Cees Van Staal on the Origin of the Appalachians
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In the podcast, Cees Van Staal tells us about the Paleozoic tectonic events that led to the formation of the Appalachians. The events are closely related to those involved in the Caledonian orogeny and the mountains it created in what is now Ireland, Scotland, east Greenland, and Norway. However, the Appalachians that we see today are not the worn-down remnants of the Paleozoic mountains. Instead, they reflect much more a topography that was created during processes associated with rifting and magmatism that accompanied the opening of the Atlantic Ocean, as well as the effects of the ice ages as recently as about 10,000 years ago.
Van Staal is Emeritus scientist at the Geological Survey of Canada and an Adjunct/Research Professor in the Department of Earth and Environmental Sciences at the University of Waterloo in Ontario.
Podcast Illustrations
Images courtesy of Cees Van Staal unless otherwise noted.
Reconstruction of continental plate locations in the early Cambrian. The terranes that later rifted away from Gondwana and accreted to Laurentia as part of the Appalachian/Caledonian orogeny include Ganderia, Avalonia, and Meguma, which is not shown but forms part of Armorican Terrane Assemblage (AMT), which is sometimes called Cadomia in other paleogeographic reconstructions.
Reconstruction of continental plate locations in the Silurian and Devonian. Between 420 and 405 Ma, Laurentia moved south at a speed that may have been as fast as 11 cm/year while the Iapetus ocean was subducting beneath the margins of Laurentia. This led to the collisions of the Caledonian and Appalachian orogenies discussed in the podcast. AM=Amazonia; WA=West Africa; L=Laurentia; BA=Baltica; SIB=Siberia.
Diagrammatic cross sections of the Laurentian margin and adjacent terranes in New Brunswick during the Silurian Salinic and Devonian Acadian orogenies, which followed earlier accretion of arc terranes during the Ordovician Taconic orogeny. The Taconic orogeny was the result of the collision between Laurentia and a volcanic island arc system in the Iapetus Ocean. The Salinic and the Acadian orogenies involved the collisions of Ganderia and Avalonia, respectively, with Laurentia. The work of Van Staal and others has deduced that a very similar orogenic evolution is manifest in Newfoundland. Top: Arrival of Ganderia at the Laurentian margin, creating the Salinic orogeny. When the last vestige of the Iapetus Ocean was closed, this orogeny terminated, and the north-dipping Salinic slab broke off. Stepping back from the subduction zone behind accreted Ganderia lies an oceanic strait called the Acadian seaway where a subduction zone initiates closure of the Rheic Ocean. Bottom: Avalonia is pulled under the Ganderian margin by the north- and shallowly-dipping Acadian subduction zone. SCLM=sub-continental lithospheric mantle; MCS=Matapedia cover sequence; BBF=Bamford Brook fault.
Figures by Van Staal; Wilson, R.A., Van Staal, C. et al. (2017), American Journal of Science 317, 449
Configurations of Laurentia, Baltica, and West Gondwana prior to the opening of the Atlantic Ocean. At a high level, this shows the relationship between the Caledonian orogeny in Greenland, Scandinavia, and the northern British Isles toward the north, and the Northern and Southern Appalachians to the south. The red dots mark locations where evidence for hyperextension (discussed in the podcast) has been observed.
BVBL, Baie Verte Brompton Line; FCL, Fairhead Clew Bay Line; HBF, Highland Boundary Fault; IAS, Iapetus suture; NR, Newfoundland re-entrant; QR, Quebec re-entrant; RIL, Red Indian Line; RP, Rockall promontory; SLP, St Laurence promontory; UA, Upper Allochthon.
Van Staal, C.R. & Dewey. J.F. (2023), Journal of the Geological Society, London, Special Publication vol. 531
Reconstruction of the Appalachian-Caledonian Mountain belt between Newfoundland, Ireland, and Scotland before the Mesozoic opening of the Atlantic Ocean. The colored regions indicate the elements of the Taconic-Grampian Tract.
For further explanation and depth beyond the scope of the podcast, see Van Staal and Dewey (2023) cited below and in the Further Reading section.
Van Staal, C.R. & Dewey. J.F. (2023), Journal of the Geological Society, London, Special Publication vol. 531
Appalachian Rocks of Newfoundland
In the podcast, Van Staal mentions the obduction of ophiolites that took place as the terranes originating from Gondwana accreted to the Laurentian margin. This image shows the gabbro of the c. 490 Ma Betts Cove ophiolite, located on the eastern shore of the Baie Verte Peninsula in northwestern Newfoundland.
Ocean-continent transition rocks of the c. 560 Ma Birchy Complex, which is located on the Baie Verte peninsula. Van Staal is fourth from left.
Van Staal studying the rocks of the highly deformed basaltic and boninitic rocks of the c. 490 Ma Advocate ophiolite on the north coast of the Baie Verte peninsula. These are part of the the Baie Verte ophiolite complexes.
The c. 478 Ma arc gabbro and associated tonalites that intruded into older Cambrian ophiolites. They form part of a volcanic arc that developed above these old ophiolites.
Cambrian trondhjemite (a type of tonalite, which is a granitoid), including xenoliths of older basalt, which include boninite.
Intrusion into the trondhjemite by younger c. 478 Ma mafic dykes.
Coastal exposures of northern Newfoundland.
Further Reading
Van Staal, C.R. and Dewey, J.F. (2023) A review and tectonic interpretation of the Taconian–Grampian tract between Newfoundland and Scotland: diachronous accretion of an extensive forearc–arc–backarc system to a hyperextended Laurentian margin and subsequent subduction polarity reversal, Journal of the Geological Society, London, Special Publication vol. 531
Van Staal, C.R. et al. (1998) The Cambrian-Silurian tectonic evolution of the northern Appalachians and British Caledonides: history of a complex, west and southwest Pacific-type segment of Iapetus, in: Blundell, D.J. & Scott, A.C. (eds) Lyell: the Past is the Key to the Present. Geological Society, London, Special Publications, 143, 199
Van Staal.C.R. (2019) North American Regional Geology Northern Appalachians, Encyclopaedia of Geology, 2nd edition https://doi.org/10.1016/B978-0-12-409548-9.12123-2
Van Staal, C.R. et al. (2021) Provenance and Paleozoic tectonic evolution of ganderia and its relationships with Avalonia and Meguma in the Appalachian-Caledonide orogen, Gondwana Research 98, 212 https://doi.org/10.1016/j.gr.2021.05.025
Van Staal, C.R & Barr, S.M. (2012) Lithospheric architecture and tectonic evolution of the Canadian Appalachians and associated Atlantic margin. Chapter 2 in Tectonic Styles in Canada: the Lithoprobe Perspecitve, Edited by Percival, F.a., and Clownes, R.M., Geological Association of canada, Special Paper 49, 41
Van Staal, C.R. & Zagorevski, A. (2022) Paleozoic tectonic evolution of the rifted margins of Laurentia, in Whitmeye, S.J. et al. eds., Laurentia: Turning Points in the Evolution of a Continent, Geological Society of America Memoir 220, 487 https://doi.org/10.1130/2022.1220(24)
Van Staal, C.R. et al. (2013) Evidence of Late Ediacaran Hyperextension of the Laurentian Iapetan Margin in the Birchy Complex, Baie Verte Peninsula, Northwest Newfoundland: Implications for the Opening of Iapetus, Formation of Peri-Laurentian Microcontinents and Taconic – Grampian Orogenesis, Geoscience Canada v. 40 http://dx.doi.org/10.12789/geocanj.2013.40.006
Van Staal, C.R. et al. (2012) Provenance and tectonic evolution of Ganderia: Constraints on the evolution of the Iapetus and Rheic oceans, Geology, 40, 987