For more than two centuries the rocks of the Southern Uplands have provided successive generations of geologists with both illumination and frustration. This area furnished vital evidence to support Hutton's ideas of petrogenesis and his cyclical view of earth history. However, for much of the succeeding century construction of a credible and consistent stratigraphy and structure eluded the best efforts of the many workers drawn to this region. It was an English-born schoolmaster, Charles Lapworth, inspired by the ideas of foreign practitioners and fascinated by the hitherto-despised graptolite, who applied a technique (zonal stratigraphy) and developed a structural paradigm (isoclinal folding) that provided an elegant and coherent solution to the myriad problems bequeathed by previous attempts.

So persuasive was Lapworth's model that the Geological Survey deployed its two finest mappers, Peach and Horne, to re-examine the entire region, using Lapworth's techniques. The outcome of that 10-year task is the monumental publication commemorated here. That work, and the ideas it espoused, remained virtually unchallenged for more than half a century and contributed to the application and elaboration of then-new geotectonic ideas, such as the geosynclinal concept.

However, in the 1950s the application of new or neglected techniques (way-up criteria, turbidite sedimentology, greywacke petrography, microstructural analysis, etc.) led to drastic reappraisal of prevailing structural and stratigraphic models and introduced a new paradigm (colloquially termed the ‘Southern Uplands paradox’) that envisaged a dominant role for strike-parallel major reverse faults. In contrast to Lapworth's shale-based approach this new model focussed on evidence derived predominantly from the thick intervening greywacke sequences. Investigations led by the Edinburgh and St Andrews schools extended and amplified this new interpretation of Southern Uplands geology, elucidated details of the palaeogeographic setting and the evolution of both the depositional basins and the source areas, and suggested comparisons with other parts of the Caledonian-Appalachian orogen.

The advent of plate tectonics revived interest in the Southern Uplands, first as a candidate subduction-related margin, then in the late 1970s an accretionary prism origin was proposed for the Southern Uplands imbricate thrust stack. The attractions of this hypothesis were manifest and it stimulated renewed activity by academics and the Geological Survey. Significant inconsistencies and perturbations in the simple accretionary prism concept have emerged from these more detailed studies and a range of convergence zone scenarios has been proposed. A Geological Society Meeting on the topic in 1986 furnished much new data and ideas but failed to yield conceptual consensus. Thus, as yet the latest Southern Uplands Controversy remains unresolved.

Changes in the level of geological research activity in the Southern Uplands can be assessed from an analysis of the numbers of relevant publications appearing over five-year intervals. This survey reveals a pattern that broadly accords with the narrative outlined above and supports the concept of two complete and one as-yet incomplete cycles of model development and adoption. Each of these developmental cycles appears to follow the progression of stages in scientific development identified by Thomas Kuhn and common to many scientific disciplines.

Lapworth, at the time of his 1878 paper on the Moffat Series, was the world's foremost expert on graptolites, and in that paper he gave the first great demonstration of the biostratigraphical value of graptolites. Peach & Home's resurvey of the Southern Uplands of Scotland extended Lapworth's ideas and his use of graptolites across the entire region. Peach's graptolitic work for the Survey is discussed: even though he identified a smaller repertoire of graptolites than Lapworth had, and often identified their general horizons rather than exact zone, his results are considered broadly correct. His faunal lists often emphasise the oldest faunas from the Moffat Shale inliers, presumably in order to stress their supposedly anticlinal structure. Subsequent work has seen a great extension of graptolite taxonomy and provided more detailed biostratigraphical subdivision, especially in the Silurian. The model of the Southern Uplands as an imbricate thrust stack is constrained by identifying the youngest (rather than oldest) fauna from the Moffat Shale inliers or, where possible, graptolites from the overlying greywacke formations. Such work has enabled the identification of about 25 thrust tracts in SW Scotland and of out-of-sequence thrusting in the Moniaive and Peebles areas to the NE.

The Southern Uplands is a major Ordovician-Silurian accretionary terrane which developed as a marine basin over a period of c.75 Ma (495-420 Ma). The terrane extends to c. 10,000 km2 in Scotland alone and correlates with the similar-scale Longford–Down terrane in Ireland. Despite tectonic complexity, a detailed lithostratigraphy has been erected. The oldest strata are mudstones, cherts and lavas of mid-Arenig age known only in the Leadhills Imbricate Zone. The next youngest rocks are of similar lithology but of late Llanvirn-early Caradoc age. These oceanic sediments are succeeded by black shales of the Moffat Shale Group which are, in turn, diachronously overlain by huge volumes of turbiditic sandstones, siltstones, mudstones and minor conglomerates (greywackes) of Caradoc to Wenlock age. Overall, the terrane is sandstone-dominated, with other components such as lavas, volcaniclastics and cherts representing only a tiny proportion of the total volume. The conglomerates have a broadly northerly provenance, whereas the sandstones exhibit both marginal (NW and SE), and axial (NE and SW) derivation. During the Ordovician, strongly contrasting sources alternated through time. The youngest sandstones (Hawick and Riccarton groups), are notably rich in detrital biogenic carbonate, a rare component in the Leadhills Supergroup and Gala Group.

Bounded by sutures and demonstrating a unique geological history and structure, the Lower Palaeozoic rocks of the Southern Uplands–Down–Longford form a definitive Caledonian suspect terrane. The geological history of the final closure of Iapetus is encrypted in their structural fabric.

Across the terrane, NW-younging turbidites predominate but graptolites invariably indicate the presence of younger sediments to the SSE. This fundamental Southern Upland paradox is soluble only by recognizing many strike-parallel faults, dividing the terrane into more than thirty tracts, each with its own variant of the stratigraphy and structure, and each having a lateral extent far in excess of what might be expected from the probable mechanical strength of the composing sediments. Structural interpretations of the terrane's unique tectonostratigraphic pattern are critically reviewed and the accretionary prism model, modified by strong sinistral transpression from the late Llandovery onward, is preferred. Transpression was apparently triggered when the converging continents of Laurentia and Avalonia made solid contact, so establishing a mechanically effective coupling of sialic crustal elements beneath and across the closing Iapetus ocean basin.

The geometry of the terrane's internal structural fabric is analysed. Tentative area-balancing calculations indicate a crustal shortening from a basin width of at least 1,000 km to the current terrane width of 75 km. Continuing sinistral transpression was expressed in fault reactivation and the development of a major shear zone. Late Palaeozoic strike-parallel extension produced W-facing half-grabens and the associated rotation may account for the easterly plunge of most fold axes.

The Scottish Caledonides, sited near the triple-junction between Laurentia, Amazonia and Baltica, is divided into at least five discrete blocks, each with a history incompatible with that of the block now lying adjacent to it. With the exception of the Hebridian margin, with its extensional Torridonian basins and Cambrian passive margin sequence, all blocks have undergone terrane-scale movements. The Moine, Central Highland Division and the bulk (if not all) of the Grampian Group have shared a common regional metamorphism, involving thickening and uplift, at c. 800 Ma and c. 450–480 Ma. This is incompatible with their being in the extensional regime that appears to characterise much of Neoproterozoic Laurentia. They, along with the polymetamorphic Dalradian block, now replace a region of passive margin and an unknown width of attendant Iapetus oceanic crust. These metamorphic blocks are grossly out-of-place.

The Midland Valley is a severely contracted block of ancient crust, once fringed by extensive oceanic basins to the N and S. An Ordovician–Devonian arc was founded on this older craton, and supplied sediment to basins on either side of it. This arc, during its Lower Palaeozoic life, matured, finally to yield relatively quartz-rich sediment, but was re-activated during the Devonian. An arc, similar to that of the Midland Valley, also supplied sediment to the Southern Uplands. Metamorphic debris in the Southern Uplands had a provenance in either this arc-basement or in a basement somewhere along the orogen. No metamorphic detritus in the Highland Border Complex has yet been demonstrated to have a Dalradian source.

Much of Scotland was assembled in a strike-slip regime. Evidence for strike-slip tectonics can be seen from the Late Proterozoic through to the Devonian. In periods of transtension, basins opened to accumulate sediment; in periods of transpression, those sediments were compressed and uplifted to yield sediments to successor basins. In the Neoproterozoic, during the phase of transpression, the basins were buried and metamorphosed, but during the Palaeozoic the basins were at a much higher level and escaped metamorphism.

A substantial volume of the Neoproterozoic–Palaeozoic sediment that accumulated in Scotland was derived from two orogens, both of which were sited some distance away. During the Neoproterozoic, the Grenville orogen was the main source, and in later (Devonian) time sediment accumulated in Scotland from the major, Late Palaeozoic continent–continent collision of Greenland–Scandinavia. These two external sources were augmented by a substantial contribution of sediment supplied from the Midland Valley arc or its lateral equivalent and by mild uplifts within the Scottish basements.

The Scottish Caledonides constitute less than 10% of the length of the Caledonian-Appalachian orogen, the rocks of which define one major margin of the Laurentian craton in Neoproterozoic-Palaeozoic times. Scotland was located, however, in a critical position at the tip of a major cratonic promontory bounded by the Caledonian and Appalachian segments of that margin. Isotopic dates from minerals and rocks collected in the Scottish Highlands have been regarded for 40 years as indicating a Neoproterozoic history of compressive orogenesis that is absent in N America and Greenland. They have therefore been taken by some authors to indicate an origin exotic to Laurentia for rocks of the Northern and Grampian Highlands S and E of the Moine thrust belt. An alternative explanation is that the Neoproterozoic rocks in the Scottish Highlands are all related to the two-stage ‘breakout’ of a discrete rift-bounded Laurentian continent from the core of the Rodinian supercontinent, believed to have assembled at the end of the Mesoproterozoic.

Traditional reconstructions of the late Neoproterozoic–Early Palaeozoic Earth oppose the proto-Caledonian/Appalachian margin of Laurentia and the W African craton of the newly assembled Gondwanaland. However, consideration of the global inventory of late Precambrian rifted margins, their relation to Grenvillian orogenic belts and of scale, leads to the hypothesis that the conjugate was the proto-Andean margin of S America. Recent recognition that the Cambrian and Lower Ordovician strata of the northwestern Argentine Precordillera and their underlying Grenvillian basement are unquestionably of Laurentian derivation, while not definitive, does point in this direction. If correct, this means that even the presence of Neoproterozoic orogenesis need not imply an exotic origin, as Neoproterozoic orogens are widespread in S America.

Traditional models show an Early Ordovician lapetus ocean basin approximately 4500 km wide, but the remarkably synchronous Ordovician collision of arcs and other terranes with the Laurentian and Gondwanan cratons from Argentina to the British Isles, suggests that this premise may be incorrect. The Appalachian–Caledonian orogen may rather have resulted from close and complex tectonic interaction between Laurentia and Gondwana, involving intervening volcanic arcs and other terranes. The interaction may have taken place during a clockwise transit of Laurentia around the proto-Andean margin to its late Caledonian–Scandian collision with Baltica, and the final suturing of Pangaea at the close of the Palaeozoic era. A modern analogue may be the interaction between Australia and Asia, involving intervening volcanic arcs and other terranes of the western Pacific Ocean basin, from ~ 50 Ma through the Present, and into the future.

A new model for the provenance, depositional environment and tectonic setting of the Northern Belt of the Southern Uplands is presented. This turbiditic sandstone-dominated sequence was deposited in a sand-rich submarine fan environment, overlying sparse hemipelagic mudstones. The oldest sandstones are rich in juvenile ophiolitic material and record the first clastic input into the Southern Uplands basin. The bulk of the Northern Belt sedimentary sequence, however, is dominated by relatively quartzose sandstones derived from a Proterozoic continental/metamorphic source represented by the Midland Valley terrane of Scotland and Ireland. The quartzose-dominated succession was punctuated by the input of fresh volcanic detritus shed from an oceanic/continental island-arc situated to the W/NW of the Northern Belt basin, with sediment dispersal turning to the NE along the axis of the basin in Scotland. The tectonic setting of the Northern Belt basin remains uncertain. The complex provenance of the sandstones and recognition of major olistostrome units within the Northern Belt succession suggest that it was tectonically active. The onset of clastic deposition within the Southern Uplands terrane broadly corresponds to uplift and erosion of earlier obducted ophiolite in both Scotland and Ireland, possibly in response to collision of Cambrian–early Ordovician island-arc systems with the Laurentian continental margin. If this interpretation is correct, then the possibility arises that the Southern Uplands–Midland Valley terranes record the dismembering of this oceanic/continental island-arc complex within an overall transpressional regime.

Subduction-related accretion of fault-defined tracts built up the Southern Uplands terrane during the final stages of closure of the Iapetus Ocean (Llandovery to Wenlock). Contrasts in depositional environment and pronounced differences in geochemical composition, provenance studies and metamorphic grade across the Laurieston Fault between the Gala and Hawick groups, suggests that it has a greater regional significance than most other tract-bounding structures. Initiated by underthrusting, and acting as a locus for subsequent sinistral strike-slip, the fault overlies a regional gravity anomaly gradient that is interpreted to be due, in part, to a concealed NW-ward dipping shallow basement surface. This is modelled as an open ramp in the NE that steepens to a near-vertical step along-strike to the SW. A change in structural geometry noted at the Laurieston Fault, with excision of accretionary tracts, is related to a period of oblique closure of the Iapetus Ocean. The youngest Gala Group tracts were accreted during a period of intense transpression to form a regional strike-slip duplex over the shallow basement ramp with termination of the tracts at the Laurieston Fault, its surface expression. The ramp acted as an obstacle to forward-breaking thrust progress, forcing the out-of-sequence thrusting and repetitive thrust imbrication noted in the eastern Southern Uplands. Upper Palaeozoic reactivation of this basement structure may have transferred strain between extensional Permian basins.

The progressive changes in the provenance of Silurian greywacke turbidites in the Southern Uplands terrane reflect geotectonic events at the Laurentian continental margin during closure of the Iapetus Ocean. In the northern Gala Group, juvenile andesitic detritus in some beds gives εNd values no lower than −4·2; more commonly, quartzo-feldspathic greywackes have εNd values in the −5·5 to −6·7 range, produced by the mixing of juvenile plutonic and Proterozoic basement detritus during arc unroofing. In the southern (younger) Gala Group, Proterozoic εNd values range down from −7·7 to −11·2 with only sporadic evidence for a juvenile component. An abrupt change is seen between the Gala Group and its tectonostratigraphical successor, the Hawick Group. In the latter, εNd values have a compact range between −4·7 and −6·6, indicating the renewed dominance of a more juvenile, plutonic provenance. Regional variations in the Sr/Rb ratio suggest that this was more evolved than the source of the Gala Group plutonic material. The Wenlock greywackes of the Riccarton Group have εNd values in the range −5·1 to −7·8, overlapping with the Hawick Group and with coeval greywackes from both the Midland Valley and Lakesman terranes. Overall, the data support proposals that the Iapetus Ocean had effectively closed by mid-Silurian times. Conversely, data from greywacke boulders in the basal Old Red Sandstone conglomerate of the Midland Valley terrane militate against its Wenlock juxtaposition with the Southern Uplands.

At Thirlestane Score and at four other localities in the Southern Uplands, graptolites of the Llandovery gemmatus Zone occur in couplets of lithologies immediately above thin ‘high-U’ bentonites. Above the bentonites, abundant graptolites, especially siculae, and a straight-line survivorship trend implies high productivity coupled with environmentally mediated mortality: the population structure expected in the early part of a plankton bloom. In the overlying facies, fewer, larger individuals and a convex survivorship curve suggest reduced productivity and internally mediated mortality. This is consistent with the later stages of a bloom where resources were waning but the ecological structure of the system was better developed. It is likely that the introduction of trace-metals, Fe or Al, to the water column via volcanic ash increased primary productivity, suggesting that macronutrients were available in the Southern Uplands system, allowing a bloom to be stimulated by the addition of volcanic products. This process is observed in modern open oceanic systems and implies a temporal continuity of control on the plankton despite complete faunal turn-over since the Silurian. These interpretations are most consistent with an open ocean geotectonic setting for the region.

Diverse and abundant brachiopod faunas, associated with unstable outer shelf and slope environments, occur through the Upper Ardmillan Group (upper Caradoc–upper Ashgill) in the Girvan district of SW Scotland. Representatives of the deep-water Foliomena fauna occur intermittently throughout the group, appearing in both the Whitehouse and Drummuck subgroups. This distinctive assemblage of small, thin-shelled brachiopods, including Dedzetina, Christiania, Cyclospira and Foliomena itself, first appeared in South China during the early Caradoc but had colonised the Laurentian margins by the late Caradoc. Within the upper Caradoc–lower Ashgill Whitehouse Subgroup, the Foliomena fauna is interbedded with a variety of other less cosmopolitan deep-water assemblages including the Onniella–Skenidioides and Lingulella–Trimurellina associations. Shallower-water environments in the middle Ashgill Lower Drummuck Subgroup hosted the Fardenia–Eopholidostrophia association in sands, and the Christiania-Leptaena association in muds and silts. The remarkable Lady Burn Starfish Beds in the upper part of the group contain a variety of brachiopod-dominated assemblages including the Eochonetes and Plaesiomys-Schizophorella associations, transported from various shelf locations, within a very diverse mid-Ashgill biota. Nevertheless, elements of the Foliomena fauna persisted to near the top of the Drummuck Subgroup, occurring as rare assemblages in more muddy and silty facies. The upper Ashgill High Mains Formation contains abundant elements of the terminal Ordovician Hirnantia fauna including Eostropheodonta, Hindella and Hirnantia itself, but also some taxa more typical of the Laurentian Edgewood Province. As a whole, the changing brachiopod biofacies monitor environmental fluctuations, on part of the Laurentian margin, driven by mainly eustatic and tectonic events.

The Silurian inliers of the Midland Valley of Scotland all exhibit a regressive sequence, and despite evident facies differences between inliers, marine successions invariably pass upwards into redbeds of continental origin. Contrasting types of shoreline facies can be seen in different inliers; here the beds in the North Esk Inlier (NEI) are compared with those previously described from Knockgardner. At Knockgardner a high energy coastal environment prevailed, but in the NEI very different conditions can be identified in near-contemporaneous deposits. In the NEI thin sandstones and siltstones of the Reservoir and Deerhope formations are succeeded by the sandstones and conglomeratic beds of the Cock Rig Formation. These are overlain by the marine mudstones of the Wether Law Linn Formation and, at the top of the sequence, the continental redbeds of the Henshaw Formation.

The Wether Law Linn Formation is interpreted, on various lines of evidence, as a lagoonal system. Conditions therein were initially fully marine, though within the photic zone, but subsequently the faunas were increasingly influenced by fluctuating salinity, prior to deposition of the redbeds. Such a lagoon would have required an offshore bar impounding it, which is represented by the Cock Rig Formation. These sedimentary rocks, previously interpreted as deposits of a laterally unconfined submarine channel, are now considered to be of shallow water origin. The succession closely conforms to classic models in which shoreface sands, consisting of small cross-bedded packets, are succeeded by tabular sandstone sheets representing foreshore beach deposits. Coarser and thicker beds, with herringbone cross-sets, linguoid ripples and trains of rounded pebbles, are interpreted as the deposits of tidal channels within the barrier complex. The barrier-lagoon system persisted throughout the whole of Cock Rig time and most or all of the time during which the Wether Law Linn Formation was deposited, and was either static or prograded seawards until the lagoon dried up during a final marine regression. The sedimentary and faunal evidence is consistent with this interpretation, and the contrast between the shoreline environments of the NEI and Knockgardner is striking. Brief reference is made to other inliers in the Midland Valley.

Leaching of Ordovician cherts from the Southern Uplands using dilute hydrofluoric acid has yielded numerous radiolaria, sponge spicules and a few conodonts. Although many of the radiolaria have suffered intense recrystallisation, it nevertheless proved possible to extract for SEM examination some examples with exquisitely preserved micro-structure. Despite this, the radiolaria have not proved sufficient, on their own, to answer the outstanding biostratigraphical question of one continuous or two separate episodes (Arenig and Llanvirn/Caradoc) of chert sedimentation in the Southern Uplands. This is partly because the biostratigraphy of Lower Palaeozoic radiolaria is still quite poorly known. A new radiolarian species, Protoceratoiciskum clarksoni Danelian, has been identified and described from cherts in the Crawford area. In addition, the distinctive sponge species Konyrium varium Nazarov & Popov, has also been found for the first time in Southern Uplands cherts. This new occurrence, in deep-water sediments, suggests a much wider habitat for this unusual sponge. Amongst the few conodont elements extracted, one can be identified as probably belonging to the genus Periodon.

Ostracodes have a wide geographical distribution in the Ordovician of Scotland. They are known from the Southern Uplands, the Girvan district, the Highland Border region and the Inner Hebrides. Overall, more than forty species are recorded. They occur in clastic and carbonate rocks indicative of a range of shallow to deeper marine-shelf environments. Though many of the faunas are allochthonous, broad patterns of ostracode palaeoenvironmental distribution can be elucidated, and elements of the shallow marine Leperditella and open marineshelf Anisocyamus associations (previously recorded from N America) are present. Indigenous faunas are absent from the deep marine sediments of the Southern Uplands Northern Belt. Ostracodes are known from the Arenig, Llanvirn, Caradoc and Ashgill series in Scotland; those of the latter two series have widest biostratigraphical value. In the Girvan district the Caradoc species ‘Ctenobolbina’ ventrospinosa, Krausella variata, Balticella deckeri and Monoceratella teres have correlative value with N America, whilst the Ashgill species Kinnekullea comma appears to be a locum for the anceps graptolite Biozone in Britain, Ireland and possibly the eastern Baltic. The ostracodes are of typical Laurentian affinity, but show progressive generic links with the Baltic region during the late Llanvirn–Caradoc interval, and by Ashgill times display species-level links with southern Britain and Ireland. These distributional patterns suggest approaching geographical proximity for the early Palaeozoic continents of Laurentia, Baltica and Avalonia, and the ability of some Ordovician ostracodes to cross the Iapetus Ocean.

In southeastern Ireland, the tectonic evolution of the northwestern Avalonian margin is rather poorly understood and only partially constrained. This is because the stratigraphy comprises in the main unfossiliferous turbidites. Nevertheless, some authors have attempted to define ‘terranes’ and ‘tracts’ in the region, aided by several significant Caledonoid-trending structural breaks that can be determined in the field. Palynological work was carried out in the early 1970s and established a range of ages from approximately mid Cambrian to late Silurian, although much of the succession proved barren. In the current study, the Lower Palaeozoic sequences W of the Leinster Granite and in the Slievenamon Inlier to the S have been palynologically re-investigated. Previous work proposed an unbroken succession from lower Ordovician Ribband Group turbidites and volcanics younging westwards conformably into upper Silurian Kilcullen Group sediments. The new study clearly shows that the Ordovician palynomorphs in the older part of the Kilcullen Group are reworked. In fact, there exists a major stratigraphical break between the Ribband Group dated as early Ordovician, Arenig, and the Kilcullen Group which is entirely Silurian, late Llandovery to early Wenlock in age. This major break has a minimum strike length of 150 km and is most likely much longer, extending some hundreds of kilometres SW to Dingle and possibly equating with a similar discontinuity in the Isle of Man to the NE. This break would thus appear to be a major feature within the succession of the northwestern Avalonian margin.

Various order duplex structures are described from oceanic sequences of basaltic and associated pelagic–hemipelagic sedimentary rocks in the Ordovician (northern) part of the Southern Uplands accretionary complex. The general structure of the terrane as a whole strikes ENE, but each component lithological tract strikes NE or more northerly, oblique to the regional trend, making an en echelon outcrop pattern. Further oblique relationships between structures and lithologies can be mapped at larger scales, up to 1 km scale or more. These duplex structures are thought to be originally SE-verging, now partly overturned to the NW. Differences in the en echelon geometry, either sinistral or dextral, are explained by variable plunge of the original structures.

Peach & Home's first regional map of the Southern Uplands suggests an en echelon pattern of lithologies, implying large-scale duplex structures across the whole terrane. Here, the duplex structures are regarded as ubiquitous at both regional and smaller scales, suggesting considerable horizontal shortening. This was accommodated by such structures during underplating and out-of-sequence thrusting, in all parts of the accretionary prism, but particularly in the deeper tectonostratigraphic levels. The duplex structures are characteristic of ancient décollement zones.

Systematic studies of metapelitic grade linked with the geological re-survey of the Southern Uplands have been used to generate a contoured metamorphic map currently covering nearly two-thirds of the terrane. These studies, based on approximately one pelite sample per 2·5 km2, have used XRD measurements of clay mineral reaction progress, particularly illite crystallinity, to delineate zones of diagenesis and low-grade metamorphism in the imbricated Ordovician and Silurian strata. The regional pattern revealed by metapelitic zonal sequences does not agree with earlier observations that grade increases across the strike, from SE to NW. Instead, the map shows considerable variations in metamorphic trends, including patterns of grade increasing from older into younger strata, indicative of accretionary burial. Other patterns identified include those generated by high strain rates in the Moniaive Shear Zone (MSZ), and by extensive low-temperature cryptic aureoles associated with late granitic intrusions. The present pattern is the result of uplift that generated normal movement on reactivated thrust faults and differential block movement on NW-trending faults.

Regional metamorphic patterns were generated by burial and underplating in an accretionary thrust stack. Subduction was initiated in the early Caradoc and probably ceased in the early Wenlock. Metapelitic patterns suggest that two levels of accretion are exposed in the terrane. Strata accreted to the toe of the prism and stacked above the décollement zone are typically at late diagenetic grade. Underplated strata below the décollement are typically at anchizonal grades with moderate to well-developed slaty cleavage. Coherent thrust-bounded tracts of strata at both levels were rotated and buried to produce a syntectonic depth-controlled pattern of meta-morphism. Shear zone metamorphism at depths of 12 km or more was probably confined to the underplated lower level of the thrust stack, and Devonian granitic intrusions were also emplaced mainly within the underplated strata.

The consensus of several geochemical studies is a polygenetic origin for the basic volcanic sequence within the Ballantrae Complex ophiolite. This overall agreement masks differences of opinion regarding local geochemical interpretation, the possible correlation of structurally isolated lava tracts, and the degree of structural imbrication responsible for the juxtaposition of the various lava types. Newly acquired data (XRF, INAA, ICP-MS) provide the best evidence yet obtained for the presence of a MORB component and establish a wider distribution for primitive tholeiitic basalts with plate-margin characteristics than had been previously reported. The two principal within-plate sequences (well established from extensive coastal outcrop) are geochemically indistinguishable, with one considered to be the deeper water equivalent of the other. Lithofacies and geochemical similarities encourage correlation of some inland and sparsely exposed examples of within-plate basalt with the well-exposed coastal sequences, and all of this lava type may have originated from a single, ocean island volcano. The diversity of outcrops formed in within-plate and plate-margin geotectonic settings is combined within a dynamic reconstruction of a Tremadoc to Arenig arc-trench system with an active back-arc spreading region. The new reconstruction reconciles for the first time all of the known geochemical and published isotopic age evidence.