Chapter 1 Caledonian structures

Introduction

J.E. Treagus

The purpose of this volume is to describe and discuss the selected Geological Conservation Review sites which demonstrate structures of Caledonian age (Cambrian to early Devonian), south of the Southern Upland Fault. The sites, of national importance, have been selected to illustrate all the principal features of the Caledonian Orogeny in Britain (Scotland, England and Wales).

The volume has been divided into three sections; the Southern Uplands, the Lake District and Wales, for both geographical and geological reasons. Each of these sections is introduced and an outline of its structural features given, putting the sites into a Caledonian context. The purpose of the following paragraphs is to introduce the principal features of the Caledonian Orogeny in Britain south of the Southern Uplands Fault, so that the three-component sections can be seen in the context both of the British area and of the wider setting of the Caledonian–Appalachian Orogenic Belt.

The Caledonian Orogenic Belt

The Caledonian–Appalachian Orogen can be traced (pre-Atlantic drift), for some 7500 km south-west to north-east, from south-eastern USA through the British Isles to Scandinavia, Greenland, and Ny Friesland (Figure 1.1). It is generally accepted, after the work of Wilson (1966) and Dewey (1969), that sedimentation and igneous activity took place at, or near, the margins of an ocean (the Iapetus) that separated the Laurentian and Gondwanaland plates, over a period from the Precambrian through the early Palaeozoic. From studies of fauna, sedimentary history, igneous activity, structural and metamorphic evolution, and palaeomagnetism on its two sides, it is considered that deformation of sediments and volcanics, resulting from the episodic closure of the Iapetus Ocean, took place through the early Palaeozoic to culminate in continental collision during the early Devonian.

Since the initial plate tectonic model for this orogen (Dewey, 1969; see (Figure 1.2)), many variations and refinements have been proposed. Subduction and obduction of the Iapetus oceanic crust is regarded as having occurred on both sides of the ocean at various times, with attendant accretion and deformation. The opening and closing of marginal basins, the collisions of volcanic arcs and microcontinents, as well as the final continent to continent collision and suturing, have all been used to explain various deformation events (Barker and Gayer, 1985). It has long been recognized that various regions within the orogen have had very different histories and different deformation timings (see (Figure 1.1)), but recently emphasis has been placed on the differences between smaller areas (terranes) with distinct geological histories within these regions. In particular, the work of Williams and Hatcher (1982) in the Appalachians, has led to the recognition that thrusting and strike-slip movements may be responsible for the very distinct geological histories of many of these terranes (see Barker and Gayer, 1985).

The British Caledonides

Scottish Highlands

In the British Isles (Figure 1.3) two groups of contrasting terranes are of long standing: the Scottish Highland Terranes (and Irish equivalents) with their early, 590–480 Ma Caledonian (= Grampian) deformation and metamorphism, and the Southern Upland, Lake District, and Welsh Terranes (and their Irish equivalents) dominated by late or end-Caledonian (= Acadian) deformation and low-grade metamorphism, dated around 400 Ma. The first of these areas itself has a complex history, but culminated with the Grampian Orogeny imposed on rocks ranging in age from Archaean basement (Lewisian) through the Proterozoic (Moine) to the Late Proterozoic to early Cambrian (Dalradian) cover. The principal deformation events took place before 490 Ma, resulting in polyphase folding, thrusting, and regional low- to high-grade metamorphism. Subsequent folding in this area has not been precisely dated, but it pre-dates intrusion of Upper Silurian to Lower Devonian granites and faulting on the Great Glen Fault system. Some of this late folding, and certainly the faulting, must correlate in time with the late-Caledonian deformation of the terranes to be considered below.

Midland Valley

Between the Scottish Highlands and the Southern Uplands lies the enigmatic terrane of the Midland Valley. Apart from some indirect evidence here of a granulite basement, the oldest exposed rocks comprise the Lower Ordovician Ballantrae Complex, commonly interpreted as obducted Iapetus Ocean crust, with a complex and little-understood terrane history. These rocks are unconformably overlain by Middle Ordovician to Middle Silurian sediments which contrast strongly in their sedi-mentology and structural and metamorphic state with the rocks of the Southern Uplands. In the south-east of the Midland Valley, they are very weakly folded and appear to be conformable with the Lower Devonian, although folding preceded Gedinnian Series deposition in the north-east in the Pentland Hills. The Middle Devonian is missing and the Upper Devonian is strongly unconformable on older rocks, with evidence, in this interval, of locally strong folding and faulting.

Another element of the Midland Valley Terrane is a narrow zone of possibly ophiolitic rocks which parallels the Highland Boundary Fault: this has small areas of Arenig Series, Middle Ordovician and Upper Ordovician sediments, each apparently with a distinct structural history.

The Midland Valley has not only a contrasting history compared with terranes to its north-west and south-east, but also a lack of features, sedimentological, magmatic, and structural, that might support continuity. This has led recent workers (for example, Bluck, 1986; Hutton, 1987) to favour the idea that the Midland Valley owes its present position largely to strike-slip movements on its two boundary faults, in preference to previous attempts to integrate it directly into Dewey's (1969; (Figure 1.2)) destructive plate margin. The history of the Midland Valley is very important to the understanding of Caledonian evolution of the British area, but because so much work is in progress, its consideration has been excluded from the current volume.

Southern Uplands

The deformation events that are the subject of this volume are generally construed to be the result of the closing of the Iapetus between the mid-Ordovician Period and the early Devonian, associated with marginally directed subduction zones. The models for this closure are largely based on Dewey (1969) (see (Figure 1.2)) and there have been many suggestions as to how both large-scale and small-scale structures may be related to these models. On the north-western margin, the arguments for NW-directed subduction in the Southern Uplands have been strengthened by the stratigraph-ical, sedimentological, and large-scale structural evidence. The distinctive stratigraphical and structural arrangement of these rocks has been used to argue, very persuasively, (McKerrow et al., 1977), for accretion above a descending oceanic plate. According to the accretionary prism model, deformation in the Southern Uplands, unlike that in the Lake District and Wales, would be expected to have developed throughout late Ordovician and Silurian times, perhaps culminating with the development of the finite cleavage in the early Devonian. Thus the upright SE-verging and steep reverse faults have been interpreted as original flat-lying, ocean-verging and NW-dipping thrusts respectively, which have been rotated into their present steep attitudes in the accretion process. The cleavage, which cuts across the folds, has been attributed to the late-stage closure of the Iapetus. However, there is still much debate concerning the detailed relationship of the structures observed to the evolution of the supposed accretionary prism (a wedge-shaped pile of deformed rocks) above the subducting plate. Indeed, recently, doubts have been cast upon the reality of the accretionary prism model, particularly as applied to the Silurian rocks (Hutton and Murphy, 1987).

Lake District

On the south-eastern margin, the argument for SE-directed subduction (Figure 1.2) rests largely on the presence of arc volcanism in the Lake District during the Llandeilo or Caradoc epochs. Structural arguments have focused on the nature and significance of the pre- and post-Borrowdale Volcanic Group unconformities, but particular features that could be related directly to subduction have not been identified. Folding in the Skiddaw Group has recently been reinterpreted as being the product of slumping (Webb and Cooper, 1988), and this may prove important with regard to the arguments for the timing of subduction and the topography of the margin. Apart from tilting and block faulting due to volcanotectonic activity (Branney and Soper, 1988), the dominant deformation in the Lake District is regarded as essentially a single event that resulted in folding, cleavage, and greenschist metamorphism during the early Devonian (Moseley, 1972 and this volume).

Wales

The Welsh Basin is now seen as a back-arc extensional basin within continental crust (cf. Dewey, 1969 and (Figure 1.2)). Its original relationship to the Lake District (and the Irish equivalents) is not clear. Between the two, lies the small, isolated area of the Precambrian rocks of Anglesey. The boundary of the Anglesey terrane with the Lake District terrane is not exposed. Its southeastern boundary, with the Welsh terrane, which has previously been interpreted as a subduction zone active in early Palaeozoic times, is now seen (Gibbons, 1987) to be a fault boundary marking Late Precambrian strike-slip docking of the small terranes that now make up Anglesey. The significant deformation related to folds and cleavage in Wales represents, as in the Lake District, essentially an early Devonian event. There are, however, many variations on a simple pattern, attributed, variously, to soft-sediment, tectonic, and volcanic activity.

Two particular structural features have provoked discussion. Firstly, there is the arcuate pattern of folds and cleavage from E–W turning to N–S, which has been most commonly attributed to basement control, and regarded by Soper et al. (1987) as part of the same curvature as that seen between the Lake District and the Craven Inliers. Secondly, there is the diminution of deformation south-eastwards, seen as the diminished affects of the Caledonian Orogeny towards the south-east margin of the orogen, represented by the basement rocks of the Welsh Borders and the English Midlands.

Conclusions

Although there are uncertainties concerning the timing and mechanisms of the Southern Upland structures, the three areas are apparently united by their common history of cleavage formation and maximum shortening which climaxed in the early Devonian. Evidence is accumulating (Soper et al., 1987; McKerrow, 1988; Soper, 1988) that this event may be equivalent to the Acadian Orogeny (Emsian in age) of the Canadian Appalachians. One feature of this cleavage that unites the two sides of the Iapetus suture, running through the Solway Firth, is the transection (cross-cutting) of folds by the cleavage, which has now been recognized widely in the Southern Uplands, Lake District and Wales. The Southern Uplands and the Lake District are also united by their flat-lying D2 folds and cleavage, which may be related, in time and space, to the major granite intrusives that characterize both areas.

Another feature which unites the latest Caledonian deformation across the whole of Britain is faulting, much of which is strike-slip and much of that sinistral (Hutton, 1987). The faults range from the Great Glen Fault system in the Scottish Highlands to the Welsh Borderland Fault system (Woodcock and Gibbons, 1988). The minor faults, in the Scottish Highlands, the Southern Uplands, and the Lake District especially, commonly show a more NNE–SSW trend and sinistral displacement. These two features, cleavage transection and faulting, have been used by both Hutton (1987) and Soper et al. (1987) to reconstruct the positions of the British Caledonian terranes and the relative movements and geometries of the margins of Iapetus itself.

Throughout the British Caledonides it is being increasingly recognized that certain structures, both folds and faults, have origins related to basin development that pre-date the main Caledonian structures. This recognition has not only allowed a clearer understanding of the early development of the area, but also removed some apparent tectonic ambiguities. For example, recent studies in both the Southern Uplands and the Lake District have recognized that certain folds are of soft-sediment origin. Similar folds have long been recognized in the Silurian of Wales (Woodcock, 1976) and other anomalous structures there (in older rocks) are also being attributed to this origin. Again, in Wales, early faults have been related to volcanic activity, as well as to facies and thickness changes. Comparable features are now being recognized in the Lake District.

In the Southern Uplands, the major strike faults are seen to have an early history that controlled the development of sedimentation in the accretionary prism, and recently, smaller-scale fractures have been attributed to shortening and extension in the accreting sediments.

Further details and references may be found in the 'Introductions' to the following chapters, which deal individually with the sites in the Southern Uplands, Lake District, and Wales.

References