Logan Medallist 1. Seeking the Suture: The Coast-Cascade Conundrum

Abstract

The boundary between rocks assigned to the Intermontane superterrane in the interior of the Canadian Cordillera and those of the Insular superterrane in the westernmost Cordillera of British Columbia and southeastern Alaska lies within/along the Coast Mountains, in which is exposed the core of an orogen 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) were near those of the Insular superterrane (Wrangellia and Alexander terranes) by the Early Jurassic (~180 Ma). This timing, as well as paleobiogeographic and paleomagnetic considerations, appears to discount a recent hypothesis that proposes westward-dipping subduction beneath an intra-oceanic arc on Insular superterrane resulted in arc-continent collision and inaugurated Cordilleran orogenesis in the Late Jurassic (~146 Ma). The hypothesis also relates the subducted ocean that had separated the superterranes to a massive, faster-than-average-velocity seismic anomaly in the lower mantle below the eastern seaboard of North America. To create such 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 no younger than Middle Jurassic (~174 Ma). This terrane, together with the probably related Bridge River terrane in the southeastern Coast Mountains, which is as young as latest Middle Jurassic (164 Ma) and possibly as young as earliest Cretaceous (≥ 130 Ma), appear to be the only candidates in Canada for the possible surface record of the seismic anomaly. SOMMAIRELa limite entre les roches assignées au Superterrane d’intermont de l’intérieur des Cordillères canadiennes et celles du Superterrane insulaire dans la portion la plus à l’ouest de la Cordillère de Colombie-Britannique et du sud-est de l’Alaska se trouvent dans et au long de la Chaîne côtière, au sein de laquelle affleure le noyau d’un orogène qui est apparu comme entité tectonique distincte entre 105 et 45 millions d’années. Des indices de la Chaîne côtière et des régions environnantes montrent que des portions du Superterrane d’intermont (dans les terranes de Stikinia et de Yukon-Tanana) se trouvaient alors près de celles du Superterrane insulaire (terranes de Wrangellia et d’Alexander) au début du Jurassique (~180 Ma). Cette chronologie, ajoutée à certains facteurs paléobiogéographiques et paléomagnétiques semblent discréditer une hypothèse récente voulant qu’une subduction à pendage ouest sous un arc intra-océanique sur le Superterrane insulaire résultait d’une collision entre un arc et le continent, initiant ainsi l’orogénèse de la Cordillère à la fin du Jurassique (~146 Ma). Cette hypothèse relie aussi l’océan subduit qui séparait les superterranes à une anomalie de vitesse sismique plus rapide que la normale dans le manteau inférieur sous le littoral maritime oriental de l’Amérique du Nord. Pour créer une telle anomalie, la subduction du plancher d’un grand océan était nécessaire. La seule indication de surface de l’existence d’un tel océan à l’intérieur de la Cordillère canadienne est le terrane de Cache Creek qui, bien qu’il se trouve dans le Superterrane d’intermont, est plus ancien que le Jurassique moyen (~174 Ma). Ce terrane, avec son équivalent probable de Bridge River dans le sud-est de la Chaîne côtière, qui est aussi jeune que la fin du Jurassique (164 Ma) et peut-être aussi jeune que le début du Crétacé (≥ 130 Ma), semblent être les seuls candidats au Canada offrant des vestiges en surface de cette anomalie sismique.

Late Jurassic flare‐up of the Coast Mountains arc system, NW Canada, and dynamic linkages across the northern Cordilleran orogen

“…Most recent studies of the northern Cordillera have recognized that reduced magmatic arc activity from 140 to 125 Ma was coeval with sinistral transcurrent displacement (>800 km) of the Coast Mountains arc, Gravina belt, and their underpinnings during the Early to mid‐Cretaceous [e.g., Monger and Struik , ; Gehrels et al ., ; Monger , ; Yokelson et al ., ]. As originally outlined by Monger et al .…”

“…[], Girardi et al . [], and Monger [] most recently summarized the Mesozoic‐Cenozoic magmatic evolution and paleogeography of the Coast Mountains batholith in British Columbia. These studies argued for Jurassic and younger intrusive rocks to have been generated in a west facing continental arc system that was built on accreted Insular and Intermontane terrane lithosphere (as originally proposed by Armstrong [] and van der Heyden []).…”

“…[] implied that original plate tectonic ties between terranes cannot be assumed, mounting evidence suggests that many Cordilleran terranes evolved together during the Paleozoic‐Mesozoic [ Monger and Nokleberg , ; Colpron and Nelson , ; Nelson et al ., ]. In the Canadian Cordillera, Mesozoic arc intrusions assigned to the Coast Mountains batholith were emplaced into terranes of both the Insular and Intermontane morphogeological belts (Figure a) [ Gabrielse et al ., ; van der Heyden , ; Monger , ]. The Insular terranes mainly consist of the Alexander terrane and Wrangellia in eastern and southeastern Alaska, southwestern Yukon, and coastal British Columbia (Figure b).…”

“…The Gravina depocenters were likely part of a belt of basins from Alaska to Mexico [ McClelland et al ., ; Anderson , ] that resulted from oblique plate convergence [ Engebretson et al ., ] and accelerated north to northwest directed motion of North America after the opening of the central Atlantic Ocean [e.g., Wynne et al ., ; Withjack et al ., ; Bird et al ., ]. The Gravina depocenters were subsequently the site of Early and Late Cretaceous transcurrent faulting and mid‐Cretaceous shortening [ Journeay and Friedman , ; Rusmore and Woodsworth , ; Haeussler , ; McClelland et al ., ; Umhoefer and Miller , ; McClelland and Mattinson , ], the latter of which was associated with the final imbrication of the Insular and Intermontane terranes and development of the Coast‐Cascade orogen in the Coast Mountains [ Rusmore et al ., ; Monger , ].…”

Along‐Strike Variation in the Magmatic Tempo of the Coast Mountains Batholith, British Columbia, and Implications for Processes Controlling Episodicity in Arcs

“…For this study, we interpret the post‐100 Ma magmatism as a single arc because, as discussed herein, magmatism migrates steadily eastward as expected in a single east‐dipping arc and lacks disruptions that would signal significant translation. Post‐100 Ma translations suggested by paleomagnetic results (Rusmore et al, ) likely took place on faults east of the batholith (Monger, ) and/or intrabatholithic faults parallel to the arc, leaving the magmatic pattern undisturbed. Better delineation of the distribution of the pre‐100 Ma arc or arcs and the boundaries of proposed far‐traveled blocks is needed to resolve these paleogeographic uncertainties.…”