Canadian Journal of Earth Sciences Musings in structure and tectonics Journal: Canadian Journal of Earth Sciences Manuscript ID cjes-2018-0192.R1 Manuscript Type: Article Date Submitted by the 30-Aug-2018 Author: Complete List of Authors: Dewey, John; University College Oxford, Keyword: tectonics, earth sciences, geology Is the invited manuscript for Understanding tectonic processes and their consequences: A tribute to consideration in a Special A.M. Celal DraftSengor Issue? : https://mc06.manuscriptcentral.com/cjes-pubs Page 1 of 66 Canadian Journal of Earth Sciences Musings in Tectonics* John F. Dewey University College, High Street Oxford OX1 4BH E-Mail: [email protected] Draft *Submitted to Canadian Journal of Earth Sciences Special Issue: Understanding tectonic processes and their consequences: A tribute to A.M. Celâl Şengör https://mc06.manuscriptcentral.com/cjes-pubs Canadian Journal of Earth Sciences Page 2 of 66 Abstract I outline and discuss my career in the context of the history of structural geology and tectonics, the progressive developments that led to plate tectonics, the people who have encouraged and influenced me, the events that changed my life, my fifty six doctoral students who have taught me so much, and my principal interests in tectonics. I discuss, in particular, nine topics of special current interest: the evolution of Tibet, the geomorphology of the British Isles, transtension, the Pre-Cambrian, the complexities of plate boundary evolution, Appalachian-Caledonian evolution, ophiolites, the structure and strength of the lithosphere, and the subducting slab. Keywords: History, memories, structure, tectonics. Rationale Draft This paper is unusual in its purpose and scope, and quite dissimilar to anything that I have written, although it is something that I have thought about for years. It is a blend of my life in structure and tectonics, the history of tectonic ideas, the people involved, and brief discussions of and opinions on many of the topics on which I have worked and am working. The paper is designed as a brief personal historical academic narrative. In my eighties, in “retirement”, looking back at my academic career in geology and looking forward to the increasing myriad and complexity of things to be found out by new generations of geologists, it seems appropriate to use this opportunity to celebrate my student, colleague, and friend Celâl Şengör’s great contributions to geology by writing, in this volume in his honour, a brief account of my views about some the things that my students have taught me and that have fascinated me over my sixty five years in geology. I hope that the reader will forgive me if my memory of details is less than perfect but I have tried to restrict myself to those events and ideas that I recall with reasonable clarity. https://mc06.manuscriptcentral.com/cjes-pubs Page 3 of 66 Canadian Journal of Earth Sciences Early years During my years (1948-1955) at Bancroft’s School in Woodford Green, Essex, my chemistry master John Hayward, an amateur geologist, sparked my interest in geology by involving me and several other teenagers in his research on the Holocene stratigraphy and palaeo- environments of the Lea Valley and by organizing three week summer bicycle tours of many parts of the UK and Ireland, the most memorable of which were Devon and Cornwall, the Lake District, the Northwest Highlands, and Ulster. During these tours, we examined an enormous variety of rocks and their relationships, scenery, history, and geomorphology. I then read geology at Queen Mary College University of London (1955-1958), which had a small geology department known for its excellent teaching of basic geology.Draft I was not disappointed. The four members of staff led by J. F. Kirkaldy (Kirk) were dedicated to the teaching and welfare of the five students in each year. Knowledge gained during the school bicycle tours was a fine foundation for the intense course in classic basic geology, firmly rooted in the field. We did two two-month pieces of independent mapping, one soft and one hard-rock; mine were the Osmington-Ringstead-Poxwell area of Dorset, where I became proficient at mapping from spring lines, changes in slope, vegetation, and auguring to three metres, and Cader Idris in North Wales, a treasure trove of extrusive and hypabyssal Ordovician volcanic rocks. We also had three-week Easter field courses in the Welsh Borderland, the Scottish Highlands, and Northern Ireland, and many weekend trips into the Weald, to Shropshire and North Wales. On a field trip to northeast Scotland, we were puzzled by the Helmsdale Fault-bounded mainly clastic Jurassic sequence with boulder beds and fallen sea stack; in the 1950’s, we had no knowledge of Jurassic rifting in the North Sea from drilling and seismic sections. The highlight and most valuable feature of the whole course was that https://mc06.manuscriptcentral.com/cjes-pubs Canadian Journal of Earth Sciences Page 4 of 66 we were given, every week, a one-inch geological map of some part of Britain, from which we had to analyse the structure with sections and block diagrams, the stratigraphic history, unconformities, and all other relevant features, and write a report on the structure and history of the area solely from information on the map. Maps of Ireland, other European countries, and North America. at various scales, were also used. In this way, the whole course was drawn together while learning a huge amount of basic geology. I was very fortunate and am deeply grateful to Queen Mary College for providing me with this meticulous grounding in geology and report-writing. The only deficiency, typical of almost all geology courses world-wide then, was that there was little or no attention paid to larger-scale features of the earth at tectonic scales. Continental drift was mentioned, as were Van Bemmelen’s ideas of tectogenes and Stille’s on geosynclines but these were presented as quite unrelated to theDraft geology that we were learning. In London intercollegiate lectures, Stanley Hollingworth outlined the idea of continental drift and remarked that it was obviously correct but seemingly irrelevant to continental geology. Lester King, the great South African proponent of continental drift gave some lectures in London that convinced most of us of its reality. Thus, the hypothesis was accepted, at least in London, by most geologists but tucked away as a curio. The only foray into the broader picture were Kirk’s stratigraphy lectures in which he gave us correlation charts for each System in Britain and encouraged us to colour them with lithologies , in as much detail as possible, and try to explain the facies distribution and likely palaeogeography, such as the carbonate platforms and shale basins of the early Carboniferous of the Pennines. On one occasion, another student, Philip Drummond, and I plotted up the age pattern of the Ordovician-Siluian turbidites of the Southern Uplands and showed that the turbidite package becomes younger southward; the concept of subduction-accretion above a northward-dipping https://mc06.manuscriptcentral.com/cjes-pubs Page 5 of 66 Canadian Journal of Earth Sciences subduction zone was then completely unknown and un-thought of. However, the principle and value of tectono-stratigraphic charts was seeded in my mind. Following Queen Mary College, I was accepted by Imperial College to do a Ph.D (1958- 1960). In those days, a Ph.D in geology generally involved experimental petrology, palaeontology, or being given an area to map and discover what geological secrets it held. I was offered a menu of carbonate biostratigraphy in the Jura (Derek Ager), Moine stratigraphy and structure in the Northwest Highlands (John Sutton), and Lower Palaeozoic rocks in the Sheeffry Mountains of western Ireland (Gwyn Thomas). I chose western Ireland for its rather exotic ambience, its very wide variety of lithologies and structures, and that it had last been mapped regionally in the 1870s. Also, William Stanton had just completed an Imperial College Ph.D on the rocks of southwest Murrisk immediately west of the SheeffryDraft Mountains, which provided a useful standard and framework upon which to hang my work. Also, I felt that, in the Highlands, I would be a mere cog in the Moine machine and, in the Jura, I might be committing myself to a lifetime in carbonates. Western Ireland, apart from the weather and flies, was Geo-Utopia. I spent about seven months mapping about four hundred square kilometres of Ordovician and Silurian rocks from Croagh Patrick, through the Sheeffry Mountains to the Partry Mountains. I mapped turbidites, lahars, ash-fall tuffs, ignimbrites, cross-bedded red-brown fluviatile sandstones, shallow marine sandstones and conglomerates, all variably and polyphase-deformed in the greenschist facies. Fossils are scarce in these rocks, graptolites in the Ordovician, brachiopods and corals in the Silurian. My time in London was spent with thin sections, reading extensively, and buttonholing and learning as much as I could from a distinguished academic staff, including Ian Carmichael, Graham Evans, John Ramsay, Doug Shearman, George Walker, and Janet Watson, who gave their time freely. https://mc06.manuscriptcentral.com/cjes-pubs Canadian Journal of Earth Sciences Page 6 of 66 Since my Ph.D work, western Ireland has been my principal geological home. I have learned and honed field observational skills there, and it has been an inspiration in much of my work in tectonics. It remains a treasure chest of wonderful geology that has inspired many geologists, and is a key to Appalachian-Caledonian evolution (Dewey, 2005; Dewey and Mange, 1999; Dewey and Ryan, 2015). Paul Ryan and I are still working on the South Mayo Trough even after over a hundred years of combined years of work. Currently, we are writing a synthesis of its structural evolution as a late Silurian transpression zone.