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rocks to a continental arc setting is the presence of

subduction-related granitoids that intrude into them.

These granitoids are thought to have formed in two

subduction zone settings. The first subduction related

magmatism occurred from about 370 to 350 Ma

(during the Late Devonian to Early Carboniferous)

and produced granitoids with a recognizable older

continental component. These granitoids are inter-

preted to have been related to the development of a

continental arc during subduction of oceanic crust

under the continental margin of the Kazakhstan

plate. A second phase of subduction magmatism oc-

curred from about 335 to 315 Ma (during the Early

to Late Carboniferous) and produced granitoids with

little, if any, continental component. These granitoids

are interpreted to have been related to melting of

the earlier continental arc during a subsequent or

continued subduction beneath the Kazakhstan

Plate margin. Magmatic activity directly related to

subduction ended after the Carboniferous.

The Foreland Thrust-and-Fold Belt

The western foreland thrust-and-fold belt and fore-

land basin of the Uralides developed from the Late

Carboniferous to the Early Triassic. The foreland

thrust-and-fold belt trends roughly north and south

and measures 50 to 150 km in width from the Main

Uralian Fault to the deformation front. Rocks in-

volved in the thrust belt include Proterozoic and

Archaean basement of the East European Craton,

Palaeozoic platform sediments of the Baltica margin,

the arc–continent collision accretionary complex, and

the Permian to Early Triassic foreland basin sedi-

ments. Along its eastern margin, rocks in the Kvar-

kush anticline (Middle Urals) and the Bashkirian

anticline (South Urals) were deformed in the Neopro-

terozoic, and structures were reactivated and the

rocks were deformed again during the Uralide Or-

ogeny. The structural architecture of the foreland

thrust-and-fold-belt is that of a west-verging thrust

stack developed above a basal detachment that

lies in the Proterozoic basement. Where it has been

determined by balanced cross-section restoration (the

procedure of restoring the layers depicted in a cross-

section to the positions they had prior to deforma-

tion), the Palaeozoic shortening is approximately

20 km or less.

Late Orogenic Strike–Slip Faulting

The internal part of the Uralides was extensively

affected by a late orogenic strike–slip fault system

that extends north and south for more than 700 km

before it disappears beneath Mesozoic and younger

sedimentary cover. Throughout much of the Middle

and South Urals, this strike–slip fault system coincides

with the East Uralian zone (see Figure 2), although

the currently defined Main Uralian fault appears to be

its western limit in the Middle Urals, so it therefore

includes the Tagil Arc. Estimates of displacement

along some strands of this fault system range from a

few tens of kilometres to more than 100 km. Isotopic

dating on one segment of the fault system indicates an

age of 247 to 240 Ma (during the Late Permian to

Early Triassic) for the development of fault-related

mylonites, and 305 to 291 Ma (during the latest Car-

boniferous) for associated metamorphic rocks. The

late orogenic strike–slip fault system was extensively

intruded by continental-type granitoids, first in the

southern part, from 292 to 280 Ma (from the latest

Carboniferous to earliest Permian), and then in the

northern part, from 270 to 250 Ma (during the Late

to Early Permian). These granitoids have an un-

usually primitive Sr and Nd isotopic composition

that is thought to have resulted from remelting of

the older continental arc.

Crustal Structure

The crustal structure of the Uralides has been deter-

mined from the integration of surface geology with a

variety of geophysical data, including reflection and

refraction seismic surveys, potential fields (gravity

and magnetics), and the thermal regime. The Europ-

robe Seismic Reflection profiling in the Urals (ESRU)

and the Urals Seismic Experiment and Integrated

Studies (URSEIS) experiments, together with Russian

reflection and refraction seismic surveys, and the

Urals Wide-Angle Reflection Seismics (UWARS) ex-

periment, provide a large dataset for interpreting the

crustal structure of the Uralides. These data show that

the Uralides still preserves its bivergent collisional

structural architecture and confirms the existence of

a crustal root along the central axis of the orogen

(Figure 4). The crustal structure has been determi-

ned by physical properties along the ESRU and, in

particular, the URSEIS transects (Figure 2).

Reflection and refraction seismic data show that the

Uralide crust, in the East European Craton, thickens

eastward, from 40 to 52 km to between 50 and

55 km, across the volcanic arcs, before thinning again

to between 40 and 45 km in the easternmost part of

the orogen. In the South (URSEIS) and Middle (ESRU)

Urals, the East European Craton part of the Uralide

crust is imaged by subhorizontal to east-dipping

reflectivity that can be related to its Uralide and

older orogenic events (Figure 5). In both datasets

the East European Craton extends eastward beneath

the Magnitogorsk–Tagil zone. The Magnitogorsk

(URSEIS)–Tagil (ESRU) zone displays moderate to

weak upper crustal reflectivity, but the middle and

90 EUROPE/The Urals

lower crust reflectivity is diffuse in the case of the

Magnitogorsk Arc, but quite strong in Tagil. The

crust-to-mantle transition, or Moho, is not imaged

beneath the Magnitogorsk zone, but is a fairly sharp

transition beneath the Tagil zone. East of the arc com-

plexes, the upper and middle crust region is imaged as

clouds of diffuse reflectivity interspersed with, or cut

by, sharp, predominantly west-dipping reflections

that extend from the middle part of the crust into the

lower crust, where it appears to merge with the Moho.

In the Middle Urals (ESRU), it is characterized by

abundant lower crustal reflectivity. In both the

URSEIS and ESRU data, the Trans-Uralian zone dips

westward beneath the East Uralian zone.

The velocity structure of the Uralide crust is best

characterized along the URSEIS transect (Figure 6A

and B). The upper crustal pressure wave velocities

) reach up to 6.3 km s

1

, and the shear wave veloci-

ties (V

) reach up to 3.9 km s

1

, with the higher values

being in the Magnitogorsk Arc. In the middle and lower

crust, V

ranges from 6.5 to 6.8 km s

1

,reaching

7.1 km s

1

above the Moho in the central and eastern

part of the transect; V

ranges from 3.7 to 3.9 km s

1

increasing to between 3.9 and 4.0 km s

1

at the Moho.

The crust–mantle boundary is marked by an increase in

to >8.0 km s

1

and in V

to >4.6 km s

1

. Using less

resolved data, the Middle Urals appears to have a V

structure similar to that of the URSEIS transect, al-

though higher values (7.6–7.8 km s

1

) are reached

near the Moho where the crust is thickest.

The Uralide heat flow density (HFD) is character-

ized by a strong minimum along the central part of

the orogen (with values as low as 10 mW m

2

reaching typical continental crustal values (up to

Figure 4 Schematic crustal cross-section along the URSEIS profile (see Figure 2), showing the bivergence of the Uralides and the

crustal root beneath the Magnitogorsk arc. The geometry of the western foreland thrust-and-fold belt is constrained by surface geology

and reflection seismic and borehole data, and has been balanced and restored.

Figure 5 Automatic line drawings of the ESRU and URSEIS reflection seismic profiles, with the major crustal boundaries and the

Moho indicated. These profiles clearly show the bivergent collision architecture of the Uralide orogen and the thickening of the crust

towards its central axis.

EUROPE/The Urals 91

60 mW m

2

) on either side (Figure 7A). The short

wavelength of the HFD anomaly is suggestive of a

shallow origin for the minimum, although the heat

production (k) data determined from surface samples

are too high to allow the HFD minimum to be simu-

lated along the URSEIS transect. This implies that

more ultramafic material is present at depth beneath

the Magnitogorsk zone, compared to below the East

European Craton crust. Another option is that

the HFD minimum is due to propagation of ground

surface temperature changes to depth as a result of

palaeoclimatic disturbances (such as recent climate

change or glaciation). Modelling assuming the first

alternative suggests that the Uralides thermal struc-

ture is characterized by relatively flat geotherms, with

a Moho temperature of around 600



C(Figure 6C).

This suggests that the root is not very cold and that

the Magnitogorsk Arc rocks do not have a completely

negligible heat production.

The Bouguer gravity anomaly in the South and

Middle Urals (Figure 7B)ischaracterizedbyalowof

between 60 and 45 mGal across the East European

Craton, indicating upper and middle crustal densities

of about 2.80 and 2.90 g cm

3

(with small local vari-

ations) and lower crustal densities of 2.98 and

3.02 g cm

3

. There is an abrupt increase in the Bouguer

anomaly to between about 0 to 40 mGal in the Mag-

nitogorsk–Tagil zone, falling to between about 70

and 40 mGal in the East Uralian zone, and about

30 to 10 mGal across the East Uralian zone. This

indicates an upper crustal density of between 2.71 and

2.80 g cm

3

(somewhat higher in the Magnitogorsk

zone), a middle crustal density of between 2.92 and

2.95 g cm

3

, and a lower crustal density of between

2.98 and 3.07 g cm

3

. The upper mantle has a density

of 3.34 g cm

3

. The magnetic signature of the South

and Middle Urals (Figure 7C)ischaracterizedby

short-wavelength features, with a long-wavelength

low that reflects the magnetic character of the East

European Craton. In the URSEIS transect, the magnetic

crystalline basement is truncated about 50 km to the

west of the Main Uralian fault, and magnetic sus-

ceptibilities of the rocks are 0 and 1.5 A m

1

to the

east and west of this truncation, respectively. Magnetic

Figure 6 (A) The URSEIS reflection profile with the V

data plotted over it; V

is generally higher in the lower crust beneath the arc

terranes. The location of the Moho is marked by the increase in

to greater than 8 km s

1

. (B) The URSEIS reflection profile with the V

data plotted over it; V

is generally higher in the middle and lower crust beneath the arc terranes. The location of the Moho is marked

by the increase in

to greater than 4 km s

1

. (C) The URSEIS reflection profile with the present-day geothermal gradient plotted over

it. The temperature at the Moho is generally around 600



C, which suggests that the metamorphic grade of the Uralide crust currently

does not reach granulite facies metamorphic conditions.

92 EUROPE/The Urals

Figure 7 (A) Contoured heat flow density map of the Middle and South Urals. Dots represent data points. (B) Bouger gravity and (C) aeromagnetic maps of the Middle and South Urals. The

maps are equal colour area and shaded relief, with illumination from the west. Maps courtesy of G Kimbell and C Ayala, BGS, British Geological Survey.

EUROPE/The Urals 93

susceptibilities of the upper crust vary between 0.25

and 0.5 A m

1

, with areas reaching 2 and even

2.5 A m

1

locally. The middle and lower crust region

has magnetic susceptibilities of 0.5 and 1.5 A m

1

Petrophysical modelling (assigning rock types based

on the seismic velocity, density, and thermal data) of

the Uralide crust along the URSEIS transect shows

clear differences between the composition of the old

continental crustal nucleus of the East European

Craton and the newly added crust of the accreted arc

terranes to the east. The crust of the East European

Craton is more felsic than that of the Magnitogorsk

and East Uralian zones, and the latter two zones have

a lowermost crust with characteristics indicating a

high garnet content (mafic garnet granulite) and/or

the presence of hornblendite. The overall composition

of the arc terranes is basaltic. The physical properties

data suggest that eclogite is not present in the lower

crust, or if present, it exists in such small amounts that

it is below the resolution of the dataset.

Topography of the Ural Mountains

There is widespread evidence that much of the Ura-

lides was eroded and peneplained by the Late Triassic

to Jurassic. If the Uralides were peneplained by this

time, when did the topography of the Ural Mountains,

which locally reaches about 1900 m, form? The geo-

morphology of the South and Middle Ural Mountains

is generally mature, being dominated by a system of

smooth, north- and south-trending ridges, but with

some younger features, such as deeply incised river

valleys and elevated river terraces, that hint at recent

uplift. The topography of the Ural Mountains is

almost exclusively associated with the foreland

thrust-and-fold belt and there is a strong correlation

of topography with thrusts (thrusts lie in the valleys)

(Figure 8). Low-temperature thermochronological

studies using apatite fission track dating suggest that,

with the exception of the Magnitogorsk Arc (which

yields an age of 262 þ 5 Ma, or Early Permian), much

of the South and Middle Urals has been relatively

stable since the Jurassic (ages range from 226 þ 8to

206 þ 9 Ma, or Late Triassic) (Figure 8), and that since

that time, very little cooling or erosion has taken

place. However, there is a disturbance in the fission

track age–altitude relationships across thrusts in the

foreland thrust-and-fold belt, which suggests that

some post-Jurassic reactivation has taken place along

these faults and that the current relief of the Ural

Mountains is therefore post-Uralide. (A fission track

age taken from a higher altitude in the hanging wall of

a thrust should be younger than one from the same

stratigraphic unit at a lower level in the footwall, but

this is not always the case in the Uralides.) Exactly

why or when the recent relief formed is, however, not

fully understood.

Further Reading

Berzin R, Oncken O, Knapp JH, et al. (1996) Orogenic

evolution of the Ural Mountains: results from an inte-

grated seismic experiment. Science 274: 220–221.

Brown D, Juhlin C, Alvarez-Marron J, Perez-Estaun A, and

Oslianski A (1998) Crustal-scale structure and evolution

of an arc–continent collision zone in the southern Urals,

Russia. Tectonics 17: 158–170.

Brown D, Juhlin C, and Puchkov V (2002) Mountain Building

in the Uralides: Pangea to Present. Geophysical Monograph

132. Washington, DC: American Geophysical Union.

Brown D, Carbonell R, Kukkonen I, Ayala C, and

Golovonova I (2003) Composition of the Uralide crust

from seismic velocities (V

and V

), heat flow, gravity,

and magnetic data. Earth and Planetary Science Letters

210: 333–349.

Figure 8 Topography of the South and Middle Urals, with

average apatite fission track ages indicated (millions of years).

Low-temperature exhumation in much of the Uralides took place

predominantly in the Late Triassic, although a notable exception

is the Magnitogorsk Arc, where it took place in the Early Permian.

94 EUROPE/The Urals

Carbonell R, Perez-Estaun A, Gallart J, et al. (1996)

A crustal root beneath the Urals: wide-angle seismic evi-

dence. Science 274: 222–224.

Echtler HP, Stiller M, Steinhoff F, et al. (1996) Preserved

collisional crustal architecture of the Southern Urals –

Vibroseis CMP-profiling. Science 274: 224–226.

Glodny J, Bingen B, Austrheim H, Molina JF, and Rusin A

(2002) Precise eclogitization ages deduced from Rb/Sr

mineral systematics: the Maksyutov complex, Southern

Urals, Russia. Geochimica et Cosmochimica Acta 66:

1221–1235.

Juhlin C, Friberg M, Echtler H, et al. (1998) Crustal struc-

ture of the Middle Urals: results from the (ESRU)

Europrobe Seismic Reflection Profiling in the Urals

Experiments. Tectonics 17: 710–725.

Knapp JH, Steer DN, Brown LD, et al. (1996) A lithosphere-

scale image of the Southern Urals from explosion-source

seismic reflection profiling in URSEIS ‘95. Science 274:

226–228.

Meyer FM, Kisters AFM, and Stroink L (1999) Integrated

geologic studies along the URSEIS ‘95 transect: contribu-

tions to the understanding of the orogenic evolution of

the southern Urals. Geologische Rundschau 87.

Perez-Estuan A, Brown D, and Gee D (1997) Europrobe’s

Uralides Project. Tectonophysics 276.

Puchkov VN (1997) Structure and geodynamics of the Ura-

lian orogen. In: Burg J-P and Ford M (eds.) Orogeny

Through Time, pp. 201–236. Special Publication 121.

Oxford: Geological Society.

Savelieva GN and Nesbitt RW (1996) A synthesis of

the stratigraphic and tectonic setting of the Uralian

ophiolites. Journal of the Geological Society 153:

525–537.

Zonenshain LP, Kuzmin MI, and Natapov LM (1990) Ura-

lian foldbelt. In: Page BM (ed.) Geology of the USSR:

A Plate-Tectonic Synthesis, pp. 27–54. American Geo-

physical Union Geodynamics Series 21. Washington, DC:

American Geophysical Union.

Permian Basins

A Henk, Universita

t Freiburg, Freiburg, Germany

M J Timmerman, Universita

t Potsdam, Potsdam,

Germany

Introduction

The Late Carboniferous to Early Permian (Stepha-

nian to Rotliegend) evolution of western and central

Europe was characterized by the formation of numer-

ous sedimentary basins as well as widespread and

voluminous magmatism in the area of the Variscan

orogen (see Europe: Variscan Orogeny) and its north-

ern foreland. This phase of intense extension and

thermal perturbation of the lithosphere led to an

almost complete destruction of the just-formed Var-

iscan orogen. Within 20 My, after convergence in the

external zone of the orogen had ceased, extension and

erosion had already removed most of the surface

topography and crustal roots, respectively, and a uni-

form Moho depth was restored in the area of the

former orogen. Towards the end of the Permian, the

crust had locally subsided even below sea-level as is

indicated by the rapid transgression of the Zechstein

Sea over part of the former orogen.

In this article, some general characteristics of the

Permo-Carboniferous evolution in western and cen-

tral Europe are first summarized. Subsequently, the fill

and structure of the sedimentary basins, as well as the

composition and petrogenesis of the magmatic rocks

formed during this epoch, are treated in some detail.

Finally, an overview of the geodynamic setting and the

potential driving forces for extension – topics that

have been controversially debated in past years – is

given.

General Characteristics

The end of convergence and the start of postconver-

gent evolution in the Variscan domain are docu-

mented by the youngest sediments that were still

deformed by compressional movements. These occur

in the most external parts of the Variscan fold belt

(Ruhr basin) and are Westphalian D, about 305 Ma.

However, the final stage of convergence was already

accompanied by localized extension in the orogen’s

interior. This is documented by the rapid syn-colli-

sional exhumation of metamorphic complexes, as

well as the lateral extrusion of crustal blocks (‘tectonic

escape’) and associated Namurian/Westphalian basin

evolution next to major orogen-parallel strike–slip

faults.

The subsequent Permo-Carboniferous (i.e., Stepha-

nian to Early Permian) evolution of western and cen-

tral Europe was largely controlled by a system of

north-west- to south-east-trending strike–slip faults

that intersected the Variscan fold belt and extended

far into its foreland. Together with reactivation of

older Variscan structures, movements along this

fault system led to the formation of numerous trans-

tensional half-grabens and pull-apart basins. Exten-

sion of the lithosphere and tectonic subsidence, as

EUROPE/Permian Basins 95

indicated by rapid subsidence and sediment accumu-

lation in these fault-controlled basins, can be dated to

between about 305 and 285 Ma. This correlates with

a phase of intense magmatism, peaking between 300

and 285 Ma, which occurred almost synchronously

over large parts of Europe – both inside and outside

the area affected by Variscan orogenic processes.

Subsequent thermal equilibration of the intensely dis-

turbed European lithosphere led to thermal subsid-

ence. During this stage, sedimentation overstepped

the initially fault-controlled basin margins. The indi-

vidual basins were connected and a coherent depos-

itional area formed in large parts of Europe. At about

250 Ma, the crust had locally subsided below sea-

level, as is indicated by the rapid transgression of the

Zechstein Sea over part of the former orogen. Even the

subsequent Mesozoic basin evolution in large parts of

Europe was still controlled by thermal equilibration

of the anomalies formed by Permo-Carboniferous

geodynamic processes.

Basin Formation

Late Stephanian to Early Permian basin evolution

started after a regional hiatus of Early Stephanian

age. This hiatus is documented within the Variscan

domain as well as in its northern foreland. There,

Stephanian and Early Permian sediments and volcanic

rocks unconformably overlie the Westphalian fore-

land basin sequence. In the external fold-and-thrust

belt, corresponding sediments were deposited on very-

low-grade to low-grade metasediments, whereas in the

internal part of the Variscides, they were often directly

deposited on amphibolite-facies metamorphic rocks

and granite intrusions. This demonstrates that by Late

Carboniferous times, the Variscides had already been

deeply eroded. Only locally Stephanian sediments un-

conformably overlie Namurian to Westphalian pre-

cursor basins. These basins, such as the Saar-Nahe,

Central Armorican Basin (i.e., Laval), and the basins

east of the Elbe Line, are usually located at major fault

zones that exhibit strike–slip movements and may be

related to tectonic escape during the final stage of the

Variscan Orogeny (see Figure 1).

The fill of the basins consists mainly of continental

siliciclastic sediments (shales, sandstones, and con-

glomerates) and interbedded volcanic and pyroclastic

deposits. During the tectonic subsidence stage, be-

tween 305 and 285 Ma, the sedimentary sequences

frequently show a typical facies evolution from flu-

vial to lacustrine and back to fluvio-deltaic depos-

itional environments. Deposition in fluviatile and

playa-lake environments was common during the

subsequent thermal subsidence stage. In some basins,

the earlier deposits contain economically important

coal measures (e.g., the Saar-Nahe, Saale, Autun, and

Sillon Houillier basins). The gradual decrease of coal-

bearing strata from the Late Carboniferous to

the Early Permian reflects the northward drift of

Gondwana from an equatorial setting. Due to the

Figure 1 Age distribution of Permo-Carboniferous magmatism in the Variscan domain.

96 EUROPE/Permian Basins

resulting changes in climate, formation of substantial

coal measures north of the Variscides had already

ceased in the Westphalian D, whereas in the southern

areas it continued until the end of the Stephanian.

The sediments and volcanic rocks accumulated

during the syn-rift and subsequent thermal subsid-

ence phases can locally reach total thicknesses of up

to 8 km. However, it should be noted that the thick-

nesses presently observed are often only erosional

remnants. At least in parts of the Variscan orogen it

can be shown that a substantial part of the sediments

deposited during the Stephanian to Early Permian

was already eroded prior to the Zechstein transgres-

sion. In the Saar-Nahe Basin, for example, estimates

based on shale compaction data and vitrinite reflect-

ance modelling indicate removal of up to 3.7 km of

sediments during this phase. Between these basins,

the basement was exposed or was directly overlain

by younger deposits.

The geometry and structural style of the individual

basins were strongly controlled by the interplay of

pre-existing structural elements and new faults that

formed in conjunction with the dextral translation of

Gondwana relative to Laurussia. A dextral strike–slip

component is inferred from basin geometry and orien-

tation, isopach maps of syn-rift deposits, migra-

tion direction of depocentres, the orientations and

structural data from syn-sedimentary faults, simul-

taneous horst and basin formation, and the arran-

gement of volcanic centres and dykes. Thus, some of

the basins trend obliquely to the structural grain of the

orogen and intersect the Variscan deformation front,

whereas others owe their formation to transtensional

reactivation of older, compression-related faults.

A prime example of the latter is the Saar-Nahe Basin

in south-west Germany, which formed by oblique

extensional reactivation of a crustal-scale fault struc-

ture, along which the internal zone of the Variscan

orogen (Mid-German Crystalline Rise, part of the

Saxothuringian Zone) was previously thrusted onto

the external fold-and-thrust belt (Rhenohercynian

Zone). Thus, the basins that were formed by transten-

sional reactivation of former thrusts frequently have

half-graben structures. In contrast, pull-apart basins

that opened during dextral strike–slip movements

tend to be located next to the dominant north-west-

to south-east-trending faults (continental-scale dex-

tral shears parallel to the Tornquist–Teisseyre Line;

e.g., the Elbe Line, Pays de Brays Fault, Bay of Biscay,

Gibraltar-Minas, and Agadir fracture zones).

Several of the faults that controlled Permo-

Carboniferous basin evolution were later reacti-

vated, particularly in connection with the inversion

of the Alpine foreland in the Late Cretaceous and

Early Permian. These later events can sometimes

obscure the Permo-Carboniferous basin geometry

and fault displacement. Thus, identification of faults

that were active in Permo-Carboniferous times re-

quires careful analysis of indicators for syn-sediment-

ary tectonics, such as abrupt variations in thickness

and facies architecture and syn-depositional deform-

ation structures.

Magmatism

Late Carboniferous to Early Permian volcanic and

plutonic activity of varying style and composition

was coeval with extension and basin formation in

both foreland and the internal Variscides, and oc-

curred in crustal domains of various ages (Figure 1).

Reliable U–Pb and

Ar/

Ar mineral ages show that

most of the activity occurred in the period 305 to

290 Ma. Magmatic rocks of mafic composition are

common in the foreland, but are relatively rare in the

internal Variscides. Layers of strongly altered air-fall

tuffs, known as bentonites or tonsteins, occur in

nearly every basin, but most of these originated

from distal volcanic sources.

Extensional faulting took place before, during, and

after volcanic activity. In the north-western part of

the North German Basin, initial faulting caused the

formation of horsts and grabens that strongly con-

trolled the placement and thickness of the volcanic

rocks. Faulting in the central North Sea mainly post-

dates the volcanic activity, whereas in the Oslo Rift,

the main east and west extension was coeval with the

main phase of trachyandesitic volcanism. In nearly

all of the basins, lavas occur interbedded with sedi-

ments, showing that magmatic activity took place

during subsidence and extension. The volcanic rocks

are interbedded and alternate with Stephanian coal

measures and Early to Late Permian, Rotliegend-

facies alluvial and fluviatile clastic sediments, and

lacustrine marls deposited in semiarid environments.

Where not eroded, these are often followed by aeolian

sandstones.

Foreland

In the foreland, a variety of volcanic and plutonic

rocks are present in the Oslo Rift in south Norway,

and large volumes of rhyolitic and andesitic volcanic

rocks occur in northern Germany below younger

cover and are known only from deep boreholes.

More mafic magmatic activity resulted in the forma-

tion of dyke swarms and sills, such as the basaltic

Whin Sill and the Midland Valley complexes in

Great Britain, the basaltic dyke swarm in South

Sweden, and the lamprophyre dyke swarms in the

western Highlands of Scotland. Smaller volumes of

mafic and felsic volcanic rock have been drilled in the

EUROPE/Permian Basins 97

central and eastern North Sea, and the Midland

Valley of Scotland is known for its large number of

alkaline mafic sills, vents, and subvolcanic intrusions

that often contain megacrysts and xenoliths of mantle

material and high-grade metamorphic crust.

Several stages of magmatism have been recognized

in the Oslo Rift. During the initial stages, felsic in-

truded Westphalian sediments, accompanied or

shortly followed by the extrusion of up to 1500 m of

primitive, alkaline basalt to phonotephrite lavas in

the southern parts of the rift. The subsequent main

rift stage resulted in fissure eruptions of thick se-

quences of porphyritic trachyandesite flows that con-

stitute the largest volume of volcanic rocks. This was

accompanied by the intrusion of large amounts of

syenitic magmas. Following the main stage of mag-

matism, intrusion of dykes and batholiths of syenitic,

monzonitic to granitic composition took place, often

related to the collapse of large volcanoes. The Oslo

Rift extends offshore into the Skagerrak to the south,

where seismic data suggest the presence of a 1-km

sequence of Late Carboniferous–Early Permian lavas.

The Whin Sill and Midland Valley complexes in

northern Britain consist of a series of dolerite sills

and east–north-east- to east–west-trending dykes that

extend eastwards into the North Sea. The dykes and

sill have subalkaline to transitional basaltic com-

positions. In Scania in South Sweden, a north-west-

trending swarm of subalkaline dolerite dykes may be

related to a few isolated, north–north-west-trending

dolerite dykes on the south-west coast and Bornholm

Island, and to dolerite sills that intrude Cambrian

shales in Va

stergo

tland. Volcanic rocks were encoun-

tered in the drill core in the Kattegat, and seismic data

suggest the presence of Early Permian volcanic

edifices offshore from eastern Denmark.

The Stephanian–Autunian volcanic rocks in the

north-western part of the North German Basin are

mainly limited to north–north-west- to north–north-

east-trending grabens, and their thicknesses vary

from a few metres to 100 m. In the north-east,

they form continuous layers up to 2 km thick and

may have covered an area of 180 000 km

. Basalt

lavas, dolerite, and gabbro intrusions are subordin-

ate, and rhyolite lavas and ignimbrites, basaltic an-

desite and andesite lavas, and tuffs form the largest

volumes. Five eruptive stages have been recog-

nized, and the total volume has been estimated at

48 000 km

, approximately 70% of which are rhyo-

litic lavas and ignimbrites that predominate in the

eastern part of the basin.

Elsewhere in the foreland, Stephanian to Autunian

magmatic activity occurred in much smaller volumes.

In the central and eastern North Sea, basalt, trachyan-

desite, and rhyolite flows and tuffs occur interbedded

with Rotliegend-facies mudstones and sandstones.

The volcanic rocks reach thicknesses of up to 160 m

in the central North Sea, and 680 m in the Horn

Graben. In northern Britain, up to 100- to 240-m-

thick sequences of alkaline basalt lavas and tuffs

occur in the Mauchline and Thornhill basins. These

unconformably overlie Westphalian sediments and

are interbedded with and overlain by Early Permian

aeolian red sandstones. The volcanic rocks are prob-

ably related to the approximately 60 mafic vents that

occur within a 20-km radius of the Mauchline Basin

and predate the aeolian sandstones.

Variscan Internides

The magmatic rocks in the internal Variscides range

from granite and diorite intrusions to rhyolitic–

andesitic volcanic and subvolcanic rocks. Mafic

rocks are limited to diatremes, small volumes of

lavas, and mafic dykes. In addition, the volcanic

rocks often comprise high proportions of ignimbrites

and tuffs, typical for explosive volcanism. Examples

are the granite and diorite intrusions in south-west

England (Cornwall), the Harz, Erzgebirge, northern

Italy, and the basement of the Alps, and the volcanic

rocks in basins in the Thuringian Forest, Pyrenees,

Iberia, northern Italy, the Provence, and the Saar-

Nahe Basin. However, not all basins contain mag-

matic rocks; for example, in the Stephanian to Early

Permian basins in the Massif Central in France, only

small volumes of volcanic rocks are present.

In south-west England, postorogenic granite em-

placement was probably accompanied by the extru-

sion of lavas, as indicated by clasts of rhyolite and tuff

within Permian sediments. The main magmatic activ-

ity occurred within the relatively short period about

4.5 Ma. Granite intrusion and subsequent uplift and

erosion partly overlapped with the formation of

narrow, east–west-trending, fault-bounded grabens.

These contain Rotliegend-facies clastic sediments

and small volumes of olivine basalt and lamprophyric

lavas and agglomerates. The lamprophyre lavas are

genetically related to the minette dykes that occur

throughout south-west England.

The Saar-Nahe Basin contains Early Permian an-

desite flows and pyroclastics that are associated with

subvolcanic dacite and rhyolite domes and diatremes

of subalkaline basalt. In the south-westwards exten-

sion in France, Early Permian andesites, rhyolites,

and rhyolitic breccias occur in boreholes and reach a

thickness of over 1 km south of Nancy. The small

Ilfeld Basin in the south Harz contains a 800-m-

thick sequence of sediments and thick layers of tuffs,

ignimbrites, and latitic, trachytic, and rhyolitic flows.

Of similar age are the undeformed or only weakly

98 EUROPE/Permian Basins

deformed granitoid and gabbro intrusions in the

nearby Thuringian Forest and Harz area, and the

high-level rhyolite sills near Halle. Differential uplift

in the Thuringian Forest, associated with granite

and diorite magmatism, led to block faulting and

the formation of horsts and pull-apart basins, the

latter filled by an up to 2-km-thick sequence of mo-

lasse sediments, lavas, and pyroclastic rocks of mainly

trachyandesitic and rhyolitic composition.

Magmatic activity in Iberia and the Pyrenees took

place in at least two stages, Late Carboniferous to

Early Permian and mid-Permian to Triassic. The first

phase is of predominantly calc-alkaline composition,

whereas the younger magmatic rocks are more alka-

line. Small basins and half-grabens contain sequences

of Late Westphalian C to Autunian terrestrial sedi-

ments and volcanic rocks, the latter mainly of pyro-

clastic character, but also comprising volcanoclastic

rocks and ash flows. Compositions range from andes-

ite, to dacite, to rhyolite, but andesites predominate.

Volcanism was accompanied by intrusion of hypabys-

sal sills, dykes, and domes, by high-level, often com-

posite and hybrid, granitoid intrusions, and by several

generations of granitic to dioritic dykes. The earliest

volcanism in the Pyrenees may partly overlap with the

intrusion of high-level calc-alkaline, often composite

granitic to dioritic, plutons that have Westphalian

(305–312 Ma) U–Pb zircon crystallization ages.

The pre-Mesozoic basement of the Alps, exposed in

the external massifs and tectonic windows, contains

Late Carboniferous to Early Permian granitoid intru-

sions and volcanic and sedimentary rocks. Narrow

pull-apart basins contain andesite to rhyolite lavas

and rhyolitic tuffs that were deposited on deformed

and metamorphosed Visean rocks, suggesting several

kilometres of uplift and erosion before 300 Ma. South

of the Alps, near Bolzano in northern Italy, Early

Permian latite, dacite, rhyodacite, and rhyolite lavas

and tuffs cover an area of 4000 km

and locally

reach a thickness of over 2 km. The Late Carbonifer-

ous to Early Permian granitoids in the basement of the

Alps have a calc-alkaline, volcanic arc character, but

in northern Italy, magmatic activity continued into

mid-Permian times and changed to more alkaline

compositions.

Petrogenesis

The varying compositions of the mafic rocks in the

foreland show that they were derived from different

mantle sources and evolved differently. The magmatic

rocks in the Midland Valley of Scotland were derived

by low-degree melting of deep mantle sources

followed by rapid ascent, as indicated by their alka-

line, magnesium-rich, and silica-poor composition,

by the absence of low-pressure differentiates, by the

presence of abundant megacrysts and of mantle and

lower crustal xenoliths, and by the style of volcanism.

The early alkaline mafic volcanic rocks in the Oslo

Rift may have been derived from mantle sources that

were metasomatically enriched by carbonatite fluids

in earliest Palaeozoic times. Lithospheric extension

causing decompression melting is a probable mechan-

ism, but geochemical evidence suggests that a mantle

plume component cannot be completely ruled out. In

contrast, the subalkaline basalts of the Whin Sill and

Midland Valley complexes indicate higher propor-

tions of melting of shallow mantle sources. Despite

being distributed over a relatively large area, their

distribution need not reflect a mantle thermal anom-

aly of the same extent. The geometry and orientation

of the dyke swarms suggest a magmatic focal region

in the vicinity of the Denmark–Skagerrak region, and

magma transport may have been horizontal, west-

wards into the North Sea and Britain. The position,

trend, number, and size of the dykes may have been

controlled by the regional dextral extensional stress

field.

The magmatic rocks in the North German Basin

and the internal Variscides have predominantly felsic

to intermediate (rhyolitic to andesitic) compositions.

Sr–Nd isotope data and the presence of garnet and

crustal xenoliths indicate that their parent melts as-

similated large amounts of crustal material, or that the

rhyolitic end-members were derived by the melting of

older crust. Furthermore, the scarcity of mafic mag-

matic rocks, in combination with the fractionated

character and degree of alteration of the felsic to

intermediate rocks, makes it difficult to establish the

nature of the mantle sources. The granite intrusions

and the felsic volcanic rocks often have calc-alkaline

compositions suggesting subduction-related volcanic

arc origins. This does not agree with the intraconti-

nental extensional setting of the magmatic activity.

Furthermore, most Variscan oceans had closed by

Visean times. Instead, the volcanic arc signature may

have been inherited from mantle sources that had been

metasomatized by previous subduction events, caused

by extensive assimilation of continental crust by the

parent melts, and/or inherited through partial melting

of older, calc-alkaline lower crust. The fact that

Stephanian to Autunian mafic rocks are much rarer

in the internal Variscides, compared to the foreland,

suggests that the mantle-derived parent melts were

unable to reach the surface directly. Instead, they

stalled in lower to mid-crustal magma chambers,

where they assimilated crustal material and fraction-

ated to more felsic compositions before erupting.

The large amount of crustal melts (rhyolites) in, for

instance, the North German Basin may have been

EUROPE/Permian Basins 99

Elsevier Encyclopedia of Geology - vol I A-E

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