Academic literature on the topic 'Grand Chapter of British Columbia and the Yukon'

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.

Serpentine plant life varies dramatically across western North America from north to south and, to a lesser extent, from the coast inland. At the latitudinal extremes in Alaska and Baja California, it follows patterns seen in other climatically harsh parts of the world (as discussed in chapter 10), but the species composition is not very distinctive and there are few endemics. In between, in Washington, Oregon, and especially in the California Floristic Province, lies a great diversity of distinctive serpentine vegetation types and endemic species. This chapter outlines the coarse patterns of variation in vegetation structure and endemic species richness across this region, as a prelude to chapter 12, which describes specific serpentine vegetation types in detail. Little has been published about the serpentine vegetation of Alaska and the Yukon. The Serpentine Slide Research Natural Area in central Alaska was described by Juday (1992) as having a mixture of white spruce (Picea glauca) and paper birch (Betula papyrifera) with Rosa acicularis, Juniperus communis, and Vaccinium uliginosum in the understory. Several herbs are shared with Swedish serpentines (e.g., Campanula rotundifolia, Minuartia rubella, Rumex acetosa, Saxifraga oppositifolia, Silene acaulis); several others showed northern range extensions on serpentine (see chapter 9). The ultramafic vegetation of Golden Mountain in southeast Alaska was described by Alexander et al. (1989) as alpine meadow containing forbs, graminoids, and low shrubs, with a transition through low shrubs and stunted lodgepole pines (Pinus contorta) down to spruce (Picea sitchensis)–hemlock (mainly Tsuga mertensiana) forest, with shrubs and some cedar (Chamaecyparis nootkatensis). The forest to alpine transition was lower on serpentine than on other soils. No serpentine endemic species are known from this region. From south–central British Columbia to central Oregon, serpentine has been described by Kruckeberg (1969, 1992) as supporting open stands of various conifers, which are either a subset of the species occurring in adjacent denser forests on other soils or represent elevational or geographic range shifts. Understories are sparse and may include graminoids, perennial forbs, and shrubs. On rocky ridgetops and at higher elevations and latitudes, these conifer plant communities give way gradually to alpine tundra.