The pre-Cretaceous structure of the outer belt of southwest Japan

Tecronophysics,

B.V., Amsterdam

THE PRE-CRETACEOUS SOUTHWEST

MICHEL

139

113 (1985) 139-162

Elsevier Science Publishers

- Printed

STRUCTURE

in The Netherlands

OF THE OUTER BELT OF

JAPAN

FAURE

Department

of Earth Scrences, Kyoiku Gakubu, Tokushima University,

770 Tokushima (Japan); L.A. 215

C.N. R.S., Universite de Paris, Paris (France); and D~partemenr des Sciences de la Terre, Universitti O&an& OrlPans (France) (Received

April 5, 1984; revised version accepted

September

7, 1984)

ABSTRACT

Faure,

M., 1985. The pre-Cretaceous

structure

of the outer belt of southwest

Japan.

Tectonophysics,

113:

139-162. Based on a study of the island of Shikoku, Japan

is proposed.

of a middle

Jurassic

Jurassic-early including

From top to bottom

reworked

to east, in ductile sedimentary

olistostrome,

Cretaceous

(second

ophiolitic

thrusted phase);

and synmetamorphic strike-slip

conditions

continent

Cretaceous

turbiditic

as the southern

The tangential Green

sequence

structure

Schist nappe

is explained

for the abduction

subparallel

to the belt.

The new zonation Hokkaido

zone. During

the Paleozoic

nappe.

as the Mesozoic outcrops

phase,

owing to

the Green

Schist

zone is a late Jurassic-early

rocks and Triassic

of an oceanic

by the continental sequences. Japan,

presently

the second

the late

Schist

(first phase) from west

interpreted

the Sanbosan

Kurosegawa

during

the Green

sediments.

It is

continent.

by the subduction

of the oceanic

formations

This basement

chain of

nappe composed

conditions

schists:

in the late Jurassic

nappes. Southwards,

is valid for all southwest

in northeast

in superficial

upon detrital

of Kurosegawa.

followed

of: (1) a superficial

of HP/MT

thrusted

of the Kurosegawa

were deposited,

responsible

western

reworking

margin

of the Outer belt of the Mesozoic

consists

the north

faults in the Kurosegawa

nappe was sliced into two post-metamorphic considered

from

(2) a nappe

debris and radiolarites,

cover of the Paleozoic

post-Cretaceous

a new zonation

the nappe structure

area where the materials

subduction

The main component and also extends

of the Kurosegawa of the displacement

up to the Kitakami

of the mass, was

massif and

Japan.

INTRODUCTION

The present Japanese islands are often considered as an island arc, but this present structure is overprinted upon a polyorogenic substratum. Southwest Japan (Fig. 1) is the 1000 km long segment running from Kyushu to the Kanto mountains, and separated from northeast Japan by the Tanakura fault. Three erogenic cycles are responsible for the pre-Miocene structures, i.e.: (1) a late Paleozoic cycle or 0040-1951/85/$03.30

0 1985 Elsevier Science Publishers

B.V.

140

Akiyoshi

cycle, (Kobayashi,

1941), unconformably

sediments:

(2) a Jurassic

Paleogene

cycle, or Shimanto

The Mesozoic cycle,

Sakawa

(the so-called

so-called

covered

covered by Triassic

by a Neocomian

and

includes

the early

since the Neocomian

Indeed erogenic

movements

deposits

were considered

in southwest

en Schists urosegawa-Sanbosan

zones are included.

of the Outer

cycle (the as synoro-

Japan should be seen as a

are not relevant to the same stress field, the lower Cretaceous taken as the mark of the completion of the Jurassic cycle.

situation

(3) a

the late stage of the Jurassic

one of the Shimanto

continuum from the Paleozoic to Present; however. as the Neocomian seals the Jurassic tangential contacts, and the Jurassic and Paleogene

Fig. 1. General

shallow water

unconformity:

orogeny.

cycle of Kobayashi

Oga phase),

Sakawa phase),

genie sediments.

cycle

unconformity deformations

unconformity

will be

-Ultra

Kurosegawa

Nappe Z.

belt of southwest

200 km

Hapan.

The Kurosegawa-

141

Japan is also divided longitudinally Line

(MTL).

crosscutting motion

In its present the Mesozoic

is known

The MTL parts

by a big strike-slip

outcropping

metamorphic

it is a Quaternary isograds,

belt on the Japan

Pacific Ocean

side. The Outer belt is divided

(e.g., Kimura,

1973; Tanaka

Sanbagawa,

Mikabu,

vided into a northern zone. The boundary Butsuzo

Tectonic

and Nozawa,

Chichibu

early

due to findings

(Ichikawa,

north

zones. The Chichibu

zone, central or Kurosegawa between the Sanbosan and

1980).

belt on the strips

to south:

the

zone is subdi-

zone and southern or Sanbosan Shimanto zones is a fault: the are generally

the tangential in the study of conodonts

fault

strike-slip

into several parallel

namely-from

Line (BTL). The zone boundaries

aspects,

Cretaceous

classically

1977)

Tectonic

right-lateral left-lateral

Sea side and the Outer

and Shimanto

and when the zonation was determined Since 1980 a revolution has occurred stratigraphic

but

to have taken place since the middle the Inner

fault: the Median

subvertical

faults

tectonics were not considered. of the geology of Japan; on

and radiolarians,

and on struct-

20 km

Fig. 2. Structural

map of Shikoku. a, b-Green

b = epimetamorphic formation, superficial Neocomian continent.

Shozanji

e = Kurosegawa,

nappe;

Ultra-Kurosegawa

nappe, Middle Jurassic deposits;

olistostrome;f

h = L,-shearing

Schist nappes,

c, e-Kurosegawa zone,

a = Highly metamorphic continent,

crustal

= Sanbosan

to the east. T-Triassic

rocks

c = Oboke with

their

zone, late Jurassic continental

Mt. Kotsu nappe,

unit,

sandstone

Mesozoic

cover;

rich d =

flysch; g = unconformable

deposits

on the Kurosegawa

142

on structural the zoning southwest towards

aspects, due to microtectonics. and a possible

Japan, northeast

Mesozoic

using the island

of Shikoku

is proposed

for the Outer

as an example.

A possible

of

belt of

extension

Japan is also suggested.

MAP OF SHIKOKU

STRUCTURAL

Using these two tools, a rehandling

evolution

(Figs. 2. 3. 4. 5)

The polyphase deformation

South of the MTL and up to the Kurosegawa recognized

on the microtectonic

1983). From the youngest Cretaceous, (third phase), They are E-W At the outcrop

and regional

zone, three phases

scales (Hara

of folding

et al.. 1977; Faure,

are 1982,

to the oldest: (1) A phase of upright folding in lower giving rise to kitometric scale antiforms and synforms.

trending en echelon folds, (Hara et al.. 1977. 1980; Ichikawa. 1980). to thin section scale. a fan shaped crenulation cleavage is conspicu-

ous. (2) A phase of NgO’E-~N120’E post-folial asy~nmetric south vergent folds (second phase). This late Jurassic early Cretaceous post-metamorphic, deformation is responsible for the tangential structures described below. (3) A phase of synmetamorphic ductile deformation (first phase). with E-W (N70”E, N1OOOE) trending hneations

,MTL

and folds.

N

S

2km

S

Fig. 3. Cross

section

of the Outer

belt in Central

Shikoku.

(’= Shozanji nappe; d = Oboke unit and Ultra-Kurosegawa olistoliths; Paleozoic Jurassic

e = superficial basement; Sanbosan

nappe,

g = Triassic @sch.

with limestone, shallow

water

a = Mt. Kotsu

nappe; b = serpentinite;

zone, including

radiolarites

and green-schist

basic rock and radiolarite

olistoliths;

f = Kurosegawa

detrital

rocks;

h = Neocomian

unconformity;

I = Late

143

Owing to the first phase structures, a distinction is made between a domain where the first synmetamo~hic structures are conspicuous-called the infrastructure, and a domain where they are lacking--caMed the superstructure, composed of second phase nappes thrusted upon the infrastructure.

Fig. 4. Structural d = superficial

map of eastern

nappe;

h = Kurosegawa

Shikoku.

e = Late Paleozoic

rocks; i = detrital

limestones,

radiolarites

Cretaceous

Shimanto

and

nappe;

b = Shozanji

f = serpentinite;

Triassic rocks upon the Kurosegawa

k = Late Paleozoic

flysch with olistoliths;

a = Mt. Kotsu

basic rock olistolith;

basic

olistoliths,

volcanites

zone; o = first phase

probably

olistoliths:

lineations,

c = Oboke

Kurosegawa

M = subcontinental

rocks;

Neocomian

an eastward

shear;

unit;

granites;

rocks; j = Late Jurassic

reworking

showing

nappe;

g = Kurosegawa

Sanbosan

I = Triassic rocks:

n =

p = pre-Neocomian

thrusts.

N

S

Fig. 5. Cross c = Oboke

section

of the chain

unit; d = superficial

in eastern

nappe;

nappe;

f = serpentinite;

g = Kurosegawa

ments;

i = Neocomian

unconfo~ty;

Paleozoic

olistoliths.

Late Jurassic-early

I -first Cretaceous;

nized in this section, probably

Shikoku.

e = Paleozoic

phase thrust,

Jurassic

3 -post-Neocomian hidden

by late faults.

thrusts.

nappe; included

rocks,

Paleozoic

Sanbosan

Iikely reworked,

Kotsu

olistoliths

h = Kurosegawa

granites; j = Late

a = Mt.

green-rock

flysch;

b = Shozanji and Mesozoic

k = Triassic

early Late Jurassic;

2 -second

The Ultra-Kurosegawa

nappe;

into the superficial olistoliths;

sedi!=

phase thrust,

zone is not recog-

144

The superficial nuppes of the superstructure

These form the geometrically

higher part of the chain.

They have been recently

recognized

in western Shikoku (Tominaga

et al.. 1979; Hada et al., 1979; Tsukuda

al.. 1981)

eastern

Charvet,

mountains

(Guidi

Shikoku

(Faure

and

et al., 1984). They are composed

(Isozaki

et al., 1981; Suyari

siltstones

and coarse-grained

sandstones

sole marks. Sandstones. of the pebbly

Several kinds of decimetric Carboniferous-early

in the

matrix

features:

olistostrome and by places

slumps,

cherts, basic volcaniclastic

et

Kanto

disrupted

sediments

are

mudstones. to kilometric

massive

Permian

also

of a middle Jurassic

with turbiditic

beds, laminations,

assic and early Jurassic

and

et al., 1982) with a black pelitic

common

elements

1983)

olistoliths

or well-bedded, reef limestones

are included:

light-colored (Kanmera.

(1) Permo-Tri-

radiolarites;

(2) Late

1968; Yokoyama

et al.,

1981); (3) massive crystalline, undated limestones and well-bedded ones alternating with red-green mudstones and volcaniclastic sediments. A few dolomites are also present. (4) Pillow lava, gabbro, hyaloclastites and volcano-detrital rocks. From the red shales, red cherts and limestones representing interpillow sediments, a late Carboniferous-early Permian age is inferred (Fig. 6; Suyari et al.. 1982). In eastern Shikoku, limestones and green rocks are often associated; they are assumed to be fragments of dismembered seamounts (Yokoyama et al., 1981). Geochemical analyses indicate that a part of the basaltic lava has alkaline affinities (Maruyama, 1976). Owing to late subvertical faults, the basal thrust plane of the nappe is not clearly observed, except in western Shikoku. However, there is a noteworthy deformation gap between

the rocks belonging

to the nappe

and to its substratum

nappe rocks do not bear the ductile deformation

Fig. 6. Sketch of the Jurassic / = green-rock donts;

olistolith,

d-hyaloclastite;

olistostrome.

c-inter-pillow e-pillow

a-sandstone sediments,

lava; f-Red

nor the Sanbagawa

block; providing

shale.

h-pebbly

mudstone.

late Carboniferous-early

(Fig. 7). The metamorphism

Jurassic; Permian

c, d, r, cono-

Fig. 7. Microstructures (left side). 1. Schistozed Highly strained Undeformed Sandstones,

of the superficial hyaloclastite.

nappe

(right side) compared

2. Cracked

pyroxene

with those of the Shozanji

into a weakly

calcite clasts with bended twins. 4. Calcite clasts undeformed.

radiolarians.

7. Sandstone

weakly schistosed,

by pressure

schists

with undulose

solution,

quartz

and quite unstrained

grains

deformed 5. Stretched

3.

radiolarians.

6.

and pressure

quartz grains.

nappe

hyaloclastite. shadows.

8:

146

conspicuous in the substratum. Moreover, close to the contact zone, brittle subhorizontal shear zones associated with southwards directed drag folds are often observed

(Fig. 8).

The substratum trending

close to the thrust plane is deformed

folds related

Both the nappe upright

folds.

occurred

and

by E-W (N5O”E-NllO’E),

to the second phase since the first phase foliation

is refolded,

its substratum

third

Therefore

are refolded

the superficial

in the late Jurassic-early

nappe

by late Cretaceous emplacement

is inferred

In

slikenslides

the surroundings,

submeridian existence

and southeastwards

sandstone

grain elongation of several

tectonic

to have

Cretaceous.

Inside the nappe, discrete flat lying brittle shear zones marked N--S) trending

phase

is weakly

overturned deformed

and volcaniclastic

(N130’E.

drag folds are observed.

by pressure

pebbles

slices into the superficial

by N-S,

are slightly nappes

solution

with

flattened.

The

is likely.

the slicing

being promoted by the heterogeneous nature of the olistostrome, because displacements occur more easily along the block boundaries. Dealing with the origin of the superficial nappe, as all the displacement criteria show a southern or southeastern motion and as there is a structural continuity of the underlying infrastructure up to the MTL, a northerly origin, i.e. from the Inner belt is likely, Age and facies similitudes between olistoliths in the superficial nappe and rocks of the Inner belt, namely the limestone agreement with this inte~retation. Moreover same sedimentological

features

as the Tanba

plateau and the Sangun zone, are in the matrix has the same age and the olistostrome

of the Inner

belt.

The j~jrasrruc~ur~ The general features

The infrastructure metamorphism

suffered

(e.g.+ Miyashiro,

a HP/MT

metamorphism.

the famous

1961; Iwasaki, 1963; Banno,

Sanbagawa

1964; Higashino,

1975)

and a contemporaneous infrastructure includes

ductile deformation (Faure, 1982, 1983, in press). The the previous Sanbagawa, Mikabu and the northernmost

fringe

zone. Moreover

of the Chichibu

the Kurosegawa

zone partly

affected

by a

Fig.% Brittle flat shear zone with southward motion. near the basal contact of the superficial nappe.

147

prehnite-pumpellyite could

be included

metamorphism

attributed

to the Sanbagawa

in this group but it should be noticed

is lacking. As the first deformation phase has already been described 1985) only the main points are summarized, Foliation, lineation are the typical

microstructures.

The E-W

trending

metamorphism

that the ductile deformation

lineation

(Faure, 1982, 1983, and intrafolial folds is composite:

minera-

logic with mica flakes, amphibole and tourmaline needles, quartz and chlorite fibers in pressure shadows; stretching with elongated radiolaria, quartz grains, magmatic vesicules,

pull-apart

axes. Curviplanar

of amphibole,

pyroxene,

and sheath folds similar

al.. 19’77); Brittany

(Quinquis

epidote

and piemontites;

to those known

in the Pyrenees

and

to the lineation,

perpendicular to the foliation asymmetric pressure shadows,

rotations

axis perpendicular

et

are incoherent;

perpendicular while

in X2

to the sections,

and parallel to the lineation, rotational criteria, i.e. sigmoidal inclusions, quartz C-axis preferred orienta-

tion, show an eastwards rotation. The following interpretation is proposed: rotation

(Carreras

et al., 1978) and Alpine Corsica (Faure and Malavieille,

1980; Mattauer et al., 1981) are often observed. The strain regime is rotational (Fig. 9). In YZ sections, foliation

microfolding

to the lineation,

as the deformation

is rotational

the first phase microstructures

and the are due

to a ductile subhorizontal shear mechanism. The direction of shearing is materialized by the stretching lineation which is then an “a’‘-kinematic lineation; and the shearing occurred from west to east. Such an inte~r~tation is similar to those proposed to account for the deformation of infrastructure in other belts, for instance in Greenland (Escher and Waterson, 1974); the Himalayas (Mattauer, 1975); and Corsica (Mattauer et al., 1977, 1981).

Fig. 9. Block diagram of first phase bearing hyaloclastite, Shozanji nappe, showing the E-W mineral lineation and the eastward rotation of pressure shadows in XZ section.

14x

The Subdivisions to lithological

Owing

the Green schist nappe,

differences,

the infrastructure

is divided

The Green Schist nuppe thrusts upon the Oboke unit during to the metamorphic second phase:

into

two

groups:

and the Oboke unit, (Figs. 2. 3, 4, 5).

grade, it is subdivided

into two nappes

the first phase. Owing

differentiated

during

the

(I) The Mt. KOLSUnappe in the north, contains rocks with the higher metamorphic grade: glaucophane schist facies in eastern Shikoku (Iwasaki, 1963) amphibolite and eclogite in central Shikoku (Banno, 1964). The Mt. Kotsu nappe micaschist and small amounts of quartz-schist (metaradiolarite). basic sediments.

Many meter to kilometer

diabase,

amphibolite,

gabbro,

This nappe

is composed of talc-schist and

thick masses of mafic rocks: pillow-lava,

and serpentinized

peridotites

is sliced into several units, whose contacts

are probably

are underlined

olistoliths. by mylonitic

schists and serpentinites (I,Zaure. 1985). During the second phase deformation, Mt. Kotsu nappe is thrusted southwards upon the Shozanji nappe.

the

(2) The Shozanji nuppe includes the low-grade part of the basic schist of the ancient Sanbagawa zone. the Mikabu zone and the northernmost part of the Chichibu zone. In eastern Shikoku, the nappe is refolded by a third phase antiform of several kilometers that the complete the Kanto between

whose southern

lithological

limb is cut by a late fault, the Mikabu

succession

mountains,

the Mikabu

the Sanbagawa,

Mikabu

is never observed.

line is missing and Chichibu

In central

line, so

Shikoku

or in

and there is no clear boundary

zones.

The succession from top to bottom is the following. Horizons of quartzite and basic schists, up to some ten meters thick, are intercalated into pelitic schist. Indeed these “green rocks” are mainly sediments: shale, breccia, sandstone reworking gabbroic minerals and looking like reconstituted gabbro, (Iwasaki, 1979a). Some diabasic masses and some ultramafic cumulates are interpreted as sheeted complex (Iwasaki, 1979b), but an olistolithic origin is also possible. Towards the south, when the basic volcanoclastites become thicker, they are called the Mikabu zone. There breccia of basic rocks are dominant and the pebble size increases. Above lies an olistostrome mainly composed of gabbroic elements ranging from pyroxenic minerals to kilometric blocks surrounded by a brown reddish matrix. Thus the basic rocks are not ophiolites in the Alpine sense, since peridotite and serpentinite are scarce and the magmatic members are olistoliths. They should be regarded as a peculiar olistostrome formed by the reworking of ophiolitic debris before the tectonic activity. The gabbroic olistostrome is capped by a conglomerate bearing pebbles of red radiolarites, shales, basalt, diabase and gabbro; plagiogranite fragments are frequent (Iwasaki, 1979b). The upper part of the sequence is made of shale and becomes more and more chaotic upwards. Chert, limestone, hyaloclastite, pillow-lava, gabbro and ultramafic cumulates are common olistoliths. CaIc-schist closely associated with basic schist provide late Triassic conodonts (Matsuda, 1978). Ill preserved Nasselaria from tuffaceous beds (Suyari et al., 1982)

149

and from the shalely (Iwasaki,

Ichikawa

part of the conglomerate

and Faure,

unpublished

overlying

data)

the gabbroic

suggest a middle

age, at least for the uppermost part of the sequence. The Oboke unit is the lowermost part of the infrastructure, third phase antiforms. lava

and

granitic

pebbles.

sphene, are common the contact recrystallized

It is a detrital Heavy

in sandstones.

sequence minerals

to late Jurassic

seen in the core of

with conglomerates such

olistostrome

as zircon,

including

apatite,

In the upper part of the sequence,

acidic

tourmaline,

that is close to

with the Green Schist nappe, basic schists and dark red strongly radiolarites are included as olistoliths. Those facies suggest a neighbor-

ing continental sedimentological

source providing the sialic detritus. According to Kojima (1973) analysis of the sandstone indicates a southern source for the

detrital rocks. The boundary between the Oboke unit and the Green Schist nappe is a tectonic contact marked by crushed zones with intense microfolding and quartz veins due to fluid circulation

during

schist blocks close to the contact

the thrusting.

are either olistoliths

Some isolated or tectonic

chert and basic

blocks.

The Kurosegawa zone

As defined by Ichikawa et al. (1956) it is a discontinuous row of lenticular masses of sialic rocks, but cartographic distribution is due to a post-Cretaceous left-lateral strike-slip

fault (Hada

et al., 1979), since Neocomian

sediments

Kurosegawa zone consists of several rocks types: (1) Crustal rocks: granitoids, tonalites, garnet-amphibolites, roxenites, flaser gabbro, providing radiometric ages around and Noda, 1969; Ishizaka, (2) Basic metamorphics whose associations 1978). Radiometric

200-240

1978), suggest either polyphase

Ma K-Ar

or perturbations

jadeite,

ones (Nakajima et al., and Ueda, 1974; Ueda

ages from muscovites

metamorphism

The

granulites, garnet-py380-400 Ma (Hayase

1972; Noda, 1973; Yoshikura et al., 1981). bearing HP minerals: glaucophane, lawsonite,

are quite different from the Sanbagawa ages cluster around 400 Ma (Maruyama

et al., 1980). However,

are involved.

(Maruyama related

et al.,

to Mesozoic

tectonics. (3) Serpentinites are frequent and squeezed tematicaly form the matrix of a serpentinite (Maruyama,

along faults. But they do not sysmelange as sometimes ascertained

1981).

(4) Siluro-Deuonian weakly to unmetamorphosed limestones, acidic tuffs and volcanoclastites. (5) A late Permian olistostrome reworking Permo-Carboniferous limestones, radiola&es, basic volcanics, early Paleozoic schists and granites. (6) Triassic shallow water sat&tones and conglomerates. In eastern Shikoku they unconformably cover the Permian olistostrome (Ichikawa et al., 1958), but they are generally in fault contact with the surrounding rocks.

150

(7) Late Jurassic shaiIow water conglomerates, sundstones and shales. They progressively grade into the turbiditic

facies of the Sanbosan

zone (cf. below).

Indeed the whole sequence is never entirely observed since it is always disrupted by strike-slip faults. It is quite impossible to separate the pre-Cretaceous deformation to that due to the strike-slip. the granite. structure

Moreover,

towards

the cataclastic

serpentinites,

texture is conspicuous

in

shear zones with asymmetric

the south are widespread.

The Ultra-Kurosegawa

zone.

This zone forms the northern been included on the structural outcrop

However,

in the flat-lying

in eastern

Shikoku

boundary of the Kurosegawa zone with which it has map (Fig. 2). Owing to strike-slip faults it does not

while it develops

in the western

part. It is composed

of

turbidites of undetermined age, but at least Triassic or younger. since Triassic lamellibranches are found in sandstones (Noda, 1954; Hada, 1481) and Permo-Triassic radiolarites are enclosed as olistoliths together with late Paleozoic limestones, basic volcanites,

granites

and metamorphic

to the Ultra-Kurosegawa

schists. Recently

zone have provided

early Jurassic

pelitic rocks belonging radiolarians

(S. Hada,

pers. commun., 1984). The red triassic radiolarites are quite different from the Triassic cherts included in the superficial nappe, but can be compared with the undated but probably early Mesozoic red cherts and shales found in the Shozanji nappe and the Oboke unit. The Ultra-Kurosegawa zone tectonically underlies the Shozanji nappe as it is observed in central Shikoku. Therefore it is tentatively correlated with the Oboke unit. It is itself sliced in several schuppen and thrusts southwards upon the Kurosegawa zone, but this thrust could be a post-Cretaceous one since a few kilometers to the south the Neocomian deposits are also affected by

Fig. 10. Detailed

cross

flysch; b = Triassic

section

showing

chert oiistoliths;

d = undated

Mesozoic

rocks,

olistostrome,

reworking

granites,

to the superficial mity.

the Kurosegawa-Sanbosan

c = limestone

covering

olistoliths,

the Triassic;

radiolarites

r = Triassic

and limestones;

nappe (along this section the Ultra-Kurosegawa

zones relationships.

T-Triassic,

J-Jurassic,

sandstones;

h = serpentinites;

a = Sanbosan (Torinosu

f = granite;

type);

g = Permian

i = basic rocks belonging

zone is missing).

u-Triassic

unconfor-

151

such thrusts (fig. 3). Generally contacts

during

speaking,

the early Tertiary

a post-Cretaceous

Shimanto

orogeny

reworking

of the Jurassic

or earlier is likely.

The Sanbosan zone This is a flyschoid zone.

formation

lying on the southern

The age of the flysch is late Jurassic,

boundary

of the Kurosegawa

Oxfordian-~mmeridgian,

owing

to

scarce ammonites and radiolarians (Matsuoka, 1981; Ishida, 1983; personal findings). Moreover, in its northern part the flysch includes Kimmeridgian-Portlandian reef limestones: tional

the Torinosu

conglomerates

shales provided Late Triassic (Ishida,

limestones Paleozoic

lower Berriasian lint-colored

covered

The northern

and associated blocks included

olistoliths

one bears mainly

basic volcaniclastic

“ upper-Torinosu”

(A. Matsuoka,

part, two conspicuous

in the Permian

southern one is characterized and basic volcanic sediments,

by early Cretaceous

Shikoku,

nannofossils

radiolarites

1979). In the southern

core of anticlines.

limestones,

and shales. In central

are dispersed olistolithic

sediments,

calcareous

pers. commun.,

1983).

in the turbidites

belts outcrop

Permo-Carboniferous

olistostrome

intraforma-

quite similar

of the Kurosegawa

in the

radiolarites, to the late zone. The

by Triassic radiolarites, Triassic micritic limestones probably of the same age. Though described as a

sedimentary sequence with unconformities, they are in fact reworked fragments of the pre-flysch substratum. The sandstone is rather coarse and under the microscope reveals an abundance of plagioclase and mica. The detritus were probably provided by the nearby Kurosegawa granites. The detailed structure is at present unknown, but schematically, the tectonic style is characterized by tight upright folds as shown by graded bedding and southward

Fig. 11. Schematic inte~retation of the deep structure of the Outer belt of southwest Japan. a = Kurosegawa basement; b = Green Schist nappe undifferentiated; c = superficial nappe; d = Neocomian unconformity; e = Sanbosan flysch. I -first phase contact, abduction of the Green Schists nappe upon the Kurosegawa basement, in early-Late Jurassic; 2 -second phase contact of the superficial nappe, Late Jurassic-early Cretaceous; 3 -post-Neocomian thrusts.

152

verging

thrusts.

marked

by a complex

sandstones,

deformations

for instance

unconformity Structural

between

imbrication are relevant

late Cretaceous

is lacking

the Kurosegawa

of schuppen

(Fig. 10). The two tangential

superficial ones,

The boundary

phases known

to the Paleogene ones cannot

in the Sanbosan

and the Sanbosan

shown

by repetition

zones is

of Triassic

to the north are lacking. The Shimanto

be excluded

orogeny.

but earlier

since the Neocomian

zone.

interpretation

The Outer belt of the Jurassic orogeny is a pile up of nappes emplaced by two tangential phases. The early one occurred in ductile, synmetamorphic conditions with

an eastwards

conditions

displacement.

with southward

The

second

or southeastward

one

occurred

displacement.

in more

The Oboke

superficial unit, corre-

lated with the Ultra-Kuroajegawa zone is interpreted as the Mesozoic sedimentary cover of an underlying Paleozoic continent, presently outcropping in the Kurosegawa zone, owing to post-Neocomian

faults (Fig. 11). Subsequently

this continent

is referred to as the Kurosegawa continent. The Kurosegawa continent must not be confused with the Kurosegawa zone which is only the presently outcropping part of a larger domain tectonically underlying the Green Schist nappes and the superficial nappe. Such an interpretation is in agreement with the seismic data (Kimura and Okano, 1980) suggesting the existence of a sialic type crust under the Outer belt of southwest Japan, between MTL and BTL. Moreover, in this new structural scheme the former Sanbagawa, Mikabu and Chichibu zones are not representative of the structure nor of the paleogeography of the Jurassic orogen. THE MESOZOIC

EVOLUTION

OF THE OUTER

BEL’I

Triassic paleogeogruphy.

The Triassic corresponds to the pretectonic stage. The superficial nappes belonging to a more septentrional domain are excluded since the structure and evolution of the Inner

belt is not considered

(Fig. 12), account

here. The following

for the evolution

domains,

from north

to south

of the Outer belt.

An oceanic areu

In this domain, the constituent rocks of the Green Schist nappe were deposited. The nature of the crust, known only by reworked fragments, has not been established: marginal sea such as the present Japan Sea or wide ocean such as the Pacific, as the size and abundance of gabbroic olistoliths decrease from south to north (Iwasaki, 1979b; Takeda et al., 1981), a meridional source is assumed. However, the cause of this submarine ophiolitic detritism has not been settled. Such a phenomenon occurs at present in several kinds of environments like ridges, marginal seas,

153

subduction

zones, fracture

al., 1982). As a working (Faure

and Iwasaki,

The Kurosegawa This domain

zones where it has been observed

hypothesis,

we explain

to the Oboke unit and the Ultra-Kurosegawa crystalline

and basic lava-are

with early Paleozoic

sediments.

rocks, it suffered

tectono-metamorphism

zone, in

mixed with reworked

schists and Paleozoic

The Kurosegawa continent Covered by Triassic shallow water detrital superficial

et fault

northern margin

corresponds

rocks-granites,

zoic evolution

of a transform

1982).

which oceanic derived rocks-radiolarites continental

in situ (Lagabrielle

it by the motion

a complex

pre-Meso-

and late Paleozoic

more

reworking.

The Hurosegawa

southern margin

In the future Sanbosan sediments were deposited Kimura et al. (1975).

zone, during

radiolarites, limestones and basic Triassic in rather shallow waters

volcaniclastic according to

Triassic

Middle

Late

Jurassic

Jurassic-early

.

. ..

......... .. .......... . .,.

Sanbosan

Cretaceous

Neocomian ‘MTL

Fig. 12. Geodynamic block assumption

evolution

of the Outer

is not discussed.

belt from the Triassic

to the Neocomian.

The pre-Ryoke

154

Laie Jurassic tectonism At that time the oceanic continent

are progressively

To account generally Uyeda,

shortened

for the HP/LT

invoked

domain

by thrusting

metamorphism,

(e.g., Miyashiro,

1974). The nature

and the northern

of the Kurosegawa

upon the Kurosegawa

a northward

1961; Matsuda

of the overriding

margin

oceanic

and Uyeda,

plate is sometimes

continent.

subduction

1971; Miyashiro assumed

to be a sialic

block called the pre-Ryoke continent. In the oceanic subduction model, neither tangential structures nor the part of the Kurosegawa continent are considered. In this paper the arguments for this assumption are not discussed, Charvet, 1984), because the question of the existence of a pre-Ryoke

is and the

(cf. Faure and block does not

influence the evolutive model for the Outer belt which is determined by the subduction of the southern plate whatever the nature of the overriding one. Therefore, a model involving oceanic subduction followed by continental subduction is proposed as the cause of the Jurassic orogeny. A plate (Kula plate?), bearing the oceanic

domain

and the Kurosegawa

continent

subducts

northwards.

But, as

continental crust cannot suffer large amounts of subduction owing to its buoyancy, chocking results, promoting the abduction of the Green Schist nappes upon the Kurosegawa occurred

continent.

As recorded

by the microstructures.

the main displacement

eastwards.

Continental subduction is a recognized process in many erogenic belts: Alpine Corsica (Mattauer et al., 1977. 1981) the Western Alps (Caby et al., 1979) the Himalayas (Mattauer, 1975). and the North American Cordillera (Roure and Blanchet,

1983). A similar

process

fits rather

well with the geological

features

of

southwest Japan. During the Late Jurassic-early Cretaceous, the deformation goes on in more shallow conditions. The Green Schist nappes are sliced into the Mt. Kotsu and Shozanji nappes and the superficial nappes are emplaced with a south-southeastwards displacement. In the Kurosegawa southern margin, the turbidites provided by the Kurosegawa continent and the previous stones

and basic lavas ones. The flysch deposits

the northern

tangential

rework Paleozoic Triassic radiolarites

are seen as a sedimentary

rocks iimeecho of

tectonics.

The Neocomian Conglomerates, sandstones and pelites cover unconformably the superficial nappe, the Kurosegawa and Ultra-Kurosegawa zones sealing the tangential contacts. Up to now the Neocomian was unknown in the Sanbosan zone of eastern Shikoku. It was recently discovered in the Kii peninsula (Yao, 1984), where it is always related to the surrounding rocks through faults. More to the south, some of the PermoTriassic-Jurassic olistohths included in the Sanbosan flysch are reworked into

155

Albian-Aptian

turbidites

to the Shimanto

belonging

orogeny

are not presented

At the same time, to the north, related

to the HT/LP

motion

induces

Ichikawa,

to the Shimanto

Ryoke

the third

related

in this paper.

the MTL starts its early left-lateral

metamorphism.

phase

zone, The deformations

en echelon

In the Outer upright

belt,

folding

motion.

It is

the strike-slip

(Hara

et al., 1980;

1980).

Basically,

in the Outer belt of southwest

Japan,

the three deformation

phases

of

the Jurassic orogeny are associated with a noteworthy movement parallel to the belt. At first, it is a tangential movement related to synmetamorphic nappes thrusted towards the east; followed by south-southeastwards more superficial the shortening is associated with a left-lateral strike-slip. LATERAL

EXTENSION

thrusts.

Finally,

OF THE STRUCTURES

Other areas in southwest Japan Kyushu Though

hidden

by recent

volcanites

and squeezed

along

faults,

all the zones

determined in Shikoku extend to Kyushu with the same characteristics. The Green Schist nappes outcrop in the easternmost part; the Kurosegawa zone in the westernmost part. The Sanbosan zone is reworked by post-Cretaceous (Murata, 1981), emplaced upon the Shimanto zone.

south-tending

nappes

The Green Schists nappes, the Oboke unit, a Jurassic olistostrome (Isozaki et al., 1981) the Kurosegawa zone and the Sanbosan zone (Yao, 1984) are found in the western part. They are much more disturbed by late faults than in Shikoku, but pre-Cretaceous central

thrusts

in the Kurosegawa zone are recognized

Kii, the Kurosegawa

large overthrust

zone disappears

upon the Shimanto

and the Sanbosan

zone (Yamato

(Maejima,

Omine Research

the eastern part, E-W trending stretching lineation is recognized schists belonging to the Green Schist nappe (Faure, unpublished

1978). In

zone is involved Group,

in a

1981). In

in pelitic and basic data).

Central Japan The pre-Cretaceous zones are considerably laminated by faults. They are less than 10 km as opposed to 50 km in central Shikoku. However, “green schist” affected by the first phase lineation and folds and by the HP/MT metamorphism are known (Watanabe, 1970). However, as the early microstructures are overprinted by fault related deformations, their interpretation is rather difficult. Moreover, a thrust probably equivalent to the basal contact of the superficial nappe is recognized( Kimura, 1961).

Kant0 mountains The structure (Cuidi

is similar

to that of Shikoku.

et al., 1984). The earty

structures. thrusting

They

are reworked

of a Middle Jurassic

one

Two tangential

is marked

by a south

by eastward

tending

superficial

phases

are described

directed

rotational

one leading

to the

ohstostrome.

Sout~~esr Japan und northeast Japan The Tanakura northeast

Japan.

fault (Fig. 1) marks The later domain

the boundary

consists

between

of the Abukuma

southwest massif.

Japan

and

the Kitakami

Green Schists Nappe Sanbosan

“W Fig, 13. ~~onstitution The superficial Arrows

nappe

Shimanto

Zone

of the Mesozoic structural is absent,

show the direction

xonation

before the Miocene opening

and the Kurosegawa-Sanbosan

zones are grouped

of motion of the Green Schist nappe during

is afterwards

cut by left-lateral

area suffered

a ciockwise

faults. parallel

rotation.

to the Tanakura

of the Japan

in southwest

the first phase. The general

fault. During

this movement,

Sea.

Japan. trend

the Kanto

157

massif and the island of Hokkaido. The Tanakura fault is often considered as the main paleogeographic and structural division between southwest and northeast Japan. Indeed it is a polyphase strike-slip fault (Otsuki, 1975), with left-lateral displacements known as Late Cretaceous. The main motion occurred in the Miocene, probably due to the opening of the Japan Sea. Moreover northeast Japan is affected by several N-S trending, left-lateral strike-slip faults disturbing the pre-Cretaceous organization. However apart from these late perturbations, the following comparisons can be made (Fig. 13). The south Kitakami massijr The massif is very similar to the Kurosegawa zone (Kimura et al., 1975; Tanaka and Nozawa, 1977), since a Paleozoic basement is unconformably covered by Triassic-Jurassic shallow water sediments. In the basement, outcrops of pre-Silurian granitoids, Paleozoic sediments, ophiolitic rocks of unknown age associated with HP metamorphic rocks are overlain by a Devonian unconformity, (Murata, 1979; Maekawa, 1981. The Hitachi metamorphic rocks These rocks are located in the southernmost part of the Abukuma massif (Fig. 1). They consist of pelitic schist, limestones, basic schist and mafic-ultramafic masses. They are metamorphosed in the green schist-epidote amphibolite facies; the grade increases westwards (Tanaka and Nozawa, 1977). The N-S to N50”E trending foliation bears a subhorizontal mineral and stretching lineation, with rotational criteria towards N or N50”E (Gusokujima, 1983; Faure, pers. observations). Towards the east they are separated by faults from Permian limestones and elastics. Thus the Hitachi rocks display close affinities with the Green Schist nappes of southwest Japan. The northern Kitakami belt This belt consists of Mesozoic turbidites including Triassic cherts and limestones, and uppermost Jurassic Torinosu-type reef limestones (Tamura 1960; Sugimoto, 1974; Yamaguchi et al., 1979). Thus the northern Kitakami belt is similar in lithology and age to the Sanbosan zone. It probably extends northwards in western Hokkaido, (Kimura et al., 1975; Sugimoto, 1977; Yoshida, 1978). However, a striking difference must be underlined. While in southwest Japan there is no evidence for a pre-Neocomian deformation in the Sanbosan flysch; in northeast Japan, the North Kitakami flysch suffered a synschistose eastwards verging deformation (Yamaguchi, 1981), before the granodioritic intrusions, dated around 130-110 Ma, and was covered by Aptian-Albian unconformity. This early Cretaceous phase, called the Oshima orogeny (e.g., Sugimoto, 1974; Tanaka and Nozawa, 1977), has up to now, never been observed, in the southern margin of the Kurosegawa continent in southwest Japan.

In order

to solve this problem,

There also, a late Jurassic-early phic deformation mity (Jolivet (Cadet

of the Kamuikotan

survey

orogeny

ophiolites,

between

is necessary.

by the synmetamor-

and the lower Cretaceous reworked

1983; Jolivet et al.. 1984). However,

for a good comparison

of Hokkaido

is proved

et al., 1984). But this phase is strongly

and Charvet,

yet adequate

a structural

Cretaceous

central

unconfor-

by Tertiary

the data available

and eastern

Hokkaido

tectonics are not and the

other areas of Japan. CONCLUSION

Structural zoning

and stratigraphic

studies

of the Outer belt of southwest

of Shikoku Japan,

result in a new definition

which better accounts

of the

for the deforma-

tion and the paleogeography of the region than the previous one. The Jurassic orogeny of southwest Japan is responsible for two sets of tangential structures. The early

phase

basically

corresponds

to the eastwards

thrusting

of ophiolitic

debris

bearing nappes in ductile and synmetamorphic context. As in other alpinotype erogenic belts, the orogeny can be explained by the continental subduction of the Kurosegawa block and the related abduction of the oceanic rocks. During this tectonic the southern margin of the Kurosegawa continent formed a basin filled by turbidites provided from the Kurosegawa continent. The structural zones defined in Shikoku, extend from Kyushu to Hokkaido over more than 2000 km, with the same features. This demonstrates that the pre-cretaceous orogeny of Japanese Islands is not related to any collage or accretion process of severat independent microblocs, but to quite a simple m~hanism; that is the oblique subduction of the Kurosegawa continent. The present disparity between southwest and northeast Japan is due to post-Cretaceous events as such as late Cretaceous-Paleogene

piutonism,

the Paleogene

Shimanto

orogeny.

opening of the Japan sea, all disturbing the pre Cretaceous chain. In order to get a more comprehensive view of the Mesozoic Japanese

islands,

a detailed

survey of the stratigraphic

and the Miocene evolution

and structural

of the

relationships

between the Sanbosan and Shimanto zones in one hand, the North Kitakami belt and Hokkaido on the other hand is necessary. Moreover, no geodynamic model can be proposed

before a reinvestigation

of the Inner

belt of the chain has taken place.

ACKNOWLEDGEMENTS

J. Aubouin, J.P. Cadet, M. Caridroit, J. Char-vet, A. Guidi, J.T. Iiyama, M. Iwasaki, L. Johvet, F. Lalevee, C. Nakagawa and K. Sano are acknowledged for their help and comments throughout the elaboration of this paper. Field expenses were partly supported by a grant from the Ministry of Education of Japan and the French C.N.R.S.

159

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