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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.