A Tectonic History of Northwest England

A tectonic history of northwest England FRANK MOSELEY CONTENTS Introduction 56x Caledonian earth movements 562 (A) Skiddaw Slate structures . 562 (B) Borrowdale Volcanic structures . 570 (C) Deformation of the Coniston Limestone and Silurian rocks 574 (D) Comment on Ingleton-Austwick inlier 580 Variscan earth movements. 580 (A) General . 580 (B) Folds 584 (C) Fractures. 587 4 Post Triassic (Alpine) earth movements 589 5 References 59 ° SUMMARY Northwest England has been affected by the generally northerly and could be posthumous Caledonian, Variscan and Alpine orogenies upon a pre,Cambrian basement. The end- no one of which is entirely unrelated to the Silurian structures include early N--S and later others. Each successive phase is partially NE to ~NE folding. dependent on earlier ones, whilst structures The Variscan structures are in part deter- in older rocks became modified by succeeding mined by locations of the older massifs and in events. There is thus an evolutionary structural part they are likely to be posthumous upon sequence, probably originating in a pre- older structures with important N-S and N~. Cambrian basement and extending to the elements. Caledonian wrench faults were present. reactivated, largely with dip slip movement. The Caledonian episodes are subdivided into The more gentle Alpine structures also pre-Borrowdale Volcanic, pre-Caradoc and follow the older trends with a N-s axis of warp end-Silurian phases. The recent suggestions of or tilt and substantial block faulting. The latter a severe pre-Borrowdale volcanic orogeny are was a reactivation of older fault lines and rejected but there is a recognizable angular resulted in uplift of the old north Pennine unconformity at the base of the volcanic rocks. massifs relative to the downwarped Irish Sea The 'pre-Borrowdales' trends and those of the Basin and the Vale of Eden. pre-Caradoc movements are variable but are i. Introduction D tl R I N G T H E L A ST T E N Y E A R S there have been substantial and controversial additions to the vast geological literature on northwest England, and the tim£ would seem appropriate for a review of the nature, origin and time sequence of the major structures of this well documented region. It is not intended to deal with every aspect and it will be noticed in particular that the igneous intrusions and mineral veins receive only passing mention. The subject is most conveniently treated chronologically, starting with the Caledonian structures and continuing J. geol. Soc. Lond. vol. x28, 1972, pp. 561-598, I2 figs., 2 pls. Printed in Northern Ireland. Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/128/6/561/4884608/gsjgs.128.6.0561.pdf by guest on 30 September 2021 562 F. Mosde.y with less complex, but by no means simple, Variscan and post-Triassic structures. Whilst separating the structural episodes in this way, it is important to remember the strong influence of each episode on succeeding episodes, and that strong movements of the latter reactivated earlier structures. 2. Caledonian earth movements It is within the area of the Lake District and Howgill Fells that the Caledonian structures are best studied, but the Cross Fell, Ingleton and Austwick Inliers also provide direct evidence critical to interpretation. It is essential to emphasize at the outset that structural styles, particularly in the Lake District are closely related to rock facies and of fundamental importance to any understanding is the 3 ° ooo foot Lake District sandwich of highly competent Borrowdale volcanic rocks between incompetent Skiddaw Slates below and relatively incompetent upper Ordovician and Silurian slates and greywackes above. Each of these three divisions has a tectonic style peculiar to itself, although in each case a wide variety of structures is exhibited. It is not the purpose of this paper to detail historical developments of geological thought in northwest England. This has been effectively reviewed elsewhere at different times, most recently by Hollingworth et al. (z954) and Mitchell (I956A). It may be said in summary that after initial con- troversy about structural relationships between these three groups of rocks, and some misreadings of general structure by Green (i915), opinion seemed to be progressively hardening towards acceptance of views initially proposed by the pioneer surveyers a century ago. There was thus general belief in a conformable passage from Skiddaw Slates to Borrowdale Volcanics with complex folds in the former a result of their incompetence whilst the unconformity at the base of the Coniston Limestone first recorded by Aveline (I872) became an accepted fact, and there seemed little doubt that the major deformation was a result of post- Silurian orogenesis. Some of these opinions have now been questioned particularly by Simpson (z967, I968 ) and by Helm (x97 o) who believe that the Skiddaw Slates were severely folded and cleaved during a pre-Borrowdale Volcanic orogeny and that the volcanic rocks now rest upon them with major unconformity, thus explaining the differences in structural style in an apparently straightforward way. The interesting problem of the relationship between these two divisions is thus re-opened and Simpson and Helm's evidence has to be weighed against other conflicting evidence. Although structurally no single division of the Lake District rocks can be prop- erly assessed in isolation from the others it is nevertheless necessary to take them separately for descriptive purposes and in the pages following reference will be made in turn to structures within Skiddaw Slates, the Borrowdale Volcanics and the (essentially) Silurian slates and greywackes, and to the junctions between these major divisions. (A) SKIDDAW SLATE STRUCTURES Simpson's (i967) structural account of the Skiddaw Slates revived interest, and has been followed by some ten articles and discussions already published, and Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/128/6/561/4884608/gsjgs.128.6.0561.pdf by guest on 30 September 2021 A tectonic history of northwest England 563 another half dozen either in the press or in preparation. None would dispute a complex polyphase structural history, the principal bone of contention, already stated, being the time of the principle orogenic event, with two important alternatives. The first states that the Skiddaw Slates suffered a major orogeny with several complex phases of folding and cleavage prior to the formation of the Borrowdale Volcanics and the second, that the pre-volcanic folding represented a minor event and the major orogeny post-dated the volcanics. These alternatives represent an outstanding issue in Lake District geology at the present time, but before attempting to resolve it a summary of the visible structures is desirable. I. Major Structures Study of small scale maps can give a rather misleading impression that the Skiddaw Slates are brought to outcrop by several major anticlines. It is quite clear however that such major structures are to be seen clearly only in the over- lying Borrowdale Volcanic rocks and major structures within the Skiddaw Slates do not appear to conform to the same pattern, although they are not known with certainty over much of the outcrop. For example, the Ullswater anticline is a distinct major anticline as far as the volcanic rocks are concerned, but the Skiddaw Slates of the same area form a fault-bounded inlier with complex minor structures but unknown major structure (Moseley 1964). It is always the case that major structures within the Skiddaw Slates are not immediately obvious and in those areas so far examined there has been no consensus of opinion. For example, axial traces of major anticlines were placed by Rose (I 954, see also Mitchell 1956A, and Eastwood et al. 1968 ) along the outcrops of the major sandstones and were sup- posed to be upfolds of the Loweswater Flags in a two fold stratigraphical sequence of Loweswater Flags followed by Mosser-Kirkstile Slates. These folds do not coincide at all with those shown by Simpson (1967) who returned partially to an earlier stratigraphical interpretation of Dixon (1925) in which each of the sand- stone outcrops was a different formation in a much thicker sequence. In assessing the published information now available, Simpson's more detailed structural analysis must be weighed against Rose's apparently more subjective summary (Rose I955) , although Rose's unpublished field maps and notes do contain a wealth of objective information, which formed the basis of his conclusions, and knowing of this it is impossible to say which is the more reasonable interpretation at the time of writing. It is to be hoped that Rose will publish his important data before long, but in any event the structures are so complex and the stratigraphy so difficult that much further work is necessary. More detailed structural studies are in fact now in progress, and together with studies of sedimentology, micropalaeon- tology and geochemistry should help towards a fuller understanding. For the time being Simpson's structural lines have been plotted on figure 12 for the ~rw fells (Bassenthwaite to Ennerdale). To the northeast is the large area of the Northern Fells much of which is covered by the recent Geological Survey Memoir (Eastwood et al. 1968 ). The fold axes plotted in this memoir are based on the hypothesis of upfolds in the Loweswater Flags (pp. II, 35 and 38), and could be suspect. Indeed D. E. Roberts (1971) in remapping part of this ground has failed to con- firm the locations of some of these fold axes. Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/128/6/561/4884608/gsjgs.128.6.0561.pdf by guest on 30 September 2021 564 F. Moseley 2. Minor Structures (i) Folds Whereas there is strong element of subjective interpretation in determination of the major folds most of the minor folds can be readily observed and measured in the field, and it is understandable that they could be of similar orientation and style to the major folds.

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