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Some aspects of the tectonic evolution of South-West England
Some Aspects of the Tectonic Evolution of South-West England by W. R. DEARMAN Received 6 May 1970
ABSTRACT: The bases are discussed for the recognition of structural zones embracing the southern margin of the Carboniferous synclinorium of south-west England, and additional structural lines are set down on the Tectonic Map. The Tintagel-type structures, in which tergiversate folds are an important element. probably have counterparts in the south-west of Cornwall; they mark a transition zone of intense deformation between the infrastructural and suprastructural zones in the Variscan orogen of south-west England.
accompanying the first Memoir of the Geological Survey De la Beche (1839)clearly depicted the broad synclinal structure of mid-Devon and north Cornwall, with a central outcrop of carbonaceous (Carboniferous) deposits resting in a trough-like depression on rocks of the Grauwacke Group (Devonian) beneath. The latest versions of his summary map (ibid., plate 1) appear in two forms, the one showing the distribution of the formations present in south-west England (Edmonds, McKeown & Williams, 1969, fig. 1) the other (Dunning, 1966) depicting only the major tectonic features. Excluding the outcrops of the five main granite cupolas (Fig. 1 ) trending east-north-east from Land's End to Exeter, the structural pattern of the region is determined by four major elements. The outcrop of Carboniferous rocks is mainly contained within a parallel-sided basin trending east-south-east, the southern margin passing on the east side of the Dartmoor granite into a thrust. Along the south coast the Devonian outcrops are limited by the Lizard-Dodman-Start Thrust. The adjacent sediments are thrown into a series of major anticlines which, with the main basin structure farther north, determine the general east-to-west structural grain of the peninsula. Because of the small scale, the tectonic map (Fig. 1) is oversimplified and this is particularly noticeable along the southern boundary of the Carboniferous outcrop. Published geological maps (Prentice, 1962, fig. 6/1; Edmonds and others, 1969, fig. 1) show alternating linear outcrops of Lower and Upper Carboniferous formations with, on larger scales (Dearman & Butcher, 1959, fig. 14), intercalations also of Upper Devonian strata. Recognition at Tintagel of the Carboniferous age of the ON THE MAPS
483 PROC. GEOL. ASS., VOL. 81, PART 3,1970
31
.e« Basin ~
Thrust p~ Wrench fault . - ' Anticline
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lLIJ Crackington Formation [[[II Crackington form.& L.Carboniferous l:23 LowerCarboniferous E::::3 Devonian ~ Granite
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Fig. 1. Geological map of the southern margin of the Carboniferous synclinorium (based on Edmonds and others, 1969, fio 1\ Ah,we : extract from the Tectonic MaD of Great Britain (Dunning, 1966)
TECTONIC EVOLUTION OF SOUTH·WEST ENGLAND
485
Tintagel Group, comprising the Barras Nose and Trambley Cove Formations with the intercalated volcanics (Edmonds and others, 1969, table 2), now allows a continuous horizon of these rocks to be traced from the Cornish coast to the western side of Dartmoor. This unit, sandwiched between Upper Devonian slates, cuts across the southern margin of the basin structure shown on the tectonic map (Fig. 1), thus invalidating the interpretation and suggesting that the details of the map between Launceston and Tavistock need revising. Now that the mapping along the coast has been completed and published (one-inch Geological Sheet 322, New Series) and the structural pattern along the north-west margin of Dartmoor is known in some detail (Edmonds, Wright, Beer, Hawkes, Williams, Freshney & Fenning, 1968), it is possible to suggest a generalised interpretation of the structure of the southern margin of the Carboniferous outcrop. The important aspects of this interpretation are related first to changes in attitude and shape of folds as the structures are traced southward and westward and secondly to a general shuffling of stratigraphical units which increases in intensity and complexity in the same directions. The Upper Devonian rocks of the area are mainly well-cleaved grey and green slates with occasional thin sandstones and limestones. They are succeeded by Lower Carboniferous silty black slates with a few lenticular limestones, followed by black shales or slates with sandstones and quartzites containing an intercalated group of lavas, tuffs and agglomerates, overlain by black cherts, shales and lenticular masses of limestone. Upper Carboniferous rocks include the Namurian Crackington Formation comprising shales with subordinate thin turbidite sandstones. A very wide outcrop of the Crackington Formation is bordered on the north by thickbedded and massive greenish-brown sandstones with thinner grey shales and greenish-brown siltstones of the overlying Westphalian Bude Formation. Folds in the massive sandstones of the Bude Formation are upright open structures; within the formation an overriding southerly tectonic drive is reflected in the increasing abundance of overfolds associated with upright folds as the southern junction with the Crackington Formation is approached (Figs. I and 2). Within the Crackington Formation belt on the coast (Freshney, 1965)and inland (Edmonds and others, 1968, fig. 10) this tendency increases southward, so that on the coast the folds are overturned to the south and become recumbent as the Lower Carboniferous inliers are approached. At Okehampton, associations of upright and overturned folds just north of the Lower Carboniferous inlier suggest that the overfolds are derived from the upright folds by a later tectonic pulse and it may be that the combinations of visible folds in the coastal structures in both the Bude and Crackington Formations have also been formed in the
486
W. R. DEARMAN
same way by successive phases of deformation (Zwart, 1964; Dearman, 1969a). South of the main outcrop of the Crackington Formation, which is up to ten kilometres wide, there is repetition of all three stratigraphical units involved in the structure of the southern margin of the main Carboniferous outcrop (Fig. 1). Individual outcrops of the Upper Devonian slates, the Lower Carboniferous rocks and the Namurian Crackington Formation are up to four kilometres wide, suggesting that folding, if present, is on a large scale. The Lower Carboniferous inlier south of Okehampton, with a core of presumed Upper Devonian slates with siltstone lenticles, is undoubtedly a complex major anticline which is overturned to the south. As with the coastal structures in Upper Carboniferous rocks, doubt must exist whether the present folds within the main anticline are original features or have developed by deformation of earlier upright folds (Dearman, 1969c,fig. 2). At Okehampton the core of presumed Upper Devonian slates is characterised by a distinctive style of small-scale tight to isoclinal folds; incidentally, the slates appear to be folded into the overfolded pattern characteristic of the Lower and Upper Carboniferous envelope. The Okehampton anticlinal inlier undergoes a significant change in structural style as it is traced to the north Cornish coast (Fig. 2). The main anticline dies out and the whole succession up to the Crackington Formation is folded into small-scale tight to isoclinal folds which at Okehampton were seen as a distinctive structural type only in the presumed Upper Devonian slates. Two outcrops of Lower Carboniferous rocks have slates and sandstones of the Crackington Formation between them and Upper Devonian to the south, the whole appearing to be repetitions of evenly and gently northward inclined groups of strata. To the south of the Okehampton anticline, along the margin of Dartmoor, the two most southerly outcrops of the Crackington Formation also have only small-scale tight to isoclinal folds (Fig. 2). It is likely that the same division into areas with and without large-scale folds also occurs in the east side of the moor where the outcrop of Lower Carboniferous rocks marked on the map borders a southern region dominated by slate-type structures. The impression that, wherever large-scale folds are absent, the major structures comprise uniform sheets of strata, right way up and inclined gently to the north but with a haphazardly shuffled repetition of the three main stratigraphical horizons, is most evident in the coastal exposures north and south of Tintagel. Bedding and slaty cleavage are parallel and sparse hinges of small isoclinal folds are visible only in thin sandstones and in lenses of siltstone and limestone. In a similar setting north of the Bodmin Moor granite where again slaty cleavage is gently and evenly inclined northward, Freshney (1969) has inferred from a borehole record the
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BF BUDe FORMATION CF CRACKINGTON FORM. .::~':i L.CARBONIFEROUS o DEVONIAN Fig. 2. Sketched horizontal sections (a) from Bude to Tintagel, and (b) from Hatherleigh to Okehampton, and along the west side of Dartmoor to Tavistock. Early structures only are shown
488
W. R. DEARMAN
presence of a large-scale isoclinal to tight fold closing to the south which has 250 m. of Upper Devonian green slates as a core enveloped and infolded in the brow region by Carboniferous grey slates. An element of large-scale inversion is implicit in the interpretation of this structure and a similar situation probably exists to the east of Tavistock, where the southernmost outcrop of the Crackington Formation appears to be in mainly inverted beds. Large folds therefore do exist elsewhere than at Okehampton but, in keeping with both a regionally uniform attitude of slaty cleavage and a general parallelism of slaty cleavage with bedding, they must be very much tighter, even isoclinal, structures than those at Okehampton. Overfolds in the Okehampton area are broken by thrusts parallel with the gently inclined normal limbs of the folds; movements were directed up the thrustplanes toward the south-south-east. Although there is no direct evidence, Freshney (1965, 179) has suggested that 'some of the more marked lowangle faults [on the coast], such as the Rusey Fault and another major fault at Trambley Cove, existed at the time of the early recumbent folding and during the formation of the southerly directed overfolds, when they would have moved as thrusts from the north'. It would seem reasonable to extend this hypothesis to include many, if not all, of the junctions between lithological and stratigraphical units within the area under discussion. It is now possible to draw some additional lines on the tectonic map. The first is a line separating the areas of markedly different structural style. This line crosses stratigraphical boundaries. It is more or less coincident with the line drawn by Simpson (1969,fig. 4) marking the boundary between his broad Structural Zone III, which is succeeded to the north by a subzone of Structural Zone II. The present author (Dearman, 1969a, 34, and fig. 3) has drawn the same line in Cornwall separating the following zones: (2) Zone of overturned folds in Namurian strata. The folds, overturned to the south, are sliced up by low-angle faults. Cleavage is prominently developed in the southern part of the zone. (3) Zone of tight to isoclinal folds with slaty cleavage. Upper Devonian (Famennian), Lower Carboniferous (Dinantian) and Upper Carboniferous (Namurian) present in shuffled thrust slices. Low-angle faults in Zone II and thrusts in Zone III demand the inclusion on the map of a whole family of thrust faults, gently inclined to the north, on which there has been a general southerly sense of overriding (Fig. 1). There are, however, complications in the tectonic history of the area, implicit in the use of the terms low-angle fault and thrust which can be used synonymously to describe only the earliest phases of tectonism. The deformation is polyphase and the main tectonic sequence (Fig. 3) may now be summarised:
TECTONIC EVOLUTION OF SOUTH-WEST ENGLAND
489
(a) Generation of early folds; open upright folds in suprastructural region, style changing progressively downward to evenly bedded units with very much smaller-scale isoclines or tight folds and crosscutting thrusts which achieve the same degree of shortening. (b) Continuation of early deformation leading to selective tightening of fold angle in the open upright folds of the suprastructure. Infrastructural response would be mainly in terms of continued movement on early thrusts giving rise to a pronounced version of Breddin's (1966) shingle-structure. (c) Overturning and other deformations of early folds in suprastructure, predominantly but not exclusively to the south, and generation of low-angle thrusts overriding southward. Thrusting and marked flattening of the infrastructure with formation of north-north-westsouth-south-east trending mineral lineation in slates and tergiversation (Dearman, 1969b) of isoclines. Over large areas dominant fold trend is around north-north-west but very variable; folds face southsouth-east, west-south-west and east-north-east. Tectonic drive was distinctly Caledonoid in direction with overriding southward.
Foldsopen upright Incipientclravage SUPRASTRUCTUR~ _
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(d) The great reversal : low-angle normal-faulting northward along existing thrusts and possibly along newly formed fault planes; folded deformation of the slates of the infrastructure and the overfolds in the suprastructure. (e) Rise of the granite batholith and generation of granite-induced structures. All earlier structures offset by steep normal faults and by Tertiary dextral wrench-faults. The anomalous north-north-west-south-south-east trend of the early folds in the Tintagel district can be traced for at least twenty kilometres south to Delabole (Wilson, 1952) from the northern margin of the structural zone. On the west side of Dartmoor the same structures crop out in the infrastructural zone over at least eleven kilometres. It is conceivable that the southern limit should be taken as far south as the confrontation with northerly facing structures at Padstow (Gauss, 1967). This would extend the southern limit of Structural Zone 3 established by Dearman (1969a, 34 and fig. 3), and would warrant the establishment of a subzone along the northern margin of Simpson's Structural Zone 3 (1969, fig. 2). Similar structures occur in south-west Cornwall (Dearman, 1969b, fig. 2). The two regions of tergiversate folds represent a transition zone between the suprastructure and the infrastructure of the Variscan orogen of south-west England. REFERENCFS BREDDIN, H. 1966. Der Schuppenbau der mitteldevonischen Gesteinsfolgen im Gebiet von Lindlar bei Koln (Rheinisches Schiefergebirge). Geol, Mitt., 7, 1--44. DEARMAN, W. R. 1964. Wren ch-faulting in Cornwall and South De von . Proc, Geol, Ass., 74, 265-87. - - - . 1969a. An Outline of the Structural Geology of Cornwall. Proc. geol. Soc. , No. 1654,33-9. - - -. 1969b. Te rgiversate Folds fro m South-West England. Proc, Ussher Soc., 2, 112-15 . - - - . I 969c. On the Association of Upright and Recumbent Folds on the Southern Margin of th e Carboniferous Synclinorium of De vonsh ire and North Cornwall. Proc, Ussher Soc.• 2, 115-21. - - & N. E. BUTCHER. 1959. The Geology of the Devonian and Carboniferous Rocks of the North-West Border of the Dartmoor Granite. Devonshire. Proc. Geol. Ass., 70,51-92. DE LA BECHE. H. T. 1839. Report on the Geology of Cornwall, Devon and West Somerset. Mem. geol. Sur v, U.K. DUNNING. F. W. 1966. Tectonic Map of Great Britain and Northern Ireland. London: Her Majesty's Stationery Office. EDMONDS, E. A., J. E. WRIGHT, K. E. BEER , J. R. HAWKES, M. WILLIAMS, E. C. FRESHNEY & P. J. FENNING. 1968. Geology of the Country around Okehampton . Mem. geol. Surv, U.K. - - - , M. C . McKEOWN & M. WILLIAMS. 1969. South-West England. Br. reg. Geol. Geol. Surv .
TECTONIC EVOLUTION OF SOUTH-WEST ENGLAND
491
FRESHNEY, E. C. 1965. Low-Angle Faulting in the Boscastle Area. Proc, Ussher Soc., 1, 175-80. - - - . 1969. Preliminary Report on the Wilsey Down Borehole, North Cornwall (Abstract). Proc, Ussher Soc., 2, 107-8. GAUSS, G. A. 1967. Structural Aspects of the Padstow Area, North Cornwall. Proc, Ussher Soc., 1,284-5. PRENTICE, J. E. 1962. The Sedimentation History of the Carboniferous in Devon. In Some Aspects of the Variscan Fold Belt, ed. K. Coe, 93-108. Manchester Univ. Press. SIMPSON, S. 1969. Geology, pp. 5-26. In Exeter and its Region. University of Exeter. WILSON, G. 1952. Field Meeting, Whitsun 1951, at Tintagel, North Cornwall. Proc. Geol. Ass., 63, 49-58. ZWART, H. J. 1964. The Development of Successive Structures in the Devonian and Carboniferous of Devon and Cornwall. Geologie Mijnb., 43,516-26.
W. R. Dearman Department of Geology The University Newcastle upon Tyne NEt 7RU
ABSTRACT: The bases are discussed for the recognition of structural zones embracing the southern margin of the Carboniferous synclinorium of south-west England, and additional structural lines are set down on the Tectonic Map. The Tintagel-type structures, in which tergiversate folds are an important element. probably have counterparts in the south-west of Cornwall; they mark a transition zone of intense deformation between the infrastructural and suprastructural zones in the Variscan orogen of south-west England.
accompanying the first Memoir of the Geological Survey De la Beche (1839)clearly depicted the broad synclinal structure of mid-Devon and north Cornwall, with a central outcrop of carbonaceous (Carboniferous) deposits resting in a trough-like depression on rocks of the Grauwacke Group (Devonian) beneath. The latest versions of his summary map (ibid., plate 1) appear in two forms, the one showing the distribution of the formations present in south-west England (Edmonds, McKeown & Williams, 1969, fig. 1) the other (Dunning, 1966) depicting only the major tectonic features. Excluding the outcrops of the five main granite cupolas (Fig. 1 ) trending east-north-east from Land's End to Exeter, the structural pattern of the region is determined by four major elements. The outcrop of Carboniferous rocks is mainly contained within a parallel-sided basin trending east-south-east, the southern margin passing on the east side of the Dartmoor granite into a thrust. Along the south coast the Devonian outcrops are limited by the Lizard-Dodman-Start Thrust. The adjacent sediments are thrown into a series of major anticlines which, with the main basin structure farther north, determine the general east-to-west structural grain of the peninsula. Because of the small scale, the tectonic map (Fig. 1) is oversimplified and this is particularly noticeable along the southern boundary of the Carboniferous outcrop. Published geological maps (Prentice, 1962, fig. 6/1; Edmonds and others, 1969, fig. 1) show alternating linear outcrops of Lower and Upper Carboniferous formations with, on larger scales (Dearman & Butcher, 1959, fig. 14), intercalations also of Upper Devonian strata. Recognition at Tintagel of the Carboniferous age of the ON THE MAPS
483 PROC. GEOL. ASS., VOL. 81, PART 3,1970
31
.e« Basin ~
Thrust p~ Wrench fault . - ' Anticline
(Zl post-Carboniferous
D:J Bude Formation
.j>.
lLIJ Crackington Formation [[[II Crackington form.& L.Carboniferous l:23 LowerCarboniferous E::::3 Devonian ~ Granite
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TECTONIC ZONES
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lIa SLICED OVERTURNED FOlDS ilia
III
THRUST SLICES RECUMBENT FOLDS r
I
30km J
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Fig. 1. Geological map of the southern margin of the Carboniferous synclinorium (based on Edmonds and others, 1969, fio 1\ Ah,we : extract from the Tectonic MaD of Great Britain (Dunning, 1966)
TECTONIC EVOLUTION OF SOUTH·WEST ENGLAND
485
Tintagel Group, comprising the Barras Nose and Trambley Cove Formations with the intercalated volcanics (Edmonds and others, 1969, table 2), now allows a continuous horizon of these rocks to be traced from the Cornish coast to the western side of Dartmoor. This unit, sandwiched between Upper Devonian slates, cuts across the southern margin of the basin structure shown on the tectonic map (Fig. 1), thus invalidating the interpretation and suggesting that the details of the map between Launceston and Tavistock need revising. Now that the mapping along the coast has been completed and published (one-inch Geological Sheet 322, New Series) and the structural pattern along the north-west margin of Dartmoor is known in some detail (Edmonds, Wright, Beer, Hawkes, Williams, Freshney & Fenning, 1968), it is possible to suggest a generalised interpretation of the structure of the southern margin of the Carboniferous outcrop. The important aspects of this interpretation are related first to changes in attitude and shape of folds as the structures are traced southward and westward and secondly to a general shuffling of stratigraphical units which increases in intensity and complexity in the same directions. The Upper Devonian rocks of the area are mainly well-cleaved grey and green slates with occasional thin sandstones and limestones. They are succeeded by Lower Carboniferous silty black slates with a few lenticular limestones, followed by black shales or slates with sandstones and quartzites containing an intercalated group of lavas, tuffs and agglomerates, overlain by black cherts, shales and lenticular masses of limestone. Upper Carboniferous rocks include the Namurian Crackington Formation comprising shales with subordinate thin turbidite sandstones. A very wide outcrop of the Crackington Formation is bordered on the north by thickbedded and massive greenish-brown sandstones with thinner grey shales and greenish-brown siltstones of the overlying Westphalian Bude Formation. Folds in the massive sandstones of the Bude Formation are upright open structures; within the formation an overriding southerly tectonic drive is reflected in the increasing abundance of overfolds associated with upright folds as the southern junction with the Crackington Formation is approached (Figs. I and 2). Within the Crackington Formation belt on the coast (Freshney, 1965)and inland (Edmonds and others, 1968, fig. 10) this tendency increases southward, so that on the coast the folds are overturned to the south and become recumbent as the Lower Carboniferous inliers are approached. At Okehampton, associations of upright and overturned folds just north of the Lower Carboniferous inlier suggest that the overfolds are derived from the upright folds by a later tectonic pulse and it may be that the combinations of visible folds in the coastal structures in both the Bude and Crackington Formations have also been formed in the
486
W. R. DEARMAN
same way by successive phases of deformation (Zwart, 1964; Dearman, 1969a). South of the main outcrop of the Crackington Formation, which is up to ten kilometres wide, there is repetition of all three stratigraphical units involved in the structure of the southern margin of the main Carboniferous outcrop (Fig. 1). Individual outcrops of the Upper Devonian slates, the Lower Carboniferous rocks and the Namurian Crackington Formation are up to four kilometres wide, suggesting that folding, if present, is on a large scale. The Lower Carboniferous inlier south of Okehampton, with a core of presumed Upper Devonian slates with siltstone lenticles, is undoubtedly a complex major anticline which is overturned to the south. As with the coastal structures in Upper Carboniferous rocks, doubt must exist whether the present folds within the main anticline are original features or have developed by deformation of earlier upright folds (Dearman, 1969c,fig. 2). At Okehampton the core of presumed Upper Devonian slates is characterised by a distinctive style of small-scale tight to isoclinal folds; incidentally, the slates appear to be folded into the overfolded pattern characteristic of the Lower and Upper Carboniferous envelope. The Okehampton anticlinal inlier undergoes a significant change in structural style as it is traced to the north Cornish coast (Fig. 2). The main anticline dies out and the whole succession up to the Crackington Formation is folded into small-scale tight to isoclinal folds which at Okehampton were seen as a distinctive structural type only in the presumed Upper Devonian slates. Two outcrops of Lower Carboniferous rocks have slates and sandstones of the Crackington Formation between them and Upper Devonian to the south, the whole appearing to be repetitions of evenly and gently northward inclined groups of strata. To the south of the Okehampton anticline, along the margin of Dartmoor, the two most southerly outcrops of the Crackington Formation also have only small-scale tight to isoclinal folds (Fig. 2). It is likely that the same division into areas with and without large-scale folds also occurs in the east side of the moor where the outcrop of Lower Carboniferous rocks marked on the map borders a southern region dominated by slate-type structures. The impression that, wherever large-scale folds are absent, the major structures comprise uniform sheets of strata, right way up and inclined gently to the north but with a haphazardly shuffled repetition of the three main stratigraphical horizons, is most evident in the coastal exposures north and south of Tintagel. Bedding and slaty cleavage are parallel and sparse hinges of small isoclinal folds are visible only in thin sandstones and in lenses of siltstone and limestone. In a similar setting north of the Bodmin Moor granite where again slaty cleavage is gently and evenly inclined northward, Freshney (1969) has inferred from a borehole record the
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BF BUDe FORMATION CF CRACKINGTON FORM. .::~':i L.CARBONIFEROUS o DEVONIAN Fig. 2. Sketched horizontal sections (a) from Bude to Tintagel, and (b) from Hatherleigh to Okehampton, and along the west side of Dartmoor to Tavistock. Early structures only are shown
488
W. R. DEARMAN
presence of a large-scale isoclinal to tight fold closing to the south which has 250 m. of Upper Devonian green slates as a core enveloped and infolded in the brow region by Carboniferous grey slates. An element of large-scale inversion is implicit in the interpretation of this structure and a similar situation probably exists to the east of Tavistock, where the southernmost outcrop of the Crackington Formation appears to be in mainly inverted beds. Large folds therefore do exist elsewhere than at Okehampton but, in keeping with both a regionally uniform attitude of slaty cleavage and a general parallelism of slaty cleavage with bedding, they must be very much tighter, even isoclinal, structures than those at Okehampton. Overfolds in the Okehampton area are broken by thrusts parallel with the gently inclined normal limbs of the folds; movements were directed up the thrustplanes toward the south-south-east. Although there is no direct evidence, Freshney (1965, 179) has suggested that 'some of the more marked lowangle faults [on the coast], such as the Rusey Fault and another major fault at Trambley Cove, existed at the time of the early recumbent folding and during the formation of the southerly directed overfolds, when they would have moved as thrusts from the north'. It would seem reasonable to extend this hypothesis to include many, if not all, of the junctions between lithological and stratigraphical units within the area under discussion. It is now possible to draw some additional lines on the tectonic map. The first is a line separating the areas of markedly different structural style. This line crosses stratigraphical boundaries. It is more or less coincident with the line drawn by Simpson (1969,fig. 4) marking the boundary between his broad Structural Zone III, which is succeeded to the north by a subzone of Structural Zone II. The present author (Dearman, 1969a, 34, and fig. 3) has drawn the same line in Cornwall separating the following zones: (2) Zone of overturned folds in Namurian strata. The folds, overturned to the south, are sliced up by low-angle faults. Cleavage is prominently developed in the southern part of the zone. (3) Zone of tight to isoclinal folds with slaty cleavage. Upper Devonian (Famennian), Lower Carboniferous (Dinantian) and Upper Carboniferous (Namurian) present in shuffled thrust slices. Low-angle faults in Zone II and thrusts in Zone III demand the inclusion on the map of a whole family of thrust faults, gently inclined to the north, on which there has been a general southerly sense of overriding (Fig. 1). There are, however, complications in the tectonic history of the area, implicit in the use of the terms low-angle fault and thrust which can be used synonymously to describe only the earliest phases of tectonism. The deformation is polyphase and the main tectonic sequence (Fig. 3) may now be summarised:
TECTONIC EVOLUTION OF SOUTH-WEST ENGLAND
489
(a) Generation of early folds; open upright folds in suprastructural region, style changing progressively downward to evenly bedded units with very much smaller-scale isoclines or tight folds and crosscutting thrusts which achieve the same degree of shortening. (b) Continuation of early deformation leading to selective tightening of fold angle in the open upright folds of the suprastructure. Infrastructural response would be mainly in terms of continued movement on early thrusts giving rise to a pronounced version of Breddin's (1966) shingle-structure. (c) Overturning and other deformations of early folds in suprastructure, predominantly but not exclusively to the south, and generation of low-angle thrusts overriding southward. Thrusting and marked flattening of the infrastructure with formation of north-north-westsouth-south-east trending mineral lineation in slates and tergiversation (Dearman, 1969b) of isoclines. Over large areas dominant fold trend is around north-north-west but very variable; folds face southsouth-east, west-south-west and east-north-east. Tectonic drive was distinctly Caledonoid in direction with overriding southward.
Foldsopen upright Incipientclravage SUPRASTRUCTUR~ _
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490
W.R.DEARMAN
(d) The great reversal : low-angle normal-faulting northward along existing thrusts and possibly along newly formed fault planes; folded deformation of the slates of the infrastructure and the overfolds in the suprastructure. (e) Rise of the granite batholith and generation of granite-induced structures. All earlier structures offset by steep normal faults and by Tertiary dextral wrench-faults. The anomalous north-north-west-south-south-east trend of the early folds in the Tintagel district can be traced for at least twenty kilometres south to Delabole (Wilson, 1952) from the northern margin of the structural zone. On the west side of Dartmoor the same structures crop out in the infrastructural zone over at least eleven kilometres. It is conceivable that the southern limit should be taken as far south as the confrontation with northerly facing structures at Padstow (Gauss, 1967). This would extend the southern limit of Structural Zone 3 established by Dearman (1969a, 34 and fig. 3), and would warrant the establishment of a subzone along the northern margin of Simpson's Structural Zone 3 (1969, fig. 2). Similar structures occur in south-west Cornwall (Dearman, 1969b, fig. 2). The two regions of tergiversate folds represent a transition zone between the suprastructure and the infrastructure of the Variscan orogen of south-west England. REFERENCFS BREDDIN, H. 1966. Der Schuppenbau der mitteldevonischen Gesteinsfolgen im Gebiet von Lindlar bei Koln (Rheinisches Schiefergebirge). Geol, Mitt., 7, 1--44. DEARMAN, W. R. 1964. Wren ch-faulting in Cornwall and South De von . Proc, Geol, Ass., 74, 265-87. - - - . 1969a. An Outline of the Structural Geology of Cornwall. Proc. geol. Soc. , No. 1654,33-9. - - -. 1969b. Te rgiversate Folds fro m South-West England. Proc, Ussher Soc., 2, 112-15 . - - - . I 969c. On the Association of Upright and Recumbent Folds on the Southern Margin of th e Carboniferous Synclinorium of De vonsh ire and North Cornwall. Proc, Ussher Soc.• 2, 115-21. - - & N. E. BUTCHER. 1959. The Geology of the Devonian and Carboniferous Rocks of the North-West Border of the Dartmoor Granite. Devonshire. Proc. Geol. Ass., 70,51-92. DE LA BECHE. H. T. 1839. Report on the Geology of Cornwall, Devon and West Somerset. Mem. geol. Sur v, U.K. DUNNING. F. W. 1966. Tectonic Map of Great Britain and Northern Ireland. London: Her Majesty's Stationery Office. EDMONDS, E. A., J. E. WRIGHT, K. E. BEER , J. R. HAWKES, M. WILLIAMS, E. C. FRESHNEY & P. J. FENNING. 1968. Geology of the Country around Okehampton . Mem. geol. Surv, U.K. - - - , M. C . McKEOWN & M. WILLIAMS. 1969. South-West England. Br. reg. Geol. Geol. Surv .
TECTONIC EVOLUTION OF SOUTH-WEST ENGLAND
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FRESHNEY, E. C. 1965. Low-Angle Faulting in the Boscastle Area. Proc, Ussher Soc., 1, 175-80. - - - . 1969. Preliminary Report on the Wilsey Down Borehole, North Cornwall (Abstract). Proc, Ussher Soc., 2, 107-8. GAUSS, G. A. 1967. Structural Aspects of the Padstow Area, North Cornwall. Proc, Ussher Soc., 1,284-5. PRENTICE, J. E. 1962. The Sedimentation History of the Carboniferous in Devon. In Some Aspects of the Variscan Fold Belt, ed. K. Coe, 93-108. Manchester Univ. Press. SIMPSON, S. 1969. Geology, pp. 5-26. In Exeter and its Region. University of Exeter. WILSON, G. 1952. Field Meeting, Whitsun 1951, at Tintagel, North Cornwall. Proc. Geol. Ass., 63, 49-58. ZWART, H. J. 1964. The Development of Successive Structures in the Devonian and Carboniferous of Devon and Cornwall. Geologie Mijnb., 43,516-26.
W. R. Dearman Department of Geology The University Newcastle upon Tyne NEt 7RU