Thrust Tectonics of S Devon

J. geol. Soc. London, Vol. 140, 1983, pp. 215-228, 11 figs, 1 table. Printed in Northern Ireland.

Thrust tectonics of S Devon

M. P. Coward & K. R. McClay

SUMMARY: The tectonics of the area from Start Point to Chudleigh, SW England, are shown to be dominated by northward-verging thrust nappes. Two major thrusts are postulated. The 'Dartmouth antiform' is consideredto be a major thrust structure, nota fold structureas previously interpreted. The Torquay limestones are allochthonous and part of a klippe of a major thrust nappe with a minimum displacement of13 km northwards. A progressive and diachronous sequence of deformation has been established with deformation starting in the S and moving N. Many of the folds and cleavages are probably related to the strains developed locally in thrust tips and may have no regionalcorrelation. More intense and complex deformation is found in the S compared to the N. The diachroneity is also reflected in the K-Ar mineralages. In the S, at Start Point, southward-verging backfolding is the last significant Hercynian deformation phase folding the thrust nappes.

In SW England, Hercynian orogenesis has affected the Devon we found thatthe structuralinterpretations Lower to Upper Palaeozoic rocks of Devon and Corn- indicated in previous publications and maps (e.g. wall. Much of this deformation post-dated the deposi- Lloyd 1933) were oversimplified. Further research has tion of Mid-Carboniferous Culm sediments but pre- revealed the presence of largerecumbent folds and dated the intrusion of the SW England granite bath- thrusts; it is the aim of this paperto describe the olith, which has yielded K-Ar mineral ages ranging evidence for these and discuss their significance in from 254 to 277 Ma (Dodson & Rex 1971). K-Ar terms of themajor tectonics. We do not aim to mineralages theon weakly metamorphosed provide a definitive synthesis of the tectonics of the Palaeozoic sediments range from Devonian to Carbo- region but rather to promote further discussion and niferous, the older ages occurring in the S (Dodson & research. Rex 1971). This suggests that deformation and accompanying metamorphism may be either diachronous or polyphase, and that some deformation pre-dated the Stratigraphy deposition of Upper Devonianand Carboniferous sediments.Sanderson & Dearman (1973) noted The classical Devonianstratigraphy, as described by changes in style of structures from S to N in Cornwall the Geological Survey (Ussher 1903, 1904, 1913; and N Devon and they correlated these changes with Lloyd 1933) and later modified by Richter (1965), the variationin mineral ages. Theyrecorded north- House (1975) andHouse et al. (1977), isgiven in ward-verging recumbent folds and thrusts in S Corn- Table 1. This lists the local rock names as well as local wall but in N Cornwall and Devonthey recorded a stage names and both will be referred to in the text. later set of southward-verging folds and thrusts. These The lowermost rocks observed in this region are the later structures steepen in dipto the N(Sanderson monotonous red mudstones, siltstones (now slates and 1979) and in N Devon and S Wales they fan through phyllites) andsandstones, known collectively as the over 130" to becomeupright to northward-verging Dartmouth Slates. They have been interpreted as shal- structures. low water, fluviatile sediments(Shannon 1921, 1928; In SE Devon no major recumbent folds have been Dineley 1966; Richter 1965). They pass up into shales reported, thoughVachell (1963) reported athrust and thin sandstone beds and lenses, termed the Mead- structure, the Marldon Beacon Nappe, in the Torbay footBeds. The sandstones are often graded, cross- area. He noted that Lower Devonian rocks occupied bedded and contain storm deposits. They have been high topographic levels, higher than the adjacent Up- interpreted as estuarine-marinesediments (Richter per Devonianrocks. More recentlyScrutton (1979) 1967). The Dartmouth Slates and Meadfoot Beds con- describedinverted stratigraphy in the Babbacombe tain basic tointermediate volcanic horizons oftuff area and Hobson (1976, 1977) has recognized several breccia,agglomerate and lava. Intermediate dykes, phases of folding in theStart Point-Brixham area possibly feeders to thevolcanics, occur in the Meadfoot (Fig. 1).However, many of the stratigraphicand Beds. palaeontologicalreconstructions of the geology and In SE Devon, the Middle Devonian rocks are char- facies distributions do nottake these structures into acterized by the widespread development of lime- account (House & Selwood 1966; House 1975; Richter stones with a local development of stromatoporoid 1967). reefs andbanks (Scrutton 1977a, b). Detailed On taking undergraduate field classes around S palaeoenvironmental studies have been carried out by 0016-7649/83/03004215$02.00 0 1983 The Geological Society

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Dartestuary

Blackpool

D '/Ci .d Cove

//---7=T J

Exeter

S E\ Hallsands l

*Start Paint

Point

FIG. 1. Map of SE Devon showing section lines of Fig. 2. Inset, location map and section line for Fig. 9.

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TABLE1: Stratigraphy of the Torbay district, after intercalated with red mudstones and calcareous shales House et al. 1977. and thence intodeep water pelagic red shales and mudstones dated as Upper Devonian by goniatite Culrn Greywackes faunas (cf. House 1963). Thismarked upward facies rNarnurian (Slates of Anstey’s change is attributed to subsidence and fragmentation of the carbonateplatform with the development of rise Upper DevonianUpper Slates of Saltern and basin (‘Schwellen’ and‘Becken’) features as 1 i :::I observed over much of the Rheno-Hercynian zone in I Saltern Cove Beds NW Europe (see review in Anderton 1979). FrasnianBabbacornbe Slates et al. Givetian Torquay Limestone Middle Devonain Calcareous shales Structure Meadfoot Beds Lower Devonian Dartmouth Slates A simplified geological map of the area is shown in Fig. 1 and detailed cross-sections in Fig. 2. These Richter (1967) and Scrutton (1977a, b) who intersections will first be described, in areas from N to S, preted the facies as originating in shallow shelf seas and then the structure will be collated into an overall with carbonate banks. The Middle Devonian lime- schematic cross-section. The structure is complicated stones passupwards into thinly-beddedlimestones by the presence of Permiansediments and later

MEADFOOT8AY

U DEV SHALES

WATER,VOE COVE BmAO SANDS SALTEHN COVE : ELBERRY COVE - _---_

START FQINT SCHISTS , *c+ S’ qv? $1 m .P-. F3 STRVCTURES2 F3 CRENULATICM yms LWMINANT l

i\ l e YOUNGINGDIRECTION CLEAV-AGE SYNFQ9M\. f LATE FAULT

FIG. 2. Crosssections along the Devon coast, section lines given on Fig. 1. m = Meadfoot Beds. See text and appendix for discussion.

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Deformation is intense and the rocks will be discussed later. havea strong cleavage which is axial-planar tothe Minor, probably imbricate thrusts have been recog- folds and generally dips southwards more steeply than nized at Petit Tor, where Middle Devonian limestones the bedding. The limestones are cut by several small are thrust over Upper Devonian shales. They are also thrusts (Fig. 2), which cut the short inverted limbs of found within the limestonesat Hope’s Nose. How- the folds. ever, these thrusts are difficult to correlate because of Between Babbacombe and Anstey’s Cove (see Figs the complication of post-Permian block faulting; a 1 and 2for locations) Middle Devonian limestones major fault, of uncertain throw, strikes E-W across overlie Upper Devonian red shales, dated as Frasnian the Torquay Headland (Figs 1 and 2). S of this fault, (House 1966; House et al. 1977). AtBabbacombe, Lloyd (1933) and Shannon (1928) recorded several re- Scrutton (1979) considered this inversion to be due to petitions of Lower Devonian strata which, if correct, alarge recumbent fold of cleavage age. However, must indicate at least two major thrusts. However, this many of the beds show cleavage dipping more steeply stratigraphic repetition is based on the interpretation of to the S than bedding, a relationship which elsewhere Devonian shelly faunaand this needs further study in the Torquay Headland shows that the rocks lie on before the presence of these faults can be confirmed. the right way up limb of a N-facing, inclined to recumbentfold. The way up criteria in the locally Torquay to Brixham(Berry Head) intensely deformed limestones and underlying Upper (Cross-sections N, 0, P & Q) Devonianred shales are ambiguous-some indicate right way up, someinversion. From a study of the Thenorthern part of this section is obscured by stromatoporoid and other reef organisms at Long Permian deposits but according to Ussher (1903) and Quarry Point, much of the limestone appears tobe the Lloyd (1933) these deposits are underlain by Meadfoot right way up (Kershaw & Riding 1980). Thus there are Beds which form a large open synform, probably of F2 two possible explanations for this out of order strati- age (Figs 1 & 2). S of Paignton, at Goodrington Sands, graphy. Either: the MeadfootBeds crop out.Here they are highly (a) there is a major thrust of cleavage age, carrying folded by asymmetrical,northward-verging, recum- dominantly right way up Middle Devonian limestones bent folds. Both cleavage and bedding dip northwards such as those at Anstey’s Cove and Long Quarry Point and the folds are downward-facing, being folded by over generally right way up but folded Upper Devo- the broad F2 syncline (Fig. 2). nian slates. Locally the rocks could beinverted by On the S side of Waterside Cove (Fig. 2, section N) folds in the hanging wall and footwall; or Upper Devonian red shales and limestone conglomer- (b) there is a major recumbent fold in which much ates (House et al. 1977; Holwill 1966) dip northwards. of the inverted limb in the limestone is missing. This Locally, the beds are vertical with a horizontal cleav- fold would pre-date the main cleavage. age, as they are located in a fold hinge. These shales There is evidence for a pre-cleavage deformation in are undoubtedly Upper Devonian in age, as evidenced that some shaly limestones immediately N of Anstey’s by their coral,goniatite and conodont faunas (van Cove contain an early cleavage and slickensides paral- Straaten & Tucker 1972; Scrutton 1965; House 1963; lel to bedding. These structures arefolded by the main Holwill 1966). The contact between these Upper cleavage and the cleavage itself is folded by later E-W Devonian shales and the Lower Devonian rocks to the trending, northward-verging folds, so that locally, N of N is now anormal fault. From the curvature of Anstey’s Cove, the main cleavage is downward-facing. bedding and cleavage and the downthrow of Permian A similar problematical structure occurs atDaddy rocks on the N side of this fault, thefault brings Lower Hole on the S side of the Torquay Headland, where Devonian rocks down from a higher structural level. thebeddingkleavage relationshipsindicate thatthe Unless there was a multiple history of faulting, the beds are right way up, butevidence from geopetal Lower Devonian rocks must have previously overlain cavities within brachiopods indicates that part of the the Upper Devonian rocks. succession is overturned (Fig. 3). This suggests the Vachell (1963) noteda similar structuralrela- presence of a pre-cleavage inclined fold, theaxial trace tionship of Lower Devonian rocks on top of Upper of which passes through Daddyhole Cove (cf. Scrutton Devonian rocks to the W of Paignton. He considered

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that the Lower Devonian rocks were part of a thrust Dartmouth Slates (Fig. 2). From minor fold and bed- sheet, which he termed the Marldon Beacon Nappe. ding cleavage relationships, these beds are right way Indeed, publishedmaps by the Geological Survey up and on the southern limb of the sameantiform. (Lloyd 1933) show thatthe LowerDevonian rocks This beddingicleavage relationship can be seen on occupy the high ground and also the core of the large either side of theDart Estuary, at Kingswear and open F2 synform (Figs 1 & 2). Dartmouth.Unfortunately, reliable sedimentary way At SalternCove (Fig. 2) and tothe S theUpper up criteria have not been identified in the Dartmouth Devonianshales are deformed in gently inclined, Slates near their contact with the Meadfoot Beds, and northward-verging folds with a well developed axial- we have no evidence to assume that there was local planar cleavage. Locally, these folds are deformed to stratigraphic inversion before this cleavage-producing become downward-facing. Several minor thrusts have deformation. been recognized where the Middle Devonian lime- Thus, assuming that the stratigraphic and palaeonto- stones overlie the Upper Devonian red shales (Fig. 2). logical evidence is correct in that the DartmouthSlates At Brixham and Berry Head (Fig. 2, sections 0, P & are older thanthe Meadfoot Beds, then theDart- Q) the Middle Devonian limestones are intensely de- mouth Slates must be thrust over the Meadfoot Beds formedinto tight, recumbent, northward-verging in this section or inverted on a large scale before the folds, gently refolded by more upright F2 structures. cleavage age folding. The outcrop of the Dartmouth Slates cannot form a simple antiform in SE Devon as Berry Head-Dartmouth suggested by Ussher (1904) and Hobson (1976). Thestructure from Berry Headto Dartmouth is illustrated in Fig. 2, sectionsP & R. At Sharkham Dartmouth-Start Point Point, S of St. Mary’s Bay, Middle Devonian lime- Between Dartmouthand Pilchard Cove the Dart- stones are folded into an inclined F1 syncline, which is mouthSlates are strongly foldedinto northward- itself refolded by a large F2 antiform which brings facing, moderately to steeply inclined folds with an Middle to Lower Devonian shales into St. Mary’s Bay. accompanying intense cleavage. The change in the dip S of Sharkham Point, the Middle Devonian rocks pass of this cleavage is accompanied by the development of upwards into structurally overturned Meadfoot Beds. later-stage kinks and crenulations. At this locality, the Meadfoot Beds are on the over- The junction between the Dartmouth Slates and the turned limb of alarge, northward-verging antiform. MeadfootBeds is obscured by the sands of Slapton The contact of Middle Devonian shales with lime- (Fig. 1). BetweenTorcross and Beesands the Mead- stones in St. Mary’s Bay may bea thrust fault (see foot Bedsform a large upright sycline (Fig. 2). The section P, Fig. 2 and locationk in Appendix). The cleavage is believed to be the same generation as that structures are not easily matched across this contact to the N but has been folded around a large backfold. and there is a major facies change between the rocks N of Beesands sedimentary evidence indicates that stratigraphically below the limestones,N and S of the folds are all upward facing, but S of Beesands Sharkham Point. cleavage-age folds face both upwards and downward. Further S in this section the rocks consistently dip The rocks must havebeen tightly folded before the southwards but bedding-cleavage relationships reveal main cleavage. In addition, S of Beesandsa steeply the presence of a large antiform at Man Sands (Fig. 2, dipping crenulation cleavage is important, and folds of section R). S of Man Sands there are parasitic folds this generationbecome the dominantstructures to- but the dominant fold sense, bedding-cleavage rela- wards Hallsands (Fig. 2, section S). This crenulation tionships and sedimentary way up criteria (cross-bed- generally dips steeply northwards, but in the southern ding and graded bedding) in the grit horizons, indicate part of this section it is itself folded by southward- that these beds are dominantly right way up and young facing, moderately inclined folds. tothe S. Further S these Lower Devonianrocks, The junction between the Start Point Series and the equivalent to the Meadfoot Beds, are overlain by the Meadfoot Beds can be seen on the beach N of Hall- sands,where it is a sharp, normalfault break.The Lkkiy Hde cow Start PointSeries consist of micaceous-amphibolitic right-wayup Itmestmes schists of a much higher metamorphic grade than the juxtaposedMeadfoot Beds. They, however, show similar structures to the Meadfoot Beds immediately NE to the N. Marshall (1962) andHobson (1977) described at least 4 phases of folding in these schists. The nochsngehtedfilgc~sme dominantstructures are steeply-inclined,southward- FIG. 3. Sectionthrough DaddyHole Cove, Tor- verging crenulations, which may becorrelated with quay. Beddingshown by solid lines, cleavage by similar (F3)structures tothe N. An antiformal dis- fine lines. tribution of green hornblende schists (Marshall 1962;

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la l It

Torbay I 24 2.1 Salrern Cove

W 3-7 I Sharkham

$/23 7 I Blackpool li 4 Fold hmges 4 8.5 4 l Torcross 4Slickensrder l A Start Pomt

FIG. 4. (a) Map of SE Devon showing directions of fold hinges and slickensides. (b)Map showing directions of the maximum extension of the strain ellipsoids as measured i-from strained objects and the preferred orientations of minerals. Mean ellipsoid ratios for 6 localities shown.

Hobson 1977) presumablyresults from earlier fold Deformation intensities phases. Lineations Strain measurements Strainmeasurements have been made from deformed reduction spots in the Upper Devonian Slates, Fold hinges and bedding-cleavage from deformedgrains in the calcareous grits in the intesection lineations Upper-MiddleDevonian limestones, and from de- Fold hinges and bedding-cleavage intersection formed fragmentsin volcanic agglomerates in the lineations generally trend at N80"E f 20" in the area Meadfoot Beds and Dartmouth Slates. The results are from Start Point to Hope's Nose (Torquay) (Fig. 4a). shown onthe map in Fig. 4b and on a Flinn plot However, N of Hope's Nose, in the Upper Devonian (Fig.5). Slatesand Middle DevonianLimestones, the fold The most intensestrains recorded are from the hinges change to an NW-SE orientation (Fig. 4). volcanics of Blackpool Sandsand the least intense strains from the grits atSaltern Cove. These latter results may reflect the competency differences be- Slickensides and stretching lineations tween the grains and host matrix. The strain recorded Thedominant trend of the slickensides found on by the grits appears to be a plane strain, whereas all bedding surfaces is NW-SE (Fig. 4a). This is similar to the other strains are strongly oblate. This may be due the maximum extension direction of the strain ellip- to extension parallel to the strike of the cleavage but soid (X axis) as measured from strained objects (see in some of the slates the oblate character may also be next section) and the preferred orientation of minerals dueto volume loss (Ramsay & Wood 1973). It is on the cleavage plane. importantto note that differential movement on S of Torcross, however, the maximum extension thrust-nappes,together with layer-parallel compres- lineation is anomalous and pitches at a low angle to sion, can also produce oblate facrics (Coward & Kim the E. This is a real horizontal stretching, not just a 1981; Coward & Potts 1981). As we believe that many result of strain superimposition, as pyrites and fossil of these strains involve non-coaxial deformation, we orthocones show boudinage and fibre infills in this do not consider it valid to use strain integrating tech- direction. niques to remove the effects of deformation in this

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nian.A weak second cleavage (post-depositional) is superimposed upon this early (pre-depositional) cleavage in the lithic fragments. S of Brixham the rocks are dominantly slates, siltstones and arenites. The fine-grained lithologies dis- play a well-developed penetrative cleavage butto- wards Dartmouth crenulationa cleavage (almost parallel tothe slaty cleavage) is also developed. In many cases it is difficult to detect, although it has a AA

more widely-spaced character than the slaty cleavage. .@@0 O A 0 It is particularly noticeable where second phase folds -.. deform the slaty cleavage, e.g. in the section between Long Sands and Scabbacombe Head. At Dartmouth the red slates and siltstones are characterized by apenetrative cleavage. Later kink and FIG. 5. Flinn plot to show the shapes of the strain crenulation folds becomemore intense southwards ellipsoids. and are well developed at Blackpool Sands. In thin section the slaty cleavage is outlined by bands of area and not enough data are available to use factor- phyllosilicates (dominantlysericite) which are crenu- ization techniques,as described by Coward & Kim lated by the later deformation. (1981). The deformation intensity increases southwards, particularly in the complexly folded section from Tor- cross toStart Point. Hereat least two penetrative Deformation microstructures schistosities are formed. In the psammitic layers the Microstructural studies of the rocks described above quartz grains exhibit undulose extinction, overgrowths confirm the structural sequence deduced from macro- and sutured grain boundaries. Phyllosilicates, how- scopic fabric elements. ever,are abundant and define either apenetrative In the Torquay-Brixham area the limestones gener- spaced schistosity (dueto metamorphicsegregation) ally develop only a poor cleavage, the intensity of or later crenulation schistosities. Towards Start Point which decreasesnorthwards. Stylolites, fracturesand several overprinting schistosities are observed in thin veins areabundant. However, local bedding-parallel sections but become increasingly difficult to separate. zones of intense shear strain (Fig. 6A) probably repre- In summary, the microstructuresillustrate an in- sent the response of the limestones to thrusting. creasing amount of internal deformation from N to S. Highly strained corals,bioturbation structures and Slightly higher metamorphic grades are also indicated, intraformationalconglomerates are found. The lime- suggesting the involvement of deeper crustal levels stones are intenselyrecrystallized, with grain sizes towards the S. varying from -10 mm to 1 mm size depending upon the original grain size, micrites to crinoidal pack- Major folds and thrusts stones. In intensely sheared limestonesstrong shape fabrics are developed (Fig. 6B), often with large calcite Fig. 7 shows a simplified composite cross-section be- porphyroclasts surrounded by a fine grained matrix tween Oddicombe in the N and Dartmouth in the S, (Fig.6C). Sorby (1856) originally recognized the de- after the removal of the post-Permian normal faults. formed nature of the limestones in S Devon. This is only one of several possible cross-sections. It is, Inthe slates and sandstones(Lower Devonian however, the simplest, involving only one major Meadfoot Beds and Upper Devonian slates), a single thrust, with several minor imbricate faults. This thrust, S-dipping cleavage is developed. Inthe arenaceous which includes the Marldon Beacon Nappe of Vachell units it is commonly aspaced cleavage (cf. pressure (1963)is termedhere T1 (Figs 2, 7, 10, 11, 12). solution cleavage, Cosgrove 1976; Gray 1977; Gray & According to the section, shown in Fig. 7, the thrust Durney 1979). which puts Dartmouth Slates on Meadfoot Beds near Thin sections of the Saltern Cove (Upper Devonian) Dartmouth is also the one which puts Meadfoot Beds sequencehave revealed an extremelyimportant fea- on Middle to Upper Devonian rocks in the fold klippe ture. These pebbly mudstones contain slaty fragments at Torbay. It may be associated with the thrusts which of varying compositions including presumed Middle carry Middle Devonian limestones over Upper Devo- Devonianlimestones. A few of the slaty fragments nian slates, N of Torquay Headland, though this cor- (probably Meadfoot Beds) have a pre-existing cleav- relation is uncertain. The section is based on simple age (Fig. 6D), which indicates thatthe deformation correlations across the normal faults; it is possible to had begun prior to the deposition of the Upper Devo- construct other more complex cross-sections involving

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FIG. 6. Microstructures in thethrust rocks: A, Strongly sheared Middle Devonianlimestones with elongate bioturbation structures parallel tothe foliation. Negative print of thin section (Grid Reference SX 913663). B, Mylonitic MiddleDevonian limestone with strongly developed foliation of fine-grained calcite with large porphyroclasts of calcite-riginally crinoid ossicles. Plane polarised light (Grid Reference SX 844701). C, De- formed Middle Devonian limestone (similar to A and B) but showing fine-grained elongate calcite grains defining the foliation.Crossed polars (GridReference SX933653). D, Deformed breccia-pebble bedfrom Upper Devonian slates, Saltern Cove. The fragments of volcanics and limestones have a pre-existing cleavage indicating deformation of the parent rock before re-deposition. Negative print of thin section. (Grid Reference SX 896582).

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. ---X, anlh w:m cor W*Nose omrombe l l

FIG.7. Simplified cross section between Dartmouth and Oddicombe after the removal of post-Permian faults. Two alternatives are given for the structure of Torquay: A; puts the Meadfoot Beds of Torquay Headland in the hanging wall of thrust T1. This is the preferred model: B; puts the Meadfoot Beds of the Headland on thrust slices below T,. The amount of movement on the St Mary’s thrust (see appendix note k and section P, Fig. 2) is uncertain. The arrows indicate the way-up of the Meadfoot Beds round the Man Sands fold. several thrusts. Note that the section shown in Fig. 7 formation, the fault may cut through the overturned shows the contact between the Dartmouth Slates and fold limb.This mechanism for fold development is the Meadfoot Beds N of the Dart Estuary as a thrust beautifully exemplified by the fold-thrust relationship rather than as the overturned limb of a pre-cleavage at Hope’s Nose. It has been described by Willis (1893) fold, for which there is no local evidence. from his Appalachianstudies and by Fischer & Co- The section shown in Fig. 7 obeys some of the rules ward (1982) and Coward & Potts (1981) from the folds of thrust geometry (Dahlstrom 1970) in that the thrust in the MoineThrust zone. If another thrustthen climbs up section so that the hanging wall contains the develops at a lower structural level (Fig. 8c), this could Dartmouth Slates in the S, but Middle Devonian develop its own fold with its own cleavage. Thus limestones at Torbay. However, at Torbay (Fig. 2) the locally, asN of Anstey’s Cove, there may be poly- Beacon Hill Nappe overlies a footwall of Upper Devo- phase deformation. In this area there was pre-cleavage nian in the S, near Saltern Cove, but overlies Middle folding related to the transport of Middle Devonian Devonian rocks near Beacon Hill. Assuming this map limestonesover Frasnian red shales.This produced is correct, the fault cuts down the stratigraphy in its strong, bedding-parallel slickenside fabrics in the footwall.However thearea inlandfrom Torbay is badly exposed: there may well be lateral ramps in the BUCKLE FOLD footwall and we feel there is not enough evidence to say whether or not this fault has local contractional or a extensionalgeometry (see Dahlstrom 1970, and Co- STICKING ON A ward, 1982, for detailed discussion on the geometry THRUST of extensional faults). Assuming a fault geometry as shown in Fig. 7, the displacement on the thrust, T1, is at least 13 km as determined from displacement of the Torquay klippe. Cleavage, folding and thrusting are approximately coeval, in that the cleavage is axial planar to the folds, which are oftenthrust along their short, overturned limbs. However, this does notmean that cleavage, folds and thrusts are the same age along the whole of c the section. It has been noted, N of Torbay, that some foldsare pre-cleavagein age andsome folds are post-cleavage.This situation is to beexpected in a NEW FOLD ABOVE LOWER THRUST thrust zone where the thrusts propagate in the trans- FIG. 8. Sketches to show thesequential development of folds and cleavage in athrust zone. port direction and folds are related to somesort of (a) Development of a fold abovea thrust tip. sticking mechanism on a thrust. Considera fault in (b) Further deformation causes the thrust to slice Fig. 8a. If this faultsticks, thenfurther movement through the more deformed overturned fold limb. would cause buckling of the beds near the fault tip. A (c) Late folds and cleavage develop above a later clearage may or may notdevelop. With further de- lower-level thrust.

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START IWINT PAIGNTON BRIXHAM DARTMOUTH mm

FIG. 9. Suggested cross section between Start Point and Chudleigh; section line S-S’ on Fig. l

limestones. There was thenthe main cleavage de- Though the contact between the Start Point schists formation at Anstey’s Cove, presumably developed on and the Meadfoot Beds is now a late normal fault, it is alower decoupling horizon,and this cleavage was considered that the original contact must have been a later folded by a local thirdphase of deformation, major shearzone orthrust; the Start Point schists Note that, if these fold phases were developed in the show amore complex structural history and higher way proposed, they should be only local phenomena. metamorphic grade than the Meadfoot Beds. Near the There shouldbe no large scale correlation of fold Start Point schists, S of Beesands, the Meadfoot Beds phases; some rocks may show severalphases while show pre-main cleavage folding and locally at least two beds nearby may show only one phase. phases of cleavage age folds;a more complex de- Similarly, the Man Sands Antiform underlies, folds formation history than at Torcross. It is suggested that and must post-date the thrust T1, yet it has a cleavage these extra deformation phases were developed close similar in style and orientation to thecleavage in rocks to,but beneath the Start Pointthrust. The outcrop of the hanging wall to T1. The Man Sands fold is position of these major thrusts is shown in Fig. 10. presumably related to the recumbent folds and thrusts N of Brixham the folds and thrusts dip only gently, between Brixham and Saltern Cove, though it is not but to the S they are moderately inclined. This change known whether these imbricate faults cut the thrust T1 is partly due to folding by the Man Sands Antiform, or join it to form a roof-thrust. However, the thrust T1 but further S the folds and cleavage are re-folded by and the underlying folds and thrusts are folded by a inclined,southward-verging crenulations. These ‘F3’ yet later set of northward-verging, open F2 structures folds are often the dominant structures in the rocks. at Saltern Cove (Figs 2 and 7). S of Dartmouth they fold the earlier cleavage into a Fold axes and bedding cleavage intersections gener- large southward verging antiform so that at Torcross ally trend to N 70-N go”, almost normal to the move- the cleavage is northward-dipping. The F3 structures ment direction as indicated by slickensides and elonga- may be analogous to the ‘backfolds’ or ‘retrocharriage’ tion directions. However, near Torquay Headland the zones in the Alps. This major backfold may be traced fold hinges trend 130-140” and the folds verge NE. It across South Devon to the Plymouth and the Perrin- is possible thatthe thrust movementdirection may porthareas in Cornwall, though its wavelength and havechanged in this northernarea, but it is more likely thatthe foldsformed oblique tothe thrust movement (or rotated after initiation), probably due to differentialmovement (cf. Coward & Kim 1981). Fig. 9 shows our suggested cross-section through SE Devon from eastern Dartmoor to Start Point. To the N of Torquay the Devonian and Carboniferous rocks are intensely deformed into a series of north-westward verging inclined folds and thrusts. Waters (1970) describedfour important thrusts in the Chudleigh- .?h Hulme district. These thrusts must underlie the thrust T1, (unless T1 is downfaulted by some large yet unde- tected normal fault), and hence they may be part of .

the same thrust and fold system which underlies the ~~~~ thrust T1 in Torbay. Fig. 10 suggests that these lower FIG. 10. Map of SE Devon showing outcrop posi- thrusts link together along a gently dipping decoupling tion of themajor thrusts. Stipple shows where zone T2. It is realized that thisconcept of a single younger (305-285 Ma) mineral ages have been lower thrust may be a gross oversimplification. obtained (after Dodson & Rex 1971).

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amplitudedecrease westwards (Shackleton et al. range in ages across Cornwall and Devon may reflect, 1982). It is this structure which has imposed an E-W therefore, different phases or pulses of movement. grain to the tectonics of South Devon. Away from this The earliestdeformation presumably occurred in structure, as in the Torbay-Newton Abbott area, the the S of Cornwall, in the Lizard and Dodman Point structures trend NE-ENE (Fig. 10). thrust zones (Sanderson & Dearman 1973) where ages of 365-345 Ma havebeen obtained from the slates (Dodson & Rex 1971). In thesethrusts deep water Age and sequence of thrusts sediments, volcanics and basic igneous rocks of ocean floor affinities (Bromley 1977) were carried over Mineral ages on the slates (Dodson & Rex 1971) in the Lower Palaeozoic greywackes and slates. There is area range from 330 to 286 Ma (down to 260-250 Ma evidence for this early deformation in SE Devon, in near the later Dartmoor Granite). In SW England as a that somepebbles within Upper Devonian slates of whole the mineral ages on slates range from 365 Ma to Saltern Cove contain well-foliated Devonian lime- 270 Ma(Dodson & Rex op. cit.). In general, the stones (Fig. 6D). These may represent flysch-like de- oldest ages occur in the S, near the Lizard Headland, posits eroded off the front of the advancing thrusts. the youngest occur in folded mid-Carboniferous rocks In SE Devon, the Start Point schists were probably of N Cornwall and Devon. Thus there is an apparent carried over the lower Devonian rocks, producing the diachroneity in cleavage formation(Sanderson & pre-cleavage folds and possibly the first cleavage age Dearman 1973). However, young mineral ages of folds in theadjacent Meadfoot Beds (Fig. 11). The 305-285 Ma have been obtained from the Start Point nature of any thrust or shear at Start Point is uncer- area (Dodson & Rex 1971) and also from N of Dod- tain; we certainly donot invoke any majorStart man Point in Cornwall. These young cleavage ages Point-Lizard thrust.The deformationthen prop- coincide with the area of F3 crenulations, kinking and agated northwards to produce the thrusts T1 and the back-folding and so these ages may representre- underlying Man Sands Fold and imbricate thrusts. The setting ages due to F3 kinking and metamorphism. The northward-verging folds and thrusts at Torbay are part

TORCROSS FOLLX MAN CLEAVAGE

MAN SAWANTIFL”

0UPPER DEVONIAN G MIDDLE - MEADFOOT BEDS m DARTMOUTH SLATES START WINT SCHISTS

FIG. 11. Schematic sections through SW Devon showing the proposed sequence, and thrusts and folds.

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of this sequence (Fig. 11). Theslates near Torbay have has many of the characteristics of a typical foreland- yielded ages of 34G310Ma (Dodson & Rex 1971) fold andthrust belt. However, insufficient dataare andN of Torbaythe deformation must be post- available to fully assess the plate tectonic implications Namurian, as Namurian Culm-Measure rocks are in- of the thrustnappes. For some discussion of this volved in the thrusts. problem, see Shackleton et al. (1982). The Hercynian The folds and thrusts were later back-folded, prob- belt of SW England lies in the continuation of the ably atabout 300 Ma asages of310-295 Ma have Appalachians, where displacements of over 200 km on been obtained from regions where small-scale F3 folds low-angle thrustfaults have beendetermined from areprominent. This back-folding involved a major detailedaccurate structural mapping (Roeder et al. change in structural vergence. Similar large, south- 1978) and from COCORP deep seismic reflection pro- ward-verging folds and thrusts have been recognized in filing (Cook et al. 1979). Future deep seismic reflection NCornwall (Sanderson & Dearman 1973), andat studies in Britain may either confirm or negatethe Tintagelthese southward-verging structures form a presence and importance of the thrusts in SW England major shear zone. S of Tintagel the rocks also show and may possibly also reveal other, deeper level, blind intense, flat-lying fabrics and strains which Ries & thrusts to the N (i.e. in N Devon and S Wales). Shackleton(unpublished lecture to TectonicStudies Group, December 1979) and Shackleton et al. (1982) ACKNOWLEDGMENTS.We wish to thankDrs R. Knipe and consider to be part of thesame southward-verging M. Leeder for critical reviews and comments. shear.However, no major southward-vergingshear hasbeen recognized in SE Devon;either this shear passes beneath the present erosional level or the displacement dies out eastwards. Appendix Notes on critical localities for the structure of Torbay-Start. Conclusions Localities are labelledon Fig. 1 andare referredto the In this paper we have demonstrated that the tectonics British National Grid, grid letters SX. of the region from Start Point to N of Torquay are (a) Coastsection between Oddicombe and Babbacombe shows folded limestones in the cliff section overlying Upper characterized by northward-verging thrust nappes. At Devonian(Frasnian) shales. The way up of the shales is least two major thrustfaults are indicated:a basal uncertain; some fine, graded bedding (at 928655) shows the decouplingzone (of undeterminedposition) and a beds to be inverted. Way-up criteria in basalt-shale contacts large thrust, T1, which rootsat least as far S as (927656) are ambiguous. Dartmouth with a minimum of 13 km displacement. (6) Limestones on the S side of Long Quarry Point, the N The deformation is diachronous, with the least de- side of Anstey’s Cove (935649), overlie Upper Devonian red formation in the N and the most intense and complex shales. The relationships are confused by slumping and nor- in the S at Start Point. Southward-verging back-folding mal faults. The limestones at Long Quarry Point are the right has deformed the thrust nappes at Start Point. way up (Kershaw & Riding 1980), though near the contact with the Upper Devonianshales their way up is uncertain The fold axesand beddingicleavage intersections andhere theyshow a locally intensepre- main cleavage are almost normal to the movement direction, except bedding-parallel fabric. in the northern part of the area, where oblique fold- (c) Anstey’s Cove (935646). Limestones overlie Upper axes suggest differentialmovement of the thrust Devonian shales. Is this an inverted stratigraphic contact or a nappes. thrust? Bedding cleavage relationships in the shale on the S Microstructuralstudies indicate thatthe deforma- side of the cove indicate that the rocks are the right way up, tion was initiated prior to the deposition of the Famen- but sedimentary criteria are ambiguous. If at a. b and c the nian sediments. The easy glide horizons for the thrust Middle to Upper Devoniancontact is an invertedstrati- faultsinclude both theshalehlate horizons of the graphic contact, then the Torquay rocks form the core of a large pre-cleavage fold nappe. If the rocks near the contact LowerDevonian sequence and also beds within the are dominantly the right way up, thecontact is a major Middle Devonianlimestones. Strongly-deformed, thrust. See Figs. 2 and 7. mylonitic limestones are found around Torbay. (d) Valley (943640), S of Anstey’s Cove, along a late The recognition of major, northward-verging thrust normal fault, the sense of displacement on which is uncertain nappes in this region has important implications for butseems to downthrowto the S. This fault marks the the palaeogeographic and palinspastic interpretations contactbetween the complex structureN of theTorquay of theDevonian. In particular, the detailed Headland and the relatively simple structure to the S. palaeogeography, palaeoecology and facies recon- (e) S side of Hope’s Nose (947634) shows excellent expo- sure of fold in hanging wall, with minor thrust in Middle structions of the MiddleDevonian limestones and Devonian limestone. Upper Devonian slates need to be reconsidered, and cf, Daddyhole Cove to Triangle Point (927628 to 928628). palinspastic maps and sections (cf. Hossack 1978) con- Beddingicleavage relationshipsindicate the Middle Devo- structed for the Devonian rocks. nianlimestones to be right way up butgeopetal evidence The Hercynian fold and thrust belt of SW England indicates the rocks to be inverted. (See text for discussion.)

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The contacts with the Meadfoot Beds are generally obscured the underlying shales. The suggestion is that most of the by faulting. limestones of Sharkham Point, Berry Head and Brixham are (g) Permiansandstones and the main Torquay town and allochthonous compared to the underlying shales, though the harbour hide the relationship between the Lower Devonian amount of displacement is unknown. rocks of theBeacon Hill Nappeand those of Torquay (l) Contactbetween Meadfoot Beds and Dartmouth Headland. Slates, S of Long Sands (922517). Beddingicleavage rela- (h) Waterside Cove (896582), N of Saltern Cove, shows a tionships indicate the rocks are on the right way up limb of a late, normalfault with post-Permiandownthrow to theN, major antiform (the Man Sands anticline). Sedimentary way juxtaposing Lower Devonian rocks against Upper Devonian up criteria have not been found near the contact. shales in the S. The shales contain pebble beds of limestone (m) Contact betweenMeadfoot Beds and Dartmouth and Upper Devonian mudstone. slates at Kingswear (882509) andDartmouth (875513) (i) Elberry Cove (Y03571) shows minor thrusts of Middle obscured by buildings but outcrops on roadsides, the river- Devonian limestones overUpper Devonian red calcareous side and nearby gardens show the same as at locality 1. muds, with associated folds. (n) S of Torcross (824417). Meadfoot Beds deformed by (j) Quarry at Berry Head (945566) shows limestones with oneset of cleavage-agestructures. Fold plunge generally tight to isoclinal, nearly recumbent folds. horizontal but locally extremely variable. Elongate volcanic (k) Sharkham Point. On the S side (937546) the Middle clasts and fibres to pyrites crystals indicate a stretching com- Devonian limestones are stratigraphically underlain by volca- ponent E-W. nics and grits, and sandstones and shales of the Meadfoot (0) Cliffs S of Beesands (819400). Intenselydeformed Beds. On the N side (at 932548) the limestones are under- Meadfoot Beds show a prominent cleavage, axial planar to lain by Lower-Middle Devonian(Emsian) shales of St folds, but also evidence of a pre-cleavage deformation, in Mary’s Bay. This is a major facies change. The structure of that gradedbedding indicates variable younging directions Sharkham Point is an inclined syncline (Fig. 2, section P). while on the same cleavage-age fold limb (see section S, Couldit also represent a major tectonicbreak where the Fig. 2). Meadfoot Beds and Middle Devonian limestones are thrust (p) N side of Hallsands Bay (818392). A steep late fault over Emsianshales? The exactcontact of limestones with juxtaposes Start Point schists next to MeadfootBeds. The shales in St. Mary’s Bay is obscured by landslips but there is structures of both rock groups are dominated by southward- no simple match of the structure between the limestones and verging late crenulations.

References

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Received 16 June 1982. M. P. COWARD, Departmentof Earth Sciences, The University, Leeds LS2 9JT. K. R. MCCLAY,Department of Geology, University of London, Goldsmith’s College, New Cross, London SE14 6NW.

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