The Anahim Volcanic Belt (AVB) is a west–east trending chain of volcanoes and related magmatic features in British Columbia, Canada. It extends from Athlone Island on the Central Coast, running eastward through the strongly uplifted and deeply dissected Coast Mountains to near the community of Nazko on the Interior Plateau. The AVB is delineated as three west-to-east segments that differ in age and structure. A wide variety of igneous rocks with differing compositions occur throughout these segments, comprising landforms such as volcanic cones, volcanic plugs, lava domes, shield volcanoes and intrusions.
Volcanic activity has occurred repeatedly in the AVB for the last 15 million years, during which time three major magmatic episodes took place 15–13, 9–6 and 3–1 million years ago. These major magmatic episodes are represented by plutons, dike swarms, volcanic fields and large shield volcanoes. Volcanic activity in the last 1 million years has been relatively minor and localized, having produced small lava flows and cones. The last eruptive period took place at the easternmost end of the volcanic belt 7,200 years ago, with magma-induced earthquakes having occurred as recently as 2007–2008.
Geology
The AVB is one of the six volcanic provinces in British Columbia that formed during the Neogene–Quaternary period. It consists mainly of alkaline to peralkaline rocks that range from oversaturated, highly evolved phonolites and rhyolites to more undersaturated lavas. Several AVB centres overlie Miocene flood basalts of the Chilcotin Group, which in turn overlie Devonian to Jurassic rocks of the Stikinia volcanic arc terrane. In some places the AVB lavas imperceptibly merge with the Chilcotin Group basalts as the two volcanic zones were active simultaneously from the Neogene to Quaternary.
This west–east trending volcanic belt is about long, extending from the Central Coast through the Coast Mountains to near the community of Nazko on the Interior Plateau. Its orientation is unique among the Neogene–Quaternary volcanic provinces of British Columbia in that it is perpendicular to the major geomorphological, structural and tectonic elements of the Canadian Cordillera. Three segments comprise the AVB: the western segment, which has been reduced to remnants of eruptive breccia, high-level plutons and dike swarms; the central segment, which consists of predominantly shield volcanoes; and the eastern segment, which consists of several small cinder cones and is the location of all modern volcanic activity.
Several tectonic models have been proposed to explain the origin of the AVB. These include rifting, a mantle hotspot, a plate-edge effect or slab window leading to magma ascent along the northern edge of the Juan de Fuca Plate, or a propagating crack controlled by stress fields related to large-scale plate tectonics of western North America. The generally preferred tectonic model is a mantle hotspot as the AVB decreases in age from west to east, a trend that is consistent with the westward motion of North America at the rate of about per year. There are also observed chemical differences between the oldest (western) and youngest (eastern) basalts. The youngest basalts are undersaturated alkaline lavas and are classified as basanite, suggesting a deeper source zone for magma in the Nazko area compared to the rest of the AVB.
Western segment
The Bella Bella and Gale Passage dike swarms on the Central Coast of British Columbia form the westernmost extent of the AVB. They are both over wide, with individual dikes measuring up to thick. Both swarms attain K–Ar ages of 12.5 to 14.5 million years and consist of a bimodal suite of basalt, trachyte and comendite. Basaltic and comenditic dikes are the most abundant, distributed in about equal proportions. Near the centre of each dike swarm are erosional remnants of rhyolite breccia with locally occurring rhyolite flows that comprise the Bella Bella Formation. The breccias contain a high content of coarse basement clasts, indicating that they are the product of explosive volcanism. Rhyolites associated with the Bella Bella dike swarm are distributed on Denny Island whereas rhyolites belonging to the Gale Passage dike swarm are found on the northern ends of Athlone Island and Dufferin Island.