Inversion tectonics along the Western margin of the Bohemian Massif

Tecronophwrcs.

93

137 (1987) 93-100

Elsevier Science

Publishers

B.V.. Amsterdam

- Printed

in The Netherlands

Inversion tectonics along the western margin of the Bohemian Massif BERNT SCHRiiDER Geolog~al Institute.

Ruhr University Bochum, P. 0. Box 102148. D-4630 Bochum (F. R. of German_v) (Received

November

3. 1985; accepted

March

13. 1986)

Abstract Schriider,

B.. 1987. Inversion

Compressional

Intra-Plate

The block-faulted malor

faults

basins

associated

became

with

this

along

Deformations

southwestern

of which

Cretaceous-Early

tectonics

margin

repeatedly fault

system

the western in the Alpine

of the Bohemian

margin

of the Bohemian

Foreland.

Massif is superimposed

reactivated

during

occurred

during

the Mesozoic the

Early

and

trending

(Fig. 2). This graben the Stephanian-Autunian induced shortly

again

graben.

of sedimentary

during

the

latest

Late came

Palaeozoic

after

graben

gravity

repeatedly reactivated. This paper deals mainly with evolution of the 120 km long and NW-SE striking block faulted area the northeast by the Franconian and west by the Eisfeld-Kulmbach-Freihung I’ 1987 Elsevicr

(Ziegler.

the tectonic 20 km wide bounded to to the southfaults

Science Publishers

Mountains,

with the Bavarian is based

profiles

which

last five years (Fuchs

of the Variscan

its consolidation

Naab

halfwhich

Pfahl fault zone

on detailed

field observa-

tions carried out over the last 100 years, a number of boreholes drilled since 1900, very limited reflection seismic lines (Breyer, 1972: KTB, 1985) and

into evidence during phase of wrench fault-

the break-up

with the Bodenwiihr

and the adjacent

The paper

1982). The eastern boundary of this Late Palaeozoic graben, which consisted of a number of differentially subsiding fault blocks, coincides with the present-day western margin of the Bohemian Massif. During the Mesozoic and Cenozoic the fault systems of the Bohemian Borderzone became

0040-1’)51/x7/$03.50

Inversion

(Fig. 2).

Bohemian Massif, delimited by the Franconian and Eisfeld-Kulmbach faults, is here referred to as the Bohemian Borderzone. It is superimposed

fold belt

Cretaceous

are associated

The study area is shown in Fig. 1. The blockfaulted zone along the southwestern margin of the

ing which

(Editor).

on a Permo-Carbomferous

and Cenozoic.

and. to a lesser degree, graben

a NW-SE

In: P.A. Ziegler

137: 93-100.

Cenozoic,

Introduction

on

Massif.

Tectonophwcs,

B.V.

were recorded

during

the

and Soffel. 1981; Ernstson.

1982). Supporting data are provided by sedimentological studies (Mielke. 1982; Klare and Schrbder, 1986), fluid inclusion analysis on quartz grains (Vollbrecht, oral commun.. 1984) and radiametric age determinations on mineralisations along the Pfahl fault zone (Horn Sediments preserved along Borderzone

are in its northern

et al.. 1983). the Bohemian

parts 2000 to 3000

m thick and in its southern parts up to 1000 m. The Permo-Carboniferous troughs contain up to 2000 m of continental elastics. Important faultcontrolled basins occur near the towns of Kronach and Weiden and two smaller ones near the Naab Mountains and the city of Regensburg (Fig. 3).

development

of the Bohemian

Borderzonr

the

reader is referred to Leitz and SchrZidrr ( 39x5 1. Late Hercynian basin development

r7-q

I

0

Cenozoic

1Mesozoic

m

km

100

I

Paleozoic f crystall. basement

Fig. 1. Location map of area of interest

Late Permian Zechstein, Triassic and Jurassic sediments reach in northern areas a thickness of some 1500 m whilst in the south some 500 m thick Early Triassic to Late Cretaceous strata are preserved. The structural development of the area under discussion (outline in Fig. 1), has been summarised by Schr&ler (1968, 1976) on the basis of five block diagrams depicting the evolution of the Bohemian Borderzone from the Late Jurassic to the recent. Figures 4 and 5 present a sequence of evolutionary cross-sections along a line crossing the Bohemian Borderzone in a SW-NE direction, north of the Naab Mountain in an area of maximum sediment preservation (Fig. 3). For a detailed review of the stratigraphic and structural

I

The occurrence of Late Palaeozoic sediments is restricted to the area of basement outcrops along the Bohemian Borderzone. The lateral arrangement of the individual basins suggests that their development was controlled by deep seated wrench faults. In the Kronach-Weiden Graben sedimentation commenced during the Stephanian in a fault controlled depression at the foot of an active fault scarp (Visscher and co-workers, oral commun., 1986). Synsedimentary fault activity persisted during the Early Permian as suggested by lateral thickness and facies changes and the analysis of pebble assemblages (cf. Leitz and SchriSder. 1985). The distribution of these, predo~Mntly red beds, up to 2000 m, is well documented by outcrop observations, boreholes and gravity data. These elastics were mainly derived from an eastern source area for which a Permo-Carboniferous uplift of some 4 to 5 km has been estimated (Helmkampf et al., 1982). The outliers of Permo-Carboniferous sediments near Schmidgaden (Helmkampf and Waeber, 1983) and Donaustauf accumulated either in separate basins and/or represent remnants of formerly larger basins which were partly destroyed by erosion prior to the transgressions of Late Permian to Triassic sediments. Such an erosional phase in the northern Stockheim and Weiden basins preeeeded the transgression of the finer-grained “Upper Rotliegend” elastics and, further to the south, in the area of Bayreuth-Weiden the transgression of the “Bunter” elastics (Figs. 4 and 5; Fuchs and Soffel, 1981; Ernstson, 1982). This erosional event was accompanied by the deformation of the Stephens-E~ly Permian graben zone. Limited reflection data indicate the occurrence of compressional deformations in the Weiden basin (KTB, 1985) and similar features have been observed in outcrops. For instance, in the Stockheim basin, N-S trending compressional features (e.g., anticlinal and synclinal axes and W-E striking overthrusts) involving PermoCarboniferous strata are unconformably overlain

r.

-

_.

.*

E : WB= SB= DB =

-

-

ErbendorF Weiden Schmidgaden ~o~~~st~u~

S 2 Stockheim

Lower Permian basins :

.Y

A E-K F NB SB WH

.T7___

= = = Freihu~g = illorthern branch 7 = Southern broric i = Hahnbach f-&$-i

Fault

-

.

__. _

m...

1

BASEMENT

.

..

’

u

1 &

Fig.

3. Generalized

the Kulmbach-Freihung gaden.

geological

map

of

fault zone (after

Bohemian Gudden,

Borderzone. 1983).

Lower

showing

structural

Permian

basins:

Upper

contours ZI-Donaustauf.

at

Cretaceous

top hasemcnt E- --Erbendorf,

level west

of

S-Schmid-

ST-Stockheim.

by Late Permian sediment (Saxonian and Zechstein). These features were exhumed by erosion as a consequence of Mesozoic-Cenozoic blockfaulting and inversion. This suggests that the present Bohemian Borderzone is superimposed on a Stephanian to Early Permian graben which underwent transpres-

sional deformation at the transition Autunian to the Saxonian.

from the

Triassic-Jurassic

Synsedimentary tectonic activity persisted into the Early Triassic but was now concentrated on

91

Hahnbach 1 pre-Late 2 uplift

20

10

0

Cretaceous

during

Kaltenbrunn

High upllft

1 latest relief

Late Cretaceous

with upthrustlng tlonal slldlng

and

2 post-Cretaceous max IL00 m

gravlta-

Franconian fracture zone

High

Cretaceous -1300m

1 Permian faulting

tectonic

the eastern

margin

cross-section

through

Bohemian

accumulated

fault zone. During Massif

at the foot of the Franconian the Early Triassic

apparently

Franconian

became

the Bohemian

uplifted

line and shed coarse

along

elastics

the

into the

adjacent lowlands. Westward these fan deposits grade into finer alluvial plain sediments which transgressed over Permo-Carboniferous 1986).

the western margin of the graben (Klare and Schroder.

Further support for Early Triassic tectonic activity is lent by hydrothermal quartz mineralisations along the Pfahl fault zone and by genetically related ore veins at its northern end. These were radiometrically dated as f 240 Ma (Horn et al., 1983). Isopachs and facies patterns of Middle and Late Triassic and Jurassic sediments indicate an area of maximum subsidence immediately west of the Eisfeld-Kulmbach and Pfahl fault zones which is referred to as the Coburg-Bayreuth Basin. Within

it these strata

attain

a thickness

and

For location

see Fig. 3 (lint .4 R )

1500 m, whereas in the actual area of the Bohemian

of the Permo-Carboniferous

graben. The geometry of Early Triassic isopachs, supported by grain-size analysis of source areas of conglomerates, indicate that a belt of alluvial fan deposits

Bordelzone.

flexure

2 prelate Cretaceous upllft and erosion of the sedimentary cover 3 latest Cretaceous upthrustlng L relative subsidence during Late Tertiary

erosion

3 latest Cretaceous upllft and activity along Pfahl fault zone 1 tectonic relief -5OOm 5 oost-Cretaceous erosion -55Om

Fig. 4. Structural

10km

30

of some

Borderzone, between the Eisfeld-Kulmbach and the Franconian fault zone. their thickness is of the order of 1000 m (Schroder, 1976). In the northern parts of the Coburg-Bayreuth Basin sedimentation was continuous from the Late Permian to the Late Jurassic and in its southern parts from the Early Triassic to the Late Jurassic. Although there is no evidence for intraJurassic faulting the axis of this basin coincides with the Permo-Carboniferous fault system.

Early Cretaceous

trace

of

the

inversion

During the Early Cretaceous tectonic activity along the Bohemian Borderzone accelerated again. The area along readily

experienced identifiable

spectacular inversion axes. This involved the

compressive reactivation of pre-existing fault systems and specifically of the border faults of the Permo-Carboniferous graben zone. Within this graben zone Mesozoic strata became tilted to the west. uplifted by some 1000 m (Schriider, 1968) and deeply truncated prior to the

I End of Upper Cretaceousl=Campanianf

I

+i

-E

Base of Upper Cfetaceous

End of Jurassic

\

Nercynion

basement

Fig. 5. Evolutionary cross-section through Bohemian Borderzone. Same location as Fig. 4.

transgression of Late Cretaceous strata (Fig. 5). Jurassic-Triassic series are generally preserved in narrow half-grabens separated by broad erosional highs, as evident in the area of Bayreuth (Fig. 3). Eastward the rate of uplift apparently increased and it is estimated that the Bohemian Massif became uplifted along the Franconian fault zone by some 1500 m. This induced the total erosion of its former Triassic-Jurassic sedimentary cover To the west of the Bohemian Borderzone the Franconian Platform also became uplifted and eqosed during the Early Cretaceous, This is illustrated by the erosion and karstification of the Late Jurassic carbonates which show an erosional pre-Late Cretaceous rehef of a few ht.&red me&es. The timing of this tectonic activity is difficult to determine as no Early Cretaceous strata occur

in the area. Mormver dating of the transgress&e basal Late Cretaceous non-marine sediments is stiil uncertain (Gudden, 1984; Leitz and schriider, 1985). In view of this it is not clear whether these deformations occurred in one or in several phases. By analogy with the stratigraphie record of the German Molasse Basin (Ba&mann et d., 1987) an early defo~~on phase may have occurred during the Late Berriasien to Early Vala+nian and a second one during the Aptian. Whether Late’ Valanginian to Barremian sediments were ever deposited in the area of the Bohemian Borderzone and were subsequently completely removed is unknown. The earhest Late Cretaceous strata are tentatively dated as Cenomanian-Turonian (see Ziegler, 1987, pp. 395-396, fig. 2).

99

Late Cretaceous

Basin development

due to a very incomplete a review

Following parent

the Early

nondeposition,

Cretaceous sedimentation

ing the Cenomanian/Turonian the Campanian.

non-marine

conglomeratic

sands.

durinto

strata

marine

Turonian

Campanian

conglomeratic

Cretaceous

sediments

mian

resumed consist

of

shaly and silty, in part

Distal

to the Late

of ap-

and persisted

Late Cretaceous

predominantly restricted

period

influences and

sands

preserved

are

Santonian.

series

and

basin,

basin

the Maastrichtian Borderzone

deformations.

ceased

to Danian

became These

of major basement

is

(1983).

in the Regensburg

transpressional the uplift

of the area the reader and Geyssant

during

For and

blocks

affected

by

resulted

in

mainly

along

the Pfahl fault zone. These fault blocks are associated with steep reverse

locally

even

Borderzone.

referred

time

and the Bohemian

the Bohe-

tion

of the Bohemian

Cretaceous

Sedimentation some

record.

Cretaceous

along

Borderzone.

Carboniferous

deformations to Bergerat

latest

are the youngest

These Late Cretaceous sediments transgressed over the truncated Jurassic, Triassic, Permobasement

Cenozoic referred

stratigraphic

of the complex

over

the

This Late

to as the Regensburg

of pre-existing

portant

fault

role during

probably

(Fig. 4). Reactiva-

systems

played

this deformation

occurred

and Paleocene.

faults

during

the Latest

This is in keeping

graphic

record

Molasse

Basin

of

the

Cretaceous

with the strati-

German

(Bachmann

an im-

phase which

and

Austrian

et al., 1987:

Nacht-

Basin, was limited to the northeast by the tectonically active Franconian and Pfahl fault zones.

mann phase

Across these fault zones coarse elastics, derived from the Bohemian Massif, were shed into the

contributed substantially to its present configuration. It was accompanied and followed by the

Regensburg Basin. At the same time large areas of the Bohemian Massif became deeply kaolinized. The marine influences in the Regensburg Basin suggest that it was in communication with the marine Helvetic Shelf Basin of which the area of the German and Austrian Molasse Basin formed a

profound strata.

part (Ziegler,

mentation resumed only during the late Oligocene and Miocene with the deposition of thin continen-

1982).

Synsedimentary documented

flexures

from Cenomanian

and

fractures

to Santonian

along the Pfahl Zone. Near Amberg Cretaceous the

subject

sedimentary of detailed

are strata

these contain

iron ores which have been investigations

(Gudden,

1984, fig. 5; Pfeufer, 1983). During the Late Cretaceous the depot-centre of the Regensburg Basin moved gradually southward. Maximum Late Cretaceous sedimentary thicknesses of the order of close to 1000 m have been recorded. From this it is concluded that the Late Cretaceous basin development was governed by continued tectonic activity along the fault systems of the Bohemian Borderzone which had already come into evidence during the Permo-Carboniferous.

and Wagner, of inversion

erosion

1987). This important second of the Bohemian Borderzone

of the Cretaceous

and

earlier

Oligocene to Miocene record In the area of the Bohemian

tal elastics.

Miocene

east of Bayreuth

and

activity

can be related

the NE-SW striking Central Bohemian Huckenholz

volcanic volcanic Massif

Schrijder,

Borderzone

sedi-

in the area

to the evolution Eger Graben (Malkovsky.

1985).

of

of the 1980;

Post-Miocene

uplift of the area of the Bohemian Borderzone is probably associated with the progressive thermal doming of the Bohemian Massif in conjunction with the evolution of the Eger Graben volcanic province. Along the Bohemian Borderzone this was accompanied by post-Middle reactivations (Leitz and Schrader, 1985).

Miocene fault 1985; Schirmer.

Conclusions Late Cretaceous to Cenozoic inversion The latest Cretaceous and Cenozoic evolution of the Bohemian Borderzone is difficult to unravel

The present structural relief in the area of the Bohemian Borderzone is at the top basement level of the order of 4000 m (Figs. 2 and 4). Fault

geometries

are of the upthrust

of transpressional Borderzone ferous

is superimposed

graben

system.

repeatedly

reactivated

during

Cretaceous,

the

Early

Paleocene

and

inversions

again

occurred

and again during The Early

North Late

during

the Early

Cretaceous-Early intra-plate

Cretaceous

Atlantic

can be

Borderzone

of

coincides

with

major

compressional

tions

in other

parts

land

(Ziegler,

1982, 1987). A distinction

of the northern

The

phase

deformaAlpine

fore-

between

not be made in the area of the Bohemian Borderzone mainly due to an incomplete stratigraphic

ward

continuation

in

the

fault

system

of

the

ing and basement

involving margin

an important to Paleocene

upthrusts,

delimiting

of the Bohemian

element in the latest Cretatranspressional deformations

foreland

(Ziegler,

Horn,

Miiller, M. and Weggen. Basin.

K., 1987. Evolution

Senke

P., Kohler,

0..

Kaiser,

der Weidener

Tectonophysics.

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1982. Das

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(Opf.).

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H. and Mtiller-Sohnius,

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(“fluid

of the

Bayerischer

Pfahl, Eastern

Bavaria.

3 (2/3):

199.

H.G.

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and

Schriider.

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des mesozoischen

Vorlandes

1985).

Mitt.

Jarhesber.

B..

1985.

Tertilrer

Teil des Eger Grabens (Exkursion

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G am 13. April

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107-124. Klare. B. and Schrlider.

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at the western

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137

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