Journal of the Geological Society, London, Vol. 150, 1993, pp. 669-672, 1 fig. Printed in Northern Ireland 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 Geology, 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-Fair Head-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 Scotland 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 fault, 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 $2 formed at 10 kbar and 6-7 kbar Complex boundary coincides with the Claggan Bay Mylonite respectively. This indicates very rapid uplift during the Zone (Fig. 1C). progressive westerly directed overshear, probably coinciding A low-angle, westerly directed ductile thrust zone, the with the development of the Central Achill Boundary Slide. Central Achill Boundary Slide, divides the nappe zone into The transition from nappe zone to steep zone is exposed western and eastern sectors (Fig.lC). To the west, in the around Dooega and coincides with the NE-SW-trending footwall of the slide, the Grampian Group Croaghaun trace of a downward-facing F2 synform (Fig.lC). At the Metasandstone Formation is inverted. It comprises thickly present erosion level, the southern limb of the synform is bedded, coarse feldspathic metasandstones containing heavy vertical, youngs SE and lies in the steep zone, while its mineral seams; it carries abundant sedimentary structures inverted northern limb is flatter, lying within the nappe 669 Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/150/4/669/4892505/gsjgs.150.4.0669.pdf by guest on 26 September 2021 670 D. H. M. HARRIS ~~.~ ACHILL ISLAND Central Achill Boundary Slide I N$ ~ CaledonianOrogenic Zon~ 500 km A Traceof suture Donega~ N [~1Dalradian ~ ~/~S/~'r TyPe ~Orthogneissb..... t 501tmk ~~X M :!O:pySX / / c~wTheFair 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