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
thumb|right|500px|The location and extent of the Anahim Volcanic Belt
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.
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.
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. 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. Four volcanic episodes characterized by highly fluid alkaline and peralkaline lava flows created an thick assemblage exposed on the north flank. A basal sequence of comenditic trachyte flows are unconformably overlain by flows and flow breccias of mugearite which in turn are overlain by a thick sequence of columnar-jointed comendite flows. Late stage volcanism resulted in the creation of scattered hawaiite dikes, plugs and minor capping flows over the north flank. The lower shield assemblage represents the oldest exposed rocks in the Ilgachuz Range. It comprises pyroclastic breccias, domes and lava flows of alkali rhyolite that have been hydrothermally altered. The overlying upper shield assemblage, which forms the bulk of the Ilgachuz Range, encompasses a series of comendite and pantellerite flows and domes that are interbedded with flows of alkali basalt and hawaiite. Formation of the shield was followed by collapse of a small central caldera in which the intracaldera assemblage was deposited. This assemblage consists of a thick sequence of tuffs that are overlain by a ponded single cooling unit of trachyte more than in thickness.
The Baldface Mountain volcanic field is a group of scattered cones east of the Itcha Range. Argon–argon dating of seven cones indicate the volcanic field was active 2.5 to 0.91 million years ago, during which time the Itcha Range and the Satah Mountain volcanic field were also areas of volcanicity. Baldface Mountain is the largest and one of the oldest volcanoes in the Baldface Mountain volcanic field. It consists of a 2.37 million year old cone of aphanitic phonolite and porphyritic trachyte with feldspar phenocrysts. A basaltic lava flow near the head of Moore Creek has an age of 3.91 million years, similar to the oldest rocks known from the Itcha Range or the Chilcotin Group basalts of the surrounding plateau. The relation of this flow to the Baldface Mountain volcanic field is in question as its source and extent have not been identified.
From 9 October 2007 to 15 May 2008, a series of earthquakes measuring up to 2.9 magnitude occurred in the Nechako Basin about west of Nazko Cone. Most of them occurred below the surface, indicating they originated within the lowermost crust. Analysis of seismic waves suggest that the earthquake swarm was caused by brittle failure and fracturing of rock at depth from magma intrusion. No volcanic eruption was likely as the number and size of the tremors were too small. Although these volcano tectonic earthquakes were too small to be felt, they generated substantial local interest as they represented a significant concentration of seismic activity within the Anahim Volcanic Belt.