Structural zones of the Variscan fold belt in SW 533

BUTCHER,N. E. i96I. Age of orogeny and granites in sw England. Nature, Lond. xgo~ 253. D~ARMA~, W. R. x959- The structure of the Culm Measures at Meldon, near Okehampton, north Devon. Q. Jl geol. Soc. Lond. xx5, 65-Io6. I967. Structural patterns in the Upper Carboniferous rocks at Welcombe Mouth, north Devon. Rep. Trans. Devon Ass. Advmt. Sci. 99, 273 -86. I969a. An outline of the structural geology of . Proc. geol. Soc. Lond. x654~ 33-9. x969 b, Tergiversate folds from South-west England. Proc. Ussher Soc. 2~ 115-2 I. x97I. Some aspects of the tectonic evolution of south-west England. Proc. Geol. Ass. 8x~ 483-9 2 • DOBSON, M. H. & REx, D. C. 197 x. Potassium-argon ages of slates and phyllites from south-west England. Q. Jl geol. So¢. Lond. x26, 465-99. t-IADLEY, J. B. 1964. Correlation of isotopic ages, crustal heating and sedimentation in the Appa- lachian region. In Lowry, W. R. (editor) Tectonics of the southern Appalachians. Mem. Virginia Polytech. Inst. Dept. Geol. Sci. x. HAm'ER, C. T. I967. The geological interpretation of potassium-argon ages of metamorphic rocks from the Scottish Caledonides. Scot. J. Geol. 3, 46-66. t-~m~eaxs, E. M. L., HousE, M. R. & RHODES, F. H. T. I97I. Evidence bearing on the strati- graphical successions in south Cornwall. Proc. Ussher Soc. 2, 27o-75. MAa'THEWS, S. C. x966. Lower Carboniferous stratigraphy in the St. Mellion area. Proc. Ussher Soc. x, 227-29. MOOm3ATI~, S. I967. Recent advances in the application and interpretation of radiometric age data. Earth Sci. Rev. 3, I I t-33. R~SAY, J. G. 1967. Folding and Fracturing of Rocks. (McGraw-Hill). R.~tSBOT'rOM W. H. C. I97o. Carboniferous faunas and palaeogeography of the region. Proc. Ussher Soc. % I44-57. ROBERTS, J. L. & S.~'~DFI~SON, D. J. I971. Polyphase development of slaty cleavage and the confrontation of facing directions in the Devonian rocks of north Cornwall. Nature, Lond. (Physical Science) 23o, 87-9. S.~a~DERSO~r, D.J. 197 I. Superposed folding at the northern margin of the Gramscatho and Mylor Beds, Perranporth, Cornwall. Proc. Ussher Soc. 2, 266-9. 1973. The development of fold axes oblique to the regional trend. Tectonophysics x6, 55-7o. SIMPSON, S. 197o. The structural geology of Cornwall: some comments. Proc. geol. Soc. Lond. x66% I- 3 . Sm~-ri, M. A. P. i965. Repeated folding between and Portreath, Cornwall. Proc. Ussh~r Soc. x, I7o-I. S~X'THE, D. K. I97 x. Viscous theory of angular folding by flexural flow. Tectonophysics x2, 415-3 o. Received 26 June i972; revised manuscript received 25 November x97~; read xo January t973. David Jardin Sanderson & William Robert Dearman, Department of Geology, University, Newcastle upon Tyne NE z 7RU.

DISCUSSION DR S. C. MATTHEWS said it was surprising that the authors made so little reference to fold plunge. They used a plunge feature in S Cornwall to separate Zones i i and I2, but there is no mention of plunge in N Cornwall and S Devon (Zones 6 and 7) where geological maps have long shown different senses of plunge. In N Devon and W Somerset (Zone I) they suggest that fold axes are subhorizontal and trend E-W. Shearman (I 960) referred to varying plunge in this region and Webby's (1965) work in the Brendon Hills and the Quantocks confirmed this. The WSW plunging folds on the south side of Exmoor involve an U. Devonian to U. Carboniferous sequence and if one of these folds acts on the base of the U. Carboniferous greywackes then it has the character that the authors prescribe

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for Zone 2. The same fold in an up-plunge direction, where the Pilton Beds out- crop, has dominant slaty cleavage, a characteristic that Sanderson and Dearman ascribe to Zone I. Such situations are frequently mentioned in the German literature under the name Stockwerktektonik. This should not be translated as 'stockwork' but as the development of structural styles in successive levels in a major structure. A Stockwerktektonik situation exists in N Cornwall with Zones 4-6 forming a recumbent fold zone with different structural effects in the different major lithologies and so plunge should again be significant, tending to run the boundaries of zones (or major lithological types) around major closures of fold- structure. But the authors' map shows the zone boundaries to run fairly straight, E-W or ESE-WNW, with no obvious indication of control by plunge even allow- ing for poor inland information (note the patch north and east of Dartmoor, where no distinctions are made among the rocks of Zones 3-6). Alternatively, the bound- aries may be everywhere faulted; an explanation that applies at the coast and few examples of the interaction of the structural characteristics of adjacent zones may then be available. If fault effects are maintained across N Cornwall on the scale seen at the coast, then what was originally a pile of structures developed in an upward sequence must now be distributed, as Sanderson and Dearman's map indicates, through a considerable south to north spread of outcrop. The three zones in N. Cornwall are possibly three different structural levels (Stockwerke) sliced apart by faulting. Southward from there, the authors rapidly switch the means of identifying zones. On the W Cornwall coast, a local strong F 2 structure makes it opportune to separate zones. In S Cornwall, a special plunge effect separates Zone 11 and Zone IO. Zone I2 is the higher grade metamorphic rocks of the Lizard and Dodman. The SW to NE line they draw linking the Lizard to the Dodman is geologically spurious. The Dodman rocks are greywackes and slates, and they occur normally and naturally along with the folded L.-M. Devonian rocks of the Roseland area of Cornwall (P. M. Sadler, pers. comm.) Their occurrence does nothing to indicate a Caledonoid grain in the structure of S Cornwall. The Zone boundaries seem therefore to be often faulted in Cornwall, and in the south are shiftily (i.e. variably) defined. The authors suggest that Dodson and Rex's K-Ar dates are best interpreted in terms of uplifts. Do they find that the younger ages are recorded from sites relatively low in the structure ? The paper refers to a 'Bretonic' effect in Cornwall. The Bretonic phase in NW France is an event affecting the character of the U. Devonian and L. Carbon- iferous stratigraphy. There is still no evidence of any parallel in the Cornish stratigraphy. In France, there is no evidence to suggest that any production of small-scaIe structures or cleavage accompanied the Bretonic stratigraphic event. Is anything achieved if we attach the label 'Bretonic' to any stratigraphically indeterminate development of deformation in the rocks of S Cornwall ? THE Atrr~oRs pointed out that they have used the attitudes of fold axes to define zones of oblique folding in which the pitch of the axis within the axial plane is oblique to the regional trend. The plunge variations referred to by Dr. Matthews involve only small departures (less than 2o °) from the general horizontal attitude of fold axes. Such variation is common to all types of fold systems both throughout

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SW England and other fold belts. The interpretation of upright and inclined folds delimiting a stockwerktektonik situation is interesting. The strict stratigraphical control of structures is, however, not seen between Zones 3 and 4, as the inclined folds northwest of Dartmoor (Zone 3) pass laterally into recumbent folds (Zone 4) at a similar stratigraphic level. Faulting does not define structural zones in N Cornwall and mid-Devon. The authors recognise the problems of joining the Lizard and Dodman areas, and hope that the work of P. M. Sadler in the Roseland area, referred to by Dr. Matthews, will establish the structural and stratigraphical relationship of the Dodman to contiguous areas. The NE-SW trend of the zone boundaries in S Cornwall follows the attitude of the folds and the finite strain ellipsoid, and satisfactorily explains the distribution of fold structures. The reference to this direction as Caledonoid by Dr. Matthews and others is unfortunate, as it may be considered by some to imply stratigraphic and tectonic relationships to the Caledonian fold belt. The relationship of K-Ar dates to structural depth is difficult to establish, as we have only a single slice through the fold belt. At Polzeath the 340-320 my and 31°-27° my ages occur at the same stratigraphic and structural level and hence the boundary between them is probably steeply dipping. In south Cornwall the effect of the granite batholith separates the 340-320 my and 365-345 my ages and hence the nature of the boundary is obscure. The development of chevron folding, implying high differential stress (Smythe I97I), would suggest that both the area and SW Cornwall were at relatively high structural levels. Bishop et al. (I 968) describe deformation, with the development of slaty cleav- age, in late Devonian and early Carboniferous times (the Bretonic phase) in west FinistSre, NW France. It is uplift associated with this deformation to which the 365-345 my thermal event in SW England is ascribed. DR D. M. MACKINTOSH drew the authors' attention to the strong normal faulting in the Variscan fold belt of SW England. The prevalence of this faulting has brought forth the comment that the fold belt has Andean affinities and shows basement control of its structural development. Widespread normal faulting has been described from the and Plymouth regions: at Exeter it cuts up the folding in Namurian turbidites and pre-dates the New Red Sandstone. In the area between Wanson Mouth and Rusey, N Cornwall (Zone 3) normal faults strike WNW subparallel to the fold axes and dip NNE at high angles. The minimum throw is frequently equal to the height of the cliffs (about i oo m). Clearly these faults play an important part in the history of the fold belt. How do the authors interpret their distribution and significance ? Secondly, could the authors comment on the relationship between the up- right folds and the recumbent folds in the region (Zone 2) ? Can these two fold attitudes be conjugate or is there clear evidence that the flat lying folds re- fold the upright folds ? In reply the AUTHORS recognise the existence of both high and low angle normal faulting in N Cornwall. This faulting is responsible for the extent of the inverted strata of the Crackington Formation, but does not appear to define the boundaries to the structural zones. The zones boundaries are defined on the basis of original variation in the attitude of primary folds in this area. The Andean nature of the

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Variscan fold belt is an interesting suggestion but discussion of this is beyond the scope of the present paper. The occurrence of two phases of folds in Zone 2 is clearly described by Dearman (i 967). DR J. L. M. LAMBERT congratulated the authors for presenting a most useful analysis of the fold belt. By dealing with observed rather than hypothetical structures they have not, at the outset, accepted the concept of a Lizard thrust as the essential structural element of SW Cornwall. Nevertheless, the authors re- gard thrusting as an important factor in the deformation of the region and identify a narrow zone, Zone I I, of second phase folds of variable trend and facing direction, the so-called tergiversate folds, adjacent to the Lizard boundary. These particular folds have been distinguished by the speaker as belonging to two sets, main folds and cross folds, trending approximately at right angles to each other. While this tectonic style is indeed most clearly displayed in the strata of the area, it can be recognized in the Gramscatho Beds through- out the region including strata exposed I O km to the north between and Porthtowan. The folds can be referred to one episode of superposed deforma- tion and suggest a phase in the orogeny when movement became more restricted. The structural cross-section presented to account for the differing attitudes of minor folds at Newquay and Perranporth shows a major recumbent fold reminiscent of the primary Dalradian folds discussed in a preceding paper. Bear- ing in mind the restricted occurrence of the inverted minor folds, what evidence is there that they are congruently related to a structure of such magnitude and regional significance ? THE AUTHORS replied that the two sets of folds described by Dr Lambert from the Helford River area can be demonstrated to involve two phases of deformation, but the F1 folds themselves have fold axes of variable trend and are oblique folds. The development of oblique folding is well seen elsewhere in south Corn- wall, for example at Pendower Beach where the F~ folds are more restricted in their development, and form the basis of the recognition of Zone I I. The develop- ment of oblique folds need not imply laterally restricted movement (Sanderson 1973). The presence of large scale folds is clearly demonstrated by the geometry of associated minor folds, the inverted nature of bedding and inverted stratigraphic sequences. The Newquay-Perranporth fold, referred to by Dr Lambert, has been described by Sanderson (197 I). PROFESSOR StrrTON said he had followed the account of the geometry of these structures with interest, but would like to know what effort the authors had made to make use of the palaeontological and stratigraphical information which was available. To ignore it as it appeared they had done could lead to an incorrect picture of the structure of southwest England. Conversely a combination of their approach with other methods of attack might be expected to unravel this fascinat- ing structure, parts of which they had so clearly described and analysed. THE AtrrHORS agree with Professor Sutton that palaeontological and stratigraphi- cal information should be incorporated into structural interpretations. The stratigraphical information available is consistent with the described structure and the broad stratigraphical divisions are incorporated in the section in Fig. I. The emphasis on structural rather than stratigraphical data reflects the interests of the authors.

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