Terrane boundaries in the Bohemian Massif: Result of large-scale Variscan shearing

Tectonophysrcs, Elsevier

151

177 (1990) 151-170

Science Publishers

B.V., Amsterdam

- Printed

in The Netherlands

Terrane boundaries in the Bohemian Massif: Result of large-scale Variscan shearing Ph. MATTE

‘, H. MALUSKI

‘, P. RAJLICH

2 and W. FRANKE

’ Laboratorre de Tectonrque et Geochronologre, U.R.A. 1371, U.S. T.L, 34060 Montpelher ’ Geological Instrtute of the Academy of Sciences, v Holesovtckach ’ Instrtut fur Geowtssenschaften

3

(France)

41. 18200 Prague 8 (Czechoslovakra)

und Lithosphare Forschung der Unruersrtat, 06300 Glessen (F.R.G )

(Received

March

20.1989;

accepted

June 16.1989)

Abstract Matte,

Ph., Maluski,

large-scale Paleozoic

SW-NE

On a NW-SE appears eroded

Drosendorf Saxothuringian. of strike-slip

section

and

Gfohl

of an oceanic

or ductile

strike-slip

in the Bohemian Belt of Europe

grouped

have all suffered

Massif:

Result

of

and Circum-Atlanttc

is based

on recent

Belt (mamly

structural,

the Massif

boundaries

Central).

trending

roughly

faults.

vergence)

the

Variscan

Carboniferous

metamorphic

to northwest in

into terranes

Ma) and the terrane

area to the Moravian

with an east to southeast

(previously

These terranes

(390-300

with two high-grade southeast

Massif

wtth the French

to the Vat&an

thrusts

orogen From

boundaries

in the Variscan

of the Bohemian

from the Saxothuringian

divergent

the Barrandian.

separated

the belt may

Moldanubian

different

geological

Variscan

history,

foredeep.

the Bohemran

belts (the Saxothuringian by a less metamorphic be divided

zone), histones

into

Barrandran.

and much

six terranes:

less

Moravian,

Mtinchberg-Tepla

and are now separated

Masstf

with northwest

and

by maJor thrusts

faults.

This configuration lasting

to Variscan

and the Moldanubian block,

rocks

and on a comparison

events are attributed

as a fan-like

vergence

studies

Terranes

177: 151-170.

of the pre-Permian

to SSW-NNE

W., 1990. Terrane

In: Ph. Matte (Editor),

Tectonophysrcs,

and radiometric

Major tectonothermal

P. and Franke,

shearing.

Grogens.

A new subdivision kinematic

H., Rajlich,

Variscan

is the result of a complex

domain

followed

by abduction,

continental

probably

collision,

including

continental

an early subduction

subduction

and

and closure

strike-shp

faulting

more than 80 Ma.

Introduction Apart from the Bohemian Massif is pre-Permian rocks km2), and it forms

Basin (Matte, 1983; Matte and Him, 1988). Terrane similarities are striking and permit good correlations (Fig. l), particularly between the southem part of the Bohemian Massif and the Massif Central.

French Massif Central, the the largest stable outcrop of in Western Europe (90,000 the easternmost rim of the

In the past the Bohemian Massif has been subdivided into various zones (Saxothuringian, Moldanubian, Moravian, etc.) based on the respective effects of the Cadomian (Pan-African) and Variscan orogenies (Kossmat, 1927; Svoboda et al., 1966; Suk et al., 1984; Zoubek et al., 1988). Many authors agree that the Variscan event had a great effect in the Bohemian area and that most of

Variscan Belt. Like the Massif Central it consists mainly of high-grade metamorphics of uncertain age, granites, and subordinate fossiliferous Palaeozoic rocks. A NW-SE traverse through the massif shows a nearly complete cross section of the Variscan Belt, this section being comparable to the traverse across the Massif Central-Paris 0040-1951/90/%03.50

6 1990 - Elsevier

Science Publishers

B.V.

Fig. 1. Tarmms and sutures in the Va~sican Belt of Europe. N.t: F. = north Vakcan front; L.R.H.S = Lizard-R.ha&h Sptwe; M.T.S = M~b~rg-T~la suture; N.C.S = Massif Central Suture; C.C.S = Coitnbra-Cordoba suture; 0. hf. S = 0s%+-Motpa suture.

the granitic batholiths are Carbomferous in age, but debate continues about the relative importance of the Precambrian and Variscan tectonothermal effects. The concepts of thrust tectonics and allochthony first suggested by Kossrnat (1927) and Suess (1926) for the Bohemian Massif and by Demay (1930, 1942) for the Massif Central have been confirmed by modern strnctural and radiometric studies that have led -to plate tectonic models for the Vat&can Beit (Weber, 1978, 1981; Tollmann, 1982; Weber and Behr, 1983; Matte, 1983,1986; Behr et al., 1984; Franke tR34; h&tte et al., 1985). New in&ght into the deep structure of the prePermian European Variscan crustbascomefrom the deep seismic pr&ling w @CURS SWAT and DFKORP), The seismic data oonfkm

the concept of ~~~~y

by showiug that thrusts outcropping at the surface are rooted in the lower crust, at the Moho, or the mantle (Gazes et al. 1985; Meissner et al., 1981; DEKORP, 1985; Meissner and Wever, 1986; Behr and Heinrich, 1987; Matte and Him, 1988; Franlce et al., 19g8; Volibrecht et al., 1989). Within the context of these concepts, we have carried out structural and radiometric studies mainly in the Moldanubian and Sax&urrngmn withthecooperareas of- the Bohemian ation of various laboratories *. ~~~ resuits

* rnstitut far GeuBten

und Li

Giesscn, F.RG.; Laboratoire de Tectonicpe et de G4ocbronolo@e de Motttpdiier and bstit& CS&oi&ue -de Remas, Fmnce; Gcologiaal Institute of tbc A&&my of Scielmq Prague, cacc~ov*.

VARISCAN

SHEARING

are the basis Massif

AND

TERRANE

BOUNDARIES,

for a subdivision

BOHEMIAN

153

MIASSIF

of the Bohemian

into six terranes.

tends from the Donau (Krems and Vienna) to Ostrava and it has been found by drilling below the Carpathian

Subdivision of terraues in the Bohemian Massif The subdivision ric relationships separated faults,

is based on large-scale between

by large each

unit

tectonothermal attention

major

thrusts

history.

assemblages

suffered

units

strike-slip a

We have paid

to the presence

morphic

iithotectonic

or ductile

having

geomet-

different particular

of high-pressure

and mafic-ultramafic

metarocks

which may be markers of sutures, to horizontal movements on large-scale shear zones indicative of plate dating

motions

and

to preliminary

of the metamorphism

related

39Ar/40Ar to these shear

zones. Six terranes have been distinguished on the basis of their distinct sedimentary and tectonometamo~~c history (Figs. 1, 2 and 5). They do not correspond exactly to the classical zones defined

by Kossmat

mind,

different

(1927)

and

plate tectonic

represent,

in our

settings.

m

Munchberg

B

Etorrondmn

comprising formed

mainly and

580 Ma

(Van

Breemen

upper intercept) by Middle

a Late

the Moravian

sedimentary

cover

upper

1982,

U/Pb

on is and

Carboniferous

carbonate

karst (Dvorak,

1982). This

to the northeast

Culm and Namurian

as

paralic

contact with the carbonate platform autochthonous cover of the Pan-African basement (Dvorak press) NW-SE

and Novotny, (Fig.

1984; Cizek and Tomek,

3). Early,

transverse

probably

faults

an important

of the deformation

The degree of deformation

in the Bohemian

Massif

in

pre-deposition,

(Dvorak,

1973,

1982,

role in the parti-

in the Carboniferous

basin, with areas becoming more formed towards the northeast.

Orosendorf

subdivision

Ma and

sequences. The Carboniferous may be divided into a nappe pile with a flat decollement at the basal

Tepio

Fig. 2. Terrane

550

isochron)

pink sandstones

continues

Visean

de-

(Fig. 4). This basement

Devonian

Devonian-Early

?ZJ Gfdhi a

et al,,

It

weakly

between

1980, Rb/Sr

overlain platform,

nappes. basement,

or

emplaced

Batik,

zircons,

middle

and external (Pan-African)

undeformed

granitoids

(Sharbert

tioning

The Moravian terrane corresponds to the Moravo-Silesian zone of Suk et al. (1984). It ex-

So~othurlnglan

foredeep

of a Cadomian

1985) have played

The Morau~an terrane

a

consists

and

more

and metamo~hism

de-

lb-At&an

LwmoGlrbenifet5us co¬e Ltform

basement Fig,

3. Cross-section

tbrc~gb

Middle - Upper Devonian

Culm

ex= Culm

the Moravian Carbonilerous lxx&~ (northeastern comer of the Jbhminn Massif) reintqaxtcd reflection s&n& data of Cizek and Tomek, in press). See location in Fig. 5.

arrandian

p

Gfdhl

m

Dtosendotf

m

Moravisn

lccl

La18 Granites

m

Early durbachites Ii Jd,

I\:‘\:,

u.i

I.i s

394r / 40AF

1

fib/

from

the boundary between the Moravian and Moldanubian terranes corresponds to a wide, low-angle, NW-dipping ductile shear zone with a large northeastward translation of the Moldanubian over the Moravian (Rajlich, 1987; Matte and Rajlich. 1988). In the boundary zone the 2000 m thick Bites gneiss unit, which is probably part of the Moravian basement, is severely sheared with NE-SW sheath folds and northeastward directed ductile motion of the hanging wall (Schulmann, this issue). In the Jesenik dome, the same type of NE-

increases towards the boundary with the Mo~d~ub~ zone and also along strike toward the no~h~t in such a way that beet and cover which are undeformed in the Brno area show strong shearing under epizonal metamorphic corx?itions in the Thaya dome and in the Svratka window. Towards the northeast in the Jesenik dorm, the Keprnik pencil gneisses are probably the equivalent of the Brno batholith and the Devoniaa volmsedimentary series is transformed to ~~~~t~ and staurolite m&xschists (Soucek, 1978). From the Thaya dome to the Kepr&k dome,

-e

b3wer

St whole rock

Fig. 4. Recent radiometric data on tbc southern part of the B&%~uI off.

f~.i. aMf 1.i.-upper and lower intercepts.

I”“,‘3

+

0

’ +

____-

---

VARISCAN

SHEARING

AND

TERRANE

shearing

occurs

by N-S

F’ and NE-SW

or even parent

but the shear planes

gently

show tight N-S zontal ion,

stretching

lineation

related

3). These

trending

which

subhori-

ages of 305-310 of

the

gneisses and micaschists (Fig. 4). In summary, the Moravian terrane

Ma

Jesenik

presents

a

external part of the Variscan Belt and even as a part of the southern foreland. The weakly deformed

Cadomian cover

Carpathian Moesia

basement

extends

foredeep

platform

with its thin epicon-

far to the east below and

near

even

perhaps

the Black

terrane,

the

Gfiihl

the

to the

Sea (Burchfiel.

recognized nappe

m sheet

and

and metagabbros.

overlies

Drosendorf klippes

of the Main

in a

anatectic,

paragneisses,

This thrust

terrane

as

Matura,

consists

in part

ortho-

amp~bo~tes the

and

1982)

of high-grade,

sillimanite-bearing

in Austria

(Fuchs

1976; Tollmann

in the

(Gfiihl,

Blansky).

Moldanubian

Thrust

nappe form

of

The lo(MMT)

at the bottom

of the Gfohl

terrane

superposition

of high-grade

over lower grade rocks,

and on the presence

low-grade metamorphism and for the section has been considered as the most

tinental

Gfiohl

1976; Thiele,

cation

is constrained

amphiboles

340 and 320 Ma.

The

large synformal

shearing.

as post-Namurian-pre-Perm-

and

between

2300

is, in our opin-

transpression

ian and by 39Ar/40Ar plateau

relatively Bohemian

faults

folds with

and

Bohemian Massif, this area also being characterized by various granitic batholiths emplaced

forming

SE-dip-

as normal

to the northeasterly

by the stratigraphy biotites

an ap-

of the cover

slaty cleavage

The age of the dextral

from

flat

in press). The Culm sediments

to NNE-SSW

axial planar

again

(Fig.

159

MASSIF

are refolded

locally,

have been interpreted

(Cizek and Tomek, fan-like

with,

displacement

to the basement

ping planes

BOHEMIAN

F3 folds and become

SE-dipping

southwesterly

relative

BOUNDARIES.

of lenses

gites and garnet peridotites origin along the boundary.

is based on the

of granulites,

found as boudins in the amphibolites disseminated in the anatectic gneisses association Fediukova,

and are in close

with the garnet peridotites (Dudek 1974; Misar et al., 1984; Dudek

Misar, 1985). Large massifs of acid granulites the highest

sheet of the nappe

1985; Scharbert, The Drosendorf

ecio-

of probable mantle Other eclogites are

pile (Matte

and and form et al.,

1988). terrane

consists

essentially

of a

1975). The Early Carboniferous molasse basin (middle Visean to Namurian) is interpreted as a syntectonic foredeep basin which received the

thick (over 6000 m) sequence of mainly pelitic metasediments (the so-called Drosendorf unit, including the Monotone and Bunte series of the

metamorphic detritus from the more western internal parts of the belt that were involved in intense orogeny and erosion between 350 and 320

Austrian geologists) with biotite-garnet-staurolite f kyanite + sillimanite schist and gneiss, sub-

Ma. The Moravian terrane was subsequently deformed at about 310 Ma, mainly along the transcurrent boundary with the Moldanubian zone. This led to transpressional decollement tectonics in the Carboniferous basin. The Moravian terrane is similar to the southern edge of the Massif Central and the eastern Pyrenees (Aquitaine Montagne and Noire terrane) where a Pan-African granitic basement

is also covered

deposits and 1986, 1988).

Culm

by shallow-water molasse

facies

Palaeozoic (Matte,

1983,

The Gfihl and Dro~endorf terranes These two terranes, formerly grouped into the Moldanubian zone, correspond to the most metamorphic and possibly the most eroded part of the

ordinate graphitic micaschists, talc-silicate gnetsses and marbles and few amphibolites and metadolerites. While part of this thick series may be Late Proterozoic in age, owing to the discovery of acritarchs Krumlov

in the graphitic micaschists of Cesky (Andrusov and Corna. 1976; Pa&ova,

1981) part is clearly of Early Palaeozoic age. The most prominent tectonic feature of both the Gfohl and Drosendorf terranes is the generally flat attitude

of the foliation

which has been folded

into large, open antiforms (the Moldanubian antiform with the central Moldanubian granitic batholith and synforms with the Gfohl and Blansky HP granulites). This foliation generally bears a strong minerai and stretching lineation with a very consistent direction around NW-SE. Stretching in a NW-SE direction is also demonstrated by intense boudinage, particularly in the Drosendorf

I60

terrane just below the MMT in the marbles of the Bunte variegated sequence at Cesky Kmmlov in Czechoslovakia (Rajlich, 1987) and near Krems in Austria (Tollmann, 1982). In some cases of very high shear strain it is possible to demonstrate that the stretching lineation corresponds to the transport direction. This is the case for the Doubravcany pencil gneisses of Kutna Hora (80 km southeast of Prague}. All the planolinear gneisses of this area, show a strong fabric (Nemec, 1965) corresponding to high-temperature prismatic quartz glide planes with C-axis concentrations along the X- or Y-directions. Augen gneisses with large K-feldspars show asymmetric S-C textures indicative of a southeastward shearing. The same type of high-temperature SE-directed shear criteria are observed in the Blansky and Gfiihl linear granulites (Matte et al., 1985, 1987) and in the Sneznik augen gneisses in Poland which are also probably part of the Gfijhl terrane. The Gfohl and Drosendorf terranes have suffered a complex multi-facies and polyphase tectonothermal history before their northeastward tr~slation onto the Moravian. The high-grade metamorphic events of the Gfiihl terrane (eclogite, grant&e and amphibolite facies) have been considered as Caledonian (Fuchs, 1976) or older (Zoubek, 1965; Holubec 1968; Chaloupsky, 1978, 1988; Zoubek et al., 1988). However, up to now there are no supporting stratigraphic or radiometric data available. Indeed, the U/Pb and Rb/Sr measurements made by Van Breemen et al. (1982), the recent U/Pb results of Kroner et al. (1988) and our preliminary 39Ar/40Ar results reveal only metamorphic events not older than 370 Ma (Fig. 4). The age of the high-grade metamorphism contemporaneous with the southeasterly shearing ranges between 460 Ma, the possible intrusion age of the sheared and metamorphosed orthogneisses, and 331 Ma, the age of the undeformed or weakly deformed late tectonic plutons such as the central Bohemian granodiorite which intrudes the Bunte series at Blatna (Czechoslovakia) (Van Breemen et al., 1982, Rb/Sr isochron). We have obtained similar 39Ar/40Ar plateau ages on the granites: 336 Ma on biotite of the earliest late tectonic granitoids intruding both the Gfijhl and Drosendorf terranes

3 I,

hl:\l’f

f

1 f ~1

and the Durbachite massif west of ‘Tahor (Czechoslovakia), and 330 Ma on the undeformed Mrakotin post-tectonic granite in the centre of the Moldanubian antiform (Fig. 4). These results provide an upper limit for the high-grade Barrovian metamorphism. The age of this metamorphism affecting the two terranes has been determined by various methods (Fig. 4) (e.g., 337-339 Ma by U/Pb ages on monazites from the anatectic Gfohl gneisses and Mohelno granulites (Van Breemen et al., 1982) and 367 and 347 Ma in the Bunte series below the Blansky klippe and on a sheared granulite west of the Svratka window respectively using W/Pb lower intercept dating on single grains of zircon (Kroner et al., 1988)). These authors consider these ages as reflecting the high-grade ~p~bo~te facies metamo~~sm. Van Breemen et al. (1982) conclude that 350 Ma is the age of the granulite facies metamorphism. In our studies of the high-grade metamorphism we have found a similar 39Ar,/40Ar plateau age (350 Ma) on muscovite in a marble. We have also obtained plateau ages of around 325 Ma on biotites in sheared granulites in the Gfiihl nappe (Matte et al., 1985) and in the planolinear orthogneisses of the Kutna Hora. These relatively young ages could reflect resetting either by the thermal effect of the huge central Moldanubian batholith or more probably by the second retrogressive metamorphism which affected the eastern part of the Bohemian Massif during the northeastward transcurrent shearing. The Drosendorf terrane, with its thick pelitic series, probably represents a thinned passive continental margin filled with Late Proterozoic and Lower Palaeozoic sediments adjacent to the Moravian continental block. The Gfijhl terrane is marked by an mundane of mafic and ultramafic rocks. It suffered the same tectonothermal history as the Drosendorf terrane (amphibolite facies metamorphism at about 350 Ma) but an important difference is the presence-of high-pressure metamorphism relicts (granulites and eclogites). The age of the high-pressure event is still keenly debated: Proterozoic according to Zoubek (1965) and Chaloupsky (X978), 446 f 35 Ma (Rb/Sr whole-rock isochron) according to Arnold and Scharbert (1973) and 350 it 5 Ma

VARISCAN

SHEARING

according Although

further

Variscan

similarities

with and

terranes may

(Matte,

is pre-

of

161

MASSIF

evidence terranes

the

be compared

to the Cevennes-Vendee

terranes

BOHEMIAN

et al. (1982). and Gfohl

Belt of France

respectively

BOUNDARIES,

geochronological

the Drosendorf

striking

Central

TERRANE

to Van Breemen

necessary, sent

AND

and Massif

1988) where the HP event

et al., 1966;

Holubec,

from bottom

to top:

1968) and

(1) The pre-spilitic slates). (2) The

spilitic

group

group,

(spilites

and keratophyres),

diorites

and

and

sedimentary

Vielzeuf.

flysch-type

1983).

associated

mantle

Drosendorf have

Moldanubian

between are interpreted

(MMCT).

lar to the Massif Central active-margin imbricated

tectonic (Lower

and

terranes

significance

Thrust

(MMT)

peridotites

and Gfiihl

the same

Central

Thrust

as the Main The Gfijhl

terrane, melange

to

Massif

terrane,

simi-

may represent in which

Palaeozoic)

the

oceanic

an

strongly crust

and

mantle, possibly of the back arc and perhaps of the more distal and thinned parts of the passive margin (Pin, 1989) were progressively telescoped together between 430 and 350 Ma.

(3) The post-spilitic

shales

composed

bros

and alum slates, graphitic

Main

(mainly

volcanics

is better constrained at about 430-400 Ma and the oceanic crust is dated at - 480 Ma (Pin and The

are as follows, and

of effusive mmor rocks

gab(pyrite

schists and black cherts). group,

pile with slates,

a thick sedimentary greywackes

and con-

glomerates. The Late Proterozoic

rocks are unconformably

overlain

by up to 2000 m of elastic

deposits

of Cambrian

sandstones

and

shallow-water

age (red and pink

conglomerates).

part has been well dated. using trilobite faunas. The Ordovician

Only

arkoses.

the upper

Middle Cambrian transgressive de-

posits (red-violet conglomerates, quartzites. shales and iron ores) may, unconformably. directly overlie the Proterozoic and are themselves overlain by a complete Silurian series of graptolite-rich black shales and limestones. The Devonian (200-500 m thick) is characterized by the development of carbonates alternating with shallow-water organo-

The Barrandian

terrane

detrital and deeper water cephalopod limestones (Praguian) and continues to the Givetian in which

The Barrandian terrane is separated Moldanubian zone by a major NE-SW Variscan

dextral

fault, the Central

Zone (Rajlich, 1987). Further the Zelezne Mountains, the tectonically Jelinek

overlies

Bohemian

Shear

to the northeast in Barrandian terrane

the Gfijhl

et al. (1984) maintain

from the trending

terrane

(Fig.

4).

that the Barrandian

block continues to the southeast of the Bohemian Massif, includes rocks of oceanic affinity (the Letovice amphibolites) and is in direct contact with the Moravian. In our opinion, the Letovice amphibolites are part of the Gfiihl terrane. The northern boundary of the Barrandian is probably a major NW-directed thrust reactivated by a dextral NE-SW trending shear zone. The Barrandian consists of a very thick Late Proterozoic (Algonkian) sedimentary and volcanic pile unconformably overlain by a well-dated Lower Palaeozoic sedimentary sequence ranging in age from Cambrian to Middle Devonian. The Late Proterozoic sequence (> 6000 m thick) has been divided into three groups (Kettner, 1917: Svoboda

arenaceous-pelitic

sedimentation

prevails.

Deformation and metamorphism randian is much less intense Moldanubian

and

of the

Bar-

than in the Its age is still

Saxothuringian.

controversial. Despite the presence of clear angular unconformities between Cambrian/Ordovician and Late Proterozoic tion

and

rocks. most of the deforma-

metamorphism

of the Barrandian

terozoic is attributed in this paper for the following reasons:

Pro-

to the Variscan.

The structural features in the Palaeozoic rocks of the Barrandian synchne and in the underlying Proterozoic are very similar. The strongest deformation occurs south of Prague (Orlik dam) where the rocks of the spilitic group (spilites. keratophyres and metadiorites) are strongly sheared with a pervasive vertical N50” foliation and an horizontal stretching lineation. Shear criteria clearly show that this deformation is due to transcurrent dextral shearing along the NE--SW boundary between the Barrandian and the Drosendorf-Gfiihl

terranes

(Rajhch,

1987. 1988).

162

This shear zone may reach 6 km in width. Locally, the Palaeozoic rocks of the Barrandian synchne are tightly folded (for instance, the Silurian and Devonian strata of La Roche Barrande south of Prague) and the Cambrian and Devonian conglomerates are strongly stretched in the same NESW direction (Rajlich, 1988; Rajlich et al., 1988). Nevertheless, the possibility of a low-grade Precambrian metamorphism in the Proterozoic rocks of the Barrandian area cannot be ruled out. The upper age limit of Variscan deformation in the Barrandian area is constrained by the presence of much less deformed Westphahan conglomerates and by the age of the Central Moldanubian pluton (331 Ma) (Van Breemen et al., 1982) which is located along the shear zone but did not suffer the main transcurrent deformation. The Barrandian terrane is a central block separating the NW and SE-verging parts of the Bohemian Variscan Belt. The existence of a PanAfrican metamorphic basement has not been demonstrated. Compared to the adjacent terranes, the Variscan deformation and rne~rno~~srn are very weak. This block pinches out westwards between two former intracontinental oceanic subduction zones which bear opposing orientations. The Barrandian terrane is litholagically comparable to the Central Brittany terrane which has

Bohemian basin in the Sudetic Mountains (Krkonosce, Kaczawskie and Sowie Gory). The Miinchberg-

Tepla terrune

The Mtinchberg terrane outcrops mainly as large klippes: In the West German part of the S~oth~~~ area it comprises the Miinchberg and the ZEV and possibly also allochthonous? eclogite-bearing gneisses east of Nabburg in the western limb of the Oberpfalz antiform (Matthes, 1978); other small khppes exist to the northeast in the DDR. The new deep reflection seismic data (DEKORP 4) recorded for the deep continental borehole (Weber and Vollbrecht, 1986) leave little doubt as to the allochthony of this inverted metamorphic pile. This concept was first proposed by Kossmat (1927). The MUnchberg terrane comprises the fohowing, from top to bottom (Behr et al., 1984): (1) A high-grade complex of gneisses, amphibelites and eclogites (Matthes, 1978) with a rn~i~-press~e met~o~~srn dated around 380-390 Ma by K/Ar (Kreuzer et al., 1989) and U/Pb

(Gebauer and Grunenfelder, 1979). The HP

metamorphism (eclogite) is probably older, at around 420-430 Ma (Quadt and Gebauer, 1988). (2) A lower grade complex with peridotites, amphibolites, metavolcanics and phy-Bites. (3) An epizonal Palaeozoic series of elastic

been subjected to a similar, simple, low-grade strike-slip tectonism. The difference is that a Precambrian epizonal metamorphism has been clearly

sediments, volcanics and radiolarian cherts rang-

demonstrated in the southern part of Central Brit-

ing from Ordovician to Devonian.

tany.

(4) A lowermost sole with wildflysch and a tectonic melange of Early Carboniferous age.

Miinchberg-

Tepla and Saxothwingian

terraws

These terranes are the main units of the Saxothuringian zone and outcrop north and northwest of the Barrandian. The contact between the Barrandian and the Saxothuringian is a major fault which is largely obscured by PermoCarboniferous and Cretaceous cover. This fault is presumed to be the root of the Munchberg, Erbendorf-Vohenstrauss Zone and Frankenberg klippes (Behr et al., 1984). It is very likely that these terranes extend northeast of the Cretacecus

This allochthonous pile, reaching 4-5 km in thickness, overlies Upper Devonian and Lower Carboniferous rocks of different sedimentary facies (Franke, 1984). All these klippes are rooted in the Tepla area on the western comer of the Barrandian block. The Teplu terrune is very similar to the Munchberg. It consists in a 5000 m thick inverted metamorphic pile of mainly mafic/ultramafic rocks dipping 30-45 o southeast. These rocks include the folIowing, from top to bottom: (1) Garnet-bearing fIaser gabbros. (2) Garnet-rutile-bearing amphibolites eclogites.

and

VARlSCAN

SHEARING

AND

TERRANE

(3) Serpentinized

amphibolites.

(5) Epizonal

volcanics

by

sequence,

(tuffs, agglomerates

of probable

acterized

by a strong lineation

terrane,

ophiolitic

1984), is structurally

gneisses.

stretching

ward shearing

Deformation

pervasive

NW-SE

corresponding

in mesozonal

which is squeezed

and the Saxothuringian

and origin

lar to the metabasalts

terranes,

blue

or

found

klippe

amphiboles

have

In

been

Poland

phism

(Guiraud

fined

39Ar/40Ar

glaucophanes

and

the youngest

may be consid-

phism

which

allochthonous

found

of the

in which

(Hofmann

Czechoslovakia

Burg,

LT-HP 1984).

et such

metamor-

A poorly

de-

age at 367 Ma on these

(Maluski,

resent

uppermost

and

plateau

the Barrandian

(Fig. 5).

conditions

at the bottom

rocks also show a glaucophane

The Tepla

alkali

of Freiberg

al.,

char-

1989).

north

overlain down-dip

conditions. between

intraplate

et al., 1986). These rocks are very simi-

is

to northwest-

ered as the roots of the Munchberg terrane

of

(Narebski Frankenberg

and Tonika,

sillimanite

163

MASSIF

indicative

and cherts.

This (Kastl

BOHEMIAN

peridotites.

(4) Striped basalts)

BOUNDARIES.

Unpubl.

data)

age limit

occurred

prior

could

of this

rep-

metamor-

to deposition

of the

Visean.

South

of Wroclaw

in Poland,

high-grade gneissose characteristics similar

the Gory

Sowie

pile exhibits metamorphic to those of the ZEV massif.

The Saxothuringian terrane This is a thick metapelitic and magmatic pile ranging in age from Proterozoic to Early

The medium-pressure high-grade metamorphism which follows the granulite metamorphism seems

Carboniferous. The oldest rocks outcrop in the Erzgebirge-Fichtelgebirge and probably also in

to have occurred at about 390-380 Ma (Van Breemen et al., 1988) as in the Munchberg terrane,

the Oberpfalz metapelitic Cambrian

antiforms,

of a

as Proterozoic-Early

1986).

orthogneisses

Locally,

lenses

(Weber

and Vol-

of granulites

and

is itself

phism.

in age (Pflug and Reitz, 1987) intruded

by 490 Ma granitic lbrecht,

which are composed

series considered

and

ultramafics are found. These deep-formed rocks mainly outcrop in the Granulitgebirge, which is in our opinion a mantled gneiss dome of granulites surrounded by Lower Palaeozoic metasediments separated from the granulitic core by a low-angle detachment fault (Fig. 5).

overlying

a lower grade

nappe

unit

amphibolite

the Sudetic area situated more to the northeast, the few published data and our own experience in the field tend

to confirm

the general

assumption

that the Mtinchberg-Tepla and the Saxothuringian terranes continue in the Sudetes: North of the Krkonosce granitic massif, the Izera granitic orthogneiss is very similar to the orthogneisses of the Erzgebirge (Borkowska et al., 1980). South and east of the Krkonosce Variscan granite, Silurian metabasalts with REE characteristics of island-arc tholeiites tectonically overlie Cambrian spilites and keratophyres which bear REE characteristics

of

peridotite,

(Znosko,

which bears oceanic

1981;

is also

metagabbro

and

Majerowicz,

1985)

REE and Sm/Nd

tics. An age of 350 Ma has been these oceanic rocks using Sm/Nd et al., 1988).

characteris-

determined isochrons

for (Pin

The most characteristic tectonic feature of the Saxothuringian area is a flat foliation related to northwestward nappe emplacement; a more comthonous

for

metamor-

as a flat nappe

regarded

plex structural

locally reaching a thickness of 2000 m. Although much less information is available

LP-HT

massif

ous, with a Lower Ordovician

series

by

gneissose

The essentially pelitic Palaeozoic sequence is continuous from Cambrian to Lower Carboniferquartzopelitic

overprinted

The Sowie Gory

history

Munchberg

is registered

in the alloch-

and ZEV units. This foliation

is gently refolded in antiforms and synforms. It bears a stretching lineation which consistently follows

a NW-SE to NlOO” direction. This lineation is particularly well developed in the Erzgebirge gneisses in Czechoslovakia and East Germany and is related

to large

flat shear

zones

which

show

general northwestward to westward displacements, Local southeastward shearing, as found by Rajlich and Synek (1987) may be related to conjugate S-dipping shear zones which are created as a result of a flattening component. This shearing may also be related to normal ductile southward faulting on the southern limb of the Erzegebirge and Granulitgebirge domes.

164

in the Palaeozoic metasedimentary series underlying the Sowie Gory massif most of the lineations trend NE-SW. These lineations could be due to an overall northeastward displacement during the late stages of transpressional tectonism which affected the northeastern comer of the Bohemian Massif. The tectonothermal history in the Saxothuringian and Munchberg-Tepla terranes is complex: The highest, most metamorphic units of the Munchberg-Tepla allochthonous terrane show granulitic to eclogitic metamorphism followed by an ~p~bo~te facies medium-pr~s~e event and locally HP-LT metamorphism (sod& amphiboles in the underlying metabasalts). The upper age limit of this event is around 380 Ma, both in the Munchberg (Kreuzer et al., 1989, K/A@ and the Sowie Gory klippes (Van Breemen et al., 1988, Rb/Sr and U/Pb). The Miinchberg and Frankenberg kiippes were transported northwestward after this metamorphism onto epizonal fossiiiferous rocks ranging in age from Ordovician to Late Visean. This movement occurred between the formation of the upper Visean Culm facies which shows slaty cleavage and the formation of the tilted but undeformed uppermost Visean mofasse-type coal-bearing deposits which are preserved in small basins. The parautochthonous Saxothuringian terrane similarly shows m&i-facies media-pressure metamorphism. In the Erzgebirge dome, metamorphism gradually increases with depth, eventually reaching granulitic conditions (muscovite-free rocks). Locally, pyroxene and p~oxe~e-g~et grant&es are found. In the Granulitgebirge dome, HP-HT granulites (Grew, 1989) are separated from the overlying pelitic gneisses by a slightly folded S-dipping normal fault (Fig. 5). The granulites are either an old basement, as indicated by a Rb/Sr whole-rock isochron around 2 Ga (Werner et al., 1982) or, more likely, equivalents of the deep crustal Variscan grant&es which outcrop in various massifs of the Variscan Belt (Pin and Vielzeuf, 1983). An age of 380 Ma has been determined by U/Pb and Sm/Nd dating on these Saxonian granulites (Quadt and Gebauer, 1988). Whatever the case

these rocks have been brought to the surface along a low-angle detachment fault during a late collapse of this thickened part of the crust in the same manner as that proposed for other orogens (Malavieille, 1987, Eisbacher et al., 1989: Malavieille et al., this issue). The age of the medium-pressure amphibolite facies metamorphism in the cover of the Granulitgebirge and in the Erzgebirge is not dated, although it is understood that this metamo~~sm occurred later than the Cambro-Ordovician and probably in part prior to the formation of the uppermost Visean molasse. Parts of the Saxothuringian zone have locally suffered a low-pressure metamo~~sm probably related to the ascent of anatectic granites (Oberpfalz antiform); this metamorphism is dated at around 330-320 Ma by the U/Pb and K/Ar methods (Carl et al., 1985; Quadt and Gebauer, 1988). The Miinchberg-Tepla and Saxothuringian terranes are obliquely cut by the Bray dextral fault (Fig. 1) and in France have not been well recorded. Positive Bouguer anomalies along the Bray fault could represent mafic rocks which may be related either to the Miinchberg-Tepla suture or to the Lizard-Rhenish suture (Gazes et al., 1985; Matte and Him, 1988). Further to the west the Saxothuringian and Miinchberg-Tepla terranes. could he beneath the English Channel as part of the pre-Mesozoic basement between the Lizard thrust in Cornwall and the nor~w~tem edge of Brittany where 380 Ma old eclogites have been found (Paquette et al., 1987). Condu&m~ the Varim

Terrmes and suture axwrekdims in Belt

The most conspicuous tectonothermal events in the Bohemian Massif are Variscan in age and occurred roughly between 430 and 300 Ma. Older HP met~o~~srn has not yet been accurately dated. By analogy with the Massif Central the eclogitic metamorphism of the Gfiihl unit could have occurred at around 450-400 Ma, during the first stages of plate convergence (Matte, 1986), but a younger age (- 350 Ma) cannot be excluded. Granitic magmatism between 600 and 490 Ma may represent Pan-African intrusive activity, as in various other parts of the Variscan Belt. Evidence

VARISCAN

AND

SHEARING

for the existence contained

TERRANE

of older

in detrital

BOUNDARIES.

Precambrian

zircons

It is difficult,

ages

is

range

to assess

the

cross

Variscan

section

Bohemian

cally very similar

the

Belt appears

France

(Matte, the same

tively

weakly

central

block flanked

ing directions

through

1986;

Matte

fan-like

and

Him,

arrangement

deformed

and

by mobile

Behr et al. (1984)

belts with oppos(1986) proposed

basement

same

to the general

Precambrian-Caledonian

and its epicontinental

sedimentary

rocks

The could

trend

offset

is slightly

of the belt could of the Brittany

terrane

found

dextral

and Leveridge,

1988) that

(Fig.

ing subduction zones are perhaps neous. The young ages (375-350 along

obhave

terrane

1). The opposnot contemporaMa) of some of

the northernmost

suture (Davies, 1984; Pin et al., 1988) and the talc-alkaline affinity of the Upper DevonianLower

Carboniferous

Vosges and northern cover.

A large

the separation

the oceanic

zones

and large-scale

fault (Holder

from the Barrandian

The northover 2000

with each other.

the Early Carboniferous

et al., 1989). and Him,

at all, is so

Saxothuringian

shortening

during

along the Bray-Vittel lique

and

in contact

seems

Forest-Vosges

block, if present

Moldanubian

faulting

accentuated

and Matte

In the Black

Massif

of the two subduction

1986; Matte

mobile belts reflect at least inand/or perhaps intraoceanic sub-

the

the

are nearly

(Eisbacher

km from Brittany through Germany to Poland and the northern stable foreland is clearly repreby

that

It

of a rela-

metamorphosed

duction zones with opposing polarities. em part of the belt shows a continuity

sented

thin

strike-slip

of thrusting.

that these tracontinental

westward.

in

1988).

in the Bohemian

be partly due to extreme

to be geometrisections

terrane

juxtaposition

NE-trending

to the more western

shows

to taper

zones

source of these zircons. The

Barrandian

area, this intermediate

rocks and ortho-

however,

165

MASSIF

crust

whose

from 2 to 2.5 Ga in sedimentary gneisses.

BOHEMIAN

an oceanic

domain

volcanics Massif

in the

Central

northern

indicate

could have existed

that

to the north

The Barrandian/ Saxothuringian boundary is related to a deep-seated northwestward directed

at this time. Franke (1989) proposed that the northern suture is in fact a double suture and

thrust reaching the Moho, and can be seen from deep seismic data (Cazes et al., 1985; BIRPS and ECORS, 1986; Behr and Heinrich, 1987; Matte and Hirn, 1988; Le Gall, in press; Bois et al., this

corresponds to two oceanic domains, one the Erbendorf-Munchberg-Tepla domain closing during the Silurian and the other the Rhenohercynian domain closing during the DevonoCarboniferous.

issue). Because remnants of oceanic crust and high-pressure metamorphism are found along this major thrust, it is tempting to interpret this boundary

as

Saxothuringian/ truely represents oceanic

basalts

more external Lizard.

the

main

northern

suture.

The

Rhenohercynian boundary, if it the roots of the Giessen nappe and sediments, northern

suture

could

represent

related

to

a the

In the same way, the Barrandian/ Moldanubian boundary as the root of large, southward displaced nappes with ophiolites and HP metamorphism is a good candidate for a southern suture in the Bohemian Massif. Although severely disrupted by a major dextral transcurrent fault. it could correlate with the suture in the northern part of Massif Central. The application of the terrane concept to the Bohemian Massif and the comparison with the Massif Central poses four questions: (1) Bidirectional subduction. The intermediate

(2) Southern boundary of the Variscan Belt. The Moravian terrane with its Pan-African basement and thin Palaeozoic epicontinental cover could be the southern foreland of the Variscan Belt. lt is assumed

to extend

below

the Carpathians

perhaps

platform.

more

The

the external

nappes

of

as far as the Moesian internal

parts

of

the

Carpathians. such as the Tatra horst. show a prePermian basement composed of high-grade rocks and granites of probable Variscan age. The Permian position of the Tatra block is difficult to define exactly but it was probably 100 to 200 km further to the south. Thus the Moravian terrane may not be the true southern foreland of the Variscan Belt but a stable microblock surrounded by two branches of the Variscan Belt. The southernmost branch outcrops in the basement of the Carpathians, the Balkans and the Pontides (Zagorcev and Moorbath. 1986). (3) Northeastern termmation of the V&scan

166

Belt. A simple look at the tectonic map of Europe

or the Magsat map shows that the Variscan Belt strikes broadly in an E-W direction over 2000 km from Ireland to Poland and ends abruptly against the Russo-Baltic shield along a NW-SE boundary. This boundary is represented by two major parallel faults, the Elbe and the Tomquist lines, which enclose a large Carboniferous basin (Pozaryski and Radwandski, 1979). Regional considerations and preliminary observations in Poland around Wroclaw indicated that these faults acted as dextral wrench faults during the Carboniferous (Arthaud and Matte, 1977). Such Late Carboniferous dextral transcurrent faults may have offset the eastern extension of the Variscan Belt to the southeast in Dobrogea (Burchfiel, 1975) and in the Caucasus where Variscan deformation, granites, metamorphism and sutures are known (Adamia et al., 1981). It is also likely that these faults existed in pre-Variscan times. The Tomquist-Tram European fault line separates two very different crusts (EUGENO-S Working Group, 1988). To the northeast, the thick (40-50 km) crust of the Russo-Baltic shield mainly consolidated between 2000 and 1500 Ma. To the southwest, a thinner crust (30-35 km), mainly composed of Late Proterozoic metasediments and granitdids, is, for the most part, 600-800 Ma old but some relicts of an older basement dated between 2 and 2.5 Ga are also present. NW-SE trending faults such as the Tomquist-Trans European line seem to have also influenced the Devono-Carboniferous sedimentation in the Moravian basin (Dvorak, 1982). It is thus very likely that the Tomquist-Elbe zone had first been a Late Proterozoic suture or a deep basin and that the major faults played the role of transform faults during the closure of the Palaeozoic oceans and during the Variscan intracontinental shortening. Despite the occurrence of a Caledonian (post-Ordovician, pre-Gedinnian) deformation in Brabant and the Ardennes (Zwart and Domsiepen, 1978; Andre et al., 1981; Delvaux et al., 1984) and in southern Denmark (EUGENO-S Working Group, 1988), the existence of a north German-Polish ocean is still debated: there are no outcrops of oceanic rocks except for the talc-alkaline mafic magmatism of

Ph

‘4.4

TT t t. I hi

Brabant (Andre et al.. 1986). However, palaeomagnetism does indicate a separation in Ordovician times between Baltica and an “Armorica plate” to the south which includes Wales and Brabant (Perroud et al., 1984) while during this period major faunal differences nevertheless exist between Baltica (including Wales and the Ardennes-Brabant) and the more southern blocks * (Paris and Robardet, this issue) thus favouring a Rheic ocean with a corresponding suture(s) running from the English Channel to the Saxothuringian area. (4) Dextral displacement on the main Central Bohemian and Moravian shear zones. The displacement on the Central Bohemian shear zone could have been accommodated along NW-SE trending steep axial planes of folds in the metamorphic series extending from Prague to Wroclaw in the eastern comer of the Bohemian Massif. Displacement on the Moravian shear zone was accommodated by the strong N-S to NE-SW folding and formation of the slaty cleavage in the Carboniferous basin. It may also be accommodated by northeastward thrusting along the Elbe line as is apparent in Poland from the Wroclaw area up to the Holy Cross Mountains (Kutek and Glazek, 1972; Mizerski, 1979).

We are particularly indebted to our guides in the Bohemian Massif: In Czechoslovakia Academician W. Zouhek, V. Harms, J. Dvorak, J. Chaloupsky, A. Dudek, J. Schovanek, B. Mlcoh, J. Holubec, J. Masek, C. Tomek, M. Urban, V. Kachlik and K. Schulmann, in Polmd, B. Wajsprych, Z. Baranowsky and A. Zelazniewicz, and in the GDR, M. Schwab, J. Rolzler, P. Bankewitz, J. Hofmann, A. Fiischbutter and M. Kurze. G. Eisbacher, D. Keppie and H.J. Zwart have reviewed the manuscript- and improved the English. We have benefited from their great experience in the Palaeozoic Belts.

*

InMmsof~~W~8nd3fttb~t~part0f

but dUkg the Ckdoti~ ogy, they were part of Baltica.

~Orica,

in

termsof palaeontol-

VARISCAN

SHEARING

We

thank

drawings

AND

TERRANE

G. Garcia

BOUNDARIES.

and

and Mrs. Roth

BOHEMIAN

M. Boeuf

for the

and Mrs. Faure

for typ-

work

K/Ar-Datierungen Cazes,

was

Czechoslovakian C.N.R.S.)

Carl, C., Dill, H., Kreuzer, Oberpfalz.

ing the manuscript. This

167

MASSIF

supported

scientific

by

the

French-

cooperation

and forms part of IGCP

Project

Raoult,

233.

Nord

J.F.,

Damotte,

A., Matte,

1985. Structure

de la France,

und

Hohenstein/

74 (3): 483-504.

G., Bois, Chr.,

A., Mascle,

I., 1985. U/Pb

Uranvorkommens

Geol. Rundsch.

M., Torreilles,

A., Him,

(D.R.C.I.-

H. and Wendt,

der

B., Galdeano,

Ph., Van Ngoc.

de la crofite

premiers

resuitats

P. and du

hercynienne

du profil

ECORS.

Bull. Sot. Geol. Fr., 1 (6): 925-941.

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