Elsevier Encyclopedia of Geology

Подождите немного. Документ загружается.

reflecting the progressive thinning of the Laurentian

crust during rifting of the Rodinia Supercontinent and

the evolution of a passive continental margin adjacent

to the developing Iapetus Ocean. Inliers of Palaeopro-

terozoic gneisses in the Inner Hebrides and north-

western Ireland can be linked with similar units in

south-eastern Greenland and probably represent the

Laurentian basement on which the Dalradian sedi-

ments were unconformably deposited. Shear zone-

bounded inliers of Early Neoproterozoic Moine-like

rocks occur in the northern Grampian Highlands and

may provide linkage across the Great Glen Fault.

Midland Valley Terrane

The Midland Valley Terrane is bounded to the north

by the Fair Head–Clew Bay Line and the Highland

Boundary Fault and to the south mainly by the South-

ern Upland Fault (in the west of Ireland it lies north of

the Doon Rock Fault). Much of the pre-Late Palaeo-

zoic geology, particularly in the Scottish Midland

Valley, is obscured by Devonian and Carboniferous

cover. However, the exposed Early Palaeozoic rocks

have oceanic affinities. Early Ordovician ophiolites

occur on Shetland, along the Highland Boundary

Fault, and at Ballantrae, Tyrone, and Clew Bay. In

western Ireland, remnants of an accretionary prism

are present in Clew Bay and on Achill Island, locally

associated with blueschists. Immediately to the south

are the fore-arc Ordovician sediments of the South

Mayo Trough and the mafic to intermediate calc-

alkaline volcanics of the Lough Nafooey Group.

Whether the unexposed continental basement that

underlies the terrane has Laurentian affinities or is

entirely exotic is unknown.

Connemara Terrane

The Connemara Terrane is located in western Ireland

between the Doon Rock and Skirds Rock faults. It

consists of metasedimentary rocks that have been

correlated with the Dalradian Supergroup, and thus

the terrane is interpreted as a tectonic slice of the

Laurentian margin that became detached and inter-

leaved with units of the Midland Valley Terrane

during strike-slip displacements.

Southern Uplands Terrane

Located between the Southern Uplands Fault and the

Solway Line, the Southern Uplands Terrane is charac-

terized by thick sequences of Ordovician and Silurian

sediments, which were deposited in deep-marine en-

vironments and subjected to complex deformation

and low-grade metamorphism. The terrane itself

comprises three major units bounded by strike-slip

faults: the Northern, Central, and Southern belts.

The Northern Belt comprises Ordovician sediments

and rare volcanics; the Central Belt is formed of Or-

dovician and Silurian sediments, and the Southern

Belt is entirely Silurian in age. Although opinions

vary as to the tectonic setting of sedimentation, the

balance of evidence suggests that the sediments repre-

sent an accretionary prism developed above the north-

wards-subducting Iapetus Ocean. In eastern Ireland,

the Iapetus suture is drawn between the Ordovician

inliers of the Grangegeeth and Bellewstown terranes,

which show Laurentian and Avalonian affinities,

respectively, in their brachiopod and trilobite faunas.

Lake District–Leinster Terrane

The Lake District Terrane is situated between the

Solway Line and an unexposed boundary thought to

lie to the north-west of Anglesey. It contains Cam-

brian–Arenig clastic rocks, mid- to Late Ordovician

basic and acid volcanic and volcaniclastic rocks, such

as the Borrowdale Volcanic Group of the English

Lake District, and the unconformably overlying Silur-

ian sediments of the Windermere Supergroup of

northern England. The Lake District represents the

northern edge of Eastern Avalonia, and the volcanic

rocks are thought to have originated in a volcanic arc

created by southwards-directed subduction on the

Gondwanan side of the Iapetus Ocean.

Monian Terrane

The Monian Terrane is bounded to the south-east by

the Menai Strait Fault System and comprises the

Mona Complex of Anglesey and the Llyn Peninsula

in north-western Wales and the Rosslare Complex of

south-eastern Ireland. The terrane is highly seg-

mented by shear zones and brittle faults. Proven Pre-

cambrian rocks include high-grade metasedimentary

gneisses, a calc-alkaline granite pluton, and a belt of

blueschist facies metamorphic rocks, which may rep-

resent an accretionary prism. Thick low-grade meta-

sedimentary sequences, including a major melange

unit, have been viewed as Precambrian but may be

Cambrian, albeit very different from the Cambrian

rocks of the adjacent Welsh Basin Terrane. The Mon-

ian Terrane may be an Allochthonous Gondwanan

Terrane that was amalgamated with Eastern Avalonia

by the Early Cambrian at the latest, after which it

formed part of the southern foreland of the orogen.

Welsh Basin Terrane

Bounded to the south-east by the Welsh Borderland

Fault System, the Welsh Basin Terrane comprises

Cambrian–Silurian sedimentary successions, largely

deposited in deep-water environments, and thick

sequences of Ordovician tholeiitic to calc-alkaline

60 EUROPE/Caledonides of Britain and Ireland

volcanics. Volcanism and sedimentation probably

occurred in a back-arc basin floored by extended

Avalonian basement, behind the Lake District–

Leinster volcanic arc. A range of Ordovician calc-

alkaline volcanic and plutonic rocks underlie the

Mesozoic and younger cover of eastern England and

may represent continuations of the Welsh Basin and/

or Lake District volcanic belts.

Midlands Terrane

The Midlands Terrane is a triangular terrane

that represents the southern foreland of Eastern Ava-

lonia during the Caledonide Orogen and acted as

a rigid indentor during the Acadian event. Late Neo-

proterozoic calc-alkaline igneous rocks and associ-

ated low-grade metasedimentary rocks, which

developed along an active plate margin of Gon-

dwana, are overlain unconformably by relatively

thin sequences of Cambrian–Silurian shallow-marine

platform sediments.

Tectonic Evolution of the Caledonides

The tectonic evolution of the Caledonides began in the

Early to mid-Ordovician, with the commencement

of closure of the Iapetus Ocean, and finished with

the Early Devonian Acadian event. The main stages

are as follows.

Early to mid-Ordovician Rifting of Eastern

Avalonia and Arc–Continent Collision on the

Laurentian Margin

Eastern Avalonia rifted away from Gondwana in the

Early Ordovician (ca. 475 Ma), initiating closure of

the Iapetus Ocean and opening, to the south, of the

Rheic Ocean. A major southwards-dipping (in the

present reference frame) subduction zone developed

beneath the leading northern margin of Eastern Ava-

lonia, and arc volcanism began in the Welsh Basin in

the Early Ordovician. Sedimentary sequences de-

posited on Eastern Avalonia vary from shallow-

marine on upstanding blocks such as the Midlands

terrane and the Irish Sea landmass (effectively the

Monian terrane) to deeper-marine sequences in the

Welsh Basin.

By the Late Cambrian–Early Ordovician, ocean-

wards-dipping subduction zones had developed

along the margins of Laurentia and Baltica, to form

intraoceanic volcanic arcs. Localized orogenic events

reflect the collision of these volcanic arcs with the

continental margins. These include the Finnmarkian

event (505 Ma) along the Baltica margin and the

Grampian event (470–460 Ma), which affected the

Northern Highlands and Grampian terranes. In

western Ireland, the Grampian Arc is represented by

the Lough Nafooey Volcanic Group, and the exist-

ence of a similar arc beneath the Late Palaeozoic

cover of the Scottish Midland Valley is indicated by

the presence within the Ordovician sediments of

clasts of contemporaneous calc-alkaline plutonic

rocks derived from a proximal source. By analogy

with a similar-aged event in Newfoundland (the

Taconic event), collision of the Laurentian margin

with the arc at the subduction zone was associated

with northward obduction of a major ophiolite nappe

onto the Laurentian margin. Ophiolitic remnants of

this nappe lie along the north-western margin of the

Midland Valley Terrane, which effectively represents

the Grampian Suture; the Ballantrae Ophiolite was

probably obducted at about the same time. Within the

Grampian Terrane, ophiolite obduction resulted in

regional deformation and Barrovian metamorphism

at greenschist–amphibolite facies of the Dalradian

Supergroup. In north-eastern Scotland and Conne-

mara, this was accompanied by the emplacement of

major gabbros and granites. Late-stage backfolding

and underthrusting of the Dalradian Supergroup

formed south-eastwards-directed regional-scale fold

nappes such as the Tay Nappe in Scotland and the

Ballybofey Nappe in north-western Ireland. Within

the Northern Highland Terrane, similar-aged folding

and high-grade metamorphism of the eastern parts of

the Moine Supergroup is assigned to the Grampian

event. Following arc–continent collision, the polarity

of subduction reversed to northward.

Mid-Ordovician–Silurian: Collision of Eastern

Avalonia, Baltica, and Laurentia

On the Avalonian side of Iapetus, south-eastwards-

directed subduction beneath Avalonia continued into

the Late Ordovician. A major calc-alkaline volcanic

arc lay through south-east Ireland and the Lake Dis-

trict, with the Welsh Basin being a volcanically active

back-arc basin. Volcanism and subduction ceased

during the Late Ordovician, probably because the

Avalonian margin overran the ridge system in the

Iapetus Ocean. Throughout the Late Ordovician

and Silurian, the stable Midland Platform and Irish

Sea landmass hosted shallow-marine or emergent

conditions surrounded by deeper-marine basins,

which were often filled with turbidites. Eastern Ava-

lonia collided with Baltica during the Late Ordovi-

cian (440 Ma) as shown by palaeomagnetic data and

evidence of progressive faunal integration. South-

western Baltica records evidence of a Late Ordovician

thermal event, contemporaneous with open folding

and uplift in the Welsh Borderlands (the Shelveian

event). This event is also synchronous with low-

grade metamorphism in the North Sea, which was

EUROPE/Caledonides of Britain and Ireland 61

probably also related to the collision between Baltica

and Eastern Avalonia.

On the northern side of Iapetus, the uplands

created by the Grampian Orogeny remained emer-

gent through the Late Ordovician and Silurian. In

the Midland Valley Terrane, the former volcanic arc

was mostly buried under a blanket of marine and

nonmarine sediments, most of which were sourced

from the emergent Grampian Highlands to the

north. Sedimentation occurred in a series of localized

sinistrally transtensive basins. Outboard, to the

south-east, the Southern Uplands Terrane is domin-

ated by thick fault-bounded sequences of complexly

deformed trench sediments, which define an accre-

tionary prism. The dominant folds, cleavages, and

associated detachments within these sediments

formed progressively as an integral part of the devel-

opment of the accretionary prism, rather than as the

result of a distinct collisional event. A switch from

early approximately orthogonal shortening to sin-

istral transpression within the accretionary prism

occurred in the Early Silurian at 430 Ma.

The earliest linkage of Laurentian-derived and

Gondwanan-derived crustal blocks occurred in New-

foundland in the mid-Ordovician, at approximately

450 Ma. However, the earliest closure events in the

British Isles appear to be Silurian (ca. 425 Ma).

With progressive collision of the two margins of Iap-

etus through the Silurian, deep-water basins tended

to shallow whilst shallow-marine basins became

nonmarine or emergent. In the Lake District, the

Silurian Windermere Supergroup was deposited in a

flexural foreland basin that resulted from overthrust-

ing of the Southern Uplands Terrane onto the leading

edge of Eastern Avalonia during the terminal stages

of collision. The oblique collision between Eastern

Avalonia and Laurentia along the Solway Line,

which resulted in the final eradication of the Iapetus

Ocean, was complete by the end of the Silurian.

The collision here was relatively ‘soft’ and did not

result in major folding or cleavage development in

the immediately adjacent terranes. In western Ireland,

the collision was ‘harder’ and was associated

with sinistral transpression, which deformed the Or-

dovician and Silurian strata of the South Mayo

Trough.

Silurian: Oblique Collision of Laurentia and

Baltica, and Closure of Northern Iapetus

The Northern Highlands Terrane was probably located

700 km along strike and to the north-east of the Gram-

pian Terrane when this part of Laurentia collided ob-

liquely with Baltica at 430–425 Ma to result in the

Scandian orogenic event. This was associated with

regional nappe stacking in Scandinavia and eastern

Greenland, with intense folding, ductile thrusting, and

metamorphism of the Moine rocks in the Northern

Highlands Terrane, and with amalgamation of the

Northern Highlands and the Hebridean terranes

along the Moine Thrust. A total north-west–south-

east shortening displacement across the Moine Thrust

of at least 150 km seems likely, and an equivalent

total amount of shortening may also have occurred

along internal ductile thrusts within the Moine.

Analysis of Scandian structures in Scandinavia, eastern

Greenland, and north-west Scotland is consistent with

overall sinistral transpression, which became progres-

sively partitioned into orogen-orthogonal components

and orogen-parallel left-lateral strike slip.

Regional-scale nappe stacking was therefore

followed by major sinistral strike-slip displacements

from 425 Ma onwards along the Great Glen, High-

land Boundary, and Southern Uplands faults. The

magnitude of the displacements is uncertain, al-

though movement of 700 km along the Great Glen

Fault and minimum movements of 200 km along both

the Highland Boundary Fault and the Southern

Uplands Fault seem likely. Strike-slip faulting was

largely synchronous with the emplacement of the

Newer Granite Suite, a series of calc-alkaline mainly

I-type plutons probably derived from the melting of

lithospheric mantle and lower-crustal sources above

the north-westwards-dipping subduction zone.

Major plutons crop out extensively in the Grampian

Highlands and in north-western Ireland, and eruption

of coeval lavas and volcanics occurred in the area

between Glencoe and Lorne in the south-west Gram-

pian Highlands. Plutons were mainly emplaced in

transtensional pull-aparts along relatively minor

faults that were splays of, or Reidel shears related

to, the major displacement faults.

Early to mid-Devonian: the Final Caledonian

(Acadian) Collision – Closure of the Rheic Ocean?

Sinistral relative displacement of the terranes north of

the Solway Line and subduction-related plutonism

and volcanism continued into the Early Devonian.

By this time, the marine basins on both margins had

been largely uplifted following crustal shortening. In

the Scottish Highlands, Early Devonian continental

sediments were deposited unconformably on deeply

eroded Moine and Dalradian rocks. The Early

Devonian sediments of the Moray Firth–Orkney–

Shetland area and eastern Greenland probably accu-

mulated in transtensional basins, reflecting a change

from orogen-parallel displacements to sinistrally ob-

lique divergence between Laurentia and Baltica–

Eastern Avalonia between 410 Ma and 395 Ma.

62 EUROPE/Caledonides of Britain and Ireland

A similar transtensional setting may also apply to

the Early Devonian basins of the Midland Valley

and the Lake District. Fluvial and alluvial sedimenta-

tion also occurred on the Irish Sea landmass and in

the Anglo-Welsh Basin, with major rivers sourced

from uplifted segments of the Caledonides to the

north and north-east.

In southern Britain, the Cornubian basins re-

cord south-deepening Devonian marine sedimenta-

tion on the southern rifted passive margin of

Avalonia, which faced Armorica across an already

very narrow arm of the Rheic Ocean. East–west

trending Early Devonian growth faults have been

identified in South Wales and North Devon. In south-

ern Cornwall, Devonian sediments are characterized

by thick sequences of distal turbidites, which were

deposited either on highly thinned continental crust

at the edge of the shelf or on oceanic crust. The

ultramafic and associated rocks of the Lizard Com-

plex in Cornwall have been interpreted as an ophio-

lite derived from the Rheic Ocean basin, which is

inferred to have existed to the south.

During the Early to Mid-Devonian, widespread

Acadian deformation and low-grade metamorphism

occurred south of the Solway Line, forming the

slate belts of the Lake District, south-eastern Ireland,

and north-western Wales. In contrast to the area

north of the Solway Line, where the Caledonian

structural grain is mainly north-east–south-west, the

trend of folds and cleavage in Avalonia is much more

variable, showing a distinctly arcuate form. This

structural pattern is apparently strongly controlled

by the triangular shape of the old rigid continental

block that underlies the Midlands terrane, which did

not experience Caledonian deformation. The Welsh

Borderlands Fault System broadly defines the south-

eastern limit of Acadian deformation. There is a

common tendency in the slate belts for the steeply

dipping Acadian cleavage to cut gently across associ-

ated folds in a clockwise-deflected sense, indicating

that the collision direction was sinistrally oblique.

Although the Acadian phase of the Caledonian Or-

ogeny has been ascribed to the closure of Iapetus, it is

not clear why the shortening and uplift associated

with this event should occur as late as the Early to

Mid-Devonian. It is unlikely that forceful northward

collision of Avalonia continued for a further 20 Ma

after the last remnant of Iapetan crust was subducted,

especially as there is growing evidence for sedimenta-

tion in sinistrally transtensive basins at this time, both

south and north of the suture. Another possibility is

that the Acadian deformation resulted from the colli-

sion of Armorica with Avalonia in the Early Devon-

ian. This is supported by isotopic data that indicate

that initial exhumation and northward-directed

thrusting of the Lizard Complex in southern Corn-

wall occurred between 400 Ma and 380 Ma, overlap-

ping in time with cleavage development in the slate

belts to the north.

Further Reading

Dewey JF and Mange MA (1999) Petrology of Ordovician

and Silurian sediments in the western Irish Caledonides:

tracers of short-lived Ordovician continent-arc collision

orogeny and the evolution of the Laurentian Appalachian-

Caledonian margin. In: MacNiocaill C and Ryan PD

(eds.) Continental Tectonics: Geological Society, London,

Special Publication 164, pp. 55–108.

Dewey JF and Shackleton RM (1984) A model for the

evolution of the Grampian tract in the early Caledonides

and Appalachians. Nature 312: 115–121.

Draut AE and Clift PD (2001) Geochemical evolution of arc

magmatism during arc-continent collision, South Mayo,

Ireland. Geology 29: 543–546.

Soper NJ, Ryan PD, and Dewey JF (1999) Age of the

Grampian Orogeny in Scotland and Ireland. Journal of

the Geological Society, London 156: 1231–1236.

Van Staal CR, Dewey JF, MacNiocaill C, and McKerrow

WS (1998) The Cambrian-Silurian tectonic evolution

of the northern Appalachians and British Caledonides:

history of a complex, west and southwest Pacific-type

segment of lapetus. In: Blundell DJ and Scott AC (eds.)

Lyell: the past is the key to the present: Geological Society

[London] Special Publication, 143, pp. 199–242.

EUROPE/Caledonides of Britain and Ireland 63

Scandinavian Caledonides (with Greenland)

D G Gee, University of Uppsala, Uppsala, Sweden

Introduction

The Caledonide Orogen of the North Atlantic region

reaches northwards from type areas in the British

Isles, along the eastern edge of Greenland and western

Scandinavia, to the Barents Shelf and the Svalbard

Archipelago. Prior to the opening of the Norwegian

and Greenland seas and the Eurasian Basin in the

Tertiary (Figure 1), this ca. 3000 km long segment of

the orogen was about 1000 km wide. A substantial

part of this width, perhaps as much as 30–40%, was

the result of a long period of post-orogenic extension,

lasting from the Late Palaeozoic into and through

the Mesozoic, which was accompanied by the depos-

ition of thick sedimentary successions; these now

compose the continental shelves and host the main

oil and gas resources of northern Europe. Thus, at the

time of orogeny in the mid-Palaeozoic, this northern

part of the Caledonides was a long and relatively

narrow (ca. 600–700 km wide) mountain belt, similar

in dimensions and majesty to today’s Himalayas, sep-

arating the low-lying old cratons of eastern Europe

and Greenland.

The Caledonian Orogeny, referred to as the Scan-

dian Orogeny in these northern regions, resulted from

the collision of two continents, Baltica and Laurentia.

The former was much smaller than the latter and, dur-

ing collision, played a similar role to that of India in

the present-day Himalayan context. The Scandian

Orogeny began in the Silurian and extended into and

through the Early Devonian; the name is derived from

the Scandes, the mountains of Norway and western

Sweden. However, it is worth remembering that

today’s mountains, along the coasts of Scandinavia

and eastern Greenland, are the result of Tertiary uplift

during the opening of the Greenland and Norwegian

seas; they are not the relics of Palaeozoic mountains,

although they are dominated by Caledonian rocks.

Before the collision of Baltica and Laurentia and

the Scandian Orogeny, these two continents were

separated by the Iapetus Ocean. The closure of the

Iapetus Ocean occurred over a period of about 80 Ma

and involved the development of subduction systems

along the margins of both Laurentia and Baltica

and a wide range of tectonothermal activity. This

complex situation, involving magmatism and sedi-

mentation, deformation and metamorphism, was an

essential part of the Early Caledonian evolution, prior

to the final collision of the continents and a Devonian

change in global stress regimes: compression and

lateral shortening gave way to regional extension.

The tectonic evolution of the northern part of the

Caledonide Orogen is discussed below, after a presen-

tation of each of the three major regions of develop-

ment – western Scandinavia, eastern Greenland, and

the Barents Shelf.

Western Scandinavia

The Scandian mountains, with many peaks reaching a

little over 2000 m, extend for nearly 2000 km along

the length of Norway; they include substantial

regions of western Sweden and the westernmost

highest parts of Finland. The Caledonide Orogen,

on land, is up to 300 km wide and extends off the

Norwegian coast for a further 200–300 km beneath

the shallow shelf areas of the Norwegian Sea. The

orogen (Figure 2) is dominated by thrust sheets trans-

ported from west-north-west to east-south-east onto

the Palaeozoic platform successions of the Baltoscan-

dian margin of Baltica. The front of the orogen is

generally marked by a prominent thrust scarp, clearly

indicating that these allochthonous (i.e. transported)

rocks originally, in the Devonian, extended much

further eastwards, perhaps as far as 100 km, onto

the platform.

Figure 1 The North Atlantic Caledonides, from eastern Canada

to the high Arctic Barents Shelf, in the Late Mesozoic.

64 EUROPE/Scandinavian Caledonides (with Greenland)

The Caledonide Orogen of Scandinavia is one of

the world’s classic areas for demonstrating the im-

portance of very large scale near-horizontal thrusting

of rocks during orogeny. Nearly 120 years ago, Alfred

ETo

rnebohm demonstrated that the lateral trans-

port of major allochthons from Norway into Sweden

must have exceeded 100 km; subsequent work has

shown that the total displacements amount to many

hundreds of kilometres.

Along the eastern front of the orogen, the lower-

most Scandian thrust sheets rest on thin (usually a

few tens of metres) Cambrian sandstones and shales;

locally, these autochthonous successions include

underlying Vendian tillites and, in some central and

southern areas, Ordovician and Silurian limestones

and shales may also be present. These mainly

Palaeozoic cover sediments were deposited on the

deeply eroded and peneplained surface of the Fennos-

candian Shield, which forms the basement along

the entire Caledonian front. The shield is dominated

by crystalline rocks, ranging in age from Late

Mesoproterozoic (ca. 1000 Ma) in the south to

Palaeoproterozoic and Archaean in the far north.

The contrast in physical properties between these

ancient granites and gneisses and the overlying sedi-

mentary cover, especially the Cambrian alum shales,

played an important role in controlling the character

and geometry of the Scandian Orogeny. The alum

shales are highly organic-rich (locally over 10%)

and even today sometimes smell of bitumen. During

Figure 2 The Scandinavian Caledonides.

EUROPE/Scandinavian Caledonides (with Greenland) 65

orogeny, these beds, up to a few tens of metres thick,

provided a well-lubricated surface along the entire

length of the mountain belt, over which the alloch-

thons could easily be transported. These Cambrian

shales are also characterized by unusually high levels

of particular trace elements, such as uranium, van-

adium, and molybdenum, allowing them to be identi-

fied further west, towards the hinterland of the

orogen, where they are more metamorphosed and

lack fossils.

The Scandian thrust sheets contain rocks that are

derived both from the Baltoscandian margin (base-

ment and cover) and from a variety of terranes that

existed in the Early–mid-Palaeozoic outboard of Bal-

tica. The latter are largely composed of ocean-floor

and island-arc assemblages that formed in the Iapetus

Ocean and were thrust onto the Baltoscandian

margin during the Scandian Orogeny. Identification

and correlation of the different allochthons across the

orogen from the foreland, where they are relatively

well preserved, to the hinterland, where they are gen-

erally highly attenuated and metamorphosed, has

been possible only because all the thrust sheets are

influenced by major late-orogenic folding with oro-

gen-parallel axes. Within the cores of the anticlines,

in tectonic windows, basement–cover complexes and

overlying thrust sheets have been mapped, and alloch-

thons have been correlated both along and across

the orogen.

The many Scandian thrust sheets can be subdivided

into four major groups of allochthons – Lower,

Middle, Upper, and Uppermost. Where the rocks

below the Lower Allochthon are disturbed but still

comparable with the autochthon, the term parau-

tochthon has been widely applied. The Lower and

Middle Allochthons are composed of rocks derived

from typical Baltoscandian-margin sedimentary-

cover successions and their underlying crystalline

basement. Mafic dykes intrude the uppermost units

of the Middle Allochthon (Sa

rv Nappes). The Upper

Allochthon is more varied and is readily divisible

into two parts: a lower group of thrust sheets

(Seve Nappe Complex) that are compositionally

similar to some of the underlying sedimentary

cover but are much more highly metamorphosed,

and an upper group (Ko

li Nappe Complex) of lower

metamorphic grade that includes a wide range of

igneous and sedimentary rocks derived from oceanic

environments and including ophiolites and island-

arc and back-arc assemblages. The Uppermost

Allochthon has also proved to be complex, contai-

ning both continental-margin lithologies and ophio-

lites, intruded by major granite batholiths; these

highest nappes were derived from the margin of

Laurentia.

Baltoscandian Platform to Outer Margin (Lower

and Middle Allochthons)

The Baltoscandian-platform successions of the Au-

tochthon generally dip 1–2



westwards and reappear

towards the hinterland in the antiformal windows.

The overlying thrust sheets of the Lower Allochthon,

riding on a de

collement surface above the Cambrian

alum shales, are dominated by a Cambro-Silurian

stratigraphy that can be readily correlated with that

in the Autochthon, but in which the formations are

generally thicker. Prominent facies changes occur in

the Ordovician, with the platform carbonates of

the Autochthon giving way westwards to basinal

shales and westerly derived turbidites. A return to

shallow-marine environments characterizes the Late

Ordovician and Early Silurian, before Late Llandov-

ery deepening and the influx of Mid-Silurian turbi-

dites (flysch). In southern Norway, in the Oslo graben

and, locally, in the Swedish Caledonides, Early Silur-

ian shallow-marine limestones and deeper-water

turbidites shallow upwards in the Wenlock to a non-

marine siliciclastic Old Red Sandstone facies indica-

tive of the development of a foreland molasse-filled

basin. This basin is inferred to have existed along the

entire Caledonian front, prior to Late Palaeozoic and

Mesozoic erosion of the mountain belt.

In foreland parts of the orogen, the Lower Alloch-

thon is dominated by low-grade sedimentary succes-

sions, with only subordinate slices of Precambrian

crystalline rocks. Further west, towards the hinter-

land, the thrusts cut deeper into the basement. The

overlying Middle Allochthon contains extensive

sheets of highly mylonitized Precambrian granites

and gneisses, along with metasedimentary succes-

sions that are generally of Neoproterozoic age. Prom-

inent mylonites separate the Middle Allochthon from

the Lower Allochthon, and the metamorphic grade

(low greenschist facies) is generally somewhat higher

in the former. Precambrian complexes (Jotun Nappe)

of the Middle Allochthon comprise the highest

mountains in Scandinavia.

Included in the upper part of the Middle Alloch-

thon are thrust sheets that have a stratigraphy similar

to that in the underlying nappes, including thick Neo-

proterozoic siliciclastic and carbonate successions,

overlain by Vendian tillites and then sandstones.

However, these allochthons (Sa

rv Nappes) are re-

markable because of the widespread occurrence of

ca. 600 Ma old mafic dyke swarms, which are often

composite and even occur as sheeted complexes. Ig-

neous rocks of Neoproterozoic or Palaeozoic age are

notably absent from all the underlying tectonic units

in the Scandes, except where they are exposed in the

deep hinterland along the Norwegian west coast.

66 EUROPE/Scandinavian Caledonides (with Greenland)

Thus, it can be inferred that these allochthons were

originally located off the coast of western Norway

and, along with the overlying nappes, have been

transported at least 300 km eastwards onto the

Baltoscandian Platform.

Outermost Margin of Baltica (lower part of the

Upper Allochthon)

The Sa

rv Nappes are overthrust by a higher grade

allochthon, which is itself dominated by psammites,

pelites, and subordinate marbles, with numerous

amphibolited dolerites and gabbros and occasional

dunites and serpentinites. These rock units comprise

the Seve Nappe Complex. Many of the lithologies are

comparable with, but more heterogeneous than, the

underlying Sa

rv Nappes; they are always of higher

metamorphic grade, reaching granulite and eclogite

facies in parts of the mountain belt. The eclogite-

facies metamorphism is of latest Cambrian to earliest

Ordovician age (ca. 500–490 Ma), and at least some

of the granulites are younger and related to the

Scandian collisional orogeny.

The Seve Nappe Complex has been treated as a

separate so-called suspect terrane, which was prob-

ably derived from the outermost edge of Baltica,

where the extensive mafic magmatism and solitary

ultramafites were concentrated in the transition

zone between continental and oceanic crust.

Iapetus Ocean Terranes (the Ko

li Nappe Complex

of the Upper Allochthon)

Well-preserved ophiolites, derived from the Early

Palaeozoic ocean floor, along with volcanic island-

arc igneous complexes and associated thick, mainly

volcaniclastic, sedimentary successions characterize

the outboard terranes of the Scandinavian Caledo-

nides. The ophiolites are mostly of Early Ordovician

(perhaps partly Late Cambrian) age, and the arc com-

plexes date from throughout the Ordovician, provid-

ing evidence of the subduction systems controlling the

closure of the Iapetus Ocean. The rock units are

generally metamorphosed in greenschist facies and,

within the sedimentary successions, fossils are lo-

cally preserved, providing evidence of both age and

faunal affinity. Of particular interest has been the

identification of Early Ordovician faunas (mainly tri-

lobites and brachiopods, but also molluscs) with

North American affinities, indicating that some of

the ophiolite-bearing allochthons were derived from

the Laurentian side of the Iapetus Ocean; others,

apparently, were more centrally placed in the ocean.

These Scandian Iapetus-derived outboard terranes

occur in the Ko

li Nappe Complex, which, in central

parts of the mountain belt, can be readily divided into

three parts – Lower, Middle, and Upper. Fragmented

ophiolites occur locally in the base of the Lower Ko

Nappes (e.g. at Otta and Hando

l). Associated black-

shale formations often host solitary ultramafites. The

latter are sometimes mantled by detrital serpentinite

and at one remarkable location (Otta) these sedi-

mentary rocks are highly fossiliferous, giving good

control of age (Llanvirn) and ocean-island affinity.

Early Ordovician island-arc and rifted-arc volcanites

dominate the lower parts of the Lower Ko

li Nappes,

which are overlain by turbidites and sandstones;

these shallow upwards in the Late Ordovician

into continentally derived quartz sandstones and

then limestones. The latter host a coral- and brachi-

pod-bearing fauna with Holorhynchus, providing evi-

dence of proximity to Baltica by Ashgill times. These

shallow-marine environments deepen into basinal

black shales, sometimes with pillow basalts, in the

mid–Late Llandovery, after which the basins are filled

with turbidites.

The Middle Ko

li Nappes are dominated by igneous

complexes with calc-alkaline volcanic-arc affinities.

Major plutons range in composition from gabbro and

gabbro–diorite to tonalite, granodiorite, and trondh-

jemite, and the host rocks are mainly basalts and

volcaniclastic formations, all generally metamorph-

osed in greenschist facies. Ordovician turbidites are

conspicuous, and black shales have locally yielded

Llandovery graptolites. The igneous complexes have

provided mid-Ordovician uranium–lead zircon ages

and evidence of Ordovician deformation; their chem-

istry indicates an origin in an oceanic setting, and

most authors have considered them to be related to

Laurentian-margin subduction systems.

The Upper Koli Nappes contain a major basal ophio-

lite (e.g. at Støren) and a range of mid-Ordovician

calc-alkaline plutons. The ophiolite is directly over-

lain by Arenig limestones containing trilobites and

brachiopods of unambiguous North American affin-

ities; it has therefore been inferred that initial obduc-

tion of this ocean floor was onto the outer continental

margin of Laurentia. Turbidites and fanglomerates

dominate the overlying successions, which are also

thought to be Ordovician in age; Silurian fossils

have not been found in the Upper Koli Nappes.

Laurentian Continental Margin (Uppermost

Allochthon)

Highest in the Scandian nappe pile, located along the

west coast of central Norway, is a characteristic

allochthon dominated by major granitoid batholiths,

intruded into amphibolite facies sedimentary suc-

cessions of continental affinity – marbles, schists,

and psammites. Within this allochthon (e.g. in the

EUROPE/Scandinavian Caledonides (with Greenland) 67

Helgeland Nappe Complex) there are fragmented

ophiolites that may have affinities with those in the

underlying Støren Nappe. The batholiths are of

Ashgill to Llandovery age; thus they were intruded

immediately prior to Scandian collision.

Scandian Collision

The Scandian collision, with thrusting of major

allochthons many hundreds of kilometres onto the

Baltoscandian platform, resulted in deep depression

of the western edge of Baltica. Eclogite facies (high

pressure and temperature) metamorphism, locally

with mineralogical evidence of very high pressures

(coesite and microdiamonds), was widespread in

mafic rocks in the Precambrian basement of south-

western Norway (Western Gneiss Region); it also

occurs further north along the coast at a similar struc-

tural level. Exhumation of these deeply depressed

rocks occurred during both Late Silurian thrusting

and the extensional collapse of the orogen, with par-

ticularly rapid uplift in the Early Devonian. Crustal

thickening during continental collision led to the

build-up of the Caledonian mountain belt, and this

was kept in balance during continued collisional com-

pression by upper-crustal extension. This gravita-

tional collapse resulted both in further migration of

the nappe pile onto the Baltoscandian platform and in

major westerly directed detachments in the hinter-

land, the latter accompanying the development of

Old Red Sandstone basins.

Eastern Greenland

The Caledonian mountains of eastern Greenland,

reaching from 70



–82



N, flank the eastern edge of

the Greenland ice dome. As in Norway, the coastline

is deeply penetrated by fjords, and the steep mountain

sides, many rising vertically for 1000–2000 m out of

the sea, provide spectacular outcrops for geological

analysis. The continental shelf reaches only about

50 km offshore in the south, but widens northwards

to about 300 km in north-east Greenland. Beneath

these shelf areas of the Greenland Sea, the Caledonian

bedrock is covered by Late Palaeozoic and younger

successions. Tertiary mafic volcanics and intru-

sions, related to the opening of the Greenland Sea,

are extensive south of Scoresby Sund (ca.70



N).

The outcrop of Caledonian bedrock in eastern

Greenland (Figure 3) is about 300 km wide in the

south, at 70



N, and narrows northwards to about

100 km, striking obliquely offshore into the northern

Greenland continental shelf. It forms the western part

of the Caledonide Orogen and is dominated by thrust

sheets, which were emplaced west-north-westwards

onto the Laurentian Craton with its Early Palaeozoic

cover. Characteristic of the autochthonous cover of

the Laurentian margin of the entire Caledonides, from

eastern Canada in the south, via north-western Scot-

land and eastern Greenland, to Svalbard, is a Cam-

bro-Ordovician carbonate succession that contrasts

markedly with the coeval succession of Baltica in

terms of both lithology and fauna. In southern parts

of the eastern Greenland autochthon, these forma-

tions, with underlying quartzites and, locally, tillites,

rest on a deeply eroded and peneplained basement of

Archaean and Palaeoproterozoic age. In northern

areas, thick Mezoproterozoic sandstones and basalts

separate this old basement from the Palaeozoic cover,

and tillites are absent.

Most of the orogen front in eastern Greenland is

covered by ice, and the relationships between the

major Caledonian thrust sheets and the underlying

cover are best seen in antiformal windows, similar

to those in the Scandes. The allochthons have been

divided into two groups and are referred to as ‘thin-

skinned’, overlain by ‘thick-skinned’. Whereas in

the far north the former is a fold-and-thrust belt

dominated by sedimentary successions, further south

it incorporates extensive slices of Palaeoproterozoic

and Archaean crystalline rocks. The upper ‘thick-

skinned’ complex differs greatly from the under-

lying thrust sheets in the area south of 76



N, by

incorporating latest Mesoproterozoic successions

(Krummedal and Smallefjord groups), which are sev-

eral kilometres thick and intruded by ca. 930 Ma

granites; these are overlain, probably unconformably,

by Neoproterozoic siliciclastic and carbonate succes-

sions (Eleonore Bay Supergroup), Vendian tillites,

and platform Cambro-Ordovician carbonate forma-

tions. Thus the entire Caledonian mountain belt of

eastern Greenland is composed of rocks derived

from the Laurentian margin, including both Ar-

chaean and Palaeoproterozoic crystalline basement

and younger generally shallow-marine sediment-

ary successions deposited on it. Outboard (e.g. Iap-

etus-related) terranes are notable by their absence.

West-north-west-directed thrust transport of the

allochthons has been estimated to be at least

200 km, along with some sinistral strike displace-

ments; thus, prior to the Caledonian Orogeny, the

platform margin of Laurentia was nearly twice as

wide as it is today.

Only in north-easternmost Greenland do the Early

Palaeozoic carbonate-dominated successions conti-

nue upwards through the Ordovician into the Silurian,

to be replaced by easterly derived turbidites in the

mid-Silurian. This evidence, together with the presence

of mid–Late Devonian Old Red Sandstones in ex-

tensional basins in southern areas and many mid-

Palaeozoic isotopic ages, indicates that, although

68 EUROPE/Scandinavian Caledonides (with Greenland)

Figure 3 The Eastern Greenland Caledonides.

EUROPE/Scandinavian Caledonides (with Greenland) 69

Elsevier Encyclopedia of Geology - vol I A-E

Подождите немного. Документ загружается.