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Logan Medallist 1. Seeking the Suture: the Coast-Cascade Conundrum Jim W.H

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Logan Medallist 1. Seeking the Suture: the Coast-Cascade Conundrum Jim W.H Document generated on 09/25/2021 7:54 a.m. Geoscience Canada Journal of the Geological Association of Canada Journal de l’Association Géologique du Canada Logan Medallist 1. Seeking the Suture: The Coast-Cascade Conundrum Jim W.H. Monger Volume 41, Number 4, 2014 Article abstract The boundary between rocks assigned to the Intermontane superterrane in the URI: https://id.erudit.org/iderudit/1062254ar interior of the Canadian Cordillera and those of the Insular superterrane in the DOI: https://doi.org/10.12789/geocanj.2014.41.058 westernmost Cordillera of British Columbia and southeastern Alaska lies within/along the Coast Mountains, in which is exposed the core of an orogen See table of contents that emerged as a discrete tectonic entity between 105 and 45 million years ago. Evidence from the Coast Mountains and flanking areas indicates that parts of the Intermontane superterrane (in Stikinia and Yukon-Tanana terranes) Publisher(s) were near those of the Insular superterrane (Wrangellia and Alexander terranes) by the Early Jurassic (~180 Ma). This timing, as well as The Geological Association of Canada paleobiogeographic and paleomagnetic considerations, appears to discount a recent hypothesis that proposes westward-dipping subduction beneath an ISSN intra-oceanic arc on Insular superterrane resulted in arc-continent collision and inaugurated Cordilleran orogenesis in the Late Jurassic (~146 Ma). The 0315-0941 (print) hypothesis also relates the subducted ocean that had separated the 1911-4850 (digital) superterranes to a massive, faster-than-average-velocity seismic anomaly in the lower mantle below the eastern seaboard of North America. To create such Explore this journal an anomaly, subduction of the floor of a large ocean was needed. The only surface record of such an ocean in the interior of the Canadian Cordillera is the Cache Creek terrane, which lies within the Intermontane superterrane but is Cite this article no younger than Middle Jurassic (~174 Ma). This terrane, together with the probably related Bridge River terrane in the southeastern Coast Mountains, Monger, J. (2014). Logan Medallist 1. Seeking the Suture: The Coast-Cascade which is as young as latest Middle Jurassic (164 Ma) and possibly as young as Conundrum. Geoscience Canada, 41(4), 379–398. earliest Cretaceous (≥ 130 Ma), appear to be the only candidates in Canada for https://doi.org/10.12789/geocanj.2014.41.058 the possible surface record of the seismic anomaly. All Rights Reserved ©, 2014 The Geological Association of Canada This document is protected by copyright law. Use of the services of Érudit (including reproduction) is subject to its terms and conditions, which can be viewed online. https://apropos.erudit.org/en/users/policy-on-use/ This article is disseminated and preserved by Érudit. Érudit is a non-profit inter-university consortium of the Université de Montréal, Université Laval, and the Université du Québec à Montréal. Its mission is to promote and disseminate research. https://www.erudit.org/en/ GEOSCIENCE CANADA Volume 41 2014 379 GAC MEDALLIST SERIES (Wrangellia and Alexander terranes) by la Chaîne côtière, au sein de laquelle the Early Jurassic (~180 Ma). This tim- affleure le noyau d’un orogène qui est ing, as well as paleobiogeographic and apparu comme entité tectonique dis- paleomagnetic considerations, appears tincte entre 105 et 45 millions d’an- to discount a recent hypothesis that nées. Des indices de la Chaîne côtière proposes westward-dipping subduction et des régions environnantes montrent beneath an intra-oceanic arc on Insular que des portions du Superterrane d’in- superterrane resulted in arc-continent termont (dans les terranes de Stikinia collision and inaugurated Cordilleran et de Yukon-Tanana) se trouvaient orogenesis in the Late Jurassic (~146 alors près de celles du Superterrane Ma). The hypothesis also relates the insulaire (terranes de Wrangellia et Logan Medallist 1. subducted ocean that had separated d’Alexander) au début du Jurassique the superterranes to a massive, faster- (~180 Ma). Cette chronologie, ajoutée Seeking the Suture: The than-average-velocity seismic anomaly à certains facteurs paléobiogéo- Coast-Cascade Conundrum1 in the lower mantle below the eastern graphiques et paléomagnétiques sem- seaboard of North America. To create blent discréditer une hypothèse récente voulant qu’une subduction à pendage J.W.H. Monger such an anomaly, subduction of the floor of a large ocean was needed. The ouest sous un arc intra-océanique sur only surface record of such an ocean le Superterrane insulaire résultait d’une Emeritus Scientist in the interior of the Canadian collision entre un arc et le continent, Geological Survey of Canada Cordillera is the Cache Creek terrane, initiant ainsi l’orogénèse de la Cordil- 1500-605 Robson St., Vancouver which lies within the Intermontane lère à la fin du Jurassique (~146 Ma). British Columbia, V6B 5J3, Canada superterrane but is no younger than Cette hypothèse relie aussi l’océan sub- E-mail: [email protected] Middle Jurassic (~174 Ma). This ter- duit qui séparait les superterranes à une rane, together with the probably relat- anomalie de vitesse sismique plus rapi- SUMMARY ed Bridge River terrane in the south- de que la normale dans le manteau The boundary between rocks assigned eastern Coast Mountains, which is as inférieur sous le littoral maritime orien- to the Intermontane superterrane in young as latest Middle Jurassic (164 tal de l’Amérique du Nord. Pour créer the interior of the Canadian Cordillera Ma) and possibly as young as earliest une telle anomalie, la subduction du and those of the Insular superterrane Cretaceous (≥ 130 Ma), appear to be plancher d’un grand océan était néces- in the westernmost Cordillera of the only candidates in Canada for the saire. La seule indication de surface de British Columbia and southeastern possible surface record of the seismic l’existence d’un tel océan à l’intérieur Alaska lies within/along the Coast anomaly. de la Cordillère canadienne est le ter- Mountains, in which is exposed the rane de Cache Creek qui, bien qu’il se core of an orogen that emerged as a SOMMAIRE trouve dans le Superterrane d’inter- discrete tectonic entity between 105 La limite entre les roches assignées au mont, est plus ancien que le Jurassique and 45 million years ago. Evidence Superterrane d’intermont de l’intérieur moyen (~174 Ma). Ce terrane, avec from the Coast Mountains and flank- des Cordillères canadiennes et celles du son équivalent probable de Bridge ing areas indicates that parts of the Superterrane insulaire dans la portion River dans le sud-est de la Chaîne Intermontane superterrane (in Stikinia la plus à l’ouest de la Cordillère de côtière, qui est aussi jeune que la fin du and Yukon-Tanana terranes) were near Colombie-Britannique et du sud-est de Jurassique (164 Ma) et peut-être aussi those of the Insular superterrane l’Alaska se trouvent dans et au long de jeune que le début du Crétacé (≥ 130 1This article is the first in a series featuring research papers from recipients of GAC's prestigious Logan and Hutchison medals. Geoscience Canada, v. 41, http://dx.doi.org/10.12789/geocanj.2014.41.058 © 2014 GAC/AGC® 380 Ma), semblent être les seuls candidats in the lower mantle as massive, near- superterrane formed the Mezcalera au Canada offrant des vestiges en sur- vertical ‘slab walls’ as much as slab wall, and brought the Intermon- face de cette anomalie sismique. 800–2000 km deep and 400–600 km tane superterrane (previously accreted thick. Today the slab walls mostly lie to the leading edge of the North INTRODUCTION below the North American continent American Plate) into contact with the The innovative and thought-provoking and its eastern seaboard. Sigloch and Insular superterrane, initially in the paper by Sigloch and Mihalynuk (2013) Mihalynuk (2013) contend the slab Late Jurassic (146 ± 24 Ma). Because challenges the ‘conventional wisdom’ walls are geographically relatively the Coast Mountains of Canada and that suggests the Cordillera is largely immobile and serve as markers, called southeastern Alaska now separate the the product of plate convergence ‘terrane stations,’ that can be used (like superterranes, any Late Jurassic – Cre- between plates flooring the Pacific mantle plumes) to track westward taceous suture is within/along that Ocean and its ancestors and the mar- movement of the North American range (Fig. 1). gin of the Laurentian craton, with arcs Plate across the lower mantle. This review examines evidence formed above subduction zones that Comparison of x-y-z positions for the existence of such a suture mostly dipped towards the craton (e.g. of slab walls with the surface record of between superterranes along the Coast Monger and Price 2002; Dickinson Cordilleran orogenesis and with global Mountains where the original terrane 2004; Nelson et al. 2013). Such scenar- plate reconstructions showing the relationships are obscured or obliterat- ios vary in detail but basically suggest sequentially westward-younging posi- ed by mid-Cretaceous – early Cenozoic that in the late Paleozoic, convergence tions of the Pacific margin of North (~105–45 Ma) granitic intrusions, created magmatic arcs offshore of America (e.g. Shephard et al. 2012) led deformation, and metamorphism. what was then northwestern Pangea in Sigloch and Mihalynuk (2013) to con- Sequentially below, Coast Mountains a setting probably analogous to the clude that collision of the margin with geology is reviewed; paleogeographic present southwestern Pacific Basin. the intra-oceanic arcs initiated moun- flags raised by paleomagnetic studies Remnants of those arcs form most tain-building. Some younger slab walls, on Late Cretaceous rocks are noted; accreted terranes in the Cordillera. By such as the northern and southern aspects of terranes flanking the Coast the Middle Jurassic (174 Ma), all major remnants of the Farallon Plate (respec- Mountains are summarized; evidence is offshore arc terranes had been accreted tively Juan de Fuca and Cocos plates) examined from south to north along to the outer craton margin, although can be traced into active east-dipping the Coast Mountains for times of ter- subsequently they were disrupted and subduction zones.
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