Journal of the Geological Society, London, Vol. 150, 1993, pp. 669-672, 1 fig. Printed in

and soft-sediment deformation structures throughout its >2300 m thickness. Younger, but structurally lower, The Caledonian evolution of the Laurentian inverted Appin Group pelites, conglomerates and lime- margin in western Ireland stones occur at Keem Bay (Fig.lC). Lying above and to the east of the slide, the Ridge Point/Dooega Head metasandstone is regarded as equivalent to the Croaghaun metasandstone D. H. M. HARRIS and is generally right way up. It can be traced into the steep Department of , University of Keele, zone where it is succeeded by Appin Group quartzites, Newcastle, Staffs ST5 5BG, UK pelites and limestones. In the nappe zone, Kennedy (1969) recorded evidence for three ductile deformation episodes of which the first two largely controlled the present distribution of stratigraphy. $2, defined by phengite, biotite and magnetite, wraps albite The Highland Border-Clew Bay--Baie Verte-Brompton porphyroblasts which contain a straight or slightly Line, which probably marks the Laurentian margin, passes through Achill Island in western Ireland as a zone of ductile transpression crenulated Si (S1) also defined by phengite, biotite and within which the Clew Bay Complex and Dalradian Supergroup are magnetite, preserving the earliest increments of the D2 juxtaposed. Dextral greenschist facies transpression fabrics in the crenulation of Si (S1) (Bell & Rubenach 1983). Clew Bay Complex record the northwest directed emplacement of The dominant nappe zone fabric ($2) generally strikes terranes, bearing high P-low T mineral assemblages, against NE-SW and dips 20-40 ° SE (Fig.lC). A stretching Laurentia, causing the Dalradian continental margin sediments to lineation, defined by quartz and phengite, plunges E within be thrust westward. There are significant differences in timing, $2, above and below the Central Achill Boundary Slide. In structural geometry and regional facing between W Ireland and the East Achill, the F2 recumbent hanging-wall antiform of the Scottish Highland Border Zone. slide plunges shallowly and consistently NE. Below and to the west of the slide is a major F2 curvilinear footwall The Highland Border-Fair Head-Clew Bay-Baie Verte- synform; the slide is therefore interpreted as a D2 ductile Brompton Line (referred to as the Clew Bay Line) has been thrust. The polarity of F2 regional facing, defined by interpreted by Dewey & Shackleton (1984) as a major Holdsworth (1988) as the direction, parallel to the stretching collisional suture/terrane boundary within the Caledonian- lineation, towards younger rocks, changes across the slide Appalachian orogenic belt (Fig.lA). The line crops out from down to the east (in the east) to up to the west (in the across and Ireland, its westernmost exposure being west). This facing pattern implies that the slide brought on Achill Island in Co Mayo, western Ireland (Fig.lB). inverted rocks from the lower limb of a major F1 fold to lie Here, the Clew Bay Complex to the south, which contains above right-way-up rocks on an F1 top limb. Llanvirn elements and consists of packages of low-grade Cleavage/bedding relationships indicate that F1 regionally marine turbidites, tufts, metabasites and black shales faced to the W and was therefore probably a westerly (Dewey & Ryan 1990; Ryan & Dewey 1991), is juxtaposed directed nappe. F1 and F2 have thus developed in the same against late Proterozoic Dalradian rocks to the north. The westerly directed nappe regime under the same metamor- Clew Bay Complex has much in common with the Highland phic conditions; they therefore record a history of Border Complex of Scotland (Harper et al. 1989). The progressive westerly directed overshear (see Sanderson Dalradian-Clew Bay Complex boundary on Achill Island 1982). Preliminary work on the geobarometry of the S1 and lies within a major zone of ductile displacement and is $2 phengite, however, suggests a virtually isothermal essentially unaffected by brittle faulting. Because elsewhere decrease in the confining pressure of metamorphism of the boundary is a , Achill Island thus offers a unique 3-4 kbar between D1 and D2 during crystallization of the opportunity to study the relationship between Dalradian and albite, a result similar to that reported by Jamieson & Highland Border Complex(= Clew Bay Complex) at an O'Beirne Ryan (1991) in the Fleur de Lys Supergroup of early stage in their juxtaposition. This paper sets out the Newfoundland. Yardley et al. (1987) used oxygen isotope nature and history of events along the boundary zone which geothermometry to estimate a peak metamorphic tempera- potentially bear on understanding the evolution of the ture of 510°C in West Achill. The mineral assemblages Caledonide/Appalachian orogen. defining S1 and $2 are very similar, indicating very little variation in the temperature of their formation. The silica Achill Island Dalradian. Structurally, Achill Island can be per formula unit (pfu) of S1 phengite, within porphyro- divided into a nappe zone to the north, involving Grampian blastic albite, is 3.4 whilst that of $2, wrapping albite, is and Appin Group Dalradian metasediments (Winchester et 3.25. Using the geobarometer of Massonne & Schreyer al. 1987) in which bedding foliation is generally shallow, and (1987, fig.3) at a temperature of 510°C, it is tentatively a southern steep belt within which the Dalradian-Clew Bay concluded that S1 and $2 formed at 10 kbar and 6-7 kbar Complex boundary coincides with the Claggan Bay Mylonite respectively. This indicates very rapid uplift during the Zone (Fig. 1C). progressive westerly directed overshear, probably coinciding A low-angle, westerly directed ductile thrust zone, the with the development of the Central Achill Boundary Slide. Central Achill Boundary Slide, divides the nappe zone into The transition from nappe zone to steep zone is exposed western and eastern sectors (Fig.lC). To the west, in the around Dooega and coincides with the NE-SW-trending footwall of the slide, the Grampian Group Croaghaun trace of a downward-facing F2 synform (Fig.lC). At the Metasandstone Formation is inverted. It comprises thickly present erosion level, the southern limb of the synform is bedded, coarse feldspathic metasandstones containing heavy vertical, youngs SE and lies in the steep zone, while its mineral seams; it carries abundant sedimentary structures inverted northern limb is flatter, lying within the nappe 669

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~~.~ ACHILL ISLAND Central Achill Boundary Slide I N$ ~ CaledonianOrogenic Zon~ 500 km A Traceof suture Donega~

N [~1Dalradian ~ ~/~S/~'r TyPe ~Orthogneissb..... t 501tmk ~~X M :!O:pySX / / Thec~wFair say,no Head" ~~Clew Bay Complex ~ Zone NWMay-'--'--~ '~ Appln GroupDalradlan ~s ountai ~,,,, GnimplanGroup Dalradlan [ outh MayoOrdovieian 4 Km Dooega ~" Connemara ~/..2 ~~ Galwaygranite XSZFIaxial ¢rwce~ Clew Bay

c,ew ~y [ B !:2 axial trace ComplexNorth Ireland Strike of beddlng / C

Fig. 1. (A) The distribution of Caledonian rocks in the North Atlantic (after a reconstruction of Bullard). (B) The trace of the Clew Bay - Fair Head Line through Ireland (after Ryan & Dewey 1991). (C) A geological map of Achill Island.

zone. At and near to the margin of the steep zone, $2 is a pods in the Ooghnadarve Zone (Kennedy 1980), consists of crenulation cleavage, deforming an upright bedding-parallel metasedimentary graphitic quartzite and pelite. In both the fabric. Further into the steep zone, upright $2 crenulation Claggan and Ooghnadarve Zones, bedding, well defined in cleavage becomes overprinted by an $3 crenulation dipping the northern part, becomes progressively obscured as strain shallowly south. Southeastwards, $3 rotates into the vertical increases southwards. By contrast, preservation of bedding and intensifies, transposing S1 and $2 into a vertical is sporadic throughout the Cloughmore Zone with no S1/$2/$3 composite foliation which becomes mylonitic as obvious strain gradients. The dominant fabric in the Claggan the Claggan Bay Mylonite Zone is approached; S1, $2 and Volcanic, Claggan and Ooghnadarve Zones is vertical and $3 fabrics are all defined by the preferred orientation of trends E-W while the main foliation in the Cloughmore phengite and quartz. Cross bedding, locally preserved Zone strikes NW-SE and dips 50°SW. The mineral throughout the Dalradian of the steep belt, indicates that elongation direction throughout the Clew Bay Complex in the sequence youngs southwards, towards the mylonite South Achill plunges 10-40°E and is associated with dextral, zone. Associated with S1, $2 and $3 at all stages of type 2 C-S fabrics (Lister & Snoke 1984), dextral development are dextral shear zones. Bedding strike swings asymmetric quartz boudins and dextral asymmetric fold from NE-SW at the northern margin of the steep zone to pairs. The South Achill Steep Zone is thus interpreted as a E-W in Ashleam Bay adjacent to the mylonite zone; the dextral strike-slip/transpression zone (see Sanderson et al. Claggan Bay Mylonite Zone therefore has a dextral shear 1980). In the Clew Bay Complex, phengite which defines the sense (Fig.lC). transpressional schistosity wraps and is included within albite porphyroblasts; albite has grown during transpression. The Clew Bay Complex. This is an assembly of four The transpressional fabrics overprint and retrogress early metamorphically distinct domains; the Claggan Volcanic metamorphic assemblages in the Cloughmore Zone and Zone, the Claggan Zone, the Ooghnadarve Zone and the Ooghnadarve Zone. In the Claggan volcanics, a transpres- Cloughmore Zone. sional actinolite + albite schistosity augens the green Immediately south of the Claggan Bay Mylonite Zone epidosite pods mentioned above. Significantly the high are the Claggan volcanics containing green pods, 1 m across, pressure amphibole, crossite, is present only in the centre of comprising epidote + crossite + albite wrapped by a blue the pods implying that it has been destroyed at their schistose matrix of actinolite + albite (Gray & Yardley 1981; periphery where transpressional shear is greater and that Yardley et al. 1987). The rest of the Clew Bay Complex, crossite is pre-transpressional. No pre-transpressional with the exception of talc + actinolite + fuchsite bearing metamorphic assemblages were found in the Claggan Zone.

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Locally within pelitic parts of the Ooghnadarve Zone, notheastern Ireland the Dalradian has been transported SSE pods 2-3 mm across and 1-2cm long, not deformed by as the c. 590 Ma Tay Nappe (Rogers et al. 1989). The dextral transpression, carry garnet + chlorite + phengite. Dalradian-Highland Border Complex(--Clew Bay Probing of the garnet revealed an inner core of pyrope Complex) contact in Scotland is the brittle Highland (42-43%) almandine (53%), an outer core of spessartine Boundary Fault. Northeast of the fault, steeply-dipping (33-36%) almandine (41-43%) and a rim of grossular Dalradian rocks occupy the southeast limb of the F3 (25%) almandine (66%). This suggests that the almost Highland Border Downbend, a structure which folds S1/$2 universal greenschist-facies transpressional fabric in the to face steeply down and SE (Shackleton 1958). Harris Oognadarve Zone overprinted assemblages indicative of (1991) suggested that the downbend and the polyphase conditions at or near eclogite facies which subsequently structures in adjacent Arenig-Ashgill Highland Border underwent blueschist/greenschist metamorphism. Unaltered Complex rocks expressed the ductile transpression as- enclaves of pre-transpressional metamorphic assemblages sociated with the impingement of the Highland Border have not yet been found in the Cloughmore Zone, but Complex terrane(s). The Dalradian and the Highland within pelites, garnet + chloritoid, disintegrated and broken Border Complex must have docked before the late Silurian down to chlorite occur. Probing revealed small, irregularly because the c. 411 Ma Lintrathen porphyry ash-flow tuff shaped paragonite flakes within the greenschist-facies rests unconformably on both (Bluck 1985). phengite; these, if remnant and originally associated with In northeastern Ireland, along strike to the southwest, the garnet + chloritoid, indicate that the Cloughmore Zone the Tyrone Igneous Complex (Fig. 1B) crops out along the had been metamorphosed to blueschist facies (Yardley southern margin of the Dalradian and was regarded by 1990). Hutton et al. (1985) as an ophiolite. A U-Pb zircon age from a tonalitic body intruded into the ophiolite during ocean floor metamorphism yielded ages of c. 472Ma (i.e. Summary and conclusions. Stretching lineations defined by Arenig-Llanvirn; Hutton et al. 1985). Dalradian rocks of quartz + phengite across Achill Island, both within the Clew the SSE-facing F2 Sperrin Nappe were transported ESE Bay Complex and the nappe zone, are subparallel. The over the ophiolite on the D3 dextral oblique Omagh Thrust dominant fabric-forming mineral in both the complex and (Alsop & Hutton 1993). Although the age of D3 in Tyrone the nappe zone is phengite which both wraps and is included is poorly constrained, Alsop & Hutton believed the within early MS2 albite. Albite grew during dextral thrusting occurred very soon after ophiolite generation. transpression in South Achill and during westerly directed Further southwest, the Connemara massif (Fig. 1B) nappe translation in North Achill. This evidence indicates represents the only fragment of Dalradian crust south of the that dextral, E-W greenschist facies transpression along the Clew Bay Line. Here, the D3 event took place Clew Bay Line was probably synchronous with westerly synchronously with the emplacement of island-arc plutons directed nappe translation in the adjacent Dalradian. Within and overprinted Barrovian metamorphic fabrics similar to the South Achill Steep Zone bounding the Dalradian those seen north of the line (Yardley et al. 1987). D3 was terrane there is evidence for early eclogite and blueschist associated with a strong E-W extensional lineation (Tanner facies metamorphism. This strongly implies that the South & Shackleton 1979) and lasted from the Arenig to the Achill Zone is a crustal lineament on which there was Llanvirn (Jagger et al. 1988; Cliff et al. 1993). sufficient displacement to exhume rocks which must have The reworking of already existing SSE-directed and travelled from deep crustal levels. Geobarometry of S1 and facing Precambrian nappes in northeastern Ireland and $2 phengite indicates significant isothermal decompression Scotland probably resulted from terrane impingement along or rapid exhumation, triggering albite growth. The South the Clew Bay Line. By contrast, as indicated by this paper, Achill Zone is probably a mid-crustal section through the the earliest, and westerly facing nappes on Achill resulted zone of continental margin collision: the Clew Bay Line from a progressive westerly-directed overshear deformation which forms the Irish sector of the Highland Border-Baie induced by terrane accretion along the Clew Bay Line. Verte-Brompton Line. It resulted from oblique collision of Achill Island and the Baie Verte Peninsula, Newfoundland a terrane which approached the Dalradian, of the (Fig. 1A) have striking tectonic similarities. A ductile shear Laurentian continental margin, from the SE. Westerly zone juxtaposed the Fleur de Lys Supergroup and the directed Dalradian nappe emplacement therefore represents Ordovician Birchy Schist Complex (-=Clew Bay Complex; margin-parallel transport resulting from dextral oblique Winchester et al. 1992). Although detailed fabric relations collision (Shackleton & Ries 1984; Ellis & Watkinson 1987). between the two terranes are unknown (Jamieson 1990) it is believed that the first structures in the Fleur de Lys are Lower Ordovician and, as in Achill, verge away from the Discussion. The regional correlation of Dalradian deforma- lineament (Dewey & Shackleton 1984). Crystallization of tion below recognizes that structures (slides, folds, fabrics early MS2 albite porphyroblasts in the Fleur de Lys was, as etc) which overprint existing folds and fabrics may express with the Achill Dalradian, coeval with isothermal later increments of the same deformation, i.e. developed decompression (Jamieson & O'Beirne-Ryan 1991). within the same stress regime. On Achill Island, D1 and D2 The models of Dewey & Shackleton (1984), Dewey & are expressed as F1 and F2 folds, recognized from their Ryan (1990) and Ryan & Dewey (1991) related D1/D2 control of stratigraphic distribution and the facing properties Dalradian nappes to obduction of an Arenig-Llanvirn of their axial planar cleavages. forearc, the remains of which are exposed south of Achill. The Dalradian and Fleur de Lys Supergroups of Whilst the presence of westerly verging structures on Achill Scotland, Ireland and Newfoundland comprise deformed and Newfoundland support this model, that of the and metamorphosed Laurentian continental-margin sedi- SSE-directed D1/D2 nappes of northeastern Ireland and ment (Dewey & Shackleton 1984). In Scotland and Scotland do not, and a major change in the nature of the

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Received 2 December 1992; revised typescript accepted 26 February 1993.

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