Generation of Eocene volcanic rocks from the Cordilleran arc of south-central British Columbia (Canada) during subduction of the Farallon and Resurrection plates and Yellowstone oceanic plateau

Eocene calc-alkaline volcanic rocks (Challis-Kamloops belt) of south-central British Columbia are part of a chain of 55-45 Ma igneous rocks running along the eastern side of the Coast Mountains Batholith that straddles the Canada-United States border. This chain has been interpreted as resulting from east-vergent subduction of oceanic plates in the Pacific Ocean under the North American continent. The volcanic rocks are mainly mafic-intermediate calc-alkaline and subordinate alkaline rocks with trace element and Sr and Nd isotope characteristics typical of subduction-related, distal continental arcs. The mafic rocks were sourced from heterogeneous, subduction-modified, subcontinental lithospheric mantle at various depths by variable degrees of melting. The volcanic rocks are associated with the formation of block-faulted basins in an extensional arc setting. Positive seismic P-wave anomalies in the northern USA image a horizontal slab between two steeply dipping, approximately 500-km-long slabs representing the present Cascadia subduction zone in the west, and a fossil subduction zone beneath the Challis-Kamloops belt. The horizontal slab is interpreted as the Yellowstone oceanic plateau that eventually choked the trench leading to a westward trench jump. Underthrusting of the Yellowstone oceanic plateau is inferred to have led to subduction erosion beneath the leading edge of the Cordilleran arc followed by extrusion as the Central Gneiss Complex into the upper plate. The combination of trench jump and subduction of the Resurrection-Kula ridge terminated the Challis-Kamloops magmatism and further opening of the slab window leading to the northward motion of the Kula Plate relative to North America that produced oblique rifts and core complexes in British Columbia.