Deformation and emplacement of the Lizard Ophiolite ...

Journal of the Geological Society, London, Vol. 154, 1997, pp. 871–885, 14 figs. Printed in Great Britain

Deformation and emplacement of the Lizard Ophiolite Complex, SW England,based on evidence from the Basal Unit

K. A. JONESGeology & Cartography Division, School of Construction & Earth Sciences, Oxford Brookes University,

Gipsy Lane Campus, Headington, Oxford OX3 0BP, UK (e-mail: [email protected])

Abstract: The Basal Unit of the Lizard Ophiolite Complex preserves evidence of a protracted deformationand accretion history. The earliest recognizable events record the construction of ocean crust and thegeneration of the Lizard Ophiolite Complex in a slow-spreading ridge-axis environment. The initial stagesof deformation involved the development of a sub-horizontal crustal-scale shear zone and its progressivedeformation by ductile thrusting and folding. Subsequent deformation progressed with the out-of-sequence stacking of ocean lithosphere in which detachment, initiated at mid-crustal levels, culminated inmantle decoupling as thrust slices were derived from progressively deeper crustal levels. The progressiveemplacement of a coherent slice of ocean lithosphere over previously accreted units and the generation ofa dynamothermal aureole are consistent with models suggesting rapid overthrusting of young oceanlithosphere. Evidence from the dynamothermal aureole indicates that syn-emplacement extensionalcollapse of the complex occurred in response to thrust loading. Foreland-directed offsliding and lateralthrusting mark the final stages in the collapse of the complex.

Keywords: Cornwall England, Variscan orogeny, ophiolite, deformation, oceanic crust.

The processes associated with the generation and emplacementof ophiolites are not well understood, although many modelshave been put forward to account for the phenomenon (e.g.Coleman 1971; Dewey 1976; Moores 1982). It is generallythought that the generation and emplacement of ophiolites isa two-stage process. Obduction sensu stricto involves theemplacement of ocean lithosphere over a continental margin,whereas a pre-obduction or intra-oceanic stage records thegeneration, detachment and deformation of ocean lithospherein the intra-oceanic environment. The dynamothermal aureoleor metamorphic sole of an ophiolite complex records theearliest stages of the emplacement history (e.g. Williams &Smyth 1973; Spray 1984; Suhr & Cawood 1993) and isproduced during displacement of the ophiolite over oceaniccrustal rocks during thrusting in an oceanic environ-ment (Pamic 1977; Jamieson 1980), often well before its finalemplacement onto a continental margin (Wirth & Bird 1992).Where complete, the aureole displays an inverted lithological,metamorphic and strain gradient, and polyphase deformationall of which result from the progressive amalgamation of thrustslices at different levels in a large scale shear zone generatedduring obduction (Jamieson 1986 and references therein). Theheat source for metamorphism may have resulted from (a)residual heating from the young hot oceanic crust (Spray1984); (b) frictional heating generated during shearing (Pavlis1986), or (c) a combination of both (Hacker 1991). The finalarchitecture of an ophiolite results from its eventual emplace-ment over a continental margin along a series of high levelthrusts (Moores 1982).The aim of this paper is to establish the style of deformation

and the processes that operate during the early intra-oceanic orpre-obduction stage of ophiolite development. In order toachieve this aim, this paper draws on the results of detailedmapping of key localities which expose different levelswithin the lowermost or Basal Unit of the Palaeozoic LizardOphiolite Complex in SW England. The results illustrate that

the Lizard Complex preserves a rare and near-completehistory of high-temperature, intra-oceanic, pre-obductiondeformation.

Geological settingThe Lizard Complex or Nappe constitutes the higheststructural level exposed of the Variscan nappe stack of SWEngland (Holder & Leveridge 1986). The Lizard ultra-mafic–mafic complex has been interpreted as the fragments ofan ophiolite suite (Bromley 1979; Styles & Kirby 1980). Thecomplex comprises three tectonic units, each separated bysub-horizontal thrusts (Bromley, 1979); the Crousa DownsUnit, the Goonhilly Downs Unit and the Basal Unit (Fig. 1a).The structurally highest, the Crousa Downs Unit, comprises arelatively undeformed section of ocean crust, consisting ofperidotite, gabbro and sheeted-dyke complex (Kirby 1979a).The Goonhilly Downs Unit has been interpreted as a deformedand dismembered ophiolite sequence and consists of inter-leaved serpentinized peridotite and associated mafic and ultra-mafic cumulate complex (the Traboe cumulate complex) and asequence of deformed pyroxene granulites (Green 1964a;Leake & Styles 1984). According to Bromley (1979) the CrousaDowns and Goonhilly Downs Units are separated by thePorthoustock–Kennack Fault, although the nature and signifi-cance of this boundary has been questioned (Leake & Styles1984; Leake et al. 1992). Recently, Power et al. (1996) havesuggested the Crousa Downs Unit represents the downfaultedupper level of the Goonhilly Downs Unit. The base of theGoonhilly Downs Unit is marked by the Basal Thrust (Sanders1955) (Fig. 1b). A comingled mafic and felsic magma suite, theKennack Gneiss, has been intruded along the Basal Thrust (seeFloyd et al. 1993 and references therein).The Basal Unit is composed of hornblende schists of Lower

Landewednack type (see below) and metasedimentary and

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metavolcanic rocks of the Old Lizard Head Series (Tilley 1937;Flett 1946; Green 1964a; Kirby 1979b); a Series that has beeninterpeted as pillow lavas, dykes and interflow sedimentstypical of the upper parts of an ophiolite sequence (Kirby1979b). The hornblende schists of the complex were originallysubdivided into the Landewednack and Traboe types (Flett &Hill 1912). The Landewednack schists were interpreted asmetabasalts and tuffs whereas the Traboe types were inter-preted as deformed gabbros (Flett & Hill 1912; Flett 1946). Onthe basis of structural position in relation to the peridotite,Bromley (1979) further subdivided the Landewednack schistsinto Upper and Lower Series. The Lower LandewednackSeries (including Traboe types) and the Old Lizard HeadSeries are thought to represent fragments of a dynamothermalaureole formed beneath a hot overridding peridotite sheet(Bromley 1979; Styles & Kirby 1980). The Upper Series isthought to represent gabbros, cumulates and pyroxenegranulites (Flett 1946; Green 1964a; Kirby 1979a, b; Styles& Kirby 1980; Vearncombe, 1980; Leake & Styles 1984)deformed by ductile extension in the lower oceanic crust in aslow-spreading ridge-axis environment (Gibbons & Thompson1991; Roberts et al. 1993; Hopkinson & Roberts 1995). AnSm–Nd mineral isochron obtained on gabbro has yielded anage of 375&34 Ma (Davies, 1984) for the formation of oceancrust.

The lowermost boundary of the Lizard Complex consists ofa semi-brittle/ductile thrust, the Old Lizard Head Thrust(Jones 1994) (Fig. 1b). A suite of foliated dioritic to tonaliticgneisses, the Man O’War Gneiss, occurs in the immediatefootwall to the Lizard Complex (Fig. 1b). Recent U–Pb datingof zircons within the Man O’War gneiss has yielded an age of499 +8/"3 Ma, which is interpreted to record crystallizationof the igneous protolith (Sandeman et al. 1995a). Sandemanet al. (1995a) have suggested the Man O’War gneiss is possiblya fragment of basement incorporated into the Lizard Complexduring obduction at c. 366 Ma.The Lizard complex is thought to have formed the floor to

the late early Devonian (c. 375 Ma, see above) Gramscathobasin (Rattey & Sanderson 1984; Barnes & Andrews 1986;Holder & Leveridge 1986). The initial stages of accretion ofthe complex are thought to have involved the amalgama-tion of ‘hot slices’ of crust in the sub-oceanic domain(Vearncombe 1980). 40Ar/39Ar age spectra obtained fromamphiboles from the Basal Unit record cooling throughc. 500)C at c. 366 Ma (Sandeman et al. 1995b) and constrainthe timing of accretion or the intra-oceanic pre-obductionstage. Cold obduction followed (Barnes & Andrews 1984)and final emplacement of the complex occurred at the top ofa series of northward-directed thrust nappes (Holder &Leveridge 1986). K–Ar whole rock data obtained on samples

Fig. 1. (a) Location of the study area. (b) Structural subdivisions of the Lizard Ophiolite Complex (modified after Bromley 1979). (c) Simplifiedgeological map of the tectonic subdivisions within the Basal Unit and lowermost part of Goonhilly Downs Unit. Location of detailed maps Figs4, 5, 6, 7 and 9 indicated.

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from the Lizard Complex are interpreted as indicating lateDevonian (c. 362 Ma) uplift of the complex (Warr et al.1991).

Structural subdivisions of the Basal Unit

Recent mapping has provided the basis upon which a defini-tion of the major tectonic units which comprise the Basal Unithas been established (see Figs 1b & 2a, b). The Basal Unit isdefined as all tectonic units or nappes occurring in the footwallto the Goonhilly Downs Unit or Basal Thrust and occurringstructurally above the Old Lizard Head Thrust (Figs 1b & 2a).The Basal Unit of the Lizard Complex comprises two allo-chthonous tectonic units, the Lower Nappe Series and theUpper Nappe Series (Figs 1b & 2a). The Lower Nappe Series,composed of slices of ocean floor stratigraphy (see Fig. 3a) isdivided into two nappes; the Polurrian Nappe and the MullionNappe (Fig. 2a). The structurally lower Polurrian Nappe iscomposed of a sequence of hornblende schists that areinterpreted as deformed gabbro-dyke complex and volcanicsequences (Fig. 3a). The overlying Mullion Nappe consists of anear-complete slice of ocean floor stratigraphy; serpentinizedperidotite, hornblende schists (deformed gabbro-dyke com-plex and volcanic sequence), and metasediments of the OldLizard Head Series (Figs 2 & 3a). The Mullion Nappe hasbeen emplaced along a major ductile thrust, the MullionThrust. Within the Upper Nappe Series two nappes aredistinguished: the Polbream Nappe and the Ogo-dour Nappe(Fig. 2a). The Polbream Nappe is composed of a sequenceinterpreted as a slice of the upper parts of the ocean floorstratigraphy: hornblende schists (deformed metagabbro-dykecomplex and volcanic sequence) and metasediments (Fig. 3b).The juxtaposition of the Mullion Nappe and Polbream Nappeoccurs along a major ductile thrust, the Polbream Cove–Predannack Cliffs Thrust (Fig. 2a). The structurally highestnappe of the Basal Unit, the Ogo-dour Nappe, consists entirelyof serpentinized peridotite and has been emplaced along asecond major ductile thrust, the Ogo-dour–Rynian Thrust(Fig. 2a).

Lithostratigraphy

The Lower Nappe Series

Polurrian Nappe. The coastal section from Polurrian Cove toMullion harbour provides a cross section through the invertedlimb of the Polurrian Nappe (Figs 3a & 4). In Polurrian Cove,epidote-bearing hornblende schists are intruded by severalgenerations of fine-grained dykes that crosscut compositionalbanding in the hornblende schists and often contain deformedphenocrystic plagioclase feldspar. The epidote-bearinghornblende schists pass transitionally into a 300 m thicksequence of strongly foliated dark massive hornblende schist,and into strongly deformed metagabbroic sequences in theimmediate footwall to the Mullion Thrust, north of MullionCove.

Mullion Nappe. The lithological sequence of the MullionNappe is interpreted as a near-complete slice of ocean floorstratigraphy; metasediments, hornblende schists and serpentin-ized peridotite (Fig. 3a & b). Rocks of the Mullion Nappe areexposed in two main areas, from Pentreath Beach to PolbreamCove (Figs 5 & 8a) and Mullion Cove to Ogo-dour Cove(Figs 4, 6 & 8b). The metasediments, exposed in a narrowcoastal strip from Pentreath Beach to Polbream Cove, consistof a series of pelites and quartzites, hornblende schists andtransitional lithologies (schists grading between amphiboliteand pelite of probable volcanic origin). Hornblende schists andtransitional lithologies dominate the lower parts of the succes-sion; pelitic and quartzitic lithologies increase in abundance upsection. In a small section on the southwest of Housel Bay,epidote-bearing hornblende schists of the Mullion Nappe areexposed in the footwall of the Polbream Cove–PredannackCliffs Thrust (see Fig. 7). A sequence of intrusive sheets anddykes of varying lithologies intrude the Old Lizard HeadSeries. The largest of these forms a sub-horizontal sheet whichis exposed along the length of the southern coast and on OldLizard Head, here termed the Old Lizard Head Basic Sheet.The Old Lizard Head Series and the Basic Sheet are crosscutby fine-grained leucogranite sheets (see Figs 5, 8a & 10a).

Fig. 2. Schematic tectonic stratigraphyof the Basal Unit and lowermost part ofGoonhilly Downs Unit. (a) Majorthrusts and tectonic units. (b) Structureand internal stratigraphy (open arrowheads indicate thrust contacts alongwhich extensional reworking has takenplace).

EMPLACEMENT OF THE LIZARD OPHIOLITE 873

In Mullion Cove, hornblende schists of the Polurrian Nappeand lenses of serpentinized peridotite of the Mullion Nappe aretectonically interleaved along the Mullion Thrust (Fig. 4). Thelowermost part of the Mullion Nappe is exposed in thehanging wall of the Mullion Thrust and consists of a thicksequence, up to 500 m, of massive and foliated serpentinizedperidotite.A 600 m section through the Mullion Nappe is exposed in a

‘tectonic window’, the Predannack Window, in the footwall tothe Polbream Cove–Predannack Cliffs and Ogo-dour–Rynianthrusts in the Predannack region (see Figs 4, 5 & 8a). Thehornblende schists in this region consist of intensely deformedmetagabbroic intrusives and crosscutting basic dykes of severalgenerations. Rare sheets of serpentinized peridotite crosscutthe sequence in the Ogo-dour region.

The Upper Nappe SeriesPolbream Nappe. The Polbream Nappe is composed of alithological sequence interpreted as a slice of the upper parts ofan ocean floor stratigraphy; hornblende schists (deformedmetagabbro-dyke complex) epidote-bearing hornblende schistsand metasediments (Fig. 3a & b). The stratigraphy of thePolbream Nappe is repeated and folded around the EastLizard Synform and the Cadgwith Antiform (Fig. 2). Rocks of

the Polbream Nappe are exposed in three areas; the largestexposure is along the southeast coast from Polbream Cove toChurch Cove (Fig. 7), limited exposure occurs at Cadgwith(Fig. 9) and a small exposure at Rynian (Fig. 4).The lowermost stratigraphic units are exposed in Church

Cove and Polbream Cove and comprise deformed metagabbroand a sheeted dyke complex (Fig. 3a) that passes transitionallyinto epidote-bearing hornblende schists. Both banded (gab-broic) and massive schists occur. These schist types passupwards into a sequence consisting of epidote-bearing horn-blende schists, massive epidosites, up to 30 cm in thickness,and thin (<2 m) metasediments. Fine-grained dykes crosscutthese sequences.At Cadgwith (Fig. 9) the stratigraphy is inverted on the

lower limb of the Cadgwith Antiform (Fig. 3b). The lowermostunit, occurring in the hanging wall of the Little Cove Thrust,consists of deformed gabbros. A sequence of massive darkhornblende schist is exposed in the footwall. The structurallyhigher sections of these schists comprise deformed gabbroicintrusives which pass downwards into massive dark horn-blende schists. The structurally lowermost (stratigraphicallyhighest) unit consists of epidote-bearing hornblende schists,epidosites and crosscutting dykes.A thin slice of the Polbream Nappe occurs in the vicinity

of Rynian (Fig. 4), within the Predannack Window. The

Fig. 3. (a) A correlation of lithostratigraphic subdivisions of the Basal Unit (stratigraphy of the Ogo-dour nappe is omitted). (b) Aninterpretation of the pre-deformation structural position of lithotectonic units.

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stratigraphic sequence is inverted with the structurally lower-most part of the sequence consisting of epidote-bearing horn-blende schists with minor epidosite horizons, and the upper

part consisting of hornblende schists with minor crosscuttingdykes.

The Ogo-dour Nappe. The Ogo-dour Nappe (see Fig. 6)consists of massive foliated serpentinite which is deformed by apervasive shear fabric. At the lowest structural levels, in theimmediate hanging wall to the Ogo-dour–Rynian Thrust,several generations of basic intrusives invade the serpentinite.Early dykes carry the shear fabric and contain granulite faciesassemblages. A later suite of intrusives crosscut the serpen-tinite and are weakly foliated. These later intrusives occur inboth the footwall and hanging wall of the Ogo-dour–RynianThrust.

Tectonic evolution

Deformation sequenceFive main deformation phases have been recognized in theBasal Unit. The earliest (D1) is associated with ocean-floor

Fig. 4. Geology of the region from Polurrian Cove to Men-t-heulheadland, NE Lizard.

Fig. 5. Geology of the region from Pentreath Beach to PolbreamCove.

Fig. 6. Geology of the region from Men-t-heul headland toPol Cornick.


construction processes, remnants of which are preservedthroughout the nappe pile. The second deformation phase(D2) corresponds to a pervasive ductile shearing and subse-quent ductile thrusting event. The main effects of this eventare recorded in the Lower Nappe Series, although relicts of D2are preserved within the Upper Nappe Series. D2 is syn-metamorphic and associated with high-temperature–low-pressure metamorphism. A third deformation event (D3) isrelated to the stacking of the Upper Nappe Series. Structuresascribed to D4 result from the emplacement of the overlyingGoonhilly Downs Unit and the development of a dynamoth-ermal aureole or shear zone in its footwall. A series ofdetachment, thrust and reactivation surfaces developed duringretrograde metamorphism are assigned to D5 and mark thefinal stages of collapse of the complex.

D1 structures

In the Mullion and Polbream Nappes, lithological and fine-scale compositional layering is interpreted as primary in origin(S0). Throughout the Polurrian, Mullion and PolbreamNappes primary structures are preserved in low-strain lensesand pods as is a consistent sequence of structures. In thelowermost parts of these nappes coarse ophitic to sub-ophitictexture is preserved within metagabbro pods that display staticamphibolite-facies overprinting and recrystallization. On themargins of these low-strain pods, a spaced amphibole fabricintensifies into a strongly developed foliation or shear fabric.An early suite of medium- to coarse-grained dykes, in somecases still preserving gabbroic and relict phenocrystic texture,crosscut, and are subsequently deformed within, the earlyfoliation. A later suite of fine-grained aphanitic dykes crosscutthese early structures. Within the lowermost parts of theGoonhilly Downs Unit a near-vertical north–south-oriented

penetrative shear fabric and associated near-vertical stretchinglineation (pyroxene augen) constitute the earliest recognizableD1 structures.

D2 structuresD2 is a single phase of progressive deformation in whichcomplex polyphase structural sequences, folds and fabrics weredeveloped during a single mid- to upper amphibolite-faciesmetamorphic episode. D2 structures are developed sub-parallelto the lithostratigraphy. Structures assigned to D2 are thefirst to have affected the post-D1 aphanitic dyke suite. D2structures are best preserved in the Mullion Nappe, in particu-lar in the regions of Most Southerly Point (Fig. 10) andPredannack (Figs 4 & 5). It should be noted that theseregions expose two different levels within the Mullion Nappestratigraphy (see Fig. 3a).The earliest recognizable D2 fabrics are preserved within

lenses of low D2 strain in amphibolites within the Old LizardHead Series. Towards the margins of these lenses an early D2fine-grained amphibolite-facies schistosity S2a is initiallycrenulated, then progressively folded into chevron-like andtight to isoclinal F2b folds (Fig. 10a & b). Away from theselenses a pervasive S2b foliation, developed axial planar to F2bfolds, is progressively deformed by a series of foliation parallelisoclinal and sheath-like folds F2c (Fig. 10b). A strong mineral(amphibole) lineation developed parallel to F2c axes plunges atshallow angles to the south (Fig. 11a). The preservation of eyeand anvil (Mies 1993) structures, and curvilinear fold hinges(Cobbold & Quinquis 1980) and the presence of syn S2bconcordant leucocratic ‘melt’ segregations (Fig. 10b) suggeststhat progressive ductile deformation occurred under upperamphibolite-facies conditions.In the Predannack region (Figs 4 & 5), S2a has not

been observed, the main fabric S2b intensifies towards the

Fig. 7. Geology of the region fromPolbream Cove to Church Cove.

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Men-t-heul Shear Zone or ductile thrust (Fig. 5). In low-strain lenses in the footwall coarse metagabbroic unitsdisplay decimetre-scale anastomosing shear zones. Onapproaching the thrust these shears coalesce to form a coarseS2b shear fabric which locally intensifies into mylonite inwhich S2b structures are transposed, flattened and refolded.The presence of syn-kinematic dynamically recrystallizedHbl (brown)+Pl+Cpx assemblages and the presence of meltsegregations migrating along minor shear planes indicatesthat deformation progressed under upper amphibolite-faciesconditions. D2b structures are progressively reworked in aseries of mid-amphibolite-facies anastomosing high-strainzones (S2c). S2b is progressively refolded by isoclinal andsheath-like F2b folds. A strongly developed mineral (am-phibole) and stretching lineation L2 is developed parallel tofold axes of both F2b and F2c fold types and plunges towardsthe south. Shear sense recorded from s-c mylonites andshear pods from throughout the Mullion Nappe indicate atop-to-the-north sense of shear.

Mullion Thrust. The Mullion Thrust (Figs 2 & 4) places therocks of the Mullion Nappe over those of the PolurrianNappe. The Mullion Thrust consists of an anastomosing seriesof ductile thrusts within which lenses of serpentinite, of theMullion Nappe, and hornblende schists, of the PolurrianNappe, are tectonically interleaved. Along the thrust contactD2b structures in both hornblende schists and foliated serpen-tinites are progressively overprinted and transposed intomylonite (S2c). A sharp pencil-line contact between foliatedserpentinite and amphibolite-facies hornblende mylonites hasbeen observed, although most contacts have suffered retro-grade (low amphibolite and greenschist-facies) reworking.Where preserved, on the shallow limbs of the West LizardSheath Fold, a strong mineral (amphibole) lineation (L2c) andthe axes of isoclinal and sheath folds (F2c) plunge at shallowangles <30) towards the south and north (Fig. 11d). Move-ment sense obtained from shear lenses and amphibole fishindicate a top-to-the-north sense of overthrusting on theMullion Thrust.

Fig. 8. Simplified cros-sections of thestudy area. (a) Lizard Point to ChurchCove. (b) Polurrian Cove to Ogo-dourCove. (c) Devil’s Frying Pan to KildownPoint. (d) Kynance Cove. Ornamentationas in Figs 4, 5, 6, 7 and 9.


The West Lizard Sheath Fold. D2 structures are progressivelyreoriented around the West Lizard Sheath Fold (Fig. 11d & e).The X–Y plane of the sheath fold is sub-horizontal with thefold nose or cap closing to the NNW and the X axis orientatedin a NW–SE direction (sheath fold terminology after Minnigh1979). A mid-amphibolite-facies S2d foliation, developed axialplanar to F2d chevron-like and asymmetric folds, intensifiesinto mylonite in a series of shear zones that are developed

parallel to the X–Y plane of the sheath fold, the most signifi-cant of which is the Predannack Shear Zone (Figs 6 & 8b). TheS2d mylonitic foliation, developed axial planar to isoclinal andsheath folds F2d, dips at shallow angles towards the southeastand a strong mineral (amphibole) and stretching lineations(L2d) plunge down dip (Fig. 11f & g). In the fold nose L2dlineations are progressively rotated into parallelism with thefold axis (Fig. 11f & g). Late quartz rodding and stretchinglineations developed on S2d surfaces in the fold nose and in thePredannack Shear Zone are orientated in an east–west direc-tion. On the upper limb of the fold, shear sense obtained froms-c mylonites and shear pods determines a top-down-to-the-southeast-directed shear whereas on the lower limb top-to-the-northwest sense of overthrusting has been determined.Consistent NNW vergence is displayed by F2d folds. Thesedata suggest that during the progressive growth of this struc-ture the fold nose advanced or protruded at a greater rate thanthe limbs. The increased curvature of L2d towards the nose isconsistent with progressive syn-late D2 protrusion of thisstructure.

D2 structures in the Upper Nappe Series. In the PolbreamNappe a similar deformation sequence as to that observed inthe Mullion Nappe is preserved in low-strain areas. Structurescorrelated with D1 are progressively deformed by D2 (a–c)fabrics. S2b dips at shallow angles <30) towards the south andL2b and c mineral (hornblende) lineations are developedparallel to F2b and c fold axes and plunge down dip (Fig. 11a).The S2 surface is folded around the regional scale recumbentisoclinal East Lizard Synform (Fig. 2b). In the core of thesynform L2b is developed parallel to the synform axis and thestructure is correlated as a large scale F2b structure.The Ogo-dour Nappe consists of massive foliated

serpentinite. The serpentinite is deformed by a pervasive shearfabric. Foliation surfaces and stretching lineations (pyroxeneaggregates) plunge at moderate c. 30–40) towards the south-west. It is only possible to speculate about the relativetiming of fabric development in the Ogo-dour Nappe; itseems plausible that these fabrics may correlate with the D2bstructures of the Mullion Nappe.

Fig. 9. Geology of the Cadgwith region.

Fig. 10. (a) Schematic structural sectionfrom Old Lizard Head to PolbreamCove. (b) Structures developed inamphibolite in area of low D2 strain.(c) Early D4 fabrics. (d) Progressivelydeformed phenocrystic assemblages inthe Old Lizard Head Basic Sheet.(e) Progressive development of D4structures in the Most Southerly PointShear Zone. (f) Extensional crenulationcleavages (e) deforming S4b.(g) Late-stage D4 structures deformingthe Old Lizard Head Basic Sheet.(h) Structures of the Old Lizard HeadThrust (e, extensional cleavages;T, minor thrust surface; stippled areaindicates granite sill).

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D3 structuresThe stacking history of the Upper Nappe Series is ascribed toD3. Evidence for the successive emplacement of the PolbreamNappe D3a, along the Polbream Cove–Predannack CliffsThrust, and Ogo-dour Nappe D3b, along the Ogo-dour–Rynian Thrust, is preserved within the Predannack Window(Figs 4, 5 & 8b). The structures are described from this regionas similar contacts in the region of Most Southerly Pointhave been subjected to intense reworking during subsequentdeformations.

The Polbream Cove–Predannack Cliffs Thrust. In the Predan-nack Window (Figs 4, 5 & 8b), this thrust truncates the D2structures of the Mullion Nappe. The footwall to the Thrust,the D2 surface is folded into a tight footwall F3a synform. Astrong mineral (hornblende) L3a lineation parallels the axes ofsmall scale parasitic isoclinal and sheath folds F3a and the axisof the footwall synform. L3a plunges at shallow angles to-wards the south (Fig. 12a). Along the ductile thrust contact,both isoclinal and sheath folds deform the D2 shear fabrics.The axial planes of isoclinal folds and the X–Y planes of sheathfolds parallel the S3a mylonitic fabric and the thrust contact.Shear sense obtained from shear pods, porphyroclast tails ands-c mylonites indicate top-to-the-north-directed overthrustingoccurred on the Polbream Cove–Predannack Cliffs Thrust. Inthe hanging wall the S2 surface is progressively deformed bytight to isoclinal and sheath folds F3a. L3a mineral (horn-blende) lineations parallel F3a fold axes which plunge atshallow angles to the south.

The Ogo-dour–Rynian Thrust truncates D2 structures of theMullion Nappe and D3a structures of the Polbream Nappe(Fig. 8b). A thin (<10 m) zone of banded hornblende myloniteoccurs in the immediate footwall to the overriding serpentinite

nappe. A weak mineral (hornblende) and stretching lineationL3b plunges down dip on mylonitic surfaces S3b, which dip atc. 50) towards the southwest (Fig. 13a), and shear senseobtained from s-c mylonites and shear pods indicates top-to-the-northeast-directed overthrusting on the Ogo-dour–RynianThrust.

D4 structuresD4 is a syn-metamorphic progressive deformation and isassociated with the development of a dynamothermal aureoleor shear zone in the footwall to the overriding GoonhillyDowns Unit. In the Lower Nappe Series the relative timingand metamorphic conditions developed during D4 are con-strained by the emplacement of the Old Lizard Head BasicSheet, a pre-D4 intrusion. Various levels of the D4 shear zoneare exposed throughout the complex and are described fromthe highest to lowest structural levels. In the immediatehanging wall of the Basal Thrust, at Cadgwith (Fig. 9) andalong a section from Pentreath Beach to Kynance Cove(Fig. 1b), early, near-vertical, D1 structures of the GoonhillyDowns Unit are increasingly deformed by a series of ductilemylonite zones that intensify towards, and are developedsub-parallel to, the thrust contact. A strongly developedstretching (pyroxene augen) L4 lineation is developed onfoliation surfaces and dips at shallow angles. Shear senseobtained from shear pods and shape fabrics indicates top-to-the-north and northwest-directed overthrusting on these earlyfabrics.At Cadgwith (Figs 8c & 9), in the footwall to the Basal

Thrust, D2b structures of the Polbream Nappe are progres-sively reworked in a series of spaced narrow anastomosingshear zones D4 which coalese and intensify into the Little Cove

Fig. 11. Lower-hemisphere equal-area projections of structural data of D2 to D4 history of the Lower Nappe Series. Arrows indicate sense ofrotation of structures during progressive deformation.


Shear Zone. On approaching the shear zone a weak S4aschistosity, developed axial planar to F4a folds, intensifies intoa pervasive shear fabric, which is progressively refolded by F4bisoclinal and sheath folds. A strong mineral (hornblende) L4lineation is developed parallel to F4a and b fold axes andplunges at shallow angles <15) towards the north and north-

west (Fig. 11d & e). The high grade nature of D4 in this regionis evident from the occurrence of clinopyroxene bearingleucocratic ‘melt’ segregations developed concordant with S4a.Evidence for the timing of emplacement of granitic sheets,

associated with the Kennack Gneiss, is observed in thissection. In the vicinity of the Basal Thrust, granitic sheets andoffshoots of the Kennack Gneiss are syn-kinematic, crosscut-ting early S4 and carrying later S4 fabrics; being sequentiallyemplaced during progressive D4 deformation. The compositeS4a/b surface and offshoots of the Kennack Gneiss are foldedaround the recumbent isoclinal Cadgwith Antiform (Figs 2a, b& 8c). In the fold core, L4 mineral lineations parallel the foldaxis and plunge at shallow angles to the northwest andsoutheast (Fig. 12d & e).Throughout the Polbream Cove to Church Cove section

evidence for progressive D4 mid-amphibolite-facies reworkingof D2 structures is preserved (see Fig. 12c). Towards thePolbream Cove–Predannack Cliffs Thrust deformation is pro-gressively more heterogeneous and partitioned into spacedhigh-strain zones separating areas of relatively lower strain. InPolbream Cove a high-strain zone is concentrated along andreworks the Polbream Cove–Predannack Cliffs Thrust.In the footwall to the Polbream Cove–Predannack Cliffs

Thrust the D3 stuctures of the Mullion Nappe, and the OldLizard Head Basic Sheet, are progressively deformed by aheterogeneous south to SE-dipping S4 foliation (see Figs 10 &11b, c). The Old Lizard Head Basic Sheet has been emplacedas a sub-horizontal sheet-like body and crosscuts the steeplyinclined D2 structure. S4a is the first fabric to have affected theprimary magmatic fabrics of the sheet (Fig. 10d). A series ofleucogranite dykes crosscut the Old Lizard Head Basic Sheet(Fig. 10a) and are themselves progressively deformed by D4.The dykes crosscut the S4a foliation within the basic sheet yetare pervasively deformed within the S4a/b shear fabrics, afeature suggesting syn-kinematic emplacement.In the surrounding metasediments S4, developed as a spaced

fabric axial planar to chevron-like and isoclinal folds F4a folds(Fig. 10b & c), intensifies into a mylonitic fabric with a series of

Fig. 12. Lower-hemisphere equal-areaprojections of structural data of D3 andD4 of the Upper Nappe Series. Arrowsindicate sense of rotation of structuresduring progressive deformation.

Fig. 13. (a) Lower-hemisphere equal-area projection of structuraldata of D3a and D5. Abbreviations: PPT, PolbreamCove–Predannack Cliffs Thrust; ORT, Ogo-dour–Rynian Thrust;GGs, Grew Graze extensional shear zones. (b) Structural data fromthe Old Lizard Head Thrust.

880 K. A. JONES

anastomosing shear zones, the most significant of which is theMost Southerly Point Shear Zone (Jones 1994) (Fig. 10a). Inthese high-strain zones the S4a surface and F4a folds areprogressively refolded by isoclinal type ‘a’ and ‘b’, and F4bsheath-like folds (Fig. 10e). Type ‘a’ folds have sub-horizontalaxes which are developed orthogonal to the main L4 lineation,and type ‘b’ are developed with axes parallel to L4. L4bmineral lineations are developed sub-parallel to F4b fold axes(with the exception of type ‘a’ isoclinal folds) which dip atshallow angles to the south and south-southeast (Fig. 11b & c)and are progressively reoriented towards the southeast duringsuccessive F4b and c fold development. Vergence obtainedfrom F4b and c folds in high-strain and shear zones indicatetop to the north and northwest sense of rotation. Shear senseobtained from s-c mylonites, ó porphyroclast tails and shearpods indicate top-to-the-north and northwest senses of shear.The mylonitic foliation S4 is deformed by a crenulationcleavage in which fold axes plunge at shallow angles to thesoutheast (Fig. 11c) and crosscut by a set of extensionalcrenulation cleavages (Platt & Vissers 1980), developed on afine scale, which dip at shallow angles to the southeast (seeFig. 10).At the structurally lowest levels of the D4 shear zone,

deformation is characterized by anastomosing high-strainzones consisting of low amphibolite- and greenschist-faciesphyllonites, these are concentrated along lithological contactsand within the micaceous metasediments between CaerthillianCove and Venton Hill Point (Fig. 5); at Polledan, and atHousel Bay (Fig. 7), where the Polbream Cove–PredannackCliffs Thrust has been intensely reworked. Shear sense fromquartz-rodding lineations and shear pods indicate top-to-the-northwest-directed overthrusting on these structures. On OldLizard Head, the composite S4 surface is refolded around a‘late’ asymmetric buckle folds F4c (Fig. 10a, e & g).

D5 structuresD5 structures preserve a record of late-stage collapse ofthe nappe pile recording either northwards foreland-directedoffsliding or westerly directed lateral thrusting.

The Basal Thrust and Goonhilly Downs Unit. The lowermostparts of the Goonhilly Downs Unit and the Basal Thrust havebeen subjected to intense reworking. In the Kynance region,the Basal Thrust has been imbricated and a ‘double’ thrustcontact is observed (Fig. 8d). In the structurally lowerthrust slice intense shearing and recrystallization of theserpentinitized peridotites has occurred. Along the westerncoastal section reactivation of the Basal Thrust is concentratedalong the thrust contact. On reactivation surfaces a weaklydeveloped stretching lineation plunges at shallow angles to thesoutheast. Shear sense obtained from shape fabric and shearlenses indicates top-towards-the-northwest thrusting. In thehanging wall of the Basal Thrust early D4 zones of myloniticserpentinite are crosscut by concentrations of narrow <10 cmretrogressive shear zones that dip at 20–40) towards the north.These zones are well developed in a section north of KynanceCove (Grew Graze). Preliminary data obtained on these zonesindicates top-down-to-the-north sense of movement.Along the eastern coastal sections from Church Cove

to Cadgwith, the Basal Thrust cuts down section on amajor semi-ductile/brittle detatchment or reactivation surfacewhich consists of anastomosing shear zones of mylonite and

phyllonite developed within a ‘melange’ of imbricated lenses ofserpentinite, hornblende schist and Kennack Gneiss (Figs 8c &9). Weakly developed stetching lineations and shear senseobtained from s-c mylonites, shape fabrics and shear lensesdetermine a top-down-to-the-north sense of displacement onthis structure.

Reactivation of the Polbream Cove–Predannack Cliffs Thrust. Afurther reactivation surface is developed along this thrust inPolbream Cove. On this surface deformation is concentratedinto a narrow zone (<10 m) along which phyllonite and s-cmylonite are concentrated. Similar top-down-to-the-northdisplacement is recorded on this zone (Fig. 13a).

The Old Lizard Head Thrust is a broad zone of complexsemi-ductile/brittle deformation which places the LizardComplex over the Man O’War Gneiss. Major thrust andfoliation surfaces dip at shallow angles to the northwest. Onthese surfaces a stretching and quartz rodding lineationplunges at shallow angles to the ESE (Fig. 13b). Shear sensedetermined from s-c mylonites and shear pods indicate top-to-the-WNW directed overthrusting on this structure. Majorthrust and foliation planes are crosscut by a spaced C* exten-sional crenulation cleavage that dips at 10–35) to the NNW(Figs 10h & 13b).

Interpretation and discussion

Generation, deformation and emplacement of the LizardOphiolite ComplexOrigin at a slow-spreading ridge-axis. Structures ascribed to D1record the construction of oceanic lithosphere. The recognitionof ophiolitic mylonites and ridge-parallel amagmatic exten-sional shear zones has led several authors to suggest an originfor the complex in a slow-spreading ridge-axis environment(Gibbons & Thompson 1991; Roberts et al. 1993; Hopkinson& Roberts 1995). A near-vertical north–south-orientedpenetrative shear fabric and associated near-vertical lineationD1 are the earliest structures preserved in the primary andrecrystallized serpentinites of the Goonhilly Downs Unit.Similar fabrics have been reported from peridotites throughoutthe complex (Green 1964b; Rothstein 1977; Vearncombe 1980;Roberts et al. 1993) and have been interpreted as resultingfrom solid state flow in the upper mantle (Rothstein 1977;Vearncombe 1980). These fabrics are consistent with near-ridge, forceful emplacement, high-temperature/low-stresshomogeneous fabrics (Nicolas et al. 1980; Ceuleneer et al.1988) that have resulted from plastic flow of upwelling mantlebeneath a spreading ridge (Boudier & Coleman 1981). Theconsistent orientation of these mantle flow fabrics would seemto point to a N–S-oriented palaeo-ridge-axis.

D2: a crustal-scale shear zone within the oceanic environment.Initial deformation of the ocean crust occurred within asub-horizontal crustal-scale shear zone and is characterizedby a high-temperature deformation history. The resultingpervasive shear fabric and associated ductile flow structurescarry a strongly developed mineral lineation that along withkinematic indicators provide evidence for northward-directedshear. The ductile thrusting or stacking of the Lower NappeSeries (see Fig. 14a) signifies the onset of significant strainpartitioning following ductile flow. The high-temperature up-per amphibolite-facies assemblages recorded within the ocean


floor sediments and throughout the ocean crust are inconsist-ent with models of ocean floor metamorphism (e.g. Humphris& Thompson 1978), and are thought to have been generated inresponse to deformation during the initial stages of oceanicslicing.During the later stages of D2 the Lower Nappe Series was

refolded on a regional scale around the West Lizard SheathFold (Fig. 14b). The progressive rotation of lineations on axialplanar foliations S2c and in localized shear zones indicate aprogressive north to northwestward propagation of this struc-ture, synchronous with mid-amphibolite-facies retrogressionand cooling of the Lower Nappe Series.

D3: emplacement of the Upper Nappe Series. The stacking ofthe Upper Nappe Series D3 is characterized by the sequentialout-of-sequence thrust emplacement of oceanic slices (Fig. 14b& c). Lineation and inferred transport directions on thePolbream Cove–Predannack Cliffs Thrust indicate a northerlytransport direction, whilst data from the Ogo-dour–RynianThrust indicate northwesterly directed thrusting. Tectonic in-terleaving of oceanic slices is envisaged. It can be inferred fromthe stratigraphy that the Polbream Nappe is a slice of the midto uppermost levels of the ocean floor (Fig. 3b) and that the

Polbream Cove–Predannack Cliffs Thrust represents a mid-crustal level detatchment. Boudier et al. (1988) have suggestedthat detachment or decoupling of oceanic lithosphere mayoccur at the c. 600)C isotherm, which according to Watts et al.(1980) separates elastic from plastic lithosphere. A similardetachment level may be envisaged for the generation of thePolbream Cove–Predannack Cliffs Thrust and the emplace-ment of the Polbream Nappe. The progression of detatchmentto lower structural levels is inferred from the emplacement ofthe Ogo-dour Nappe. The consistently oriented homogeneousshear fabric developed throughout the Ogo-dour Nappe isinterpreted as having an origin in a ductile shear zone. Theconcordance of fabrics in the Ogo-dour Nappe, with thoseof the Ogo-dour–Rynian Thrust, suggests progressiveemplacement along a single flow path (Fig. 14).

D4: mantle decoupling and the emplacement of the GoonhillyDowns Unit. The final phase in the construction of the complexis marked by the emplacement of an entire slice of oceanlithosphere, the Goonhilly Downs Unit (Fig. 14c & d). Thepresence of mylonitic serpentinites and a dynamothermalaureole along the base of this unit is consistent with re-cent models which suggest mantle decoupling and tectonic

Fig. 14. Deformation and emplacementhistory of the Lizard Ophiolite Complex.(a) Initiation of thrusting, along theMullion Thrust (MT), of oceanic slicesfollowing development of a crustal scaleshear zone. (b) Progressive overthrustingof the Mullion Nappe, note the inversionof the upper parts of the PolurrianNappe in the footwall to the MullionThrust (MT). (c) Refolding of the LowerNappe Series around the West LizardSheath Fold and the initiation ofout-of-sequence thrust stacking of theUpper Nappe Series, along the PolbreamCove–Predannack Cliffs Thrust (PPT)and the Ogo-dour–Rynian Thrust(ORT), and the Goonhilly Downs Unit,along the Basal Thrust (BT), as thehanging wall to the Basal detachmentprogresses to deeper structural levels.(d) The progressive overthrusting of therelatively coherent Goonhilly DownsUnit leads to the onset of partial meltingmanifest in the emplacement of theKennack gneiss and associatedintrusives, and the development of adynamothermal aureole in its footwall(not shown). The initial stages of internalcollapse along extensional shears isillustrated by the arrows. (e) Late stagecollapse of the nappe pile and itsemplacement onto the continentalmargin along the Old Lizard HeadThrust (OLHT).

882 K. A. JONES

emplacement occurring in close proximity to an extinct spread-ing centre (Boudier & Coleman 1981; Spray 1984; Suhr &Cawood 1993). These models show thrusting, facilitated bymelting, initiates at the relatively weak asthenosphere/lithosphere boundary (LVZ). Boudier et al. (1988) have calcu-lated that the depth to this boundary in hot immature crustmay be as shallow as 12 km in <1 Ma lithosphere, with rapidcooling and thickening to up to 25 km within 5 Ma. Calculated1000–1200)C temperatures at this boundary (Boudier et al.1988) are consistent with two-pyroxene assemblages and tem-perature estimates recorded in the aureole (900–1050)C, Stylesunpublished data as quoted in Floyd et al. 1993) and otherophiolite complexes (Jamieson 1986). Recent pressure esti-mates of 3–4 kbar (Sandeman et al. 1995b), obtained fromlithologies in the footwall to the Goonhilly Downs Unit, are inaccord with young ocean lithosphere some 9–12 km in thick-ness. The presence of a dynamothermal aureole may providesome constraints on the processes involved in obduction. Ashas been pointed out by Nicolas (1989) subduction zoneenvironments are too cold to allow formation of high-gradezones beneath ophiolites, a conclusion supported by Suhr &Cawood (1993). However, the thermal models of Hacker(1991) indicate that in order to generate an aureole by over-thrusting of ocean lithosphere at a spreading centre, young<2 Ma ocean crust is required, crust as old as 4–9 Ma cannotgenerate thermal aureoles in these zones (Hacker 1991). Theoverlap in age data from gabbro emplacement in the CrousaDowns Unit (375&35 Ma, Davies 1984), to high-temperaturefoliation development in the footwall hornblende schists(366&4 Ma, Sandeman et al. 1995b), are consistent with rapidoverthrusting of young ocean lithosphere over the adjacentocean crust. The lack of internal imbrication and the presenceof a coherent slice of ocean lithosphere, the Goonhilly DownsUnit, at the highest structural levels needs some explanation.During the initial stages of overthrusting ocean lithosphere ishighly rigid (e.g. Brace & Kohlstedt 1980) and moves over theweaker asthenosphere within which shearing is concentrated.In the hanging wall, deformation is facilitated by shearing inthe ‘basal’ peridotites but as obduction proceeds, and coolingensues, deformation switches to the footwall, therefore retain-ing the coherent nature of the overriding ocean lithosphere(Hacker 1991). It has been suggested that as thrusting contin-ues, deformation is concentrated in weaker crustal lithologies(Pavlis 1986; Hacker 1991), which are progressively accreted,in a downward stacking mechanism (Spray 1988; Jamieson1980, 1986), as the hanging wall climbs to shallower crustallevels. It is apparent from the present study that the emplace-ment of the Goonhilly Down Unit occurred at a late stage inthe accretion history, and that the Basal Thrust or detachmentprogressed to deeper crustal levels.

Pre- to syn-D4 magmatism. The overthrusting or subduction ofocean lithosphere has major consequences for magma gener-ation. Boudier et al. (1988) indicate that one consequence ofthis process would be the melting of the overridden lithosphereduring high-temperature metamorphism (as illustrated onFig. 14d). In this case anatexis of hydrous metagabbros andmetasediments would initiate at 3–4 kbar and 700–750)C. Thepre- to syn-D4 timing of emplacement of a basic–felsic mag-matic suite, including the Kennack Gneiss, the Old LizardHead Basic Sheet and associated granites, places some struc-tural constraints on their probable origins; the magmas wereemplaced during thrust emplacement of the Goonhilly DownsUnit. Some controversy still arises over the origin of the

Kennack Gneiss magmas (for a review see Floyd et al. 1993and references therein) althought recently Sandeman et al.(1995b) have suggested that temperatures in the Basal Unitwere insufficient (c. 600)C) to have caused anatexis of thehornblende schists and metasediments as an origin for theKennack Gneiss. However, the presence of two-pyroxeneassemblages and clinopyroxene-bearing melt segregations inthe dynamothermal aureole to the Goonhilly Downs Unit,suggests that temperatures well in excess of 700–800)C wereachieved during overthrusting. Therefore an origin for theKennack gneiss by anatexis of metabasic schists and meta-sediments, as suggested by earlier workers (Sanders 1955;Strong et al. 1975; Styles & Kirby 1980; Malpas & Langdon1987) cannot be ruled out.

The dynamothermal aureole as an extensional shear zone.Limited data indicate that at least during the later stages of itshistory the dynamothermal aureole underwent a phase ofductile extension. Evidence for extension is best preservedalong the Most Southerly Point Shear Zone where thedevelopment of a pervasive S4b extensional crenulationcleavage is synchronous with prograde mid to upperamphibolite-facies metamorphism. The coincidence ofextension and prograde metamorphism may have resultedfrom the mechanical weakening of lithologies in response toheating and thrust loading following the emplacement of a hotperidotite slab (Goonhilly Downs Unit), with additional heatprovided by magma emplaced into the extending shear zone(the Kennack Gneiss and associated intrusives).

D5 Collapse of the nappe pile. Several structures provideevidence for the late stage collapse of the entire nappe pileunder waning metamorphic conditions. Foreland-directed off-sliding is indicated by northwards dipping shear zones (northof Kynance Cove), reactivation surfaces (Polbream Cove–Predannack Cliffs Thrust in Polbream Cove) and reworking ofthe Basal Thrust (Cadgwith). Lateral extrusion by thrustinghas been accommodated along the Basal Thrust (Kynanceregion) and the Old Lizard Head Thrust. These structuresidentify the final stages in the emplacement of the complex andits collapse (Fig. 14d & e).

Conclusions

The mechanism of ophiolite emplacement has been the subjectof much debate and to date the only surviving record of whatis thought to be the emplacement history has been provided bydetailed studies of dynamothermal aureoles preserved beneathophiolites and the associated overlying peridotites. The BasalUnit of the Lizard Ophiolite Complex preserves a detailedrecord of the previously unrecognized earliest and final stagesin the emplacement history of this ophiolite. Crustal scaleshear zone development and the tectonic assembly of oceanicslithers occurred prior to mantle decoupling and the emplace-ment of a single ophiolite slab along a major low-angledetatchment. A consequence of this emplacement history isthat the dynamothermal aureole is superimposed on previouslyaccreted units and represents a late stage in the emplacement ofophiolites. The final stage is marked by syn-emplacementextensional collapse and coeval prograde metamorphism andmajor magma generation. The recognition of an extensionalcollapse phase provides a mechanism which may go some way


towards explaining models which infer ‘tectonic thinning’during ophiolite emplacement (see Jamieson 1986).

Thanks are due to R. A. Strachan for his continued discussions andcomments on various forms of the original manuscript. I am verygrateful for the financial support from Oxford Brookes Universityand Cheltenham & Gloucester College of Higher Education. I amalso very grateful to S. Sherlock for help and assistance during thefieldwork part of this project.

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Received 17 July 1996; revised typescript accepted 20 February 1997.Scientific editing by David Snyder.


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