SLAB WINDOWS DRIVE EOCENE FOREARC MAGMATISM ON VANCOUVER ISLAND

A new model of Paleogene ridge subduction and slab window formation beneath the central North American Cordillera has been constructed in a fixed-hotspot reference frame to account for Eocene forearc magmatism on Vancouver Island. According to previous models, subduction of the Kula-Farallon and Pacific-Juan de Fuca ridge systems occurred beneath western North America in Late Cretaceous to Recent time. From ca. 80-39 Ma the Kula-Farallon ridge subducted below North America as it migrated northward. At Chron 18r (39 Ma), the Kula and Pacific plates fused, spreading at the Kula-Farallon ridge ceased, and subduction of the Pacific-Juan de Fuca (Vancouver) ridge began. Subduction of the Juan de Fuca plate and subduction-to-transcurrent motion of the Pacific plate along the Queen Charlotte fault has continued to the present. These aspects of plate history are widely accepted but the locations of the ridge-trench intersections and their resultant slab windows are controversial. Our model uses U-Pb geochronology from forearc intrusions on Vancouver Island to help constrain the locations and timing of ridge-trench intersections.

Vancouver Island has been in a forearc position since at least Early Eocene time. Plutonic and volcanic rocks of Early to Late Eocene age are common on Vancouver Island and indicate that the forearc was anomalously hot during this interval, probably due to mid-ocean ridge subduction. Eight new U-Pb dates from hypabyssal felsic intrusions combined with existing data indicate forearc magmatism occurred in two discrete episodes. The first occurred from ca. 52-48 Ma (supported by new dates of 51.2 +/- 0.4 and 48.8 +/- 0.5 Ma), and is attributed to northward passage of the Kula-Farallon slab window from southern Vancouver Island to Queen Charlotte Islands. The second occurred from ca. 39-35 Ma (supported by new dates of 38.6 +/- 0.1, 38.6 +/- 0.2, 37.4 +/- 0.2, 36.9 +/- 0.2, 35.4 +/- 0.2 and 35.3+/-0.3 Ma), and is attributed to subduction of the Juan de Fuca ridge and formation of the Pacific-Juan de Fuca slab window.