Older Mountain Belts
The previous ten chapters have described the system of mountain belts and associated tectonic features that have developed over the last 200Ma during the Mesozoic and Cenozoic Eras and have resulted in the present-day distribution pattern of active seismicity and vulcanicity. The processes that gave rise to this pattern are in most cases relatively easy to reconstruct by tracking successive plate movements using the magnetic stripe data from the ocean floors, as explained in chapter 3. However, the oldest ocean crust that can be dated in this way is of Jurassic age, which means that the starting point for the reconstruction of Mesozoic–Cenozoic plate movements is the Pangaea Supercontinent at the end of the Palaeozoic, as shown in Figure 14.1. However, the geological evidence for the previous existence of mountain belts throughout most of Earth history is clear, and this chapter gives examples of some of the pre–Mesozoic belts that are recognised, and of the different methods used to reconstruct them in the absence of ocean-floor data.
Figure 14.1 Reconstruction of the continents during the late Permian (c.250Ma). Orogenic belts: Ap, Appalachian; Ca, Caledonian; CAB, Central Asian; In, Inuitian; Ou, Ouachita; Sam, Samfrau; Ur, Urals; Var, Variscan. Continental blocks: Ka, Kara; Kazakh, Kazakhstania; SEA, SE Asia; Ta, Tarim. Geographic features: BS, Black Sea; CS, Caspian Sea; Gd, Greenland. After Cocks & Torsvik, 2006.
Methods of reconstruction of older mountain belts
Old mountain belts may or may not betray their presence in the form of topographic elevations, and the older they are the more chance that the effects of erosion have reduced or even completely removed any topographic expression. The investigation of pre-Mesozoic belts has therefore focused on the evidence for the various geological processes that are involved in the creation of a mountain belt – these orogenic processes that collectively are responsible for the creation of orogenic belts.
A major advantage of studying older orogenic belts is that erosion has made it easier to access the deeper parts of such belts and therefore to gain information about tectonic processes in the deep crust. It should not be surprising that much of the information about ductile deformation processes and the metamorphic conditions that accompany them have been gained from regions such as the Scottish Highlands.
The main over-riding processes involved in the formation of an orogenic belt are subduction and collision. Evidence of subduction may be in the form of obducted ophiolite complexes, or of the magmatic products of subduction-related volcanism. Evidence of collision may be in the form of differing geological histories on either side of a suture line, or of the structural and metamorphic effects of crustal thickening and deformation. It is important to recognise, however, that reconstructions of former orogenic belts invariably involve a degree of speculation, which becomes increasingly more significant the older the belt. Intense disagreements have accompanied the study of many of these older belts, and interpretations are being continually revised. Accordingly, all reconstructions should be viewed with a degree of scepticism!
The Palaeozoic world
At the end of the Palaeozoic Era, during the late Permian Period, almost all the continental masses were arranged together as shown in Figure 14.1, in the supercontinent of Pangaea. The geological evidence for this arrangement was briefly discussed in chapter 3. Had geological observers been around then, 250Ma ago, several great mountain ranges, rivalling the Himalayas in scale, would have been evident. These are of two types: collisional belts traversing the supercontinent where the various component parts had come together; and subduction-related belts around the margins. There are three main collisional belts: the Caledonian–Appalachian–Variscan belt along the join between North America on one side, and Europe and Africa on the other; the Urals between Europe and Asia; and the Central Asian Belt between Siberia and Khazakhstania.
The circum-Pangaea belt follows the present western boundary of the Americas, and is the precursor to the Cordilleran and Andean chains of the present day. The Palaeozoic history of these belts has been largely obscured by the Mesozoic–Cenozoic orogenic activity responsible for the existing mountain belts. Another section follows the outer margin of Gondwana, with representatives in southernmost Africa, western Antarctica and eastern Australia, where it is known as the Tasman Belt. The existence of this orogenic belt was recognised in the early twentieth century as evidence for continental drift by Wegener and Du Toit and referred to as the ‘Samfrau Geosyncline’. The latter belt is relatively unaffected by subsequent orogenesis.
The collisional belts that cut through the Pangaea Supercontinent record how the various continental masses that preceded it were assembled to form the supercontinent. The main continental units that existed during the early Palaeozoic are shown in Figure 14.2; the largest of these by far was Gondwana, which comprised most of the present continents of South America, Africa, Australia, Antarctica and India. The other continents were Laurentia, which consisted of the core of North America plus Greenland; Baltica, which represents the old Precambrian core of Europe, and includes Scandinavia and much of east-central Europe; and Siberia, the Precambrian core of Asia. There were also numerous smaller continental terranes that had split off from the major continents and travelled independently across the oceans.
The Palaeozoic history of the major continents has been reconstructed with a fair degree of accuracy by means of palaeomagnetic information from within each of the various continental units, supported by geological indicators of climatic conditions. Palaeolatitudes can usually be closely constrained by this means but there is no method of determining pal-aeolongitude, so that the exact position of a continent on a particular palaeolatitude can only be guessed.
The assembly of Pangaea
During the Cambrian Period, a subduction zone lay along the margin of Gondwana facing west towards Baltica and Siberia, and as a consequence, back-arc rifting caused several continental fragments to be split off from the Gondwanan margin (Fig. 14.2A). By the mid-Ordovician, one of these fragments, Avalonia, had become detached from Gondwana and had travelled independently towards Laurentia and Baltica (Fig. 14.2B). The subsequent collision of Avalonia with Laurentia during the Silurian gave rise to the southern sector of the Caledonian–Appalachian Belt (Fig. 14.3A). During the same period, Baltica was also moving towards Laurentia and collided with it during late Silurian to early Devonian times to create the Scandinavian sector of the Caledonides. As a result of this collision, the western margin of Baltica came into contact with Avalonia to form the eastern branch of the Caledonides (now obscured by the later Hercynian Belt), which traverses Europe from Denmark to the Black Sea (see Fig. 14.1). On the northern side of Laurentia, the small continental terrane of Arctica collided with Laurentia to form the Inuitian Belt, which met the northern end of the Caledonian Belt at Svalbard.
Figure 14.2 Reconstruction of the continents, I: A, at the end of the Precambrian (c.550Ma); B, in the mid-Ordovician (c.460Ma). Av, Avalonia; En, England; Gd, Greenland; S, Scotland; SP, South Pole. After Cocks & Torsvik, 2006.
Figure 14.3 Reconstruction of the continents, II: A, in the Silurian (c.420Ma) showing the Caledonian–Appalachian belt (in blue) resulting from the collision with Baltica in the northern sector and East and West Avalonia in the southern. Arrows show the convergence direction. Note the positions of Scotland (S) and England (E); B, in the late Devonian (c.370Ma). The small terranes in green are destined to collide with Laurussia to form the Hercynian Belt, the blue terranes combine to form Kazakhstania, which collides with the Baltic margin of Laurussia to form the Urals Belt. Arc, Arctica; E, England; Gd, Greenland; K, Kazakhstania; Nf, Newfoundland; P, Perunica; S, Scotland; Sv, Svalbard. A, after Dalziell, 1997; B, after Cocks & Torsvik, 2006.
During the Devonian, several other terranes became detached from Gondwana and travelled northwards towards the combined continent of Laurentia–Baltica, now known as ‘Laurussia’ (Fig. 14.3B). During the Carboniferous, these terranes successively amalgamated with the Baltic and Laurentian margins to create the Hercynian Orogenic Belt (shown in brown in Fig. 14.1), which extends across central and southern Europe (where it is known as the Variscan) and along the southern Appalachians. The last two episodes in this assembly process were the collision of the West African margin of Gondwana with southeastern Laurentia to complete the Southern Appalachian Belt, which occurred in the late Carboniferous, and the collision of Khazakhstania and then Siberia with eastern Baltica to create the Urals Belt, which was finally completed in the early Permian to create the Pangaea assemblage as shown in Figure 14.1.
Of the Lower Palaeozoic orogenic belts, the Caledonides is the one that has been most intensively studied and on which much of the research into orogenic processes has been concentrated. Two contrasting sectors are discussed: Scandinavia–Greenland and British Isles–Newfoundland.
The Scandinavian Caledonides
The Caledonian Orogenic Belt extends over 1700km along the Atlantic coast of Scandinavia (Fig. 14.4) and a matching belt of a similar length lies along the east coast of Greenland. Figures 14.2B and 14.3A show how this orogenic belt resulted from the convergence of Baltica with Greenland during the Ordovician and their ultimate collision in the Silurian. The belt was subsequently split in two by the opening of the North Atlantic Ocean in the early Cenozoic.
The Scandinavian mountain chain occupies a belt varying in width from about 250km in the north to nearly 500km in the south. There are many peaks over 2000m in height, the highest being Galdhøpiggen, 2469m, on the Jotunheimen Plateau. The orogenic belt is narrower, varying from about 100km to 400km in width, and occupies a coastal belt 1700km long, ending on the north coast of Norway, and reappearing in the Svalbard archipelago in the Arctic Ocean, a further 500km to the north.
Structure and composition of the Scandinavian belt
In mainland Scandinavia, the belt consists of a series of nappe complexes divided into four tectonic zones, each of which has been thrust eastwards onto the Baltica craton (Fig. 14.4). The lowest of these zones, known as the Lower Allochthon, consists of nappes containing a mid-Proterozoic basement, similar to that of the foreland, overlain by a late Proterozoic to Lower Palaeozoic shallow-marine platform cover. This sedimentary cover is exposed mainly in the east of the orogenic belt, whereas the outcrops of the Precambrian basement appear in a series of basement ‘windows’, the largest of which is a coastal area about 100km wide extending from Trondheim in the north to Bergen in the south.
The overlying Middle Allochthon zone consists of greywacke sandstones intruded by a dolerite dyke swarm, and interpreted as the thinned passive continental margin of Baltica with its overlying continental slope sediments. It is overlain by the Upper Allochthon zone consisting of a late Proterozoic to Lower Palaeozoic clastic sedimentary sequence containing a calc-alkaline volcanic suite, and includes an ophiolitic assemblage. This unit is interpreted as the product of an intra-oceanic volcanic arc produced by a subduction zone dipping towards Laurentia rather than Baltica, since there is no sign of a volcanic arc on the Scandinavian side. However, the Ordovician to Silurian sediments contain Scandinavian rather than Laurentian faunas, indicating their relative closeness to Baltica.
Differences in the age of the metamorphic event accompanying the arc emplacement suggest that two different arc collisions may have occurred: one in the north at c.505Ma, termed the Finnmarkian event, and one in the central sector at c.493–482Ma, termed the Trondheim event.
Figure 14.4 Tectonic units of the Scandinavian Caledonides. All boundaries between the various units are thrusts. The cross-section is taken across the central part of the orogen along the line A–B. LA, Lower Allochthon; MA, Middle Allochthon; UA, Upper Allochthon; UMA, Uppermost Allochthon. G, Galdhøppen (Mountain); JP, Jotunheimen Plateau. After Gee et al., 2010.
The structurally highest zone, known as the Uppermost Allochthon, is composed of a quite different assemblage, which is exotic to Scandinavia and considered to be derived from Laurentia (i.e. Greenland). This unit is composed of a basement of gneisses and schists overlain by Ordovician to Silurian sediments containing Laurentian faunas, together with volcanics. It is considered to have been formed initially by the overthrusting of a volcanic arc onto the Laurentian margin, then subsequently backthrust onto the Baltic continent as a result of the main Laurentia–Baltica collision (the Scandian event) during the Silurian.
Coarse clastic deposits of late Silurian to early Devonian age occur in intra-montane sedimentary basins at several places along the west coast. These formed as a consequence of syn-orogenic and post-orogenic gravitational collapse, which prompted extensional movements on some of the pre-existing thrusts.
Tectonic history
This is summarised in Figure 14.5. Sedimentation commenced on the western margin of Baltica during the late Proterozoic, on a mid-Proterozoic basement of gneisses and granites, with a sequence of continental and shallow-marine sediments, including glacial deposits of the Varangian Glaciation at c.668Ma.
Figure 14.5 Tectonic evolution of the Scandinavian Caledonides: interpretative cartoon profiles. UMA, Uppermost Allochthon; U, Upper Allochthon; M, Middle Allochthon; L, Lower Allochthon. 1, Finnmarkian event: collision of microcontinent and arc terrane in the northern sector of the Baltica margin. 2, Trondheim event: collision of volcanic arc terrane in the central sector. 3, Taconian event: collision of volcanic arc terrane with Laurentia. 4, Scandian event: collision of Laurentia and Baltica and overthrusting of Laurentian Uppermost Allochthon onto Baltica. 5, Devonian extensional faulting and creation of sedimentary basins in half-graben. After Roberts, 2003.
For most of the Cambrian Period, the western margin of Baltica was a stable platform covered by shallow-marine deposits, typically of black shales and carbonates. During the Cambrian, at least two volcanic island arcs formed outboard of the Baltic margin, within the Iapetus Ocean, above a west-dipping subduction zone, and accretionary prisms developed within their foredeep basins. In the late Cambrian, one of these arcs collided with the Scandinavian margin during the Finnmarkian event (Fig. 14.5.1).
In the earliest Ordovician, black shale deposition continued on the Baltic shelf; however, a widespread unconformity marked an episode of uplift at about 478Ma, during the Trondheim event, which is ascribed to the collision of a second volcanic arc with the Baltic margin (Fig. 14.5.2). Differences of at least 20Ma in the age of the metamorphism associated with these events suggest that two different arc collisions may have occurred, the earlier one in the north, and the later in the central sector of the orogen.
A very similar event occurred rather later, between 470 and 465Ma, in the mid-Ordovician, although in this case the arc was obducted onto the Laurentian margin and is correlated with the widespread Taconic event in North America (Fig. 14.5.3). In the later Ordovician, gradual mixing of the Baltic faunas with Laurentian species indicates the approach of the two continents.
By the end of the Ordovician, the closure of the Iapetus Ocean separating the two continents was signalled by the emplacement of the Laurentian basement of the Uppermost Allochthon onto the Baltic margin (Fig. 14.5.4). The early Silurian limestones of the foreland are overlain by shales and turbiditic greywackes that are interpreted as the products of a foreland basin created by the depression of the Baltic margin by the Upper Allochthon. By the mid-Silurian, the nappes of the Upper Allochthon had been transported eastwards beneath the Uppermost Allochthon and were subjected to metamorphism, while greywacke deposition continued in the foreland basin. These deposits were succeeded by non-marine clastic sediments as the nappe complex was uplifted and eroded. Translation of the Upper and Middle Allochthons across the foreland basin continued into the late Silurian, incorporating elements of the basin into the complex. The nappes of the Lower Allochthon were formed in advance of the Middle Allochthon, and involved units of the foreland platform cover.
Movements in the nappe complex continued into the early Devonian. During this period, the Atlantic coastal zone was uplifted into a basement antiform due to isostatic rebound of the crust in response to the removal of the overlying nappes. This removal was due partly to erosion, but was mainly a response to an extensional reversal of the thrust-sense shear zones causing gravitational sliding of the nappes away from the tectonically thickened central zone of the orogeny (Fig. 14.5.5). The uplift revealed basement rocks that had experienced high-temperature and high-pressure metamorphism caused by their having been underthrust beneath the Laurentian continental margin during the Baltica–Laurentia collision.
The present-day height of the mountain range, together with a crustal thickness of 40–45km, indicate that the orogenic belt has been reduced in scale by perhaps 50% compared with modern examples such as the Alps. Nevertheless, the Scandinavian Caledonides are still an impressive crustal structure despite the 400Ma or so of uplift and erosion that have affected them since the Devonian.
The Caledonides of the British Isles and Newfoundland
The Caledonian orogenic belt, or Caledonides, extends southwards from Norway and East Greenland, through the British Isles to Newfoundland and eastern North America (Fig. 14.6) where it merges with the Appalachian Belt. The original extent of the belt only becomes obvious when the effects of the opening of the Atlantic Ocean in the Cenozoic Era have been restored. Figure 14.3A showed a reconstruction of the relative positions of the continents during the mid-Palaeozoic, from which the Caledonian–Appalachian orogenic belt (shown in blue) is seen to represent a collisional orogen between Laurentia (North America plus Greenland), Baltica (northern mainland Europe) and two (possibly joined) microplates, East and West Avalonia. According to this reconstruction, the northern part of the British–Irish sector has resulted from collision between Baltica and Laurentia, whereas the southern part, including England and Wales, is ascribed to collision with the Avalonia microplate that has migrated from Gondwana.
Regional context
Figure 14.6 is an interpretation of how the zones of the British–Irish sector of the Caledonides might link up with Norway and Greenland to the north and Newfoundland to the south. The Northern Highland and Grampian Highland zones of Scotland are part of a regional metamorphic core complex, represented also in Ireland, Newfoundland and East Greenland, which has been thrust to the northwest onto the Laurentian continent. In Newfoundland, Ireland and Shetland, this zone is overthrust on its southeastern side by Ordovician arc terranes.
Figure 14.6 The Caledonide belts of the North Atlantic Region. Reconstruction after restoring the effects of Cenozoic Atlantic opening. Note the major NW-directed thrusts north of the suture and SE-directed thrusts south of the suture. ABFB, Anglo-Brabant Fold Belt; Dk, Denmark; GGF, Great Glen Fault; GH, Grampian Highlands; HBF, Highland Boundary Fault; HPFB, Heligoland-Pomerania Fold Belt; IS, Iapetus Suture; LD, Longford Down Massif; LkD, Lake District; LM, Leinster Massif; MP, Midlands Platform; MV, Midland Valley; Ne, Netherlands; NH, Northern Highlands; MT, Moine Thrust; OH, Outer Hebrides; Sd, Shetland; SU, Southern Uplands; WB, Welsh Basin; U, Unst ophiolite. Note orientation of North. After Leslie et al., 2008; and Dewey & Shackleton, 1984.
South of these zones are southeast-directed units, which include Ordovician accretionary prisms in Newfoundland, Scotland and Ireland, and volcanic arc terranes on the northern margin of Avalonia. Between Avalonia and Baltica, there are two further branches of the Caledonides: the Anglo-Brabant Fold Belt, which defines the eastern margin of Avalonia, and the Heligoland–Pomerania Fold Belt, which follows the western margin of Baltica. These two belts, which are separated by a narrow stable block, are not well known, being almost completely obscured by younger cover.
An important role is played by major strike-slip faults – the Great Glen, Highland Boundary and Southern Uplands faults being only the more obvious. The total sinistral strike-slip displacement on these is unknown, but estimates have ranged from a few hundred to over 1000 kilometres. Consequently, none of the terranes within the Caledonian Belt can be directly linked to its neighbour, which makes interpretation difficult.
The strike-slip faulting is a result of the late Silurian to early Devonian collision between Laurentia and Avalonia, which must have been oblique to the plate boundary such that the convergence direction was partitioned into components at right angles to, and parallel to, the boundary. The earlier (mid-Silurian) collision between Laurentia and Baltica seems to have been more nearly at right angles to the Baltica plate margin. The effects of this Scandian event on the British Isles south of the Northern Highlands are not obvious. This may be the result of the movements on the Great Glen Fault, which have juxtaposed terranes that were previously far to the southwest, away from the influence of the Scandian collision.
This sector of the Caledonides occupies the key position at the centre of the Caledonide Belt, where the three separate branches meet, and provides the most complete cross-section through the belt. It has also benefited from intensive study by several generations of geologists since the late nineteenth century.
The belt here is about 300km wide and can be divided into nine tectonic zones separated by major fault boundaries (Fig. 14.6). These are, from northwest to southeast: the Northwest Foreland (i.e. Laurentia), the Moine Thrust Belt, the Northern Highlands, the Grampian (or Central) Highlands, the Midland Valley, the Southern Uplands, the Lake District, the Welsh Basin and the Midlands Platform, or Southeast Foreland, part of the East Avalonia microplate. The Northern and Grampian Highlands zones together belong to the central metamorphic core of the orogenic belt. The zones were established in Scotland and England; however, the Northern Highlands, Grampian Highlands, Southern Uplands and Lake District zones can also be traced across into Ireland, though with considerable differences in detail.
The Northwest Foreland
This zone consists of a mainly gneissose basement of early Proterozoic age (the Lewisian Complex) comparable with the formerly adjacent early Proterozoic belt in East Greenland at the southeastern margin of the Laurentian continent. This basement is unconformably overlain by continental red-bed sequences of late Proterozoic age, and by a Cambrian to early Ordovician shallow-marine shelf sequence (Fig. 14.7.1–2).
The Moine Thrust Belt
Here the foreland sequence is involved in several major thrust packages, each of which is divided internally by smaller thrust slices. These are overlain by an allochthonous nappe consisting of the Moine Complex of the Northern Highlands, resting on the Moine Thrust (Fig. 14.8A). Elongation lineations indicate a WNW-directed shear sense. The present outcrop width of the zone is very narrow – from less than a metre to a maximum of only 10km. However, the zone must originally have extended for a considerable distance both eastwards beneath the Moine thrust, and also westwards to incorporate the Caledonian thrusting in the basement of the Outer Hebrides, where the platform cover has been removed by erosion.
The thrusts seem mainly to have propagated forwards, towards the foreland, such that the youngest thrusts, involving only Cambrian sediments, carry older Lewisian basement on their roofs, with the oldest, the Moine Thrust, overlying the whole package. The Moine Thrust differs from the younger thrusts in being characterised by a thick band of mylonite, indicating derivation from considerable depth. The thrust movements are attributed to the mid-Silurian Scandian orogenic phase.
The Moine Thrust Belt is well exposed in the Assynt district of NW Scotland, which lies in the North West Highlands Geopark, and is known internationally as the area where Ben Peach, John Horne and their colleagues of the British Geological Survey (Peach et al., 1907) first mapped and explained the complex thrust geometry in the latter part of the nineteenth century. Much of the geology can be readily appreciated in the scenery from the roadside, and some of the key exposures are explained at viewpoints and at a visitor centre at Knockan Crag, where a section through the Moine Thrust is exposed.
The Northern Highlands
This zone contains the Moine Supergroup, which consists of a thick sequence of late Proterozoic marine clastic sediments. They rest on a basement of Lewisian gneisses, and are overlain in the east by post-orogenic Devonian cover (the Old Red Sandstone). The Moine Supergroup has been intensely deformed and metamorphosed, and has experienced three separate orogenic events. The earliest of these, the Knoydartian, took place around 800Ma and is represented by granitic intrusions, pegmatites and metamorphic ages. However, the nature and extent of the Knoydartian structures have been obscured by the younger orogenic phases. The second phase, the Grampian orogeny, is the main tectonic event to affect the Grampian zone to the south, but its effects in the Northern Highlands are less obvious. The Scandian orogeny, attributed to the Silurian collision between Baltica and Laurentia, is the most important tectono-thermal event to affect the northern part of the Northern Highlands. Scandian structures include ductile thrusts and recumbent folds that are overthrust towards the west-northwest (Fig. 14.7.3). These structures were re-folded by more upright folds with a NE–SW to NNE–SSW trend. Folding was accompanied by high-grade metamorphism dated at 435–420Ma (mid- to late Silurian); these dates also correspond to the date of the movements on the Moine Thrust.
Figure 14.7 Tectonic evolution of the British–Irish Caledonides: cartoon profiles. Note that these are composite profiles as no direct correlation is possible because of the large displacements on the major strike-slip faults. GGF, Great Glen Fault; HBF, Highland Boundary Fault; LDA, Lake District Arc; MTB, Moine Thrust Belt; MVA, Midland Valley Arc; SUF, Southern Uplands Fault.
The Grampian (Central) Highlands
This zone contains a thick marine clastic sequence, the Dalradian Supergroup, thought to have been laid down on the extended passive margin of Laurentia in several half-graben (Fig. 14.7.1). The base of the sequence is of late Proterozoic age; towards the top, greywacke deposits and mafic lavas culminate in a Lower Cambrian limestone. The upper part of the sequence is therefore the deeper-water equivalent of the shelf deposits of the foreland. The basement in the west is composed of early Proterozoic gneisses, but in the northeast, the Dalradian rocks lie on gneisses that are correlated with the Moine Supergroup.
The earliest deformation to affect the Dalradian rocks consists of NW-directed ductile thrusts and recumbent folds; these are refolded by overfolds that are also NW-directed in the northwest part of the zone, but in the southeast are flat-lying and appear to be overthrust towards the southeast (Fig. 14.8B). Later folds are more upright with a NE–SW trend. The main regional metamorphism, dated at c.475–460Ma, accompanied the earlier folding and is attributed to the mid-Ordovician Grampian event. The later folds were formed during retrogressive metamorphism.
Figure 14.8 Caledonian structures: (A) in the Moine Thrust Belt, and (B) in the Grampian Highlands. A. Note the imbricate structure of the lower middle nappe, consisting of thin slices of Cambrian strata sliding on a weak layer of shales. The higher nappes were emplaced first and deformed as the lower nappes progressed forwards. B. This cross-section through the central part of the Grampian Highlands shows the earliest recumbent folds (F1) refolded by younger overfolds (F2) and finally by later upright folds (F3). A, after Elliott & Johnson, 1980; B, after Treagus, 2000.
The Grampian event is attributed to a collision between Laurentia and an oceanic arc terrane represented, in part, by the Midland Valley Zone. The presence of a thick ophiolite sequence in eastern Shetland, which includes a substantial upper mantle component, has prompted the suggestion that the Dalradian was overlain by a large ophiolite nappe, now removed by erosion, emplaced prior to the collision (Fig. 14.7.2). The presence of extensive ophiolite sequences in Newfoundland and Norway is evidence that oceanic arc terranes of this type were regionally important (see Fig. 14.6). The later folds in the southeastern part of the zone are attributed to the collision with Avalonia in the Silurian.
A series of syn-orogenic granite plutons intruded in the period 470–460Ma are attributed to crustal melting in the thickened orogen during the Grampian event. However, the many large post-orogenic granite plutons, such as the famous Glencoe and Ben Nevis complexes, are linked with a later (Silurian) episode of subduction (Fig. 14.7.5), discussed below.
The Midland Valley
Lower Palaeozoic rocks are only exposed in a few small inliers near the northern and southern margins of the Midland Valley, the Caledonian history of which is mostly concealed by Upper Palaeozoic cover. The oldest rocks in the inliers are of lowermost Ordovician age and consist of an ophiolite assemblage succeeded by Ordovician to mid-Silurian sediments containing volcanic clasts ranging up to boulder size. The sediments are unmetamorphosed and gently folded. The Midland Valley is interpreted as a Lower Palaeozoic oceanic volcanic arc terrane (Fig. 14.7.3), better represented in Norway. Fossil assemblages both here and in the Southern Uplands to the south have Laurentian rather than Gondwanan affinities.
Geophysical evidence indicates that the Ordovician cover lies on a crystalline basement similar in properties to that underlying the Dalradian to the north, and in Ireland, ophiolites are thrust over gneissose basement, which is likely therefore to belong to the stretched margin of Laurentia. These ophiolites, together with the more extensive oceanic assemblages in Newfoundland, East Greenland and Norway that have been thrust over the Laurentian and Baltic forelands respectively, are thought to represent former back-arc basins.
Major sinistral faults
Both the Great Glen and Highland Boundary Faults have experienced large lateral displacements, since the rocks on each side do not match up. Estimates of the amount of displacement on the Great Glen Fault vary from c.160km to over 500km, but the presence of Moine rocks with similar dates on both sides of the fault suggests that it is probably not a major terrane boundary. However, the sequences presently juxtaposed across the Highland Boundary Fault cannot be directly matched, indicating that this fault is a terrane boundary representing a suture zone between Laurentia and a separate Midland Valley Terrane. Minor structures associated with the Highland Boundary Fault zone have been attributed to sinistral transpression. Both these major faults, together with the Southern Uplands Fault, are attributed to the late Silurian to early Devonian collision between Laurentia and Avalonia.
The Southern Uplands
This zone consists of several fault-bounded packages of steeply dipping Ordovician to Silurian strata bounded in the north by the Southern Uplands Fault, which has experienced at least 10km of sinistral strike-slip displacement. The beds in each individual package become younger northwards, although the more southeasterly packages contain younger material. The individual successions in the north contain at their base early Ordovician basalts and cherts overlain by black shales, succeeded by sedimentary sequences dominated by greywacke turbidites. The steeply inclined strata trend uniformly NE–SW and are affected by asymmetric upright folds. The shales possess a slaty cleavage.
The set of faults that occur throughout the belt are considered to represent steepened thrusts that have been re-activated in a strike-slip sense. The total sinistral offset across the zone is probably considerable and took place during the closing stages of the Caledonian orogeny along with the other strike-slip faults further north.
The Southern Uplands zone is bounded on its southeastern side by the Iapetus suture, which is not exposed, but has been seismically imaged as a major discontinuity inclined at a moderate angle northwestwards, and lies at a depth of around 12km in the central part of the zone. The basement beneath the suture is interpreted as part of the Avalonian Terrane. This zone is represented in Ireland by the Longford Down massif. The Southern Uplands has long been regarded as an accretionary prism, formed above a NW-dipping subduction zone at the northwestern margin of Avalonia (Fig. 14.7.5).
The Lake District zone
The Caledonian rocks of the English Lake District consist of a sequence of Ordovician arc-type volcanics, succeeded by Silurian marine deposits; these rest on a late Precambrian basement, which is exposed in Anglesey and NW Wales. The southwestern continuation of this zone in Ireland is represented by the Leinster massif. The zone is interpreted as an Ordovician volcanic arc situated at the northern margin of Avalonia. These Lower Palaeozoic rocks were deformed and subjected to slate-grade metamorphism in the early Devonian, at the same time as those of the Welsh Basin, and reflect the final collision of Laurentia and Avalonia.
The Welsh Basin
The Lower Palaeozoic rocks of Wales have been intensively studied by generations of geologists and were regarded as an example of a ‘eugeosyncline’ in the 1930s. They comprise around 10km of Cambrian to Silurian sediments, including a large proportion of turbidites. Volcanics are an important constituent, especially in the northwest, in Snowdonia. The rocks of the zone were deformed in the early Devonian; tight folds with associated slaty cleavage in the north give way to more gentle folds in the southeast. The zone has been interpreted as a back-arc basin situated behind the Lake District arc on thinned Avalonian crust (Fig. 14.7.4–5).
Tectonic history
This is summarised in Figure 14.7. Note that profiles 1–3 relate to the northern part of the British Isles (i.e. the Laurentia–Baltica relationship) and profiles 4 and 5 to the southern (the Laurentia–Avalonia relationship).
1Late Proterozoic–mid–Ordovician. Deposition of shelf and continental slope sedimentary sequences on the thinned margin of the Laurentian continent.
2Mid-Ordovician. The Grampian event: NW-directed overfolding and thrusting attributed to the emplacement of a volcanic arc terrane over the Grampian Highlands. A similar event occurred in Scandinavia, involving eastwards overthrusting onto the margin of Baltica.
3Mid–late Silurian. The Scandian event: refolding of Grampian and Northern Highlands, and thrusting of the Moine Thrust Belt, attributed to collision with the Midland Valley volcanic arc terrane and the convergence and collision with Baltica.
4Late Silurian–early Devonian. Folding and thrusting in the Southern Uplands, Lake District and Welsh Basin, followed by sinistral strike-slip faulting, attributed to the collision between Laurentia and Avalonia.
Upper Palaeozoic Belts
The three main collisional belts that formed during the Upper Palaeozoic are shown in brown in Figure 14.1: the Appalachian–Ouachita Belt along the eastern and southern margins of North America; the Urals Belt along the eastern side of Baltica; and the Central Asian Belt between Siberia and Kazakhstania. The Northern Appalachians, from Newfoundland to New York, are part of the Caledonides (see Fig. 14.6) and experienced little deformation after the Acadian orogenic event – the Appalachian equivalent of the main Caledonian (late Silurian to early Devonian) orogenic phase. The Ouachita Belt and the Southern Appalachians, together with its counterpart in Northwest Africa, form the western end of a broad Upper Palaeozoic orogenic belt that extends across southern Europe to the western shores of the Black Sea. The European sector of this belt experienced further tectonic activity in the Alpine Orogeny with the collision between the Eurasian and African–Arabian plates (see chapters 4–6).
The Southern Appalachians and the Ouachita Belt
The Southern Appalachian mountain chain extends in a southwesterly direction from the Hudson River in the north to central Alabama in the south, passing through the States of Pennsylvania, Virginia, West Virginia, North Carolina and Georgia – a distance of about 2000km (Fig. 14.9). The mountain belt is narrower in the north but broadens to around 200km wide in the south, where some of the higher peaks are. The highest summit is Mount Mitchell (2037m), in the Black Mountains of North Carolina – part of the Blue Ridge Province. The mountain belt consists of numerous parallel ranges and individual ridges, many of which reflect the geological structure in a direct way, such that individual folds can be easily traced in aerial views.
Tectonic overview
While the Caledonian Orogeny was affecting the Northern Appalachians, the British Isles and Norway, sedimentary deposition continued on the thinned passive margin of the Laurentian continent further south, which was not seriously disturbed until the Alleghenian Orogeny during the Permian, which culminated in the collision between Laurentia and Africa, resulting in the formation of the Southern Appalachian Belt.
The Southern Appalachian Belt consists of four main tectonic zones: the Appalachian Basin, the Valley and Ridge Province, the Blue Ridge Province and the Piedmont Zone (Fig. 14.9).
The Appalachian Basin
This zone is a foreland basin situated on the passive margin of the Laurentian continent, containing a sedimentary succession consisting mainly of Carboniferous strata, which are undeformed in the main part of the basin but gently folded near the eastern margin. These Carboniferous rocks are an important source of oil and gas deposits.
The Valley and Ridge Province
This zone contains strata ranging from Cambrian to early Permian without any appreciable break (i.e. neither the Taconic nor the Acadian orogenies of the Northern Appalachians has affected them). This Palaeozoic sequence has experienced folding and thrusting during the Permian in the Alleghenian Orogeny – the North American equivalent of the Hercynian Orogeny of Europe. The zone has long been used as a type example of a thin-skinned fold-thrust belt.
The Blue Ridge Province
This zone consists of a mid-Proterozoic (Grenville) crystalline basement with a late Proterozoic to early Palaeozoic sedimentary cover. The zone has been thrust westwards over the Valley and Ridge Province for a distance of over 240km along a major low-angle fault.
The Piedmont Province
This zone occupies the eastern foothills of the Appalachian Mountains and the Coastal Plain, where it is obscured by younger cover. It consists of a deformed and metamorphosed pre-Carboniferous complex, intruded by granitic and gabbroic plutons of mid-Carboniferous to early Permian age, and is interpreted as a volcanic arc. The zone includes Lower Palaeozoic greywackes and slates containing a European-type fauna comparable with the Welsh slate belt. The Piedmont Province is interpreted as a group of exotic terranes of probable Gondwanan origin, similar to the Avalonian Terrane of the northern Appalachians, accreted to the Laurentian continent during the Alleghenian orogeny.
Figure 14.9 The Southern Appalachians and the Hercynian belts of NW Africa. Simplified map of the Hercynian tectonic belts on the opposite sides of the Atlantic Ocean, restored to their approximate pre-opening position. AS, Adrar Souttouf Belt; Con, Connecticut; Da, Dakhla; Del, Delaware; Dh, Dhlou; HR, Hudson River; Ma, Maryland; Mas, Massachussets; MM, Mt Mitchell; NH, New Hampshire; NJ, New Jersey; Nouak, Nouakchott; NY, New York; RI, Rhode Island; St LR, St Lawrence River; Ten, Tennessee; Ver, Vermont; W, Washington DC. After Michard et al., 2008 (NW Africa); and US Geological Survey (Appalachians).
The effects of the Hercynian Orogeny are preserved in several places along the western and north-western margins of Africa, as a result of the Gondwana–Laurasia collision, which has resulted in pieces of the eastern rim of the Appalachian orogen having been emplaced onto the margin of the West African Craton. In Western Mauretania and southwestern Morocco, reworked Precambrian basement has been thrust eastwards over the Reguibat Shield, part of the Archaean West African Craton, but Alleghenian deformation is relatively weak. There are three separate belts, from south to north: the Mauretanides in Western Mauretania, and the Adrar Souttouf and Dhlou belts in Southwest Morocco.
Further north, in Northwest Morocco, the Alleghenian orogeny has affected both the Western Meseta and Anti-Atlas regions. In the Western Meseta, the Precambrian basement, already strongly deformed and metamorphosed during the late Precambrian to early Cambrian Pan–African event, has been affected by further deformation and metamorphism of late Carboniferous age and intruded by granitic plutons. This belt, which is the Hercynian counterpart of the Northern Appalachians, is cut off on its southern side by the Alpine Atlas Belt. The Pan-African basement of the Anti-Atlas Belt on the southern side of the Alpine Atlas is also affected by Alleghenian deformation, but to a lesser extent, and is bounded to the south by post-Palaeozoic cover. Figure 14.9 shows that the Hercynian Orogenic Belt extended north-eastwards from the northern end of the Southern Appalachians and crossed to North Africa to include the Atlas region, but left the Northern Appalachians relatively unscathed on its northern side.
The Ouachita Belt
The Ouachita Belt (Fig. 14.10) forms a northwardly convex arc following the Ouachita Mountains of Arkansas and Oklahoma, and a westward continuation of the belt forms the Llano, Marathon and Solitaro Uplifts of Texas. The topography is relatively subdued – the highest mountain in the Ouachitas is only 839m high.
In geological terms, the Ouachita–Marathon Belt is a thin-skinned foreland fold-thrust belt developed on the southern passive margin of the Laurentian continent. The fold belt includes a typical passive-margin sedimentary sequence of Ordovician to Lower Carboniferous age. In the Upper Carboniferous (Mississippian), thick flysch deposits heralded the approach of a continental terrane from the south fringed by a south-directed subduction zone. This terrane is completely obscured by the post-orogenic cover of the coastal plain, but is considered to have finally collided with Laurentia in the late Upper Carboniferous to early Permian.
Figure 14.10 The Ouachita–Marathon Fold Belt. RG, Rio Grande. After Hickman et al., 2009.
The Variscan Orogenic Belt
The Hercynian Orogenic Belt extends eastwards from the Appalachians and NW Africa, as shown in Figure 14.9, to form a wide zone in central and southern Europe, where it is known as the Variscan (see Fig. 14.1). The Variscan Belt begins at the western side of the Iberian Peninsula, and ends at the western margin of Baltica in Poland – a distance of around 2500km. It is cut off on its southern side by the younger Alpine Belt. Exposures of Variscan rocks in Europe are limited to upstanding massifs, such as the Massif Central in France and the Schwarzvald in Germany, showing generally low relief – typically less than 1500m – and surrounded by Mesozoic and Cenozoic sedimentary deposits.
Tectonic overview
The Variscan Orogenic Belt is a complex assemblage of terranes that have accreted to the southern margin of Laurussia (the combined continent of Laurentia plus Baltica) during the Devonian and Carboniferous (see Fig. 14.1). In West and Central Europe, the northern boundary of the orogenic belt, the Variscan Front, is the frontal thrust of a foreland thrust belt known as the Rheno-Hercynian Zone (RHZ), which can be traced from southern Ireland and southwest England across the Ardennes to the Rhenisches Schiefegebirge and the Harz Mountains in Germany (Fig. 14.11). The RHZ has been thrust across the southern passive margin of the Avalonian foreland, which extends beneath it at least to the southern margin of the RHZ and possibly beyond.
To the south of the RHZ is a series of exotic terranes collectively termed the Armorican Terrane Assemblage (ATA). The various units of the ATA are exposed in several Palaeozoic massifs that are now separated by Mesozoic and Cenozoic cover. Although the massifs have been extensively studied, the link between them is somewhat speculative. Figure 14.11 shows the simplest correlation between the terranes, but there are other possible interpretations. Several Gondwana-derived continental terranes have been recognised, of which the best preserved are Armorica and Perunica (otherwise known as Bohemia). Other pieces of Gondwanan continental crust, together with several volcanic arc terranes, have been incorporated within the complex accretionary regions to the north and south of the two major continental blocks.
Figure 14.11 The Variscan Belt of Europe. Variscan Massifs: AM, Armorican; Ar, Ardennes; BM, Bohemian; CM, Cornubian; H, Harz Mountains; MC, Massif Central; Rsg, Rhenisches Schiefergebirge; SW, Schwarzwald; V, Vosges. Variscan Terranes: As, Asturias; Ga, Galician; MZ, Mauges; SAZ, South Armorican; SF, Southern Foreland (Gondwana); SPZ, South Portuguese; TB, Tepla-Barrandian. Py, Pyrenees; RS, Rheic Suture. Iberia is rotated to its approximate pre-Mesozoic position. After Ballèvre et al., 2009; and Martinez-Catalan et al., 2009.
Armorica and Perunica
The name ‘Armorica’ was traditionally attached to the Armorican Massif of Northwest France, but its use has been expanded to mean the large continental terrane underlying much of Western Europe, including the Iberian Peninsula and most of France. This terrane consists of a late Proterozoic to early Cambrian ‘Pan-African’ basement (known in Europe as the ‘Cadomian’) with a Lower Palaeozoic sedimentary cover originating in northern Gondwana. Its Cambrian to Devonian faunas are of Gondwanan affinities, and the terrane is believed to have been detached from Gondwana at the end of the Silurian when the Palaeo-Tethys Ocean opened (see Fig. 14.3B).
The other main crustal addition to Europe at this time, Perunica, includes the central part of the Bohemian Massif in the present Czech Republic, and is known there as the Teplá-Barrandian Zone (Fig. 14.11). This zone also contains a Cadomian basement, and is considered to have split from Gondwana and travelled across the Rheic Ocean separately during the Devonian (see Fig. 14.3B). Late Silurian faunas in the massif, like those of Armorica, have Gondwanan affinities and differ from those of Northern Europe.
The accretionary suture zones
The two continental terranes, Perunica and Armorica, are bounded on their northern and southern sides by complex zones containing both continental and oceanic material, and which contain the sutures where the intervening ocean basins have been consumed. The suture zone to the north is known as the Léon Zone in the west and the Saxothuringian Zone in the east. The southern suture zone is known as the Mauges Zone in France and the Moldanubian Zone in the Bohemian Massif.
South and west of the Mauges Zone lies the South Armorican Zone, which has been interpreted as part of the Gondwanan foreland. It is here that the first contact with the main Gondwana continent probably occurred at the end of the Carboniferous (see Fig. 14.1). Each of these zones can be traced into the northwestern part of the Iberian Massif, where they curve through more than 90° in a structure known as the Iberian Arc.
A number of Variscan terranes also occur within the Alpine Orogenic Belt in southern Europe, and along the southern margin of Baltica north of the Alpine Front. These terranes include sections of the Laurussian continental foreland, some of which have been involved in the Alpine Orogeny, and independent terranes that have been added to the Laurussian plate during the Mesozoic.
Tectonic evolution of the Variscides
Several continental terranes including Armorica, Teplá-Barrandia and Central Iberia separated from Gondwana during the Lower Palaeozoic, this separation probably promoted by back-arc spreading from a circum-Gondwana subduction zone.
During the Silurian, a volcanic arc formed within the Rheic Ocean caused by a north-dipping subduction zone, which resulted in pulling the ATA terranes away from Gondwana towards Laurussia. In the early Devonian, the Rheno-Hercynian back-arc basin formed above this subduction zone and eventually spread onto the Avalonian passive margin.
Continued expansion through the Devonian of the Palaeo-Tethys Ocean between the ATA and Gondwana resulted in the closure of the Rheic ocean basin and the subsequent amalgamation of the northern terranes of the ATA. The convergence direction of the combined terranes was oblique to the Tornquist Zone at the margin of Baltica, resulting in a combination of clockwise rotation and dextral shear in the marginal parts of the ATA. Continued expansion of the Palaeo-Tethys Ocean into the early Carboniferous caused the development of a subduction zone at the southern margin of Armorica and Teplá-Barrandia, leading to the development of the Mauges–Moldanubian accretionary complex.
During the mid-Carboniferous there was a change in convergence direction between Gondwana and Laurussia causing Gondwana to rotate anti-clockwise such that North Africa now faced North America, and the final collision took place between the NW African part of Gondwana and the section of Laurussia from the Appalachians to Iberia (see Fig. 14.1). Tectonic activity in more easterly sectors of the Central European Variscides was revived during the Alpine Orogeny (see chapters 4–6).
The Urals and Central Asia
Two further orogenic belts of Hercynian age complete the Palaeozoic assemblage that stitched Pangaea together in the Permian: the Uralides and the Central Asian Belt. These belts are the result of the accretion of volcanic arcs, oceanic terranes and micro-continents to the passive margins of Siberia in the north, Baltica in the west, and the Tarim and North China continental blocks in the south over a long period of time, commencing in the late Proterozoic and ending with final collision in the late Carboniferous through to the early Mesozoic.
During the early Palaeozoic, the Aegir and Turkestan Oceans separated Siberia both from Baltica and from a number of micro-continental blocks: Tarim, North China (Sino-Korea) and several other terranes, which collectively formed Kazakhstania (Fig. 14.3B). The (present) eastern border of Baltica and the southern and western borders of Siberia seem to have behaved as passive margins until the Carboniferous Period and accumulated platform sediments up until that point. However, within the Aegir and Turkestan Oceans, volcanic island arcs developed, fringed by subduction zones, and these ultimately became caught up in the late Carboniferous to early Mesozoic collision between the larger continents.
Figure 14.12 is a simplified map showing the complex assemblage of terranes making up a roughly triangular area sandwiched between Baltica, Siberia, and the Tarim and North China continental blocks. The western part of this assemblage, forming the separate continent of Kazakhstania, was completed in the late Silurian, and consists of Lower Palaeozoic subduction-accretion complexes juxtaposed with elongate bands and lensoid masses of continental crust of Gondwanan derivation. The Tarim and North China Blocks, which joined the assemblage in the Permian and Triassic respectively, are likewise founded on Gondwanan Precambrian basement.
This assemblage is bounded in the east by a large volcanic arc terrane that extends for over 1500km from the eastern margin of the Uralides to the Junggar Basin in Northwest China. The eastern side of this terrane is defined by the Chara Suture, which contains ophiolites and tectonic mélanges, and marks the eastern boundary of Kazakhstania, separating it from the terranes that have accreted to Siberia. The southern part of the Chara Suture and its continuation through Mongolia, known as the Main Mongolian Lineament, runs through the Altai Mountain Range. The southern margin of Kazakhstania lies on the southern side of the Tian Shan Range, and is defined by another suture separating it from the Tarim Block to the south.
Figure 14.12 The Central Asian Orogenic Belt. CS, Chara Suture; KOK, Kokchetav Block; MML, Main Mongolian Lineament; SG, South Gobi Block; TM, Tuva-Mongol Massif. Geographic names: AM, Altai Mountains; Ch, China; Mon, Mongolia; SR, Stanovoy Range; Rus, Russia; TS, Tian Shan (Mts); YR, Yablonovy Range. After Windley et al., 2007.
The Uralides
The Ural Mountains form the eastern boundary of the European sub-continent and extend for a total distance of over 3,200km from the City of Orenburg near the Kazakhstan border to the northern end of the Novaya Zemlya islands in the Arctic Ocean. The mountain range is relatively narrow over most of its length but broadens to around 300km in width around Magnitogorsk, where the highest peaks are situated – the highest being Gora Yamantau at 1638m. Formed during the late Palaeozoic to early Mesozoic Uralian Orogeny, the Ural Range is the only component of the Hercynian orogenic system to offer a complete section across a collisional orogenic belt between two major continents, and in that sense is comparable with the more recent Himalayan belt.
Plate-tectonic context
The origins of the Uralian belt lie in a sequence of plate-tectonic movements that took place through the Silurian and Devonian Periods. Figure 14.3B shows a wide ocean, known as the Aegir Ocean, separating Baltica (then part of Laurussia) from the Siberian continent, with several small terranes between them. At that time, and throughout the early Carboniferous, the (present) eastern border of Baltica was a passive margin overlain by a wide shallow-marine shelf. By the late Silurian, several of the small terranes had combined to form the continent of Kazakhstania, which, in the mid-Carboniferous, collided with the eastern margin of Baltica, followed in the Permian Period by collision with Siberia to the north (see Fig. 14.1).
Main tectonic features of the Uralian Belt
The Uralian Orogenic Belt (Figure 14.13) follows the Ural Mountains that divide European Russia from Siberia. The orogenic belt divides into two branches in the north, separated by the Pechora Basin, the western branch following the late Proterozoic Timan Belt. Further south, the belt varies from about 200km to 600km in width, bounded in the west by the Uralian Foredeep Basin and in the east by the West Siberian Basin. At its southern end, the belt disappears beneath post-orogenic cover of the Peri-Caspian Basin.
The southern part of the Uralian belt can be divided into six separate tectonic zones, from west to east: the Uralian Foredeep, West Uralian, Central Uralian, Magnitogorsk, East Uralian and Trans-Uralian Zones.
The Uralian Foredeep Basin
This developed during the Carboniferous, and occupies the passive margin of Laurussia (i.e. originally Baltica). It grades westwards into a shallow-marine, mostly carbonate, shelf and is bounded in the east by the main boundary thrust of the Uralian Orogenic Belt. It contains continental clastic molasse deposits derived from the rising Ural Mountains. To the south, it is replaced by the shallow-marine deposits of the Peri-Caspian Basin, and to the north it divides into two branches, respectively following the western and eastern branches of the orogenic belt.
The West Uralian Zone
This is a typical foreland fold-thrust belt, and involves basement and sedimentary cover belonging to the East European Craton. The principal structure in the southern part of the orogen, illustrated in Figure 14.13B, is the large Bashkirian Anticlinorium, which exposes Precambrian Laurussian basement in its core. The thrusts in this zone dip uniformly eastwards, and end on a gently inclined mid-crustal detachment thrust which eventually meets the Main Uralian Fault (or Thrust) and thereby descends to the base of the crust at a depth of around 55km.
The Central Uralian Zone
This relatively narrow zone consists of folded and thrust high-grade metamorphic rocks (presumed to be part of the Laurussian basement) with a sedimentary cover of Carboniferous flysch deposits. The eastern part of the zone is occupied by a band of basic and ultrabasic ophiolites. The zone is bounded in the east by the Main Uralian Thrust, which forms the eastern margin of Laurussia, the presence of the ophiolites indicating that it marks a suture zone separating the Laurussian Plate on its western side from various exotic Asian terranes on its eastern side. The thrust descends to the base of the crust at a depth of about 55km, and traces of its continuation through the upper mantle are believed to mark the former position of the east-dipping subduction zone that descended beneath the Magnitogorsk terrane to the east (Fig. 14.13B).
Figure 14.13 The Uralian Orogenic Belt. Simplified map (A) and cross-section (B) showing the main tectonic features. BA, Bashkirian anticlinorium; FS, fossil subduction zone?; FT, Frontal Thrust; G, granite intrusion; K?, Kazakhstania basement?; MUF, Main Uralian Fault; MZ, Magnitogorsk Zone; PKP, Pay-Khoi Peninsula; TF, Trans-Uralian Fault; TskF, Toisks Fault. A, after Juhlin et al., 1998; B, after Berzin et al., 1996.
The Magnitogorsk Zone
This zone represents an oceanic volcanic arc complex, folded in the west and cut by west-dipping thrusts, in contrast to the east-dipping structures west of the suture. The zone is bounded on its eastern side by the East Uralian Zone; the nature of the boundary itself is uncertain but presumed to be a thrust. A major strike-slip fault, the Toistsk Fault, outcrops close to the boundary. The city of Magnitogorsk hosts a steelworks that was once the largest in Europe and was based on iron ore from Magnitnaya, the local ‘magnetic mountain’.
The East Uralian Zone
This zone consists of metamorphic basement rocks belonging to a small continental terrane and is intruded by numerous granitic bodies of typical magmatic arc type. It is bounded in the east by a major west-dipping thrust, the Trans-Uralian Fault, which separates it from the Trans-Uralian Zone.
The Trans-Uralian Zone
This zone consists of a Palaeozoic volcanic arc complex. It is regarded as a separate arc terrane accreted to the East Uralian zone along a west-dipping subduction zone that is marked by sporadic ophiolite bodies. This zone is only recognised in the south of the Urals belt; its northerly continuation is obscured beneath the West Siberian Basin. The presence of a strong west-dipping seismic reflector beneath the Trans-Uralian Zone probably marks the top of an underthrust continental terrane (part of Kazakhstania).
Tectonic evolution
1During the Silurian Period, the wide Aegir Ocean separated Baltica (Laurussia) from the nearest continental plate, Siberia. Within this ocean were several continental terranes and oceanic volcanic arcs, some of which, by the end of the Silurian, had amalgamated to form the continent of Kazakhstania. Between Kazakhstania and Laurussia were the Magnitogorsk volcanic arc and another continental terrane, which became the East Uralian Zone.
2In the late Devonian, the Magnitogorsk Arc collided with the (present) eastern passive margin of Laurussia, and was thrust over it. A west-dipping subduction zone then formed beneath the Magnitogorsk Arc, bringing the East Uralian Terrane closer to the developing orogen. Another oceanic volcanic arc, which would become the Trans-Uralian Zone, formed above a second west-dipping subduction zone between the East Uralian Terrane and Kazakhstania.
3In the early Carboniferous, the East Uralian Terrane underthrust the Magnitogorsk Zone, causing deformation and uplift of the orogenic belt to commence. Kazakhstania continued to approach Laurussia as the intervening oceanic plate was subducted beneath the Trans-Uralian Arc.
4The mid- to late Carboniferous saw the climax of the Uralian Orogeny in the south; the Trans-Uralian Terrane collided with and underthrust the East Uralian Terrane, which in turn met and was underthrust by the leading edge of the Kazakhstanian assemblage, causing further compressive deformation throughout the orogeny and spreading into the Laurussian foreland to produce the foreland fold-thrust belt. The orogenic belt experienced considerable crustal thickening; even at the present day, the base of the crust beneath the central part of the orogen is still at more than 55km depth, compared to around 40km in the adjacent Laurussian platform.
5Further north, commencing in the Permian and lasting into the early Jurassic, the Siberian continent collided with the northern sector of the Uralian belt, including Novaya Zemlya and the Taimyr Peninsula.
The Central Asian Belt
The Central Asian Orogenic Belt extends from the Ural Mountains in the west to the coast of the Sea of Okhotsk in eastern Siberia, a distance of around 6000km and nearly 80° of longitude. Much of the western part of the orogenic belt, in Central Kazakhstan, is relatively low-lying, but from Longitude 70°E a series of high mountain ranges stretches across eastern Kazakhstan, Mongolia and Northern China in a broad belt around 1000km across. These include the Tian Shan ranges, already discussed in chapter 8, together with the Altai Mountains in Western Mongolia, and the Yablonovyy and Stanovoy ranges along the China–Siberia border. The highest peak in the Altai Range is the 4506m-high Mt. Belukha; the eastern ranges are lower, below 3000m.
Tectonic overview
The terrane assemblage fringing Siberia forms a broad arc around the western side of the Siberian craton to Lake Baikal, then strikes north-eastwards (see Fig. 14.12). The Archaean–early Proterozoic core of Siberia is ringed by a late Proterozoic to early Cambrian orogenic belt known as the Baikalides. Around this margin are wrapped a complex array of terranes, similar to those composing Kazakhstania, consisting of continental blocks, oceanic volcanic arc terranes and subduction–accretion complexes ranging in age from latest Proterozoic to Carboniferous.
On the southeastern side of Siberia, an elongate block of Archaean to late Proterozoic basement, believed to be of Gondwanan origin, together with its platform cover, extends for over 2000km in a northeasterly direction. The northern side of this massif lies along the Yabonovyy and Stanovoy mountain ranges, and the massif is bordered on its south side by Palaeozoic subduction–accretion complexes and another basement terrane, the South Gobi Block, on the south side of which is the Solonker Suture, separating the accretionary belt from the North China Craton, which joined in the late Triassic.
Sinistral movements on several major strike-slip faults, including the Chara Suture and the Main Mongolian Lineament, which are curved around parallel to the margin of the Siberian Craton, are attributed to late Hercynian anti-clockwise rotation of Siberia relative to both Baltica and the southern cratons.
Orogeny in the Precambrian
It is generally believed that plate-tectonic processes essentially similar to those discussed above in relation to the Phanerozoic orogenic belts operated also in the Proterozoic, but there has been considerable debate over whether that is equally true for the Archaean. The consensus now is that, although there may be differences in detail – for example in the rate of heat flow through the crust, the thickness of the lithosphere, and the temperature and composition of magmas – the earliest crust known was produced by the same kinds of subduction–accretion processes that have been discussed for the Phanerozoic.
However, the major difference between investigations of the Phanerozoic belts and those formed during the Precambrian is that there is considerably more uncertainty regarding the sequence of plate-tectonic events responsible for the latter. In order to reconstruct these, it is necessary to restore the successive positions of the major continents before the Pangaea Supercontinent was formed, in the same way that was done in Figures 14.2 and 14.3 to end up with the reconstruction of Figure 14.1.
Most students of Precambrian tectonic history agree that a late Proterozoic supercontinent, known as Rodinia, broke up around the beginning of the Cambrian and that the various pieces rearranged themselves into the Pangaea Supercontinent. However, there is currently no generally agreed solution to the problem of how the various components of Rodinia fitted together. This problem is even greater for the postulated mid-Proterozoic supercontinent known as Nuna, and for the even more speculative end-Archaean supercontinent.
To conclude, therefore, although much is known about certain Precambrian orogenic belts such as the Grenville and Hudsonian belts of North America or the Sveco-Karelian and Sveco-Norwegian belts of Scandinavia, without more information concerning their relationships to their opposing continental hinterlands, it is not yet possible to reconstruct their tectonic history with any degree of certainty.
