The South Connemara Group reinterpreted: a subduction-accretion complex in the Caledonides of Galway Bay, western Ireland

Journal of Geodynamics 37 (2004) 513–529 www.elsevier.com/locate/jog

The South Connemara Group reinterpreted: a subduction-accretion complex in the Caledonides of Galway Bay, western Ireland Paul D. Ryana,*, John F. Deweyb,1 a

Department of Earth and Ocean Sciences, NUI, Galway, Ireland b University College, Oxford, England, UK

Abstract The Caledonian geology of western Ireland records the collision of two arc complexes with the Laurentian Margin during the closure of the Iapetus Ocean. An earlier complex collided with this hitherto passive margin in the mid-Ordovician during the Grampian Orogeny. Subsequently, arc magmatism developed along the Laurentian margin and continued until the late Silurian collision between Laurentian and Avalonia. The Ordovician volcanic and sedimentary rocks comprising the South Connemara Group lie along the Southern Uplands Fault, the terrane boundary separating these two arc complexes. Palaeontological dating indicates an Arenig-Llanvirn age for part of this complex (Williams, Armstrong and Harper, 1988), making it contemporaneous with the earlier arcs. However, most authors correlate this complex with the northern belt of the Southern Uplands (Morris, 1983; Williams, D.M., 1984. The stratigraphy and sedimentology of the Ordovician Party Group, south-eastern Murrisk, Ireland. Geological Journal, 19, 173–186; Williams et al., 1988), associated with post-Grampian subduction of north directed polarity. We present new field evidence that the South Connemara Group is tectonically disrupted by bedding parallel shear zones and that contacts previously interpreted as conformable are marked by units of tectonic me´lange. We present structural and provenance arguments consistent with the me´langes forming above a north-dipping subduction zone after 463Ma. This Group is reinterpreted as occurring within a subduction–accretion complex that was generated by the accretion of early Ordovician mafic seamounts into a post-Grampian trench, thus reconciling the age of the Group with its generally accepted tectonic setting. We discuss the regional significance of this finding with respect to the Caledonide-Appalachian orogeny and argue that this is the site along which the Iapetus Ocean closed. # 2004 Elsevier Ltd. All rights reserved.

* Corresponding author. E-mail addresses: [email protected] (P.D. Ryan), [email protected] (J.F. Dewey). 1 Present address: Geology Department, UC Davis, Davis, CA, USA. 0264-3707/$ - see front matter # 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.jog.2004.02.018

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1. The Geology of the South Connemara Group 1.1. The regional tectonic setting The Caledonian geology of western Ireland can be divided into four major terranes (Fig. 1), which record the opening and suturing of the Iapetus Ocean (see Dewey and Mange, 2000 for a review). The Grampian terrane, north of the Highland Boundary Fault, Fair Head- Clew Bay Line (HBF-FCL, Fig. 1) comprises the Dalradian Supergroup and small basement inliers which are interpreted as the Neoproterozoic margin of Laurentia. The South Mayo Terrane is assigned to an earliest Ordovician island arc complex that formed above a south-dipping (relative to modern geography) subduction zone, (Dewey and Ryan, 1990; Dewey and Mange, 2000; Draut and Clift, 2001). The destruction of the intervening Iapetus oceanic lithosphere and northern migration of this arc to collide with the Laurentian margin caused the Grampian Orogeny

Fig. 1. Location map showing the positions of the Grampian Terrane, bound to the north by the Great Glen Fault (GGF) and to the south by the Highland Boundary Fault (HBF- Scotland), Fair Head- Clew Bay line (FCL- Ireland), which is generally taken as the southern limit of rocks deposited on Laurentian basement. The early to mid Ordovician arc and associated rocks of the South Mayo Terrane lie to the south of the FCL but to the north of the Southern Uplands Fault (SUF). The Connemara terrane is believed to be tectonically emplaced outboard of the South Mayo Terrane during the late Ordovician. The Southern Uplands- Longford Down terrane is interpreted as forming after post-Grampian subduction flip. The South Connemara complex lies immediately south of the westward extension of the SUF.

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(Dewey and Shackleton, 1984; Ryan and Dewey, 1991, Dewey and Mange, 2000). This event is dated at 474–463Ma (Friedrich et al., 1999; Soper et al., 1999) and late stages are associated with a subduction flip (Ryan and Dewey, 1991; Dewey and Mange, 2000) causing Iapetus lithosphere subducting northwards beneath Laurentia. The Connemara terrane has strong geological parallels with the Grampian Terrane and is believed to have been tectonically emplaced during late stages of the Grampian event (Leake et al., 1983; Leake and Tanner, 1994). The Longford-Down Terrane is a westwards continuation of the Southern Uplands Terrane of Scotland which is generally thought to have formed as an accretionary prism developed above the post-Grampian, north-dipping subduction zone. These rocks were deformed during the final closure of Iapetus to produce the Caledonian Orogeny in latest Silurian times (Dewey and Strachan, 2003). The South Connemara Group (SCG) is generally assigned to the Longford-Down Terrane (Williams, et al., 1988) and its location has been used to define the major post-Grampian terrane boundary in western Ireland (Fig. 1), the Southern Uplands Fault (SUF) (Leake, 1963; Max and Ryan, 1975; McKerrow, 1988; Hutton, 1987). However, there has been some considerable debate about the location and nature of the post-Grampian arc(s) lying to the south of the SUF (Morris, 1983, 1987). The matter is further complicated by cherts (Golam Formation, see below) within the SCG yielding fauna that are being broadly coeval with those recovered from arc complexes in the South Mayo Terrane (Williams et al., 1988). This raises a fundamental problem. Either the Southern Uplands arc system had to exist prior to the Grampian Orogeny, and could not have been formed after post-Grampian subduction flip; or the stratigraphic and structural arguments for the position of the SUF in western Ireland, supported by ample geophysical evidence (Armstrong et al., 1998; Smith et al., 1997; O’Reilly et al., 1999), are incorrect. This study reinvestigates the field relationships of the SCG with a view to trying to resolve this problem. 1.2. The South Connemara Group The SCG occurs as a roof pendant of the early Devonian Galway Granite cropping out over an area of some 10 square km on the islands of Gorumna, Lettermullen and Crappagh, NW Galway Bay (Fig. 2). It also crops out in fault contact with the Dalradian of Connemara in the small offshore islands of Skirdmore and Doonguddle, some 16 km further west in the Atlantic Ocean. This account deals with the onshore outcrops only; the offshore rocks are extremely difficult to access and we rely upon the account of Ryan and Max (1975). The rocks are extremely wellpreserved in spite of pervasive hornblende hornfels facies contact metamorphism and exposure is typically between 30–100%. All units dip steeply to south but young to north. A gravity survey suggests that they comprise a wedge shaped body about 250 m thick at southern end (Fairhead and Walker, 1977) fading to a feather edge in the north. All authors since McKie and Burke (1955) have proposed a similar stratigraphy and almost identical maps for the SCG, with the work of Ffrench and Williams (1984) representing the current state of knowledge (Table 1 and Fig. 3). A lower mafic volcanic and sedimentary unit (Lough Faoilea´n Formation) passes upwards into a thick clastic sedimentary sequence (Ryan’s Farm, Golam and Lettermullen Formations) which is overlain by a second thick mafic volcanic unit (The Gorumna Formation). Cherts of the Golam Formation have yielded a fauna of ArenigLlanvirn age (Williams et al., 1988). The coarse clastic sequences of the Lettermullen Formation have been interpreted as arc-derived trench fill (Ffrench and Williams, 1984). Previous authors

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Fig. 2. Geological map of the South Connemara Complex.

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Table 1 Comparisons of the various stratigraphic successions proposed for the South Connemara Group McKie and Burke

Ryan and Max

Upper Basic Group 1524 m Greywacke Group 762 m Siliceous Group 274 m Lower Basic Group 457 m

Gorumna Formation Lettermullen Formation

Golam Formation

gap Skird Formation

Ffrench and Williams 7000 m

Gorumna Formation 1500 m Lettermullen Formation 1035 m Golam Formation 350 m Ryan’s Farm Formation 112 m Loch Faoilea´n Formation 234 m

gap Skird Formation

This work

Lettermullen Formation 618 m Golam Formation 324 m Tectonic me´lange Gorumna Formation < 750 m (N.B. This formation is now placed at the base of the sequence as we recognise that it has been structurally emplaced over the other formations and is equivalent to the mafic volcanic rocks within the Lough Faoilea´n Formation). Not considered here

have always assumed that this was a primary stratigraphy with no structural repetition because younging is consistently to the north, the very few small-scale reversals were associated with slumping and the well-preserved sedimentary sequences do not contain any previously-recognised thrust, or significant strike-parallel faults. However, in addition to the enigma of the age of this Group discussed above, the published stratigraphies place a sedimentary thickness of some 1400 m stratigraphically beneath a thick sequence (Gorumna Formation) exhibiting MORB geochemistry (Ryan et al., 1983). It is, of course, possible for basalts to erupt in a trench by ridge subduction and/or at a migrating triple junction involving a ridge and trench as two of the three components. We dismiss this for the SCG because there are none of the intrusive and thermal effects that one would expect. We present a new map (Fig. 2) for the SCG which shows many hitherto-unrecognised bedding parallel shear zones that commonly occur within the basalts or at the basalt-sediment contacts (Fig. 2); few lithological contacts are primary. Many of the shear zones are within the basalt horizons rather than the more ductile sedimentary portion of the sequence implying a tectonic assemblage for the complex, rather than disruption by later thrusting. The sequence is repeated tectonically across these shear zones and occurs in six imbricate slices, not in one continuous stratigraphy. We also recognise tectonic me´langes that were previously attributed to a sedimentary origin (Ryan and Max, 1975; Ffrench and Williams, 1984). We use this map to reinterpret the stratigraphy of the SCG (Fig. 3).

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Fig. 3. Comparison of the previously established stratigraphy for the SCG (Ffrench and Williams, 1984) with the tectono-stratigraphy proposed in this study.

2. Stratigraphy 2.1. Gorumna Formation The Gorumna Formation, the base of which is not seen, is the oldest formation within the SCG and consists of mafic volcanic rocks and shallow intrusives. We define the type area as the south shore of Gorumna island where the relationships with the overlying units are well exposed. The formation occurs in six imbricate slices, three of which occur on the island of Lettermullen, and all six along the SE coast of Gorumna Island. We use the name Gorumna Formation, as have previous authors, for the basic rocks of Gorumna Island, but assign them to the base, not the top, of the SCG. This is because in the few sections where unsheared primary contacts exist, such as in the type area, the mafic volcanics always young upwards towards the contact with the Golam Formation. This reinterpretation implies that the mafic volcanic component of the Lough Faoilea´n Formation is the same stratigraphic unit as the Gorumna Formation. The name Gorumna

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Formation has precedence in the literature (Ryan and Max, 1975) and we, therefore, use this name for the mafic volcanic rocks within the SCG. Fig. 3 attempts to show how this mafic unit is repeated structurally to produce the apparent stratigraphy recognised by earlier authors. Most of the Formation consists of featureless, fine-grained, sheared basalts whose shear fabric has been partly annealed by hornblende-hornfels contact metamorphism. This monotonous texture led Ffrench and Williams (1984) to propose that much of this formation may be intrusive. However, the local preservation of volcanic or sub-volcanic textures in low strain zones suggests that this is a sequence of sheared volcanic rocks, not the coarser gabbros one would expect if it were a sill of 1 km thickness. The basalts vary from massive to pillowed with scoriaceous and hyaloclastite units. Vesicular pillows up to one meter with bun-shaped tops have interstitial limestone and red chert on Lettermullen, Gorumna, and Golam. The lavas on Lettermullen clearly young to the north and dip steeply southward. They are intercalated with a unit of pillow lava breccia, whose contact is slightly oblique to the structurally overlying me´lange zones to the north. Commonly, the pillow lavas can be shown to be isolated balls but, more rarely, crosssections of elongate lava lobes occur. On Gorumna, the pillow lavas are cut by fine grained ophitic sills and are in direct contact with the overlying Golam Formation (M.R. L887207). Locally (Lettermullen M.R. L830214), coarser grained ophitic dolerites similar to those found at relatively deep levels in sheeted dike complexes occur as tectonic slices. Previous authors have described many sedimentary intercalations with the Gorumna Formation, including both turbidite and chert horizons. We attribute these to tectonic imbrication, a relationship that is welldisplayed along the south-eastern shores of Gorumna Island (Figs. 2 and 3). We specifically define the Gorumna Formation as including only mafic volcanic rocks. 2.2. Provenance of the Gorumna Formation The association of volcanic textures such as vesicular small pillows, pillow lava breccias, dykes and sills with MORB geochemistry is consistent with an oceanic origin for this formation. We believe the carbonate within the pillow interstices to be sedimentary, and not ophicalcite, because the enclosing pillows show no alteration of their once glassy margins. Also, limestone fragments occur in adjacent me´langes (Ryan and Max, 1975) and a single remnant of a shelly fauna is found in the me´lange units (Williams et al., 1988). This and the vesicularity of the pillows, implies eruption in shallow water. The considerable thickness of overlying pelagic cherts (Golam Formation) suggest that the mafic rocks were formed considerably before their emplacement and obduction would be unlikely in view of their thermal maturity. Therefore, we interpret the Gorumna Formation as being part(s) of an accreted sea-mount. 2.3. Golam Formation The Golam Formation is dominated by white-weathering grey chert with maroon, green and grey argillite and cherty argillite interbeds, typically on a centimetric or millimetric scale. The type area is at Golam Head. The base of the formation occurs in southern Gorumna, Lettermullen and on Golam Head, where manganiferous cherts rest directly upon pillow lavas or fine-grained ophitic dolerite, commonly in pockets up to 1 m. The lower levels of the formation are highly manganiferous; locally the cherts have specific gravities of up to 3150 kg.m 3 and contain up to

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19% pyrolusite, although thin seams contain up to 90% pyrolusite (Fig. 4c). The MnO2 content systematically decreases up section indicating dilution of the manganiferous cherts by more siliceous components associated with a colour change from deep red or deep green at the base to lighter reds and whites near the top of the formation. These gradational changes indicate that the thickness on Lettermullen is at least partly original and not a result of structural repetition. However, this is a post-accretion thickness (see below). We agree with earlier authors on the nature of the Golam Formation but assign the slivers and larger bodies of chert (Fig. 2) that had been previously attributed to the Gorumna or Lettermullen Formations to the Golam

Fig. 4. Textures and structures within the South Connemara Complex me´lange unit and the Golam Formation which we suggest represent an inner trench wall mega shear zone like that on the Kashima cross section of Fig. 7b. (a) Red chert lozenge in lenticular sheared chert/argillite matrix; (b) Lenticular fine-medium grained sandstone phacoidal lenses with shear laminated argillite margins; (c) Fold in recrystallised Golam Formation chert layer, showing annealed triple junction quartz fabric, with pyrolusite rich cherty argillite (dark layer). (4) White and green lenticular chert phacoids in cherty argillite matrix; (e) Grey- maroon chert lenticule incased in a sheared phacoidal sandstone matrix similar to that of 4b. Asymmetric tails are consistent with left lateral shear; (f) Sub-rounded elongate chert lozenges in a sheared cherty argillite to fine sandstone matrix with a thin unit of phacoidal sheared fine sandstone (under the lens cap).

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Formation. In every case where exposure permits direct observation, mafic rocks overlying cherts are in tectonic contact with them. 2.4. Provenance of the Golam Formation The lower part of the Golam Formation comprises typical deep sea cherts with high manganese content that formed below CCD. Upper levels have been diluted with distal continentally-derived detritus (see Ffrench and Williams, 1984) similar to that comprising the overlying Lettermullen Formation with which they show a gradational lateral and vertical contact. We interpret this as resulting from cherts lying on oceanic crust being transported towards a continental margin (see below) and becoming intercalated with distal turbidite flows, as occurs in the Nankai Trough (Taira et al., 1989). Even where no major de´collements are recorded, the thickness may not be original. In the Kurile trench, the pelagic sediments have been thickened tectonically by a factor of 2 as they approached the trench, and inter-tongue with distal turbidites before being subducted or accreted (Schnu¨rle et al., 1995). 2.5. Lettermullen Formation The type area of the Lettermullen Formation is on Lettermullen Island (Ryan and Max, 1975; Ffrench and Williams, 1984). The formation is some 618m in thickness and its base occurs on SW Lettermullen where the formation exhibits a gradational contact with the underlying Golam Formation (Fig. 2). We follow Ffrench and Williams (1984) in defining this contact at the first thin graded sandstone horizon. These turbiditic sandstones, with slump folds up to one meter, coarsen upwards to thicker granule-grade beds of over one meter containing conglomeratic layers and then fine upwards. Ffrench and Williams (1984) divided the Formation into 5 facies (Table 2). We recognise facies 1 to 4, but attribute facies 5 to a structural repetition of the Golam Formation. Making allowances for such structural repetition we propose a lesser thickness than did Ffrench and Williams (1984), see Table 1 and Fig. 2. Table 2 Facies within the Lettermullen Formation Facies Number (Ffrench and Williams, 1984)

Lithology

Comments (this work)

Facies 1

Boulder, cobble and pebble conglomerates with subordinate coarse sandstones, with rare channelling. Thick sandstones (0.2–1.1 m) with thin mudstone interbeds Medium grained thin sandstones (0.05 m) with mudstone interbeds (0.01 m) Fine grained sandstones, siltstones and mudstones (0.02 m) Bedded cherts, locally slumped and disrupted, locally found on Lettermullen

Range from 245 m to 30 m in thickness

Facies 2 Facies 3 Facies 4 Facies 5

Locally contain seismites (N. Rast, personal communication 1997)

Interpreted as a structural repetition of the Golam Formation (Fig. 2)

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2.6. Provenance of the Lettermullen Formation Ffrench and Williams (1984) give a detailed account of the sedimentology of the Lettermullen Formation. The conglomerates are dominated by rounded to sub-rounded igneous and metamorphic clasts from 1cm to 1m, and the sandstone petrology, with abundant detrital almandine, suggests a recycled orogen provenance. Ffrench and Williams (1984) concluded that the source area was reasonably proximal and lay to the west and that these rocks were a possible trench fill sequence. This conclusion is consistent with the SCG forming in an accretionary complex. The following clast lithologies are recorded in Facies 1: high level quartz-rich granites both foliated and unfoliated; foliated tonalites; migmatitic grey andesine/oligoclase gneisses; metagabbros (rare); pebbly quartzites; quartzites showing refolds; marble (rare); biotite, muscovite, and amphibole semipelitic schists; granodiorite; quartzdiorite; greenschist phyllites and slates; quartz-plagioclase porphyries; and intraformational sedimentary clasts. The bulk proportions of clast types are: igneous 37%; metamorphic 50%; sedimentary 13%. Metamorphic clasts increase up-section. The association of foliated granitoids, migmatites and metagabbros with metasediments, particularly pebbly quartzites, is typical of Connemara which lies immediately to the north. Therefore, we conclude that the Lettermullen Formation was derived from the Barrovian metamorphic complex of Connemara, or some very similar part of the Grampian Terrane, after the emplacement of the metagabbros and polyphase folding during the Grampian orogeny (474–463 Ma, Friedrich et al., 1999). 2.7. Me´lange zones The Gorumna Formation, in SW Lettermullen, is tectonically bounded to the north by me´lange units. The me´lange matrix varies from muddy, volcanic, sandy to calcareous material, in well defined layers on a scale of 0.1 m to several meters. The sub-rounded to sub-angular blocks are elongate in foliation, comprise mainly white, red, and epidotic green chert, metabasalt, sandstone and limestone from 1cm to 1m, and show varying states of strain. Although there is a strong layer-parallel shape fabric (Fig. 4a,d and f), blocks vary from angular and relatively undeformed (Fig. 4d and f) to highly-deformed (Fig. 4a and e), behaving as porphyroclasts within a mylonitic matrix. The angular blocks do not show pressure shadows, whereas the lenticular blocks may show tails that are either symmetric (Fig. 4a) or sinistrally asymmetric (Fig. 4e). There are all gradations from boudinaged chert and fine sandstone layers to intensely sheared and shredded lenses. Some intensely-foliated cherty argillite zones, up to one meter thick, do not contain blocks. The sandier me´lange layers are commonly phacoidal (Fig. 4b and e). A large pillow lava ‘knocker’, more than 10 metres across, capped by limestone, (MR L830213) separates this unit from a me´lange that contains large blocks of conglomeratic and sandy Lettermullen Formation and Golam Formation. Ffrench and Williams (1984) attributed the southerly me´lange unit containing limestones and metavolcanics to the Loch Faoilea´n Formation and the northerly clastic dominated unit to the Ryan’s Farm Formation. The varying state of strain of the knockers clearly implies a tectonic, not stratigraphic, origin for these units, and we recommend that the use of these two formation names be discontinued.

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2.8. Discussion The SCG contains 3 primary formations: The Gorumna Formation (basic volcanics), the Golam Formation (cherts and distal turbidites) and the Lettermullen Formation (proximal turbidites). The complex stratigraphy reported by earlier authors does not allow for the structural repetition of these units (Fig. 2). This simplified stratigraphic scheme has several consequences. First, the locality used for determination of the Arenig-Llanvirn age for cherts (MR L 826241, see Fig. 2) within the SCG lies within the me´lange zone described above. Therefore, its stratigraphic position is unknown but may reasonably be attributed to the manganiferous deep sea cherts of the Golam Formation. The thicknesses of chert within the Golam Formation may represent a considerable time interval (Williams et al., 1988) suggesting that the Gorumna Formation may be somewhat older. The Lettermullen Formation is best dated by correlation with events in Connemara, which underwent rapid uplift from 470–461Ma. This implies that the Group ranges in age from, perhaps, Arenig to Caradoc.

3. Structure The bedding generally dips to the south and youngs to the north (Fig. 5a) with local small to medium scale folds plunging gently westwards (Fig. 5c), commonly occurring near the contact between the Golam and Lettermullen Formations. Cleavage, associated with these folds is often sub-parallel to bedding, but dips to the south slightly less steeply (Fig. 5b and d), implying that bedding is upward-facing on the cleavage (Fig. 5e). The cleavage broadly parallels shear zones in the Gorumna Formation and the layer-parallel fabric within the me´lange zones (Fig. 5d). Cleavage transects bedding by 12o clockwise (Fig. 5d) consistent with sinistral transpression. Folds, which vary from open to tight, show south-side-down vergence and, in plan view, are S-shaped implying sinistral shear. In places, the shear foliation is folded by S-shaped folds. Ramp cut-offs, especially in the cherts, indicate a south-side-down sinistral sense of shear (Fig. 5d and e). A stretching lineation and mullions occur locally, mostly within the layer parallel fabric of the me´lange zones, that have variable orientation (Fig. 5c) but whose mean lies within the mean cleavage plane and makes an angle of 74 clockwise with the mean fold axis orientation. If the fold axes were initially statistically horizontal, this implies a steep eastward pitch, consistent with north-side-up sinistral transpression (Fig. 5e). In five localities, within the me´lange zone on Lettermullen, the fibre lineation plunges steeply SSE (Fig. 5d). If the fabric is rotated so that it dips northwards, these lineations plunge steeply eastward, consistent with sinstrally oblique transport. The bedding becomes right way up and still remains upwards facing (Fig. 5e).

4. Interpretation We interpret the Gorumna Formation as being parts of an accreted seamount(s) (Fig. 6) part of which was above CCD (vesicles, carbonates). Its disrupted nature we attribute to accretion after break-up caused by extensional faulting as the plate passed over the flexural bulge of the associated trench. A modern analogue is perhaps the accretion of the Kashima seamount in the Japan

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Trench (Konishi, 1989; Taira et al., 1989). Sedimentary me´lange units may have initiated as the seamount broke up along normal faults and slumped into the thickening cherts and distal turbidites. However, the fabrics suggest that most me´langes were formed tectonically during accretion. We infer the manganifeous cherts of the Golam Formation to be a deep sea deposit, formed before the Caradoc change in pelagic fauna. This formed a drape over the sea floor and may have onlapped onto the lower slopes of the Gorumna seamount for a considerable time. This sequence has been thickened tectonically by thrusting and perhaps by dewatering associated with layer parallel shortening prior to its accretion. The upward passage of these cherts into a fine grained turbidite sequence occurred as the ocean floor deposits approached the trench, and were intercalated with distal flows originating from the arc side of the trench (Fig. 6). The Nankai Trough is a modern analogue (Taira et al., 1989). We believe that the Lettermullen Formation was derived from an arc and that the trench in this vicinity was near the mouth of a fan. Ffrench and Williams (1984) review the evidence for this formation being deposited in a trench which includes: easterly, trench-parallel transport directions; proximal to distal facies relationships; the lack of a typical distal fan sequence and a

Fig. 5. Structural data for the South Connemara Complex, see text for explanation. (a) Poles to bedding (S0, N=51); (b) Poles to cleavage (S1, N=36); (c) Axes of minor folds, stretching lineations and mullions; (d) Mean S0 and Mean S1 showing S1 is12o clockwise of S0. Poles to five shears which record top down to the SSE and axes and axial planes of dextral kinks are also plotted; (e) Cartoon to show that if the structural data is rotated from its present north dipping attitude to the south dipping attitude inferred at its time of formation, then the sense of shear is sinistrally oblique, top to the south. Bedding remains upward facing.

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Fig. 6. (a) Section of the Nankai Trough subduction zone (after Taira et al., 1989) showing early off-scraped accreted trench turbidites, late accreted cherts and subducted oceanic crust. (b) Section of the Kashima no. 1 seamount (after Taira et al., 1989) showing its extensional break-up as it enters the trench and the mega shear zone that truncates the footwall seamount and hanging wall folded sediments.

sheet flow transport mechanism for many of the conglomeratic units. They also note that no suitable source lay to the south at this time. We argue that the variety of conglomerate clasts strongly implies a source from Connemara to the north, or some very similar terrane, that had undergone the Grampian Orogeny (474–463 Ma: Soper et al., 1999). Although the meta-sedimentary clasts are of typical Dalradian lithologies, the migmatites and calc-alkaline meta-gabbros are only found in the Dalradian of Connemara. The granites and lesser silicic volcanics are not preserved in any quantity in the Dalradian of Connemara. We interpret these as being either derived from the arc whose collision caused the Grampian Orogeny (Ryan and Dewey, 1991) or from renewed volcanism associated with the development of the South Connemara trench. Fig. 7 shows a cartoon summarising the development of the SCG immediately prior to accretion of the Gorumna seamount.

5. Regional significance The lithology, structure and tectonic position of the SCG, along the SUF terrane boundary, are consistent with a subduction-accretion complex origin. The clasts of the Lettermullen Formation

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Fig. 7. Palaeotectonic cartoon showing the late-Ordovician approach of the Gorumna seamount(s) to the South Connemara trench.

imply a post- or syn-Grampian Orogenic age for the Group. The older age for the Golam Formation reflects its deposition within an ocean basin prior to its accretion. The thickness of the Golam Formation, although it may be attributed partly to post-depositional processes, implies that this Formation may have taken some time to be deposited (Williams et al., 1988). The lower units show no sign of dilution with continental material, both of these facts imply they were deposited some considerable distance away from the site of the South Connemara trench, well before accretion. The overlap in age with part of the South Mayo Terrane arc rocks to the north is explained if the Gorumna and Golam Formations were part of an oceanic plate which lay outboard (southwards) of this arc, possibly generated at the main Iapetus spreading centre in the early Ordovician. The structural data in conjunction with the likely provenance of the Lettermullen Formation are consistent with accretion above a north-dipping subduction zone. The SCG is a particularly well-exposed segment of a zone of Ordovician subduction-accretion that runs from Connemara, along the northern edge of the Longford-Down Massif to the Northern Belt of the Southern Uplands of Scotland, where alkali and tholeiitic ‘‘within plate’’ basalts, associated with limestones, occur as tectonic slices or knockers in the Caradoc Marchburn Formation (Barnes et al., 1995; Phillips et al., 1995). Controversy has surrounded the origin and incorporation of these basalts, some of which, in the Moffatt Shale, have an arc affinity. We believe that they are all tectonically-accreted fragments of seamount and island arc volcanoes. An island arc volcano arriving in a trench, perhaps by the migration of a trench/trench/trench triple junction, collides with an inner trench wall in much the same way as a seamount. In Newfoundland, the Annieopsquatch mafic complex has tectonic contacts along the southern margin of the post-flip Notre Dame arc (Van Staal et al., 1998) in the same tectonic position as the SCG and the

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Northern Belt of the Southern Uplands and may similarly represent a portion of a clipped off seamount. We visualize the tectonic milieu of the SCG during the middle Ordovician (Llanvirn) as a trench along the southern margin of Laurentia to which the Connemara Grampian terrane was the backstop against which trench and oceanic material was accreted that involved combined processes very similar to those now operating in the Nankai Trough (Trench) and on the Kashimi No.1 Seamount. In the Nankai Trough, hemipelagites, on oceanic crust, with a conformable cap of trench turbidites, travel arc-wards. Packets of trench turbidites are clipped off and accreted, whereas the hemipelagites travel beneath a detachment zone basal to the accreted turbidite packages eventually to be accreted where the accreted zone rapidly increases in vertical thickness. The trench turbidites and, to a lesser extent the hemipelagites, are layer-parallel shortened before accretion. The basaltic oceanic crust is subducted. This is the steady state background that we believe explains the Golam hemipelagites and Lettermullen turbidites. The accretion of basalt slices (Gorumna Formation) can be understood as the approach and collision of seamounts such as the Kashimi No 1. The seamount and its limestone carapace, are extensionally faulted as they travel over the outer trench lithosphere bend. Talus breccias of basalt, limestone and chert accumulate along fault-line scarps. On the inner trench wall, folded siltstones overly a mega-shear zone with toe breccias that slice across the broken seamount (Taira et al., 1989). We suggest that the SCG me´lange zone was generated initially as talus and toe breccias of basalt, chert, turbidite, and limestone, which were then incorporated and deformed in an inner trench wall shear zone. Thus, the intermittent arrival of seamounts in a trench perturbs the normal ‘‘steady state’’ deposition of hemipelagites and turbidites to generate mini-collisions in which the seamount is broken and accreted in slices against the inner trench wall. We, therefore, propose that the SCG developed in a subduction-accretion complex formed after the Grampian orogeny above a north dipping subduction zone. This is consistent with the arc collision, subduction flip model of Ryan and Dewey (1991) for the west of Ireland that has been extended throughout the northern Appalachians (Van Staal et al., 1998). Subduction-related volcanism continued in western Ireland until Wenlock times (Menuge et al., 1995) as did the evolution of the Southern Uplands- Longford Down accretionary complex. The Iapetus ocean closed during Pridoli times (Dewey and Strachan, 2003). Subduction related magmatism had essentially ceased along the Avalonian margin by the Caradoc, implying that the final closure of Iapetus which led to the collision of Avalonia with Laurentia, took place along this northerly subduction zone, which runs through Galway Bay.

6. Conclusions We propose a revision for the stratigraphy of the SCG, recognising in stratigraphic order the Gorumna, Golam and Lettermullen Formations. We propose that this stratigraphy was assembled into a subduction-accretion complex by the accretion of seamount(s) into the fore-arc wedge of the trench along which the Iapetus Ocean closed. This revision of the stratigraphy further supports the correlation of the SCG with the Northern Belt of the Southern Uplands- Longford Down and supports the westwards continuation of the Southern Uplands Fault along the north shores of Galway Bay. We suggest tectonically bound mafic complexes of similar age along strike

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in both Scotland and Newfoundland may have been formed in a similar manner. Finally, we note that it is remarkable that the 2000 square kilometre Lower Palaeozoic inlier of western Ireland preserves an almost complete record of an Ordovician arc collision–subduction flip event.

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