Showing papers in "Geological Society, London, Special Publications in 1979"

Pelite facies series and the temperatures and pressures of Dalradian metamorphism in E Scotland

Abstract: Summary An attempt is made to synthesise data on the regional metamorphism of pelites in the eastern Scottish Dalradian. Zonal sequences (facies series) of mineral assemblages are presented and variants of the traditionally recognised Barrovian and Buchan metamorphism are separated. A Schreinemakers net and petrogenetic grid for the system KFMASH is presented and fixed in P-T space using experimental data. The P-T data for the various facies series are used in conjunction with other constraints to develop a model of isotherm and isobar distribution in the region, and the effects of post metamorphic folding of the isopleths are noted. P-T gradients normal to isobars suggest overall ‘geothermal’ gradients for most of the area to be convex to the T-axis, and it is suggested that they result from magma intrusion at depth. The region adjacent to the Highland Boundary fault shows strong horizontal and vertical temperature gradients which are tentatively interpreted to indicate the presence of a synmetamorphic tectonic boundary to the Dalradian metamorphic belt.

Petrogenetic significance of Rb-Sr and U-Pb isotopic systems in the 400 Ma old British Isles granitoids and their hosts

Abstract: Summary Rb-Sr isotopic data for 17 Caledonian granitic plutons in Scotland and northern England define low 87Sr/86Sri (<0.709) for all granitoids emplaced between ∼435 and 390 Ma with the exception of Moy and Aberdeen (which may be older). There is no apparent correlation of 87Sr/86Sri with distance from the proposed Iapetus suture near the Southern Uplands. Zircons from the Moinian and Dalradian rocks and Southern Uplands Ordovician greywacke roughly define U-Pb discordia lines with upper concordia intersections of 1200–1800 Ma and the position along the discordia related to the age of the sedimentary unit. Zircons from the Strontian tonalite and granodiorite and the Loch Doon granite contain little if any Pb isotopic memory, whereas the Strontian central granite contains a marked inherited zircon component. We conclude that, in strong contrast to the c.550 Ma and c.460 Ma granitoids, the c.400 Ma magmas were largely hybrids derived from the mantle and, or, ‘new’ lower crust with varying degrees of incorporation of upper crustal-type material.

New evidence for a Central Highland Root Zone

Abstract: Summary The major nappe structures of the west Central Highlands of Scotland have been found to diverge from the upward facing Ossian-Geal Charn steep belt, trending NE-SW through the Ben Alder Forest. As the deep equivalent of the Loch Awe synclinal complex to the southwest, it more probably represents a root zone than the Tummel steep belt which is largely controlled by later deformational events. A structural synthesis of the west Central Highlands describes how the large elongated Drumochter-Dochard dome influences at least three subsequent phases of deformation.

South-east Ireland: Lower Palaeozoic stratigraphy and depositional history

Abstract: Summary Four lithostratigraphical groups are distinguished in the Lower Palaeozoics of SE Ireland. The sand-dominant Lower-Middle Cambrian Bray Group consists of laterally-derived flysch. The conformably overlying, and perhaps in part time-equivalent, mud-dominant Middle Cambrian-Llandeilo Ribband Group comprises axially-transported flysch with minor volcanics. Due to tilting and subsequent differential erosion of these trough sediments, the overlying mainly Caradoc Duncannon Group rests unconformably on the Ribband Group except in the west where there is a conformable passage. The Duncannon Group is a highly faulted, predominantly volcanic, platform sequence with local basal calcareous beds. In the northwest of the district, the Bray and Duncannon Groups are absent and a continuous period of flysch sedimentation is reflected by the upward passage of lower Ordovician Ribband Group sediments into sand-dominant turbidites of the Kilcullen Group (lower Ordovician to at least Llandovery). In contrast, the Slievenamon inlier in the southwest contains Ribband Group strata of Upper Cambrian age which palynological evidence suggests are separated by a major stratigraphical break from the entirely Silurian Kilcullen Group. In the Comeragh Mountains to the south, the Kilcullen Group is conformably underlain by the Ribband and Duncannon Groups. These features point to large-scale irregularities within the Leinster Basin, the main Lower Palaeozoic palaeogeographical feature of SE Ireland.

The tectonic evolution of the Welsh Caledonides

Abstract: Summary The succession, metamorphism, geochronology and major structure of the Mona Complex, Arvonian and Lower Palaeozoic rocks of N Wales are outlined. This results in a model which relates early (pre-Ordovician) NW-verging structures to the closing of an early Caledonian ocean and obduction of deep water sediments, slide breccia and basic volcanic rocks on to the sediments previously metamorphosed, locally at high temperatures, and intruded by granites in central and N Anglesey. The pattern of folds, cleavages, thrusts and finite strain states in N Wales are described. A maximum of 43 km shortening between the Menai Straits and the Welsh Borderlands is estimated across the more highly deformed part of central Snowdonia, during end-Silurian deformation. The amount of shortening decreases to the northeast (Denbighshire Basin) and south (Harlech Dome) ends of the arc. This is attributed to compression of Snowdonia against a rigid block situated in the Berwyn Hills. The SE-verging structures are thought to pass downwards, with listric geometry, into a decollement zone at some unknown depth in the basement, which accommodated Caledonian crustal shortening across Wales, and must itself either descend into the mantle or root in any Caledonian suture beneath the Irish Sea.

Dating of the tectono-metamorphic history of the southwestern Moine, Scotland

Abstract: Summary New Rb-Sr whole-rock data are presented which confirm a Grenvillian age for widespread, early, regional metamorphism of the Moine in the SW Highlands of Scotland. It further suggests that Caledonian metamorphic activity occurred between c.470 and 410 Ma ago. Previous isotopic analyses of mineral separates from schists, gneissose pelites and pegmatites are reviewed in the light of the new data, and the work of Purdy & Jaeger (1976). This suggests a cooling history, following the Caledonian metamorphism, which involved sequential uplift of crustal segments.

Basement-cover relationships at Lewisian inliers in the Moine rocks

Abstract: Summary Lewisian basement of the N Highlands occurs (a) as an autochthonous massif, the Strathy Complex, (b) in parautochthonous fold cores, e.g. Morar, and (c) as slices along zones of high strain, e.g. the Sgurr Beag Slide. Structural features, within the Morar Division psammites adjacent to the Glenfinnan Division and to Lewisian inliers, have been used to define zones of increasing strain within some of which Lewisian strips now occur. The psammites show evidence of high strain where slides occur and massive, cross-bedded psammites give way to thinly banded platey psammites with a total absence of sedimentary structures. Folds progressively tighten. Quartz veins are re-orientated into parallelism with the foliation in the psammite and discordant fabrics are lost. In contrast to psammites at slides, those adjacent to Lewisian rocks in parautochthonous fold cores show only some of these features and those adjacent to the autochthonous massif show none of these features.

Thermal evolution of metamorphic rocks in the Central Highlands of Scotland

Abstract: Summary Conditions of the metamorphic maxima in lower Dalradian kyanite zone pelites from Kinloch Rannoch, Schichallion, Glen Tilt, Glen Ey and Glen Avon are calculated from analyses of coexisting kyanite, garnet, biotite, plagioclase and quartz (staurolite or chlorite also coexist in some samples). Temperatures of 550–620°C were reached at pressures of 9–12 kb corresponding to depths of burial of 35–40 km. Compositional gradients in zoned garnets indicate that pressures were falling at the time maximum temperatures were reached. The high values of pressure are confirmed by observations of the sub-assemblages kyanite-zoisite (not anorthite-margarite) and kyanite-garnet-chlorite-quartz (not staurolite or chloritoid). The pressure of intrusion of a Newer Igneous complex in upper Glen Tilt is 5–6 kb. The present results are contrasted with conditions at a similar structural/stratigraphical level 50 km WNW where comparable temperatures were reached at depths around 20 km. It is concluded that either erosion rates early during metamorphism were lower or the tectonic cover was greater in the Central Highlands than to the west.

The north-western margin of the Iapetus Ocean

Abstract: Summary A review of the stratigraphy, sedimentology, petrography, and structure of the Ordovician and Silurian rocks of the Southern Uplands (Scotland) and the Longford-Down zone (Ireland) identifies many features which are typical of accretionary wedges developed at Mesozoic-Recent subduction zones. The bedding becomes gradually more inverted in the west and northwest parts of the Longford-Down zone. Slumping becomes increasingly important along strike southwestwards from Scotland to Ireland. The roles of transform movement along the continental margin and of back arc spreading are also considered.

The ‘Central Highland Granulites’: cover-basement tectonics in the Moine

Abstract: Summary The Central Highland Granulites have been split into a cover assemblage of medium grade, little disturbed metasediments named the Grampian group, separated by a zone of sliding referred to as the Grampian Slide, from an underlying basement assemblage of high grade gneissose and migmatitic metasediments called the Central Highland Division. The Central Highland Division rocks are correlated on lithological, tectonic, metamorphic and radiometric grounds with the high grade Grenvillian rocks of the western Highland Moine, and are believed to represent their extension southeast of the Great Glen fault. The discovery of Precambrian (c.720 Ma) pegmatites sheared by Caledonian (<570 Ma) sliding in the Central Highlands, enables the separation of Caledonian from Precambrian orogenic events in the rocks of both assemblages; their Precambrian orogenic history is so different, that the Central Highland Division rocks are considered to form a Grenvillian basement to the younger (Morarian) cover succession of the Grampian group. The western Highland Moine region is examined in the light of the tectonic cover-basement relationship postulated for the Central Highlands. The Loch Eil Division contains rock assemblages indistinguishable from the Central Highland Division, and others similar to the Grampian group. Evidence is put forward for a post Grenvillian, Morarian age for the Morar Division. The distribution of metamorphic zones in the Moine would seem to reflect the distribution of high grade basement and lower grade cover rocks, and may be in part determined by movements on a series of Caledonian slides, though this has not been demonstrated.

Basement-cover relations in Shetland

Abstract: Summary The relationship between basement and cover can be studied in two places in Shetland. In NW Shetland, orthogneisses with a minimum age of 2900 Ma are followed to the east and southeast by Caledonian metasediments and greenschists and these in turn by schists and gneisses with a minimum age of 860 Ma. The metasediments contain bands of mylonite and blastomylonite. Wide belts of superposed schistosity occur in the schist and gneiss unit and also at the junction of the orthogneisses with the metasediments. This area is interpreted as including the Caledonian front. In NE Shetland, two nappes of ultramafic and basic rocks, one above the other, have been thrust over the surface of the earth onto Caledonian (lower Dalradian?) schists and gneisses. During transport the nappes have suffered erosion, and sediments, including nappe detritus, have been deposited on and in front of them and subsequently been over-ridden and metamorphosed by the nappes. The occurrence forms a typical obducted ophiolite complex.

Geochemistry of the appinite suite

Abstract: Summary The suite is highly enriched in many of the trace elements, both those typical of basaltic rocks (Co, Ni, Cr) and those more frequently richer in calc alkaline suites (Ba, Sr). The suite is rich in these elements and in K in all the members of the suite and the geochemical characters are indicative of normal igneous differentiation of a volatile, K and trace element-rich basaltic magma. The suite is not similar to any of the more widespread basaltic or andesitic suites although the rocks mostly vary in composition from a rather ultramafic basalt to a silica-poor andesite. There are also abundant acidic and alkaline differentiates but these do not show any more extreme enrichment in major or trace elements than the more basic rocks.

Caledonian minor intrusions of the N Highlands of Scotland

Abstract: Summary Three major suites of minor intrusions cut the Caledonian rocks north of the Great Glen fault. (1) The amphibolite suite was intruded between D1 and D2 deformations as a swarm of dolerites (locally gabbroic) trending sub-parallel to the fold belt. (2) The microdiorite suite, consisting of a wide variety of rock types, was emplaced during the closing phases of Caledonian tectonism principally in the southern part of the area. Early members were locally involved in the final phase of folding and metamorphism. The majority, however, occur as sheared sheets occupying easterly dipping planes which transgress all the Caledonian fold structures. These sheared intrusions exhibit a variety of internal fabrics, the geometry of which indicates that they were recrystallised under an ESE, subhorizontal maximum stress. Over most of the N Highlands, the recrystallised mineral assemblages belong to the greenschist facies metamorphism, but in the south an area of amphibolite facies has been defined. This is believed to reflect a residual Caledonian temperature gradient. (3) The minette suite, centred on the Ratagain granite is a post tectonic dyke swarm which marks the end of igneous activity in the N Highlands Caledonides. Members of the swarm are widely dispersed throughout the Moine and are also present in the foreland rocks west of the Moine thrust. Variations in K-Ar ages from these dykes may relate to differential uplift or to the continuing presence of a local Caledonian thermal high.

Structure and stratigraphy of the Dalradian rocks of the Bennabeola area, Connemara, Eire

Abstract: Summary The geology of an area of 400 sq km of Lower and Middle Dalradian rocks is described. It consists mainly of rocks belonging to the Appin and Argyll Groups which have been strongly deformed during four stages (F1–F4) with the major development of tectonic slides occurring before F3. Eighteen major F3 folds, whose geometry is described, are correlated across the Connemara antiform (F4); they refold an earlier nappe-like (? F2) antiform, the Derryclare fold. All of the Appin Group rocks and the Cleggan Boulder Bed Formation are contained within the core of the Derryclare fold, which locally has attenuated limbs. A zone of vertical foliation, the ‘steep belt’, is present in the south of the area. It is superimposed upon the originally steeply inclined, south-facing Derryclare fold and appears to have developed during F4.

The environmental significance of some faunal changes in the Upper Ardmillan succession (upper Ordovician), Girvan, Scotland

Abstract: Summary A series of discrete, distinctive faunules has been established for the Upper Ardmillan succession in the Girvan district. The faunules, collected from a diverse suite of clastic sedimentary rocks in a well defined stratigraphical framework, belong to the Whitehouse Group, the Shalloch Formation, the Drummuck Group, and the High Mains Formation; these rock units range in age from late Caradoc (Onnian) to late Ashgill (Hirnantian). The brachiopod associations identified within each faunule are outlined; few are considered indigenous. The series of faunal changes is fundamental to the assessment of the depositional environment of the sequence. The changing composition and structure of the brachiopod associations appear to reflect deposition in a predominantly unstable and often turbulent offshore continental slope environment. The faunal and sedimentary evidence supports the existence of a submarine fan throughout at least much of Upper Ardmillan times at Girvan.

Caledonide volcanism in Britain and Ireland

Abstract: Summary This paper reviews the state of knowledge of the volcanic activity which accompanied the development of the Caledonide orogen in Britain and Ireland. Unlike the relative abundance of stratigraphical data, geochemical information is scarce in earlier accounts, but in the last decade or so there has been a surge of interest coincident with the introduction of rapid analytical techniques and with the development of methods for distinguishing primary petrographical and chemical characteristics from those of subsequent alteration. Many of the Caledonide volcanic rocks were erupted into or beneath water, interbedded with sediments and then affected by orogenic deformation and metamorphism, yet this has apparently had less effect than might be expected on the gross geochemistry of the volcanic rocks. In the paratectonic Caledonides, submarine weathering and low-grade hydrous metamorphism have been the principal causes of element mobility, and the effects produced are, in many cases, remarkably similar to those which affect present-day eruptives in oceanic environments.

Geometry and emplacement of nappes in the Central Scottish Highlands

Abstract: Summary A deformation history related to polyphase nappe formation is presented for the Central Scottish Highlands as a model for the Dalradian tract. The genesis of a four-fold pattern of tectonic belts recognisable throughout this tract is discussed. The fan structure of the Tummel steep belt is argued to be a deep-seated rotation zone. Movements within this zone spanned the climax of Barrovian metamorphism and were responsible for creating the major belt of inverted rocks—the Loch Tay inversion—which forms the lower limb of the Tay (fold) nappe. The early-formed Tay nappe, nucleated at a relatively high tectonic level, was translated as a result of movements within the deeper rotation zone progressively towards the SE principally by simple shear. The core of the Tay nappe is recognised within the Highland Border steep belt at Aberfoyle, but does not root within the deeper levels of the Central Highlands Dalradian. The Highland Border steep belt at Dunkeld was formed by late orogenic downbending of the previously flat-lying inverted limb of the Tay nappe probably resulting from sub-vertical movements within the basement along the ancestral line of the Highland Boundary fault. A kinematic model explains the nappe geometry by density inversion of the Dalradian succession consequent on the pre-orogenic evolution of the Dalradian (marginal) basin.

New evidence for the age of the Manx Group, Isle of Man

Abstract: Summary Acritarch assemblages which are sufficiently well preserved to date the rocks in which they occur have been obtained from five formations in the Manx Group of the Isle of Man. The acritarch evidence indicates that the Niarbyl Flags, Glen Dhoo Flags, and Lonan Flags are most probably Tremadoc in age, the last two very late Tremadoc. The Maughold Banded Group is probably Arenig and the Lady Port Banded Group is probably either late Arenig or early Llanvirn. The minimum time interval represented by the Manx Group is late Tremadoc to late Arenig and a correlation of the Manx Group with the Skiddaw Group in NW England and the upper part of the Ribband Group in SE Ireland is proposed. The contemporaneous volcanic rocks in the Manx Group at Peel, here named the Peel Volcanic Formation, are probably upper Arenig in age.

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

Abstract: 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.

A metamorphic map of the British and Irish Caledonides

Abstract: Summary The metamorphic rocks of the British and Irish Caledonides fall into two groups. The first group constitutes the northern belt. These rocks have been subjected to a lower Ordovician metamorphism (Grampian event of Lambert & McKerrow 1976) which was probably, in part at least, diachronous. The metamorphic pattern ranges from the low- to the intermediate-pressure facies series and from the low greenschist- to the upper amphibolite-facies groups. The distribution of facies within the belt is complex and may relate to a depth-controlled metamorphism with locally induced thermal highs. The second group constitutes the southern belt. Here the rocks have been subjected to a late-Silurian event which, in general, causes slight crystallisation but very locally rises to upper greenschist facies.

Stratigraphical and structural correlation between the Dalradian rocks of the SW and Central Highlands of Scotland

Abstract: Summary The stratigraphical continuity between the SW and Central Highlands of Scotland is clarified with particular reference to the lower Dalradian rocks. The structural disposition of the rocks reveals the presence of the same two structural elements as found in the SW Highlands, namely, the Ardrishaig anticline and the Ben Lui fold. The mushroom structure of the Central Highlands is rejected, while the Boundary Slide is shown to be a diffuse D2 structure, which is probably of no great mechanical importance.

Sedimentary environments of the basinal Llandovery of mid-Wales

Abstract: Summary The position of the Ordovician-Silurian junction in the mid-Wales rock sequence remains elusive. There is no evidence of a break in sedimentation between the periods but conditions of deposition changed abruptly about then; presumed Ashgill rocks, mainly argillaceous and disturbed, are overlain by Llandovery strata with even and regular bedding, yielding Glyptograptus persculptus (Salter). The former are barren and accumulated rapidly, near the foot of a slope facing west from the ‘shelf’, synchronously with a marine regression and a glaciation in N Africa. The regresson may therefore have been a sympathetic eustatic fall. Of the succeeding strata, those of lower Llandovery age embody much pelagic mud while the middle and upper Llandovery strata record an invasion by turbidite fans. Current indicators in the turbidites reveal westward (‘lateral’) flow below the Aberystwyth Grits (low turriculatus Zone), later augmented or replaced by northwards (longitudinal) flow. This change stemmed probably from a tectonic event to the south. Characteristic turbidite rhythms have been noted, varieties of which are used to define many of the formations. Such formations are probably highly diachronous.

A structural profile of Caledonian deformation in Down

Abstract: Summary Coastal exposure in County Down provides an almost complete cross section of the continuation of the Southern Uplands fold belt in Ireland. Intensely folded Ordovician and Silurian turbidite successions are segmented by at least ten major strike parallel faults. In each segment the fold envelope descends northward but the strike faults throw up progressively younger sediments to the south. A system of upright folds, with slaty cleavage axial plane, but locally transecting, becomes progressively complicated southward by the clear development of two further phases of folding and cleavage with constant geometry and opposing vergence. The chronology of cleavage, folding, later kink-band and brittle deformation is punctuated by successive late Caledonian lamprophyre dyke swarms. The total structure is consistent with a model of diachronous deformation and northward translation and rotation at a destructive plate margin.

Slopes, submarine fans, and syn-depositional faults: sedimentology of parts of the Middle and Upper Dalradian in the SW Highlands of Scotland

Abstract: Summary The depositional palaeogeography of the metasediments between the Jura Quartzite and Craignish Phyllites was dominated by a series of fault-bounded blocks and basins. The position of the basin margins, and hence the faults, is inferred from the location of submarine fan and slope facies and lateral thickness variations. These syn-depositional faults had trends either approximately parallel with, or normal to, the subsequent Caledonoid grain and had throws of up to several kilometres. Such faults may have exerted a major control over sedimentation throughout the Middle and Upper Dalradian. Many of the Dalradian sedimentary bodies therefore have a complex geometry, and persistent stratigraphical horizons may reflect regional climatic and tectonic events, rather than persistent sedimentary facies.

The status of the Banff nappe

Abstract: Summary The status of the major horizon of gneisses (Cowhythe, Donside, Ellon and Inzie Head gneisses) in NE Scotland has been reinterpreted, on the basis of geochronological and fabric evidence, as the basal element of a major nappe, the Banff nappe. It is suggested that these gneisses represent a pre-Caledonian basement gneiss-complex, thrust into its present structural position during the Grampian phase of the Caledonian orogeny, after the formation of the Tay nappe. The basal zone of this unit is characterised by the development of distinctive mylonitic assemblages. In this interpretation, the rocks of the overlying Banff division form part of the allochthonous assemblage, as a Caledonian cover sequence, uncoupled from the basement along the Boyne line during emplacement.

Deformation in the Caledonides of England, Ireland and Scotland

Abstract: Summary The past few years have witnessed a remarkable change in our view of the British Caledonides resulting from radiometric work, application of plate tectonic models as well as basic field work. Overshadowing all else has been the acceptance of a later Proterozoic (probably Grenville) orogeny in the Moine rocks occurring between the Moine thrust and the Great Glen fault. The northern Moines are clearly polymetamorphic in character, but several matters are still controversial, for example the separation of Proterozoic and Caledonian fabrics and the areal extent of the older orogeny. This northern block was subjected to intense and repeated tectonism and, above all, large thrust translation during the Caledonian orogeny and the rocks are now juxtaposed with the older Proterozoic and Archaean gneisses of the Caledonian foreland. In Ireland small areas of Grenville basement are in slide contact with the younger Moine and Dalradian rocks. South of the Great Glen fault above a postulated Grenville basement, are late Proterozoic sediments (Central Highland Granulites and Dalradian) which were orogenically deformed in lower Ordovician times. In this area the essence of the structural pattern established some years ago stands unquestioned for the most part, but significant modifications have been made, notably the recognition of a slice of ‘Cadomian’ basement in the Dalradian pile of NE Scotland, and possible pre-Caledonian basement in W Ireland. In Ireland the structural pattern in the Scottish Highlands may be recognised in the north, but becomes considerably modified in the west where the strike becomes E-W. The picture of the Scottish Highlands as a metamorphic Alpine-type orogen, dating from the early Ordovician, and undergoing uplift during Ordovician and later times, is complemented by the recent recognition of the Southern Uplands zone (later, non-metamorphic Caledonides) as being an accretionary prism, on the Aleutian model, in which imbricate thrusts developed in series towards the south above the subducting oceanic crust of Iapetus. The Iapetus suture lies between the Southern Uplands and northern England and passes through central Ireland. The pattern of Ordovician vulcanicity in northern England, Wales and SE Ireland has been attributed to southerly directed subduction of the Iapetus crust beneath the opposing continental margin. Final closure of Iapetus in Silurian and Devonian times, seems to have been marked by relatively mild tectonic activity in the ‘metamorphic Caledonides’, but by considerable compression across the slate belts of the Lake District, Wales and SE Ireland with polyphase structural evolution.

The British Caledonides: comments and summary

Abstract: An extensive and systematic review of Caledonian deformation is included in this volume (Johnson et al.) so in these comments I attempt only to assess our present understanding of what I think are some of the major problems, in the light of the new data presented at the conference and published recently. The emplacement mechanism of the Dalradian nappes and the problem of a root zone The primary (D1) folds in the Dalradian have long been known to face northwestwards in the Ballachulish area and southeastwards in the Tay nappe but the relation between these oppositely facing nappes has been uncertain. This is partly because much of the intervening ground is obscured by granites. In the SW Highlands the anticlinal core of the Tay nappe is represented by the D1 Ardrishaig anticline. This lies SE of the D1 Loch Awe synclinal complex across which the primary schistosity fans over into the northwest-facing D1 Islay anticline. Thus in the SW Highlands, the Tay nappe appears to have a clearly defined anticlinal root and if so, that root must continue northeastwards as far as the nappe itself. The eastward continuation was formerly thought to occur in the Tummel steep belt (Sturt 1961, Harris 1963) but this steep belt is now known to be a zone of tight D3 folds involving no major primary (D1) folds (Bradbury et al. this vol.). Instead, Roberts & Treagus (1977 and this vol.) proposed that in Argyllshire the axial trace of the primary anticlinal root and the

East Greenland Caledonides—an extension of the British Caledonides

Abstract: Summary The major part of the Caledonian fold belt in East Greenland comprises gneiss complexes, which in the south have yielded Archaean and early Proterozoic isotopic ages; their features may be compared to those of the Scourian and Laxfordian divisions of the Lewisian. They are overlain by middle Proterozoic supracrustal sequences, which in some areas are involved in migmatitic activity and granite emplacement of Grenville age; parallels may be drawn with events recorded in the Morar Division of the Moine in Scotland, and in Ireland with those in the Annagh gneiss complex in N Mayo and perhaps in the Ox Mountains sequence. The late Precambrian lower Eleonore Bay Group is similar in development to the Grampian Group (Dalradian) of the British Isles, and the upper Eleonore Bay Group bears comparison with the Appin Group of the Dalradian. The Vendian glacial event is well represented in both regions. The early Palaeozoic succession in Greenland shows marked similarities with the Durness Limestone succession. The pattern of Caledonian deformation, metamorphism and plutonic activity show many similarities but also differences. The marked marginal thrust zone in the west of the E Greenland fold belt is the logical continuation of the Moine thrust belt. Late Palaeozoic continental deposits overlie Caledonian folded units in both regions.

The Capel Curig Volcanic Formation, Snowdonia, North Wales; variations in ash-flow tuffs related to emplacement environment

Abstract: Summary A detailed correlation of the tuffs of the Capel Curig Volcanic Formation is presented. The depositional environment of the sediments associated with the tuffs varied from subaerial to submarine. Variation in the ash-flow tuffs are related to this changing environment. Subaerially emplaced tuffs are characterised by even bases, distinctive recrystallisation textures, the absence of reworked tops and sparse siliceous nodules. Tuffs emplaced in a subaqueous environment show irregular undersurfaces with welding to the lower contact, reworked tops and common siliceous nodules.

Seismic constraints on the deep geology of the Caledonides of northern Britain

Abstract: Summary Interpretation of compressional and shear wave data from the 1974 L.I.S.P.B. seismic experiment has resulted in a detailed crustal cross-section across the Caledonian orogenic belt of northern Britain. This section provides clear evidence of lateral variations in deep structure comparable with the gross geological variations: in particular crustal structure is asymmetric across the orogenic belt. The axis of this asymmetry seems to vary with depth within the central part of the belt but the evidence suggests that any Caledonian destructive plate boundary was located south of the Southern Uplands fault.