NW Ireland Research Articles

The clew bay group: A displaced terrane of highland border group rocks (Cambro‐ordovician) in Northwest Ireland

AbstractMarine turbidites, tuffs, black mudstones and conglomerates of the Cambro‐Ordovician Clew Bay Group, were deposited in the E–W elongate transtensional Clew Bay Graben that is centred on Clew Bay, NW Ireland. The group is characterized by extensive sedimentary deformation and mass movement on slides; olistostromes, autoclastic breccias and course proximal turbidites are interbedded with apparently less disturbed but often overturned sediments.The Clew Bay Group lies structurally above serpentinized dunite/harzburgite breccias, schistose carbonate peridotites, and other basic and ultrabasic igneous rocks that have ophiolitic geochemical affinities; the sediments may have been in part deposited upon oceanic crust. Ophiolites and sediments that now rest on the Clew Bay Thrust abut Silurian shallow water strata in which the main tectonothermal history, associated with sinistral transcurrent faulting along the thrust zone, is dated at about 410 Ma. The sole thrust dips northward and coalesces with a major deep structure along the Fair Head‐Clew Bay Line (FCL) that is the western continuation of the Highland Border Fault of Scotland. Blueschist relics in the Dalradian immediately to the north of the FCL indicate that subduction was active early in the history of the late Cambrian–early Ordovician Grampian orogeny. The Clew Bay Thrust was a sinistral, transpressional shear zone late in its history, but it probably originated as an obduction complex.The Clew Bay Group cannot be traced into sedimentary, metamorphic or structural continuity with the adjacent Dalradian to the north or Ordovician and Silurian rocks in the South Mayo Trough to the south. It should be considered as a distinct terrane (Clew bay Terrane) or a subterrane of Highland Border‐type rocks along the southern margin of the Grampian Terrane.

The analysis and interpretation of stratification cycles

Beds in carbonate successions often form regular patterns. Either there is a cyclic repetition of beds with different thicknesses, or bundles of beds occur separated from each other by some distinct layer. Any such group will be called a stratification cycle. Cycles of this type are from 0.5 m to several meters in thickness, and they seem to be present throughout the geological column. The analysis of sedimentary cycles involves three stages.(1) Regional mapping is necessary to understand the facies of the sediment. Establishing the lateral persistence of bedding eliminates the possibility that bedding is caused by diagenetic processes.(2) Petrographic examination and, in particular, analyses of primary structures are used to reconstruct the sedimentation conditions during cycle formation. Such conditions are extremely important for judging the time content of individual beds.(3) The quantitative analysis of vertical sections using power spectral methods can find superimposed cycles and enables one to judge their degree of regularity. The latter approach is illustrated using lower Carboniferous limestone sections from NW Ireland as an example. Spectral analysis finds a stratification cycle of 1 to 3 m in thickness. Each cycle consists of between five to seven limestone beds. Upper Jurassic limestone marl successions from southern Germany and late Cretaceous and early Tertiary successions from Italy show a very similar cyclic pattern. The scale of the cycles and their regularity suggest that they are ultimately caused by variations in the earth's orbit, possibly by climatic cycles as was postulated by Milankovitch. However, the exact mechanism which links sedimentation with orbital variations is not known.

Discussion on a tectonic model for the emplacement of the Main Donegal Granite, NW Ireland

Mr M. J. Arthur writes: Hutton (1982) suggested that the Main Donegal Granite (MDG) was accommodated in a NE–SW zone of tension ('elongate hole'), contemporaneously with NE–SW sinistral shearing around that zone about 400 Ma ago (end Silurian–early Devonian times). Calculations from strain studies suggested that the sinistral displacement was about 20 km at its maximum, across the NW boundary of the MDG, but only about 5 km where the NW and SE boundaries united and continued to the SW (Hutton 1982). No figure was given for the offset across the SE boundary, though it was considered to be a major sinistral shear zone (Hutton 1982). However, Hutton (1982) proposed an unorthodox ‘crack opening model’ for the emplacement, which is adequately explained by the well established pull-apart model. The MDG has the rhomboidal/elliptical shape (Fig. 1) typical of a pull-apart produced at a releasing bend (cf. Crowell 1974, fig. 3), resulting from the intersection of the two throughgoing parts of a left-stepping sinistral wrench fault (cf. Ramsay 1980, fig. 18). The MDG is about 50 km long and 11 km wide and thus has a length/width ratio of about 4.5, whilst the ratio for pull-aparts is commonly between 2 and 5 with an average of about 3 (Aydin & Nur 1982). Emplacement into an ‘elongate hole’ contemporaneously with shearing around it, is precisely a characteristic expected of a pull-apart. Although pull-aparts are perhaps best known for their occupation by sedimentary deposits they also often contain contemporaneous igneous

Late Proterozoic High‐pressure granulite facies meta‐morphism in the north‐east Ox inlier, north‐west Ireland

Abstract High‐pressure granulite‐facies gneisses in the NE Ox inlier in NW Ireland have undergone extensive Caledonian retrogression. In the local area of Slishwood, however, reworking was negligible and the gneisses (psammites, semipelites, pelites, metabasites and ultramafites) preserve evidence of P–T changes at high grade which mainly post‐date pre‐Caledonian polyphase deformation. Temperatures reached 850–900°C (based on garnet‐clinopyroxene geothermometry and the presence of mesoperthite) during and after decompression from earlier eclogite‐facies conditions (inferred from textural evidence of plagioclase release in sieve‐textured augite). Subsequent cooling at high pressure is inferred from the unequivocal replacement of sillimanite by kyanite.A Sm–Nd mineral isochron (gt–cpx–plag–WR) of 605 ± 37 Ma is taken to date a point on the cooling path, and confirms the hitherto suspected pre‐Caledonian age of the high‐grade metamorphism. Geochemical and Sm–Nd isotopic data indicate that the protoliths were probably late Proterozoic arkosic sediments and tholeiites. Following metamorphism they apparently came to reside near the base of the crust where they slowly cooled. The eventual exhumation of these gneisses is attributed to Caledonian crustal imbrication, followed by rapid isostatic recovery.

Discussion on a tectonic model for the emplacement of the Main Donegal Granite, NW Ireland

P rofessor W. S. P itcher remarked: Dr Hutton has enthusiastically addressed himself to the main problem highlighted by the previous work: the reason for the wide variety of emplacement mechanisms within coeval and contiguous plutons at the same level in the crust, central to which is the relationship in time between the Main Donegal Granite and the localized but intense deformation. This new structural analysis clearly establishes the presence of an important sinistral shear component, and my reservations concern only the timing and the magnitude of the calculated displacements. Concerning the latter, the evidence is that where the ductile shear zone leaves the Main Donegal Granite pluton it seems not to displace the contact zone, and where it runs off SSW it barely displaces an earlier dislocation known as the Knockateen slide. I am equally in doubt of the extension of the postulated shear zone far into northern Fanad, unless it has been conveniently cut out by movement on the post-shear Cashelmore Fault. Within the granite outcrop itself, where the shear zone runs along the apparent boundary between the Trawenagh Bay lobe and the Main Granite, to the S of Meenderryherk, a train of xenoliths straddles the zone of deformation, by which it is distorted but not dislocated. I conclude that whilst sinistral rotational strain has been proved, the displacements suggested seem excessive. I am much attracted to the proposed model whereby magma was emplaced into space created by a curvature in an active shear zone, particularly so as the early

Fracture controlled feldspar shape fabrics in deformed quartzo-feldspathic rocks

The behaviour of feldspar in the brittle-ductile transition region has often been discussed in models dominated by mechanisms of extension fracturing. In this example, a small Caledonian granitic pluton from NW Ireland, feldspar shape changes are primarily accomplished by small displacements upon numerous shear fractures. These fractures developed as Riedel and anti-Riedel shears as the granite was synkinematically deformed in a regional shear zone. The deformation took place under greenschist facies conditions at strain rates estimated between 10 -13 and 10 -14 s -1.

Age and significance of the Old Red Sandstone around Clew Bay, NW Ireland

ABSTRACTThe stratigraphy of red bed successions in the Clew Bay area of County Mayo (Ireland) is described, and new palynological evidence for the age of some of the rocks is presented. The Old Red Sandstone in the Clew Bay area comprises several lithostratigraphic units. The Louisburgh succession is interpreted as part of a Silurian marginal marine sequence. Devonian strata are represented by the Islandeady Group which has yielded plant spores which suggest that these rocks should be assigned to the Lower Devonian (Siegenian to Emsian) and the Beltra Group which contains Middle Devonian (Eifelian) spores. These Lower and Middle Devonian strata are probably separated by an unconformity, and are overlain with clear unconformity by the highest unit, the Maam Formation, which is Dinantian (Carboniferous) in age. The Clew Bay Old Red Sandstone succession, therefore, shows important differences from that of the Midland Valley of Scotland, with which it has previously been compared.

Deformational history of an area with well-developed tectonic slides: Dalradian rocks of Horn Head, NW Irish Caledonides

ABSTRACTA structural reinterpretation of Appin Group metasediments of the Dalradian Supergroup in part of the northwestern marginal zone of the Irish Caledonides is presented. No major F1 folds have been recorded although bedding occasionally faces upwards NW in S1. The structure is dominated by a series of NW-facing F2 recumbent folds and associated D2 tectonic slides. The slides, which have thrust the overlying rocks to the NW, are generally oblique to the fold axial planes and commonly cut out major F2 fold hinges. Rotation of the hinges towards the NW-SE (Caledonian) stretching direction in areas adjacent to the slides may have contributed to fold excision: an example of a major dislocated sheath fold is described. DM3 produces major SE-verging inclined folds. Although D4, D5 and D6 are of minor importance in the area D6 intensifies towards the SE to become the principal fabric in a major NE–SW-trending sinistrai shear zone that contains the c. 400 Ma Main Donegal granite. The early major structures of this area are compared with those in Dalradian Supergroup of adjacent parts of Scotland. It is suggested that the absence of a major F1 fold (analagous to the Islay anticline) and the occurrence of D2 recumbent folds and low angle thrusts in NW Ireland reflect the close proximity of the foreland, composed of the Lewisian complex, to the NW; such a situation is analagous to the Moine thrust zone in Scotland. The progressive development of low angle structures and thrusts to the W of the Scottish mainland is attributed to a westward shallowing of the Lewisian basement between the SW Highlands of Scotland and NW Ireland.

Some problems associated with the analysis and interpretation of mixed carbonate and quartz beach sands, illustrated by examples from North-West Ireland

Mixes of carbonate and quartz sands, which are commonly encountered in Recent coastal sediments, require careful analysis if they are to be correctly interpreted. Grain-size data fall into multimodal or segmented zig-zag distributions which may require some kind of component separation if they are to be summarised by conventional statistics, and before they can be assigned to a particular hydrodynamic depositional process or environment. Unfortunately, separation techniques are only spasmodically applied, usually without due regard to the consequences. Such artificially filtered or truncated distributions are of little subsequent use. Using a range of samples from two beaches in NW Ireland, where carbonate proportions range from nearly zero to over sixty percent, the interrelationships of the two dominant components were examined. Where only a small carbonate proportion is incorporated into a large quartz one, predictable modifications of the size-curve are apparent. However, the components are more independent if mixtures are near equal. The occurrence of a number of distinctive combinations of simple straight lines and complex zig-zag curves probably reflects the relatively dynamic nature of the carbonate fraction.

A tectonic model for the emplacement of the Main Donegal Granite, NW Ireland

The Main Donegal Granite is shown to lie in a major, late stage, NE-SW trending transcurrent sinistral shear zone in the Caledonides of NW Ireland. The zone, which is bent both laterally and upwards around the pluton, has displacements which diminish SW along its length from a maximum of approximately 20 km at the NE end of the pluton. The granite has also been deformed by this sinistral shear but it has a strain component of its own, the nature and development of which indicate that the pluton was accommodated rather than forcefully emplaced in the shear zone. Strain variations between granite and wall rock, and an early plagioclase fabric associated with the petrographic banding of the granite, suggest that intrusion and deformation were more or less synchronous (at around 400 Ma). It is suggested that unconsolidated granite in the shear zone locally created high strain rates and instability which caused the zone to bend, split lengthwise, and progressively create an internal low pressure zone into which the Main Granite was emplaced. The wide variety of emplacement styles of the other, radiometrically synchronous, plutons of the batholith can be attributed to the progressive development of the shear zone: Thorr and Fanad in tension between the zone and a complimentary structure to the N; Ardara and Toories in local compression at the shear zone termination; Rosses in tension on the outer arc of the bending zone; and Trawenagh Bay, as displaced Main Granite magma, in a similar tensional setting during the final stages of deformation.

The plate tectonic setting of NW Britain and Ireland in late Cambrian and early Ordovician times

Deposition of Dalradian sediments in a stretching ensialic basin peripheral to Iapetus continued into Cambrian times. Late in the history of Dalradian deposition a southern continental landmass was removed by strike-slip motions and a subduction zone then developed beneath the Dalradian sediments. Deformation and moderately high pressure metamorphism of Dalradian rocks occurred and was followed by thermal relaxation and development of a volcanic arc with magmatic centres in Connemara by Tremadoc times and in NE Scotland by the Arenig. The S Mayo Trough was a back-arc basin that opened shortly after the initiation of the arc, the oldest volcanic rocks seen there being the extrusive equivalents of the basic intrusions of Connemara. Dalradian rocks outcropping in Ordovician times over the Highlands and probably extending southwards are a possible source for high pressure metamorphic clasts at Ballantrae, while clasts of arc volcanics may have come from N of the Highland Boundary Fault. Plate models that involve a separate arc in the Midland Valley are rejected because the tectonic elements are impossibly small by comparison with modern convergent plate margins. The arc rooted in Dalradian rocks ceased to be active at the end of the Llanvirn as a result of a further change in plate motions, and there are considerable spatial problems in locating an arc coupled to the Southern Uplands accretionary prism after Llanvirn times.

The strain history of a dalradian slide: Using pebbles with low fluctuations in axis orientation

The strain associated with the Horn Head Slide, a major tectonic break in the Dalradian rocks of NW Ireland, is recorded by pebbles in an adjacent quartzite horizon. Mean X/Y ratios of the deformed pebble shapes in excess of 8.0 are seen closest to the slide and the field of three-dimensional shapes lies along the K = 1 line. The usual methods of separating initial shape ratio ( R i ) and tectonic strain ratio ( R s ) from the deformed shape ratio ( R f ) of ellipsoidal markers are based on measurements of variation in fluctuation (e.g., the ( R f /φ technique). However, due to the high X/Y strains in this situation and since the pebbles initially lay parallel to bedding and to a principal plane of the finite strain ellipsoid, fluctuation is generally very low. Thus, except for the least deformed X/Y data, the R f/φ technique is inapplicable and other methods are used. For X/Y data with mean ( R f > 4.0 : R s is calculated as the harmonic mean of R f ; maximum R i values only are obtained from the range of R f data. For all Y/Z and X/Z data: R i is calculated from the logarithmic range ( ω log ) of R f ; R s is simply obtained from the geometric mean of R f modified by R i . It is concluded from this that a varying prolate tectonic strain ( K ∼- 1.5) reaching X/Y values in excess of 8.00 was coaxially superimposed on an initial oblate shape fabric to produce the present field of deformed pebbles in the quartzite near the slide.

The continuation of the Canadian Appalachians into the Caledonides of Britain and Ireland

Summary Three zones can be recognised in the Canadian Appalachian-British Caledonian belt. The NW Marginal Zone extends from NW Scotland and Ireland through N Newfoundland to Quebec. It consists of late Precambrian to early Palaeozoic clastic sequences and an extensive Cambro-Ordovician carbonate platform. Orthotectonic deformation and metamorphism occurred in the early Ordovician, and middle Ordovician allochthons and obducted ophiolite characterise this zone in N America. The SE Marginal Zone extends from Wales and the English Midlands through SE Ireland to E Newfoundland, Nova Scotia and S New Brunswick. It is characterised by extensive late Precambrian volcanicity, late Precambrian orthotectonic deformation and metamorphism and the development of lower Palaeozoic basins in Wales and Nova Scotia. Late Precambrian orogenic activity locally involved ophiolite. The Axial Zone can be subdivided into a number of subzones, some of which do not extend for great distances along strike. It contains local ophiolites, trench complexes and calc-alkaline volcanics ranging in age from Cambrian to Silurian. Volcanicity was most pronounced in the Ordovician on both sides of this zone and extended into the marginal zones. Middle Ordovician deformation and metamorphism is locally intense but elsewhere Ordovician and Silurian sequences are apparently conformable. The Axial Zone and adjacent parts of the marginal zones have all suffered Siluro-Devonian, generally paratectonic, deformation and metamorphism. The history of the whole belt indicates that subduction was active on the SE in the late Precambrian, on the NW and in the Axial Zone in the lower and middle Ordovician and extended into the Silurian. Final closure of the Iapetus ocean was delayed until the upper Silurian-Devonian. Plate-tectonic models must involve multiple subduction zones.

Pre-Caledonian basement to the paratectonic Caledonides in Ireland

Summary In NW Ireland the exposed pre-Caledonian basement adjacent to the paratectonic Caledonides contains Grenville gneisses in N Mayo. A pre-Caledonian age is preferred for the amphibolite- and granulite-facies metasediments of the Ox Mountains, Lough Derg and Central Tyrone inliers. Gneissic xenoliths in lower Carboniferous agglomerates indicate that a comparable basement extends southwards beneath a considerable part of the Longford-Down zone in central Ireland. Pre-Caledonian basement is also exposed in SE Ireland. An important change of basement composition is inferred across central Ireland from regional magnetic data and from lead isotope data. This change occurs in the vicinity of the Iapetus suture both in Ireland and Britain.

The distribution of pre-Caledonian basement in the Ox Mountains inlier, Ireland

Summary Studies in the Ox Mountains inlier of NW Ireland in recent years have led to conflicting views by two groups of workers as to whether the inlier comprises (1) mainly pre-Caledonian basement with small areas of Dalradian (Caledonian) rocks, or (2) mainly Dalradian rocks excepting a pre-Caledonian area in the northeast. Both views are outlined here with the main arguments of the protagonists.

TECTONIC CONTROL OF OFFSHORE SEDIMENTARY BASINS TO THE NORTH AND WEST OF IRELAND

A newly compiled tectonic map of Ireland and part of the offshore area can be used to show that a pattern of rejuvenated Caledonian structures was superseded by newly imposed structures at about the time of the opening of the northern North Atlantic Ocean. There are three distinct patterns of structural control: 1. Older Caledonian structures, 2. Younger Caledonian structures and 3. ‘Oceanic’ structures. Younger structural patterns are each superimposed on older, so that locally interference patterns are developed. Each of the structural regimes has controlled sedimentation in particular zones, and where superimpostion has occurred, mixed shelf‐continental and ‘oceanic’ sedimentation can be expected at different structural levels.One of the major control structures in and immediately off NW Ireland is the Great Glen Fault system, which has been repeatedly rejuvenated since its development in Caledonian times. Other major control structures are associated with the Porcupine Seabight, which probably developed during the initiation of the immediately adjacent North Atlantic Ocean basin. Where these structural systems intersect at the head of the Seabight, sedimentational provinces can be defined.A poorly defined E‐W structural line at about 53° 25′ across the northern part of the Seabight may reflect deep wrench faulting related to a subjacent transform. Major structures along this line would have influenced sedimentation and consequent growth structures. This line may be an eastward analogue of the Gibbs Fracture Zone against which the axial zone of ‘quasi‐oceanic’ or attenuated continental crust in the Porcupine Seabight could terminate.

Faunal Provinces in the Ordovician of North Atlantic Areas

WILLIAMS1 has recently pointed out a number of quantitatively significant relationships between Ordovician brachiopod faunas on both sides of the Atlantic. In the Early Ordovician and the early stages of the Middle Ordovician three brachiopod faunal provinces persisted in the area of the British Isles. The provinces occurred as belts in the NE–SW strike direction of the Caledonian geosyncline. Along the northern margin of the geosyncline, extending through NW Ireland and Scotland, there was a province with mainly North American faunal affinities (American Province). Williams suggests that the migration of the brachiopods in this province took place by a current transporting the brachiopod spat towards the NE. The central part of the geosyncline was occupied by anaerobic bottoms with graptolitic mud, devoid of significant brachiopod fauna. South of this belt there was a strip extending through SE Ireland and NW Wales (Anglesey), in which the Lower–Middle Ordovician brachiopod faunas are most closely related to stratigraphically equivalent faunas of the Baltic area (Baltic Province). Williams shows that these faunas probably spread with a current system flowing towards the SW. The same current system was responsible for the migration of the Anglo-Welsh fauna occupying the southern marginal area of the geosyncline.