Elsevier Encyclopedia of Geology

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The present-day Variscan Belt of central Europe

is not a linear feature. In Early Carboniferous times,

the paralinear or linear subduction and collision

zones were transported north-westwards, and drag

along the south-western margin of Baltica caused it

to rotate clockwise through approximately 90



(Figures 1 and 5). In about the latest Early Carbon-

iferous, the south-eastern flank of this ‘Bohemian

Arc’ was truncated by a huge transpressional fault

zone (the Moldanubian ‘Thrust’), which carried the

south-eastern blocks south-westwards, possibly for a

distance of about 1500 km.

Geological Record: Western Europe

An even more prominent arc structure is observed in

the western part of the Variscan belt: if Iberia is

rotated into its pre-Mesozoic position by closing the

bay of Biscay, Variscan structures define a tight arc

with a curvature of 180



in its internal part (the

‘Ibero-Armorican’ arc) (Figure 1).

The terranes of north-west Iberia are clearly

linked with those of Brittany and the Massif Central

in France and can be traced further eastwards into

the Bohemian Massif, thus forming parts of the

Armorican Terrane Assemblage. As in central Europe

(see above), cross-sections through the two arms of

the Ibero-Armorican virgation show a fan-like orogen

with opposite vergences (verging towards the north-

east and south-west in Iberia, and towards the south

and north in France), i.e. convergent on its concave

side and divergent on its convex side.

Two main sutures are visible in the internal meta-

morphic parts of the belt, and are the roots of nappes

with opposing vergences (Figures 6 and 7). The

Beja suture resulted from the closure of an oceanic

realm between Avalonia (the south Portuguese fore-

land) and Armorica. There are no pre-Carboniferous

rocks exposed in south Portugal, thus it remains un-

certain whether this oceanic realm was the early

Palaeozoic Rheic Ocean or a younger Devonian fea-

ture (as in south-west England and Germany). In any

case, the Beja suture can be correlated with the

Rheno-Hercynian ophiolites of the Lizard complex

in south-west England and the Gießen-Harz Nappe

in Germany. The second suture resulted from the

closure of the Galice-Brittany ocean and can be traced

from the French Massif Central, through the southern

Vosges and Black Forest, into the Moldanubian belt

of the Bohemian Massif (Figure 1). In contrast to

Figure 4 Plate kinematic evolution of central Europe, after Franke 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.

80 EUROPE/Variscan Orogeny

Figure 5 (A) Tectonic and palaeogeographical subdivision of the Variscan basement in central Europe. (B) Simplified tectonic cross section. A, Anticline; KTB, German Continental Deep

Drilling site; M, Massif; S, South (as in S-Bohemian Pluton); Tepla

-Barrand, Telpa

-Barrandian; Centr. Boh. Pluton, Central Bohemian Pluton; S-karkon, South-Karkonasze.

EUROPE/Variscan Orogeny 81

the situation in the central European section, there

appears to be only one Armorican microcontinent

(instead of Saxo-Thuringia and Bohemia). Nappes

and sutures of the Ibero-Armorican domain are

characterized by oceanic lithospheric rocks and

early (420–370 Ma) ultrahigh-pressure, high-pressure

and medium-pressure metamorphism. The most ex-

ternal parts of the belt comprise low-grade to non-

metamorphic Carboniferous marine to paralic basins,

which were deformed between 320 Ma and 290 Ma.

The structures and tectonothermal histories of the

Iberian and French sections of the Variscan Ibero-

Armorican arc are described below (Figures 6 and 7).

The Iberian section (Figures 6 and 7A) combines a

southern segment from the South Portuguese Zone

to the Central Iberian Zone and a northern seg-

ment based upon observations in north-western

Spain (Figure 1). The northern segment originated

from the closure of the Galicia-Brittany–Massif

Central–Moldanubian ocean. Westward-directed sub-

duction (in present-day coordinates) under an Armor-

ican terrane affected first oceanic and then continental

rocks between about 400 Ma and 300 Ma. Collision

produced an eastward-facing accretionary wedge,

in which deformation migrated from west to east.

The lower autochthonous part consists of Early

Palaeozoic shallow-water sediments deposited on

the margin of Gondwana. The upper allochthonous

part includes, from bottom to top, rocks of a passive

margin thinned during the Ordovician, ophiolitic

rocks (400–480 Ma) of arc or oceanic mantle,

representing the remnants of the Galicia-Brittany–

Massif Central ocean, and an uppermost ultra-

nappe, which may represent the extended eastern

margin of Armorica.

The southern segment of the Iberian section shows

an opposite orogenic polarity, with south-west-facing

folds and thrust sheets. Four main units are separated

Figure 6 Plate kinematic evolution of the Variscan belt in Iberia: 1, mantle; 2, continental crust; 3, oceanic crust; 4, volcanic island

arc; 5, Early Palaeozoic sediments; 6, Carboniferous foreland sediments. CCSZ, Coimbra-Cordoba-Shear Zone.

82 EUROPE/Variscan Orogeny

by major faults and/or sutures and are discussed

below.

The Central Iberian Zone is the southern continu-

ation of the autochthonous series of the northern

Iberian section, with Early Palaeozoic rocks uncon-

formably overlying low-grade Proterozoic sediments.

A very important intracontinental sinistral shear

zone separates the Central Iberian Zone from the

Ossa Morena Zone and marks the boundary between

tectonic domains with north-east and south-west ver-

gences. This shear zone either represents or overprints

the suture zone in which the north-west Iberian alloch-

thon is rooted. There is evidence of very peculiar

Ordovician magmatism with alkaline to peralkaline

gneisses.

The Ossa Morena Zone shows a relatively com-

plete fossiliferous Early Palaeozoic record from the

lowermost Cambrian through to the Silurian and

Devonian. Lower Cambrian sandstones and lime-

stones unconformably overlie Upper Proterozoic

rocks that are characterized by the presence of black

cherts metamorphosed and intruded by Cadomian

granitoids (550–500Ma) as in northern Brittany. In

some places (e.g. Cordoba) calc-alkaline andesites

erupted during the latest Proterozoic/Early Cambrian.

The Ossa Morena Zone shows large south-west-facing

recumbent folds emplaced before the Early Carbon-

iferous (which is, in part, coal-bearing). The Variscan

metamorphism (epizonal to catazonal) is of a high-

temperature–low-pressure type, and an important

Variscan magmatism (diorites and granodiorites) is

present. The present-day southern boundary of the

Ossa Morena Zone is considered to be a suture,

based on the existence of a belt of oceanic amphibolites

(Beja-Acebuches) thrust over a possibly Devonian

accretionary prism with slices of oceanic metabasalts.

This north-west–south-east-trending boundary with

the South Portuguese Zone has been reworked as a

sinistral wrench fault.

The South Portuguese Zone exposes only Devonian

and Carboniferous sediments and is characterized

by important bimodal Tournaisian volcanic deposits,

which contain the largest copper ore bodies in West-

ern Europe (the South Portuguese Pyrite Belt). Time-

equivalent volcanic rocks are known from south-west

England and are widespread in the Rheno-Hercynian

Belt of Germany. The Rheno-Hercynian and, hence,

Avalonian affinities of the South Portuguese Zone are

also suggested by a thick turbidite sequence deposited

in a foreland basin (‘Culm’ facies), with a change to-

wards paralic deposits in the south-west. It comprises

the Vise

an to Westphalian D. The South Portugese

Zone is characterized by south-west-facing folds

and thrusts. Deformation occurred during the Late

Carboniferous.

The French section (see Figure 1 for location) can be

subdivided into three segments. The Massif Central

segment is the most complete section on the southern

flank of the Variscan belt and is exposed over a dis-

tance of 400 km from the Montagne Noire to the

southern margin of the Paris Basin. The Paris Basin

segment is buried beneath Mesozoic sediments, but

has been sampled in rare drillings and imaged at

depth by a wide-angle reflection profile. Potentially

equivalent rocks are exposed to the west in central

Brittany. The Ardennes section is partly buried be-

neath the sediments of the Paris Basin, but is well

known at depth from drilling, coal mining, and a

Figure 7 Simplified tectonic cross sections through the Variscan belt in (A) the Iberian peninsula and (B) France: pink, pre-

Palaeozoic basement and early Palaeozoic granitoids (undifferentiated); yellow, Early Palaeozoic sediments; green, thrust sheets

with Early Palaeozoic ophiolites; dots, Carboniferous foreland basin deposits; VF, Variscan Front; Bray F, Bray Fault, NASZ, North

Armoricn Shear Zone; SASZ, South Armorican Shear Zone; CCSZ, Coimbra-Cordoba-Shear Zone.

EUROPE/Variscan Orogeny 83

deep seismic-reflection profile. Approximately

100 km of section are well exposed along the Meuse

River in the Ardennes.

In the Massif Central segment, three main tecto-

nostratigraphical units may be distinguished. First,

the Montagne Noire at the southern extremity of the

Massif Central consists of fossiliferous Palaeozoic sedi-

ments including Lower Cambrian shallow-water sand-

stones and limestones, Lower Ordovician shales and

sandstones, Devonian limestones, and a thick turbiditic

syntectonic Visean–Namurian series. The whole of

this lithostratigraphical sequence is involved in large

(10 km) recumbent folds facing south to south-west.

The lowermost para-autochthonous unit (Zone Axiale)

consists of Upper Proterozoic sediments that were

intruded by Ordovician granites at 450–460 Ma. De-

formation and low-pressure metamorphism occurred

between 330 Ma and 310 Ma.

Second, the ‘Schistes des Ce

vennes-Albigeois’ are a

very thick (possibly around 4000 m) greenschist-grade

series, probably derived from both Early Palaeozoic

and Late Proterozoic protoliths. They were intruded

by Cambrian–Ordovician diorites and granites at

540–460 Ma. The Ce

vennes series shows a northwards

gently dipping slaty cleavage related to a pervasive

southward or south-westward shearing. Barrovian-

type metamorphism increases towards the top of the

pile (inverted). Deformation and metamorphism oc-

curred between 350 Ma and 340 Ma. Large granitic

plutons were emplaced between 330 Ma and 305 Ma.

Third, the complex Leptyno-Amphibolitic Group

consists of mafic and ultramafic rocks characterized

by high-pressure to ultrahigh-pressure metamorphism

(in places they consist of coesite-bearing eclogites). It is

a very large (300 km) nappe with ophiolite fragments.

The probable root zone in the southernmost part of

the Paris Basin is marked by a significant positive

gravity anomaly. The Leptyno-Amphibolitic Group is

overlain by pelitic gneisses with slices of high-pressure

granulites and peridotites. Metamorphism has been

dated at 420 Ma (high-pressure) to 380 Ma (Barro-

vian). All these units are intruded by various types of

Variscan granites, which were emplaced between

360 Ma and 300 Ma.

The Ardennes segment is well documented from

drilling, coal-mining, and surface outcrops in the Ar-

dennes. Like the more easterly Rhenish Massif, this

segment represents the northern flank of the Euro-

pean Variscan Belt. Palaeozoic fossiliferous sediments

with a thick Devonian clastic wedge are involved in a

large north-west-facing fold and thrust belt, which

has been well imaged by ECORS (France) and

DEKORP (Germany) deep reflection profiles. The

main frontal thrust (Faille du Midi), exposed at the

France–Belgium border, carries Devonian rocks over

the Late Carboniferous coal basin. Deformation

occurred at around 300 Ma.

Features Characteristic of the

Variscan Belt

The central parts of the Variscan belt are intruded by

huge volumes of granite. In contrast to the situation

in many other orogenic belts, most of these granites

were not formed over oceanic subduction zones but

originated from the melting of metasediments in

the continental crust. Heating and melting of the

crust probably occurred initially when the crust was

thickened (two mica and cordierite leucogranites) and

then later when subducted parts of the subcontinental

lithospheric mantle became detached and sank back

into the asthenospheric mantle. This permitted the

upward ascent of hot asthenospheric mantle and

advection of heat to the crust.

The Variscan crust is rich in unstable isotopes

(mainly potassium, uranium, and thorium), whose

decay may produce up to 30% of the heat flow ob-

served in continental rocks. These heat-producing

isotopes were extracted from the mantle by repeated

magmatic episodes, shortly before and during Varis-

can plate convergence (Cadomian subduction mag-

matism, Cambro-Ordovician rift magmatism, and

subduction- and collision-related magmatism in the

Devonian and Carboniferous). These elements are

also contained in mica and feldspar, two main con-

stituents of the thick Early Palaeozoic shelf deposits,

which piled up during the collision of the major and

minor plates.

For these reasons, the Variscan orogen was ‘hot’ in

comparison with ‘cold’ orogens such as the Alps (see

Europe: The Alps), the Caledonides (see Europe: Scan-

dinavian Caledonides (with Greenland); Caledonides

of Britain and Ireland), and the Urals (see Europe: The

Urals). The high temperatures prevailing during con-

tinental collision effected mechanical weakening of the

crust. This is documented by the ‘squeezing out’ of

melts or low-viscosity metamorphic rocks towards

the forelands. The same effect is responsible for the

rapid destruction of the orogenic ‘root’. When heated,

the deeper parts of the thickened crust spread laterally

like oil on water. Therefore, the base of the continental

crust (the Mohorovicic discontinuity) had already

levelled out at a depth of about 30–35 km by Late

Carboniferous or Permian times, i.e. shortly after the

termination of crustal thickening (see Moho Discon-

tinuity). This process was aided by the ascent of

mantle-derived melts during the Permian, which initi-

ated the break-up of Pangaea (see Pangaea).

84 EUROPE/Variscan Orogeny

Further Reading

Burg JP, Leyreloup A, Marchand J, and Matte P (1984)

Inverted metamorphic zonation and large scale

thrusting in the Variscan Belt: an example in the

French Massif Central. In: Hutton DHM and Sanderson

PJ (eds.) Variscan Tectonics of the North Atlantic Region.

pp. 47–61. Special Publication 14. London: Geological

Society.

Fortey RA and Cocks LRM (2003) Palaeontological evi-

dence bearing on global Ordovician–Silurian continental

reconstructions. Earth Science Reviews 61: 245–307.

Franke W (2000) The mid-European segment of the Varis-

cides: tectonostratigraphic units, terrane boundaries and

plate tectonic evolution. In: Franke W, Haak V, Oncken

O, and Tanner D (eds.) Orogenic Processes: Quantifica-

tion and Modelling in the Variscan Belt, pp. 35–62.

Special Publication 179. London: Geological Society.

Franke W and Stein E (2000) Exhumation of high-

grade rocks in the Saxo-Thuringian Belt: geological con-

straints and geodynamic concepts. In: Franke W, Haak V,

Oncken O, and Tanner D (eds.) Orogenic Processes:

Quantification and Modelling in the Variscan Belt, pp.

337–354. Special Publication 179. London: Geological

Society.

Franke W and Zelazniewicz A (2002) Structure and evolu-

tion of the Bohemian Arc. In: Winchester JA, Pharaoh

TC, and Verniers J (eds.) Palaeozoic Amalgamation of

Central Europe, pp. 279–293. Special Publication 201.

London: Geological Society.

Kossmat F (1927) Gliederung des varistischen Gebirgs-

baues. Abhandlungen des Sa¨chsischen Geologischen

Landesamtes, Neue Folge 1: 1–39.

McKerrow WS, MacNiocaill C, Ahlberg PE, et al. (2000)

The late Palaeozoic relations between Gondwana and

Laurussia. In: Franke W, Haak V, Oncken O, and

Tanner D (eds.) Orogenic Processes: Quantification and

Modelling in the Variscan Belt, pp. 9–20. Special Publi-

cation 179. London: Geological Society.

Martinez-Catalan JR, Arenas R, Diaz Garcia F, and Abati J

(1997) Variscan accretionary complex of northwestern

Iberia: terrane correlation and succession of tectonother-

mal events. Geology 25: 1103–1106.

Matte P (1998) Continental subduction and exhumation of

HP rocks in Paleozoic belts: Uralides and Variscides.

Journal of the Geological Society of Sweden 120:

209–222.

Matte P (2001) The Variscan collage and orogeny (480–290

Ma) and the tectonic definition of the Armorica micro-

plate: a review. Terra Nova 13: 122–128.

Owen AW, Harper DAT, and Rong Jia-Yu (1991) Hirnan-

tian trilobites and brachiopods in space and time. In:

Barnes CR and Williams SH (eds.) Ordovician Geology,

pp. 179–190. Ontario, Canada: Geological Survey of

Canada.

Robardet M, Verniers J, Feist R, and Paris F (1994) Le

Pale

ozoique ante

-varisque de la France, contexte pale

ographique et ge

odynamique. Ge

ologie de la France 3:

3–31.

Scho

nlaub HP (1992) Stratigraphy, biogeography and

paleoclimatology of the Alpine Paleozoic and its implica-

tions for plate movements. Jahrbuch der Geologischen

Bundesanstalt 135: 381–418.

Scotese CR, Boucot AJ, and McKerrow WS (1999) Gond-

wanan palaeogeography and palaeoclimatology. Journal

of African Earth Sciences 28: 99–114.

Simancas JF, Carbonell R, Gonzalez Lodeiro F, et al. (2003)

The crustal structure of the transpressional Variscan oro-

gen of SW Iberia: the IBERSEIS deep seismic reflection

profile. Tectonics 22. DOI 10.1029/2002TC001479.

Stampfli GM (1996) The intra-alpine terrain: a Paleoteth-

yan remnant in the Alpine Variscides. Eclogae Geologi-

cae Helvetiae 89: 13–42.

Suess E (1888) Das Antlitz der Erde, vol. IV. Prague, F.

Tempsky.

Tait J, Scha

tz M, Bachtadse V, and Soffel H (2000) Palaeo-

magnetism and Palaeozoic palaeogeography of Gon-

dwana and European terranes. In: Franke W, Haak V,

Oncken O, and Tanner D (eds.) Orogenic Processes:

Quantification and Modelling in the Variscan Belt, pp.

21–34. Special Publication 179. London: Geological

Society.

Torsvik TH, Smethurst MA, Meert JG, et al. (1996) Con-

tinental break-up and collision in the Neoproterozoic and

Palaeozoic: a tale of Baltica and Laurentia. Earth Science

Reviews 40: 229–258.

Van der Voo R (1993) Paleomagnetism of the Atlantic,

Tethys and Iapetus Oceans. Cambridge: Cambridge

University Press.

Wegener A (1915) Die Entstehung der Kontinente und

Ozeane. Braunschweig: Vieweg.

EUROPE/Variscan Orogeny 85

The Urals

D Brown, Instituto de Ciencias de la Tierra ‘Jaume

Almera’, CSIC, Barcelona, Spain

H Echtler, GeoForschungsZentrum Potsdam,

Potsdam, Germany

Introduction

The Uralide Orogen was one of the main mountain

belts built during the Palaeozoic assembly of the

supercontinent Pangaea. Since the breakup of Pan-

gaea in the Mesozoic, the Uralides have remained

intact and are today located in the interior of the

Eurasia Plate. The current extent of the orogen is

seen by its roughly north- and south-oriented mag-

netic signature, which abruptly interrupts that of

Baltica, Kazakhstan, and Siberia, the tectonic plates

that collided to form the Uralides (Figure 1A). Most

of what is known about the Uralide orogen is con-

fined to the present-day Ural Mountains, a narrow

range of low to moderate topography extending for

nearly 2500 km from near the Aral Sea in the south

to the islands of Novaya Zemlya in the Arctic Ocean

(Figure 1B). East and south of the Ural Mountains,

much of the orogen is buried beneath Mesozoic and

Cenozoic sediments of the West Siberian and Precas-

pian basins and is not well known. For descriptive

purposes, the geology of the Uralide Orogen has been

divided into a number of longitudinal zones that are

largely based on the ages and palaeogeography of the

dominant rocks within them. From west to east, these

zones are the Pre-Uralian zone, the West Uralian

zone, the Central Uralian zone, the Magnitogorsk–

Tagil zone, the East Uralian zone, and the Trans-

Uralian zone (Figure 1C). Additionally, the Uralides

have been divided geographically into the South,

Middle, North, Cis-Polar, and Polar Urals; the

following discussions focus on the South and Middle

Urals (Figure 2).

The Pre-Uralian, West Uralian, and Central Uralian

zones, which together make up the western foreland

thrust-and-fold belt, contain Late Carboniferous

to Early Triassic sediments of the foreland basin,

Palaeozoic platform and slope sediments of the

Baltica margin, and Archaean and Proterozoic rocks

of the East European Craton (that part of the cra-

tonic nucleus of Baltica that took part in the Uralide

orogeny) (see Europe: East European Craton).

The Magnitogorsk–Tagil zone is made up of two

volcanic arcs, the Magnitogorsk Arc (South Urals),

which is composed of Lower Devonian to Middle

Devonian basalts that are overlain by Upper

Devonian volcanoclastic sediments, and the Tagil

Arc (Middle Urals), which is composed of Silurian

to Lower Devonian basalts and volcanoclastic sedi-

ments that are locally overlain by Lower and Middle

Devonian sediments. The East Uralian zone is com-

posed predominantly of deformed and metamorph-

osed volcanic arc fragments with minor amounts

of Precambrian and Palaeozoic rocks thought to

represent continental crust. The East Uralian zone

was extensively intruded by Carboniferous and

Permian granitoids, forming the ‘main granite axis’

of the Uralides. The Trans-Uralian zone is composed

of Devonian and Carboniferous volcanic and plu-

tonic complexes overlain by terrigenous redbeds and

evaporites. Ophiolitic material (oceanic crust) and

high-pressure rocks have also been reported.

Tectonic Evolution

The tectonic evolution of the Uralides (Figure 3)

began during the Devonian as intra-oceanic subduc-

tion formed the Magnitogorsk and Tagil island arcs

(island arcs form along convergent margins when one

oceanic plate subducts beneath another oceanic

plate). This was followed by the entry of the contin-

ental margin of Baltica into the subduction zone and

the emplacement of an accretionary complex over the

continental margin, and the exhumation of high-

pressure rocks (the process of bringing the rocks in

question from depth toward the surface) along the

arc–continent collision boundary. At the same time,

subcontinental subduction (in which oceanic crust

subducts beneath continental crust) and volcanic arc

formation appear to have been taking place along

the margin of Kazakhstan, and to have continued

throughout the Carboniferous. Throughout much of

the Carboniferous, there was a deformation hiatus

along the Baltica margin, which by then included

the accreted volcanic arcs, and shallow-water plat-

form margin sedimentation continued undisturbed.

By the latest Carboniferous to Early Permian, the

Uralian ocean basin had closed completely and the

continent–continent collision between the Kazakh-

stan and Baltica plates had begun. As this collision

progressed through the Early Permian to the Early

Triassic, the western foreland thrust-and-fold belt

and foreland basin of the Uralides developed, while

widespread strike–slip faulting, with exhumation of

lower crustal material accompanied by melt gener-

ation and granitoid emplacement, took place in the

interior part of the orogen. An episode of Early Trias-

sic extension and volcanism followed in the eastern

86 EUROPE/The Urals

Figure 1 (A) Total field aeromagnetic map. The north-east-trending short-wavelength anomalies represent the Uralides. Courtesy of National Geophysical Data Center. (B) Topography of

the Ural Mountains. Note that the topography corresponds only to the western part of the orogen. Courtesy of National Geophysical Data Center. (C) Map of the Uralide Orogen outlining the

extent of the different zones and geographical areas discussed in the text.

EUROPE/The Urals 87

part of the Middle Urals and northward; by the

mid-Jurassic, the intraplate Old Cimmerian deforma-

tion event led to the uplift of the Timan Range and

formation of the Pay–Khoy–Novozemelian foldbelt in

the northernmost Uralides (Figure 1C), and localized

the deformation southward.

Arc–Continent Collision

Throughout geological time, intra-oceanic island arc

development and its subsequent collision with a con-

tinental margin have been important processes in

collisional orogenic belts, and among the most im-

portant means by which Earth’s continental crust has

grown. In the case of the Uralides, the Tagil and

Magnitogorsk island arcs developed from the Silurian

(Tagil) and Early Devonian (Magnitogorsk) and

began to collide with the margin of Baltica in the

Middle Devonian (Magnitogorsk) and the Early

Carboniferous (Tagil) (Figure 3 shows this evolution

schematically for the Magnitogorsk Arc). The Tagil

Arc was pervasively deformed and metamorphosed

to lower greenschist facies. Deformation in the

Figure 2 Geological map of the South and Middle Urals. The location of the Europrobe Seismic Reflection profiling in the Urals

(ESRU) and Urals Seismic Experiment and Integrated Studies (URSEIS) transects are shown.

88 EUROPE/The Urals

Magnitogorsk arc is low, with only minor, open fol-

ding and minor thrusting. The metamorphic grade is

prenhite–pumpellyite facies. The Magnitogorsk arc–

continent collision resulted in the development of

an accretionary complex (Figure 3) that involved

continental slope and platform sedimentary rocks

(Suvanyak Complex) (Figure 2) that were detached

from the margin of Baltica and thrust westward over

the continental margin. The Suvanyak Complex is

overthrust by syncollisional volcanoclastic sediments

sourced from the accretionary complex and the Mag-

nitogorsk arc (Zilair Formation). These units are

flanked to the east by the high-pressure eclogite- and

blueschist-bearing gneisses of the Maksutovo Com-

plex (Figure 2), which records a peak metamorphic

pressure of 20  4 kbar (0.3 kbar is roughly equiva-

lent to burial 1 km deep in Earth’s crust) and tempera-

ture of 550  50



C, and a peak metamorphic age of

380 to 370 Ma (during the Middle Devonian). The

highest structural level of the accretionary complex is

the Sakmara Allochthon in the south (Figure 2)and

the Kraka lherzolite massif (a piece of oceanic mantle)

in the north. The east-dipping Main Uralian fault,

along which the arcs are sutured to the continental

margin, is a fault melange that contains, among other

things, several kilometre-scale fragments of oceanic

crust and mantle.

Subcontinental Subduction

Little is known about what happened on the margin

of the Kazakhstan Plate prior to or during its collision

with Baltica, because no rocks that can be unequivo-

cally assigned to the plate have been recognized in the

Uralides. However, studies suggest subcontinental

subduction and the development of a continental vol-

canic arc did occur. In part, the evidence for this

comes from Silurian- to Devonian-age mafic to felsic

gneisses and volcano-sedimentary rocks in the East

Uralian zone that appear to represent a volcanic arc

complex. The key piece of data for assigning these

Figure 3 A simplified model for the tectonic evolution of the Uralides from the Early Devonian to the Early Triassic along the latitude

of the Magnitogorsk Arc, highlighting the geodynamic processes that were active at each stage.

EUROPE/The Urals 89

Elsevier Encyclopedia of Geology - vol I A-E

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