Elsevier Encyclopedia of Geology

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thrusting may have started in the Late Ordovician, the

timing of the collisional orogeny in eastern Greenland

coincided with that in the Scandes. In marked contrast

to the Scandes, however, major post-kinematic granite

plutons are present.

Svalbard

The Svalbard Archipelago (Figure 4), located on

the north-western corner of the Barents Shelf, is

dominated by a larger island, Spitsbergen, which is

flanked to the east by Nordaustlandet, Barentsøya,

and Edgeøya. Numerous other smaller islands are

part of the archipelago, which reaches as far east as

Kvitøya and south to Bjørnøya. The western and

northern parts of the archipelago are dominated by

Caledonian bedrock, including Old Red Sandstones;

Carboniferous and younger successions unconform-

ably overlie this ‘basement’, occurring in a major

syncline that dominates the structure of the central

and southern parts of the archipelago. These younger

successions extend eastwards and southwards over

much of the Barents Shelf. Along the western coast

of Spitsbergen, a Tertiary east-verging fold-and-thrust

belt is superimposed on the older structures.

Prior to the opening of the northernmost Atlantic

and the Fram Strait, Svalbard’s Caledonian bedrock

was a direct northern extension of the East Greenland

Caledonides (Figure 1). On Svalbard, the Caledonian

rocks generally strike northwards and are split by

major north-trending faults and Old Red Sandstone

graben. Various provinces, or terranes, have been

recognized, and nearly all can be related directly to

the East Greenland Caledonides. However, the excep-

tion is important and occurs along the west coast of

Spitsbergen.

Svalbard’s eastern province (Figure 4) is readily

divisible into two terranes, Nordaustlandet (includ-

ing Kvitøya and eastern Ny Friesland) and West Ny

Friesland. The Nordaustlandet Terrane is character-

ized by the typical Laurentian Cambro-Ordovician

carbonate platform succession overlying tillites and

thick Neoproterozoic dolomites, limestones, sand-

stones, and shales. Major unconformities separate

Figure 4 The Svalbard Caledonides.

70 EUROPE/Scandinavian Caledonides (with Greenland)

this succession from a Late Grenvillian basement of

andesites and rhyolites and underlying meta-turbidites

of latest Mesoproterozoic or earliest Neoproterozoic

age, intruded syntectonically by ca. 950 Ma gran-

ites. Upright to west-verging folding characterizes

this easternmost Svalbard Terrane, and the grade

of Caledonian metamorphism increases eastwards,

with migmatization of the eastern parts of Nordaust-

landet and the island of Kvitøya. Late-orogenic gran-

ites (ca. 420–410 Ma) also characterize this terrane,

which, in all respects, compares closely with the

highest allochthon of the central East Greenland

Caledonides.

The Nordaustlandet Terrane is thrust westwards

onto a succession of high amphibolite facies orthog-

neisses and isoclinally folded metasediments that is

nearly 10 km thick – the Atomfjella Complex. The

metasediments are dominated by quartzites, but also

include marble and schist formations; the ortho-

gneisses are mainly metagranites, dated at 1750 Ma.

In both the metagranites and the quartzites, metado-

lerites are ubiquitous. Dating of detrital zircon has

shown that the quartzites are Mesoproterozoic in age,

but probably not younger than 1300 Ma (the age

of the metadolerites), and that the marble and schist

formations are latest Mesoproterozoic or younger.

The Late Palaeoproterozoic ‘basement’ metagran-

ites are repeated by thrusting at least three times in a

major north-trending antiform that dominates the

structure of western Ny Friesland. Only Caledonian

(ca. 430–410 Ma) argon–argon ages of meta-

morphism have been obtained from the Atomfjella

Complex; evidence of Grenvillian tectonothermal

activity is notable by its absence.

The West Ny Friesland Terrane is closely compar-

able with the ‘thick-skinned’ allochthon of north-east

Greenland, in terms of both the stratigraphy and the

character of the Caledonian deformation and meta-

morphism. Taken together with the evidence (above)

for the comparability of the Nordaustlandet Terrane

and the central East Greenland allochthons, there can

be little doubt that Svalbard’s eastern Caledonian

terranes are a direct northerly continuation of the

East Greenland Caledonides (Figure 5). In much of

Figure 5 The Arctic Caledonides and Laurentia–Baltica relationships in the Early Mesozoic.

EUROPE/Scandinavian Caledonides (with Greenland) 71

the previous literature on the Svalbard Caledonides, a

hypothesis has been favoured that these eastern ter-

ranes are derived from an off-shore area to the east of

central East Greenland, implying sinistral transport

over a distance of at least 1000 km; this hypothesis is

not supported by recent investigations of Svalbard

and Greenland.

Svalbard’s eastern Caledonides are separated from

the western terranes by the 50 km wide Andree

land–

Dicksonland Old Red Sandstone graben. The Cale-

donian bedrock, from north-western Spitsbergen

southwards via the west-coast Tertiary fold-and-thrust

belt to southernmost areas and further south to Bjør-

nøya, provides evidence of both Laurentian-platform

affinities and an outboard subduction-related terrane.

Unambiguous Laurentian lithological and faunal

signatures are found on Bjørnøya and southernmost

Spitsbergen, where Cambro-Ordovician carbonate

successions are closely related to strata of the same

age in north-eastern Greenland. The underlying Neo-

proterozoic carbonate and siliciclastic successions are

also comparable with those in this part of Greenland,

but, interestingly, the metamorphic complexes uncon-

formably underlying these sedimentary rocks are of

Grenvillian age, a characteristic that is unknown in

north-eastern Greenland.

In central western Spitsbergen, Neoproterozoic

successions, including thick tillites, are overthrust by

a blueschist and eclogite assemblage (the Vestgo

ta-

breen Complex). The high pressure–low temperature

metamorphism occurred in the Early Ordovician and

was followed by thrusting to high structural levels,

erosion, and deposition of a mid–Late Ordovician

conglomerate and limestone succession, passing up

into Silurian turbidites. The subduction-related Vest-

tabreen Complex is thought to be related to the

Pearya Terrane of northernmost Canada (Ellesmere

Island), both being foreign to Laurentia.

As shown in Figure 6, the Caledonian terranes of

Svalbard comprise the western part of an orogen that

extends northwards from the Scandes beneath the

Barents Shelf to the edge of the Eurasian Basin. This

part of the orogen is called the Barentsian Caledo-

nides. Prior to the opening of the Eurasian Basin in

the Tertiary, the Barentsian Caledonides are thought

to have crossed Lomonosova (today’s Lomonosov

Ridge) and may have continued into the continental

shelves of the Amerasian Basin.

The eastern side of the Barentsian Caledonides

is flanked by the Timanides, a Late Neoproterozoic

orogen that dominates the bedrock of north-eastern-

most Europe. The Timanides strike north-westwards

Figure 6 Tectonic elements of the western Eurasian Arctic in the Early Tertiary, during the initial opening of the Eurasian Basin and

the Norwegian and Greenland seas (light blue, shelf and ridges; deep blue, ocean floor).

72 EUROPE/Scandinavian Caledonides (with Greenland)

from a type area in the Timan Range, beneath the

Pechora Basin, into the Barents Sea. A deep borehole

on Franz Josef Land sampled folded Vendian

turbidites at depth of 2 km; this evidence, along with

geophysical data, suggests that the Barentsian Cale-

donides occur beneath Late Palaeozoic, Mesozoic,

and Tertiary strata over much of the Barents Shelf.

Tectonic Evolution of the Northern

Caledonides

The first interpretations of the North Atlantic Cale-

donides in terms of continental drift and plate tecton-

ics inferred that Laurentia and Baltica were part of a

single supercontinent (Rodina) in the Late Neoproter-

ozoic. The Caledonian cycle was thought to involve

rifting and separation of these continents, with the

opening of a substantial ocean by seafloor spreading,

followed by closure, resulting in a mid-Palaeozoic

configuration that was similar to that which existed

before ocean opening. Palaeomagnetic evidence has

subsequently suggested that Baltica rotated about

120



between the Vendian and the Middle Ordovi-

cian; thus the model of simple opening and closing

(the so-called ‘Wilson cycle’) is not generally accepted

today. However, the structure of the Scandinavian

Caledonides does provide evidence of a tectonic his-

tory that is readily divisible into three parts:

1. a period of Neoproterozoic rifting, leading to the

separation of Baltica from a larger continental

assemblage by the Early Cambrian;

Figure 7 Caledonian tectonic evolution, from the Vendian to the Late Silurian. e; eclogite.

EUROPE/Scandinavian Caledonides (with Greenland) 73

2. opening of the Iapetus Ocean in the Cambrian

and closing during the Ordovician, with subduc-

tion along the margins of both Laurentia and

Baltica; and

3. Scandian collision of Baltica and Laurentia in

the mid-Silurian and Early Devonian, with

underthrusting of the latter by the former.

The Caledonides of eastern Greenland and Svalbard

provide complementary evidence on the character

of the Laurentian margin and allow the reconstruc-

tion of the tectonic evolution of the whole orogen,

which is summarized schematically in Figure 7.

The vast Himalayan-type thrust systems of western

Scandinavia and eastern Greenland, with many hun-

dreds of kilometres of crustal shortening across the

orogen, testify to more-or-less orthogonal collision

of Laurentia and Baltica. Support for this conclusion

is found in the kinematic evidence of west-north-west

and east-south-east transport of the allochthons and

the correlation of thrust complexes over distances

of up to 1000 km along the orogen. This northern

segment of the Caledonides differs markedly from

the type areas of the Caledonide Orogen in the UK

and further to the south-west, where sinistral trans-

pression appears to have dominated the Caledonian

collision of Avalonia and Laurentia.

Further Reading

Andreasson PG (1994) The Baltoscandian margin in Neo-

proterozoic – early Palaeozoic times. Some constraints on

terrane derivation and accretion in the Arctic Scandi-

navian Caledonides. Tectonophysics 231: 1–32.

Birkenmajer K (1981) The geology of Svalbard, the western

part of the Barents Sea, and the continental margin of

Scandinavia. In: Nairn AEM, Churkin M Jr, and Stehli

FG (eds.) The Arctic Ocean, pp. 265–329. The Ocean

Basins and Margins, Part 5. New York: Plenum.

Fortey RA and Bruton DL (1973) Cambrian–Ordovician

rocks adjacent Hinlopenstretet, north Ny Friesland,

Spitsbergen. Geological Society of America Bulletin 84:

2227–2242.

Gee DG (1975) A tectonic model for the central part of the

Scandinavian Caledonides. American Journal of Science

275A: 468–515.

Gee DG and Sturt BA (eds.) (1985) The Caledonides

Orogen – Scandinavia and Related Areas. Chichester:

Wiley.

Gee DG and Tebenkov AM (2005) Svalbard: Frag-

ments of the Laurentian Caledonian Margin. In: Gee

DG and Pease VL (eds.) The Neoproterozoic Timanide

Orogen of Eastern Baltica. London: Geological Society

Memoir.

Grenne T, Ihlen PM, and Vokes FM (1999) Scandinavian

Caledonide Metallogeny in a plate tectonic perspective.

Mineralium Deposita 34: 422–471.

Harland WB (1997) The Geology of Svalbard. Memoir 17.

London: Geoogical Society of London.

Henriksen N, Higgins AK, Kalsbeek F, and Pulvertaft TC

(2000) Greenland from Archaean to Quaternary.

Description text to the Geological map of Greenland

1:2500000. Geology of Greenland Survey Bulletin

185: 1–96.

Higgins AK and Leslie AG (2000) Restoring thrusting

in the East Greenland Caledonides. Geology 28:

1019–1022.

Stephens M (1988) The Scandinavian Caledonides: a

complexity of collisions. Geology Today 4: 20–26.

Stephens MB and Gee DG (1989) Terranes and polyphase

accretionary history in the Scandinavian Caledonides.

Geological Society of America Special Papers 230:

17–30.

Trettin HP (1989) The Arctic Islands. In: Bally AW and

Palmer AR (eds.) The Geology of North America – An

Overview, pp. 349–370. Boulder, Colorado: Geological

Society of America. V. A.

74 EUROPE/Scandinavian Caledonides (with Greenland)

Variscan Orogeny

W Franke, Johann Wolfgang Goethe-Universita

Frankfurt am Main, Germany

P Matte, University of Montpellier II, Montpellier,

France

J Tait, Ludwig-Maximilians-Universita

t, Mu

nchen,

Germany

Introduction

Western and Central Europe were consolidated during

an Upper Palaeozoic orogenic event, which has been

named the ‘Variscan’ orogenyafteralegendaryGer-

manic tribe in north-eastern Bavaria, mentioned by the

Roman author Tacitus. The term ‘Variscan’ should be

given preference over ‘Hercynian’, which is often used

for the same orogeny. The latter name is derived from

the Harz Mountains in northern Germany, which do

contain Variscan basement, but which represent a fault

block thrust up during the Late Cretaceous.

Variscan rocks can be traced from Portugal to

Poland and from the British Isles to the Mediterranean.

The Variscan basement is exposed in ‘massifs’,which

emerge from under younger Late Palaeozoic or Meso-

zoic cover. The most important massifs are the Iberian

Massif in Spain, the Armorican Massif, the Massif

Central, and the Maures Massif in France, parts of

Corsica and Sardinia, the Vosges and the Black Forest,

the Ardennes, the Rhenish Massif and the Harz Moun-

tains in west-central Europe, and the Bohemian Massif

further east. In the Alps and Carpathians, as well as

in the Mediterranean realm, the Variscan basement

has been much reworked by the Mesozoic–Cenozoic

‘Alpine’ orogenic processes (see Europe: The Alps).

The extraordinary width of the Variscan belt is the

result of a complex palaeogeographical situation. The

Variscan orogen is actually a collage of major and

minor continental plates (Figures 1 and 2), which

were once separated by at least three oceanic areas.

These plates first spread apart (520–450 Ma), then

converged, and eventually collided with each other

(ca. 420–300 Ma). Sequential collision produced the

huge landmass of Pangaea. By the Early Permian,

the orogenic edifice was largely eroded, and was sub-

sequently covered by Permian, Mesozoic, and Ceno-

zoic deposits. The angular unconformity between

Variscan basement and the cover is one of the key

features of European geology (Figure 3).

There are two complementary ways to reconstruct

such large-scale and long-term processes. Studies

of palaeomagnetism (see Palaeomagnetism) can

reveal fossilized magnetic fields in ancient rocks. The

palaeodeclination reveals rotation of the sampling

area with respect to the present-day magnetic merid-

ian. The palaeoinclination records the palaeolatitude:

inclination is vertical at the magnetic poles and horizon-

tal at the magnetic equator. Since the Palaeozoic oceans

in Europe were orientated more or less east–west,

their closure implies changes in palaeolatitude, which

are well documented in the palaeomagnetic record.

Palaeomagnetic data can be compared with palaeocli-

matic indicators, such as evaporites, coral reefs, tropical

forests, or glacial deposits, and biogeography (areal

distribution of fossil faunas and floras). With errors of

500 km, palaeomagnetism, palaeoclimatology and

biogeography yield estimates of palaeolatitude and

can therefore be used to deduce the larger-scale move-

ments of the plates. It is important to note, however,

that palaeomagnetism provides no information about

palaeolongitude.

The second approach is geology. Even in ancient

mountain belts, it is possible to identify characteristic

elements of the plate-tectonic cycle, such as continen-

tal rifts, volcanic belts (magmatic arcs), remnants of

oceanic lithosphere (ophiolites), and belts of high-

grade metamorphic rocks, some of which have been

subducted into the mantle before reascending to the

surface. These features allow us to identify ancient

active and passive plate margins, to establish a relative

sequence of geological processes, and to date events.

While the amount of ocean floor lost by subduction

can be assessed only by palaeomagnetic methods, the

deformation of the continental crust must be recon-

structed by unravelling the polyphase tectonic deform-

ation and metamorphism. In most orogenic belts,

collisional deformation has reduced the width of the

colliding blocks to 50% or even less.

In this article, we first address the large-scale mi-

gration of the continental blocks that are now welded

together into the ‘united plates of Europe’, and then

proceed to sketch out the ‘ground truth’ revealed by

geological studies.

Palaeomagnetic and

Biogeographical Record

The Palaeozoic geography of the crustal segments now

contained in the continents bordering the Atlantic was

dominated by the large plates of Laurentia, Gondwana,

and Baltica. Between these plates there were a number

of smaller microplates, which are now incorporated

along the eastern margin of North America and

EUROPE/Variscan Orogeny 75

Figure 1 Tectonic and palaeogeographical subdivision of the Variscan basement in Europe, after Frank W (2000) The mid-European segment of the Variscides: tectonostratigraphic units,

terrane boundaries and plate tectonic evolution. In: Franke W, Haak V, Oncken, O, and Tanner D (eds.)

Orogenic Processes: Quantification and Modelling in the Variscan Belt, pp. 35–62. Special

Publication 179. London: Geological Society.

76 EUROPE/Variscan Orogeny

Figure 2 Positions of the continents and microplates bordering the Atlantic in (A) the Early Ordovician, (B) the Late Ordovician,

Figure 3 Angular unconformity between folded Late Carboniferous deep-water sandstones and Triassic fluvial deposits. Telheiro

beach near Cabo Sao Vicente, Algarve, southern Portugal. Photograph taken by P. Matte.

EUROPE/Variscan Orogeny 77

the southern border of Eurasia. These terranes include

Avalonia, the Armorican Terrane Assemblage, and the

proto-Alps, which, in Cambrian to Early Ordovician

times, were still attached to the north Gondwanan

margin. Subduction of oceanic crust under this margin

caused the Cadomian orogen (named after Cae

n, the

Roman Cadomus, in Normandy). Cadomian basement

is present in nearly all parts of the Variscan belt.

The positions and drift histories of Gondwana,

Baltica, Laurentia, Avalonia, the Armorica Terrane

Assemblage, and the proto-Alps are now fairly well

constrained by palaeomagnetic data, and faunal and

palaeoclimatic indicators are generally in good agree-

ment. Palaeomagnetic data suggest the following

palaeogeographical evolution (Figure 2).

Cambro-Ordovician

In the Early Ordovician, the northern margin of Gon-

dwana was situated at high southerly palaeolatitudes,

Baltica was situated between 30



Sand60



S and was

inverted with respect to its present-day position, and

Laurentia was in an equatorial position. The Cado-

mian basement rocks of Avalonia and the Armorican

Terrane Assemblage clearly indicate that in Cambrian

times these terranes were contiguous with the northern

margin of Gondwana, and remained marginal to Gon-

dwana until the Early Ordovician. By the late Early

Ordovician (Tremadoc), data indicate that Avalonia

had started to drift northwards, away from Gondwana,

opening the Rheic Ocean in its wake (see Palaeozoic:

Ordovician). It continued to move northwards through-

out the Ordovician, gradually closing the Tornquist Sea

and Iapetus Ocean, which separated it from Baltica and

Laurentia, respectively. Palaeomagnetic data from dif-

ferent elements of the Armorican Terrane Assemblage

indicate a similar, but independent, movement of these

microplates, with separation from Gondwana being

initiated slightly later in the Ordovician. No palaeo-

magnetic data are yet available from Ordovician rocks

of the proto-Alps, but geological evidence suggests a

continued Gondwanan affinity during this period.

Late Ordovician

By the Late Ordovician, Gondwana had moved some



northwards, and northern central Africa was

situated over the south pole according to palaeomag-

netic data from western Australia. Baltica was now in

its present-day orientation, and its northern margin

was at the equator. Laurentia, which did not move

much throughout the Palaeozoic, remained strad-

dling the equator and was separated from Baltica

and Gondwana by the Iapetus Ocean. By Ashgillian

times, palaeomagnetic and biogeographical data in-

dicate that the Tornquist Sea, separating Baltica

and Avalonia, had closed. Collision of Avalonia with

Baltica created a narrow belt of deformation and

metamorphism (the Polish Caledonides; Figure 1).

The Rheic Ocean still separated Avalonia/Baltica

from the Armorican Terrane Assemblage, which was

situated at more southerly palaeolatitudes in the Ash-

gillian, based on palaeomagnetic evidence from the

Bohemian Massif. The presence of glaciomarine sedi-

ments and cold-water faunas throughout the Armor-

ican Terrane Assemblage reflects the Late Ordovician

period of global cooling, which enabled the colon-

ization of previously warmer-water realms by cold-

water faunas. It has been shown that these glacial

sediments of central Europe were deposited by sea-

sonal or floating ice, in agreement with palaeomag-

netic data from the Bohemian Massif, which clearly

indicate intermediate to low palaeolatitudes. Strong

faunal and lithological similarities in the Ordovi-

cian–Devonian successions of different massifs of

the Armorican Terrane Assemblage indicate similar

ecological conditions, demonstrating that they were

all part of the same palaeogeographical domain.

The proto-Alps were positioned at higher palaeolati-

tudes, between northern Gondwana and the southern

margin of the Armorican Terrane Assemblage. This

conclusion is based predominantly on faunal evi-

dence, which indicates separation from northern

Gondwana.

Siluro-Devonian

The palaeogeographical position of Gondwana from

Silurian to Late Devonian times remains controversial

on the basis of palaeomagnetic evidence, as two dif-

ferent models have been proposed in the literature.

The more conservative model involves gradual north-

ward movement of northern Gondwana throughout

the Palaeozoic, with final closure of the ocean separ-

ating northern Africa from southern Europe in the

Late Carboniferous. The alternative model is based

primarily on palaeomagnetic data from south-east

Australia and requires rapid northward movement

of Gondwana in the Silurian, followed by rapid south-

erly movement in the Devonian. However, whether or

not it is viable to use palaeomagnetic data from

this region of Australia, whose autochthony with cra-

tonic Australia is questioned, remains open to debate.

In summary, taking all the palaeomagnetic, palaeo-

climatic and biogeographical data into account, the

more conservative model, involving gradual north-

ward movement of Gondwana throughout the

Palaeozoic, is considered more plausible.

Final closure of the Iapetus Ocean between Bal-

tica/Avalonia and Laurentia occurred in the Siluro-

Devonian, after which Baltica and Laurentia (Laurussia)

78 EUROPE/Variscan Orogeny

remained in equatorial palaeolatitudes until the end of

the Palaeozoic era.

Late Silurian–Early Devonian palaeomagnetic data

from a number of different crustal blocks of the

Armorican Terrane Assemblage indicate palaeolati-

tudes of 20



–30



S. This implies gradual migration

towards the southern margin of Baltica/Avalonia

and closure of the intervening Rheic Ocean.

Late Devonian

By the Late Devonian, the Iapetus Ocean between

Laurentia and Baltica/Avalonia had closed. Similarly,

the Rheic Ocean between Avalonia and the Armorican

Terrane Assemblage closed in the late Mid-Devonian.

Closure of the ocean was essentially a longitudinal

process and thus cannot be accurately constrained by

palaeomagnetic data; however, invertebrate faunal dif-

ferences between Bohemia and Avalonia persisted until

the Emsian or Givetian. The now amalgamated Laur-

asian landmass moved southwards in Late Devonian

times, but its northern border remained equatorial.

In Gondwana, however, the Late Devonian remains

one of the more controversial periods. High-quality

palaeomagnetic data from Australia clearly place

central Africa over the south pole, requiring an ocean

between the northern margin of Gondwana and the now

amalgamated Laurasia. Faunal data from the southern

Alps show poor similarity with the coeval fauna of

northern Africa, and the sedimentary sequences of the

southern Alps reflect a period of continuous sedimen-

tation until the Late Carboniferous, with little evi-

dence of any major deformation until Carboniferous

times. However, the similarity of the fossil fish records

of Gondwana (Australia) and Laurasia suggests that

there was no oceanic separation of these two contin-

ents from late Early Devonian times onwards. These

discrepancies in the faunal record, and between biogeo-

graphical and palaeomagnetic evidence, remain, as

yet, unresolved. Nevertheless, there is general consen-

sus that the collision of Gondwana with Laurasia to

form the supercontinent Pangaea occurred in Late

Carboniferous to Permian times.

A modern analogue of Palaeozoic plate dispersal

might be the Indian Ocean: like Avalonia and the

Armorican blocks, India, the Seychelles, and Mada-

gascar have separated from Africa and, at least partly,

made their way towards Asia (Laurussia).

Geological Record: Central Europe

Geological observations provide more detailed

information about the plate-tectonic processes in-

volved. Figure 4 is a diagrammatic representation of

the plate-kinematic evolution of central Europe.

Northward migration of the Armorican Terrane

Assemblage was accommodated by northward-

directed subduction of the Rheic Ocean beneath the

southern margin of Avalonia, forming a magmatic

arc, which is now preserved in the Mid-German Crys-

talline High (Figures 1, 4, and 5). By the late Early

Devonian, the Rheic Ocean had more or less closed.

At the same time, narrow oceans or seaways between

the Armorican islands – Saxo-Thuringia and Bohemia –

and between Bohemia and Moldanubia (possibly rep-

resenting northern Gondwana) were being closed. The

generally convergent movements were interrupted only

by renewed extension along the Avalonia–Armorica

boundary, creating a new narrow oceanic basin (the

Rheno-Hercynian Ocean) whose remains can be traced

from southern Portugal via south-west England and

the Rhenish Massif into the Harz Mountains, with

an easterly extension into Moravia (Figure 1).

The Rheno-Hercynian Ocean closed from the late

Middle Devonian onwards, thus joining in the general

convergence. The final collision of Avalonia with Fran-

conia (the northernmost Armorican terrane) occurred

in the earliest Carboniferous. Collision between the

Armorican terranes further south (Saxo-Thuringia, Bo-

hemia, and Moldanubia) took place in Late Devonian

times.

Plate convergence was accommodated by a bilat-

eral array of subduction or collision zones: the narrow

Rheno-Hercynian and Saxo-Thuringian oceans were

subducted towards the south, while the seaway be-

tween Bohemia and Moldanubia was subducted

towards the north. This resulted in three collisional

belts: the Rheno-Hercynian, Saxo-Thuringian, and

Moldanubian (Figure 5), which were originally

proposed by F Kossmat in 1927.

Collision resulted in the exhumation of previously

subducted oceanic and continental rocks, which were

emplaced as thrust sheets over the foreland areas. In

each of the three collisional belts, the frontal thrust

migrated towards the foreland, thus accreting fore-

land rocks to the orogenic wedge (Figure 5). Thrusting

and folding reduced the widths of the original micro-

plates considerably: collisional shortening in the three

belts amounts to a minimum of ca. 800 km. This

figure does not include the unknown width of oceanic

crust subducted between the microplates.

Foreland basins filled with synorogenic clastic

debris migrated ahead of the tectonic fronts. In the

Rheno-Hercynian Belt, deposition kept pace with

subsidence, so that sedimentation occurred in a shal-

low-marine to fluvial environment. The extensive

coastal forests that developed in these tropical regions

provided the raw material for economically import-

ant coal seams, which are exploited in Wales, the

northern part of the Rhenish Massif, and Silesia

(Figure 1).

EUROPE/Variscan Orogeny 79

Elsevier Encyclopedia of Geology - vol I A-E

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