M a
East Asia showing a continent-continent collision orogen between the two Precambrian
cratons that is the Sino-Korean and
Yangtze blocks (modified from lsozaki &
Maruyama 1991) The collision suture featuring the 250 M a ultrahigh-pressure rnetamorphic belt of the Qinling-Dabie zone in
China extends eastward to the Hida belt and
Hitachi-Takanuki belt in Japan character
ized by the 250 M a medium-pressure metamorphism The pathway of the suture in the
Korean peninsula is still controversial as two
alternative interpretations are proposed, one
along the Ogchon zone and the other along
the Imjingan zone
1991). Thus the apparent episodicity in the formation of AC in Japan may also indicate the episodic
preservation of AC rather than formation. Nonetheless such episodic growth of AC is a primary
characteristic of oceanic subduction-related orogens that persist for 100 million years or longer.
The formation of high-PIT metamorphic and
coeval granitic belts also appears episodic as these
occur in highly restricted time intervals. As they
occur roughly every 100 million years, their formation may also be episodic and/or periodic.
In contrast, the classic concept of the Cordilleran-type orogeny proposed by Dewey and Bird
(1970) assumed a steady-state orogenic process,
including the simultaneous formation of subduction
Anatomy and evolution of Japanese Islands
307
complexes ( = AC), granitic batholiths, regional
metamorphic belts, and relevant geologic structures. Later publications, h.owever, emphasized the
non-steady-state nature of the arc-subduction zone
setting (Dewey 1980). Although various interpretations are given, causes for the episodicity in
subduction-related orogeny has not been well clarified. A possible cause for the episodic exhumation
of high-PIT metamorphosed AC and coeval granitic activity associated with low-P/T regional
metamorphism will be discussed later.
Subhorizontal piled nappe structures generally
characterize continent-continent collision-type orogens, like the European Alps or the foreland foldand-thrust belt in the Appalachian and Canadian
Rocky mountains. Development of similar structures in an arc-trench setting dominated by subduction tectonics is noteworthy and should be
tested in other AC-dominated orogens older than
100 million years.
SUBHORIZONTAL PILED NAPPE STRUCTURE OF AC
The along-arc zonal arrangement of these ancient
AC is most clearly demonstrated in southwest
Japan (Fig. 14a). Remarkably, these AC show an
oceanward younging polarity in map view without
windows and klippes. In addition, ages of regional
metamorphic belts and granitic batholiths also suggest an oceanward younging polarity. This apparent polarity is a function of the piled nappe structure of the AC (Figs 6,12f). Downward younging
polarity is also recognized within an individual AC
nappe; this was clearly demonstrated in the Jurassic AC, which comprises six or more subnappes
(Isozaki in press b). Such oceanward and downward younging polarity is consistent with the
growth patterns of modern AC.
Although detailed information is scarce, the
Ryukyus and northeast Japan plus East Hokkaido
also appear to display younging polarity. The
present zonal arrangement in northeast Japan,
however, does not fit with the oceanward younging
polarity in southwest Japan (Fig. 14b). This is due
t o secondary strike-slip faulting related to the
opening of the Japan Sea in the Miocene which
modified the primary orogenic configuration.
Most of the geotectonic boundaries between adjacent AC units in southwest Japan are low-angle
faults, and therefore the AC units are considered to
be piled nappes (Fig. 6). Recent microfossil/
chronometric mapping documented the occurrences
of regional klippes and windows of these nappes
even in thickly vegetated areas. These observations
indicate that the subhorizontal piled nappe structure governs the fundamental tectonic framework
of the 450 million-year-old orogen in southwest
Japan (this study: see also Hara et al. 1977;
Charvet et al. 1985; Faure 1985). Primary orogenic structures in the Ryukyus and northeast
Japan have not yet been fully mapped but a
subhorizontal piled nappe structure controlled by
subsurface blind thrusts has been predicted
(Tazawa 1988; Isozaki & Nishimura 1989;
Fig. 10).
To a first approximation, such a subhorizontal
piled nappe structure is consistent with subductionrelated deformations, particularly to the subhorizontal shortening caused by underplating through
which new materials are added to the sole of the
previously formed accretionary wedge through
step-wise activation of a subhorizontal decollement. The size of an individual ancient AC nappe is
nearly 200 km in width across the arc, similar to
the size of the widest modern AC wedge (von
Huene & Scholle 1991).
The documentation of a predominant subhorizontal structure in Japan is contrary to the traditional
view that vertical tectonics dominated horizontal
tectonics. There are, in fact, some along-arc vertical faults of strike-slip nature (Neo-M.T.L.,
T.T.L.), but most of them became active in the
Cenozoic and were driven by microplate activities.
The total amount of displacement along the vertical fault, however, is too small to account for the
present zonal arrangement of belts in Japan that
extend along the arc for more than 1000 km.
DOWNWARD YOUNGING POLARITY IN AC NAPPES
TECTONIC SANDWICH OF HIGH-P/T AC UNIT
There are three geotectonic units composed of
high-P/T schist in the Japanese Islands formed a t
around 450-300 Ma, 200 Ma, and 100 Ma. The
450-300 schists can be further subdivided into two
distinct units. All of these high-P/T units also
occur as subhorizontal nappes (Fig. 6). The most
striking feature of the high-P/T nappes is their
sandwich-like structure, where a high-PIT nappe is
tectonically interleaved between two unmetamorphosed AC nappes (Maruyama 1990; Isozaki &
Maruyama 1991). The nappes composed of highP/T metamorphosed units can be traced laterally
for more than 500 km along the arc, even though
they are usually thinner than 2 km. For example,
the 100-70 Ma Sanbagawa schists (Sb) occur as a
308
Y.Isoxaki
SW-Japan / Ryukyus
NE Japan
Sino - Korean crafon
Shn
\
\
Precambrian craton
collision zone
ophiolite / high-P/T metamorphosed AC
non- to weakly metamorphosed AC
low-PrT metamorphosed part
Fig. 14 Cartoon showing stepwise oceanward growth of southwest Japan with a high-P/T sandwich structure after removing windows and klippes (see text)
a map view and a contrasting feature in northeast Japan which was secondarily modified by strike-slip shredding lo obscure the primary structure
nappe sandwiched between the overlying Jurassic
AC (MT) and the underlying Cretaceous AC (Sh)
(Kawato et al. 1992; Sasaki & Isozaki 1992).
There are high pressure gaps between the highP/T nappe in the middle of the sandwich and the
adjacent AC nappes. These gaps, which are greater
than several kilobars in pressure, correspond to
10-20 km of crustal thickness. In order to preserve
these pressure gaps without metamorphic annealing, the tectonic insertion of the high-P/T nappe
must have been rapid and caused by tectonic
exhumation of a high-PIT nappe into a lowpressure domain. In fact, all of the high-P/T
nappes are bounded by a set of subhorizontal faults
along their top and bottom surfaces, however, the
sense of dislocation between these faults is opposite. To compensate for the pressure gap between
the hanging wall and foot wall, the upper fault is
normal while the bottom fault is reverse. Synchronous activation of such paired faults is believed to
result in the insertion of the thin high-P/T nappe
into the low-pressure domain.
These observations strongly suggest that the
exhumation of the high-PIT nappes was tectonic
in
and episodic rather than caused by steady-state
processes wherein buoyant uplift of a metamorphic
domain together with over- and underlying unmetamorphosed units was proposed. This episodic
tectonic exhumation mechanism called ‘wedge extrusion’ was first suggested by Maruyama (1990;
see also Maruyama in press). A very similar model
was also proposed for Himalayan medium-pressure
gneisses by Burchfiel et al. (1992). The downward
younging polarity among AC is not disturbed by
the intermittent intercalation of high-P/T nappes.
This suggests that subduction-driven burial of the
protolith AC, high-PIT metamorphism, and tectonic exhumation of metamorphosed AC all occurred in the same structural horizon, likely along
the Wadati-Benioff plane. The large-scale sandwich
(containing a thin well-done steak) with downward
younging polarity cannot be formed through any
other known exhumation process (Suppe 1974;
Cloos 1982; Platt 1986).
Based on radiometric ages determined by various
methods, the timing of metamorphic peak temperature and the subsequent cooling history of highP/T units in Japan (Itaya & Takasugi 1989; Nishi-
Anatomy and eTolution qf Japanese Islands
mura et al. 1989; Shibata & Nishimura 1989;
Takasu & Dallmeyer 1990), the exhumation of
high-P/T nappes occurred episodically. It is noteworthy that these episodic events apparently coincide with the formation of granitic batholith in
low-P/T metamorphic belts on the continent side
(Isozaki & Maruyama 1991; Fig. 15). The best
example of such paired metamorphic belts, described by Miyashiro (1961), is the 100-70 Ma
high-P/T Sanbagawa belt on the trench side and
the low-P/T Ryoke belt on the arc side of southwest Japan.
The formation of the Cretaceous paired belts was
probably related to subduction of the Kula/Pacific
ridge along the Asian margin. An observation on
contemporary OPS in mid-Cretaceous AC in the
Shimanto belt in southwest Japan (Taira e t al.
1988) suggested a progressive decrease in the age
of the subducted slab in the Late Cretaceous
(Fig. 16). At about 75 Ma, an AC was formed by
subduction of a brand new oceanic plate (a midoceanic ridge per se).
In addition, based on hot spot tracks, the reconstructed paleo-plate motion in the Pacific domain
over the last 150 million ,years indicates that the
Kula-Pacific mid-oceanic ridge subducted obliquely
beneath the eastern Asian continental margin
around 100-70 Ma (Engebretson et al. 1985).
Thus a TTR (trench-trench-ridge) triple junction
may have migrated from southwest to northeast
off-shore of Japan.
The phenomenon of ridge-subduction and its
tectonic effects on the continental margin have long
been discussed (Pitman & Hays 1968; Wilson
1973; Uyeda & Miyashiro 1974); however, its
tectono-thermal effects and orogenic consequences
are still controversial (Farrar & Dixon 1993; Thorkelson 1994). A recent compilation on the age of
Mesozoic granite emplacement in East Asia sug-
Fig. 15 Simplified diagram of geotectonic
profile of the Japanese Islands (modified
from lsozaki & Maruyama 1991) Note the
oceanward and tectonically downward
younging growth of AC nappes incluijing
high-P/T blueschist (BS)nappe Formation
of the sandwich structure of BS nappe occurs
intermittently roughly every 100 million
years, accompanying coeval granite batholith belt with low-P/T metamorphic belt
which is usually positioned -100-200 km
continentward from coeval BS belt The iritermittent oceanward propagation of the BS belt
and granite belt appears to have occurred
when mid-oceanic ridges episodically coltided and subducted beneath proto-Japan
gests an eastward along-arc younging polarity for
these intrusions (Nakajima et al. 1990; Kinoshita
1995). This observation is consistent with the
oblique subduction of the Kula/Pacific ridge beneath Asia as mentioned above (i.e. northeastward
passage of the TTR triple junction of the Eurasia/
Kula/Pacific plates, because the subducted ridgerelated slab window may have been a heat source).
The coincident timing of high-P/T nappe exhumation and granite belt formation suggests a causal
relationship to ridge subduction. Further research is
needed to elucidate the mechanism of the subhorizontal ejection of high-PIT nappes under a buoyant
subduction regime induced by the movement of
young oceanic crust. The preservation of high-PIT
units in the fore-arc is possible even under high heat
flow from the subducted ridge if the exhumation of
the high-PIT nappe preceded the arrival of the
ridge-crest a t the trench. The metamorphic/cooling
ages of the high-P/T unit (100-70 Ma) are slightly
older than the subduction timing of the brand new
oceanic plate (ca 75 Ma), and this may support the
interpretation provided above.
Prior to the 100-70 Ma event, exhumation of the
high-PIT nappe occurred probably three times in
the 600 million year history of the Japanese Islands
(Fig. 15). Such episodic occurrences of the highPIT sandwich structure suggests that these were
also consequences of episodic ridge-subduction in
the Paleozoic and early Mesozoic (Fig. 16). The
distribution of pre-Cretaceous granites in Japan is
highly limited; most of them are fragmented.
Nonetheless the ages of the granites that are
preserved suggest episodic formation of a granitic
belt in the intervals of 450-400 Ma, 350 Ma, and
250 Ma.
If those three pre-Cretaceous geologic episodes
are regarded as the result of episodic ridge subduction, the tectonic history of the Japanese Islands
Stepwi$e oceanward propaRation of BS belt and granite belt
b
N
High-PIT metamorphic AC (BS) nappe
Low-pressure A C iiappe
Granite intrusron
309
/
310
Y. Isoxaki
400
,
Age ( M a )
Formation of
high-PIT sandwich
(tectonic exhumation)
'engel
Age of subducted
slabdeduced from
OPS o f A C
330-280Ma
230-210~a
100-70Ya
Renge I1
Sang""
Sanbsgawa
*
~
*
1
-~
,'
,'
io
- LOO
Ridge subduction
*
I
I
I
(\
/
,
'\\ 7
,' ?
150
-- 200Mainteracted oceanic plate
0
I
*
--0
100
200
300
450-40018
w e 7
Unnamed
Farallon
?nn
$1
Izanagi
77n
can be simplified to 450 million years of oceanward
accretionary growth punctuated four times by
ridge subduction. For each event, granites and
associated low-P/T metamorphic belts formed on
the continent side of Japan, probably separated by
200 km or more from the coeval high-PIT schist
belt (Fig. 15). The nearly 100 million year periodicity in sandwich formation may correspond to the
consumption of a major oceanic plate dissected by
mid-oceanic ridges on both sides. The Japanese
Islands experienced subduction of a t least five
major oceanic plates (from young to old, Pacific,
Kula-Izanagi, Farallon, an unnamed older oceanic
plate, and the plate which originated during initial
rifting; Fig. 16).
The anatomy of the subduction-related orogen in
Japan and the tectonic interpretation of these
structures strongly contradict earlier views of
'Cordilleran-type' orogeny. The most significant
difference lies in the recognition of episodicity in
orogenic activity related to ridge subduction. Previous models for exhumation of high-P/T units
assumed a steady-state and long-term process related to subduction of the normal ocean floor. For
such an oceanic subduction-related orogenic process involving episodic culmination by ridge subduction, Isozaki and Maruyama (1 99 1) introduced
a new term, 'Miyashiro-type orogeny'. The name is
given after Akiho Miyashiro's outstanding contributions to subduction zone tectonics, particularly
the first perception of paired metamorphic belts
(Miyashiro 1961) prior to the birth of plate tectonics in the mid-l960s, and a keen perspective on
ridge subduction and relevant geologic phenomena
in the 1970s (Uyeda & Miyashiro 1974).
SECONDARY MODIFICATION: MICROPLATE TECTONICS
The primary orogenic structures of the Japanese
Islands are controlled by the spatial arrangement
,*"
Kula
$1
15
Pacificp
Fig. 16 Timetable of ridge subduction and
orogenic culmination in Japan indicated by
formation of the high-P/T sandwich structure,
age of subducted oceanic slab deduced from
OPS analysis and paleoplate interaction
based on hot spot track analysis (modified
from Isozaki & Maruyama 1991) The names
for oceanic plates that interacted with Japan
are adopted from the reconstructed plate interaction in the Pacific domain by Engebretson e t a / (1985) Note the temporary coincidence among the zero age of subducted
oceanic plate at 75 M a exhumation of the
high-P/T Sanbagawa metamorphic belts, and
passage of KulaiPacific ridge off Japan
of main components (i.e. ancient AC, regional
metamorphic rocks and granitic batholiths).
Subduction-related orogens in Japan are characterized by subhorizontal piled nappes of AC with
downward younging polarity, and by the episodic
occurrence of a sandwich structure of high-PIT
nappes and unmetamorphosed AC. The primary
orogenic features, however, were modified or destroyed in some cases by secondary tectonic processes. In particular, microplate-related tectonics
has the profound ability to secondarily reorganize
primary structures (Miyashiro 1982). In Japan,
there are three important secondary tectonic processes (Fig. 17; fore-arc sliver movement, back-arc
basin opening, and arc-arc collision) that are all
orogenic manifestations of microplate activities
(Isozaki 1989; Isozaki & Maruyama 1991).
A fore-arc sliver is a decoupled microplate of a
frontal arc driven by a strike-slip component of the
oblique subduction of an oceanic plate. In the
Japanese Islands, there are three active fore-arc
slivers (the South Ryukyu sliver, Nankai sliver in
southwest Japan, and Kurile sliver in East Hokkaido; Fig. 1). Along-arc movement of fore-arc
slivers can create three distinct features that destroy the primary orogenic edifice (i.e. along-arc
strike-slip fault on are-side margin, and across-arc
compressional structure in front, plus an extensional one in rear of the sliver). The best example
of an along-arc strike-slip fault is the Quaternary
Neo-M.T.L. in the Kii peninsula and on Shikoku
Island that demonstrates a remarkably linear surface trajectory for more than 500 km (Figs 3,5).
This right-lateral strike-slip fault was driven by
the westward movement of the Nankai fore-arc
sliver (Fig. 17b), cutting the older low-angle feature of the Tertiary paleo-M.T.L. (Isozaki 1989;
Yamakita et al. 1995; Fig. 6). This westward sliver
translation is also associated with across-arc compression along the Bungo Strait between Shikoku
Anatomy and evolution of Japanese Islands
311
(b) Fore-arc sliver
(a) Arc-arc collisian
Philippine Sea Plate
A\
(c) Opening of back-arc basin
I’
Fig. 17 Three representative modes of secondary modification of primary orogenic structure related to microplate tectonics (modified from lsozaki & Maruyama
1991). (a) Formation of syntaxis in the older accretionary orogenic system and accretion of exotic arc crust occur at an arc-arc collision front around the Izu
peninsula where the Izu arc penetrates almost perpendicularly to the Southwest Japan arc (b) Formation of along-arc strike-slip fault (Neo-M T.L ) and across-arc
extensional and compressional structure by along-arc movement of the Nanakai fore-arc sliver (c) Formation of along-arc extensional structure (Miocene rifted
basins with bimodal volcanism), compressional structures (thrusting along paleo-M.T L ) and the strike-slip fault (T T.L.) occurred when the Japan segment
detached from mainland Asia through the rifting-opening of the Japan Sea. Note these Neogene to Quaternary structures have modified considerably the pre-existing
major structures of the ca 450 Ma accretionary orogen
and Kyushu Islands, and across-arc extension in
Ise Bay. Across-arc compression related to a forearc sliver is best observed in the elevated Hidaka
mountains in central HoIkkaido. Kimura (1985)
explained the exposure of lower crustal rocks of
the Hidaka belt (Komatsu et al. 1989) as a tectonic
manifestation of westward frontal collision of the
Kurile fore-arc sliver to northeast Japan. Seismic
reflection research (Ikawa et al. 1995) recently
documented the crust-cutting detachment surface
that dips eastward from the Hidaka main thrust
(Fig. 11).
Back-arc spreading is another tectonic process
that has occurred frequently in the western Pacific
since the latest Mesozoic, and is also a powerful
modifier of primary orogenic structures in Japan.
For example, when the Japan Sea opened in the
Miocene, it split pre-existing orogenic structures
into several blocks (Figs 9,10,14). Although a cer-
tain amount of rotation was involved, the opening
of the Japan Sea basin has been attributed to
dislocation of a pair of north-south running strikeslip faults along the basin margin (Fig. 17c). The
chaotic alignment of belts in northeast Japan is
primarily due to the series of left-lateral strikeslips along the eastern margin of the Japan Sea.
The T.T.L. presently dividing southwest Japan and
northeast Japan, and the parallel Futaba and Hatagawa faults are typical examples of an eastern
margin strike-slip fault that obliquely dissects an
older along-arc zonal arrangement. In contrast, a
segment of the right-lateral strike-slip fault marking the western margin in east Korea was recently
noted (Yoon & Chough 1995). In addition, a domain of compression in the fore-arc may have been
associated with back-arc spreading (Fig. 17c). It is
difficult to explain the side-by-side juxtaposition of
the low-P/T Ryoke metamorphic belt and high-PIT
312
Y. Isoxaki
Sanbagawa metamorphic belt in modern southwest
Japan (Fig. 5 ) . These belts should have been separated from each other by a t least 100-200 km
when they formed in the Late Cretaceous arctrench system (Fig. 18a). The origin of the lowangle Paleo-M.T.L. between these two belts
(Fig. 6) may be related to development of a horizontal detachment surface in the arc crust and to
trenchward dislocation of the upper crust along
6
this surface, probably by back-arc spreading
(Isozaki & Maruyama 1991; Fig. 18b). For further
details on the opening of the Japan Sea, refer to
Jolivert et al. (1994), Otofuji (1996) and Yamashita et al. (1996). The present-day opening of the
Okinawa trough (Kimura et al. 1988) is still in a
nascent stage of back-arc spreading (Fig. 1). This
extensional regime appears to propagate northward into mid-Kyushu (note the Beppu-Shimabara
a
?
A
B
Akryorhr-Sangun
c
usually 100-200 k m
pre-Juraralc
ca 5 0 k m
(100-701
___b.
C
posslble
external llmlt o f
volc. tront
trench
S
-
-
h i g h - P / T Sanbagawa met.
Mlno-lanba
J u r a r a l c complex
100 k m
subducted
Kula-Paclflc
rldge
Orlglnal conflguratlon of the Cretaceous
palred metamorphlc belta in SW Japan
p i~
1
- n'B'
1
= Intra-arc shortenlng
130-40 M a ]
fore-arc contraction
I
= dlslocatlon 01 delslchmenl Iault
A ' , B'
100 Ma voic
volcanlc
back -arc
[rant
C'
front
100 Ma trench
Paleo-MTL
trench
Shlmanto
Palaogana complex
extension
mlsalnp volume of the R y o k c fore-arc rcplon
= accreted volume 0 1 the Shlmanto complex
Secondary juxtapositlon of
hlgh-P/T and low-P/T motamorphlc belts
the primary configuration of the Cretaceous arc-trench system in southwest Japan (a) and secondary modification
shortening associated with back-arc (Japan Sea) spreading (b) Note the juxtaposition of the Cretaceous paired metamorphic belts induced by
the trenchward translation of upper arc-crust along an intracrustal horizontal detachment surface
Fig. 18
Schematic cross-section showing
involving intra-arc
Anatomy and evolution of Japanese Islands
graben with active Aso and Unzen volcanoes),
accordingly, Kyushu Island, now composed of several geotectonic units, may eventually split into
two.
Arc-arc collisions are the third process that can
modify the primary geotectonic structures in Japan. The ongoing collision of the Izu-Bonin arc
against the southwest Japan arc is a typical example of this process (Fig. 17a). The northward buoyant subduction of an intra-oceanic arc can indent
into a pre-existing orogenic structure, leaving a
clear V-shaped mark called ‘orogenic syntaxis’, in
the west of Tokyo (Fig. 4). In addition, the accretion of arc crust basement has been achieved in a
step-wise manner, adding a significant volume in
central Japan (Taira et al. 1989). Several tectonic
blocks of intra-oceanic arc origin around Mt. Fuji
(Amano 1986) thus represent bona fide ‘allochthonous or exotic terranes’ in Japan.
CONCLUSION
The Japanese Islands represent a segment of a
Phanerozoic subduction-related orogen developed
along the western margin of the Pacific Ocean.
Significantly, this orogen has grown oceanward
nearly 400 km during the last 450 million years.
The anatomy of the Japanese orogen, best preserved in southwest Japan, is characterized by a
subhorizontal piled nappe structure, which involves
multiple AC nappes including those of high-P/T
metamorphosed AC. Orogenic growth is unusual in
that it includes: (i) step-wise growth of AC units;
(ii) tectonically downward younging polarity; and
(iii) intermittent sandwich structure of high-PIT
nappes. Episodic subduction of mid-oceanic ridges
( =migration of TTR triple junctions) appears to
explain the episodic exhumation of high-P/T
nappes and the formation of granite/low-P/T
metamorphic belts every 100 million years. Microplate tectonic processes such as the movement of
fore-arc slivers, back-arc spreading and arc-arc
collisions, secondarily modified the primary structure of Japan.
In order to establish a general model for
subduction-related orogenic processes, the new
geotectonic model for the origin of Japan needs to
be tested on other orogens formed in similar tectonic environments. For example, the occurrence of
a subduction-related orogen characterized by a
similar anatomy would be expected in CircumPacific Phanerozoic orogens in western North
America, western South America, Western Antarc-
313
tica, and eastern Australia, as the Paleo-Pacific
ocean floor was subducted along these continental
margins after the 750-700 Ma breakup of Rodinia.
Reconnaissance studies (Sedlock & Isozaki 1990;
Isozaki et al. 1992; Isozaki & Blake 1994) including a compilation of previous works for the
Klamath-Franciscan belt in California (Isozaki &
Maruyama 1992; Maruyama et al. 1992) suggests
fundamental similarities between the geotectonic
evolution of California and that of Japan. It appears promising to extrapolate the above-described
new aspects and analyzing schemes for ancient AC
to other orogenic belts of different time-space
co-ordinates in the Earth’s history, including the
Precambrian.
ACKNOWLEDGEMENTS
The author would like to thank S. Maruyama, Y.
Nishimura, T. Itaya, T. Matsuda and many students from Yamaguchi University for a decadelong discussion in various tectonic aspects of the
Japanese Islands. Sincere thanks are also due to
A. J. Kaufman, S. Maruyama, G. Kimura, and
L. Ivany who constructively reviewed the manuscript. P. F. Hoffman gave valuable comments on
the neo Proterozoic distribution of continental
blocks.
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APPENDIX: HISTORICAL REVIEW ON STUDIES OF
OROGENY AND GEOTECTONIC SUBDIVISION OF THE
JAPANESE ISLANDS
Japanese Islands can be roughly divided into four
stages, based on the orogenic concept that governed the understanding of contemporary geologists; these are: (i) the pre-geosyncline concept
stage (1865-1940); (ii) the stage of importing the
geosyncline concept (1941-1955); ( 5 ) the stage
of popularization of geosyncline-based orogeny
(1956-1975); and (iv) the stage of plate tectonicsbased orogeny (1976-present).
The first stage is represented by activities of E.
Naumann, B. Lyman and other foreign geologists
who imported modern geology as well as other
sciences to Japan immediately after the Meiji revolution in 1868, after two century-long diplomatic
isolation. During this stage, foreign and domestic
geologists recognized fundamental structures of
the Islands, particularly major tectonic boundaries
such as the Median Tectonic Line (M.T.L.) and
Itoigawa-Shizuoka Tectonic Line (I.-S. T.L.); how-
In order to establish a geotectonic subdivision for a
particular region, conventional field mapping on a
regional scale is essential, and this usually requires
painstaking work by numerous geologists over an
extended period of time, particularly in thickly
vegetated regions like the Japanese Islands that
are a t a humid mid-latitude. In the 100-year history of geological research in Japan, many efforts
have been made to synthesize all available geological information into an overall tectonic framework.
Although later criticized and discarded, several
outstanding geotectonic syntheses were compiled,
and these all came together with the revised summary of the geotectonic subdivision of the Japanese
Islands. The history of geological studies of the
Anatomy and evolution of Japanese Islands
ever, the overall geotectonic subdivision of the
Islands was still a rough sketch. In hindsight, this
era up to the 1930s in Japan may be called a time
of fundamental ‘find-and-describe’ in preparation
for the following stage of importing the concept of
the geosyncline.
During the second stage, T. Kobayashi was the
first Japanese geologist to synthesize a grand view
of the geotectonic evolutim of the Islands on the
basis of the stratigraphic and megafossil age data
(Kobayashi 1941). He recognized most of the important geotectonic units currently known, but his
subdivision (Fig. 19a) was strongly biased by the
geosyncline concept. In particular, he emphasized
Sakawa Orogen
Akiyoshi Orogen
~~~~~~
MTL
Opa Nappa
:b)
IctoScalel
l c h i k a w a e r a / 1970
N
___
Shimanto Orogen
Honrhu Orogcn
Cp
Mz
MT
Ry
Sb Ch
~-
Hdk
S
Mr
km
MTL
.
[i
30
, 5 0 km,
,
Fig. 19 Classic examples of geotectonic map and profile of the Japanese
Islands (modified from (a): Kobayashi 1941, (b): lchikawa et a/. 1970).
Compare these classic geotectonic subdivisions and profiles backgrounded by
geosynclinal viewpoint with the current version shown in Figs 3,4 Before
1975, major geotectonic units in Japaii were all explained as Precambrian
sialic basement and overlying Paleozoic to Mesozoic geosynclinal sediments.
Hd. Hida b , Ok: Oki b . , Cg: Chugoku b., Mz, Maizuru b., MT: Mino-Tanba b.;
Ry: Ryoke b.; Sb: Sanbagawa b.; Ch: Chichibu b.; Hdk. Hidakagawa b. ( = Sh:
Shimanto b.); Mr. Muro b.( = NK. Nakamura b.); Km: Kitakami marginal b.;
KI: Kitakami b Sm. Soma b.; Ab: Abukuma b., As: Ashio b.; Jo: Joetsu b.
~
319
the importance of distinguishing orogenic phase in
H. Stille’s sense. His summary was the first application of the then world-popular geosyncline concept to a terra incognita named Japan. His
achievement, however, marks a prime milestone in
geological studies in Japan and his subdivision
provided a foundation for later works. One point to
note is his emphasis on nappe structures throughout southwest Japan, as this idea was revived in
the 1990s in the plate tectonic framework.
The third stage was governed by geologists of
the post-World War I1 generation who strongly
criticized Kobayashi’s summary. They emphasized
the higher resolution and accuracy of their data set
through detailed mapping and the advantage of
their new tectonic models (Yamashita 1957: Minato
et al. 1965: Ichikawa et al. 1970), however, their
perspective was more or less the same as that of
Kobayashi, that is, popularization of the classic
geosynclinal concept with minor modification in the
context of regional geology. Although the world
according to fixism was still there, the quality of
the regional geological description of the Japanese
Islands improved much during this stage, with the
use of megafossil dating, particularly fusulinid
biostratigraphy. Regional distribution of geotectonic units and locations of mutual boundaries were
clarified, and the geotectonic subdivision made in
this stage appears very similar to the present one
in map view except for ‘vertical ’ major boundary
faults (Fig. 19b). Accordingly, sporadically exposed
older mid-Paleozoic rocks, including high-grade
metamorphic rocks and serpentinites, were all explained as geosynclinal basement rocks squeezed
out from deeper levels along putative ‘crustpenetrating vertical faults’. In the early 1970s,
several avant-garde geologists from Japan were
the first to propose plate tectonic interpretations
for the evolution of the Japanese Islands, with
special emphasis on subduction-related tectonics
(Matsuda & Uyeda 1971: Uyeda & Miyashiro
1974). The majority of scientists, however, still
held conservative understandings linked to the geosyncline concept, and the geotectonic subdivision
was not revised from the plate tectonic viewpoint.
The most significant reform in the geotectonic
subdivision occurred in the fourth stage. Nearly a
decade after the fundamental construction of plate
tectonics in the late 1960s, younger Japanese
geologists started to prefer mobilism rather than
fixism. Down a long and winding road with many
arguments, including the conversion from a geosyncline world to a plate tectonics world, both in
individuals and in society (Kanmera 1976), re-
320
Y. Isoxaki
gional geologic information was constantly accumulated on a nationwide basis. The great reform
came in two waves, the first one in the early 1980s
and the second in the early 1990s.
The first wave was brought by two factors combined: (i) the remarkable enhancement in microfossil (conodont and radiolaria) biostratigraphy, in
particular the high resolution dating of the preCretaceous or ‘so-called Paleozoic eugeosynclinal
sedimentary rocks’ in Japan (Koike 1979; Isozaki
& Matsuda 1980; Tanaka 1980; Yao et al. 1980,
Yamato-Omine Research Group 1981; Nakaseko et
al. 1982); and (ii) the acceptance of a subductionaccretion concept (Kanmera 1980; Taira et al.
1983) which was constructed mainly through deepsea drilling and regional seismic profiling on modern AC. As a result, most of the ‘so-called Paleozoic eugeosynclinal rocks’ in Japan were revealed
to be Jurassic AC by the mid-1980s. Age of
accretion for each AC unit was precisely dated, and
this enabled mutual discrimination of neighboring
and similar-looking Paleozoic to Cenozoic AC in
Japan. This reform initiated a considerable redrafting of the geotectonic history of the Islands in
terms of accretion tectonics, as well as their geotectonic subdivision. Utility of microfossil dating
for ancient AC and the relevant results in geotectonic subdivision of Japan up to the late 1980s are
summarized in Ichikawa et al. (1990).
On the other hand, concerning tectonic interpretation, the ‘allochthonous or suspect terrane’
concept (Jones et al. 1977; Coney et al. 1980;
Howell 1985) invaded Japan in the early 1980s
and allowed many geologists in Japan to believe
the occurrence of exotic continental and/or oceanic
blocks and to use the term ‘terrane’ to describe
various orogenic units (Saito & Hashimoto 1982;
Mizutani 1987; Ichikawa et al. 1990). On the
geotectonic subdivision, the ‘terrane’-based understanding of the attitude of boundary faults
between the orogenic units is of note because most
of the ‘terrane boundaries’ were regarded as
vertical faults of a strike-slip nature related to
‘terrane dispersion’ (Taira et al. 1983). The nappe
tectonics, on the contrary, was revived also in
the early 198Os, and its significance with subhorizontal boundary faults was emphasized by French
and domestic geologists (Hara et al. 1977;
Yamato-Omine Research Group 1981; Faure 1985;
Charvet et al. 1985; Hayasaka 1987). French
geologists, in particular, interpreted the napperelated subhorizontal structure as a result of an
ancient continent-microcontinent collision, however, evidence for the putative collided micro-
continent per se was not persuasive. Independent
from these works, relative plate motions were
partly reconstructed for the late Mesozoic and
Cenozoic Pacific region by Engebretson et al.
(1985), and the correlation between the plate
interactions in East Asia and orogenic events recorded in Japan was first discussed by Maruyama
and Sen0 (1986). Owing to the mixed effects of
these various interpretations and existence of problematic ‘gray zones’ mentioned below, understanding of the 3D structure and tectonic evolution of
the Japanese orogen was in a state of confusion in
the 1980s.
The second wave in the early 1990s that provided the final tool for redrawing the subdivision of
the Islands came in the form of success in chronometric dating of weakly metamorphosed AC. AC
well-recrystallized by regional metamorphism, such
as the Sanbagawa blueschists, had already been
dated by radiometric methods in the 1970s, however, less recrystallized metamorphic AC had not
been dated a t all owing to difficulty in mineral
separation techniques. Similarly, while microfossil
analysis is powerful in dating non- to weakly
metamorphosed AC, those metamorphosed to the
greenschists facies were mostly left untouched
owing to difficulty in microfossil extraction. Such a
dilemma had left a considerable number of undated
weakly metamorphosed AC units, the ‘gray zones’,
in Japan even after the 198Os, and this delayed the
completion of the geotectonic subdivision of the
Japanese Islands. In the late 1980s, an advanced
technique of fine-grained mineral separation was
developed (Nishimura et al. 1989) that solved the
‘gray zone’ problem (Isozaki et al. 1990a, 1992;
Isozaki & Itaya 1991; Suzuki et al. 1990; Takami
et al. 1990; Kawato et al. 1991). Not only did this
permit the dating of the greenschist facies AC
rocks, but it allowed subdivision of high-PIT regional metamorphosed units on the same basis as
non- to weakly metamorphosed ones (Isozaki &
Maruyama 1991). By 1993, most of the important
geotectonic boundaries were clearly defined by age
difference and were re-examined in the field. This
clarified that these boundary faults, that is, the
nappes, are essentially subhorizontal, except for
several vertical ones of secondary origin. In other
words, there is no deep crust-cutting vertical fault
zones nor a collage of exotic terranes in Japan. The
predominance in subhorizontal structures of the
Japanese Islands are currently accepted by many,
and are utilized, for example, in the latest version
of a geologic map of the islands issued by the
Geological Survey of Japan (1992).