Journal of the Geological Society, London, Vol. 148, 1991, pp. 869-880, Printed in Northern

On the trace of the Iapetus suture in Ireland and Britain

S. P. TODD’, F. C. MURPHY’ & P. S. KENNAN Department of Geology, University College, Belfeld, Dublin 4, Ireland Present addresses: BP Exploration, 415 Long Walk, Stockley Park, Uxbridge, Middlesex UBll lBP, UK ERA-Maptec, Level 8, 371 Queen Street, GPO Box 2927, Brisbane, Queensland 4001, Australia

Abstract: The Iapetus suture in Ireland and Britain is that line which separates Caledonian rocks of theLaurentian and Avalonian continents. The suture iscryptic: nowhere is there an exposed fault-zone containing ophiolite remnants, blue-schist melanges, or trench deposits. Instead a suture line may be traced with varying degrees of confidence through a series of faults that traverse the Iapetus suture zone,which contains two or more tectonostratigraphic . Several data sets are utilized to constrain this trace. The distributionof faunal provinces caused by oceanic separation can be used most reliably to define early Ordovician terranes. Fauna1 intermingling in the mid-Ordovician reduces provinciality and the confidence of identification using faunal data. The late Ordovician and particularly Silurian tectonostratigraphic histories of terranes either side of the suture suggest that amalgamation of the terranes, elimination of the and development of the suture zone had begun by the Ashgill. Therefore the resolution of the suture trace becomesunreliable where Ordovician rocks do not crop out, becauseSilurian turbidite fans of Laurentian provenance may have dispersed across a significant width of Avalonian crust. Similarly, the precision of structural identification of the suture zone, by correlating the suture with a major fold-facing confrontation, may beweakened by late overthrusting. Tectonic interleaving of crustal flakes in the zone is supported by geophysical evidence which suggests that some major boundaries in thelower crust do not always coincide with theirinferred projections in outcrop.Tectonic and sedimentary mixing of crustal blocks and sediments in the zone is further reflected by Nd and Pb isotopic patterns; the Rb-Sr patternsof record a stronger distinction between north and south of the suture. Despite these difficulties,we depict a possible suture trace in Ireland that departs significantly from the traditionally placed line by following a NE-SW-aligned trace through the Slane fault and the Navan-Tipperary lineament, and thence through a dog-leg to pass north of the Dingle Peninsula.

Since Wilson’s (1966) proposal thatan ocean was the the Solway-Navan-Silvermines line (Fig. 2) (Phillips et al. migration barrier between provincial faunas inLower 1976; McKerrow & Soper 1989), which is but one possible Palaeozoic rocks of Britain andIreland, it hasbecome solution (Murphy & Hutton 1986). widely accepted that the British and Irish Caledonides were constructed by the opening andclosure of the Lapetus Ocean (e.g. Dewey 1969). TheIapetus suture, astectonica Cryptic sutures and tectonostratigraphic terranes boundaryrepresenting consumed oceanic lithosphere, Uncertainties in defining a uniquesuture trace in Britain results from the amalgamation in a three-plate configuration and Ireland stem largely from the lack within the zone of an of and with , which is a exposed suture (sensu stricto), i.e. a high-strain fault-zone continental fragment or superterrane of Gondwanan affinity that probably penetratesthe lithosphereand contains (Soper & Hutton 1984; Hutton 1987; Keppie 1989; Murphy ophiolitic remnants (as in Newfoundland), or blue-schist et al. 1991) (Fig. 1). According to the three-plate model, the melanges, or other trench-related deposits (Fig. 3) (Dewey British and Irish Iapetus suture should everywhere separate 1977). Thus the Iapetus suture has been traced through the distinctive Caledonianrocks of Avalonian and Laurentian Solway line (Fig. 2), with the premise that the line is the affinity. To the east of Britain, the suture divides into two southern limit of the Ordovician-Silurian Southern Uplands arms. One of these arms passes NNEintothe of Scotland which may comprise an accretionary prism Scandinavian-GreenlandCaledonides while the other arm formed along the Laurentian margin (Leggett et al. 1983). passes SE along theTournquist Line intothe Polish However, this argument carries less weight given a body of Caledonides (Fig. 1) (Soper & Hutton 1984). To the west, in evidence which indicates that the Southern Uplands is not Newfoundland, the Irish suture is probably continuous with an accretionary prism of oceanic trench deposits (Murphy & the boundaryzone between theDunnage and Gander Hutton 1986; Morris 1987; Hutton & Murphy 1987; Stone et terranes, which is locally marked by ophiolites (Keppie al. 1987). 1989; Pickering et al. 1988). A cryptic suture may be defined as a fault, or a series of Despite the expected distinctive stratigraphic, faunal and faults, which probably neither contain remnant oceanic or tectonic characteristics of theLaurentian and Avalonian trench-related rocks nor cut the lithosphere, but nonetheless continents, the diagnosis of a unique trace, particularly in separate rocks which evolved on either side of the ocean. Ireland, has remainedcontroversial (Phillips et al. 1976; Cryptic sutures may result from thedeep of old Archer 1981; Max et al. 1983; Murphy & Hutton 1986; sutures with concomitant removal of upper crustal ophiolites McKerrow & Soper 1989). It is traditionally placed along and trench deposits, from the overthrusting of the original

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deposits in the succeeding foreland basin produced as the two continentalplates converge. In such asetting where post-suturing sediments progradefrom both margins into the successor basin, the position of the main depocentre may not necessarily be coincident with the trace of the original suture. The Silurian of central Ireland may overstep an Ordovician Iapetus suture in this way (Hutton & Murphy 1987). In the absence of an exposed oceanic suture sensu stricto, the recognition of a singular suture line in the British and Irish Caledonides depends critically on contrasts in stratigraphic,tectonic and faunal characteristics of rocks derived from either side of the ocean. Unfortunately such contrasts are not always clear cut, even in Ireland where the suture zone is probably best exposed. Moreover, the faunal provinces, upon which the existence of Iapetus (and its suture) is ultimately based (e.g. Fortey & Cocks 1988 and references therein),are not always sharply defined in EARLY DEVONIAN 410 Ma TERRANE ACCRETION MODEL Ireland (Harper & Parkes 1989). Suture definition is in some ways aided by the concept of Fig. 1. Sketch of the palaeogeography and location of the Iapetus tectonostratigraphic terrane analysis. This technique has led suture in the British and Irish Caledonides during the Early tothe definition of several terranes in the Appalachian- Devonian (after Soper 1986) LP: London platform; Mec: Caledonianorogen that have internally consistent strat- Mid-European Caledonides; BC: Bohemian craton; Ac: Armorican igraphic, tectonic and faunal characteristics distinct from craton; Lvc: Ligurian-Vosgian cordillera. neighbouring terranes (e.g.Murphy 1987a; Soper et al. 1989; Keppie 1989; Murphy et al. 1991). On the other hand, suture by crustal flakes, orfrom the transcurrent or the difficulties of asearch for a single suture may be extensional dislocation of an original suture (Fig. 3) (Dewey compounded by the definition of terrane boundaries either 1977). as '. . .faults, or melanges representing a trench complex, or Further difficulty in defining sutures arises from cryptic suture zones across which neighbouring terranes (1) concealment by post-suturing cover rocks. The difficulty is may have distinct geological records not explicable by facies increased by theoverstep of an early suture byflysch changes (i.e.exotic terranes),or (2) may have similar

Lake District

-FAULT ...... IAPETUS SUTURE CAMBRIAN (C) POST-CALEDONIAN

Fig. 2. Map of the Caledonian geology of the Iapetus suture zoneof Ireland and adjacent Britain (after a numberof sources but particularly Hutton 1987 and Murphy er al. 1991). The abbreviationsof the Caledonian faults are as follows: BMZ, Ballycogly mylonite zone; DBL, Dingle Bay lineament; DRF, Doon fault; ECDZ, East Carlow deformation zone; FSZ, Fergus shear zone; LLF, Lowther Lodge fault; MSFZ, Menai Straits fault zone; NF, Navan fault; NGF, North Galtees fault; NKL, North Kerry lineament; OBF, Orlock Bridge fault; SF, SIane fault; SiF, Silvermines fault; SIL, South Ireland lineament; SL, Solway line; SRF, Sklrd Rocks fault; SUF, Southern Uplands fault.

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Fig. 3. Cartoon to illustrate the com- plexities in the definition of terrane boundaries and/or sutures (modified after Dewey 1977). Boundaries are labelled as a, oceanic suture; b, strike-slip fault; c, overthrust; d, normal fault; e, overtstepping sedimentary linkage. Only boundary type a is an oceanic suture (sensu stricto) at outcrop; boundaries b-d are cryptic sutures. All the boundaries a-d are also terrane boundaries.

geological records (i.e. proximal terranes) that may only be & Murphy 1987). The Lake District- terrane is part distinguished by the presence of a terrane boundary of an assemblage of terranes that constitute the composite representingtelescoped oceanic lithosphere' (Keppie 1989; Avalon superterrane of Gondwananan affinity (Keppie Murphy et al. 1991). This definition allows the possibility 1989). Apart from the complication of possible smaller, that any terrane boundary may also be an oceanic suture. laterally discontinuous, suspect terranes of uncertain affinity In this paper we attempt to trace the fault or series of sandwiched betweenthese two larger terranes (Murphy faults that separates terranes with basement and/or cover of 1987a), the boundarybetween the LaurentianSouthern Laurentian affinity from those with basement and/or cover Uplands-Central terrane and the Avalonian Lake District- of Avalonian affinity. This is a fundamental boundary in the Leinster terrane is the Iapetus suture (Fig. 4). Caledonian orogen on the scale of observation of Ireland Below we review the evidence forthe trace of this and Britain and it is this line that we term the Iapetus suture boundary using five individual types of data:faunal, (Figs 2 & 4). tectonostratigraphic,structural, geophysical and isotopic. We review the available geological data used to deduce We conclude thatthe Iapetus suture is no longer the course and geometry of a suture in Britain but especially represented atoutcrop by the remnant of oneor more in Ireland. The review pertains to the debates on where and subduction or obduction zones (a suture sensu stricto), but when the ocean closed (Murphy & Hutton 1986; Hutton & instead is replaced by several steep, anastomosingfaults Murphy 1987; McKerrow 1988; Pickering et al. 1988; through which a cryptic suture may be traced with varying McKerrow & Soper 1989). Many authors now regard degrees of confidence. Iapetus as a largely Ordovician phenomenon, diminished in the Silurian, and entirelyeliminated by theonset of the Devonian. Its location is broadly defined asa NE-SW- The faunal 'suture' trending suture zone in Britainand particularly Ireland, It iswell established thatthe early Ordovician provincial some 100 km wide, traversed by several anastomosing faults faunas became gradually mixed at generic and then species each of which may have importance in palaeogeographic level during the late Ordovician (McKerrow & Cocks 1976; reconstructions and terrane recognition. Within the suture Fortey & Cocks 1988 and references therein). By the end of zone the two major, laterally continuous although possibly theCaradoc, the brachiopod and trilobitefaunas were composite terranesare the Southern Uplands-Central largely cosmopolitan. The provincialism of the benthic terraneand the Lake District-Leinster terrane (Fig. 4) ostracodes, which apparentlylacked aplanktonic larval (Hutton 1987). The Southern Uplands-Central terrane, of stage,and of freshwater fish persisted intothe Silurian Laurentian affinity, was accreted to the Laurentian margin (Fortey & Cocks 1988; butsee Pickering et al. 1988). It eitheras an accretionaryprism, a series of accretionary follows that only earlyOrdovician benthic faunas can be prisms (Leggett et al. 1983), or as thrust-imbricated back-arc used to trace an effective migration barrier with confidence. and foreland basins (Stone et al. 1987; Morris 1987; Hutton The less distinctive later Ordovician and Silurian biota imply

Fig. 4. Sketch map of the main tectonostratigraphic terranes of Ireland and their counterparts in Britain (after Hutton 1987; Murphy 1987a; Murphy et al. 1991).

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thatthe former migration barrier(Iapetus) hadnarrowed These are stratigraphically overlain by rocks with a newly considerably. recognized mid-Llanvirn shelly fauna of the Celtic province Thethree brachiopodprovinces, namely theNorth which characterizes peri-insular settings (Harper,Parkes, American, Baltic and Mediterranean provinces, are broadly Hoey & Murphy, pers.comm.). Upper Llanvirn shelly equivalent tothe cratons of Laurentia, Baltica and faunas show an affinity with the Mediterranean province of Gondwana respectively (Murphy et al. 1991). The the Gondwanan craton, while Caradocfaunas are more correspondingtrilobite provinces arethe Bathyurid, typically Anglo-Welsh. Asaphid and Selenopeltisprovinces, while the Pacific and The Ordovician rocks atGrangegeeth (Figs 2 & 4), Atlanticgraptolite provinces occupied equatorialand 10 km to the northwest of Bellewstown, also contain temperate latitudes. Other faunal provinces include the Llanvirn Atlantic province graptolites.However, the peri-insular Celticprovince that developed around early succeeding Caradoc rocks containbenthic faunas, which Ordovician islands in Avalonia, the peripheral Laurentian although originally assigned tothe Baltic Province by Toquima-Table Head realm and Scoto-Appalachian fauna, Harper (1952), can mostly beaccommodated in the and the Anglo-Welsh faunathat characterized thelate Scoto-Appalachian fauna (Harper & Parkes 1989). Ordovician of British and Irish Avalonia (Harper & Parkes The faunal (Harper & Parkes 1989) and related contrasts 1989). in substrate (Murphy 1987a) between the Bellewstown and In Britain, early Ordovician provincial shelly faunas of Grangegeeth suspect terranes deserve future clarification, the North American Province occur in Girvan and as drifted but the possible implication is thatthe interveningSlane biota in the Northern Belt of the Southern Uplands. The fault (Murphy 1985) represents a faunal suture across which Iapetean faunal barrier lay betweenthese areasand the a migration barrier of oceanicseparation was eliminated Ordovician shelly of Gondwanan aspect in Cross Fell during the Ordovician. The Bellewstown terrane was and the Lake district (Fortey & Cocks 1988; McKerrow & probably derivedfrom theLake District-Leinster terrane Soper 1989). (Murphy 1987a), from which It is separated by the Lowther Thiscomparatively simple faunalseparation inBritain Lodge fault(Murphy 1985). A more oceanicsetting is contrasts with more variable provincial assemblages envisaged forGrangegeeth, atapering terrane which is between,and even within Ordovician terranes inIreland bounded on its northern side by the Navan fault,the (Harper & Parkes 1989). Middle Arenig benthic faunas in traditional suture trace (Phillips et al. 1976). The Navan South Mayo are part of the Toquima-Table Head realm, a fault may also represent faunala suture between the peripheraldevelopment of theNorth AmericanProvince Llanvirn rocks of Grangegeeth, which containAtlantic (Harper & Parkes 1989). Slightly older graptolites from the graptolites, andthe Llanvirn of thenorthern belt, which same area have Pacific province affinities (Harper & Parkes contains Pacific-province forms.However, because the 1989 andreferences therein).Caradoc brachiopods in Grangegeeth terrane is characterized by the Scoto- Tyrone belong to the Scoto-Appalachian fauna, which has Appalachian fauna with apparently no or few Gondwanan strong affinities with the North American province. elements, it probably originated north of the Iapetus suture, Across the Southern Uplands fault, the early Ordovician or at least in the northern part of the suture zone (Harper & of the Northern Belt of the Central terrane is characterized Parkes 1989). Therefore, although it is difficult to position a by graptolitic biofacies of the Pacific province, although its faunal ‘suture’ within oceanic environments, the Slane fault, Scottish equivalentcontains a transported Scoto- rather than the Navan fault, may be the most important Appalachian benthic fauna. No distinctive provincial faunas faunal boundary in the Iapetus suture zone. have been found in the late Ordovician and Silurian Central and Southern Belts of theCentral terrane. The late The tectonostratigraphic suture Ordovician Foliomenu brachiopod faunathat occurs in Majortectonostratigraphic contrasts exist between the northern Slieve Aughty was comparatively widespread and Ordovician rocks of the Southern Uplands-Central terrane is not distinctive toone side of thesuture or theother and the Lake District-Leinster terrane (Fig. 4). However, (Harper & Parkes 1989). in central Ireland Silurian turbiditic deep-sea fan sediments South of the suture zone, the Leinster terrane contains blanketed thesuture zone, as in an overstep or linking Llandeilo to early Caradoc Scoto-Appalachian forms in sequence(Murphy & Hutton 1986; Hutton & Murphy deep-waterlimestones, but is dominated by Anglo-Welsh 1987). In this manner Silurian sediments of Laurentian faunas (Harper & Parkes 1989). It also contains some Baltic provenance may have been deposited over Ordovician rocks elements. The North American (Scoto-Appalachian) forms of Avalonian origin. Therefore a suture may be traced with in this Avalonian terrane probably indicate mid-Ordovician more confidence between pre-Wenlock rocks and, as with faunal migration and mixing, perhaps facilitated by marine the faunas, most reliance can be placed on the Ordovician transgression (Fortey & Cocks 1988). rocks. In eastern Ireland, unlike Britain, Caledonianrocks of In Britain, thesuture may betraced atoutcrop with the medial part of the Iapetus suture zone arerelatively well some confidence along the Solway Line between the exposed. Two small ‘suspect’ terranes, originally defined on LaurentianSouthern Upiands of Scotland and the the grounds of tectonostratigraphiccontrasts, are exposed Avalonian Lake District of northern England (McKerrow & between the larger Centraland Leinster terranes (Fig. 4) Soper 1989). The Northern, Central and Southern Belts of (Murphy 1987~;Harper & Parkes 1989). Fauna1 contrasts the Southern Uplands are formed of Ordovician to Silurian between these terranes support their distinction from each , ,cherts and greywackes of oceanic or other and from their neighbours on either side of the suture quasi-oceanic affininty. TheLake District containsa zone (Harper & Parkes 1989). Early Ordovician rocks in the sequence of Cambrian to earlyOrdovician shales and Bellewstown terrane of the Balbriggan inlier (Figs 2 & 4) greywackes, followed by later calc-alkaline Ordovician (Murphy 1987~)contain graptolites of the Atlantic province. volcanic rocks and a Silurian flysch sequence.

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In Ireland, the Cambrian to Lower Ordovician rocks of sediments and mugearite flows. Theseare followed the Leinster terrane record flysch-like sedimentation in a unconformably by Llandeilo to lower Caradoc volcanic NE-SW-aligned basin boundedboth north and south by conglomerates, upperCaradoc black shales, Ashgill Precambrianbasement (e.g. Briick et al. 1979 and mudrock and volcaniclastic , Llandovery to references therein).The Leinster succession, like its Wenlock greywackes and siltstones (Murphy 1987~).The equivalents in the Lake District(Murphy 1987a), typically volcanic rocks of this terraneare characterized by a containsa mid-Ordovician unconformity (Holland1981a) ‘within-plate’ geochemical signature (Stillman & Williams followed by the eruption of Llandeilo to Caradoc volcanic 1978). The Grangegeeth terrane may represent an ocean rocks that wererelated to southeastwardsubduction of island (e.g. Neuman 1988) caught up in the suture zone. Iapetusoceanic crust (Phillips et al. 1976; Stillman & The Slane fault separates the Grangegeeth terrane from Williams 1978). Gravitational collapse of the volcanic arc the Bellewstown terrane to the south (Murphy et al. 1991). sequence(Branney & Soper 1988), possibly relatedto a The oldest exposed rocks of the Bellewstown terraneare transtensional regime (Murphy1987a) was followed, in Tremadoc to Arenigmudrocks, paraconglomerates, silt- places unconformably, by heterogeneous Ashgill shales and stone, sandstone and rare coticule (garnetiferous quartzite). . Theseare succeeded by Llandovery, Wenlock Thesesediments areinterpreted asslope or basin-floor and Ludlow flysch-like deposits over a hemipelagic base deposits and are followed conformably by Llanvirn acid (Bruck et al. 1979; Hutton & Murphy 1987). Incentral lavas, ignimbrites,keratophyres, paraconglomerates and Ireland (Slieve Bernagh, Slieve Bloom, Slieve Phelim, mudrocks. This volcanic sequence differs from that of the Galtees and Slievenamon; Fig. 2), symmetrical Llandovery Grangegeeth terraneand is thought to have evolved turbiditedispersal systems progradedfrom boththe separately(Murphy 1987~).An Upper Llanvirn unconfor- northern Laurentian and southern Avalonian margins. The mityis overlain by a thin, discontinuous that is main depocentreinto which theopposed flysch systems followed by Llandeilo to Caradoc mudrocks and vol- prograded lay along a line between the Galtees and Slieve caniclastic turbiditicsandstones. Thereare no proven Phelim inliers (Hutton & Murphy1987). TheLaurentian Llandovery rocks in the Bellewstown terrane and the system became dominant in the mid-Wenlock and dispersed Ordovician rocks are instead overlain by Wenlock to across the suture zone (Hutton & Murphy 1987). A similar Ludlow greywackes andsiltstones with rare bentonites situation may hold forthe Upper Wenlock of theLake (Murphy 1987~).The Ordovician-Silurian contact is District (Leggett et al. 1983; McKerrow & Soper 1989). interpreted as faulteda unconformity of post-middle In the Longford-Down massif, the Arenig to Caradoc Caradoc to pre-Wenlock age (Murphy et al. 1991). succession in thenorthern belt of theCentral terrane The Tremadoc to Arenig mudrock-dominated succession preserves an almostintact succession of spilitic volcanic of Bellewstown has close correlatives in similarly aged rocks rocks, cherts, pelagic shalesand turbiditic greywackes in the Leinster terrane. The flysch-like Silurian sequences of (Leggett et al. 1979; Holland 1981; Morris 1987). This zone the two terranes, bounded by the Lowther Lodge fault, are is readily correlated with the Scottish Northern Belt. also similar. The dissimilarity between the upper Ordovician Palaeocurrents depicting lateral SE and axial SW and NE rocks of the two terranes may warrant the distinction of the sediment dispersal are compatible with sedimentation within Bellewstown succession as a terrane that dispersed from the an oceanic trench along the Laurentian margin (e.g. Kelling redocked with Leinster during the late Ordovician (Murphy et al. 1987). However,turbidites rich in volcanogenic et al.1991). Alternatively, the stratigraphiccontrasts detritus shed into the northern belt from the SE are more between the two areas may represent facies changes across suggestive of back-arca setting behind substantiala fault-blocks in a greatly extended continental margin (cf. the calcalkaline island arc during the Caradoc (Morris 1987). To French Alps, Graciansky et al. 1989). the south of the Orlock Bridge fault (Fig. 2) (Anderson & The stratigraphicand faunal contrasts between the Oliver 1986), the Central Belt comprises the southern part Bellewstown terraneand the Grangegeeth terraneto the of the Longford-Down massif and contains pelagic shales, north are much more striking. The Grangegeethstrat- minor lavas and greywackes,ranging fromCaradoc to igraphic and faunal succession bears very little similarity to Wenlock in age.Palaeocurrent patterns generallyrecord either of the sequences of the Bellewstown or Leinster lateral SE and axial NE and SW palaeoflow. terranes until the ubiquitoussedimentary linkage of the Atectonostratigraphic suture may be drawnin Britain Wenlock flysch. Thus the Slane fault is probably a terrane along the Solway line (Phillips et al.1976), the boundary boundary that represents telescoped between between the Avalonian Lake District andthe Laurentian the definitively Avalonian Bellewestown andLeinster Southern Belt strongly indicate an origin on the Laurentian terranesand the apparentlyoceanic Grangegeeth terrane. margin, the lack of older rocks meansthat overstep of The Navan fault, the northern boundary of the,Grangegeeth non-Laurentianbasement cannot be discounted,and may terrane, assumes equal importance with the Slane fault as a indeed be likely (e.g. Leggett et al. 1983). tectonostratigraphic terrane boundary. Inthe absence of In eastern Ireland, the Southern Belt of the equivalent definitive evidence of the origin of the basement in the central terrane has yet to be defined or is missing, and the apparently oceanic Grangegeeth terrane,thegreater suture zone is represented as a zone of Ordovician suspect Laurentian affinity of its benthic faunas (Harper & Parkes terranes amalgamated by post-Caradocsinistral strike-slip 1987, p. 415) indicate that it is the Slane fault along which faulting between the Central Belt of the Longford-Down the Iapetus suture may best be traced. terranes amalgamated by post-Caradoc sinistral strike-slip The tectonostratigraphic zones of theSouthern faultingbetween thecentral belt of the Longford-Down Uplands-Central terrane may betraced laterally tothe massif and the rocks of the Leinster massif (Figs 2 & 4) southwest intocentral Ireland. The mainly Ordovician (Murphy 1987~). The Grangegeeth terrane is composed of cherts, shales and greywackes of northern Slieve Aughty volcaniclastic sediments,overlain by Llanvirn marine may be correlated with the Northern Belt of the Southern

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Uplands and Longford-Down. The Ordovician pelagic Devonian) basin, the Dingle Basin which was generated by shales to mainly Silurian greywackes of southern Slieve Acadiantranspression and filled by more than 4 km of Aughty (Fig. 2) (Holland 1981) closely resemble the Central molasse-like lacustrine and fluviatile sediments (Todd et al. Belt (Murphy et al. 1991). The inliers of Slieve Bernagh, 1988; Todd 1989~). Thebasin was inverted in the climax of north of the Silvermines fault (SiF in Fig. 2), and the inliers the Acadian , probably in the Emsian (Todd et al. of Slieve Phelim and Slieve Bloom, south of the Silvermines 1988; Todd 1989~). Thesedimentary fill is dominated by the fault, can all be correlated with the Southern Belt of the deposits of a large sandybraided stream system which Southern Uplands. Following the reasoning for the Solway flowed along the axis of the basin towards the ENE. line in Britain, this correlation across central Ireland implies However, sediment, some of it conglomeratic, was also that a boundary south of Slieve Phelim is the Iapetus suture dispersed laterally into the basin from hinterlands lying to and not the Silvermines fault as traditionally taken. (Phillips the NW and SE (Todd et al. 1988; Todd 1989~).These et al. 1976; cf. Archer 1981). However, as discussed above, conglomeratic formations are considered to beearliest it is the nature of pre-Silurian rocks that best indicates the Devonian (Pragian) and their provenance provides a useful affinity of the Caledonian rocks of central Ireland and the impression of the geology of the terranes neighbouring the position of the suture. In southern Slieve Aughty (Fig. 2), a Dingle Basin prior to the climax of the Acadianorogeny sequence of Late Llandeilo to Caradoc /trachyte lavas (Todd1989b). Their provenanceindicates thatthe and tuffs have subalkalineocean island affinities (Emo & configuration of the terranes in the Irish suture zone was Smith 1978) and have faulted relationships to the Southern much the same by the earliest Devonian as it is today (Todd Uplands-like stratigraphy of Llandeilo to Llandovery dark 1989b). grey shales, olive-grey shales,metabentonites and rare greywackes Holland 1981; Murphy et al. 1991). The volcanic rocks bear some similarity to the coeval sequence of the The structural ‘suture’ Grangegeeth terrane.The gravimetric and aeromagnetic The suture zone in Ireland (Fig. 2) is cut by a number of character of central Ireland has been interpreted to indicate fault zones, several of which record a protracted history of the presence of such Ordovician volcanic rocksas Caledonian movements, culminating in the late Caledonian fault-bounded blocks subcropping the Silurian and younger (Acadian; Soper et al. 1987; McKerrow 1988) phase of succession of the region (Brown & Williams 1985). The sinistral transpression,indicated by the dominantly clock- southern Slieve Aughty volcanic outcrops may therefore wise transection of folds by the associated cleavage and provide a small window to a larger zone consisting of the other kinematic indicators in the wallrocks of the faults (e.g. Grangegeeth terrane or a series of similar terranes that are Phillips et al. 1979; Soper & Hutton 1984; Murphy 1985; overlain and/oroverthrust by late Ordovician to Silurian Anderson & Oliver 1986; Anderson 1987; Todd 1989). The Central/Southern Belt sediments of Laurentian provenance. Acadian transpression in the suture zone was broadly coeval With the dearth of pre-Silurian outcrops in central Ireland, with early Devonian (c. 400 Ma) plutonism (Murphy the apparent Laurentian character of the late Ordovician to 1987b; Hutton 1987; Soper et al. 1987) and with Lower Old Silurian of Slieve Bloom, Slieve Bernagh, Slieve Aughty and Red Sandstonesedimentation (Todd et al. 1988; Todd Slieve Phelim indicates that the sutureis best traced as a line 1989a). significantly to the south of its traditional course (cf. Archer Although the Acadiandeformation of the British and 1981) (Fig. 2). Irish paratectonicCaledonides represents the final con- Further to the southwest, evidence for the trace of the solidation of Avalonia with Laurentia(Soper et al. 1987), Iapetussuture is enigmatic,as only Silurian rocks are deformationrelated to theiroblique convergence began proven in the critical area of the Dingle Peninsula (Fig. 2) before the Devonian and was spatially and temporally (Holland 1981, 1988; Parkin 1976). Moreover,the variable. For example, the Ordovician and Silurian deep-sea uncertainty of the Dingle area is increased by the disparate fan sediments of the Southern Uplands-Central terrane nature of its Silurian succession compared to contiguous were folded and imbricated shortly after deposition (Knipe flysch-like successions of greywackes and siltstones in central & Needham 1985; Kemp 1987; Morris 1987). Structural Irelandsome 60km to the ENE. The Upper Wenlock to evidence shows that this deformation involved a sinistral Ludlow succession of the Dingle Peninsula is composed of strike-slip component by the Llandovery (Anderson 1987) basic tointermediate lavas, acid pyroclastic rocks inter- andstratigraphic evidence suggests that strike slip on the bedded with fossiliferous shallow marine , Laurentian margin may have commenced earlier, in the siltstones and rare limestone, plus non-marine red to purple Ordovician (Kelling et al. 1987; Morris 1987). In the mudrocks and sandstones (Holland 1988; Parkin 1976). The Leinster terrane, deformationepisodes are suggested by rocks are interpreted to have been formed on the flanks of a middle and upper Ordovician unconformities (Powell & Silurian volcanic island (Sloan & Williams 1991).The Phillips 1985; Murphy1987a; Hutton & Murphy 1987). calcalkaline geochemical signature of the lavas indicatesa Bennett et al. (1989) relatethe mid-Ordovician unconfor- subduction-related componentfor the magma generation, mity tothe earliest cleavage-forming deformation of the although it is not necessarily indicative of contemporaneous Cambro-Ordovician rocks of southeastLeinster. This oceanic subduction (Sloan & Bennett 1990). Some fauna1 deformation might representapartial inversion of the isolation of the Wenlock volcanic island of west Dingle is Leinster Basin, in response to SE-directedsubduction of- indicated by two endemic brachiopods (Bassett et al. 1976), Iapetus oceanic crust beneath Avalonia. Hutton & Murphy although thereare close similarities between the Dingle (1987) and Murphy (1987~)relate the second Ordovician faunas and those of the Welsh Borderland (Holland 1988) unconformity to extensional or transtensional fault-block and Gotland, Sweden (Watkins 1978). movements, perhaps in response to flexuring of Avalonian The Dingle Peninsula is also significant in that it crust by the overthrustingLaurentian margin afterthe preserves a Lower (Piidoli to early complete consumption of oceanic crust.

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Tectoniccontrols onthe development of the Silurian (e.g. Max et al. 1983; Hutton 1987). Hence, because the marine basins of the suture zone may have been a response extrapolated seismic boundary beneaththe Solway line to the oblique convergence of the Laurentian and Avalonian should cropout in theLake District,Klemperer & margins, possibly by the thrusting of the Southern Uplands Matthews (1987) suggested thatthe inclined geophysical above theLake District (Legett et al. 1983). Todd et al. boundarysteepens in theupper crust, or thatthe Lake (1988) consider theintrasuture Dingle Basin to have District is allochthonous, and now straddles a deeper suture developed as a flexural response to early Acadian sinistral (Freeman et al. 1988). oblique thrusting (see also Todd 1989,). The WIRE profiles off the west coast of Ireland record a During transpression, the principal across-strike tectonic prominent northerly dipping reflector assumed to represent transport directions are indicated by the fold vergence and thesuture (Klemperer 1989; Klemperer et al. 1991). The cleavage facing directions of the major folds (Fig. 5). While extrapolated outcrop trace of this acoustic boundary would backthrusting and refolding can produce reversals of facing, lie just north of the Dingle Peninsula and, as apotential it may be possible from the dominant facing directions to suture, would concur with the structuraland tec- define structural boundaries where major fold-facing divides tonostratigraphicinterpretations of this area.In central and confrontations occur. Such major structural confronta- Ireland the structure of the projected suture zone beneath tions may mark the surface expression of previous opposing the cover rocks is picked out by aeromagnetic lineaments, of subductionzones in theIapetus suture zone. Thusthe which the Navan-Tipperary lineament was given promin- Solway line inBritain separatesthe mainly south-verging ence by Max et al. (1983; see also Morris 1989), although it Siluro-Devonian structures of theSouthern Uplandsfrom does not coincide with the Navan-Silvermines fault,the the upright Acadian folds of the Lake District. However, in traditionalcourse of thesuture (Phillips et al. 1976). The the Longford-Down region, the southerlyvergence and Navan-Tipperary lineament is more closely coincident with sinistral oblique transport of the Silurian rocks of the the trace of the suture as defined here on stratigraphic and Central terrane is complicated by areas of flat-lying and structural evidence between Slieve Phelim and theNorth locally downward-facing S1 cleavage. Nevertheless, a major Galteesfault (Figs 2 & 4). This coincidence of the anticlinal culmination occupies the central ground in eastern aeromagnetic lineament, which probably reflects the Ireland between the Navan and Slane faults (Figs 2 & 5). To structure of the basement at about 5-10 km depth (Max et the southeast, in the Balbriggan region, the late Caledonian al. 1983), with theoutcrop suture trace implies thatthe sinistral transpressive D1structures involved animportant suture may be a steep to vertical structure. component of north-directedthrusting (Murphy 1985), a geometry opposedto that in the Longford-Down region. The fold and thrust confrontation between the Navan and Slane faults (the latter being the fauna1 suture) would thus The isotopic ‘suture’ representstructurala boundary or ‘suture’in eastern Isotopic data from sediments, clasts in conglomerates, and Ireland. fromgranites may be used to define possible terrane In central Ireland, southerly F1 fold vergence in Silurian boundaries or sutures, because the isotopic systems of rocks rocks continues across the Silvermines fault (Archer 1981), of disparate terranes should have evolved from different and a structural confrontation occurs at the North Galtees starting points under contrasting influences (e.g. Haughton fault (Figs 2 & 5), which defines aboundary with the 1988). As one of the most fundamental boundaries in the north-facing folds of the Galtees, which are typical of the Caledonides, the Iapetus suturemight be expected to form a northern part of the Leinster terrane. If a structural suture is distinct isotopic boundary. For example, Davies et al. (1985) to be defined in this region, then rather than the Silvermines considered that model Nd ages (TDM) of Lower Palaeozoic fault (the traditionalcourse of thesuture; Phillips et al. sedimentsindicated different crustal residence times for 1976), a line between Slieve Phelim and the Galtees (Figs 2 rocks on eitherside of thesuture. Whenplotted on a & 4) would seem most appropriate (Archer 1981). In the frequency histogram (Fig. 6), model Nd ages for British and Dingle Peninsula,NNW-directed Acadian sinistral oblique Irish suture-zone sediments display a regional pattern that is thrusting was coeval with and post-datedLower Old Red clearly related to theirgeographic (and hence palaeoge- Sandstonesedimentation (Todd 1989a; see alsoHolland ographic) context. However, there is considerable overlap in 1981). Thus a structuralsuture lies tothe north of the the suture zone, with Southern Uplands sediments yielding Dingle Peninsula. TDM ages in the range 1.0-1.9 Ga, and the Lake District, Welsh Basin and Leinster sediments giving model ages in the range 1.2-1.7 Ga (Fig. 6). The overlap across the suture zone is exemplified by the TDMages derived from samples The geophysical ‘suture’ taken from turbidites of the quartz-rich petrofacies of the British Institutions’ Reflection Profiling Syndicate (BIRPS) centralIreland Silurian (Hutton & Murphy 1987). These deep seismic reflection profiles across the suture zone image sediments, which werederived from both thenorth and a prominenta N-dipping reflector that is taken as the south and deposited in a vestigial Iapetean basin (Hutton & present-day acoustic expression of the suture (Klemperer & Murphy 1987), yield similar TDM ages in arange from Matthews 1987; Freeman et al. 1988; Klemperer 1989; 1.3-1.7Ga (Fig. 6). Although a distinct boundary does not Klemperer et al. 1991). The principal problem is that the exist, the Nd data depict a pattern across the Caledonides projectedsurface outcrop of this reflective boundary does (Davies et al. 1985) that probably reflects the mixed isotopic not always coincide with the traditional suturetrace signature of the source terranes. (Klemperer & Matthews 1987; McKerrow & Soper 1989: Kennan et al. (1979) attempted to use Pb-isotope data see also Beamish & Smythe 1986). This is also true of the from ores hosted in Palaeozoic rocks to locate an isotopic predictedsurface trace of some aeromagneticlineaments ‘suture’. Althougha pattern of increasingly radiogenic Pb

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A. Southern Uplands - Central Terrane (Lower Palaezoic) initial 87Sr/x6Sr ratios ignored, theyprovide cleara distinction between the granites of the Longford-Down- ~~ SouthernUplands region (i.e. theSouthern Uplands- rI 0 CentralSilutian Central terrane) and the Lake District-Leinster region (i.e. Southern Uplands the Lake District-Leinster terrane) (Fig. 7). The difference n in the granites of the two terranes is expressed in two ways. n Firstly, fora given isotopic Rb/Sr value,granites that intrude the Lake District-Leinster terrane contain a greater proportion of radiogenic "Sr than their counterparts in the c Southern Uplands-Central terrane (Fig. 7). This difference f must reflect contrasta in graniteprotolith. Secondly, P 5 Southern Uplands granites yield a greater proportion of low z *'Rb/"Sr values. These define a relatively steep gradient suggesting, at first sight, an older apparent age. Halliday et al. (1980) have illustrateda similar phenomenon in an individual, zoned granite pluton with more basic and acidic n components.A greater mantlecomponent with a greater proportion of 87Rbrelative to =Srin the granites of Tdm Longford-Down and the Southern Uplands is indicated. In Britain, the isotopic 'suture' defined by the Rb/Sr data is B. Lake District - Leinster Terrane (Lower Palaeozoic) 81 Anglo-Welsh Terrane (Lower Palaeozoic) coincident with the Solway line (Fig. 7).Unfortunately, while the graniteisotopic signature of the Centraland 30 Leinster terranes is continuous with their respective British H Anglo-Welsh equivalents, the lack of granite exposure in the central parts E Lake District-Leinster of the Irish suture zone limits the value of this technique in Ireland. -8 5a c Timing of Iapetus closure Which criteria are the reliable indicators of closure of the Iapetus Ocean and thesubsequent formation and location of a suture?Our view of the approaching Laurentian and Avalonian margins, as heralded by fauna1 intermingling in the mid-Ordovician, is that first contact of the plates is represented by the late Ordovician unconformity and variable deformation in the suture zone, signifying a 'soft' collision (Hutton & Murphy 1987; cf. Pickering et al. 1988). Td m This collision was probably oblique and initially incomplete Fig. 6. Frequency histogram of the model Nd ( TDM)ages along indented and previously much attenuated continental (according to the depleted mantle model of DePaolo 1981) of margins. Notwithstanding later subduction-related magmat- samples of sedimentary rocks from the suturezone of the British ism in southwest Ireland (Sloan & Bennett 1990) and in and Irish Caledonides. One square is equivalent to one sample. The northern Britain (Thirlwall 1988), the diachronous overstep data are from Davies et al. (1985) and other sources listedin the of the thick Llandovery to Wenlock Laurentianturbidite Supplementary Publication(see Fig. 5). sequences onto Avalonian crust implies the complete demise of intervening Iapetean oceanic lithosphere at least by the mid-Wenlock in central Ireland and possibly slightly later in southwards is clearly evident, aquestion remains asto northern Britain where north-derived turbidites first arrive whether this pattern is inany way broken by an isotopic in the southern Lake District in the late Wenlock (see also boundary. Recent studies of Irish ore Pb by O'Keefe (1986), Soper & Woodcock 1990). Caulfield et al. (1986) and Dixon et al. (1990) do not identify Another possible linking feature in theIapetus suture a break that might be equated with the suture as defined by zone are lamprophyre dyke suites that occur on both sides Phillips et al. (1976). Instead a central Ireland mixing zone is of the suture in the Scottish Highlands (Rock et al. 1988; identified that lies between South Mayo in northwest Ireland Wright & Bowes 1979), NW Ireland (Elsdon & Todd 1989), and Kildare in eastern Ireland. The margins of the mixing the Southern Uplands (Barnes et al. 1986), Longford-Down zone, as defined by O'Keefe (1986), demarcate breaks in the (Reynolds 1931) and Leinster(Brindley 1970). The regional Pb-isotope pattern between which an isotopic similarity of these dyke swarms suggests that there was little 'suture' must lie. separation across thesuture duringtheir emplacement The Rb-Sr isotope patterns of Caledoniangranites, (Rock et al. 1988). Some of the dykes in Scotland have ages presumably partially or wholly derived from protoliths in or between 418 to 398 Ma (Ludlow to Lochovian) (Rock et al. on the opposing continental plates on either side of Iapetus, 1988). Some Scottish appiniteintrusions, which are might also be expected to provide an indication of the trace genetically allied tothe lamprophyres(Elsdon & Todd of the suture. If the present-dayRb-Sr isotope ratios are 1989), have ages between 431 to418Ma (Llandovery to plotted on a single isochron diagram, with their ages and Ludlow) (Rogers & Dunning 1991). Thus the lamprophyre

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/ f Leinster;Lake District "fi I 1 Fig. 7. Rb-Sr isochron diagram with . S.Uplands;Longford-Down complete data set from granites in the suture zone of the British and Irish Caledonides. A 400 Ma isochron line /// Southern Uplands separates the data into two distinctsets corresponding to different granite proto-

S liths on either side of the suture. A full

Longford-Down reference list to the published sources of District data is available (see Fig. 5). Additional data for Newry from I. Meighan (pers. i comm.) and for Crossdoney from P. S. P Kennan (unpublished). Ch, Cheviot;

Leinster LD, Loch Doon; C, Cairnsmore of Fleet; C-D, Criffel Dalbeattie; N, Newry; Cr, Crossdoney; W, Weardale; S, Skiddaw; T, Threlkeld; Sh, Shap; En, I 1 2 3 4 CIOOEnnerdale; Es, Eskdale; L, Leinster.

and appinite suite may also indicate closure of Iapetus by suture may be traced through these steep faults that do in the mid-Silurian. places separate Ordovician rocks of Laurentian and Within thebroad suture zone defined above,the Avalonian affinity. Someconcealment of thesuture was shallowing Silurian marine basin passes conformablyfrom effected by Silurian rocks that were deposited in the basin the Ludlow into the Piidoli with the onset of continental which succeeded initial Iapetus closure. Later deformation red-bedsedimentation (Todd et al. 1988). Convergent in the suture zone may have resulted in the overthrusting of defomation duringthe Piidoli to Pragian (late Silurian to tectonic flakes across the original suture. For example, the early Devonian) in the suture zone caused the development Silurian rocks of the Laurentian Southern Uplands-Central of theintrasuture Dingle Basin (Todd et al. 1988; Todd terrane may havebeen deposited, then imbricated and 1989~).Acadian sinistral transpression,culminating in the thrustsoutheastwards across the Avalonian crust of the Emsian (Soper et al. 1987) representsthe 'hard' collision Leinster terrane. Such deformation would have carrieda when the previously extended Avalonian margin ex- cryptic suture southwards. Later thrusting in the Irish perienced net shortening and cleavage development, with Iapetus suture zone, such as that in the Dingle Peninsula, accompanying granite plutonism. may representbackthrusting of the Leinster terrane ina northerly direction. Althoughtectonic interleaving of terranes within the Conclusions zone de-emphasizes the role of a unique suture line (Hutton The existence of a wide IapetusOcean, and thus the 1987), the balance of evidence and interpretations presented presence of its suture within the paratectonic Caledonides of in this paper allow depiction of a possible suture trace in Ireland and Britain, derives from the interpretation of the Ireland (Fig. 2). Several problems remain in the delineation provincial characterto Cambrian to Ordovician faunas as of this boundary. One is the affinity of the Grangegeeth resulting froman oceanic migration barrier.Other terraneand similar volcanic and associated rocks of stratigraphic, structural, geochemical and geophysical data probable oceanic island origin that crop out a small faulted are allied to this fundamental hypothesis. Both the faunal inlier in southern Slieve Aughty.A mid-Iapetus origin is characteristics and geological, geophysical, structura! and indicated forGrangegeeth by the tectonostratigraphic and isotopic attributes of the Laurentian and Avalonian terranes faunal evidence, and the possibility remains that these rocks appear to be most distinct in pre-Silurian rocks. The only represent the outcrop of a larger zone of volcanic rocks, as possible exception is the Dingle area which containsa suggested by geophysical data that subcrop the central and Silurian succession that is different from coeval sequences in southern belts of the Southern Uplands-Central terrane (cf. the suture zone. The distinct although gradational isotopic Brown & Williams 1985). Such azone of volcanics may characteristics of the sediments and granites of the two sets correlate with similar rocks in theDunnage terrane of of terrailes may indicate thatLaurentian and Avalonian Newfoundland (Keppie 1989). Nevertheless, the bias basement are very different and the two regions were not towards Laurentian provincial faunasGrangegeethat attached in the , as is inadvertently implied by suggest an origin of this terrane north of the Iapetus suture. the classic platetectonic models of Iapetus(e.g. Wilson The trace of the Iapetus suture delineated here differs 1966; Dewey 1969; see also Soper et al. 1989, p. 366). substantially from the traditional suture trace (Phillips et al. However, the shared late deformation episodes 1976; McKerrow & Soper 1989), and lies significantly across in Avalonia (Cadomian) and recently proven in the strike tothe southeast. The dog-legged trace may be Dalradian of Scottish Laurentia (Rogers et al. 1989) may yet influenced by Variscan contraction in the southwest, but it indicate a pre-Iapetus connection. may also be a primary feature inherited from the shape of The Iapetus sutureis apparently no longer represented at the colliding margins or basement tectonic grain, rather than outcrop by the vestiges of a subduction zone. The faults that laterbending, because other lineamentsdistant from traverse the Iapetus suture zone are mainly steep and do not Variscan effects (e.g.the Orlock Bridge fault) display containoceanic rocks (e.g. ophiolites).Instead a cryptic similar deflections. The bend in the suture trace from Slieve

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Phelim to the Shannon Estuary (Fig. 1) may also be due to inverted margin of the French Alps and foreland basin inversion. In: Acadian displacement on a NW-SE aligned basement fault COOPER,M. A. & WILLIAMS,G. D. (eds) Inversion Tectonics. Geological indicated by aeromagnetic data (Morris 1989). Society, London, Special Publication, 44, 87-104. DEPAOLO,D. J. 1981. Neodymium isotopes in the Colorado Front Range and Outcrop and shallow levels of thesuture zone are crust-mantle evolution in the Proterozoic. Nature, 291, 193-196. dominated by steep to vertical fault zones with Caledonian DEWEY,J. F. 1969. Evolution of theCaledonian/Appalachian orogen. sinistral and reversedisplacements. Deeper crustal levels Nature, 222, 124-129. appear to be dominated by more gently dipping shear zones, -1977. Suture zone complexities: a review. Tectonophysics, 40, 53-67. DIXON, P. R., LEHURAY, A.P. & RYE,D. M.1990. Basement geology and although steeper faults may be difficult to image on seismic tectonic evolution of Ireland as deduced from Pbisotopes. Journal of the reflection data(Klemperer 1989). Therefore while the Geological Society, London, 147, 121-132. vestiges of subduction zones may lie deeper in the crust, the ELSDON, R. & TODD,S. P. 1989.A composite spessartite-appinite intrusion surface expression of Iapetus oceanic trenches are replaced from Port-na-Blagh, County Donegal, Ireland. Geological Journal, 24, by steep faultzones along only a cryptic suture may be 97-112. EVANS,J. A., STONE,P. & FLOYD,J. D. 1991. Isotopic characteristics of traced. Ordovician greywacke provenance in the Southern Uplands of Scotland. In: MORTON,A. C., TODD,S. P. & HAUCHTON,P. D. W. (eds) The evolution of thisreview has benefited considerably from the Developments in Sedimentary Provenance Studies. Geological Society, discussion by colleagues too numerous to name although staff and London, Special Publication, 57, 161-172. postgraduatecolleagues Universityat College Dublin are FORTEY,R. M. & COCKS, L. R. M. 1988. Arenig to Llandovery fauna1 particularlymentioned. The paper was given at the Geological distributions in the Caledonides. In HARRIS, A.L. & FEITES, D. J. (eds) The Caledonian-AppalachianOrogen. Geological Society, of, London, Society of Londonmeeting, ‘Palaeozoic terranes in the British Special Publication, 38, 233-246. Isles’, in June 1988 and is a contribution to IGCP Project 233. The FREEMAN,B., KLEMPERER,S. L. & HOBBS,R. W. 1988. The deep structure of Society’s referees are thankedfor improving the manuscript by their northern England and the Iapetus Suture zone from BIRPS deepseismic comments. reflectionprofiles. Journal of the Geological Society,London, 145, 727-740. References HALLIDAY,A. N., STEPHENS,W. E. & HARMON, R.S. 1980. Rb-Sr and 0 isotopic relationships in 3 zoned Caledonian granitic plutons, Southern ANDERSON,T. B. 1987. The onset and timing of Caledonian sinistral shear in Uplands, Scotland: evidence for varied sources and hybridization of County Down. Journal of the Geological Society, London, 144, 817-825. magmas. Journal of the Geological Society, London, l37, 329-348. - & OLIVER,G. J. H. 1986. The Orlock Bridge Fault: a majorLate HARPER,D. T. & PARKES, M.A. 1989. Palaeontological constraints on the Caledonian sinistral fault in the Southern Uplands terrane, British Isles. definition of Irish Caledonian terranes. Journal of the Geological Society, Transactions of the Royal Society of Edinburgh: Earth Science, 77, London, 146,413-415. 203-222. HARPERJ. C. 1952. The Ordovician rocks between Collon (Co. Louth) and ARCHER,J. B. 1981. The Lower Palaeozoic rocks of the northwestern part of Grangegeeth (Co. Meath). 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W. 1987. Strike-slip terranes and a model for the evolution of Ireland. Palaeontology, 19, 615-625. the British and Irish Caledonides. Geological Magazine, W, 405-425. BEAMISH, D.& S~E,D. K. 1986. Geophysical images of the deep crust: - & MURPHY,F. C. 1987. The Silurian of theSouthern Uplands of theIapetus Suture. Journal of the GeologicalSociety, London, 143, Scotland and Ireland as a successor basin to the end-Ordovician closure 489-497. of Iapetus. Journal of the Geological Society, London, 144, 765-772. BENNETT,M. C., DUNNE,W. M. & TODD,S. P. 1989. Re-appraisal of the KELLING, G., DAVIES,P. & HOLROYD,J. 1987. Style, scale and significance of ‘Cullenstown Formation’: implications for the Lower Palaeozoic tectonic sand bodies in theNorthern and Central Belts, southwest Southern evolution of SE Ireland. Geological Journal, 24, 317-329. Uplands. Journal of the Geological Society, London, 144, 787-805. BRANNEY,M. J. & SOPER,N. J. 1988. Ordovician -tectonics in the KEMP, A.E. S. 1987. 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Received 10 January 1991, revised typescript accepted 25 March 1991

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