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Department of Geological Sciences & Geological Engineering Distinguished Speaker Program Dr. John W. F. Waldron, Department of Earth & Atmospheric Sciences, University of Alberta “Subduction initiation in the Appalachian- Caledonide system: Implications for the Wilson Cycle” When: Tuesday, February 16, 2016 10:00 am Where: Board Room, Bruce Wing 527 All Welcome! SUMMARY: The Appalachian-Caledonide orogen was the first to be interpreted as a zone of plate-tectonic collision. Wilson's original question "Did the Atlantic close and then reopen?" addresses only part of what was subsequently termed the "Wilson Cycle". The transition from an expanding to a closing ocean was not addressed by Wilson, and the initiation of subduction in new oceans remains a poorly understood part of the supercontinent cycle. The history of oceans formed since the breakup of Pangaea suggests that spontaneous subduction initiation at passive margins (or margin inversion) is rare. In the Appalachian-Caledonide system, rifting continued to at least ~550 Ma, producing an ocean with numerous hyperextended passive margins and microcontinental blocks. These include both peri-Laurentian and peri-Gondwanan terranes; the latter have been grouped into domains characterized by platformal Cambrian environments (E. and W. Avalonia) and deeper-water successions (Ganderia and Megumia). W. Avalonia represents relatively juvenile continental crust, whereas Ganderia is more evolved; E. Avalonia may be heterogeneous, and Megumia shows a transition up-section from juvenile to more mature sources. Arcs were present in the developing Iapetus Ocean by 505 Ma as recorded by ophiolites from New England to Scandinavia. Several arguments indicate that these arcs were not generated by passive margin inversion. Many show juvenile isotopic signatures. Also, the E. to M. Ordovician Taconian/Grampian orogens are interpreted as products of collision between arcs and the Laurentian passive margin, implying prior existence of subduction zones offshore. Approximately simultaneous collisions on the margin of Gondwana, leading to the Penobscot and Monian deformation events, is also hard to reconcile with coincidental margin inversion on SE side of Iapetus. In an alternative hypothesis, we infer that subduction was initiated by incursion of arc systems from the external ocean into the young Iapetus, comparable to the recent migration of the Caribbean and Scotia arcs in the Atlantic. Almost simultaneous deformation on the Gondwanan and Laurentian margin of Iapetus can then be explained by interaction with a single, though complex and sinuous arc system. In preliminary test of this hypothesis, the kinematics of Penobscot deformation in coastal Maine show that the predominantly mafic Ellsworth terrane was thrust northwestward onto Ganderian continental margin units of the St. Croix terrane, consistent with the incursion model. These observations, and Wilson's original comparison with the Atlantic, suggest that spontaneous inversion of passive margins is an unlikely driving mechanism for the transition from ocean opening to ocean closing BIO: John Waldron was born in Denmark, but grew up entirely in the UK, mainly in the Essex suburbs of London. From about 1968 my parents indulged his enthusiasm for collecting fossils by arranging their summer vacations in classic areas of British geology and paleontology where John could collect. Attending Cambridge University from 1974, he was was subverted from paleontology into studies of orogenic belts by the glamour of the new theory of plate tectonics. From 1977 to 1981 he worked as a postgraduate student at Edinburgh University, on the geology of the Antalya Complex, southwestern Turkey. In January 1981 he came to Memorial University of Newfoundland where he held a post-doctoral research fellowship (though embarrassingly he was only truly 'post' doctoral for 13 days in this position.) From 1981 until 2000 he worked at the Geology Department of Saint Mary's University, Halifax Nova Scotia, where he taught sedimentary and structural geology, field methods, and several different flavors of introductory geology. He ventured into web-based teaching with a course entitled "The Earth: Atlantic Canada Perspective". In the summer of 2000 he moved to a position in the Department of Earth and Atmospheric Sciences at the University of Alberta in Edmonton. From 2004-2008, he took on the job of associate chair with responsibility for undergraduate studies. His research deals with deformed sedimentary rocks from both sedimentary and structural geological perspectives (For a long time he resisted being classified as either a sedimentary or a structural geologist, though he sometimes admitted to being a "deformed sedimentologist"). He recently served as chair of the structural geology and tectonics division of the Geological Association of Canada, the Canadian Tectonics Group. His recent research interests are in the Caledonian-Appalachian geology of Atlantic Canada and the British Isles, in the Archean Slave province of the Canadian Shield, and in sedimentary basins within the Canadian Cordillera. He received the Canadian Tectonics Group Dave Elliott award for his paper “How was the Iapetus Ocean infected with subduction?” published in Geology in 2014. He has a strong interest in the teaching of Earth Science, and especially Field Geology. To that end, he has developed an outdoor teaching facility known as the Geoscience Garden at the University of Alberta with the assistance of the University of Alberta teaching and Learning Fund, and a team of collaborators.