The Southern Uplands Controversy

Journal of the Geological Society, London, Vol. 144, 1987, pp. 735-736. Printed in Northern Ireland The Southern Uplands Controversy W. S. McKERROW Department of Earth Sciences, Parks Road, Oxford OX1 3PR, UK Report of a discussion meeting heldat Burlington quartz-rich. A back-arc situation is thus envisaged for the House on 12 February 1986 Southern Uplands, with a mature continental landmass to the north and a rifted continental fragment containing an The idea for a discussion meeting followed fromremarks active volcanic arcto the south. After oblique collision made by Professor C. H. Holland (1986) in his Presidential during the Llandovery, Stone and his colleagues suggest that Address to this Society in 1985, when he expressed concern underthrusting of thesouthern margin initiatedSE-a aboutthe uncritical application of the accretionary prism propagating thrust stack, which may eventually have ramped model tothe Southern Uplands of Scotland. It has since over the eroded remains of the arc. They consider that the become apparent that divergent views are held both among Hawick Group and the Wenlock sequences of the Southern those against the model and thosein favour of it.This Belt may have beendeposited in a southward-migrating meeting served to air some of these different opinions, even foreland basin ahead of the rising thruststack. They also if it has not immediately resolved many of the arguments. conclude that,after the Llandoveryoblique collision, a Most of thecontributors to the meeting(see the Society transpressional regime permitted the intrusion of Newsletter for January 1985) have produced written papers lamprophyre dykes andgranites. These dykes, dated at for this volume, and so far only one or two have modified 418-395 Ma, do not appear tohave been related to an active their views between writing their abstract and submitting margin (Rock et al. 1986). their paper. D. W. Hutton & F. C. Murphy cite evidence to show The concept of an accretionary prism model developed that, in the central inliers of Ireland,the derivation of gradually. A comparison of Scotland with Burma suggested Llandovery sediments was from both north and south of the that the Southern Uplands were like western Burma where remnant Iapetus; by contrast, the Wenlock sediments were trenchand oceanic sediments are being underthrust by derived from an arc to the north, and prograded across the subduction processes (Mitchell & McKerrow 1975). With suture. They claim that this arc has subsequently been cut thedevelopment of modern analoguesfrom the Pacific out by strike-slip along the line of the Orlock Bridge Fault (Seeley et al. 1974; Karig & Sharman 1975), the model was at the end of the Silurian, and that there is no evidence for applied tothe Southern Uplands with moreemphasis subduction of oceanic crust after the Late Ordovician. Some (McKerrow et al.1977; Leggett et al. 1979a, b, 1982). of thesepoints have already been published (Murphy & Moseley (1977, 1978) was one of the first to express doubts Hutton 1986a) anddebated (Anderson & Oliver 1986a; about this model; he believed that the Iapetus Ocean had Kemp et al. 1986; Murphy & Hutton, 19866;Stone et al. closed by the end of the Ordovician, and thus, like many 1986) elsewhere. others subsequently, had difficulty in accepting the concept J. H. Morris recognizes a distal continental source region of Silurian subduction of oceanfloor. This is still not tothe NW of theNorthern Belt in the Longford-Down accepted by all. Walton (1983, pp. 158-60) has noted other massif, and a proximal arc source tothe south, and he problems to the model. The structure, facies distributions concludes that thesesediments developed in a back-arc and stratigraphicrelationships are clearly more complex setting, which closed at theend of the Ordovician. He than first envisagedin 1977. Even with the evidence considers that the rocks of the Central and Northern belts presented at this meeting there is still no general agreement. were deposited in a fore-arc setting, and were subsequently It is to be hoped that this discussion may help the diverse thrust northwestwards during the Late Silurian. views to converge, but, before any general concensus can be Kelling et al. produce new sedimentological and reached, more field evidence is needed: new fossil localities, biostratigraphic data from the Rhinns of Galloway and morestudies insedimentology and sedimentary petro- adjacent areas, and show that aseries of submarinefans graphy,and further work todetermine the times of developed from the northwestern margin of an asymmetrical deformation in individual tracts are all priorities. basin during the Late Ordovician and Early Silurian, while J. K. Leggett illustrates the range of variation in modern pelagic facies were accumulating further to the SE. This is fore-arcregions. Inthe SW Japan trenchsome of the closely analogous to many moderntrench systems. complications seen in theSouthern Uplands, such as However, they also disclose some anomalies to a simple turbidites with current directions away from theocean, analogue; in particular, theLate Ordovician Portpatrick appear to be present. It is perhaps doubtful that any single Group, in Tract 3 of Leggett et al. (1982), appears to have modern fore-arc shows all the features seen in the Southern been derived from a proximal volcanic source to the south Uplands. Leggett also makes the point that the history of and SW. They conclude that during the Late Ordovician an pelagic sediments in the Moffat Shales (from the Caradoc to active continental margin arc was juxtaposed against the the late Llandovery) suggests that a wide ocean existed to Southern Uplands fore-arc trench; SE prograding fans were the south of Scotland until well into the Silurian. formed in a relatively narrow trench at the same time as a P. Stone and his colleagues from the BGS at Edinburgh coarse volcanilithic sediment apron migratednorth- presentevidence of some sequences which contain eastwards, as the arc was displaced relative to the trench. By southerly-derivedturbidites with freshandesitic detritus Llandovery time, the fore-arc trench region was dominated (which suggest an arc situated to the south of the Southern by a variety of fan systems which were all derived from the Uplands) while northerly-derivedturbidites are more NW. This contribution, like those of Stone et al., Hutton & 735 Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/144/5/735/4889512/gsjgs.144.5.0735.pdf by guest on 03 October 2021 736 W. S. McKERROW Murphy and Morris,point to complications in the simple et al., Morris and Kelling et al. discuss the evidence for Late fore-arc accretionarymodel first postulated(McKerrow et Ordovician sediments containing arc-derived material from al. 1977),but unlike theothers, Kelling et al. optfor a the south. The solution put forward by Kelling et al. for an fore-arc/trench setting throughout the development of the additional arc tothe south appearsattractive to me; but SouthernUplands accretionaryhistory, though with an perhaps this is because it permits the accretionary model to additional arc to seaward during the Late Ordovician. be modified with minimum distortion. There are also other A detailed comparison of the sediments and structures in anomalies, especially in the diverse structures described in three coastal sections in the Central Belt of Galloway and the Central Belt by Barnes et al. County Down by R. P. Barnes and others shows that while Stone et al. opt forcontinental collision in the the overall tectonics of the northern tracts of the belt are Llandovery;they areperhaps supported by Hutton & comparable with theNorthern Belt (and the accretionary Murphy, who suggest that there is no subduction after the prism model) thestructures in thesouthern parts of the end of the Ordovician. I am convinced, however, that the Central Belt differ markedly from this pattern. Large absence of regional stratigraphical breaks at the end of the amounts of predominantly southward-younging strata occur Ordovician, at the end of the Llandovery and at the end of in each section, and in Galloway northerly-verging D1 fold the Silurian in England and Scotland suggests that such early pairs are consistent with observed fault movement opposite closure is unlikely. in sense to the northerly-underthrusting model. Barnes and I am sure that further field work will eventually resolve others point out that landward vergence and overthrusting thesedivergent views, as will the continued friendly has been described on the continental slope off Washington discussions held regularly in the field during the annual andOregon, where the presence of a basal layer of low Southern Uplands Workshop meetings, at which most of the shear strength may suppress the seaward-verging thrusts. contributors are regular attenders. Widening of the Central Beltin Ireland is associated with an increase in the number of structural blocks and a decrease of References the age differences betweenthe sediments in adjacent blocks. Many of the tracts are not continuous along strike ANDERSON,T. B. & OLIVER,G. J. H. 1986a. Comment on “Is the Southern fora very greatdistance. In theArds and Wigtown Uplands really an accretionary prism?”. Geology, 14, 1043-4. - & - 19866. The OrlockBridge Fault: a major Late Caledonian peninsulas,‘normal’ seaward vergence is present again, to sinistral fault in the Southern Uplands terrane, British Isles. Transactions the south of the anomalous tracts, in the Hawick Group and of the Royal Society of Edinburgh-Earth Sciences, V,203-22. the Southern Belt. HOLLAND,C. H. 1986. Doesthe golden spike still glitter? Journal of the In a discussion based on deformation patterns in County Geological Society, London, 143, 3-21. KARIG,D. E. & SHAMAN,G. F. I11 1975. Subductionand accretion in Down, T. B. Anderson concludes that sinistral transpression trenches. Bulletin of the Geological Society of America, 86, 377-89. was not effective until afterthe emplacement of Upper KEMP,A. E. S., MCKERROW, W.S. & LEGGETT,J. K. 1986. Is the Southern Llandovery sediments in the middle part of the Central Belt.

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