Crustal-scale balanced cross sections through the Variscan fold belt, Germany: the central EGT-segment

Tecfonophysics,

196 (1991) l-21

Elsevier Science Publishers

B.V., Amsterdam

Crustal-scale balanced cross sections through the Variscan fold belt, Germany: the central EGT-segment J. Behrmann

a, G. Drozdzewski

b, T. Heinrichs

‘, M. Huch d, W. Meyer

e and 0. Oncken

f

a Institut ftirGeowissenschajten

und Lithosphiirenjorschung, Universitdt Giessen, D-6300 Giessen, FRG h Geologisches Landesamt Nordrhein- Westjalen, D-4150 Krejeld, FRG ’ lnstitut ftir Geologie und Dynamik der Lirhosphdre, Uniuersitiit Giittingen, D-3400 Giittingen, FRG d Institut ftir Geophysikalische Wissenschajten, Freie Universitdt Berlin, D-1000 Berlin 33, FRG e Institut ftir Geologie, Universitiit Bonn, D-5300 Bonn, FRG I Inslitut ftir Geologie, Universitiit Wiirzburg, D-8700 Wiirzburg, FRG (Received

February

5.1991;

revised version accepted

April 23.1991)

ABSTRACT Behrmann, J., Drozdzewski, G., Heinrichs, T., Huch, M., Meyer, W. and Oncken, 0.. 1991. Crustal-scale sections through the Variscan fold belt, Germany: the central EGT-segment. Tectonophysics, 196: 1-21.

balanced

cross

This paper presents crustal-scale, geometrically and isostatically balanced cross sections through the Rhenohercynian and Saxothuringian zones of the Variscan fold belt of Germany. The cross sections are based on surface geological data and on results of the DEKORP deep reflection profiles. They were constructed by means of computer-assisted, iterative forward modelling. The Rhenohercynian zone, corresponding to the Extemides of the Variscan belt, developed out of a some 300 km wide rifted continental margin sedimentary prism, limited to the south by the narrow Giessen oceanic basin. During Carboniferous imbrication this shelf sequence is stripped of its basement above one or more decollements and shortened by some 150 km. Closure of the Giessen Ocean and emplacement of the ophiolite bearing Giessen nappe entailed a minimum additional 200 km of crustal shortening. The structure of the mode1 correlates well with the present outcrop pattern of rock units, their metamorphic history, the seismic and magnetotelluric architecture of the crust, and its present regional distribution of thickness. The deeper crust beneath the Rhenish Massif is probably devoid of extremely weak decollement horizons and pronounced rheological stratification. In the southward adjacent Saxothuringian zone, nappe development involving the stacking of crustal slices during Middle Devonian to Vi&n times accounts for a further 170 km of crustal shortening above an infracrustal, partly blind thrust system with flat-ramp-geometry. Total crustal shortening in the analyzed profiles is estimated to be of the order of 520 km; this figure is a conservative value. Prominent uncertainties include the unknown importance of crustal wrenching, the state of strain within thrust units, the distribution of strain within the lower crust, and major changes in thrusting directions during the course of erogenic history.

Introduction

geological

During the past decade the continental crust of West Germany has been intensively studied by

0 1991 - Elsevier Science Publishers

At present

the discussion

by controversial issues significance of outcrop-

ping as well as buried structures (e.g., Weber and Behr, 1983; Ziegler, 1986; Matte, 1986; Behr and Heimichs, 1987; Franke, 1989). The high quality of data along some of the seismic sections and their support by published detailed geological surface observation suggest that interpretation could be aided by quantitative tectonic modelling. Thus tighter constraints could be imposed on

deep seismic reflection and refraction profiling (e.g., Meissner et al., 1980; Giese et al., 1983; Mechie et al., 1983; DEKORP Research Group, 1985, 1988; Franke et al., 1990). Geographically the seismic studies focus on the areas formed or at least affected by the Variscan orogeny. One of the major challenges posed by these sections is their 0040-1951/91/$03.50

interpretation.

seems to be dominated regarding the geotectonic

B.V.

Fig. 1. Mid-European seistic

pro&s.

V&scan

erogenic zones and the trace of the European Gmtraverse,

as well as those of the DEKORP-2

and -4

Bold lines paralleling seismic protiles, trace the parts covered by section balancing in Figs. 4.5, and 6.

crustal movements and the architecture of the Variscan orogen of Central Europe. This paper presents and discusses crustal-scale balanced sections along parts of the central EGTsegment, which explain the geometry of pertinent geological and physical features of the &ust, the pre-orogenic basinal facies-pattern, and its thermal evolution through its development from the pre-Variscan to the present state. A consistent explanation of the mentioned aspects by retrodeformable cross sections is regarded as a comerstone for the structural interpretation. The two sections chosen for analysis by computer-aided section balancing were the DEKORP-ZNorth section (Franke et al., 1990), which crosses the Rhenoherqnian extemides of the Variscan chain and its suture zone towards the south, and the DBKQRP4 se&m (DEKORP faeseareh Group 1988) which crosses parts of the internal, strongly contracted Variscan chain (see Fig. 1).

The differentiation of the central-Europeau Variscan chain into several segments with differ-

ent tectonometamorphic histories goes back to Kossmat (1927, see Fig. 1). Recent intcrpretatiMls interpret these units to be minor plates or terranes -with Paleozoic continental basin *tation (Weber, 1986; Matte, 1986; Ziegler, 1986; Franke, 1989)-which were successively accreted from early Devonian times until the Late Carbo&ferous to form a broad mountain belt. This process probably involved SE-directed subduction of minor basins floored by oceanic or strongly stretched continental crust. Final continental co&lirson occurred along two suture zones (separating the Rhenohercynian from the Saxothuringian and the latter from the Moldanubian zone; Fig. 1). Possibly there are one or two other largely ohecured, NW-dipping geosutures south of the Moldanubian zone, with their structural analogues in the Black Forest (e.g., Eiabacher et al., 1989). The Rhenohercynian zone on the nartharn flank of the belt remained stable throughout the Early Paleozoic (e.g., Franke, 1989, for review). The phase of crustal stretching and subs&nce with the formation of a thick basin fill& (3-12 km) of shallow-marine elastics and m intercalated with abundant bimodal volcanies is re-

CROSS

SECTIONS

stricted ward

THROUGH

THE

to the Devonian. migration

the Upper

the prograding contraction

Ahrendt

et al., 1983).

at the outcrop increasing and

(Oncken,

and

-

from

Franke,

metamorphism result

stacking

belt which

alents;

Molasse mark

formation

south

volcanics

and flysch of Mid-Devonian

below cherts

to Late Devonian

age

which was parent to these rocks, has to be rooted between the Rhenohercynian and Saxothuringian

deduced

by orthogonal

convergence

by the end of the Devonian. Late-erogenic kinematics of the Rhenish Massif involves oblique convergence and subsequent dextral strike-slip along the suture zone during the Late Carboniferous (Oncken

1988, 1989, Schafer

The decipherable basin

record

starts with Cambrian

unknown

continental

Cadomian

age. Bimodal

1989).

volcanism

and

a largely

siliciclastic platform assemblage suggest rifting in a continental environment during the Ordovician. The

allochthonous

greenschists,

gneisses,

and

(Ziegler,

Paleozoic”

near the Saxothuring-

1986;

stage

of basin

by major

Oncken,

of large although

strike-slip

1989:

Schafer,

volumes restricted

of the

crust

of postto the

was

by the end of the Permian.

Rhenohercynian

of Permian facies

seismic

zone

(for details section

(cf.

and

see Franke

shows

reflectivity (see Fig. three major units:

largely

as can be sedimenta-

Lorenz

and

its

Subvariscan

et al., 1.990). The

several

distinct

3a) that

can

patterns

of

be related

to

-Upper crust (O-3 s TWT): highly reflective in the Subvariscan Foreland, less reflective in the

bounded and the erties of -Lower

the “Erbendorf

of this

The 220 km long line DEKORP-2N provides a continuous deep reflection seismic section across

stacking to the south of the Saxothuringian Basin, involving oceanic crust of Cambrian and possibly younger age (Gebauer and Grtinenfelder, 1979).

ian/Moldanubian boundary) contain chromite together with pebbles of characteristic stable-foreland lithologies of lower plate provenance (Ludwig, 1968). In the course of the collision and the following intra-continental contraction the proximal (“Bavarian”) facies assemblage was dismembered and cannibalized into a wildflysch, and then thrust

parts

from the pattern

Rhenohercynian -Middle crust

the (in

zone) or the Upper

are accompanied

HP-eclogites in the nappe pile of the Munchberg Massif (MM) are evidence of Acadian crustal

Collision probably had already started before Famennian, as the oldest graywackes preserved

of the

the late Lower

Seismic studies

the

suspected

early

tion and the associated Nicholls, 1984).

foreland

of

(Saxothuringian

re-equilibration

of the Saxothuringian

basement

during

(Rhenohercynian-Saxothuringian

accomplished

sedimentation

onto an

unconformity

collapse

Saxothuringian zone however, began during the Visean and lasted into the earliest Permian. Isostatic

and was closed

(Franke,

III.

a major

crust

1989). The upsurge kinematic granites,

(Engel et al., Grosser and Dijrr, 1986). A small (minimum width: 100 km) former ocean basin,

zones

upon

equiv-

flysch sedimen-

of extensional

Hercynian

The

motion system nappe,

onset

Carboniferous

the

assemblage

until the Visean

Carboniferous

facies

Carboniferous

deposits

the

thickened

distal

“Thuringian”

lasted

suture).

1989).

MORB-type

parautochthonous the

PT-condi-

Relics of a former supracrustal nappe are preserved in the rocks of the Giessen which contains

its

tation

in an approximately

towards

over

1983;

(of rocks today

synorogenic

3

1984). Synorogenic

deformation

fold-and-thrust

strain,

tectonic

north-

Carboniferous

Concomitant

150 km wide foreland shows

GERMANY

front of synmetamorphic (Engel

level)

BELT,

sedimentation

to the Upper

erogenic

and very low-grade

FOLD

The subsequent

of flysch

Devonian

-reflects

tions

VARISCAN

northern

zone. (3-6

s TWT):

transparent

zone

by bands of strong reflectors at the base top (compare with magnetotelluric propthis zone-EREGT-group, 1990). crust (below 6 s TWT): a transparent segment

is separated

by a wedge-shaped

transition zone from a southern strongly reflective segment. The rise of the Moho from approximately 11 s TWT in the north-northwest to 9 s TWT in the south-southeast (Fig. 3) is partly a Variscan feature which, however, is overprinted by Cenozoic crustal extension associated with the Rhinegraben rift system (Ziegler, 1988).

Fig. 2. Geologjcal sketch map of the eastern Rhenish Massif, with the main thrusts and the DEKORP-2N line. I = Lower Pale&c to Lower Devonian undifferentiated, 2 = Gedbmian, Siege&n, and Lower Emsian, 3 = Upper Emsian, Middle Devonian, 4 = Upper Devonian, Carboniferous, 5 = sediments of G-iessen Nappe. White = post-Carboniferous cover. Geographical locations (Wttterau etc.) refer to Figs. 4 and 5.

Two contrasting interpretations of the DEKORP-2N line are under debate (see Franke et al., 1990). One model prefers the interpretation of the curvilinear reflections at the top of the middle crust as thrust planes. The usually steeper thrusts at the surface of the Rhenish Massif to flatten out at depth into the transparent zone (34 s TWT), which is presumed to constitute a broad zone of detachment. Inclined reflectors of the lower crust are supposed to be thrusts pertaining to a later deformation (late or post-Variscan).

The alternative interpretation takes all reflections in the profile to result from primary lithological differences. The strong reflective band at the top of the transparent zone could be the base of the sedimentary rocks (Cambrian to Precambrian) as might be deduced from outcropping Ordovician rocks in the I&m&eid and Ebbe anticlines. A structure of regional importance is a conjugate set of thrust faults below the I&be and Mtisen anticline, which is interpreted-like the “ fish-tail”-structures below the Ruhr district in

m I__

/

w

NNW

CROSS

SECTIONS

THROUGH

THE

VARlSCAN

FOLD

BELT,

11

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the north (cf. Drozdzewski 1979) as a feature indicating lateral compression. The structure consists of a northward dipping zone in the lower crust that has been displaced by at least two southward dipping thrusts in the middle crust. The effects of this blind th~sting is geometrically compensated by folding in the upper crust, and effects uplift of Lower Devonian and pre-Devonian rocks in the central part of the Rhenish Massif, above a major crustal ramp. Reflections-in the middle and lower crust are mainly inclined to the southeast, although locally to the northwest. These reflections are often cut by other, south- and northward-dipping reflectors. The transition to the Saxothuringian zonenorthern part of seismic line DEKORP-2-S (Fig. 3a; see also DEKORP Research Group 1985)-is shown by a marked change of reflectivity patterns. This is mainly attributed to the post-Variscan evolution of the southern Taunus border fault (Fig. 2; cf. Anderle, 1987) which involves a succession of major strike-slip and dip-slip displacement increments (of the southern unit) and thus juxtaposes different structural levels of the previously stacked erogenic wedge. The mainly SE-dipping reflectors south of the fault probably trace the internal geometry of the Variscan suture between the medium-grade rocks of the Central German Crystalline Rise to the south and the very low-grade, high-pressure rocks of the southern Rhenohercynian zone (cf. Behr and Heinrichs, 1987). The Saxothuringian zone proper is analysed in our study of the northwestern half of the 180 km long seismic line DEKORP-4 (Fig. 3b; see also DEKORP Research Group 1988) and is believed to be representative of the crustal structure of the southern Saxothuringian (Fig. 1). It extends from the Erbendorf Greenschist Zone (EGZ) near the Saxothu~n~an/Moldanubi~ boundary, across the granite-cored Fichtelgebirge antiform and the synform of the Munchberg nappe complex and northward through the Berga anticline with its core of Frasnian spilites, to the Teuschnitz syncline in which Lower Carboniferous flysch is exposed. This part of the seismic section shows a highly reflective upper crust contrasting with a less reflective lower one. Pro~nent upper crustal

reflectors are from northwest to southeast (Schmoll et al., in prep.): (1) A subhorizontal band of reflections at 1 s TWT extending from beneath the Teuschnitz syncline to the Berga anticline and farther south. It is interpreted to be related to the Frasnian spilite/sediment interfaces. (2) A synformal pattern beneath the Mtinchberg complex from about 0.7 s TWT to 1.3 s TWT and-with a slightly less coherent character-reaching down to about 2 s TWT. In the present study the first break in reflectivity is taken to coincide with the base of the uppermost three Miinchberg nappes characterised by amphibolite facies assemblages. The lowermost part of these reflection bundle near 2 s TWT may be correlated with the suspected Frasnian spilites discussed above. (3) A wide antiformal reflector pattern slightly south of the Berga anticline between 1.5 and 2.5 s TWT. (4) At about 3 s TWT, there is a zone of increased reflectivity extending from the northwest end of the section southward beneath the northern edge of the Munchberg complex. Beyond this point it seems to be replaced by several discontinuous reflector bands dipping south between 3and6sTWT; (5) below the Fichtelgebirge a poorly reflective zone to 1.5 s TWT corresponds to the exposed post-kinematic Fichtelgebirge granites that reach 3 km depth according to 3-D gravimetric inversions (Behr et al., in press). The granites straddle the Fichtelgebirge antiform, which is imaged seismically between 1.5 and about 3 s TWT. Balancing techniques

Usually geological and tectonic sections must be based on extrapolations because of the limited amount of surface data. Balanced cross sections of a tectonised part of the earth’s crust {e.g., Dahlstrom, 1969) should be retro-deformable to a reasonable initial geometry without having voids or overlaps (criterion of strain compatibility). The virtue of crustal balancing is not to produce a unique, essentially correct explanation of a tectonic problem, but to eliminate impossible solutions.

The following basic assumptions apply to our study, and form the basis for the computer program used (see below): (1) Lengths and thicknesses of crustal layers remain constant during deformation. This implies that in cross sections the area of the deformed prism is conserved and any internal deformation of thrust slices must be due to layer-parallel simple shear. (2) Thrusts propagate upwards and forward through the stack of crustal layers. All thrusts merge into basal decollements located within the continental crust, at the Moho, or within the upper mantle, depending on the model chosen. A logical consequence of ass~piion (1) is that during deformation of the crust, no material may be moved in or out of the cross sectional plane. Strike-parallel displacements within the Rhenohercynian domain and the Central German Crystalline rise (CGCR) may not be dismissed as a whole (e.g., Weber, 1986; Oneken, 1988). However, the high degree of cylind~~ity and lateral continuity of structures in the Rhenohercynian domain makes it unlikely that large volumes of crust have been laterally moved into or out of the plane of section of DEKORP-2S and -2N seismic profiles by strike-slip tectonics. Original layer lengths and formation thicknesses can be roughly reconstructed from regional data (see Geological framework) and the line drawing interpretations of the DEKORP sections (see Seismic studies). The CGCR is not internally structured in our model computations, but its non-cylindrical nature is evident on any geological map. As a result initial cross-sectional lengths of the CGCR may be under- or overestimated. In the Saxothuringian- Moldanubian domain, pervasive ductile deformation (e.g., Stein, 1988) severely limits the application of balanced section techniques. Here the approach taken, is to treat the present NW-SE extent of exposure of a tectonostratigraphic unit as being the minimum primary thrust sheet length. This is a reasonable assumption for the modelling of late-stage emplacement of the Munchberg nappe pile (e.g. Franke, 1984) at high crustal levels, but certainly not for any displacements associated with earlier ductile flow.

The computer-program package THRUSTBELTII (Jones and Linsser 1984) used in this study applies the inverse technique of forward balancing for crustal deformation. The philosophy of forward balancing is to iteratively change the input data set for the basin model and for thrust configuration until the deformed section reproduces and thus “explains” the present surface geology. There are limitations concerning the types of structures that can be produced by the modelling process: internal straining of thrusted units is restricted to folding over thrust ramps and thrust branch lines. Backthrusting and out-of-sequence thrusting cannot be simulated. There is no possibility to model syn-erogenic mass transfer by erosion and sedimentation (flysch deposits). The THRUSTBELT-type model simulations produce horizontally shortened crustal sections that are not in isostatic equilibrium. We have applied a computer program (H. Buness, pers. commun., 1990) to the data sets describing the deformed crustal sections that provides for Iocal isostatic compensation, using the predictions of the Airy model (e.g., Turcotte and Schubert, 1982, pp. 225ff.). Isostatic compensation was performed after thrusting and erosion (see cross sections in Figs. 4, 5 and 6). The data input consists of the same crustal layers as defined by the THRUSTBELT II computations. Realistic specific density values are assigned to each crustal layer. The integrated specific densities for each individual column of the thrustbelt model then determines the amount of uplift or subsidence after isostatic compensation. However, at present the physical validity of this type of isostatic compensation is limited. As a zero elastic or plastic yield strength is assigned to the lithosphere, short-wavelength folding appears in the profiles that is unlikely to develop in a crust with a defined yield strength. Models and reds

The chosen section paraIIels the DEKQRP-2N and northern part of DEKORP-2S seismic retition lines (Figs. 1 and 2), both of which are

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13

oriented nearly perpendicular to the strike of the erogenic belt, form the borehole Mijnsterland 1 (Subvariscan foredeep) to the Spessart hills (Central German Crystalline Rise). The iterative approach to a best-fit solution resulted in two models for overth~sting and crustal thickening; these were analysed (models 1 and 2) on the basis of an identical best fit basin configuration, but with different thrust fault geometries (compare Figs. 4a and 5a). The first model assumes detachment of the sedimentary cover from most of the crystalline basement. The second involves the entire basement in crustal stacking and shortening above a decollement at and below the crust-mantle boundary. The geological input data fit a multi-layer model (see Figs. 4 and 5) consisting of an upper mantle, a two-fold divided crystalline basement (because of the different seismic response, as discussed above), and a four-layered segments cover. The “Giessen ocean” situation is resolved into only three layers: mantle, oceanic crust, and a wedgeshaped sedimentary cover. The CGCR is shown as an undifferentiated crystalline unit already devoid of its former cover. Sedimentary layer thicknesses are based on compilations by Meyer and Stets (1980) and Oncken (1987). The assumed width of the Rhenohercynian basin corresponds to the estimates of Wunderlich (1964) and correlates well with the integral deformed bed length derived from more detailed section. Bed lengths are not unstrained which, since recent strain data (U. Dittmar, pers. commun., 1990) yield a bulk synorogenie stretching of bed length < lo%,, gives a minor overestimate of the original basin width. A minimum width for the Giessen ocean (100 km) is given by Engel et al. (1983). The thickness of the pre-Paleozoic basement reflects the minimum crustal stretching necessary to explain the accumulation and the thickness variations of the Devonian and Carboniferous sedimentary prism. The continental nature of the basement is inferred from xenolith studies (see Mengel et al., in press). Moreover, the modelled thicknesses of middle and lower crust (layers 2 and 3) correspond to those observed in the seismic sections (Fig. 3a). The 7-layer model was chosen in order to be

able to simulate a rheologically stratified lithosphere (e.g., Kusznir and Bott, 1977) that can give rise to distinct flat-and-ramp trajectories of thrust faults. The best-fit positions of thrust flats (decollement horizons) in model 1 (Fig. 4) roughly coincide with the bundles of strong seismic reflectors observed at the upper and lower boundaries of the “transparent” middle crust (layer 3) in section DEKORP-2N (Fig. 3a). This is supported by the observation that reflection bundles in the upper crust, which can be related to outcropping thrust faults that converge at depth to form single reflectors at mid and lower crustal levels (the lower crust is thus taken to “deform” by bulk heterogeneous simple shear). In model 2 (Fig. 5), a second important SE-dipping decollement horizon is located in the lower crust and partly within the upper mantle. The rationale for choosing this model is given by the observed offsets of the Moho in the DEKORP-2N traverse. The layout of the thrusts in model 2 is such that three independently operating thrust systems are produced. Two of these systems collect the displacements along thrusts reaching the earth’s surface south of the Ebbe anticline; the third system links the Ebbe overthrust and all major displacement surfaces north of it. The locus of subduction of oceanic crust (Giessen ocean) is positioned at the northern margin of the CGCR. The Giessen greywacke nappe (nonstructured in our models) thus reflects the remains of an accretionary wedge in front of the CGCR. Dips, positions and displacements of the thrust faults are taken from surface data (see Franke et al., 1990) and from the line drawing interpretations of DEKORP-2S and -2N (see Fig. 3). Displacement estimates are conservative. The upper datum line of the models represents the top of the Upper Devonian in the south (only model l), the top of the lower Carboniferous in the middle, and the top of the Westphalian in the north (horizontal distances correspond to the Devonian-Carboniferous boundary throughout). This choice incorporates all sediments involved in orogenie deformation, and reflects the northwestward migration of the deformation front. Thus the inability of the computer program to perform synerogenic mass transfer was avoided.

CROSS

SECTK3NS

THROUGH

THE

VARISCAN

FOLD

BELT,

NNW

1

4

I

SSE

kml Fig. 5. Model 2 balanced section of the Rhenohercynian zone. (a) Best-fit basin configuration. (b) Model after thrusting and stacking. Model 2 after isostatic compensation and erosion. !3ee Fig. 2 for position of geographical locations; legend as in Fig. 4. Horizontal and vertical scaks are identical. The. right margios of deformed suxions are &termined by the right margin of the uppermost thrust unit in the undeformed section. Overthrusted footwall units are cut off right of this datum line and do not appear in the deformed cross-section. Cross-sectional area is conserved in the model. but not shown.

Figure 4 shows the geometric evolution of model 1 after thrusting and erosion to the present level, including isostatic compensation. A number of important aspects can be observed: (1) The large-scale folds in the north are modelled correctly and therefore probably result from (isostaticahy compensated) ramp-flat thrust geometries in the upper crust. In part these thrusts are inversion structures of normal faults associated with the formation of the Rhenohercynian basin. (2) The pressure-temperature evolution of the southern Rhenish Massif necessitates burial beneath a larger nappe system. The regional extent of anchixonal/epizonal metamorphism imposes constraints on the regional extent of this nappe.

The nappe system mod&d yidds a reasonable overburden to account for the synorogenic temperatures and pressures reached in the southern Rhenish Massif, as well as for the observed northward decrease in the metamorphic grade. (3) The present remains of the Giessen nappe system are easily explained by huge-scale offscraping and thrusting of the Giessen ocean basin filling, with nezuly complete destruction of the basin floor. Erosion of the nappe is complete with the exception of remains in local structural depressions. (4) The model computations allow the assessment of some fundamental geometric data: Since the possible former extent of the Giessen nappe

CROSS

SECTIONS

THROUGH

THE

VARISCAN

FOLD

BELT,

15

GERMANY

system is appro~mately known (see above), the width of the former oceanic basin on which it developed can be estimated ( > 100 km), assuming that no sediments were subducted. A larger amount of sediment subduction, which is considered less likely, would result in a larger width of the Giessen ocean. Moreover, part of the former accretionary wedge probably cont~buted to the duplex system between the stacked Rhenohercynian and Saxothuringian crustal units. The intervening oceanic crust was subducted completely while a minimum of c. 100 km of lower continental crust, stripped of its sedimentary cover, was overridden by CGCR. The resulting erogenic root has a width of some 100 km and an approximate maximum thickness of 50 km (partly dependent on the preorogenic crustai thickness). The CGCR thus developed above the evolving subduction zone during the Late Devonian and Early Carboniferous as a magmatic arc (cf. Okrusch and Richter, 1986). Total shortening due to thrusting amounts to c. 350 km or some 60% of the original length (minimum values due to the rule of conservative construction). (5) The final depth to Moho in Fig. 4 (c. 30 km) corresponds to the present depth including a minor rise to the south. A problem, however, is posed by the relics of the former root zone and the Moho offsets in the southern part of the model, which are not observed at present (for details see discussion). The short wavelength Moho topography is caused by the isostatic balancing procedure (see above) which also amplifies morphological features of the thrust surfaces. Their primary ramp-flat geometry is strongly overprinted (camp. to low-amp~tude bending of mid-crustal reflector horizons in Fig. 3a). (6) The geometry of the upper, decollementbounded layer of he crystalline basement is, including its “ wavy” appearance, in excellent accord with a level of high intracrustal conductivity (see EREGT-group, 1990). The latter climbs to a higher structural level at the northern border of the Rhenish Massif. T’he peculiar conductivity pattern of the crust at the Saxothuringian-Rhenohercynian boundary appears to coincide with the modelled duplex system of steeply dipping thrust sheets.

Model 2 (Fig. 5) only envisages the structural evolution of the Rhenohercynian zone. After deformation, a thrust-and-fold pattern very similar to model 1 is observed. Local thickening of the middle and lower crust by thrust sheet imbrication is minimized by avoiding a strong ramp-flat topography of basal d&ollement surfaces, Violation of this principle would have had the effect of producing ridges of metamorphic rock high in the crust after thrusting. After isostatic compensation all major structures are reproduced akin to those in model 1. Both models seem to be equally well compatible with surface geology and the known present depth to Moho.

Southern Saxothuringian Zone The section selected for our model runs along the northern half of the seismic reflection line DEKORP-4 (Fig. 1) as it is the only line available in an area of continuous exposure of the Saxothuringian fold belt. Although the model input is comparable to the Rhenohercynian case with respect to the quality of the geometric information on the present structure of the parautochthonous Saxothuringian and the Miinchberg nappe pile (see Fig. 6) setting up a well constrained starting model is severely hampered because: (1) Post-kinematic granite intrusions distort the original geometry to an unknown extent. (2) Geometric controls of the late-kinematic LP/HIT event are not well understood. (3) The backfolding phase D, (Stein, 1988) may have led to bulk thickening of the crust. However, regional strain data are not available. (4) Remnants of the entire Bavarian facies ashalf of the former semblage, the southern Saxothuringian basin, exist only as dismembered thrust slices within the Munchberg nappe pile. (5) Early thrust directions may differ considerably from later ones (M~hrmann et al., 1989); For the time being we have disregarded the first three points. Nevertheless, it remains attractive to carry out a metamorphic balancing exercise on the Munchberg nappe pile because it most clearly embodies the concept of a metamorphic crust stacked by collision. However, modeling does not

zone. (a) Best-fit basin configuration.

section. Over&rusted footwall units are cut off right of this datum tine and do not appear in the deformed cross-section.

-

__._

Cross-sectional

and thrust unit in the undeformed

.I.

_

.

area is conserved in the model, but not shown.

by the right margin of the uppermost

(b) Model after thrusting and stacking. (c) Model 1 after isostatic compensation

erosion. Horizontal and vertical states are identical. The right margins of deformed sections are determined

Fig. 6. Balanced section of the southern ~~othu~n~n

SE

5 r: ; % T

CROSS

SECTIONS

incorporate

THROUGH

THE

VARISCAN

the established

placement

increments crustal

BELT,

of the thrusts

dis-

component,

“Randschiefer”

RSF-The

anchimetamorphic

(Moehrmann

thus only detects

shortening

may be a gross underestimate

17

GERMANY

early SW-directed

et al., 1989). Retro-deformation the NW-SE

FOLD

which

diments

In order following

to arrive

(1) Balancing discrete

at a starting

assumptions

by ramp-flat

thrust

metamorphic

geometry

the

patterns

and

planes

thrust

reasonably

begins

for

quartzo-feldspathic

upper

three Mtinchberg

white

mica

K-Ar

380 Ma (Kreuzer

closing

cooled

et al., 1989).

the

the present

380 Ma can be inferred.

known

ently

preserved

length

thrust

sheets

mates original

length

nappe

Erbendorf

Although

suspected reflection

(1984)

estimate

it is considered

in the by us as

The

from south of the

Paleozoic late-stage

eclogites nappe

HP

near its base, and the “Liegendserie”

have been treated

superposition RAM-The

with

as one unit since their

is of pre-380

“Randamphibolite”

Ma age. nappe

following

of tholei-

Phyllite

Series”

nappe

is

characterized by a distinctly lower metamorphic grade. It shows strong similarities to the Erbendorf Greenschist Zone, 30 km to the south, with regard to cooling ages around 365 Ma and protoliths of tholeiitic to talc-alkaline basalts with island arc affinities (Kreuzer et al., 1989). Both units are therefore assumed to be parts of a continuous PPS nappe.

anchimetamorstack.

It must facies

as-

anchimetamorphic 1984).

warping,

e.g. the Fichcan be modelled rising from the

are some

as indicated

by the

of the results

of the

exercise (Fig. 6). All recognizable the Mtinchberg

is brought

nappes

branch

are repro-

that is rooted beneath Zone. According to the

of a top to bottom

lower PPS nappe

itic parentage (Oppermann in Kreuzer et al. 1989) has its leading edge buried about 6 km southeast of the preserved tip of the Liegendserie nappe. PPS-The “Prasinite

principle

Wild-

and detaching the ‘Thuringian at about 9 km depth along the

duced by a thrust system the Erbendorf Greenschist

nappe,

the

(Franke,

HS/

“Hangendserie”

with

“ Bavarian

Thuringian

is

balancing

LS-The

and one in

of the southernmost

top of basement, seismic pattern.

within

of im-

in agreement

the basal

The entire Munchberg stack has been modelled by six thrust units, from top to bottom (Fig. 6):

points

“Kiesel-

pattern.

forms

which

Megascopic

middle crust metasediments

this is not a minimum

one in front

nappes,

telgebirge and the Berga anticline using a separate thrust system

nappes yet underestimates thrust displacement since all nappes occur as erosional remnants only. sense of Franke conservative.

horses,

outcrop

is (due to the lack of any

parts of the

Silurian

of the Miinchberg

have travelled semblage,

of the preserved

(Fig.

slices. They were modelled

outcrop

nappe

the pres-

strain data) taken as the original length at the start of the modelling. This probably overesti-

model

up of a number

Lower Carboniferous

flysch” phic

the individual

of the starting

the rear of the upper

record (Stosch and Lugmair, 1987; Bltimel, 1986) a depth estimate of 14-16 km for these nappes at (2) For

facies assemblage.

discontinuous

radiolarite

BA F-The

P-T

their

metaseand minor

at the top of the major

is made

as two isolated

The

at around

From

unit

tran-

below

temperature

rather

schiefer” bricated

assemblages.

nappes

ramp

when

rocks reach the brittle-ductile

sition

of Ordovician

an

6a). KIS-The

are made:

placed

crustal

forms

alkali basalts

felsic rocks of the Bavarian upper

ment.

unit

with intraplate

It is therefore

of the total displace-

nappe

thrust

sequence,

by the model

the

to cool

later than the upper three nappes, in accordance with the younger isotopic closing dates observed. Also conditions known for the uplift path of the Mtinchberg nappes and the anchimetamorphic lower plate cover can be met, provided erosion removed in the order of 12 km of the upper plate before the Munchberg nappe stack was emplaced on to the Erbendorf Paleozoic. However, if the model were not constrained by the assumption of low total displacement, it would be possible to trade larger thrust displacement for smaller instantaneous load by stacking at a given point. In the model presented by us total crustal shortening between 380 and 330 Ma amounts to 170 km.

Discussion

Modelling the crustal deformation in the Rhenohercynian and the CGCR reveals some interesting aspects of the mechanical behaviour of the crust in this part of the Variscan belt. In our simulations, the need to avoid large scale ramping hints to the fact that there may be no extremely weak inclined ddcollement horizons in the lower crust and upper mantle beneath the Rhenish Massif. The degree of rheological stratification of the lithosphere (e.g., Kusznir and Bott, 197’7) is low, and the most appropriate ~te~retation of our results is that displacements that gave rise to folding and thrusting in the upper crust, correspond to diffuse, penetrative shearing of the lower crust and perhaps the uppermost mantle. Another fundamental problem arises from oblique, out-of-section displacements. Recent work by Magi et al. (1989) has shown that displacement directions of the tectonic mega-units of the Mtichberg nappe systems rotate through about 90 Of starting with a phase of top-SW-thrusting, and ended with a pulse of NW-SE directed shortening. Apart from a lack of knowledge on the state of strain of the deformed sequences, retrodefo~a~on in these cases only detects the NWSE crustal shortening component, which may be a gross underestimate of the total displacement. Similarly, the present configuration of the Rhenohercynian-Saxothuringian boundary cannot be regarded as a simple suture zone involving only progressive crustal stacking perpendicular to erogenic strike. Recent inv~tigations show that the geometry of the zone is strongly determined by late-Variscan (Oncken 1989) and still younger (Anderle, 1987; Schwab, 1987) dextral strike-slip displacement along steeply dipping faults. Development of an intramontane molasse basin (e.g., SchHfer, 1989) probably relates to this stage of kinematic evolution. These movements, oblique to the section plane, are obviously not balanced in the sections presented; their neglect in the balancing procedure seemed justified because the clear cylindrical geometry of the belt is not greatly affected by these post-erogenic wrench deformations. However, the oblique ~spla~ment may have some

I

.

JOpkrn.

I

Fig. 7. Geometry of Rhenohercynian-Saxothuringian suture after stacking and isostatic balancing showing entire crustal volumina involved in erogenic contraction at the northern termination of DEKORP-2S. Balanced section is computergenerated. Presently preserved crust is unornamented; tbickness of eroded layer is estimated from published PT data. Layer between present Moho and p~t-~l~sion~ Moho represents minimum crustal volume required from section balancing to form “vanished” erogenic root (question mark). Introduction of further thrusts or duplexes within the subducted Rhenohercynian lower crust would yield an even thicker, but not so wide root zone.

unforeseen effect on the unresolved geometrical evolution of the lower crust. The Moho offset and the or& root zone apparently evohred adequately in the balanced section Both effects must be accounted for by other m-s since the program used is not able to cope with oblique motions, out of sequence faulting, and post-stac~~g extension. Several scenarios are conceivable to solve the problem of the “missing t~og&c root” (Fig. 7 shows the main geometrical aspts): (1) Pre-erogenic crustal thicknesses smaller than assumed in our mod&&g and/or reduced stacking of continental crust (due to a larger former ocean) would also reduce t&e ma&&g root thickness (probably by less than 10 km). (2) Post-erogenic, isostati~y-driven upwarping of the root zone (while keeping Mob he length constant) would result in stretching of the upper crust by arching, uplift and erc&o~ to midcrustal ievels. However, no more than about IO% of stretching can be achieved @quiva;tawS to less than 1 km buik crustal th&kness reduction), and -from p&ro&ica.l and surface geological evidence-the CGCR cannot be eroded to the then unavoidable deep crustal levels. If so, the COICR rocks of the Spessart mountains would have to be completely eroded by today, and instead, e&o&e

CROSS

SECTIONS

THROUGH

or blueschist affinity

grade

THE

VARISCAN

basement

FOLD

BELT.

applied

of Rhenohercynian

crustal

computer-aided

ble. This is especially

would be exposed.

(3) Major

19

GERMANY

extension

by gravity-driven

turally

desirable

program

data in the vicinity

of the DEKORP-2S

traverse.

ent models

On the other hand,

the Saar-Nahe

basin

towards

by their

ability

a major

example

of bulk

features,

especially

(Henk,

1991)

lithospheric horizontal starts

presents stretching

extension

(Schafer,

through

the Early

bimodal

volcanics

(Lorenz

provide

independent

ting of crustal volcanics erogenic

stretching.

Associated

Nicholls,

The occurrence

of: by

petrological

One consequence of two different

versus petrophysical Mengel and Kern,

magmatic

modification and melt

of this would be the Mohos: petrological

be represented in this case Moho” in Fig. 7.

with minor

to explain

of the erogenic

presence

and location

become

detectable

compensation

uable

not well understood

internal

at present

geometry

only

modelled

of the to a

tool

to

develop

at

least

qualitatively for the geo-

Conclusions (1) According to our model computations the earth’s crust in Central Europe, i.e. the Rheno-

and mag-

during

Moho uplift

(Ziegler,

figure

present

crustal

thicknesses

and west (Hunsrtick

to preclude

tion by Cenozoic subcrustal Variscan root destruction. On the whole it appears process of Moho flattening

the

by

the effects of isostatic

evaluated and contrasting scenarios metric evolution of erogenic belts.

the course needs

a major

processes

contribu-

to the post-

that the late Variscan and reequilibration is

at present.

The attempt to balance the geometrical evolution of the entire orogenically deformed crust, necessarily meets with several problems. Information on deep crustal geometry is scarce. Since the rheological behaviour and layer-to-layer interaction of stratified crust is difficult to model with simple geometrical techniques, shortcomings of the

of the Variscan

to be understood

tectono-stratigraphic crustal movements

seems

of the belt

extension

east (Harz mountains)

mountains)

the

displace-

rough approximation, can be assessed. On the whole, computer-aided crustal-scale balancing, although still in its infancy, promises to be a val-

evidence

similar

the megascopic

For example,

and can then be evaluated

on

although

geological

of out of section

Moreover,

estimate, fragments analysed,

of

belt.

and their effect on the geometry

other techniques. crust,

and compared

observable

when regarding

structure ments

effects of differ-

can easily be evaluated

1988). The pattern of Late Permian sedimentation, however, suggests a reequilibrated, stable crust already at the close of the Paleozoic; moreover, farther

geometrical

by the

techniques).

hercynian and Saxothuringian zones, was shortened by at least 520 km in a NW-SE direction

crustal

has supported

on balancing

Rhinegra-

(5) Last but not least, the Cenozoic matic activity

of the

Moho (see discussion in 1990. A fossil petrologically

defined Moho would by the “post-collisional ben rifting

set-

along the axis of the in our model experi-

of the lower crust by mafic underplating extraction. formation

1984)

for a regional

ments. This leads to the possibility (4) Orogenic root destruction These include

1989) and

Permian. and

evidence

is concentrated root constructed

processes.

sub-

of up to 30%. Sedimentation

in the latest Namurian

continues

with

the general

of struc-

are not permitted

(see the section

However,

are unavoida-

so when a number

options

collapse of thick erogenic crust (e.g., Dewey, 1988). However, this is as yet not supported by surface

the southwest

procedure

orogeny.

This

as a conservative

as in some parts of the section only of the deformation history have been and in others possible excisions of

or stacking

units by out-of-section are not considered. Subduction

of at least some 100 km of continental

crust at the sutures is necessary to form the orogenie roots-of c. 50 km thickness--which are destroyed through some as yet unknown mechanism or combination of mechanisms before Late Permian

times.

(2) The deeper crust beneath the Rhenish Massif is probably devoid of extremely weak decollement horizons and pronounced rheological stratification. Thus much of the seismic echo at least in the DEKORP-2N section is probably due to lithological contrasts. Strong strain localization in an anisotropic lower crust during lateral shortening

would have created ramp-flat topographies of thrust surfaces. This in turn would have resulted in the formation of basement-cored mega-anticlines much akin to the External Massifs of the Western Alps. Hence, the upper crust appears to have been largely detached from its basement along a broad mid-crustal zone of deformation. (3) The use of computers facilitates the application of rigorous geometrical criteria in the construction of crustal-scale cross sections. These criteria need not necessarily be correct in adequately describing deformation on a sub-km scale, but help in constr~ng the nature and tectonic significance of deep crustal seismic reflectors, the evolution of crustal geometry, and major features at the earth’s surface. Acknowkdgements We are most grateful to the European Geotraverse project for hosting us during the Rauischholzhausen workshop. The Dept. of Geophysics, Free University of Berlin, provided perfect hardware and software support and drafting facilities. This contribution would not have been written without the ideas and enthusiasm of all those who helped us during working group sessions, especially W. Franke, P. Giese, and K. Mengel. References Anderle, H.-J., 1987. The evolution of the South Hunsrtlck and Taunus borderzone. Tectonophysics, 137: 101- 114. Ahrendt, H., Clauer, N., Hunziker, J.C. and Weber, K, 1983. Migration of folding and metamorphism in the Rh&isches Schiefergebiie deduced from K-Ar and Rb-Sr age determinations. In: H. Martin and F.W. Eder (Editars), Intracontinental Fold Belts: Case Studies in the Variscan Belt of Europe and the Damara Belt in Namibia. Springer, Berlin, pp. 323-338. EREGT-group, 1990. An electrical resistivity transect from the Alps to the Baltic Sea (Central Segment of the EGT). In: R. Freeman and St. Mueller (Editors), Proc. 6th Workshop EGT. European Science Foundation, Strasbourg, pp. 299314. Behr, H.J. and Heinrichs, T., 1987. Geological interpretation of DEKGRP 2-S: a deep seismic reflection profile across the Saxothtuingian and possible im@ieations for the Late Variscan structural evolution of Central Europe. Tectonophysics, 142: 173-202. Behr, H.J., Grosse, S., Heinrichs, T., Wolf. U., 1989. A reinterpretation of the gravity field in the surroundings of the KTB-irnp~~tio~ for granite plutonism and terrane

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